Title of Invention

"A METHOD FOR PRODUCTION OF A MIXTURE OF ANTIBODIES"

Abstract

The present invention relates to a method for production of a mixture oi antibodies in a single recombinant host, the method comprising the step of: expressing in a recombinant host cell a nucleic acid sequence or nucleic acid sequences encoding one common light chain and at least three different heavy chains that are capable of pairing with said one common light chain, such that the produced antibodies comprise common light chains.

Full Text

Field of the invention The present invention relates to the field of medicine, more in particular to the field of production of antibodies, more in particular to the production of mixtures of antibodies. if Background of the invention ■r The essential function of the immune system is the defence against infection. The humoral immune system combats molecules recognized as non-self, such as pathogens, using immunoglobulines. These immunoglobulines, also called antibodies, are raised specifically against the infectious agent, which acts as an antigen, upon first contact (Roitt, Essential Immunology, Blackwell Scientific Publications, fifth edition, 1984; all references cited herein are incorporated in their entirety by reference). Antibodies are multivalent molecules comprising heavy (H) chains and light (L) chains joined with interchain disulfide bonds. Several isotypes of antibodies are known, including IgGl, ■i IgG2, IgG3, IgG4, IgA, IgD, IgE, and IgM. An IgG contains two heavy and two light chains. Each chain contains Constant (C) and variable (V) regions, which can be broken down into domains designated CH1, CH2, CH3 VH, and CL, VL (Fig. 1) . Antibody binds to antigen via the variable region domains contained in the Fab portion, and after binding can interact with molecules and cells of the immune system through the constant domains, mostly through the Fc portion. B-lymphocytes can produce antibodies in response to exposure to biological substances like bacteria, viruses and their toxic products. Antibodies are generally epitope specific and bind strongly to substances carrying these epitopes. The hybridoma technique (Kohler and Milstein 1975) makes use of the ability of B-cells to produce monoclonal antibodies to specific antigens and to subsequently produce these monoclonal antibodies by fusing B-cells from mice exposed to the antigen of interest to immortalized murine plasma cells. This technology resulted in the realization that monoclonal antibodies produced by hybridoma's could be used in research, diagnostics and therapies to treat different kinds of diseases like cancer and autcimmune related disorders. Because antibodies-'that are produced in mouse hybridoma's induce strong immune responses in humans, it has been appreciated in the'art that antibodies required for successful treatment of humans needed to be less or preferably non-immunogenic. For this, rnurine antibodies were first engineered by replacing the murine constant regions with human, constant regions (referred to as chimeric antibodies). Subsequently, domains between the complementarity determining regions (CDRs) in the variable domains, the so-called framework regions, were replaced by their human counterparts (referred to as humanized antibodies). The final stage in this humanization process has been the production of fully human antibodies. In the art, also bispecific antibodies, which have binding specificities for two different antigens, have been described. These are generally used to target a therapeutic or diagnostic moiety, e.g. T-cell, a cytotoxic trigger molecule or a chelator that binds a radionuclide, that is recognized by one variable region of the antibody to a cell that is recognized by the other variable region of the antibody, e.g. a tumor cell (see for bispecific antibodies Segal et al, 2001) . One very useful method known in the art to obtain fully human monoclonal antibodies with desirable binding properties employs phage display libraries. This is an in vitro, recombinant DNA-based, approach that mimics key features of the humoral immune response (see for phage display methods e.g; CF Barbas III et al. Phage Display. A laboratory manual.. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York, 2001}. For the construction of phage display libraries, collections of human monoclonal antibody heavy and light chain variable region I- genes are expressed on the surface of bacteriophage particles, usually in single chain Fv (scFv) or in Fab format. Large libraries of antibody fragment-expressing phages typically contain more than 10^ antibody specificities and may be assembled from the immunoglobulin V regions expressed in the B lymphocytes of immunized- or non-immunized individuals. Alternatively, phage display libraries may be constructed from immunoglobulin variable regions that have been partially assembled or rearranged in vitro to introduce additional antibody diversity in the library (semi¬synthetic libraries} (De Kruif et al, 1995b). For example, in vitro assembled variable regions contain stretches of synthetically produced, randomized or partially randomized DNA in "those regions of the molecules that are important for antibody specificity. The genetic information encoding the antibodies identified by phage display, can be used for cloning the antibodies in a desired format, e.g. IgG, IgA or IgM, to produce the antibody with recombinant DNA methods (Boel et al, 2000). An alternative method to provide fully human antibodies uses transgenic mice that comprise genetic material encoding a human immunoglobulin repertoire (Fishwild et al, 1996; Mendez eti al, 1997). Such mice can be immunized with a target antigen, and the resulting immune response will produce fully'human antibodies. The sequences of these antibodies can be used in recombinant production methods. Production of monoclonal antibodies routinely is performed by use of recombinant expression of the nucleic acid sequences encoding the H and L chains of antibodies in host cells (see e.g. EP0120694; EP0314161; EP0481790; US patent 4,816,567; WO 00/634C3). To date, man^/ different diseases are being treated-with either humanized or fully human monoclonal antibodies. Products based on monoclonal antibodies that are currently approved for use in humans include Herceptin™ (anti-Her2/Neu) , ReoproTM (anti-Glycoprotein IIB/IIIA receptor), Mylotarg™ (anti-CD33), Rituxan^" (Rituximab, anti-CD20) , Simulect™ {anti-CD25), Remicade™ (anti-TNF), Synagis™ (anti-RSV), Zenapax™ (IL2-receptor), CAMPATH™ (anti-CD'52) . Despite these successes, there is still room for new antibody products and for considerable improvement of existing antibody products. The use of monoclonal antibodies in cancer treatment has shown that so-called 'antigen-loss tumor variants" can arise, making the treatment with the monoclonal antibody less effective. Treatment with the very successful monoclonal antibody Rituximab (anti-CD2C) has for instance shown that antigen-loss escape variants can occur, leading to relapse of the lymphoma (Massengale et al, 2002). In the art, the potency ofimonoclonal antibodies has been increased by fusing them to toxic compounds, such as radionuclides, toxins, cytokines, and the like. Each of these approaches however has its limitations, including technological and production problems and/or high toxicity. Furthermore, it appears that the gain in specificity of monoclonal antibodies compared to traditional undefined polyclonal antibodies, goes at the cost of loss of efficacy. In vivo, antibody responses are polyclonal in nature, i.e. a mixture of antibodies is produced because various B-cells respond to the antigen, resulting in various specificities being present in the polyclonal antibody mixture. Polyclonal antibodies can also be used for therapeutic applications, e.g. for passive vaccination or for active immunotherapy, and currently are usually derived from pooled serum from immunized animals or from hutnans who recovered from the disease. The pooled serum is purified into the proteinaceous or gamma globulin fraction, so named because it contains predominantly IgG molecules. Polyclonal antibodies that are currently used for treatment include anti-rhesus polyclonal antibodies, gamma-globulin for passive immunization, anti-snake venom polyclonal (CroFab), Thymoglobulin™ for allograft rejection, anti-digoxin to neutralize the' heart drug digoxdn, and anti-rabies polyclonal. In currently marketed therapeutic -antibodies, an example of the■higher efficacy of polyclonal antibodies compared.to monoclonal antibodies can be found in the treatment of acute transplant rejection with anti- T-cell antibodies. The monoclonal antibodies on the market (anti-CD25 basiliximab) are less efficacious than a rabbit polyclonal antibody against thymocytes (Thymoglobulin™) (press releases dated March 12, April 29, and August 26, '2002, on www.sangstat.com). The use of pooled human sera however potentially bears the risk of infections with viruses such as HIV or hepatitis, with toxins such as lipopolysaccharide, with proteinaceous infectious agents such as prions, and with unknown infectious agents. Furthermore, the supply that is 'f available is limited, and insufficient for widespread human treatments, 'iproblems associated vith the current application of polyclonal antibodies derived from animal sera in the clinic include a strong immune response of the human immune system against such foreign antibodies. Therefore such polyclonals are not suitable for repeated treatment, or for treatment of individuals that were injected previously with other serum preparations from the same animal species. The art describes the idea of the generation of animals with a human immunoglobulin repertoire, which can subsequently be used for immunization with an antigen to obtain polyclonal, antibodies against this antigen from the transgenic animals (WO 01/19394}. However, much r technological hurdles still will have to be overcome before such a system is a practical reality in larger animals than mice, and it will -ake years of development before such systems can provide the polyclonal antibodies in a safe.and consistent manner in sufficient quantities. Moreover, antibodies produced from pooled sera, whether being from human or animal origin, will always comprise a high amount of unrelated and undesired specificities, as only a small percentage of the antibodies present ih a given serum will be directed against the antigen used for immunization. It is for instance known that in normal, i.e. non-transgenic, animals about 1-10% of the circulating immunoglobulin fraction is directed against the antigen used for hyper-immunization, hence the vast majority of circulating immunoglobulins is not specific. One approach towards expression of polyclonal antibody libraries it has been described (WO 95/20401; US patents 5,789,208 and 6,335,163). A polyconal library of Fab antibody fragments is expressed using a phage display vector, and selected for reactivity towards an antigen. The selected heavy, and light chain variable region gene combinations are transferred in mass, as linked pairs, to a eukaryotic expression vector that provides constant region genes, to obtain a sublibrary of intact polyconal antibodies. Upon transfection of this sublibrary into myeloma cells, stable clones produce monoclonal antibodies that can be mixed to obtain a polyclonal antibody mixture, 'while in theory it would be possible to obtain polyclonal^i antibodies directly from a single recombinant production process using this method by culturing a mixed population of transfected cells, potential problems would occur concerning the stability of the mixed cellv population, and hence the consistency of the produced polyclonal antibody mixture. The control of a whole population of different cells in a pharmaceutically acceptable large (i.e. industrial) scale process, is a daunting task. It would seem, that characteristics such as growth rates of the cells and production rates of the antibodies should remain stable for all the individual clones of the non-clonal population in order to keep the ratio of antibodies in the polyclonal antibody mixture more or less constant. Thus, while the need for mixtures of antibodies may have been recognized in the art, no acceptable solutions exist to make mixtures of antibodies economically in a pharmaceutically acceptable way. It is the object of the present invention.,to provide novel means for producing a mixture of antibodies in recombinant hosts. Brief description of the figures Fig. 1 Schematic representation of an antibody. The heavy and light chains are paired via interchain disulfide bonds (dotted lines). The heavy chain can be either of the a, Y,,[i, 5 or e isotype. The light chain is either X or K. An antibody of IgGl isotype is shown. Fig. 2 Schematic representation of a bi-specific monoclonal antibody. A bi-specific antibody contains two different fuctional F(Ab) domains, indicated by the different patterns of the VH-VJ, regions. I rig. 3 Sequence alignment of VL and VH of K53, UBS-54 and 02-237. DNA sequence of common V^ of UBS54 and K53 is iEQ. ID..NO. 1, while the amino acid sequence is given as SEQ. ID. NO. 2. DNA sequences of V,, of 02-237, and VH of 'BS54, K53 and 02-237 are SEQ. ID. NOs. 3, 5, 7 and 9, ■espectively, while the amino acid sequences are given in EQ. ID. NOs. 4, 6, 8 and 10, respectively. Fig. 4 Overview 'of plasmids pUBS3000Neo and pCD46_3000 {Neo}. Fig. 5 A. lEF of transiently expressed pUBS3000Neo, pCD46_3000 (Neo) and a combination of both. B. .The upper part shows a schematic representation of the expected molecules when a single ligh" chain and a single heavy chain are expressed in a cell, leading to monoclonal antibodies UBS-54 or K53. The lower part under the arrow F shows a schematic representation of the combinations produced when both heavy chains and the common light chain are co-expressed in a host cell, with theoretical amounts when both heavy chains are expressed at equal levels and pair to each other with equal efficiency. The common light chain is indicated with the vertically striped bars. Fig. 6. Schematic, representation of a possible embodiment of the method according to the invention (see e.g. Examples 9). (1) Introduction of nucleic acid sequences encoding one light chain and three different heavy chains capable of pairing to the common light chain to give functional antibodies into host cells; (2) selection of stable clones; (3i) clones can be screened for e.g. expression levels, binding; (4) expand clones; (5) production of functional mixtures of antibodies. Some or all of steps 2-5 could be performed simultaneously or in -different order. Fig. 7. Sequence of v^ and VL of phages directed against' CD22 (clone B28),^CD72 .(clone 11-2} and HLA-DR, (class II; clone 1-2). DNA sequences of V^ of clones B28, 11-2 and I- 2 are'sEQ. ID. NOs. H, 13 and 15, respectively, while^ the amino acid sequences are SEQ. ID. NOs. 12, 14 and 16, respectively. DNR sequence of the common light chain of these clones is SEQ. ID. NO. 17, while the amino acid sequence is SEQ. ID. NO. 18. Fig. 8. Map of pUBS54-IgA (pCRU-LOl encoding human IgAl against EPCAM). Fig. 9. Dimeric bi-specific IgA with a single light chain (indicated by horizontally striped bar). The method of the invention will produce a mixture of forms wherein different heavy chains can be paired, and only the most simple form is depicted in this schematic representation. A J-chain is shown to join the two monomers. Fig. 10. Pentameric multispecific IgM with a single light chain (indicated by horizontally striped bars). The method of the invention will produce a mixture of many I different form.s wherein different heavy chains can be paired, and only the most simple form when 5 different heavy chains are expressed with a single light chain, and all 5 different heavy chains are incorporated in the pentamer and paired to the sam.e heavy chain, is depicted in this schematic representation. Pentamers with less specificities can also be formed by incorporation of less than 5 different ^heavy chains. Especially when the J-chain is not expressed, hexamers can also be obtained. Fig. 11. Expression of a mixture of human IgG isotypes consisting of a common light chain, but with different binding specificities in a single cell to avoid the formation of bispecific antibodies. The different binding specificities are indicated by the different colours of the VH sequences. The comrr.on light chain is indicated with the vertically striped bars. The XgGl isotype is indicated with the grey Fc, ihe IgGB isotype is indicated with the black Fc part. Fig. 12. DNA and protein sequences of variable domains heavy and light chains of K53, UBS54 and 02-237 IgG. ■I Fig. 13 Alignment of the variable sequences of the heavy chains of K53, 02-237 .=nd UBS54. CDRl, CDR2 and CDR3 regions are indicated in bold. Fig. 14 BIACORE-^analysis of K53 and 02-237. Affinity purified human CD46 from LS1T4T cells was coupled {640 RU) to CM5 chips (BIACORE BR~1000-14) . Binding of 1000 (A), 500 (B) , 250', (C), 125 (D) , 63 (E) , 31 (F), 16 (G) , 8 (H) or 0 (I) nM 02-237 or k53 purified from stable PER.C6TM derived cell lines t:o the CD46 was monitored using a BIACORE 3000 system at 37*C. Using this experimental set-up, a Kd of 9.1 X io-7 and 2.2 X 10-8 was found for K53'and 02-237, respectively. Fig. 15 Binding of K53 and 02-237 to LS174T cells. Serial dilutions of purified 02-237 (■), K53 {*) and the negative control GBSIII (O) conjugated to biotin were incubated with LSi47T cells preincubated with normal human serum to block Fey receptor interaction. Binding (MFI, ordinate) was determined by FACS after incubation .with streptavidin'conjugated-phycoerythrin. Fig. 16. SDS-PAGE analysis of purified IgG fractions. Three pg purified IgG was analysed on a non-reduced (A) and reduced (B) 4-20% Nupage gel (Novex) according to recommendations of the manufacturer. Proteins were visualized by staining with colloidal blue (Novex cat. No LC6025) according to recommendations of the manufacturer. Clone identity is indicated on top of the SDS-PAGE. Each gel contains a control, which is either purified 02-237 or K53. NR, Non-reduced; R, reduced. Fig. 17. lEF analysis of purified IgG fractions. Ten pg purified IgG was analysed on an Isogel 3-10 gel (BMA) according to recommendations of the manufacturer. Proteins were visualized by staining with colloidal blue (Novex, cat. No LC6025) according to recommendations of the manufacturer.' Clone identity is indicated on top of the lEF. Each gel contains a control, consisting of a 1:1:1 mixture of ,02-237, K53 and UBS54. Fig. 18. lEF analysis of polyclonal mixtures 241, 280, 282, 361 and 402 in comparison to single K53, 02-237 and UBS54. Ten pg purified IgG was analysed en an Isogel 3-10 i gel (BMA) according to recommendations of the manufacturer. Proteins were visualized by staining with colloidal blue {Novex, cat. No LC6025) according to ' recommendations of the manufacturer. IgG identity is indicated on top of the IFF. Fig. 19. Mass chromatograms of CDK3 peptides of K53, 02-237, U3S54 and the two unique light chain peptides Ll-K53/U3S54 and Ll-237 in IgG fraction Polyl-280. On the right-hand side of each mass chromatogram, the isotopic pattern of the peptide is shewn. The doubly charged ion at m/z 1058.98 (Mw 2115.95 Da) results from peptide Kll-K53, The doubly charged ion at m/z 1029.96 (Mw 2057.92 Da) results from peptide Hll-02-237. The triply charged ion at m/z 770,,03 (Mw 2307.09 Da) results from peptide H9-UBS54. The doubly charged ion at m./z 1291.08 (Mw 2580.16 Da) results from peptide L1-K53/UBS54. The doubly charged ion at m/z 1278.11 (Mw 2554.22 Da) results fron peptide Ll-02-237. Purified IgG was dissolved in a 0.1% RapiGestTM (Waters) in 50 mM NH4HC03. The disulfides were reduced using 1 M DTT (1,4-dithio-DL-threitol), followed by incubation at 65°C for 30 minutes. Then, for alkylation of all sulfhydryl groups, 1 M iodoacetamide was added, followed by incubation at room temperature for 45 minutes in dark. Alkylation was stopped by addition of 1 M DTT. The buffer was exchanged to 25 mM NH4HC03, pH 7.5. Finally, the antibodies were digested overnight at 37 "C by addition of a freshly prepared trypsin solution in 25 mM NH4HC03, The peptide mixture was analysed by LC-MS. The LC-system consisted of a Vydac reversed-phase C18 column, that was eluted by applying a gradient of solvent A (5/95/1 acetonitrile, water, glacial acetic acid v/v/v) and solvent B (90/10/1 acetonitrile, water, glacial acetic acid v/v/v) . The LC was on-line coupled to a Q-T0F2 mass spectrometer (Micromass), equipped with an electrospray source operated at 3 kV. Mass spectra were recorded in positive ion mode from m/z 50 to 1500 at a cone voltage of 35 V. The instrumental resolution of >10,000 enabled unambiguous determination of the charge, and therefore the mass of most ions up to at least +7. In -this way, all peptides were identified according to their molecular weight. The amino, acid sequence of the peptide was confirmed by MS/MS-experiments. MS/MS spectra were recorded in positive ion mode from m/z 50-2000 with collision energy between 20 and 35 eVolts. Fig. 20. BIACORE analysis of polyclonal 280. Affinity purified human CD46 from LS174T cells was coupled (640 RU) to CMS chips (BIACORE BR-1000-14). Binding of 1000 (A), 500 (B), 250 (C), 125 (D), 63 (E), 31 (F), 16 (G), 8 (H) or 0 (I) nM Polyl-280 to CD46 was monitored using a BIACORE 3000 system at 37'C. Fig. 21. lEF analysis of sub-clones from clones poly 1-241, poly 1-280 and poly 1-402 producing a mixture of antibodies. A. Clones poly 1-241 and poly 1-280. Lane 1 contains a pi marker (Amersham, Cat. No. 17-0471-01). Lane 2 contains isolated IgG from the parent clone poly 1-241- (as in Fig. 18). Lanes 3, 4 and 5 respectively contain isolated IgG from 3 independent sub-clones derived from poly 1-241 by limiting dilution. Lane 6 contains isolated IgG from the parent clone poly 1-280 (as in Fig. 18}. Lanes 7, 8 and 9 respectively contain isolated IgG from three independent sub-clones derived form poly 1-280 by lirrdting dilution. B. Clone poly 1-402. Lanes 1 and 7 contain a pi marker. Lane 2 contains isolated IgG from the parent clone poly 1-402 (as in Fig. ,18). Lanes 3, 4 and 5 respectively contain isolated IgG from 3 independent sub-clones derived from poly 1-402 by limiting dilution. Lane 6 contains a control {a 1:1:1 mixture of 02-237, K53 and UBS54). Fig. 22. FACS analysis of mixtures of antibodies produced from sub-clones of poly 1-241 {A}, poly 1-280 (B) and poly 1-402 (C). Binding of the mixtures of antibodies to cells trsnsfected with cDNA cf CD46, EpCAM, or a negative control {CD38), was determined with FACS analysis, and mean fluorescent intensity (MFI) is shown for the various parent clones and three independent sub-clones of each. Control antibodies GBS-III (negative control), anti-CD72 (02-004; negative control) and the single antibodies UBS54, 02-237 and K53 are also included. Summary of the invention In one aspect the present invention provides for a method of producing a mixture of antibodies in a recombinant host, the method comprising the step of: expressing in a recombinant host cell a nucleic acid sequence or nucleic acid sequences encoding at least one light chain and at least three different heavy chains that are capable of pairing with said at least one light chain. A further aspect of the invention is the elimination of the production of potentially non¬functional light-heavy chain pairing by using pre¬selected combinations of heavy and light chains. It has been recognized that phage display libraries built from a single light chain and many different heavy chains can encode antibody fragments with very distinct binding properties. This feature can be used to find different antibodies having the same light chain but different heavy chains, against the same target or different targets, wherein .a ^target can be a whole antigen or- an epitope thereof. Such different targets may for instance be on the same surface (e.g. cell or tissue). Such I! antibody fragments obtained by phage display can be cloned into vectors for the desired format, e.g. IgG, IgA or IgM, and the nucleic acid sequences encoding these formats can be used to transfect host cells. In one approach, H and L chains can be encoded by different constructs that, upon transfection into a cell wherein they are expressed, give rise to intact Ig molecules. When different H chain constructs are transfected into a cell with a single L chain construct, H- and L-chains will be assembled to form all possible combinations. However, in contra'st to approaches where different light chains are expressed such as for the production of bispecific antibodies, this method will result only in functional binding regions. It would be particularly useful when the host, e.g. a single cell line, is capable of expressing acceptable levels of recombinant antibodies, without the necessity to first amplify in •i said cell the nucleic acid sequences encoding the antibodies. The advantage is that cell lines with only a limited copy number of said nucleic acids are expected to be genetically more stable, because there will be less recombination between the sequences encoding the heavy chains, than in cell lines where a multitude of these copies is present * A cell line suitable for use in the methods accordingij to the invention is the human cell line PER.C6™. Using this method a mixture of antibodies with defined specificities can be produced from a single cell clone m a safe, controlled, and consistent manner. The invention provides a method for production of a mixture of antibodies in a recombinant host, the method comprising the step of: expressing a nucleic acid sequence or nucleic acid sequences encoding at least one light chain and at least three different heavy chains that are capable of pairing with said at least one light chain in a recombinant host cell. In a preferred aspect, the recombinant host cell comprises a nucleic acid sequence encoding a common light chain that is capable of pairing with said at least three different heavy chains, such that the produced antibodies comprise a common light Cham. Obviously, those of s.-cill in the art will recognize that "corrjnon" also refers to functional equivalents of the light cha:-n of which the amino acid sequence is not identical. Many variants of said light chain exist wherein mutations (deletions, substitutions, additions) are present that co not materially influence the formation of functional binding regions. The invention '[further provides a composition comprising a mixture of reco:t±)inantly produced antibodies, wherein at least three different heavy chain sequences are represented in said mixture. In one i embodiment, the light chains of such mixtures have a common sequence. The mixture of antibodies can be produced by the method according to the invention. Preferably, the mixture of antibodies is more efficacious than the individual antibodies it comprises, more preferably, the mixture acts synergistically in a functional assay. The inventioniifurther provides a recombinant host cell for producing mixtures of antibodies, and methods for making such host cells. Independent clones obtained from the transfection of nucleic acid sequences encoding a light chain and more than one heavy chain may express the different antibodies in the mixture at different levels. It is another aspect of the invention to select a clone using a functional assay, for the most potent mixture of antibodies. The invention therefore further provides a method for identifying at least one host cell clone that produces a mixture of antibodies, wherein said mixture of antibodies has a desired effect according to a functional assay, the method comprising the steps of: (i) providing a host cell with nucleic acid sequence encoding at least one light chain and nucleic '^cid sequences encoding at least two different heavy chains, wherein said heavy and light chains are capable of pairing with each other,- (ii) culturing at least^ one clone of said host cell under conditions conducive to expression of said nucleic acid sequences; (iii) screening said at least one clone of the host cell for production of a mixture of antibodies having the desired effect by a functional assay; and (iv) identifying at least one clone that produces a mixture of antibodies having-'the desired effect. This method according to the invention can be performed using high-throughput procedures, if desired. The clones identified by the method can (be used to produce antibody mixtures according to the invention. The invention further provides transgenic non-human animals and transgenic plants or transgenic plant cells capable of expressing mixtures of antibodies, and mixtures of antibodies produced by these. The invention further provides pharmaceutical compositions comprising a mixture of recombinantly produced antibodies and a suitable carrier. The invention further provides mixtures of antibodies for use in the treat-ient or diagnosis and for the preparation of a medicament for use in the treatment or diagnosis of a disease or disorder in a human or animal subject. The invention further provides a method for producing a mixture of ant.ibcdies comprising different '« isotypes from a single host cell clone. The invention further provides a method for indentifying a mixture of antibodies having a desired effect in a functional assay. The invention further provides a method for producing a mixture of antibodies that are capable of binding to a target, the method comprising the steps of: i) bringing a phage library comprising antibodies into contact with material comprising a target, ii) at least one step of selecting phages binding to said target, iii) identifying at least two phages that comprise antibodies binding to said target, wherein said at least two antibodies comprise a common light chain, iv) introducing a nucleic acid sequence encoding the light chain and a nucleic acid sequence or sequences encoding the heavy chains of said at least two antibodies into a host cell, v} culturing a clone of said host cell under conditions conducive to expression of said nucleic acid sequences. Detailed description of the invention It is an object of the present invention to provide a method for producing a mixture of antibodies in a recombinant host,,the method comprising the step of; expressing in a recombinant host cell a nucleic acid sequence or nucleic acid sequences encoding at least one light chain and at least three different heavy chains that are capable of pairing with said at least one light chain. According to the invention, the light and heavy chains when paired form functional antigen binding domains. A functional antigen binding domain is capable of specifically binding to an antigen. In one aspect according to the invention, the method for producing a mixture of antibodies according to the invention further ^'comprises the step of recovering the antibodies from the cell or the host cell culture to obtain a mixture of antibodies suitable for further use. In one embodiment of the invention, a method is provided for production of a mixture of antibodies, the method comprising the step of: expressing in a recombinant host cell a nucleic acid sequence encoding a common light chain and nucleici acid sequence or sequences encoding at least three different heavy chains that are capable of pairing with said common light chain, such that the antibodies that are produced comprise common light chains. In one aspect the common light chai-n is identical in each light chain/heavy chain pair. The term "antibody" as used herein means a polypeptide containing one or more domains that bind an epitope on an antigen, where such domains are derived from cr have sequence identity with the variable region of an antibody. The structure of an antibody is schematically represented in Fiq. 1. Exam.ples of antibodies according to the, invention include full length antibodies, antibody fragments, bispecific antibodies, iiTjnunoconjugates, and the like. An antibody according to the invention may be isotype IgGl, IgG2, IgG3, IgG4, IgAl, IgA2,. IgD, IgE, IgM, and the like, or a derivative of these. Antibody fragments include Fv, Fab, Fab', F(ab')2 fragments, and the like. Antibodies according to the invention can be of any origin, including murine, of more than one origiin, i.e. chimeric, humanized, or fullv human antibodies. Immunoconjugates comprise antigen binding domains and' a non-antibody part such as a toxin, a radiolabel, an enzyme, and the like. An "antigen binding domain" preferably comprises variable regions cf a heavy and a light chain, and is responsible for srecific binding to an antigen of interest. Recombinant ai-cibodies are prepared by expressing both a heavy and a light chain in a host cell. Similarly, by expressing two c'-ains with their respective light chains (or a common light chain), wherein each heavy chain/light chain has its own specificity, so-called "bispecific" antibodies can be prepared. "Bispecific antibodies" comprise two non-identical heavy-light chain combinations {Fig. 2), and both antigen binding regions of a bispecific antibody may recognize different antigens or different epitopes on an antigen. ;i A "common light chain" according to the invention refers to light chains which may be identical or have amino acid sequence differences. Said light chains may comprise mutations which do not alter the specificity of the antibody when combined with the same heavy chain, without departing from the scope of the present invention. It is for instance possible within the scope of the definition of common light chains as used herein, to prepare or find light chains that are not identical but still functionajlly equivalent, e.g. by introducing and testing conservative amino acid changes, changes of •amino acids in regions that do not or only partly contribute to binding specificity when paired with the heavy chain, and the like. It is an aspect of the present invention to use as common light chain one identical light chain to combine with different heavy chains to form antibodies with functional antigen binding domains. The use of one common light chain avoids the formation of heterodimers in which pairing of light and heavy chains results in antigen,binding domains that are not functional, in other words which are not capable of binding to the target antigen or antigens. The use of a common light chain' and two heavy chains has been proposed by (Merchant et al, 1998; WO 98/50431) for a different purpose, viz. to Increase the formation of functional I bispecific antibodies at the expense of antibody mixture complexity. These publications teach a method for preferentially producing one defined and desired bispecific antibody, thereby minimizing the complexity of the produced mixture. Hence, Merchant specifically teaches to prevent the production of monospecific antibodies, because these are undesired byproducts in the process for bispecific antibody production described in those publications. Clearly, there is no teaching in the prior art to prepare a complex mixture of antibodies from a recombinant host^ cell avoiding the formation of non¬functional binding domains or the benefits thereof, let alone how. In the^method according to the present invention, at least three different' heavy chains that are capable of pairing with the common light chain are. expressed. In other embodiments, the host cell according to the invention is provided wiiih nucleic acid sequences encoding for 4, 5, 6, 7, 8, 9, 10, or more heavy chains capable of pairing with the comm.on light chain, to increase the complexity of the produced mixture of antibodies. "Different heavy chains" according to t.he invention may differ in the variable region and have the same constant region. In other embodiments, where it is clear from the context, they may have the same variable region and differ in the constant region, e.g. be of a different isotype. The use of a mixture of antibodies having different constant regions, such as the Fc-portion, may be advantageous if different arms of the immune system are to be mobilized in the treatment of the human or animal body. In yet other embodiments, also to be clear from the context, both the variable and the constant regions may differ!. A "mixture of antibodies" according to the invention comprises at least'two non-identicai antibodies, but may comprise 3, 4, 5, 6, 7, 8, 9, 10, or more different antibodies, and may resemble a polyclonal or at least an oligoclonal antibody mixture with regard to complexity and number of functional antigen binding molecules. The mixtures produced according to the present invention usually will comprise bispecific antibodies. If desired, formation of monospecific antibodies in the mixture can be favoured over the formation of bispecific antibodies. When n heavy chains and one common light chain are expressed accordin'g to the invention in a host cell at equal levels, the theoretical percentage of bispecific antibodies produced by the method according to the invention is (1-1/n)*100%. The total number of different antibodies in the mixture produced by the method according to the invention is theoretically n+{ {-n^-n)/2}, of which {n^-n/2) are bispecific antibodies. Distortion of the ratio of expression levels of the different heavy chains may lead to values deviating from the theoretical values. The amount; of bispecific antibodies can also be decreased, compare'd to these theoretical values, if not all heavy chains pair with equal efficiency. It is for instance possible to engineer the heavy chains, e.g. by introducing specific and complementary interaction surfaces between selected heavy chains, to promote homodimer pairing over heterodimer pairing, contrary to what has been proposed by Merchant, supra. Heavy chains may also be selected so as to minimize heterodimer formation in the mixture. A special form of this embodiment involves heavy chains of two or more different isotypes (e.g. IgGl, IgG3, IgA). When heavy chains of different isotype are expressed in the same host cell in accordance with the present invention and one light chain that can pair to these heavy chains, the amount of bispecific an1;ibodies will be reduced, possibly to very low or even to undetectable levels. Thus, when bispecific antibodies are less desirable, it is possible to produce a mixture of antibodies according to the invention, wherein a nucleic acid sequence encoding a common light chain and nucleic:acid sequences encoding at least two different heavy chains with a different variable region capable of pairing to said common light chain are expressed in a recombinant host, and wherein said heavy chains further differ in their constant regions sufficiently to reduce or prevent pairing between the different heavy chains. The mixtures of antibodies according to the invention may be produced from a clone that v.'as derived from a single host cell, i.e. from a population of cells containing the same recombinant nucleic acid sequences. It will be understood that the different heavy chains according to the invention can be encoded on separate nucleic acid molecules, but may also be present on one nucleic acid molecule comprising different regions encoding said at least three heavy chains. The nucleic acid molecules usually encode precursors of the light and/or heavy chains, which when expressed are secreted from the host cells, thereby becoming processed to yield the mature form. These and other aspects of expressing antibodies in a host cell are well known to those having ordinary skill in [the art. A "recoml^inant host cell" as used herein is a cell comprising one or more so-called transgenes, i.e. recombinant nucleic acid sequences not naturally present in said cell. These transgenes are expressed in said host cell to produce recombinant antibodies encoded by these nucleic acid sequences, when these cells are cultured under conditions Conducive to expression of said nucleic acid sequences. The host cell according to the invention can be present in the form of a culture from a clone that is derived from a single host cell wherein the transgenes have been introduced. To obtain expression of nucleic acid sequences encoding antibodies, it is well known to those skilled in t:he art that sequences capable of driving such expression can be functionally linked to the nucleic acid sequences encoding the antibodies. Functionally linked is meant to describe that the nucleic acid sequences encoding the antibody fragments or precursors thereof is linked to the sequences -capable of driving expression such that these sequences can drive expression of the antibodies or precursors thereof. Useful expression vectors are available in the art, e.g. the pcDNA vector series of Invitrogen. Where the sequence encoding the polypeptide of interest is properly inserted with reference to sequences governing the transcription and translation of the encoded polypeptide, the resulting expression cassette is useful to produce the polypeptide of interest, referred to as expression. Sequences driving expression may include promoters, enhancers and the like, and combinations thereof. These should be capable of functioning in the host cell, thereby driving expression of the nucleic acid sequences that are functionally linked to them. Promoters can be constitutive or ,regulated, and can be obtained from various sources, including viruses, prokaryotic, or eukaryotic sources, or artificially designed. Expression of nucleic acids of interest may be from the natural promoter or derivative thereof or from an entirely heterologous promoter. Some well-known s.nd much used promoters for expression in -eukaryotic cells :,comprise promoters derived from viruses, such as adenovirus, e.g. the ElA promoter, promoters derived from cytomegalovirus (CMV), such as the CMV immediate early (IE) promoter, promoters derived from Simian Virus 40 (SV40), and the like. Suitable promoters can also be derived from eukaryotic cells, such as methailothionein (MT) promoters, elongation factor la (EF-la) promoter, actin promoter, an immunoglobulin promoter, heat, shock promoters, and the like. Any promoter or enhancer/promoter capable of driving expression of the sequence of interest in the host cell is suitable in the invention. In one embodiment the sequence capable of driving expression comprises a region from a CMV promoter, preferably the region comprising nucleotides -735 to +95 of the CMV immediate early gene enhancer/promoter. 11 The skilled artisan will be aware that the expression sequences used in the invention may suitably be combined with elements that can stabilize or enhance expression, such as insulators, matrix attachment regions, STAR elements (WO 03/004704), and the like. This may enhance the stability and/or levels of expression- Protein production in recombinant host cells has been extensively described, e.g. in Current Protocols in Protein Science, 1995, Coligan JE, Dunn BM, Ploegh HL, Speicher DW, Wingfield PT, ISBN 0-471-11184-8; Bendig, 1988. Culturing a cell is done to enable it to metabolize, and/or grow and/or divide and/or produce recombinant proteins of interest. This can be accomplished by methods well known to persons skilled in if the art, and includes but is not limited to providing nutrients for the cell. The methods comprise growth adhering to surfaces, growth in suspension, or combinations thereof. Several culturing conditions can be optimized by methods well known in the art to optimize protein production yields. Culturing can be done for instance in dishes, roller bottles or in bioreactors, using batch, fed-batch, continuous systems, hollow fiber, and the like. In order to achieve large scale (continuous) production of recombinant proteins through cell culture it is preferred in the art to have cells capable of growing in suspension, and it is preferred to have cells capable of being cultured in the absence of animal- or human-derived serum or animal- or human-derived serum -components. Thus .purification is easier and safety is enhanced due to the absence of additional animal or human proteins derived from the culture medium, while the system is also very reliable as synthetic media are the best in reproducibility. "Host cells" according to the invention may be any host cell capable of expressing recombinant DNA molecules, including bacteria such as Eschericia (e.g. E.coli), Enterobocter, Salmdnalla, Bacillus, Pseudomonas, Streptomyces, yeasts such as S.cerevisiae, K.lactis, P.pastoris, Candida, or yarrowia, filamentous fungi such as Neurospora, Aspergillus oryzae, Aspergillus nidulans and Aspergillus niger, insect cells such as Spodoptera frugiperda SF-9 or .SF-21 cells, mammalian cells such as Chinese hamster ovary (CHO) cells, BHK cellS/ mouse cells including SP2/0 cells and NS~0 myeloma cells, primate cells such as COS and Vero cells, MDCK cells, BRL 3A cells, hybridomas, tumor-cells, immortalized primary cells, human cells such as W138, HepG2, HeLa, HEK293, HT1080 or embryonic.retina cells such as PER.C6™, and the like. Often, the expression system of choice will involve a mammalian cell expression vector and host so that the antibodies are appropriately glycosylated. A human cell line, preferably PER.C6™, can advantageously be used to obtain antibodies with a completely human glycosylation pattern. The conditions for growing or multiplying cells (see e.g. Tissue Culture, Academic Press, Kruse and Paterson, editors (1973)) and the conditions for expression of the recombinant product may differ somewhat, and optimization of the process is usually performed to increase the product yields and/or growth of the cells with respect to each ether, according to methods generally known to the person skilled in the art. In general, principles, protocols, and practical techniques for maKimizing the productivity of mammalian cell cultures can be found in Mammalian Cell Biotechnology: a Practical Approach (M. Butler, ed., IRL Press, 1991) . Expression of antibodies in recombinant host cells has been^ extensively described in the art (see e.g. EP0120694; EP0314161; EP0481790; EP0523949; US patent 4816567; WO 00/63403) . The nucleic acid molecules encoding the light and heavy chains may be present as extrachromosomal copies and/or stably integrated into the chromosome of the host cell. With regard to stability of production, the latter is preferred. The antibodies are expressed in the cells according to the invention, and may be recovered from the cells or preferably from the cell culture medium, by methods generally known to persons skilled in the art. Such methods may include precipitation, centrifugation, filtration, size-exclusion chromatography, affinity chromatography, cation- and/or anion-exchange chromatography, hydrophobic interaction chromatography, and the like. For a mixture of antibodies comprising IgG molecules, protein A or protein G affinity chromatography can be suitably used (see e.g. US patents 4,801,687 and 5,151,504). In one embodiment, at least two antibodies from the mixture produced according to the invention comprise a heavy-light chain dimer having different specificities and/or affinities. The specificity determines which antigen or epitope thereof is bound by the antibody. The affinity is a measure for the strength of binding to a particular antigen or epitooe. Specific binding is defined as binding .with .an affinity . (-Ka) of at least 5 * 10E4 liter/mole, more preferably 5 * 10E5, more preferably more than 5 * 10E6, still more preferably 5 * 10E7, or more. Typically, monoclonal antibodies may have affinities which go up to lOElO liter per mole, or even higher. The mixture of antibodies produced according to the present invention may contain at least two antibodies that bind to different epitopes on the same antigen molecule and/or may contain at least two antibodies that bind to different antigen molecules present in one antigen comprising mixture. Such an antigen comprising mixture may be a mixture of partially or wholly purified antigens such as toxins, membrane components and proteins, viral envelope proteins, or it may be a healthy cell, a diseased cell, a mixture of cells, a tissue or mixture of tissues, a tumor, an organ, a complete human or animal subject, a fungus or yeast, a bacteria or bacterial culture, a virus or virus stock, combinations of these, and the like. Unlike monoclonal antibodies that are able to bind to a single antigen or epitope only, the mixture of antibodies according to the present invention may therefore have many of the aavantages of a polyclonal or oligoclonal antibody mixture. In a preferred embodiment, the host cell according to the method of the invention is capable of high-level expression of human immunoglobulin, i.e. at least,1 pg/cell/day, preferably at least 10 pg/cell/day and even more preferably at;least 20 pg/cell/day or more without the nee,d for amplification of the nucleic acid molecules encoding the heavy and light chains in said host cell. Preferably, host cells according to the invention contain in their genome between 1 and 10 copies of each recombinant nucleic acid to be expressed. In;the art, amplification of the copy number of the nucleic acid sequences encoding.a protein of interest in e.g. CHO cells can be used to increase expression levels of the recombinant protein by the cells (see e.g. Bendig, 1988; Cockett et al, 1990; US patent 4,399,215). This is currently a widely used method. However, a significant time-consuming effort is required before a clone with a desired high copy number and high expression levels has been established, and moreover clones harbouring very high copy numbers (up to hundreds) of the. expression cassette often are, unstable (e.g. Kim et al., 1998). It is therefore a preferred embodiment of the present invention to use host cells that do not require such amplification strategies for high-level expression of the antibodies of interest. This allows fast generation of stable clones of host cells that express the mixture of antibodies according to the invention in a consistent manner. We provide' evidence that host cells according to the invention can be obtained, subcloned and further propagated for at least around 30 cell divisions (population doublings) while expressing the mixture of antibodies according to the invention in a stable manner, in the absence of 'selection pressure. Therefore, in .certain aspects the methods of the invention include culturing the cells for at least 20, preferably 25, more preferably 30 population doublings, and in other aspects the host cells according to the invention have undergone at least 20, preferably 25, more preferably 30 population doublings and are still capable of expressing a mixture of antibodies according to the invention. The invention also provides a culture of cells producing a mixture of immunoglobulins from a single-cell, said mixture comprising at least three different heavy chains. The invention also provides a culture of cells producing at least three different monospecific immunoglobulins from a single cell. In certain preferred aspects, said culture produces said mixture or said at least three different monospecific immunoglobulins in a single cell for more than 20, preferably more than 25, more preferably more than 30 population doublings. Preferably host cells according to the method of the invention are derived from human retina cells that have been immortalized or transformed with adenoviral El sequences. A particularly preferred host cell according to methods of the present invention is PER.C6™ as deposited under ECACC no. 96022940, or a derivative thereof. PER.C6-derived clones can be generated fast, usually contain a limited number of copies (about 1-10) of the transgene, and are capable of high-level expression of recombinant antibodies (Jones et al, 2003). Therefore, such clones are expected to maintain a stable copy number over many generations, which is an advantage in the production of biopharmaceuticals. PER-CS"^" cells have been extensively characterized and documented, demonstrating good process of scaling up, suspension growth and growth factor independence. Furthermore, PER.C6™ can be incorporated into a suspension in a highly reproducible manner, making it particularly suitable for large-scale production. In this regard, the PER.C6™ cell line has been characterized for bioreactor growth^ where it can grow to very high densities. The use of PER-Ce'" for recombinant production of anuibodies has been described in detail in publication WO 00/63403 and in (Jones et al., 2003) . It is another aspect of the present invention to provide a mixture of antibodies that is obtainable by a method according to the invention. Such mixtures of antibodies are expected to be more effective than the sole components it comprises, in analogy to polyclonal antibodies usually being more effective than monoclonal antibodies to the same target. Such mixtures can be prepared against a' variety of target antigens or epitopes. It is another aspect of the present invention to provide a recombinant host cell comprising a nucleic acid sequence encoding a light chain and a nucleic acid sequence or nucleic acid sequences encoding at least three different heavy chains of an antibody, wherein said light chain and heavy chains are capable of pairing, preferably to form a functional binding domain. The paired heavy and light chain form functional antigen binding regions against the target antigen or target antigens. The host cells according to the invention are useful in the method according to the invention. They can be used to produce mixtures of antibodies according to the invention. It is another aspect of the present invention to provide a composition comprising a mixture of recombinantly produced antibodies, wherein at least three different heavy chain sequences are represented in the mixture of recombinant antibodies. Monoclonal antibodies are routinely produced by recombinant methods. The present invention discloses mixtures of antibodies useful for diagnosis or treatment in various fields. The compositions according to the invention comprise mixtures of at least three different heavy chains paired to light chains in the form of antibodies. Preferably, the light chains of the antibodies in said m.ixtures have a common light chain. The mixtures may comprise bispecific antibodies. The mixtures may be produced from a clone that was derived from a single host cell, i.e. from a population of cells containing the same recombinant nucleic acid sequences. The mixtures can be obtained by methods according to the invention, or be produced by host cells according to the invention. In other embodiments, the number of heavy chains represented in said mixture is 4, 5, 6, 7, 8, 9, 10, or more. The optimal mixture for a certain purpose may be determined empirically by methods known to the person skilled in the art, or by methods provided by the present invention. Such compositions according to the invention may have several of the advantages of a polyclonal antibody mixture, without the disadvantages usually inherently associated with polyclonal antibody mixtures, because of the manner in which they are produced. It is furthermore expected that the mixture of antibodies is more efficacious than separate monoclonal antibodies. Therefore the dosage, and hence the production capacity required, miay be less for the mixtures of antibodies according to the invention than for monoclonal antibodies. It has for instance been described that although no single monoclonal antibody to botulinum neurotoxin (BoNT/A) significantly neutralized toxin, a combination of three such monoclonal antibodies {oligoclonal antibody) neutralized 450,000 50% lethal doses of BoNT/A, a potency 90 times greater than human hyperimmune globulin (Nowakowski et al, 2002) . This result demonstrates that oligoclonal mixtures of antibodies comprising only 2 to 3 different specificities may have very high potency. Furthermore, the chances of a mixture of the invention losing its activity due to target or epitope loss is reduced, when compared to a single monoclonal antibody. In particular embodiments, 2, 3, A, 5, 6, 7, 8, 9, 10, or more of the antibodies present in the mixture according to the invention have different specificities. Said different specificities may be directed to different epitopes on the same antigen -and/or may be directed to different antigens present in one antigen comprising mixture. A composition according to the invention further may also comprise 2, 3, A, b, 6, 1, 8, 9, 10, or more antibodies having different affinities for the same epitope. Antibodies with differing affinities for the same epitope may for instance be generated by methods of affinity maturation, known to the person skilled in the art. In a particularly preferred embodiment, the composition according to the invention has an effect that is greater than the effect of^ each individual monospecific antibody present in said composition. Said effect can be measured in a functional assay. A ^'functional assay" according to the present invention is an assay that can be used to determine one or more desired parameters of the antibody or the mixture of antibodies subject to the assay conditions. Suitable functional assays may be binding assays, .apoptosis assays, .antibody..dependent- cellular cytotoxicity (ADCC) assays, complement dependent cytotoxicity (CDC) assays, inhibition of cell growth or proliferation (cytostatic effect) assays, cell killing (cytotoxic effect) assays, cell signaling assays, assays for measuring inhibition of binding of pathogen to target cell, assays to measure the secretion of vascular endothelial growth factor (VEGF) or other secreted molecules, assays for bacteriostasis, bactericidal activity, neutralization of viruses, assays to measure the attraction of components of the immune system to the site where antibodies are bound, including in situ hybridization methods, labeling methods, and the like. Clearly, also in vivo assays such as animal models, including mouse tumor models, models of autoimmune disease, virus-infected or bacteria-infected rodent or primate models, and the like, can be used for this purpose. The efficacy of a mixture of antibodies according to the invention can be compared to individual antibodies in such models by methods generally known to the person skilled in the art. It is another aspect of the present invention to provide a method for identifying at least one host cell clone that produces a mixture of antibodies, wherein said mixture of antibodies has a desired effect according to a functional assay, the method comprising the steps of: (i) providing a host cell comprising a nucleic acid sequence encoding at least one light chain and nucleic acid sequence or sequences encoding at least two different heavy chains, wherein said heavy and light chains are capable of pairing with each other; (ii) culturing at least one clone of said host ceil under conditions conducive to expression of said nucleic acid sequences; (iii) screening said at least one clone, of the host cell for production of a mixture of antibodies having the desired effect by a functional assay; and (iv) identifying at least one clone that produces a mixture of antibodies having the desired effect. Preferably, said host cell comprises a nucleic acid seguence encoding a common light chain that is capable of pairing with said at least two different heavy chains, such that produced antibodies comprise common light chains, as described above. In specific embodiments said culturing in step (ii) and said screening in step (iii) of the method is performed with at least two clones. The method may optionally include an assay for measuring the expression levels of the antibodies that are produced, which assay may be during or after step (ii) according to the method, or later in the procedure. Such assays are well known to the person skilled in the art, and include protein ■I concentration assays, immunoglobulin specific assays such as ELISA, RIA, DELFIA, and the like. In particular embodiments of said method according to the invention, the host cell, comprises nucleic acid sequence or sequences encoding at least 3, 4, 5, 6, 7, 8, 9, 10, or more heavy chains capable of pairing with said at least one light chain. Functional assays useful for the method according to the invention may be assays for apoptosis, ADCC, CDC, cell killing, inhibition of proliferation, virus neutralization, bacterial opsonization, receptor-mediated signaling, cell signaling, bactericidal activity, and the like. Useful screening assays for anti-cancer antibodies have for instance been described in US patent. ,6,1,80,357. Such assays .may also ,be .used to identify a clone according to the method of the present invention. It is for instance possible to use enzyme linked immunosorbent assays (ELIS.-_s) for the testing of antibody binding to their target. Using such assays, it is possible to screen for antibody mixtures that most avidly bind the target antigen (or mixture of .target antigens against which the mixture of antibodies is to be tested). Another possibility that can be explored is to directly screen for cytotoxicity or cytostatic effects. It is possible that upon such a different screen, other or the same clones producing mixtures of antibodies will be chosen than with the ELISA mentioned above. The screening for cell killing or cessation of growth of cancerous cells may be suitably used according to the invention. Cell death can be measured by various endpoints, including the absence of metabolism or the denaturation of enzymes. In one possible embodiment of the present invention, the assay is conducted by focusing on cytotoxic activity toward cancerous cells as an endpoint. For this assay, a live/::;e5d assay kit, for example the LIVE/DEAD® Viability/Cytotoxicity Assay Kit (L-3224) by Molecular Probes, can suitably be used. Other methods of assessing cell viabili-y, such as trypan blue exclusion, ^^Cr release, Calcein-A:-5, Alamar Blue™, LDH -activity, and similar methods car. also be used. The assays may also include screening of the mixture of antibodies for specificity to the desired antigen comprising tissue. The antibodies according to the invention may have a limited tissue distribution. It is possible to include testing the -ix-ures of antibodies against a variety of cells, cell -yres, or tissues, to screen for mixtures of antibodies that preferably bind to cells, cell types or tissues of interest. Irrespective of a functional assay as described above, the present invention also teaches ways to determine the identity of the antibodies expressed by a clone, using methods such as iso-electric focusing (lEF), mass-spectrometry (MS),, and the like. It is therefore an aspect of the invention to provide use of MS and/or lEF in selecting a clone that expresses a mixture of antibodies according to the invention. When monoclonal antibodies are produced by recombinant host cells, a screening step is usually performed to assess expression levels of the individual clones that were generated. The addition of more heavy chains to produce mixtures adds a level of complexity to the production of antibodies. When host cells are transfected with nucleic acid molecules encoding the light and heavy chains that will form the mixture of antibodies desired, independent clones may arise containing the same genetic information, but nevertheless differing in expression levels, thereby producing different ratios of the encoded antibodies, giving rise to different mixtures of antibodies from the same genetic repertoire. The method according to the invention is useful for identifying a clone that produces an optimal mixture for a certain purpose. The culturing and/or screening according to steps (ii) and (iii) respectively, may be suitably performed using high-throughput procedures, optionally in an automated fashion. Clones can e.g. be cultured in 96-well or other multi-well plates, e.g. in arrayed format, and screened for production of a desired mixture. Robotics may be suitably employed ..for-this purpose. ..Methods-to implement high-throughput culturing and assays are generally available and known to the person skilled in the art. It will be clear that also for this method according to the invention, it is beneficial to use host cells capable of high level expression of proteins, without the need for amplification of the nucleic acid encoding said proteins in said cell. In one embodiment, said host cell is derived from a human embryonic retinoblast cell, that has been immortalized or transformed by adenoviral El sequences. In a preferred embodiment, said cell is derived from PER.C6™. This cell line has already been shown to be amenable to high-throughput manipulations, including culturing (WO 99/64582). In specific embodiments of the present invention, said mixture of antibodies according to the method of identifying at least one host cell according to the invention, comprises at least 2, 3, 4, 5, 6, 1, 8, 9, 10, If or more antibodies having different specificities and/or affinities. A potential advantage of the method will be that it will allow exploring many possible combinations simultaneously, the combinations inherently including the presence of bispecific antibodies in the produced mixture. Therefore more combinations can be tested than by just mixing purified known monoclonal antibodies, both in number of combinations and in ratios of presence of different antibodies in these co.Tb>inations. The clone that has been identified by the method according to the invention, can be used for producing a desired mixture of antibodies. I- is therefore another aspect of the preisent invention ~o provide a method of producing a mixture of antibodies, the method comprising the step of: culturing a host cell clone identified by the method of identifying at least one host cell clone that produces a mixture of antibodies according to the invention, said culturing being under conditions conducive to expression of the nucleic acid molecules encoding the at least one light chain and the at least two different heavy chains. The produced antibodies may be recovered from the host cells and/or from the host cell culture, e.g. from the culture medium. The mixture of antibodies can be recovered according to a variety of techniques known to the person skilled in the art. It is yet another aspect of the present invention to provide a mixture of antibodies that is obtainable by the method according to the invention, described above. Said mixtures can be used for a variety of purposes, such as in treatment or diagnosis of disease, and may replace, or be used in addition to, monoclonal or polyclonal antibodies. The methods according to the present invention may suitably use nucleic acid molecules for encoding the antibodies, which 'nucleic acid molecules have been obtained by any suitable method, including in vivo, e.g. immunization, methods or in vitro, e.g. antibody display methods (A. Pliickthun et al. In vitro selection and evolution of proteins. In: Adv. Prot. Chem. , F.M. Richards et al, Eds, Academic Press, San Diego, 2001, vol 55: 367-403), such as phage display, ribosome display or mRNA display (C. Schaffitzel et al,, In vitro selection and evolution of protein-ligand interactions by ribosome display. In: Protein-Protein Interactions. -A Molecular Cloning Manual, E. Golemis, Ed., Cold Spring Harbor Laboratory Press, New York, 2001, pp 535-567), and yeast display (e.g. WO 99/36569). Methods of identifying antibodies to a certain target, which target may be a known antigen or an unknown antigen present in an antigenic mixture, by phage display are known to the person skilled in the art. In general, a library of phages that express an antigen binding domain or derivative thereof on their surface, said antigen binding domain encoded by genetic material present in said phages, is incubated with the antigen or antigen mixture of interest, after which binding of a subpopulation of the phages that display antigen binding sites binding to the desired antigen is obtained whereas the non-binding phages are discarded. Such selection steps may be repeated one, two, or more times to obtain a population of phages that are more or less specific for the antigen of interest. Phage display methods to obtain antibodies, parts or derivatives thereof have been extensively described in e.g. (CF Barbas III et al. Phage Display. A laboratory manual. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York, 2001). The library used for such screening may be generated by using the genetic information of one or more light chains, combined with genetic information encoding a plurality of heavy chains. The library described by De Kruif et al. (1995b) comprises 7 light chains. Therefore, in a panel of phages binding to a target, which pan e.g. be obtained by methods described in De Kruif et al (supra); US patent 6,265,150; not more than 7 different light chains will be represented, and if the panel is large enough, several phages with the same light chain coupled to unrelated heavy chains may be found. Such phages can be used to obtain the nucleic acid molecules useful in the methods according to the present invention. It is another aspect of the present invention to provide a method for producing a mixture of antibodies to a target, the method comprising the steps of: i) bringing an antibody display library comprising antibodies or antibody fragments into contact with material comprising a target, ii) at least one step of.selecting antibodies or antibody fragments binding to said target, iii} identifying at least two antibodies or antibody fragments binding to said target, wherein said at least two antibodies or antibody fragments comprise a common light chain, iv) introducing a nucleic acid sequence encoding the light chain and a nucleic acid sequence or nucleic acid sequences encoding the heavy chains of said at least two antibodies into a host cell, v) culturing a clone of said host cell under conditions conducive to expression of said nucleic acid sequences. The antibody display library may be a phage display library, a ribosome display library, an mRNA display library, or a yeast display library. Step i) and ii) may optionally be repeated one or more times. The nucleic acid sequences encoding the antibodies obtained by the phage display, ribosome display or yeast display method may be converted to encode any desired antibody format such as IgGl, IgG2, IgG3, IgG4, IgA, IgM, IgD, IgE, before introducing them into a host cell, using standard molecular cloning methods and means known to the person skilled in the art (e.g. described in Boel et al, 2000). It .will .be clear to.the skilled-person that .libraries in which only one light chain is represented are especially useful in light of the present invention, since all antibodies that can be obtained from such a library, will have a common light chain that is functional in binding target antigen with each of the heavy chains. In other words, in accordance with the methods of the invention the formation of non-functional light chain-heavy chain dimers is avoided. Phage antibody display libraries having extensive H chain repertoires and unique or very few L chain sequences have been disclosed in the art (Nissim et al, 1994; Vaughan et al, 1996). In general, the specificity of an antibody appears to be determined to a large extent by its heavy chain. It is even possible to screen for and identify light chains that do not contribute significantly to binding of the antibody, which light chains also could be suitably used according to the invention. It may also be possible to follow the teachings of the present invention, but use one heavy chain and vary the light chains. However, the .use of a common light chain and different heavy chains appears preferable, and the following observations support the idea that the specificity of an antibody appears to be dominated by its heavy chain sequence. In the process of receptor editing, a mechanism of 3-cells to monitor if their immunoglobulin receptor encodes a potentially harmful autoantibody, B-cells expressing an autoantibody replace the expressed heavy chain with another heavy chain while retaining the the expressed light chain. Thus, a new antibody specificity is generated that does not encode an autoantibody. This shows that a single light chain can successfully dimerize with multiple heavy chains to form different antibody specificities (Nemazee, 2000; Casellas et al, 2001). Series of transfected cell lines using a single heavy chain gene with different light chain genes 'have been reported, the antibodies produced to a large extent maintaining their specificity, regardless of the light chain (Radic et al, 1991) . Different antibodies have been obtained from a library that has been constructed using a single light chain (Nissim et al, 1994). We have obtained several antibodies from the library described by De Kruif et al (1995), which was constructed using 7 light chains, that have the same light chain but different specificities (see e.g. example 1: antibodies binding to EpCAM and to CD46, described in WO 01/48485 and WO 02/18948, respectively). Besides screening a phage library against a target, it will be also possible to start with an antibody that has already proven its merits, and use the light chain of this antibody in the preparation of a library of heavy chains combined with this particular light chain only, according to methods known to the person skilled in the art, such as phage display. Using this strategy, a monoclonal antibody can be used to obtain a mixture of antibodies according to the invention, functionally resembling a polyclonal or oligoclonal antibody to the same target. Alternatively, a method reminiscent of the method described by Jespers et al (1994) to obtain a human antibody based on a functional rodent antibody can be used. The heavy chain of a known antibody of non-human origin is first cloned and-paired as a template chain with a repertoire of human light chains for use in phage display, after which the phages are selected for binding to the antigen or mixture of antigens. The selected light chain is in turn paired with a-.repertoire .of human" heavy chains displayed on a phage, and the phages are selected again to find several heavy chains that when paired with the light chain are"able to bind to the antigen or mixture of antigens of interest. This enables creating a mixture of human antibodies against a target for which thus far only a non-human monoclonal antibody is described. It is possible that a mixture according to the present invention already has beneficial functional effects when the individual antibodies do not have high affinities for the target, whereas high affinities are often required for monoclonal antibodies to be.effective. This would,have the advantage that affinity maturation may be required in less instances for methods and mdxtures according to the present invention than when an approach with monoclonal antibodies is envisaged. The heavy and light chain coding sequences can be introduced simultaneously or consecutively into the host cell. It is also an aspect of the invention to prepare a host cell comprising a recombinant nucleic acid encoding a light chain of an antibody. Such a cell can for instance be obtained by transfection of said nucleic acid, and optionally a clone can be identified that has a high expression of the light chain. An established clone may then be used to add genetic information encoding 2, 3, 4, 5, 6, 1, 8, 9, 10, or more heavy chains of the invention by introducing the nucleic acid molecules encoding these into cells of the clone that already contains the light chain. The nucleic acid molecules encoding the heavy chains may be introduced into said host cell concomitantly. It is of course also possible to introduce them consecutively, e.g. by using different selection markers, which can be advantageous if not all heavy chains can be. introduced simultaneously because the cells do not take up enough copies of recombinant nucleic -acid molecules. Methods to introduce recombinant nucleic acid molecules into/host cells are well known to the' person skilled in the art,,and include,transfection, electroporation, calcium-phosphate precipitation, virus infection,, and the like. The skilled person has several ■possibilities to introduce rrLore vectors with nucleic acid sequences of interest into the same host cell, see e.g. Sambrook, Fritsch and Maniatis, Molecular'Cloning: A .Laboratory Manual,* 2"^-edition, 1989; Current Protocols'in Molecular Biology, Ausubel FM, et al, eds, 1987; the series Methods in'-Enzymology (Academic Press; inc.} . Suitable dominant selection markers for introducing '-nucleic acids into "eukaryotic host cells according to the ■invention may be G418 or neomycin (geneticin) ,."■ hygromycin or mycophenolic acid, puromycin, and the'" like, ' for which genes encoding resistance are. available on expression vectors. Further possibilities include for instance the use of vectors containing DKFR genes or glutamate synthetase to select in the presence of methotrexate in a DHFR" cell or the absence of glutamine in a glutamine auxotroph, respectively. The use of expression vectors with different selection markers enables subsequent transfections with heavy chain sequences of interest into the host cell, which already'stably contains other heavy chains introduced previously by use of other, selection markers. It is" also-.possible to use selection markers that can be used more than once, e.g. when containing mutations, introns,^'Or weakened promoters that render- -them ■concentration dependent - (e . g... EP0724639-; -WGOl/329'01; US patent 5,733,779). Alternatively, a selection marker nucleic acid molecules consecutively. In yet another aspect according to the invention, a method is provided for making a recombinant host cell for production of a mixture of antibodies, the method comprising the step of: introducing a nucleic acid.sequence or nucleic acid sequences encoding 2, 3, 4, 5, 6, 1, 8, 9, 10, or more different heavy chains into a recombinant host cell comprising a nucleic acid sequence encoding a light chain capable of pairing with at least two of said heavy chains. In case it would appear that a recombinant host cell according to the invention does not express sufficient light chain to dimerize with all of the expressed at least two heavy chains, extra copies of the nucleic acid molecules encoding the light chain may be transfected into the cell. Besides random integration after transfection, methods to integrate the transgenes in predetermined positions of the gepome resulting in favorable expression levels, can also be used according to the invention. Such methods may for instance employ site-specific integration by homologous recombination (see e.g. WO 98/41645), or make use of site-specific recombinases (Gorman and Bullock, 2000). It is yet another aspect of the present invention to provide a transgenic non-human mammal or a transgenic plant comprising a nucleic acid sequence encoding a light chain and a nucleic acid sequence or nucleic acid sequences encoding at least two different heavy chains that are capable of-pairing-;';i-th said-light chain, wherein said nucleic acid S^QX^QTIO^S encoding said light TO and heavy chains are under the control of a tissue-specific promoter. Promoters in plants may also be non-tissue specific, and general gene expression elements, such as the CaMV 35S promoter and nopaline synthase polyA addition site can also be used. Said light chain is a common light chain according to the invention. In specific embodiments, the transgenic animal or plant according to the invention comprises 3, 4, 5, 6, 7, 8, 9, 10, or more heavy chain sequences. Besides cell culture as a production system for recombinant proteins, the art also discloses the use of transgenic animals, transgenic plants, and for instance transgenic chickens to produce proteins in the eggs, and the like to produce recombinant proteins of interest (Pollock et al, 1999; Larrick and Thomas, 2001; WO 91/08216). These usually comprise the recombinant gene or genes encoding one or more proteins of interest in operable association with a tissue-specific promoter. It has for instance been shown that recombinant antibodies can be produced at high levels in the milk of transgenic animals, that contain the nucleic acids encoding a heavy and a light chain behind a m.ammary gland specific promoter (e.g. Pollock et al, 1999; WO 95/17085). Particularly useful in this respect are cows, sheep, goats, pigs, rabbits, mice, and the like, which can be m.ilked to obtain antibodies. Useful promoters are the casein promoters, such as the ^-casein promoter, the aSl-casein promoter, the whey acidic protein (WAP) promoter, the p-lactoglobulin promoter, the a-lactalburain promoter, and the like. Production of biopharmaceutical proteins in the milk of transgenic mammals has been extensively described (e.g. Pollock et al, 1999). Besides mammary gland specific promoters, also other tissue- specific promoters may be usee, directing the expression to the blood, urine, saliva, and the like. The generation of transgenic animals comprising recombinant nucleic acid molecules has been extensively documented, and may include-micro-injection of oocytes {see e.g. Wilmut and Clark, 1991), nuclear transfer after transfection (e.g. Schnieke et al, 1997), infection by recombinant viruses (e.g. as patent 6291740), and the like. Nuclear transfer and cloning methods for mairjnalian cells are known to the person skilled in the art, and are e .g. - described in (Campbell et al, 1996; Wilmut et al, 1997; Dinnyes et al, 2002; WO 95/17500;.WO 98/39416}. It is nowadays possible to clone most of these animals, to generate lines of animals that are genetically identical, which renders it possible for a person skilled in the art to create such a line once an individual animal producing the desired mixture of antibodies has been identified. Alternatively, classical breeding methods can be used to generate transgenic offspring. Strategies for the generation of transgenic animals for production of recombinant proteins in milk are described in Brink et al, 2000. Transgenic plants or plant cells producing antibodies have also been described (Hiatt et al, 1989; Peeters et al, 2001), and useful plants for this purpose include corn, maize, tobacco, soybean, alfalfa, rice, and the like. Constitutive promoters that can for instance be used in plant cells are the CaMV 35S and 19S promoters, Agrobacterium promoters nos and ocs. Other useful promoters are light inducible promoters such as rbcS. Tissue-specific promoters can for instance be seed-specific, such as promoters from zein, napin, beta-phaseolin, ubiquitin, or tuber-specific, leaf-specific (e.g. useful in tobacco), root-specific, and the like. It is also possible to transform the plastid organelle by homologous recombination, to express proteins in plants. Methods and means for expression of proteins in recombinant plants or parts thereof, or recombinant plant cell culture, are known to the person skilled in the art and have been for instance been described in (Giddings et al, 2000; WO 01/64929; WO 97/42313; US patents 5888789, 6080560; See for practical guidelines: Methods In Molecular Biology vol. 4 9 ^'Plant Gene Transfer And Expression Protocols", Jones H, 1995). Other transgenic systems for producing recombinant proteins have also been described, including the use of transgenic birds to produce recombinant proteins in eggs (e.g WO 97/47739), and the use of transgenic fish (e.g. WO 98/15627), and can be used in combination with the teachings of the present invention to obtain mixtures of antibodies. It is also possible to use an in vitro transciption/translation or in vitro translation system for the expression of mixtures of antibodies according to the present invention. It will be clear to the skilled person that the teachings of the current invention will allow producing mixtures of antibodies in systems where recombinant nucleic acid encoding the light chain and heavy chains can be introduced and expressed. Preferably such systems are able to produce antibodies encoded by said nucleic acid sequences, without the use of amplification of said nucleic acid sequences in said systems. In another aspect of the invention, a cell from a transgenic non-human animal or a transgenic plant according to the invention is provided. Such cells can be used to generate the animals or plants according to the invention, using techniques known to the person skilled in the art, such as nuclear transfer or other known methods of cloning whole organisms from single cells. The cells according to the invention may also be obtained by introducing the light and at least two heavy chain sequences into isolated cells of non-human animals or plants, which cells are capable of becoming part of a transgenic animal or plant. Particularly useful for such purposes are embryonic stem cells. Thes.e. can contribute to the germ line, and therefore the genetic information introduced into such cells can be passed to future generations. In addition, plant cell cultures of cotton, corn, tomato, soybean, potato, petunia, and tobacco can be utilized as hosts, when transformed with the nucleic acid molecules encoding the light chain and the heavy chains, e.g. by use of the plant transforming bacterium Agrobacterium tumefaciens'or by particle bombardment, or by infecting with recombinant plant viruses. It is another aspect of the present invention to provide a pharmaceutical composition comprising a mixture of recombinantly produced antibodies and a suitable carrier, wherein at least two different heavy chains are represented in said mixture of recombinantly produced antibodies. Pharmaceutically acceptable carriers as used herein are exemplified, but not limited to, adjuvants, solid carriers, water, buffers, or other carriers used in the art to hold therapeutic components, or combinations thereof. In particular embodiments, 3, 4, 5, 6, 7, 8, 9, 10, or more different heavy chains are represented in said-mixture. Said mixture can-be obtained by mixing recombinantly produced monoclonal antibodies, but may also be obtained by methods according to the present invention. Said mixture may therefore comprise a coirition light chain for said antibodies. Said mixture may comprise bispecific antibodies. Said mixture may be produced from a clone that was derived from a single host cell, i.e. from a population of cells containing the same recombinant nucleic acid molecules. The term "recombinantly produced" as used herein refers to production by host cells that produce antibodies encoded by recombinant nucleic acids introduced in such host cells or ancestors thereof. It does therefore not include the classical method of producing polyclonal antibodies, whereby a subject is immunized with an antigen or antigen comprising mixture, after which the antibodies produced by this subject are recovered from the subject, e.g. from the blood. It is another aspect of the present invention to provide a mixture of antibodies wherein at least two heavy chains are represented, for use in the treatment or diagnosis of a human or animal subject. In another aspect, the invention provides the use of a mixture of antibodies wherein at least two different heavy chains are represented, for the preparation of a medicament for use in the treatment or diagnosis of a disease or disorder in a human or animal subject. In particular embodiments, 2, 3, 4, 5, 6, 1, 8, 9, 10, or more^heavy chains are represented in said mixture. Said mixtures of antibodies may be mixtures of antibodies according to the invention, or obtained by methods according to the invention. Antibodies present in said mixture may preferably comprise a common light chain. The mixtures may comprise bispecific antibcdies, and may be recombinantly produced from a clone that was derived from a single host cell, i.e. from a population of .cells containing the same recombinant nucleic acid molecules. The targets may be used to, screen an antibody display library, as described supra, to obtain 2, 3, 4, 5, 6, 7, 8, 9, 10, or more antibodies comprising a common light chain that bind to the target, and produce a mixture of these according to the teachings of the present invention. Virtually any area of medicine where monoclonal antibodies can be used is amenable for the use of the mixtures according to the invention. This can e.g. include treatment of auto-immune diseases and cancer, including solid tumors of the brain, head- and neck, breast, prostate, colon, lung, and the like, as well as hematologic tumors such as B-cell tumors. Neoplastic disorders which can be treated with the mixtures according to the present invention include leukemias, lymphomas, sarcomas, carcinomas, neural cell tumors, squamous cell carcinomas, germ cell tumors, metastases, undifferentiated tumors, seminomas, melanomas, myelomas, neuroblastomas, mixed cell tumors, neoplasias caused by infectious agents, and other malignancies. Targets for the antibody mixtures may include, but are not limited to, the HER-2/Neu receptor, other growth factor receptors such as VEGFRl and VEGFR2 receptor, B-cell markers such as CD19, CD20, CD22, CD37, CD72, etc, T-cell markers such as CD3, CD25, etc, other leukocyte cell surface markers such as CD33 or HLA-DR, etc, cytokines such as TNF, interleukins, receptors for these cytokines such as members of the--TNF-receptor -family, -and the like. It is anticipated that the use of such mixtures of antibodies in the treatment of cancerous tissues or other complex multi-antigen comprising cells such as microorganisms or viruses will give :rise to less occurrence of epitope-loss escape variants than the use of single monoclonal antibodies. Several treatments nowadays use polyclonal mixtures of antibodies, which are derived from immunized humans or animals. These treatments may be replaced by use of the mixtures according to the present invention. Use of these mixtures can also include use in graft-versus-host rejections, known in the art of transplantation, e.g. by use of anti-thymocyte antibodies. It is:anticipated that the mixtures of antibodies are superior to monoclonal antibodies in the treatment of complex antigens or antigen comprising mixtures such as bacteria or viruses. Therefore, use according to the invention can also include use against strains of bacteria and fungi, e.g. in the treatment of infectious diseases due to pathogenic bacteria such as multidrug resistant S.aureus and the like, fungi such as Candida albicans and Aspergillus species, yeast and the like. The mixtures according to the invention may also be used for post exposure profylaxis against viruses, such as members of the genus Lyssavirus e.g. rabies virus, or for therapeutic or prophylactic use against viruses such as Varicella-Zoster Virus, Adenoviruses, Respiratory Syncitium Virus, Human Immunodeficiency Virus, Human Metapneumovirus, Influenzavirus, West Nile Virus, the virus causing Severe Acute Respiratory Syndrome (SARS), and the like. Mixtures according to the inventions can also be used to protect against agents, both bacteria and viruses, and against toxic substances that are potential threats of biological warfare. Therefore, use according to the invention can also include use against strains or bacteria such as Bacillus anthracis, Clostridium hotulinum toxin, Clostridium perfringens epsilon toxin Yersinia Pestis, Francisella tulariensis^ Coxiella burnetii^ Brucella.species. Staphylococcus enterotoxin B, or against viruses such as Variola major, alphaviruses causing meningoencephalitis syndromes (EEEV, VEEV, and WEEV), viruses known to cause hemorrhagic fevers such as Ebola, Marburg and Junin virus or against viruses such as Nipah virus, Hantaviruses, Tickborne encephalitis virus and Yellow fever virus or against toxins e.g. Ricin toxin from Ricinus communis and the like. Use of the mixtures according to the invention can also include use against unicellular or multicellular parasites. Recombinant mixtures of antibodies according to the invention may become a safe alternative to polyclonal antibodies obtained from pools of human sera for passive immunization, or from sera of hyper-immunized animals. The mixtures may be more efficacious than recombinant monoclonal antibodies in various therapeutic applications, including cancer, allergy, viral diseases, chronic inflammation, and the like. It has been described that homodimerization of tumor-reactive monoclonal antibodies markedly increases their ability to induce growth arrest or apoptosis of tumor cells (Ghetie et al, 1997). Possibly, when antibodies against receptors or other surface antigens on target cells, such as tumor cells or infectious microorganisms, are produced according to the present invention, the bispecific antibodies present in mixtures according to the invention may also crosslink different receptors -or other antigens on the surface of target cells, and therefore such mixtures may be very suitable for killing such cells. Alternatively, when bispecific antibodies are less desirable, the present invention also provides methods to recombinantly produce mixtures of antibodies comprising mainly monospecific antibodies. It has been described that the efficacy of treatment with Rituximab™ (anti-CD20 monoclonal antibody) was increased when anti-CD59 antibodies were added (Herjunpaa et al, 2000). Therefore, it is expected that inclusion of antibodies against CD59 in a mixture according to the invention comprising anti-tumor antibodies in the form of B-cell receptor recognizing antibodies increases the sensitivity of tumor cells to.complement attack. It has also been shown that a triple combination cocktail of anti-CD19, anti-CD22, and anti-CD38-saporin immunotoxins is much more effective than the individual components in the treatment of human B-cell lymphoma in an imir.unodeficient mouse model (Flavell et al, 1997). Many other combinations may also be feasible.and can be designed by ■the person skilled in the art. In general, the use of antibody mixtures that are capable of recognizing multiple B-cell epitopes will likely decrease the occurrence of escape variants. Another possible target is a transmembrane tyrosine kinase receptor, encoded by the Her-2/Neu (ErbB2) proto-oncogene (see e.g. US patents 5,7"2,997 and 5,783,185 for anti-Her2 antibodies). Her-2 is overexpressed on 30% of highly malignant breast cancers, and successful antibodies against this target, marketed under the name Herceptin^"^ (Trastuzumab) , have been developed. It has been shown that targeting multiple Her-2 epitopes with a mixture of monoclonal antibodies results in improved antigrowth activity of a human breast cancer cell line in vitro and in vivo (Spiridon et al, 2002} . Her-2 may therefore be a good target for antibody mixtures according to the present invention. Antibodies useful for this purpose can be obtained ty methods described in the present invention, including antibody display methods. Human antibodies are capable of eliciting effector function via binding to iirmuncglobulin receptors on immune effector cells. Human IgG, and in particular IgGi and IgG3, fix complement to induce CDC and interact with Fey receptors to induce antibody dependent cell mediated cytotoxicity (ADCC), phagocytosis, endocytosis, induction of respiratory burst and release of inflammatory mediators and cytokines. Human IgA interacts with FcaR, also to result in efficient activation of ADCC and phagocytosis of target cells. Hence, due to the differential distribution of FcyR and FcaR on peripheral blood cells (Huls et al., 1999), using a mixture of antibodies directed against the target and consisting of both IgG and IgA would potentially maximize the recruitment and activation of different immune effector cells. Such a mixture of both IgG and IgA could be obtained by producing the IgG and IgA monoclonal antibody in a separate production process using two distinct production cell lines, but could also be obtained from a single cell line producing both the IgG and the IgA monoclonal antibody. This would have the advantage that only a single production process has to be developed. Thus when different heavy chains are mentioned, also heavy chains differing in their constant regions are encompassed in the invention. The principle of using a common light chain can also be used for the production of a mixture of isotypes from a host cell. It is therefore yet another aspect of the present invention to provide a method for producing a mixture of antibodies comprising different isotypes from a host cell, the method comprising the step of: culturing a host cell comprising a nucleic acid sequence encoding a light chain and nucleic acid sequences encoding at least two heavy chains of different isotype that are capable of pairing with said light chain, under conditions conducive to expression of said nucleic acid sequences. According to this aspect of the invention, different heavy chains may have identical variable regions, and only differ in their constant regions ;{i.e. be of different isotype and have the same specificity). In a particular embodiment, said isotypes comprise at least an IgG and an IgA and/or IgM, preferably IgGl or IgG3 and IgA. Other combinations of IgGl, IgG2, IgG3 and IgG4 can also be used. In these embodiments, bispecific antibodies will not be produced because the variable regions are the same. In other embodiments according to this aspect of the invention, not only the constant regions of the heavy chains may differ; but also the variable regions, thereby giving rise to different specificities, paired with the same light chain. When bispecific antibodies are not desired for a given purpose, e.g. because the mixtures "of antibodies are less efficacious" because of the presence of the bispecific antibodies, it is possible to use at least two heavy chains combined with the comrrion light chain according to the invention wherein said heavy chains differ sufficient in their constant regions to reduce or prevent-pairing between the different heavy chains, e.g. by using heavy chains of different isotypes, e.g. an IgGl and an IgG3 (see Fig. 11 for a schematic representation). It is anticipated that the heavy chains of different isotype will pair much less efficient, if at all, compared to the same heavy chains. Alternatively, it is also possible to engineer the different heavy chains in their constant region such -hat homodimerization is favored over heterodimerizaticn, e.g. by introducing self-complementary interactions (see e.g. WO 98/50431 for possibilities, such as "protuberance-into-cavity" strategies (see WO 96/27011)). It is therefore another aspect of the present invention to provide a method for producing a mixture of antibodies in a recombinant host, the method including the step of: expressing in a recombinant host cell a nucleic acid sequence encoding a common light chain and nucleic acid sequences encoding at least two different heavy chains that differ in the variable region and that are capable of pairing with said common light chain, and wherein said heavy chains further differ in their constant regions sufficiently to reduce or prevent pairing between the different heavy chains. In one embodiment, said heavy chains are of different isotype. In specific embodiments, 3, 4, 5, 6, 7, 8, 9, 10, or more different heavy chains are expressed. Mixtures of antibodies obtainable by this method are also embodied in the present invention. Such mixtures will comprise mainly monospecific antibodies. The teachings of the present invention can also be used to obtain novel multispecific antibodies or mixtures thereof. Therefore, in another aspect, the invention ■provides amethod'for producing a-mixture of antibodies comprising dimeric IgA isotype {(IgA)2} antibodies in a recombinant host, wherein at least part of said dimeric IgA antibodies have different binding regions in each of the IgA subunits, the method comprising the step of: expressing in a recombinant host cell a nucleic acid sequence encoding a common light chain and nucleic acid sequences encoding at least two different heavy chains of IgA isotype capable of pairing to said common light chain, wherein said different heavy chains differ in their variable region. Dimerization of the IgA molecules can be enhanced by co-expressing J-chain (Yoo et al, 1999). Said dimeric IgA antibodies have two specificities (see Fig. 9 for a schematic representation of one possible form produced and present in the mixture), In yet another aspect the invention provides a method for producing a mixture of antibodies comprising an IgM antibody having at least two different specificities, the method comprising the step of expressing in a recombinant host cell a nucleic acid sequence encoding a common light chain and nucleic acid sequences encoding at least two different heavy chains of IgM isotype, wherein said heavy chains are capable of pairing to said common light chain and form functional antigen binding regions. Up to five specificities can be comprised in an IgM pentamer in the presence of J-chain, and up to six in an IgM hexamer in the absence of J-chain (Yoo et ax, 1999). Therefore, in specific embodiments, 3, 4, 5, or 6 IgM heavy chains are co-expressed with the common ligh~ chain according to this aspect of the invention. See Fig. 10 for a schematic representation of one of the possible forms that can be produced and present in the mixture according to this aspect of the invention, when five different heavy chains are expressed with a common light chain. The invention also provides for IgA dimers, IgM pentamers or hexamers having at least two different specificities. These molecules can be produced from a clone of a single host cell according to the invention. Such molecules, harbouring antigen binding regions with different specificities, can bind different epitopes on the same antigen, different antigens on one cell, or different antigens on different cells, thereby crosslinking the antigens or cells. It is yet another aspect of the present invention to provide a method for identifying a mixture of antibodies having a desired effect in a functional assay, the method comprising the steps of i) adding a mixture of antibodies in a functional assay, and ii) determining the effect of said mixture in said assay, wherein said mixture of antibodies comprises antibodies having a common light chain. In a preferred embodiment said mixture is comprised in a composition according to the present invention. The invention also provides a method for recombinant expression of one or more proteins in a single host cell, wherein at least four different polypeptides are expressed in said single host cell. Each polypeptide is independently expressed, and may be under control of a heterologous promoter. The protein or.proteins may be isolated separately or as a mixture from a culture of said host cell. Preferably, the host cell of this embodiment is a human cell, and/or may be derived from a retina cell,, .more .preferably a cell- comprising -adenovirus El sequences in its genome, most preferably a PER.C6 cell. Examples The following examples are provided to illustrate the invention, and are not to be construed in any way to limit the scope of the invention. The practice of this invention will employ, unless otherwise indicated, conventional techniques of immunology, molecular biology, microbiology, cell biology, and recombinant DNA, which are within the skill of the art. See e.g. Sambrook, Fritsch and Maniatis, Molecular Cloning: A Laboratory Manual, 2"^* edition, 1989; Current Protocols in Molecular Biology, Ausubel FM, et al, eds, 1987; the series Methods in Enzymology (Academic Press, Inc.); PCR2: A Practical Approach, MacPherson MJ, Hams BD, Taylor GR, eds, 1995; Antibodies: A Laboratory Manual, Harlow and Lane, eds, 1983. Exau^le 1 Production of a mixture of monoclonal antibodies with a common light chain and two different heavy chain variable regions in a single cell Clone U3S-54 and Clone K53 were previously isolated by selections on the colorectal cell line SW40 (Huls et al., 1999) and on a heterogeneous mixture of mononuclear cells of a patient with multiple myeloma (WO 02/18948), respectively, with a semi-synthetic library (de Kruif et al., 1995b). Further studies revealed that clone UBS-S'S and K53 bound to the EP-CAM homotypic adhesion molecule (Huls et al.. , 1999) and the membrane cofactor protein CD46 (WO 02/18948), respectively. DNA sequencing of the clones revealed that they were unique in the Heavy chain CDRs, but that they contained an identical light chain sequence (Fig. 3) . The VH and V\. of clones UBS-54 and K53 were inserted into an expression vector containing the HAVT20 leader sequence and all the coding sequences for the constant domains of a human IgGl with a Kappa light chain by a method essentially as described (Boel et al, 2000), which resulted in plasmids pUBS3000Neo and pCD46_3000(Neo) (Fig. 4). These plasmids were transiently expressed either alone or in combination in PER.C6'™ cells. In brief, each 80 cm^ flask was transfected by incubation for A hours with 140 \il lipofectamine + 10 \ig DNA (either paBS3000Neo, pCD45_3000 (Neo) or 10 \ig of both) in serum-free DMEiy] mediuin at Sl^C. After 4 hours, this was replaced with DMEM + 10% FBS, and the cells were grown overnight at 31°C. Cells were then washed with PBS and the medium was replaced with Excell 525 medium- (JRH Bioscience). The cells were allowed to grow at 37°C for 6 days, after which the cell culture supernatant was harvested. Human IgG specific ELISA analysis (described in WO 00/63403) indicated that IgG was present at approximately 10 ng/ml for all flasks containing expression plasmids. No IgGl was present in a control flask which was not transfected with expression plasmid. Human IgG from each supernatant was subsequently purified using Protein A affinity chromatography (Hightrap Protein A HP, cat.no. 1-040203) according to standard procedures, following recommendations of the manufacturer (Amersham .Biosciences) . After elution, samples were concentrated in a Microcon YM30 concentrator (Amicon) and buffer exchanged to 10 mM sodium phosphate, pH 6.7. Twelve pg of purified IgG was subsequently analyzed on Isoelectric focusing gels (Serva Pre-cast lEF gels, pH range 3-1,0, cat.no. 42866). The samples were loaded on the low pH side and after focussing stained with colloidal blue (Fig. 5). Lane 1 shows transiently expressed K53, Lane 2 shows transiently expressed UBS-54 and Lane 3 shows the .IgG sample of the cells in which both antibodies were co-transfected. Clearly, K53 and UBS-54 each have a unique pi profile and the sample form the cotransfection showed other unique isoforms, with the major isoform having a pi in between those of K53 and UBS-54, This is also anticipated on the basis of the theoretic pi when calculated with the ProtParam tool provided on the Expasy homepage (http://www.expasy.ch; Appel et al., 1994). K53 and UBS-54 have a theoretic pi of 8.24 and 7.65, respectively, whereas an isoform representing a heterodimer of one UBS-54 heavy chain and one K53 heavy chain has a theoretical pi of 8.01. Assembly of such a heterodimer can only occur when a single cell translates both the heavy chain of K53 and the heavy chain of UBS-54 and assembles these into a full length IgG molecule together with the common light chain. Therefore, this experiment shows that it is possible to express two unique human IgG molecules in a single cell and that a heterodimer consisting of these two unique binding specificities is also efficiently formed. Example 2 Production of a mixture of antibodies against human B-cell markers in a PER.C6™ cell line derived clone. A method for producing a mixture of antibodies according to the invention, usin^ expression in a recombinant host cell of a single light chain and three different heavy chains capable of pairing to the single light chain to form functional antibodies, is exemplified herein and is schematically shown in Fig. 6. Phages encoding antibodies capable of binding proteins present on human B-cells, i.e. CD22, CD72 and Major Histocompatibility Complex {MHO class II (further referred to as HLA-DR) were previously isolated from a se-.i-synthetic phage library (de Kruif et al., 1995; van der Vuurst de Vries S Logtenberg, 1999). DNA sequencing of the VH and VL sequences of the phages clone 328 (anti-CD22), clone 1-2 (anti-HLA-DR) and clcne II-2 (anti-CD72) revealed that they all contain a unique Vu sequence, but a common light chain sequence {VX3) with an identical CDR region (Fig. 7). The VH and-VL sequences of clones B28, I-l and II-2 are cloned behind the HAVT20 leader sequences of an expression plasmid comprising a heavy chain. An example of such a plasmid is pCRU-KOl (contains kappa heavy chain sequences, that can easily be interchanged for lainbda heavy chain sequences if desired, by a person skilled in the art), as deposited at the ECACC under number 03041601. The cloning gives rise to plasmids encoding a full length human IgGi with binding specificities for CD22, CD72 and HLA-DR. These plasmids will further be referred to as pCRU-CD22, pCRU-CD72 and pCRU-HLA-DR, respectively. Stable PER.06*^" derived cell lines are generated, according to methods known to the person skilled in the art (see e.g. WO 00/63403), the cell lines expressing -antibodies encoded by genetic-information on either pCRU-CD22, pCRU-CD72 or pCRU-HLA-DR and a cell line expressing antibodies encoded by all three plasmids. Therefore, PER.C6™ cells are seeded in DMEM plus 10% FBS in tissue culture dishes (10 cm diameter) or T80 flasks with approximately 2.5 x 10^ cells per dish and kept overnight under their normal culture conditions (10% CO2 concentration and 37°C) . The next day, transfections are performed in separate dishes at 31^C using Lipofectamine (Invitrogen Life Technologies) according to standard protocols provided by the manufacturer, with either 1-2 ^g PCRU-CD22, 1-2 ]ig pCRU-CD72, 1-2 yg pCRU-HLA-DR or 1 Vig of a mixture of pCRU-CD22, pCRlj-CD72 and pCRU-HLA-DR. As a control for transfection efficiency, a few dishes are transfected with a Lac2 control vector, while a few dishes will be not transfected and serve as negative controls. After 4 to 5 hours, cells are washed twice with DMEM and refed with fresh medium without selection. The next day, medium are replaced with fresh medium containing 500 pg/ml G418. Cells are refreshed every 2 or 3 days with medium containing the same concentrations of G-aiS. About 20-22 days after seeding, a large number of colonies are visible and from each transfection at least 300 are picked and grown via 96-well and/cr 24-well via 6-well plates to T25 flasks. At this stage, cells are frozen (at least 1, but usually 4 vials per sub-cultured colony) and production levels of recombinant human IgG antibody are determined in the supernatant usir.g an ELISA specific for human IgGi (described in WO 00/634 03). Also, at this stage G418 is removed from the culture -edium and never re¬applied again. For a representative number of colonies larger volumes will be cultured tc purify the recombinant human IgGi fraction from the conditioned supernatant using Protein A affinity chromatography according to standard procedures. Purified human IgG. from the various clones is analyzed on, SDS-PAGE, Iso-elecrric focusing (lEF) and binding to the targets CD22, CD72 and HLA-DR using cell transfectants expressing these human antigens on their cell surface (transfectants expressing CD72 and HLA-DR have been described by van der Vuurst-de Vries and Logtenberg, 1999; a CD22 transfectant has been prepared according to similar standard procedures in PER.C6™). Colonies obtained from the co-transfeetion with pCRU-CD22, pCRU'CD72 and pCRU-HLA-DR are screened by PCR on genomic DNA for the presence or absence of each of the three constructs. ' The identity' of the PCR products is further confirmed by DNA sequencing. Next, it is demonstrated that a clonal cell line accounts for the production of each of the three binding specificities, i.e. proving that a single cell is able to produce a mixture of more than two functional human IgG's. Therefore, a limited number of colonies, which screened positive for the production of each of the three binding specificities (both by PCR at the DNA level as well as in the specified binding assays against CD22, CD72 and HLA-DR), are subjected to single cell sorting using a fluorescence activated cell sorter (FACS) (Becton & Dickinson FACS VANTAGE SE) . Alternatively, colonies are seeded at 0.3 cells/well to guarantee clonal outgrowth. Clonal cell populations, hereafter designated as sub¬clones, are refreshed once a week with fresh medium. Sub¬clones are grown and transferred from 96-wells via 24-and 6-wells plates to T25 flasks. At this stage, sub¬clones .are. frozen (at least .1, but-usually 4 vials per sub-clone) and production levels of recombinant human IgG: antibody are determined in the supernatant using a 'human IgGi specific ELISA. For a representative number of sub¬clones, larger volumes are cultured to purify the recombinant human IgGi fraction from the conditioned supernatant using Protein h affinity chromatography according to standard procedures. Purified human IgGi from the various sub-clones is subsequently analyzed as described above for human IgGi obtained from the parental clones, i.e. by SDS-PAGE, Iso¬electric focusing, {lEF) and binding to the targets CD22, CD72 and HLA-DR. Sub-clones will also be screened by PCR on genomic DNA for the presence or absence of each of the three constructs pCRU-CD22, pCRU-CD72 and pCRU-HLA-DR. The identity of the PCR products is further confirmed by DNA sequencing. Other methods such as Southern blot and/or FISH can also be used to determine whether each of the three constructs are present in the clonal cell line. Sub-clones that are proven to be transgenic for each of the three constructs are brought into culture for an extensive period to determine whether the presence of the transgenes is stable and whether expression of the antibody mixture remains the same, not only in terms of expression levels, but also for the ratio between the various antibody isoforms that are secreted from the cell. Therefore, the sub-clone culture is maintained for at least 25 population doubling times gither as an adherent culture or as a suspension culture. At every 4-6 population doublings, a specific production test is performed using the human IgG specific ELISA and larger volumes are cultured to obtain the ceil pellet and the supernatant. The cell pellet is used to assess the presence of the three constructs in the genomic DNA, either via PCR, Southern blot and/or FISH. The supernatant is used to purify the recombinant human IgGi fraction as described supra. Purified human IgGi obtained at the various population doublings is analyzed as described,' i.e. by. SDS-PAGE, Iso-electric focusing (lEF) and binding to the targets CD22, CD72 and HLA-DR using cell transfectants expressing these antigens. Exan^le 3 Screening of clones expressing multiple human XgGs for the most potent mixture of functional human IgGs Functionality 'of the antibody mixture is analyzed in cell-based assays to determine whether the human IgGi mixture inhibits proliferation and/or induces apoptosis of B-cell lines, such as for example Ramos. Other cell lines can also be used. In addition the antibody mixtures are analyzed for their potential to induce antibody dependent cellular toxicity and complement dependent. cytotoxicity of for example'Ramos cells. In each of the following experiments the functionality of the antibody mixture recognizing the targets CD22, CD72 and HLA-DR is analyzed and can be compared to each of the individual IgGi antibodies and to an equimolar combination of the three individual IgGi specificities. To assess the ability of the antibody mixtures to inhibit the proliferation of Ramos cells, these cells are incubated in 96-well plates (0.1 - 1.0 x 10^/ml) with several concentrations. .(5 -. .20 ,vig/ml) of the antibody mixtures .against CD22, CD72 and HLA-DR for 24 hours. The proliferation of the cells is measured by ^H-thymidine incorporation during another 16 hours of culture. Inhibition of growth is determined by plotting the percentage of ^H-thymidine incorporation compared to untreated cells (taken as 100% reference value). To analyze apoptosis induction of Ramos cells, these cells are stimulated in 48-well plates (0.2 - 1.0 x 10^/ml) with several concentrations (5 - 20 ]ig/ml) of the antibody mixtures against the targets CD22, CD72 and HLA-DR for 24 or 48 hours. After the incubation period the phosphatidyl serine exposure on apoptotic cells is analyzed (Koopman G et al, 1994}. Therefore, the cells are harvested, washed twice with PBS and are incubated at RT for 10 min with 100 \il FITC-labelled annexin V (Caltag) diluted 1:25 in annexin V binding buffer (Caltag). prior to the analysis of the samples by flow cytometry (FACSCalibur, Becton Dickinson, San Jose, CA) propidium iodide (PI)(Sigma) is added to a final concentration of 5 '\ig/ml to distinguish necrotic cells (annexin V-/PI+) from apoptotic cells {annexin V+/PI-, early apoptotic cells; annexin V-f/PI + , late apoptotic cells). In an alternative assay, apoptosis is induced by crosslinking the antibody mixtures against CD22, CD72 and HLA-DR on the cell surface of Ramos cells with 25 yg/ml of F{ab)2 of goat-anti-human (Fc-specific) polyclonal antibodies (Jackson Immunoresearch Laboratories, West Grove, PA) during the incubation period. In another alternative assay, apoptosis is induced by incubating the Ramos cells with several concentrations (5 ~ 20 pg/ml) of the antibody mixtures against CD22, CD72 and HLA-DR while co-incubating them with the chemosensitizing agents doxorubicin (Calbiochem) or dexamethasone (UMGU,Utrecht, the Netherlands). Antibody Dependent Cellular Cytotoxicity of the antibody mixtures is analyzed using peripheral blood mononuclear cells as effector cells in a standard ^^Cr release assay (Huls et al, 1999). To this purpose, 1-3 x 10^ Ramos cells are labelled with 100 fiCi {Amersham, Buckinghamshire, UK) for 1 hour at 37°C. After three washes with medium, the ■ Ramos target cells are plated in U bottom 96 well plates at 5 X 10^ cells/well. Peripheral blood mononuclear cells that are obtained from healthy donors by Ficoll-Hypague density gradients,are then added to each well at effector;target ratios ranging from 80:1 to 10:1 in triplicate. The cells are incubated at 37°C in the presence of various concentrations of the antibody mixtures (5 - 20 ^g/mi) in a final volume of 200 jil. After 4 hours of incubation part of the supernatant is harvested and ^^Cr release is measured. The percantage of specific lysis is calculated using the following formula: % specific lysis = ([experimental cpm - spontaneous cpm] / [maximal cpm - spontaneous cpm] x 100%). Maximal ^^Cr release is determined by adding triton X-100 to a final concentration of 1% to the target cells and spontaneous release is determined after incubation of the target cells with medium alone. Complement dependent cytotoxicity is determined in a similar assay. Instead of the effector cells, now 50 fil human serum is added to the target cells. Subsequently, the assay is performed in the same manner. Alternatively, ADCC and CDC of the antibody mixtures is determined using a Europium release assay (Patel and Boyd, 1995) or using an LDH release assay {Shields et al, 2001). Example 4 Use of phage display to isolate multiple phages with an identical Vj, sequence against a predefined target (Her-2} , and production in a recombinant host cell of a mixture of antibodies capable of binding this target. Phages displaying. scFv fragments capable of binding multiple epitopes present on the same protein^ for example the epidermal growth factor receptor Her-2/ can be isolated from a semi-synthetic phage library (de Kruif et al., 1995a,b). It is possible to identify several of such phages and select the ones comprising the same light chain sequence, for further use according to the invention. The semi-synthetic library is formed by mixing 7 sub-libraries that each contain a different light chain (de Kruif et al, 1995a,b). It is therefore particularly practical to use such a sub-library, containing only one light chain and many heavy chains, for screening so that multiple antibodies with an identical Vj, sequence are obtained, and further used for expressing the antibody mixtures according to the invention. For the selection of phages against Her-2 several fusion proteins are generated comprising different parts of the extracellular domain of Her-2 that are fused to the CK2 and CH3 domains of human IgGl. For this purpose a pCDNA3.l2eo expression vector (InVitrogen) has been constructed that contains in its multiple cloning region an Xhol restriction site in the hinge region in ^rame prior to the CH2 and CHS domains of human IgGl. Qsing a Her-2 cDNA clone as a template PCR fragments is generated using standard molecular biology techniques known to a person skilled in the art. These fragments consist .of a. unique 5' restriction site, a start codon followed by a eukaryotic leader sequence that is linked "in frame to either the total extracellular (EC) domain of Her-2 or to a part of the EC domain of Her-2 that is followed in frame by an Xhol restriction site. These PCR fragments are subsequently cloned in frame with the CH2-CH3 IgGl ■ region into the pCDNA3.1zeo expression vector. In addition to the fusion protein containing the total EC domain of Her-2, several smaller fusion proteins, are generated containing non-overlapping fragments of the Her-2 EC domain. These constructs encoding the Her-2-Ig fusionproteins are used for- transient transfection of 293T cells using the lipofectamine reagent (Gibco), Five days after transfection the supernatants of the 293T cells are harvested and Her-2-Ig fusion proteins are purified using protein A affinity chromatography according to standard procedures. Her-2-Ig fusion proteins containing non-overlapping fragments of the Her-2 EC domain are coated for 2 hours at 37° onto the surface of Maxisorp™ plastic tubes (Nunc) at a saturating concentration (0.5 - 5 pg/ml) . The tubes are blocked-for 1 h in 2% fat free milk powder ■ dissolved in PBS (MPBS) . Simultaneously, 500 yl (approximately lO" cfu) of a semi-synthetic phage display library (a sub-library according to the terminology used above) in which only one Vkappal light chain is represented..prepared-as described by De Kruif et al {"l995a,b) and references therein, is added to two volumes of 4% MPBS. In addition, human serum is added to a final concentration of 15% and blocking is allowed to proceed for 30-60 min. The Her-2- Ig-coated tubes arej emptied and the blocked phage library if IS added. The tube is sealed and rotated slowly for 1 h, followed by 2 h of incubation without rotation. The tubes are emptied and washed 10 times in PBS containing 0.1% Tween-20, followed by washing 5 times in PBS. 1 ml glycine-HCL, 0.05 Mj pH 2.2 is added, and the tube is rotated slowly for 10 min. The eluted phages are added to 500 |il IM Tris-HCl pH 7.4. To this mixture, 3,5 ml of exponentially arowina XL-1 blue bacterial culture is r added. The tubes are incubated for 30 min at 37°C without shaking. Subsequently, the bacteria are plated on 2TY agar plates containing ampicillin, tetracycline and glucose. After overnight incubation of the plates at 37'C, the colonies are scraped from the plates and used to prepare an enriched phage library, essentially as described by De Kruif er ai. {1995a). Briefly, scraped bacteria are used to inoculate 2TY medium containing ampicillin, tetracydline and glucose and are grown at 37°C to an ODgoowr. of -0.3. Helper phages are added and allowed to infect the bacteria afrer which the medium is changed to 2TY conta'ining ampicillin, tetracycline and kanamycin. Incubation is continued overnight at 30°C. The next day, the bacteria are removec from the 2TY medium by centrifugation after which the phages are precipitated using polyethylene glycol 6000/NaCl. Finally, the phages are dissolved in a small volume of P3S-1% BSA, filter-sterilized and used for a next round of selection. The selection/re-infection procedure is performed twice. After the second round of selection, individual E.coli colonies are used to prepare monoclonal phage antibodies. Essentially, individual colonies are grown to log-phase and infected with helper phages after which phage antibody production'is allowed to proceed overnight. ij Phage antibody containing supernatants are tested in ELISA for binding activity to Her-2-total EC-Ig coated 96 wells plates. Selected phage antibodies that are obtained in the screen described above, are validated by ELISA for specificity. For this purpose, Her-2-Ig fusion proteins containing non-overlapping fragments of the Her-2 EC domain are coated to Maxisorp ELISA plates. After coating, the plates are blocked in 2% MPBS. The selected phage antibodies are incubated in an equal volume of 4% MPBS. The plates are emptied, washed once in PBS, after which the blocked phages are added. Incubation is allowed to proceed for 1 h, the plates are washed in PBS 0.1% Tween-20 and bound phages lare detected using an anti-Ml3 antibody conjugated Jto peroxidase. The procedure is performed simultaneously using a control phage antibody directed against thyroglobulin (De Kruif et al. 1995a,b), which serves as a negative control. In another assay the selected phage antibodies are analyzed for their ability to bind BT474 human breast cancer cells that express Her-2. For flow cytometry analysis, phage antibodies are first blocked in an equal volume of 4% MPBS for 15 min at 4'='C prior to the staining of the BT474 cells. The binding of the phage antibodies to the cells is visualized using a biotinylated anti-M13 antibody (Santa Cruz Biotechnology) followed by streptavidin-phycoerythrin (Caltag). Alternatively, phage antibodies recognizing multiple epitopes on Her-2 ate selected using a method based upon competition of phage binding to Her-2 with binding of the well characterized murine anti-Her-2 antibodies HER50, HER66 and HERTO [Spiridon et al, 2002}. To this purpose 2x10^ BT474 cells are incubated at 4°C with approximately 10^^ cfu (0.5 ml) of a semi-synthetic phage display library in .which only one Vkappal light chain is represented prepared as described supra and blocked with 2 volumes of medium containing 10% of FBS. The mixture is slowly rotated at 4°C for 2 hours in a sealed tube. Subsequently, non-bound phages are removed by two washes with 50 ml of cold medium containing 10% FBS. Hereafter, phages recognizing multiple epitopes on Her-2 are eluted by resuspending the BT474 cells in 1 ml of cold medium containing saturating concentrations (5-20 )xg/ml) of the 1) HER50, HER66 and HER70 murine anti-Her-2 antibodies. The cells are left on ice for 10 min, spun down and the supernatant containing the anti-Her-2 phage antibodies is used to reinfect XLl-Blue cells as described supra. ii From the panel of Her-2-specific phage antibodies generated by de screens described above, three phage antibodies are selected that are recognizing three different non-overlapping epitopes on the Her-2 protein. The VH sequences and the unique Vkappal light chain sequence of these clones, provisionally designated ip VK1HER2-1, VK1HER2-2 and VK1HER2-3, are cloned behind the HAVT20 leader sequences of expression plasmid pCRU-KOl t (ECACC deposit 03041601), or a siirdlar expression plasmid, to obtain plasmids encoding a full length human IgGi-kappa with binding specificities for Her-2. These plasraids are provisionally designated as pCRU-VKlHER2-i, PCRU-VK1HER2-2 and pCRU-VKlHER2-3, respectively. Stable PER.C5™ derived cell lines are generated, according to methods known to the person skilled in the art, the cell lines expressing antibodies encoded by genetic information on either pCRU-VKlHER2-l, pCRU- VK1HER2-2 or pCRU-VKlHER2-3 and a cell line expressing antibodies encoded by all three plasmids. Therefore, PER.C6™ cells are seeded in DMEM plus 10% F3S in tissue culture dishes (10 cm diameter) or T80 flasks with approximately 2.5 x 10^ cells per dish and kept overnight I under their norm.al culture conditions (10% COa- concentration and 37°C) . The next day, transfections are performed in separate dishes at 37°C using Lipofectamine {Invitrogen Life Technologies) according to standard protocols provided!by the manufacturer, with either 1-2 ]ig pCRU-VKlHER2-l, ;i-2 ]iq pCRU-VKlHER2-2, 1-2 pg pCRU- VK1HER2-3 or 1 pg of a mixture of pCRU-VKlHER2-l, pCRU- VK1HER2-2 and pCRU-VKlHER2-3. As a control for transfection efficiency, a few dishes are transfected with a Lac2 control vector, while a few dishes are not transfected and serve as negative controls. After 5 hours cells are washed twice with DMEM and refed with fresh medium without selection. The next day, medium is replaced with fresh medium containing 500 pg/ml G418. Cells are refreshed every 2 or 3 days with medium containing the same concentrations of G418. About 20-22 days after seeding, a large number of colonies are visible and from each transfection at least 300 are picked and grown via 96-well and/or 24-w.ell via 6-well plates to T25 flasks. At this stage, cells are frozen (at least 1, but usuall'y 4 vials per sub-cultured colony) and production levels of recombinant human IgG antibody are determined in the supernatant using an ELISA specific for human IgGi. Also, at this stage G418 is removed from the culture medium and never re-applied again. For a representative number of colonies larger volumes are cultured to purify!the recombinant human IgGi fraction from the conditioned supernatant using Protein A affinity chromatography according to standard procedures. Purified human IgGi from the various clones is analyzed on SDS-PAGE, Iso-electric focusing (lEF), assayed binding to Her-2-Ig fusion proteins by ELISA, and analyzed for binding to Her-2 on the surface of BT474 cells by flow cytometry. Clones obtained from the co-transfection of pCRU-VKlHER2-1, pCRa-VKlHER2-2 knd •pCRU-VKlHER2-3 are screened by PCR on genomic DNA for the presence or absence of each of the three constructs. The identity of the PCR products is further confirmed by DNA sequencing. Next, it is demonstrated that a clonal cell line accounts for the production of each of the three binding specificities. Therefore, a limited number of colonies, which screened positive for the production of each of the three binding specificities (both by PCR at the DNA level as well as in the specified binding assays against Her-2), are subjected to single cell sorting using a fluorescence activated cell sorter (FACS) (Becton S Dickinson FACS VANTAGE SE). Alternatively, colonies are seeded at 0.3 cells/well to guarantee clonal outgrowth. Clonal cell populations, hereafter designated as sub¬clones, are refreshed once a week -with fresh medium. Sub¬clones are grown and transferred rrom 96-wells via 24- and 6-wells plates to T25 flasks. At this stage, sub¬clones are frozen (jat least I, but usually 4 vials per sub-clone) and production levels of recombinant human IgGi antibody are determined in the supernatant using a human i! IgGi specific ELISA'. For a representative numbet of sub¬clones, larger volumes are cultured to purify the '■I recombinant human IgG^ fraction from the conditioned supernatant using Protein A affinity chromatography according to standard procedures. Purified human IgGi from the various sub-clones is subsequently analyzed as described above for human laGi obtained from the parental clones, i.e. by SDS-PAGE, Iso¬electric focusing (lEF) and binding to Her-2. Sub-clones will also be screened by PCR on genomic DNA for the presence or absence of each of the three constructs pCRU-VK1HER2-1, pCRU-VKlHER2-2 and pCRU-VKlHER2-3. The identity of the PCR products is further confirmed by DNA sequencing. Other methods such as Southern blot and/or FISH can also be used to determine whether each of the three constructs are present in the(clonal cell line. Sub-clones that are proven to be transgenic for each of the three constructs are brought into culture for an extensive period to determine whether the presence of the transgenes is stable and whether expression of the antibody mixture remains the same, not only in terms of expression levels, but also for the ratio between the various antibodiesi that are secreted from the cell. Therefore, the sub-clone culture is maintained for at least 25 population doubling times either as an adherent culture or as a suspension culture. At every 4-6 population doublings, a specific production test is performed using the human IgG specific ELISA and larger volumes are cultured to obtain the cell pellet and the supernatant. The cell pellet is used to assess the presence of the three constructs in the genomic DNA, either via PCR, Southern blot and/or FISH. The supernatant is used to purify the recombinant human Igd fraction as described supra. Purified human IgGi obtained at the various population doublings is analyzed as des'cribed, i.e. by SOS-PAGE, Iso-electric focusing (lEF) and binding to Her-2 by ELISA and by flow cytometry using BT474 cells. I Functionality of the antibody mixture of anti-Her-2 antibodies is analyzed in cell-based assays to determine whether the human'IgGi mixture inhibits proliferation and/or induces apoptosis of BT474 cells. In addition the antibody mixtures are analyzed for their potential to induce antibody dependent cellular toxicity and complement dependent cytotoxicity cf BT47'3 cells. IF In each of the experiments described below the functionality of the antibody mixture recognizina Her-2 can be analyzed and compared to each of the individual IgGi antibodies and to an equimolar coir^bination of the three individual monospecific IgGi molecules. ■I To assess the ability of the antibody mixtures to inhibit the proliferation: of BT474 cells, these cells are allowed to adhere overnight in 96-well places [1.5 x 10^/well) and are subsequently incubated with several concentrations (5 - 20 fag/ml) of the antibody mixtures against Ker-2 for 72 hours. The proliferation of the cells is measured by ^H- thymidine incorporation during the last 6 hours of culture. Inhibition of growth is determined by plotting the percentage of ^H-thymidine. incorporation compared with untreated cells (taken as 100% reference value). if To analyze apoptosis induction of BT474 cells^ these cells are allowed to adhere overnight in 48-well plates (2.5 X lOVweli injlml) and are subsequently incubated with several concentrations (5-20 \ig/ml) of the antibody mixtures against Her-2 for 4 hours. Hereafter the cells are harvested by tr^-psinization, are washed twice wirh PES and are incubated at RT for 10 min with 100 Ml FITC-labeli'ed annexin V (Caltag) diluted 1:25 in annexin V binding buffer (Caltag). Prior to the analysis of the samples by flow cytometry (FACSCalibur, Becton Dickinson, San Jos'e, CA) propidium iodide (PI) (Sigma) is added to a final concentration of 5 (ig/ml to distinguish necrotic cells {annexin V'/PI"^) from apoptotic cells (annexin V^/PI", early apoptotic cells; annexin V^/PI*, late apoptotic cells). ■f Antibody Dependent Cellular Cytotoxicity of the antibody mixtures is analyzed using peripheral blood mononuclear cells as effector'cells and BT474 cells as target cells in a standard ^^Cr,release assay as descibed supra (Huls et al, 1999) . Complement dependent cytotoxicity is determined in a similar assay. Instead of the effector cells, now 50 fil human serum is added to the target cells. Subsequently, the assay is performed as described supra. .f Alternatively, ADCC and CDC of the antibody mixtures is ,b determined using a Europium release assay (Patel and Boyd, 1995) or using an LDH release assay (Shields et al, 2001). The functionality'of the antibody mixtures against Her-2 is also tested using in vivo animal models, such as for instance described in Spiridon et al, 2002. Exaittple 5 Expression of different functional huiaan IgGs in the milk of transgenic animals The VH and VH sequences of phages against proteins present on human B-cells,,^ i. e. CD22 clone B28) , CD72 {clone II-2) and HLA-DR (clone 1-2) (Fig. 7) are cloned into expression plasmid pBCl (as provided in the pBCl Mouse Milk Expression System, Invitrogen Life Technologies) to obtain mammary-gland and lactation-specific expression of these human IgG molecules in transgenic animals, according to the manufacturers instructions. These mammary-gland specific expression vectors, encoding the antibody sequences for anti-CD22, anti-CD72 and anti-HLA-DR are introduced into the murine germline according to the manufacturers instructions. Obtained pups are screened for the presence of each of the three constructs by PCR on DMA isolated from the tail. Pups, either male or female, confirmed for being transgenic for each of the three antibodies,.|| are weaned and ir.atured. Female transgenic mice are fertilized at zhe age of 6-8 weeks and milk samples are- obtained at several time points after gestation. Male transgenic r.ice are mated with non-transgenic females and female transgenic offspring (as determined with PCR as described above) is mated and milked as described above for the female transgenic founders. Whenever'needed, female or male transgenic founders are mated i'for another generation to be able to obtain sufficient amounts of transgenic milk for each founder line. Transgenic milk,is analyzed for the presence of human IgG ivith a human IgG specific ELISA, which does not cross-react with mouse IgG or other mouse railk components. Human IgG is purified from transgenic mouse milk using Protein A affinity chromatography according to standard procedures. Purified human IgG is analyzed on SDS-PAGE, Isoelectric focusing and binding on the targets CD22, .CD72 and HLA-DR. Functionality of the antibody mixture is analyzed as described supra. Example 6 Production of an IgA/igG mixture against a predefined targetjin a PER.C6™ derived clone. The VH-VL sequences of the phage UBS-54 directed against II the homotypic adhesion molecule EP-CAM {Huls et al., 1999) was not only cloned into a vector encoding the constant domains of a human IgGl with Kappa light chain (expression vector pUBS3000Neo), but also into an expression vector Encoding the constant domains of a human IgAl with Kappa light chain (expression vector pUBS54-IgA, Fig. 8). Hence, antibodies derived from pUBS3000Neo and pUBS54-IgA do bind to the same epitope on EPCAM. The only differences antibodies derived from pUBS3000Neo and pUBS54-lgA are in the sequences encoding the constant domains of the heavy chain, resulting in either an IgG^ or^'IgA: isotype. The Kappa light chain sequences of these-two vectors are identical. stable PER.C6™ derived cell lines expressing antibodies encoded by genetic information on pUBS3000Neo and pUBS54-IgA are generated by procedures well known to persons skilled in the art:. Therefore, PER.C6^" cells are seeded in DMEM plus 10% FBS in tissue culture dishes (10 cm diameter) or T80 flasks with approximately 2.5 x 10^ cells per dish and kept overnight under their normal culture conditions (10% CO2 concentration and 37°C). The next day, transfections are) performed in separate dishes at 37°C using Lipofectamine (Invitrogen Life Technologies) according to standard protocols provided by the manufacturer, with either 1-2 ]xg pUBSSOOONeo and pUBS54-IgA. As a control for transfection efficiency, a few dishes are transfected with a LacZ control vector, while a few dishes are not transfected and serve as negative controls. After 4-5 hours, pells are washed twice with DMEM and refed with fresh medium without selection. The next day, medi-JiT; is replaced with fresh medium containing 500 pg/ml G413. Cells are r|efreshed every 2 or 3 days with medium containing the same concentrations of G418. About 20-22 days after seeding, a large number of colonies is visible and from each tra'nsfection at least 300 are picked and grown via 96-well and/or 24-well via 6-well plates to T25 flasks. At this stage, cells are frozen (at least 1, but usually 4 vials per sub-cultured colony) and production levels of recombinant human IgG and human IgA antibody are determined in the supernatant using an ELISA specific for hu-man IgGl as: well as an ELISA specific for human IgA. Also, at this stage G418 is removed from the culture medium and never 'bre-applied again. For a representative number of colonies larger volumes are cultured to purify the recombinant human IgGi and human IgA fraction from the conditioned supernatant using for instance (a combination of) Protein L or LA affinity chromatography, cation exchange chromatography, hydrophobic interaction chromatography and gel filtration. Purified human immunoglobulins from the various clones are analyzed on SDS-PAGE, Iso-electric focusing (lEF) and binding to the target EPCAM using cell lines having a high expression of this molecule. The clones will also be screened by PCR on genomic DNA for the presence or absence of pUBS300CNeo and pUBS54-IgA. The identity of the PCR products is further confirmed by DNA sequencing. A limited number of clones, which are screened positive for the production of both EPCAM IgGi and EPCAM IgA, are subjected to single cell sorting using a fluorescence activated cell sorter (FACS) (Becton Dickinson FACS VANTAGE SE) . Alterna'^ively, colonies are seeded at 0.3 'I cells/well to guaranjtee clonal outgrowth. Clonal cell populations, hereafter designated as sub-clones, are refreshed once a week with fresh medium. Sub-clones are grown and transferrecl from 96-wells via 24- and 6-wells plates to T25 flasks; At this stage, sub-clones are frozen (at least 1, but usually 4 vials per sub-clone) and production levels of recombinant human IgGi and IgA antibody are determined in the supernatant using a human IgGi specific ELISA and a human IgA specific ELISA. For a representative number of sub-clones, larger volumes are cultured to purify the recombinant human IgGi and human IgAi fraction from the conditioned supernatant using for instance (a combination of) Protein L or LA affinity chromatography, cation exchange chromatography, hydrophobic interaction chromatography and gel filtration. Purified human immunoglobulins from the various clones are analyzed on SOS-PAGE, Iso-electric focusing (lEF) and binding to the target EPCAM using cell lines having a high expression of this molecule. Sub-clones will also be screened by PCR on genomic DNA for the presence or absence of pUBS3000Neo and pUBS54-IgA. The identity of the PCR products is further confirmed by DNA sequencing. Other methods such as Southern blot and/or FISH may also be used to determine whether both constructs are present in the clonal cell line. Example 7 Production of a human IgGi/lgGa mixture against multiple targets in a clonal PER.C6™ cell line Phage clone UBS-54 and Clone K53 (Fig. 3) were obtained as described in Example 1. The VH and V^ of clone UBS-54 was inserted into an expression vector containing the HAVT20 leader sequence 'and all the coding seauences for the constant domains of a huirian IgGi with a Kappa light chain by a method essentially as described (Boel et al, 2000) . The resulting plasmid was designated a.s pUBSBOOONeo (Fig. 4). It will be clear that expression vectors containing heavy chain constant domains of any desired isotype can be constructed by routine methods of molecular biology, using the sequences of these regions that are all available in the art. The VH and VT, sequences of Phage clone K53|are cloned intc an expression vector containing the HAVT20 leader sequence and all the coding sequences for the constant domains of a heavy chain of a human IgGs with a Kappa light chair, by a method II essentially as desc'ribed (Boel et al, 2000) . This expression vector is designated, as pK53IgG3. These piasmids are transiently expressed either alone or in combination in PER.C6™ cells. In brief, each 80 cm^ flask is transfected by incubation for 4 hours with 140 pi lipofectamine +'|10 ^g Dt^A (either pUBS3000Neo, pK53lgG3 or 10 ug of both) in serum-free DMEM medium at 37°C. After 4 hours', this is replaced with DMEM + 10% FES, and the cells are grown overnight at 37°C. Cells are then washed with F33 and the medium is replaced with Excell 525 medium (JRH Bioscience). The cells are allowed to grow at 37°C for 6 days, after which the cell culture supernatant is harvested. Human IgG specific ELISA analysis, i.e. measuring all IgG sub-types, is done to determine the IgG concentration in transfected and non-transfected PER.C6™ cells. Human IgG from each supernatant is subsequently purified using Protein A affinity chromatography (Hightrap Protein A HP, cat.no. 1-040203) according to standard procedures, following recommendations of'the manufacturer (Amersham Biosciences). After elution, samples are concentrated in a Microcon YM30 concentrator (Amicon) and buffer exchanged to 10 mM sodium phosphate, pH 6.7. Samples are analysed for binding to the targets EPCAM and CD46 using cell lines having a high expression of these molecules such as LS174T cells. Twelve pg of purified IgG, either transiently expressed UBS-54 IgGl, K53 IgG3 or IgG from the cells in which-both antibodies were co-transfected, is subsequently analyzed on Isoelectric focusing gels (Serva Pre-cast lEF gels, pH range 3-10, cat.no. 42866). Samples are loaded on the low pH side and. after focussing stained with colloidal blue. The pi values of the major isoforms for each sample are determined to illustrate whether there has been expression of UBS-54 IgGl, K53 IgG3 or bispecific heterodimers, depending on how the cells were transfe'cted. The identification of heterodimers would indicate that single cells have translated both the IgG3 heavy chain of K53 and the IgGl heavy chain of UBS-54 and assembled these into a full length IgG molecule together with the common light chain. The absence of bispecific heterodimers indicates that it is possible to translate both the IgG3 heavy chain of K53 and the IgGl heavy chain of UBS-54 in single cells, but that these do not,assemble into a full length IgG molecule together with the common light chain, i.e. there is preferential binding of IgGl and IgG3 heavy chains. This could however also be explained by the lack of co-expression of UBS^54 IgGl and K53 IgGs. Therefore, stable clonal cell lines^expressing both pUBS3000Neo and pK53IgG3 are generated by procedures as such well known to persons skilled in the art. PER.06"^" cells are seeded in DMEM plus 10% FBS in tissue culture dishes (10 cm diameter) or T80 flasks with approximately 2.5 x 10^ cells per dish and kept overnight under their normal culture conditions (10% CO2 concentration and 37°C) . The next day, transfections arej performed in separate dishes at 37°C using Lipofectamine (Invitrogen Life Technologies] if according to standard protocols provided by the manufacturer, with either 1-2 ]ig p"JBS3000Neo, pK53IgG3 or both. As a control for transfecticn efficiency, a few dishes are transfected with a LacZ control vector, while a few dishes will be not transfected and serve as •I negative controls'. After 4-5 hours, cells are washed twice with DMEM and refed with fresh medium without selection. The next day, medium is replacedi with fresh -edium containing 500 ug/ml G418. Cells .are refreshed every 2 or 3 days with medium containing the same concentrations of G4ia. About 20-22 days after seeding, a large number of colonies is visible and from each trari'sfection at least 300 are picked and grown via 96-well and/or 24-well via 6-well plates to T25 flasks. At this st'age, cells are frozen {at least 1, but usually 4 vials per sub-cultured colony) and production levels of recombinant human IgG antibody are determined in the supernatant using an ELISA specific for ail sub-types of human IgG. Also, at this stage G418 is removed from the culture medium and never re-applied again. For a representative number of colonies larger volumes are cultured to purify the recombinant human IgG from the conditioned supernatant using Protein A affinity chromatography (Hightrap Protein A HP, cat.no. 1-040203) according to standard procedures, following recommendations of the manufacturer {Amersham Biosciences) . Purified human immunoglobulins from the various clones ar6 analyzed on SDS-PAGE, Iso-electric focusing (lEF) and binding to the targets EPCAM and CD46 using cell lines having a high expression of these molecules such as^.LS174T cells. The clones are also screened by PCR on genomic DNA for the presence or absence of pUBS3000t^eo and pK53lgG3. The identity of the PCR products is further confirmed by DMA sequencing. A limited number of clones, which are screened positive for the production of both EPCAM IgGl and K53 IgG3, are subjected to single cell sorting using a fluorescence activated cell sorter (FACS) [3ecton Dickinson FACS VANTAGE SE). Alternatively, colonies ate seeded at 0.3 cells/well to guarantee clonal outgrowth. Clonal cell populations, hereafter designated as sub-clones, are refreshed once a week with fresh medium. Sub-clones are grown and transferred from 96-wells via 24- and 6-wells plates to T25 flasks. At this stage, sub-clones are frozen (at least 1, but usually 4 vials per sub-clone) and production levels of recombinant human IgG antibody are determined inifthe supernatant using a human IgG specific ELISA. For a representative number of sub¬clones, larger volumes are cultured to purify the recombinant human IgG fraction from the conditioned Supernatant using Protein A affinity chromatography {Hightrap ProteinfA HP, cat.no. 1-040203) according to standard procedures, following recommendations of the i| manufacturer (Amersham Biosciences). Purified human immunoglobulins f|rom the various clones are analyzed on SDS-PAGE, Iso-electric focusing (lEF) and binding to the targets EPCAJM and CD46 using cell lines having a high expression of this molecules, such as for instance LS174T ■I Cells, or transfectants expressing these molecules. Sub-clones are also screened by PCR on genomic DNA for the presence or absence of pUBS3000Neo and pK53lgG3. The identity of the PCR products is further confirmed by DNA Sequencing. Other methods such as Southern blot ancl/or FISH may also be used to determine whether both constructs are present in the clonal cell line. Once the clonal sub-clones are available and confirmed positive for the,, expression of both aBS-54 IgGl and K53 I9G3, the presence of functional K53 and UBS-54 shows that it is possilDle to generate a mixture of functional IgG's with different isotypes with the common iight cnam in a single cell. Analysis of the expression of bispecific antibodies binding both EpCAM and CD46 will reveal to what extent the different heavy chains having a different subtypeIwill pair, which will influence the amount of bispecific antibodies produced. It is expected that no or very low levels of bispecific antibodies will be found in this case. Example 8. Selection of phage carrying single chain Fv fragments specifically recognizing rabies virus glyco protein (RVGP) using RVGP-Ig fusion protein, and expression of mixtures of antibodies against the rabies virus. This example^describes the production of mixtures of antibodies against the rabies virus, as another potential target. As an antigen, the Rabies Virus Glycoprotein (RVGP) is chosen, but other rabies antigens may be chosen or included as well for this purpose. Several monoclonal antibodies recognizing RVGP have already been described in the art, and polyclonal antibodies have been recognized to be useful in treatment of rabies infections as well (e.g. EP0402029; EP0445625) . Antibody fragments are selected using antibody phage display libraries and MAbstract™ technology, essentially as described in US patent 6,265,150 and in WO 98/15833. All procedures are performed at room temperature unless stated otherwise. The sequence of RVGP is available to the person skilleck in the art, for cloning purposes {e.g. Yelverton et al, 1983), An RVGP-Ig fusion protein consisting of whole RVGP fused genetically to the CH2 and CH3 domains of human IgGl is produced using vector PCDNA3.1 Zeo-CH2-CH3 expressed in PER.C6^" and coated for 2 hours at 37° onto the surface of Maxisorp™ plastic tubes (Nunc) at a'concentration of 1.25 ]ig/ml. The tubes are blocked for 1 h in 2% fat free milk powder dissolved in PBS (MPBS). Simultaneously, 500 pi (approximately 10^^ cfu) of a phage display library expressing single chain Fv fragments (scFv's) essentially prepared as described by De Kruif et al {1995a,b) and references therein, is added to two volumes of 4% MPBS. In this experiment, selections are performed using fractions of the original library constructed using only one single variable light chain gene species (e.g. a 'VKl'-library). In addition, human serum is added to a final concentration of 15% and blocking is allowed to proceed for 30-60 min. The RVGP-Ig-coated tubes are emptied and the blocked phage library is added. The tube is sealed and rotated slowly for 1 h, followed by 2 h of incubation without rotation. The tubes are emptied and washed 10 times in PBS containing 0.1% Tween-20, followed by washing 5 times in PBS. 1 ml glycine-HCL, 0.05|M, pH 2.2 is added, and the tube is rotated slowly for 10 min. The eluted phages are added to 500 ^il IM Tris-HCl pH 1.4. To this mixture, 3.5 ml of exponentially growing XL-1 blue bacterial culture is added. The tubes are incubated for 30 min at 37'=C without shaking. Then, the bacteria are plated on 2TY agar plates containing ampicillin, tetracycline and glucose. After overnight incubation of the plates at 37°C, the colonies are scraped from the plates and used to prepare an if enriched phage library, essentially as described by De Kruif et al. (1995a,b). Briefly, scraped bacteria are used to inoculate 2TY medium containing ampicillin. tetracycline and glucose and grown at a temperature of 37°C to an ODgoonm of -0.3. Helper phages are added and allowed to infect the bacteria after which the medium is changed to 2TY containing ampicillin, tetracycline and kanamycin. Incubation is continued overnight at 30°C. The next day, the bacteria are rerr.oved from the 2TY mediuHi by centrifugation after which the phages are precipitated using polyethylene glycol 6000/NaCl. Finally, the phages are dissolved in a small volume of P3S-1% BSA, filter-sterilized and used for a next round of selection. The selection/re-infection procedure is performed twice. After the second round of selection, individual E.coll colonies are used to prepare rr.onoclonal phage antibodies. Essentially, individual colonies are grown to log-phase and infected with helper phages after which phage antibody production is allowed to proceed overnight. Phage antibody containing supernatants are tested in ELISA for binding activity to human RVGP-Ig coated 96 wells plates. Selected phage antibodies that are obtained in the screen described above, are validated in ELISA for specificity. For this purpose, human RVGP-lg is coated to Maxisorp ELISA plates. After coating, the plates are blocked in 2% MPBS. The selected phage antibodies are incubated in an equal volume of 4% MPBS. The plates are emptied, washed once in PBS, after which the blocked phages are added. Incubation is allowed to proceed for 1 h, the plates are washed in PBS 0.1% Tween-20 and bound phages are detected using an anti-M13 antibody conjugated to peroxidase. As a control, the procedure is performed simultaneously using a control phage antibody directed against thyroglobulin (De Kruif et al. 1995a,b}, which serves as a negative control. The phage antibodies that bind to human RVGP-Ia are subsequently tested for binding to human serum IgG to exclude the possibility that they recognized the Fc part of the fusion protein. In another as^ay the phage antibodies are analyzed for their ability to bind PER.C6™ cells that express RVGP To this purpose PER.Ce™ cells are transfected with a plasmid carrying a cDNA sequence encoding RVGP or with the empty vector and stable transfectants are selected using standard techniques known to a person skilled in the art (e.g. Coligan, J.E. etal. (2001) Current protocols in protei'in science, volume I. John Wiley & Sons, Inc. New York). For flow cytometry analysis, phage antibodies are first blocked in an equal volume of 4% MPBS for 15 min at 4°C prior to the staining of the RVGP- and control transfected PER.06"^" cells. The blocked phages are added to a mixture of unlabelled control transfected PER.ce"^" cells and RGVP transfected PER.C6™ cells that have been labelled green using a lipophylic dye {PKH67, Sigma). The binding of the phage antibodies to the cells I! IS visualized using a biotinylated anti-MlS antibody (Santa Cruz Biotechnology) followed by streptavidin- phycoerythrin (Caltag). Anti RVGP scFv selectively stains the ■pER.ce^-"' RVGP transfectant while they do not bind the control transfectant. An alternative way of screening for phages carrying single chain Fv fragments specifically recognizing human RVGP, is by use of RVGP-transfected PER.CS'^'' cells. PER.C6™ cells exp^ressing membrane bound RVGP are produced as described supra. Phage selection experiments are performed as described supra, using these cells as target. A fraction of the phage library comprised of scFv-phage particles using only one single scFv species (500 |il, approximately 10^^ cfu) is blocked with 2 ml RPMI/10%FCS/1%NHS -for 15' at RT. Untransfected PER.C6™ cells (-10*10^ cells) are added to the PER.C6-RVGP cells {-1,0*10^ cells). jThis mixture is added to the blocked light chain restricted phage-library and incubated for 2,5 hr while slowly rotating at 4°C. Subsequently, the cells are washed twice and were resuspended in SOOul RPMI/10%FCS and incubated with a murine anti-RVGP antibody (Becton Dickinson) followed by a phycoerythrin (PE)-conjugated anti-mouse-IgG antibody (Calltag) for 15' on ice. The cells' are washed once and transferred to a 4 ml tube. Cell sorting is performed on a FACSvantage fluorescence-activated cell sorter (Becton Dickinson) and RVGP (PE positive) ceils are sorted. The sorted cells are spun down, the supernatant is saved and the bound phages If are eluted from the cells by resuspending the cells in 500pl 50mM Glycin pH2.2 followed by incubation for 5 min. at room temperature. The mixture is neutralized with 250iil IM Tris-HCl pH 7.4 and added to the rescued supernatant. Collectively these phages are used to prepare an enriched phage library as described above. The selection/re-infection procedure is performed twice. After the second round of selection, monoclonal phage antibodies are prepared and tested for binding to RVGP- PER.C6 cells and untransfected PER.C6™ cells as described supra. "Phages that are positive on RVGP- transfected cells are subsequently tested for binding to the RVGP-igG fusion protein in ELISA as described supra. The selected scFv fragments are cloned in a human IgGl' format, according to methods known in the art (e.g. Boel et al, 2000). To this purpose, the VL fragment shared by the selected scFv is PCR amplified using oligo's that add appropriate restriction sites. A similar procedure is used for the VH genes. Thus modified genes are cloned in expression pCRU-KOl {ECACC deposit 03041601), which results in expression vectors encoding a complete huIgGl heavy chain and a complete hiiman light chain gene having the same specificity as the original phage clone. By this method, three different heavy chains are cloned into separate expression vectors, while only one of the vectors needs to comprise the common light chain sequence. These expression vectors are provisionally designated pCRU-RVGP-1, pCU-RVGP-2, and pCRU-RVGP-3. Alternatively, these three vectors may lack DNA encoding the VL region, which can then be encoded in a fourth, separate expression vector not encoding a heavy chain. It is also possible to have VL sequences present in all three or two of the three vectors comprising the il different VH sequences. Stable PER.06™ derived cell lines are generated, according to methods) known to the person skilled in the art {see e.g. WO 00/63403), the cell lines expressing antibodies encoded by genetic information on either pCRU- RVGP-1, pCRU-RVGP-2 or pCRU-RVGP-3 and a cell line expressing antibodies encoded by all three plasmids. Therefore, PER.C6™ cells are seeded in DMEM plus 10% FBS in tissue culture dishes (10 cm diameter) or T80 flasks with approximately 2.5 x 10^ cells per dish and kept overnight under their normal culture conditions (10% CO2 concentration and 37°C). The next cay, transfections are performed in separate dishes at 37°C using Lipofectamine (Invitrogen Life Technologies) according to standard protocols provided by the manufacturer, with either 1-2 pg pCRU-RVGP-1, 1-2 pg pCRU-RVGP-2, 1-2 pg pCR[J-RVGP-3 or 1 pg of a mixture dt pCRQ-RVGP-l, pCRU-RVGP'2 and pCRU- RVGP-3. As a control for transfection efficiency, a few dishes are transfected with a LacZ control vector, while a few dishes will nbt be transfected and serve as it negative controls. After 4 to 5 hours, cells are washed twice with DMEM and refed with fresh medium without selection. The next day, il medium are replaced: with fresh medium containing 500 pg/ml G418. Cells are refreshed every 2 or 3 days with medium containing the same concentrations of G418. About 20-22 days after seeding, a large number of colonies are visible and from each transfection at least 300 are picked and grown via 96-well and/or 24-well via 6-well plates to T25 flasks. At this stage, cells are frozen (at least 1, but usually 4 vials per sub-cultured colony) and production levels of recombinant human IgG antibody are determined in the supernatant using an ELISA specific for human IgGi (described in WO 00/63403). Also, at this stage G418 is removed from the culture medium and never re¬applied again. For a representative number of colonies larger volumes will be cultured to purify the recombinant human IgGi fraction from the conditioned supernatant using Protein A affinity chromatography according to standard procedures. Purified human IgGi from the various clones is analyzed on SDS-PAGE, Iso-electric focusing (lEF) and binding to the target RVGP using an RVGP PER.C6- j transfectant described above. Colonies obtained from the co-transfection with pCRu-RVGP-1, pCRU-RVGP-2 and pCRU-RVGP-3 are screened by PCR on genomic DNA for the presence or absence of each of the three constructs. The identity of the PCR products is further confirmed by DNA sequencing. A limited number of colonies, which screened positive for the production of each of the three binding specificities (both by PCR at the DNA level as well as in the specified binding assays against RVGP), are subjected to single cell sorting using "a fluorescence activated cell sorter (FACS) {Becton & Dickinson FACS VANTAGE SE). Alternatively, colonies are seeded at 0,3 cells/well to guarantee clonal outgrowth. Clonal cell populations, hereafter designated as sub-clones, are refreshed once a week with fresh medium. Sub-clones are grown and transferred from 96-wells via 24- and 6-wells plates to T25 flasks. At this.stage, sub-clones are frozen (at least 1, but usually 4 vials per sub-clone) and production levels of recombinant human IgGi antibody are determined in the supernatant using a human IgGi specific ELISA. For a representative number of sub-clones, larger volumes are cultured to purify the recombinant human IgGi fraction from the conditioned supernatant using Protein A affinity chromatography according to standard procedures. Purified human IgGi [from the various sub-clones is subsequently analyzed as described above for human IgGi obtained from the parental clones, i.e. by SDS-PAGE, Iso- I electric focusing (lEF) and binding to the target RVGP. Sub-clones are also screened by PCR on genomic DNA for the presence or absence of each of the three constructs pCRU-RVGP-1, PCRU-RVGP--2 and pCRU-RVGP-3. The identity of the PCR products is further confirnied by DNA sequencing. Other methods such as Southern blot and/or FISH can also be used to determine whether each of the three constructs are present in the clonal cell line. Sub-clones that are proven to be transgenic for each of the three constructs are brought into culture for an extensive period to determine whether the presence of the transgenes is stable and whether expression of the antibody mixture remains the same, not only in terms of expression levels, but also for the ratio between the various antibody isoforms that are secreted from the cell. Therefore, the sub-clone culture is maintained for at least 25 population doubling times either as an adherent culture or: as a suspension culture. At every 4-6 population doublings, a specific production test is performed using the.human IgG specific ELISA and larger volumes are cultured to obtain the cell pellet and the supernatant. The cell pellet is used to assess the presence of the three constructs in the genomic DNA, either via PCR, Southern blot and/or FISH. The supernatant is used ,to purify the recombinant human IgGi fraction as described supra. Purified human IgGi obtained at the various population doublings is analyzed as described, i.e. by SDS-PAGE, Iso-electric focusing (lEF) and binding to the target RVGP. The efficacy of the antibody mixtures against rabies is tested in in vitro cell culture assays where the decrease in spread of rabies virus is measured, as well as in in vivo animal models infected by rabies. Such models are known to the person skilled in the art, and are e.g. described in EP0402029. Example 9. Production of a mixture of antibodies with a common light chain and three different heavy chain variable regions in a single cell. A method for producing a mixture of antibodies according to the invention, using expression in a recombinant host cell of a single light chain and three different heavy chains capable of pairing to the single light chain to form functional antibodies, is exemplified herein and is schematically shown in Fig. 6. Human IgG's UBS54 ^nd K53 against the EP-CAM homotypic adhesion molecule {Huls et al,, 1999) and the membrane cofactor protein CD46 (WO 02/18948), respectively, are described in Example 1. Another clone that was identified to bind to cofactof protein CD46 was clone 02-237 (sequence of VH provided in Fig. 12). DNA sequencing of this clone revealed that it contained the same light chain as UBS54 and K53, but a unique heavy chain variable sequence (see alignment in Fig, 3). As a result, the CDR3 of the heavy chain of 02-237 differs at 4 positions from that of K53 (see alignment in Fig. 13). The heavy and light chain variable sequences of phage 02-237 were cloned into the expression plasmic pCRU-KOl,(pCRU-KOl is deposited at the European Collection of Cell Cultures (ECACC) under number 03041601), which contains the heavy and light chain constant domains for an IgGl antibody-The resulting plasmid was designated pgG102-237. Due to the cloning strategy followed, the resulting N-terminus of the light chain' of 02-237 as er.coded by pgG102-237 differed slightly from the N-terrr::.nus of UBS54 and K53 as present by pUBSBOOONeo, pCD46_300C'(Meo) , respectively (Fig, 3). Plasmid pgG102-237 was "ransiently produced in human 293 (T) cells or stably in PH:R.C6 cells. It appeared that purified 02-237 IgG had a much higher affinity for purified CD45 (Fig. 14) than K53 IgG, i.e. the affinity had increased from 9.1 x 10"^ M to 2.2 X 10"^ M for K53 and 02-237, respectively. Also, 02-237 bound much better to CD46 on human coloncarcinoma LS174T cells than K53 (Fig. 15) . Stable PER.CS'^ derived cell lines expressing a combination of the plasmids pUESSODONeo/ pCD46_3000(Neo) and pgG102-237 encoding human IgG 02-237 were generated according to methods known as such to the person skilled in the art {see e.g. WO 00/63403). Therefore, PER.C6™ cells were seeded in DMEM plus 10% FBS in tissue culture dishes (10 cm diameter) with approximately 2.5 x 10^ cells per dish and kept overnight under their normal culture conditions (10% C02, concentration and 37°C}. The next day, transfections were performed in separate dishes at 37°C using Lipofectamine (Invitrogen Life Technologies) according to standard protocols provided by the manufacturer, with 2 \ig of an equimolar mixture of pUBS3000Neo, pCD46_3000(Neo) and pgG102'237. As negative control for selection, a few dishes were not transfected. After 4 to 5 hours, cells were washed twice with DMEM and refed with fresh medium without selection. The next day, medium was replaced with fresh medium containing 500 |ig/ml G418. Cells were refreshed every 2 or 3 days with medium containing the same concentrations of G418. About 20-22 days after seeding, a large number of colonies was visible and about 300 were picked' and grown via 96-well and/or 24-well via 6-well plates to T25 flasks. During sub-culturing, production levels of recombinant human IgG antibody were determined in the supernatant using an ELISA specific for human IgGl (described in WO 00/63403). About 25% of all colonies appeared to be positive in this highly specific assay. The production levels measured at this stage were comparable to the levels when a single IgG is expressed in PER.ce™ cells (expression of a single IgG described in Jones et al., 2003). It is important to ■I stress that these high expression levels were obtained without any methods for amplification of the transgene and that they occur at a low copy number of the transgene. The 30 best producing colonies were frozen down in vials and the 19 highest producing clones were selected for purification of the "IgG (Table 1). They were sub-cultured in T80 flasks and human IgG from each clone was if subsequently purified using Protein A affinity chromatography. Therefore, 15-25 ml of conditioned medium was loaded on a 5 ml Protein A FF Sepharose column (Amersham Biosciences). The column was washed with 4 mM phosphate buffered saline, pH 1.4 (P3S) before elution with 0.1 M citrate pH 3.0. The eluted fraction was subsequently desalted on a Sephadex G25 Fine HiPrep Desalting column (Amersham Biotech) to PBS. The concentration of the purified IgG fraction was determined by absorbance measurement at 280 nm using a coefficient of 1.4 for a 0.1% (w/v) solution (Table 1). The purified IgG samples were analysed on non-reduced and reduced SDS-PAGE and lEF. Non-reduced SDS-PAGE (Fig. 16A) showed that all IgG samples migrated comparable to the control K53 or 02-237, as an asseir>bled, intact IgG molecule of approximately 150 kDa. On reduced SDS-PAGE (Fig. 16B), the IgG samples migrated as heavy and light chains of about 50|and 25 kDa, respectively, comparable to the heavy and light chain of the control K53 or 02-237. On lEF, the purified IgG fractions were first compared tc a mixture of equal amounts of K53, UBS54 and 02-237 (Fig. 17). Clearly, some of the samples contained isoforms with a unique pi profile, when compared to the mixture containing purified K53, UBS54 and 02-231. Some major unique isoforms have a pi in between the pi of K53 and 02-237 on one hand and uBS54 on the other hand. This is also anticipated on the basis of the theoretic pi when calculated with the ProtParam tool provided on the Expasy homepage (http://www.expasy.ch; Appel et al., 1994) . K53, 02-237 and UES54 have a theoretic pi of 8.24, 8.36 and 7.65, respectively, whereas an isoform representing a heterodimer of one UBS54 heavy chain and one K53 heavy chain has a theoretical pi of 8.01. Assembly of such a heterodimer can only occur when a single cell translates both the heavy chain of K53 and the heavy chain of UBS54 and assembles these into a full length IgG molecule together with the common light chain. Hence these results suggest that certain clones at least express two functional antibodies. To confirm the unique identity of some of the isoforms, samples of the most interesting clones were run in parallel with K53, UBS54 and 02-237, either alone or in a mixture (Fig. 18). This furthermore showed that some clones expressed at least two antibodies {241, 282, 361). Moreover, it provided evidence that some clones express all three functional antibodies (280 and 402) . To confirm that the.clones expressed IgG mixtures comprising all three heavy chains, peptide-mapping (Garnick, 1992; Gelpi, 1995) was used to analyse the polyclonal IgG fraction. We previously employed peptide mapping to recover;99% of the protein sequence of K53. Based on the protein sequence provided in Fig. 12, the mass of the theoretical tryptic peptides of K53, UBS54 and 02-237 was calculated (Table II and III). A few unique peptides for each IgG, could be identified, i. e. for instance the CDR3 peptides for K53, 02-237 and UBS54 with a Mw of 2116.05, 2057.99 and 2307.15 Da, respectively. Next,'! a tryptic digest of Polyl-280 was prepared and this was analysed using LC~MS (Fig. 19). Peptides with Mw of 2116, 2057 and 2308 Da, representing the unique CDR3 peptides of K53, 02-237 and UBS54, respectively, were detected. The precise amino acid sequence of these peptides (as listed in Table III) was confirmed by MS-MS analysis (Tables IV, V and VI). We also confirmed the presence of the two unique N-terminal light chain peptides with Mw of 2580 and 2554 Da, respectively. The peptide mapping data unequivocally il showed that a mixture of antibodies comprising a common light chain and three different heavy chains 'was expressed by PER.C6™ clone Polyl-280. Also clones 055, 241 and 402 were screened by peptide mapping. Clones 241 and 402 were confirmed positive for all three heavy chain sequences, whereas clone 055 only showed expression of the heavy chains of JIKSB and 02-237, and not of UBS54. This confirms the lEF screening (Fig. 18), where no UBS54 related band was seen in sample 055. Polyl-280 was analysed by BIACORE for binding to CD46 (Fig. 20). The affinity of polyl-2S0 for CD46 was 2.1 X 10"^ M, which shows that the IgG mixture contains CD46 binding molecules haying the same affinity as 02-237 IgG alone. Taken together, this experiment shows that it is possible to express a mixture of functional IgG molecules comprising three unique heavy chains in a single cell and that next to the homodimers, heterodimers consisting of two binding specificities are ^also formed. Furthermore, the frequency of clbnes expressing three different heavy chains suggests that it will also be possible to obtain clones expressing at least 4, 5 or more heavy chains, using the same procedure. In case it would be difficult to obtain clones expressing higher numbers of heavy chains, a clone expressing at least 3 heavy chains according to the invention can be used to introduce more heavy chains in a separate round of transfection, for instance by using a different selection marker. Next, it was demonstrated that a single cell is able to produce a mixture of more than two functional human I IgG's. Therefore, clones 241, 280 and 402, which were screened positive for the production of each of the three IgG's, both by lEF and MS, were subjected to limiting dilution, i.e. seeded at 0.3 cells/well in 96-wells plate to guarantee clonal-j outgrowth. 't Clonal cell populations, hereafter designated as sub¬clones, were refreshed once a week with fresh medium. Sub-clones were grown and transferred from 96-wells via 24- and 6-wells plates, T25, T80 'and T175 flasks. At the T80 stage, sub-clones were frozen. Production levels of recombinant human IgGi antibody were determined in the supernatant using a human IgGi specific ELISA. For each parental clone, 3 sub-clones were chosen and cultured in a few T175 flasks to obtain sufficient conditioned medium for purification using Protein A affinity chromatography as described above. Purified human IgGi from the sub-clones was subsequently analyzed as described above for human IgGi obtained from the parental clone,i by Iso-electric focusing CIEF). The result is shown in i'Fig. 21. Subclones from clone poly 1-241 each have the same pattern, but differ from the parental clone in that they appear to miss certain bands. Subclones from clone poly 1-280 all appear to differ from each other and from the parental clone. Patterns obtained by lEF for subclones from parental clone poly 1-402 are identical for all three subclones and the parent clone. From these data it can be concluded that clone 402 is stably producing a mixture of antibodies. This demonstrates that it is feasible to produce a mixture of antibodies according to the invention from a single cell clone. The clones have undergone about 25 population doublings (cell divisions) from the transfection procedure up to the,'first analysis (shown in Fig. 18) under selection pressure, and from that point on further have undergone about 30 population doublings during the sub-cloning procedure in the absence of selection pressure, before the material analysed in Fig. 21 was harvested. Therefore, the production of a mixture of antibodies from a clone from a single cell can be stable over at least 30 generations. Purified IgGi from the parental 241, 280 and 402 clones, and subclones, were also analysed for binding reactivity towards the CD4 6- and SpCAM antigens. To this end, cDNA of EpCAM, CD4 6 and control antigen CD38 were cloned into expression vectors jscDNA (Invitrogen). These vectors were transfected into CHO (dhfr-) cells using Fugene (Roche) according to the protocol supplied by the manufacturer. Cells were cultured in Iscove's medium containing 10% ?BS and HT supplement (Gibco). After culturing for 2 days, cells were harvested by trypsinization and suspended in PBS-1%BSA (PBSB) for use in FAC3 analysis. Purified IgGl of the clones producing the mixtures of antibodies and control IgGl samoles of anti-GBSIII, an anti-CD72 antibody :(02-004), as well as antibodies from anti-EpCAM clone UBS54 and anti-CD46 clones K53 and 02-237 were diluted in PBSB to a concentration of 20 ^g IgGl /ml.'20 ^il of each was added to 200.000 transfected cells and incubated on ice for 1 hour. Thereafter, cells were washed once in ice-cold PBSB. Bound IgG was then detected using incubation with goat-anti-human IgG-biotin followed by streptavidin-PE.(After a final washing step, cells were suspended in PBSB containing 1 p.q/w.1 propidium iodide. The samples were analysed on a FACS (FACSvantage, Becton Dickinson). Live cells were gated and Mean Fluorescent Intensities (MFI) were calculated from the FACS plots. The results are represented in Fig. 22. As expected, UBS54 bound selectively to EpCAM transfected cells and 02-237 and', K53 bound selectively to CD46 transfectants, whilei unrelated antibodies did not bind to these transfectants.li ■i The results demonstrate that binding activities towards both EpCAM and CD46 were present in the purified IgGl preps of most clones expressing a mixture of antibodies according to the invention, demonstrating that a mixture of functional antibodies was produced by sub-clones that have undergone more than 30 cell divisions, and that result from .a single :celi. In subclone 280-015 binding patterns towards CD4 6 and EpCAM were similar as in the parent clone poly l'-280, in contrast to the other clones. It should be stated that the quantitative aspect of this assay is not completely clear. Routine screening, e.g. by a functional test, can be used to find a clone with the desired expression profile. Quantitative aspects may also be included in such screens. Such screening allows for the identification !of desired clones, which express the mixture of antibodies with a given functionality in a quantitatively stable manner. il Table I. Overview of the clones used for purification of IgG. Clone Polyl- Screening ELXSA (jig/ml) Purification Cone. in feed (pg/ml) Purified (mg) 209 6.1 98 1.37 233 10.0 53 0.75 234 8.0 51 0.71 241 6.6 91 1.42 250 12.5 117 2.10 280 6.3 36 0.80 282 1 8.5 67 1.48 289 8.2 33 0.54 304 7.2 161 3.91 320 6.3 A3 0.33 322 15.2 168 3.27 340 6.0 109 2.64 361 10.4 71 1.73 379 9.5 78 1.75 402 39.9 135 3.14 022 16.2 83 1.69 040 7.8 67 1.43 048 6.5 43 0.94 055 11 55 1.04 Table II. Tryptic peptides of the variable domains of the light chain of K53/'UBS54 and 02-237. Peptide First Last AA Monoiso- topic Mw (Da) K53/UBS54 Monoiso-topic Mw 02-237 LI 1 24 2580.31*^^ 2554.28'^^ L2 25 59 4039.02 4039.02 L3 60 66 700.35 700.35 L4 67 7'b 1302.61 1302.61 L5 80 82 374.23 374.23 L6 83 107 2810.29*^' 2810.29'^' L7 108 111 487.30 487.30 L8 112 112 174.11 174.11 ^* AA, amino acid ^^ One Cysteine residue alkylated Table III. Tryptic peptides of variable domains of heavy chains of K53, 02-237 and UBS54. K53 02-237 UBS54 A B C D A B C D A B C D HI - 1 12 1267.63 HI 1 12 1267.68 HI 1 12 1267.68 H2 13 19 6B5.41 K2 13 19 685.41 H2 13 19 729.41 H3 20 23 492.24 H3 20 23 492.24 H3 20 23 492.24 H4 24 38 1693.81 H4 24 38 1693.81 H4 24 38 1587.77 H5 39 63 2783.28 H5 39 63 2783.28 H5 39 63 2646.33 H6 64 67 472.23 H6 64 67 472.28, H6 64 67 506.26 H7 68 84 1906.87 H7 68 84 1906.87 H7 68 87 2174. 04 H8 85 87 374.23 H8 85 87 374.23 - - - — H9 88 98 1319.55 H9 88 98 1319.55 H8 88 98 1333.56 HIO 99 102 493.21 ' HIO 99 102 4 75.25 H9 99 119 2307.15 Hll 103 122 2116.05 Hll 103 122 2057.99 - - - - Key: A: peptide i B: first amino acid , C: last amino acid 10 D: monoisotopic Mw (Da) Remarks: 1) for HI, amino acid residue 1 is a pyroglutamic acid 2) peptides H3 and H9 from K53 and 02-237, and peptides H3 and H8 of UBS54 contain one alkylated cysteine residue 15 3) Unique peptides that can be used to confirm the presence of the respective IgG's are indicated in bold italics i Table IV. MS/MS-data of CDR3 peptide (HID- of K53,,. obtained by collision induced dissociation of doubly charged m/z 1059.06. I Underlined m/z-values are main peaks in the MS/MS-spectrum. ^ n.d. is not detected. Table V. MS/MS-data of CDR3 peptide (Hll) of 02-237, obtained by collision induced dissociaticn of doubly charged m/z 1030.02. ^ Underlined m/z-values are main peaks in the MS/MS-spectrum. ^ n.d. is not detected. Table VI. MS/MS-data of' CDR3 peptide (H9) of' UBS-54, obtained" by collision induced dissociation of triply charged m/z 770.09. I ^ Underlined m/z-values are main peaks in the MS/MS-spectrum. ^ n.d. is not detected. References Appel RD, Bairoch A, Hochstrasser DF. 1994, A new generation of information retrieval tools for biologists: the example of the ExPASy WWW server. Trends Biochem. Sci. 19: 258-260. Bendig MM. 1988. The production of foreign proteins in mammalian cells. Genet Eng 7: 91-127, Boel E, Verlaan S, Poppeiier MJ, Westerdaal NA, Van Strijp JA, Logtenb'erg T. 2000. Functional human monoclonal antibodies of all isotypes constructed from phage display library-derived single-chain Fv antibody fragments. J Irtmunol Methods 239:153-156. Brink MF, Bishop MD, Pieoer FR, 2000. Developing efficient strategies for the generation of transgenic cattle which produce biopharmaceuticals in milk. Theriogenology 53: 139-148. Campbell KH, McWhir J, Ritchie WA, Wilmut I. 1996, Sheep cloned by nuclear transfer from a cultured cell line. Nature 380: 64-66. Casellas R, Shih TA, Kleinewietfelt M, Rakoniac J, Nemazee D, Rajewski K, Nussenzweig MC. 2001. Contribution of receptor editing to the antibody repertoire. Science 291: 1541-1544. Cockett MI, Bebbington CR, Yarranton GT. 1990. High level expression of tissue inhibitor of metalloproteinases in Chinese hamster ovary cells using glutamate synthetase gene i amplification. Bio/technology 8: 662-667. De Kruif J, Terstappen L, Boel E and Logtenberg T [1995a) Rapid selection of cell subpopulation-specific human monoclonal antibodies from a synthetic phage antibody library. Proc Natl Acad Sci'USA 92:3938 De Kruif J, Boel E and Logtenberg T {1995b) Selection and application of human single chain Fv antibody fragments from a- semi-synthetic- phage antibody display library with designed CDR3 regions. J Mol Biol 248:97 Dinnyes A, De Sousa P, King T, Wilmut I. 2002. Somatic cell nuclear transfer: recent progress and challenges. Cloning Stem Cells 4: 81-90. I Flavell DJ, Noss A, Pulford KA, Ling N, Flavell SU. 1997. Systemic therapy with 3BIT, a triple combination cocktail of anti-CD19, -CD22, and ~CD38-saporin immunotoxins, is curative of hioman B-cell lymphoma in severe combined immunodeficient mice. Cancer Res 57: 4824-4829. If Fishwild DM, O'Donnell SL, Bengoechea T, Hudson DV, Harding F,-Bernhard SL, Jones D, Kay RM, Higgins KM, Schramm SR, Lonberg N. 1996. High-avidity human IgG kappa monoclonal antibodies from a novel strain of minilocus transgenic mice. Nat Biotechnol.l4: 845-51. Garnick RL. 1992. Peptide mapping for detecting variants in protein products. Develop. Biol. Standard 76: 117-130. Gelpi E. 1995. Biomedical and biochemical applications of liquid chromatography-mass spectrometry. J. Chromatography A 703: 59-80. ■I Ghetie M-A, podar EM, Ilgen A, Gordon BE, Uhr JW, Vitetta, ES. 1997. Homodimerization of tumor-reactive monoclonal antibodies markedly increases their ability to induce growth arrest or apoptosis of tumor cells. Proc. Natl. Acad. Sci. USA 94:. 7509-7514. Giddings G, Allison G, Brooks D, Carter A. 20Q0. Transgenic plants as factories for biopharmaceuticals. Nat. Biotechnol. 18: 1151-1155. Gorman C, Bullock C. 2000. Site-specific gene targeting for gene expression in eukaryotes. Curr Opin Biotechnol 11: 455-460. Hiatt A, Cafferkey R, Bowdish K (1989) Production of antibodies in transgenic plants. Nature. 342: 76-78. Huls, G. A., Heijnen, I. A., Cuomo, M. E., Koningsberger, J. C.r Wiegman, L., Boel, E. , van der Vuurst de Vries, A. R., Loyson, S. A., Helfrich, W. , van Berge Henegouwen, G. P., van Meijer, M., de Kruif, J. & Logtenberg, T. 1999. A recombinant, fully human monoclonal antibody with antitumor activity constructed from phage-displayed antibody fragments. Nat Biotechnol 17: 276f281. Jespers LS, Roberts A, Mahler SM, Winter G, Hoogenboom HR. 1994. Guiding the selection of human antibodies from phage display repertoires to a single epitope of an antigen. Biotechnology (N Y} 12: 899-903. Jones, D., Kroos, N. , Anema, R., Van Montfort, B,, Vooys, A., Van Der Kraats, S., Van Der Helm, E., Smits,' S., Schouten, J., Brouwer, K., Lagerwerf, F., Van Berkel, P., Opstelten, D-J., Logtenberg, T., Bout, A. (2003). High-level expression of recombinant IgG in the human cell line PER.C6™. Biotechnol Prog. 19, 163-168. Kim SJ, Kim Ns, Ryu CJ, Hong HJ, Lee GM. 1998. Characterization of chimeric antibody producing CHO cells in the course of dihydrofolate reductase-mediated gene amplification and their stability in the absence of selective pressure. Biotechnol Bioeng 58: 73-84. Kohler G and Millstein C. 1975. Continuous cultures of fused cells secreting antibody of predefined specificity. Nature 256: 495-497. Koopman G, Reutelingsperger CP, Kuijten GA, Keehnen RM, Pals ST, van Oers MH. 1994. Annexin V for flow cytometric detection of phosphatidylserine expression on B cells undergoing apoptosis. Blood 84: 1415-1420. Larrick JW, Thomas DW. 2001. Producing proteins in transgenic plants and animals, Curr. Opin. Bio-echnol. 12: 411-418. Massengale WT[, McBurney E, Gurtler J. 2002, CD20-negative relapse of cutaneous B-cell lymphoma after anti-CD20 monoclonal antibody therapy. J. Am. Acad. Dermatol. 46: 441- 443. Mendez MJ, Green LL, Corvalan JR, Jia XC, Maynard-Currie CE, Yang XD, Gallo ML, Louie DM, Lee DV, Erickson KL, Luna J, Roy CM, Abderrahim H, Kirschenbaum F, Noguchi M, Smith DH, Fukushima A, Hales JF, Klapholz S, Finer MH, Davis CG, Zsebo KM, Jakobovits A. 1997. Functional rransplant of megabase l( human immunoglobulin loci recapitulates human antibody response in mice. Nat Genet. 15: 146-56- Merchant AM, Zhu Z, Yuan JQ, Goddard A, Adams CW, Presta LG, Carter P. 1998. An efficient route to human bispecific IgG. Nat. Biotech!^ 16: 677-681. Nemazee D. 2000. Receptor editing in B cells. Adv. Immunol. 74: 89-126. Nissim A, Hoogenboom hR, Tomlir.son IM, Flynn G, Midgley C, Lane D, Winter G. 1954. Antibody fragments from a "single ij pot" phage display library as immunclogical reagents. EMBO J. 13: 692-698. Nowakowski A, Wang C, Powers C3, Amersdorfer P, Smith TJ, Montgomery VA, Sheridan R, Blake R, Smith LA, Marks JD. 2002. Potent neutralization of botulinum neurotoxin by recombinant oligoclonal antibody. Proc, Natl. Atac. Sci. USA 99: 11346-11350. Patel AK, Boyd PN. 1995. An irrrroved assay for antibody dependent cellulai cytotoxicity based on time resolved fluorometry. Journal of Immunological Methods 184: 29-38. Peeters K,. De Wilde C, De Jaeger G, Angenon i^r uepic.ter A. 2001. Production of antibodies and antibody fragments in plants. Vaccine 19: 2756-2761. Pollock DP, Kutzko JP, Birck-Wilson E, Williams JL, Echelard Y, Meade HM. 1999, Transgenic milk as a method for the production of recombinant antibodies. J Immunol Methods. 231: 147-157. Radio MC, Mascelli MA, Shan H, Weigert M. 1991. Ig H and L chain contributibns to autoimmune specificities. J. Immunol. 1-36: 176-182. Schnieke AE, Kind AJ, Ritchie WA, Mycock K, Scott AR, Ritchie M, Wilmut I, Colman A, Campbell KH. 1997. Human factor IX transgenic sheep produced by transfer of nuclei from transfected fetal fibroblasts. Science. 278: 2130-2133. Segal DM, Weiner GJ, Weiner LM. 2001. Introduction: bispecific antibodies. J Immunol Methods. 248:1-6. Shields, RL, Namenuk AK, Hong K, Gloria Meng Y^ Rae J, Biggs J, Xie D, Lai J, Stadlen A, Li B, Fox JA and Presta LG. 2001. High resolution mapping of the binding site on human IgGl for FcgRI, FcgRII, FcgRIII and FcRn and design of IgGl variants with improved binding to the FcgR. J Biol Chem 276: 6591-6604. Spiridon CI, iGhetie MA, Uhr J, Marches R, Li JL, Shen GL, Vitetta ES. 2002. Tartgeting multiple her-2 epitopes with monoclonal antibodies results in improved antigrowth activity of a human breast cancer cell line in vitro and in vivo. Clin. Cancer Res. 8: 1720-1730. Van der Vuurst de Vries A, Logtenberg T. 1999. Dissecting the human peripheral B-cell compartment with phage display-derived antibodies. Immunology 98: 55-62- Vaughan TJ, Williams AJ, Pritchard K, Osbourn JK, Pope AR, Earnshaw JC, McCafferty J, Hodits RA, Wilton J, Johnson KS. 1996. Human' antibodies with sub-nanomolar affinities isolated from a large non-immunized phage display library. Nat. Biotech. 14: 309-314. Wilmut I. Clark AJ. 1991- Basic techniques for transgenesis. J Reprod Fertil Suppl 43: 265-275, Wilmut I, Schnieke AE, McWhir J, Kind AJ, Campbell KH. 1997. Viable offspring derived from fetal and adult mammalian I" cells. Nature 385: 810-813. Wilson TJ, Kola I. 2001. The LoxP/CRE system and genome modification. Methods Mol Biol. 158:83-94. Yelverton E, Norton S, Obijeski JF, Goeddel DV. 1983. Rabies virus glycoprotein analogs: biosynthesis in Escherichia coli. Science 219: 614-620. Yoo EM, Coloma MJ, Trinh KR, Nguyen TQ, Vuong LU, Morrison SL, Chintalacharuvu KR. 1999. Structural requirements for polymeric immunoglobulin assembly and association with J chain. J Biol Chem. 274: 33771-33777. WE CLAIM: 1. A method for production of a mixture of antibodies in a single recombinant host, the method comprising the step of expressing in a recombinant host cell a nucleic acid sequence or nucleic acid sequences encoding one common light chain and at least three different heavy chains that are capable of pairing with said one common light chain, such that the produced antibodies comprise common light chains. 2. The method as claimed in claim 1, further comprising the step of: recovering the antibodies from the host cell or the host cell culture. 3. The method as claimed in any one of claims 1-2, wherein at least two antibodies comprising a heavy-light chain dimer in said mixture of antibodies have different specificities and/or affinities. 4. The method as claimed in any one of claims 1-3, wherein said cell is derived from a human embfyonic retina cell that has been immortalized or transformed by adenoviral El sequences. 5. The method as claimed in claim 6, wherein said host cell is derived from a PER.C6 cell. ■I 6. The method as claimed in any one of claims 1-5, wherein said mixture of antibodies comprises antibodies of isotype IgGl, IgG2, IgG3, IgG4, IgAl, IgA2, IgD, IgE or IgM. 7. The method as claimed in any one of claims 1-6, wherein antibodies present in said mixture bind to different epitopes of the same antigen and/or to different antigens present in one antigen comprising mixture. 8. The method as clahned in any one of claims 1-7, further comprising adding a suitable carrier to the mixture of antibodies to form a pharmaceutical composition. 9. A method for making a recombinant host cell for production of a mixture of antibodies, the method comprising the steps of: introducing into said host cell a nucleic acid sequence encoding a common light chain and nucleic acid sequences encoding at least three different heavy chains that are capable of pairing with said common light chain, wherein said nucleic acid sequences are introduced consecutively or concomitantly. 10. The method as claimed in any one of claims 1-9, wherein the nucleic acid sequence or sequences encoding at least one of said light and/or heavy chains have been obtained by a method comprising at least one antibody display selection step.

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