Low binding affinity interleukin-12 beta receptors

Abstract: The present invention is directed to IL-12 receptor proteins comprising a complex of the beta1 receptor protein with the beta2 receptor protein, which complex is capable of binding to human IL-12 with high affinity. When expressed in host cells the nucleic acid gives rise to substantially homogeneous IL-12 receptor proteins. Further, the invention relates to antibodies capable of binding to cells expressing the IL-12 receptor molecules.

Description: This invention relates generally to Interleukin-12 receptors, especially to human Interleukin-12 receptors. Interleukin-12 (IL-12), formerly known as cytotoxic lymphocyte maturation factor or natural killer cell stimulatory factor, is a 75-kDa heterodimeric cytokine composed of disulfide-bonded 40-kDa (p40) and 35-kDa (p35) subunits that has multiple biological activities including stimulation of the proliferation of activated T and NK cells (Gately, M. K., et al., 1991, J. Immunol., 147:874) (Kobayashi, M., et al., 1989, J. Exp. Med., 170:827), enhancement of the lytic activity of NK/LAK cells (Kobayashi, M., et al., 1989, J. Exp. Med., 170:827; Stern, A.S., et al., 1990, Proc. Natl. Acad. Sci. USA, 87:6808), enhancement of cytolytic T-cell responses (Gately, M.K., et al., 1992, Cell. Immunology, 143:127), induction of interferon gamma by resting and activated T- and NK-cells (Kobayashi, M. et al., 1989, J. Exp. Med., 170:827; Chan, S. H., et al., 1991, J. Exp. Med., 173:869), and promotion of T h 1-type helper cell responses (Manetti, R., et al., 1993, J. Exp. Med., 177:1199; Hsieh, C.-S., et al., 1993, Science 260:547). The biological activity of IL-12 is mediated by the binding of the IL-12 molecules to cell surface, or plasma membrane, receptors on activated T- and NK cells; however, the contributions of the individual subunits, p35 and p40, to receptor binding and signal transduction remain unknown. Studies with labeled IL-12 have shown that this binding occurs in a specific and saturable manner. IL-12 delivers a signal to target cells through a receptor that was initially characterised on phytohaemagglutinin (PHA)-activated CD4+ and CD8+ T-cells and on IL-2 activated CD56+ NK-cells (Chizzonite, R., et al., 1992, J. Immunol., 148:3117; Desai, B., et al., 1992, J. Immunol., 148:3125). A survey of over 20 human cell lines belonging to the T-, B-, NK- and myelomonocytic lineages only identified a single CD4+, IL-2 dependent human T-cell line (Kit 225/K6) that constitutively expresses the IL-12 receptor and responds to IL-12 (Desai, B., et al., 1992, J. Immunol., 148:3125; Desai, B., et al., 1993, J. Immunol. 150:207A). Freshly prepared PHA-activated peripheral blood mononuclear cells (PBMC) and the Kit 225/K6 cell line thus represent two convenient cell sources to study the biochemistry of the functional IL-12 receptor; there may be others. Equilibrium binding experiments with 125 I-labeled IL-12 identified three sites with binding affinities for human IL-12 of 5-20 pM, 50-200 pM, and 2-6 nM on IL-12 responsive T-cells (Chizzonite, R., et al., 1994, Cytokine 6(5):A82a). A cDNA encoding a low affinity IL-12 receptor was previously cloned (Chua, A., et al, 1994, J. Immunology 153:128; European Patent Application No. 0,638,644). Based on a previously suggested nomenclature (Stahl and Yancopoulos, 1993, Cell 74:587), the initially isolated human IL-12 receptor chain is called the beta1 chain. Brief description of the drawings: Figure 1: DNA sequence of human IL-12 receptor beta2 cDNA. (start codon = nucleotide 641; stop codon = nucleotide 3226.) (SEQ ID N-Q:1). Figure 2: Amino acid sequence of human IL-12 receptor beta2 protein. (single underlined amino acid residues at the N-terminal sequence = signal peptide; amino acid nos. 623-646 = transmembrane area, marked by double underline; 9 potential N-linked glycosylation sites in the extracellular portion are marked by bold italics and are also underlined; conserved box 1 and 2 motifs in the cytoplasmic domain are shaded [amino acid residues nos. 667-669, 699-704, 786-798]) (SEQ ID NO:2). Figure 3: DNA sequence of human IL-12 receptor beta1 cDNA (start codon = nucleotide 65; stop codon = nucleotide 2050) (SEQ ID NO:3). Figure 4: Amino acid sequence of human IL-12 receptor beta1 protein. (underlined amino acid residues of N-terminal sequence = signal peptide sequence; amino acid residues nos. 541 to 571 = transmembrane area marked by ------ ; 6 potential N-linked glycosylation sites in the extracellular portion marked by -------; conserved box 1 and 2 motifs in the cytoplasmic domain are marked by -------- [amino acid residues nos. 577 to 584 and 618 to 629]) (SEQ ID NO:4). Figure 5A: Scatchard analysis of recombinant human IL-12 binding to transfected COS cells expressing human IL-12 beta1 receptor protein. Figure 5B: Scatchard analysis of recombinant human IL-12 binding to transfected COS cells expressing human IL-12 beta2 receptor protein. Figure 5C: Scatchard analysis of recombinant human IL-12 binding to transfected COS cells coexpressing human IL-12 beta1 receptor protein and human IL-12 beta2 receptor protein. Figure 6: Analysis of proliferation, in the presence of various concentrations of human IL-12, of Ba/F3 cells stably transfected with cDNA for human IL-12 beta1 receptor protein (-- ◆ --), with cDNA for human IL-12 beta2 receptor protein (-- ○ --), or with cDNA for both human IL-12 beta1 receptor protein and human IL-12 beta2 receptor protein (--  --), by measuring incorporation of tritiated thymidine. The present invention relates to a low binding affinity interleukin-12 (IL-12) beta2 receptor protein, or a fragment thereof which (a) has a binding affinity for IL-12 of from about 1 to about 10 nM and (b) when complexed with a IL-12 beta1 receptor protein forms a complex having a binding affinity to IL-12of from about 5 to about 100 pM, wherein the IL-12 beta2 receptor protein is encoded by the nucleic acid sequence SEQ ID NO:1 or a nucleic acid having a sequence that hybridises under stringent conditions to the complementary strain of nucleic acid sequence SEQ ID NO:1; and the IL-12 beta1 receptor protein is encoded by the nucleic acid sequence SEQ ID NO:3 or a nucleic acid having a sequence that hybridises under stringent conditions to the complementary strain of nucleic acid sequence SEQ ID NO:3. In a preferred embodiment the nucleic acid encoding the IL-12 beta2 receptor protein is encoded by a nucleic acid sequence that hybridises under stringent conditions to nucleic acid sequence SEQ ID NO:1 or which shares at least 80%, more preferably at least about 90%, and most preferably at least about 95% sequence homology with the polypeptide having the SEQ ID NO:1. Especially, the invention relates to the human IL-12 beta2 receptor protein having for example the amino acid sequence of SEQ ID NO:2 or allelic forms or variants thereof. In addition, the invention relates to a complex capable of binding to IL-12 with high affinity, comprising interleukin-12 (IL-12) beta2 receptor protein, or a fragment thereof as defined in any of claims 1-4 complexed with IL-12 beta1 receptor protein as defined in claim 1, or a fragment thereof which (a) has a binding affinity for IL-12 of from about 1 to about 10 nM, and (b) when complexed with an IL-12 beta2 receptor protein forms a complex having a binding affinity to IL-12 of from about 5 to about 100 pM. In a preferred embodiment the above complex comprises an IL-12 beta1 receptor protein has an amino acid sequence which is substantially homologous to SEQ ID NO:4 or which is encoded by a nucleic acid which is substantially homologous to SEQ ID NO:3. In a more preferred embodiment the nucleic acid encoding the IL-12 beta1 receptor protein is encoded by a nucleic acid sequence that hybridises under stringent conditions to nucleic acid sequence SEQ ID NO:3 or which shares at least 80%, more preferably at least about 90%, and most preferably at least about 95% sequence homology with the polypeptide having the SEQ ID NO:3. Especially, the invention relates to the human IL-12 beta1 receptor protein having for example the amino acid sequence of SEQ ID NO:4 or allelic forms or variants thereof. The present invention also relates to the above proteins or complexes which are soluble. An aspect of the present invention is a protein or complex encoded by a nucleic acid which comprises two subsequences, wherein one of said subsequences encodes a soluble protein as defined above, and the other of said subsequences encodes all of the domains of the constant region of the heavy chain of human Ig other than the first domain of said constant region. The invention also includes proteins encoded by a first and a second nucleic acid, wherein the first nucleic acid comprises two subsequences, wherein one of said subsequences encodes a soluble fragment of an IL-12 receptor beta2 protein mentioned above and the other of said subsequences encodes all of the domains of the constant region of the heavy chain of human Ig other than the first domain of said constant region, and the second nucleic acid comprises two subsequences wherein one of said subsequences encodes a soluble fragment of a IL-12 receptor beta1 protein and the other of said subsequences encodes all of the domains of the constant region of the heavy chain of human Ig other than the first domain of said constant region. The term "human IL-12 beta2 receptor protein" refers to (1) the protein of SEQ ID NO:2, or (2) any protein or polypeptide having an amino acid sequence which is substantially homologous to the amino acid sequence SEQ ID NO:2 and which has the following properties: 1) The protein or polypeptide has low binding affinity for human IL-12, and 2) The protein or polypeptide, when complexed with human beta1 eceptor protein forms a complex having high binding affinity for human IL-12. The term "human IL-12 beta1 receptor protein" refers to (1) the protein of SEQ ID NO:4, or (2) any protein or polypeptide having an amino acid sequence which is substantially homologous to the amino acid sequence SEQ ID NO:4 and which has the following properties: 1) The protein or polypeptide binds to has low binding affinity for human IL-12, and 2) The protein or polypeptide, when complexed with human beta2 receptor protein forms a complex having high binding affinity for human IL-12. As used herein, the terms "IL-12 beta2 receptor protein" and "IL-12 beta1 receptor protein" includes proteins modified deliberately, as for example, by site directed mutagenesis or accidentally through mutations. The terms also includes variants which may be prepared from the functional groups occurring as side chains on the residues or the N- or C-terminal groups, by means known in the art, and are included in the invention as long as they remain pharmaceutically acceptable, i.e. they do not destroy the activity of the protein and do not confer toxic properties on compositions containing it. These variants may include, for example, polyethylene glycol side-chains which may mask antigenic sites and extend the residence of the proteins in body fluids. Other variants include aliphatic esters of the carboxyl groups, amides of the carboxyl groups by reaction with ammonia or with primary or secondary amines, N-acyl derivatives of free amino groups of the amino acid residues formed with acyl moieties (e.g. alkanoyl or carbocyclic aroyl groups) or O-acyl derivatives of free hydroxyl groups (for example that of seryl- or threonyl residues) formed with acyl moieties. "Substantially homologous", which can refer both to nucleic acid and amino acid sequences, means that a particular subject sequence, for example, a mutant sequence, varies from the reference sequence by one or more substitutions, deletions, or additions, the net effect of which do not result in an adverse functional dissimilarity between the reference and subject sequences. For purposes of the present invention, sequences having greater than 80 %, more preferable greater than 90% homology and still more preferably greater than 95% homology, equivalent biological properties, and equivalent expression characteristics are considered substantially homologous. For purposes of determining homology, truncation of the mature sequence should be disregarded. Sequences having lesser degrees of homology, comparable bioactivity, and equivalent expression characteristics are considered substantial equivalents. Generally, homologous DNA sequences can be identified by cross-hybridisation under high stringency hybridisation conditions. "A fragment of the IL-12 beta2 receptor protein" means any protein or polypeptide having the amino acid sequence of a portion or fragment of a IL-12 beta2 receptor protein, and which (a) has low binding affinity for IL-12, and (2) when complexed with a IL-12 beta1 receptor protein, forms a complex having high binding affinity for IL-12. "A fragment of the IL-12 beta1 receptor protein" means any protein or polypeptide having the amino acid sequence of a portion or fragment of IL-12 beta1 receptor protein, and which when complexed with a IL-12 beta2 receptor protein, forms a complex having high binding affinity for IL-12. A "soluble fragment" refers to a fragment of a IL-12 receptor protein having an amino acid sequence corresponding to all or part of the extracellular region of the protein and which retains the IL-12 binding activity of the intact IL-12 receptor protein. For example, a soluble fragment of a IL-12 beta2 receptor protein is a fragment of a IL-12 beta2 receptor protein having an amino acid sequence corresponding to all or part of the extracellular region of a human IL-12 beta2 receptor protein. In accordance with the invention, a "complex" comprising IL-12 beta2 receptor protein, or a fragment thereof, complexed with IL-12 beta1 receptor protein, or a fragment thereof, may be expressed on the cell surface of the host cell. When expressed on the cell surface of the host cell, the complex has a high binding affinity for IL-12, whereas the IL-12 beta1 receptor protein and the IL-12 beta2 receptor protein alone each have a low binding affinity for IL-12. In accordance with this invention, the IL-12 beta2 receptor protein may be expressed on the surface of a host cell . In accordance with this invention, not only the IL-12 beta2 receptor protein may be obtained, but also fragments of IL-12 beta2 receptor protein which (1) have low binding affinity for IL-12 and (2) which when complexed with a IL-12 beta1 receptor protein forms a complex having high binding affinity. The fragments of IL-12 beta2 receptor protein may be obtained by conventional means, such as (i) proteolytic degradation of the human IL-12 beta2 receptor protein, (ii) chemical synthesis by methods routine in the art, or (iii) standard recombinant methods. For purposes of the present invention, a human IL-12 receptor protein which has a high binding affinity for human IL-12 is a protein which binds to human IL-12 with a binding affinity of from about 5 to about 100 pM. For purposes of the present invention, a human IL-12 receptor protein which has a low binding affinity for human IL-12 is a protein which binds to human IL-12 with a binding affinity of from about 1 to about 10 nM. The binding affinity of a protein for IL-12 can be determined by conventional means, such as described in R. Chizzonite et al., 1992, J. Immunol., 148:3117 and as set forth in Example 5. Fragments of IL-12 beta2 receptor protein can also be measured for binding affinity for IL-12 by conventional means, such as described in R. Chizzonite et al., 1992, J. Immunol., 148:3117 and as set forth in Example 5. The fragments of IL-12 beta2 receptor protein may be measured for binding affinity for IL-12 either alone or complexed with IL-12 beta1 receptor protein, or a fragment of IL-12 beta1 receptor protein which when complexed with a IL-12 beta2 receptor protein forms a complex having high binding affinity. The present invention also relates to nucleic acids, e.g. DNA, cDNA, RNA, mRNA, etc. encoding the above proteins, for example a complex capable of binding to human IL-12 with high affinity, the complex comprising human IL-12 beta2 receptor protein, or a fragment thereof, and human IL-12 beta1 receptor protein, or a fragment thereof. Preferably these nucleic acids encode the human IL-12 beta2 receptor protein such as a nucleic acid having the SEQ ID NO:1 and/or the IL-12 beta1 receptor protein such as a nucleic acid having the SEQ ID NO: 3. The present invention also relates to recombinant vectors comprising an above nucleic acid, to expression vectors, and especially to expression vectors wherein the above nucleic acid is operably linked to control sequences recognised by a host cell. The invention includes eukaryotic and prokaryotic host cells transformed with one or more of the above vectors and especially to host cells wherein the proteins or complexes are expressed on the surface of the host cells and to host cells wherein these cells proliferate in the presence of IL-12. The above host cells may be transformed with a first vector comprising a nucleic acid encoding the IL-12 receptor beta2 protein as defined above and a second vector comprising a nucleic acid encoding the IL-12 receptor beta1 protein as defined above or with a single vector comprising a nucleic acid encoding an IL-12 receptor beta2 protein and a nucleic acid encoding an IL-12 receptor beta1 protein. As used herein, "nucleic acid" refers to a nucleic acid polymer, in the form of a separate fragment or as a component of a larger nucleic acid construct, which has been derived from a nucleic acid isolated at least once in substantially pure form, i.e., free of contaminating endogenous materials and in a quantity or concentration enabling identification, manipulation, and recovery of the sequence and its component nucleotide sequences by standard biochemical methods, for example, using a cloning vector. Such sequences are preferably provided in the form a DNA or a cDNA with an open reading frame uninterrupted by internal nontranslated sequences, or introns, which are typically present in eukaryotic genes. However, it will be evident that genomic DNA containing the relevant sequences could also be used. Sequences of non-translated DNA may be present 5' or 3' from the open reading frame, where the same do not interfere with manipulation or expression of the coding regions. "Expression vector" is a genetic element capable of replication under its own control, such as a plasmid, phage or cosmid, to which another nucleic acid segment may be attached so as to bring about the replication of the attached segment. It comprises a transcriptional unit comprising an assembly of (1) a genetic element or elements having a regulatory role in gene expression, for example, promoters and enhancers, (2) a structural or coding sequence which is transcribed into mRNA and translated into protein, and (3) appropriate transcription initiation and termination sequences. "Clone" is a group of identical DNA molecules derived from one original length of DNA sequence and produced by a bacterium or virus using genetic engineering techniques, often involving plasmids. In addition, the invention refers to a purified, recombinant protein comprising two different polypeptide chains (a heterodimeric protein) which may be prepared by known methods. The two different polypeptide chains are each encoded by a different chimeric polynucleotide which has two nucleic acid subsequences fused in frame. The first nucleic acid subsequence of the first chimeric polynucleotide, located at its 5' end, is an isolated nucleic acid sequence encoding a soluble fragment of a IL-12 beta2 receptor protein. The second nucleic acid subsequence of the first chimeric polynucleotide, located at its 3' end, is an isolated nucleic acid sequence encoding all domains of a human Ig heavy chain (preferably IgG) except the first domain of the constant region. The first nucleic acid subsequence of the second chimeric polynucleotide, located at its 5' end, is an isolated nucleic acid sequence encoding a soluble fragment of IL-12 beta1 receptor protein. The second nucleic acid subsequence of the second chimeric polynucleotide, located at its 3' end, is an isolated nucleic acid sequence encoding all domains of a human Ig heavy chain (preferably IgG) except the first domain of the constant region. The starting materials for the purified, recombinant proteins of the invention may be obtained by methods known in the art. In particular, on the basis of the DNA sequence coding for human IL-12 beta2 receptor protein described in Figure 1 and of the already known nucleic acid sequences for certain receptors, those partial nucleic acid sequences which code for a soluble fragment of IL-12 beta2 receptor protein can be determined and engineered from the DNA sequence coding for human IL-12 beta2 receptor protein described in Figure 1 using known methods, see Sambrook et al., "Molecular Cloning", 2nd ed., Cold Spring Harbor Laboratory Press (1989). Similarly, on the basis of the DNA sequence coding for human IL-12 beta1 receptor protein described in Figure 3 and of the already known DNA sequences for certain receptors, those partial DNA sequences which code for a soluble fragment of human IL-12 beta1 receptor protein can be determined and engineered from the DNA sequence coding for human IL-12 beta1 receptor protein described in Figure 3 using known methods, see Sambrook et al., "Molecular Cloning", 2nd ed., Cold Spring Harbor Laboratory Press (1989). Sources for isolated DNA sequences coding for constant domains of human immunoglobulins are known in the art and disclosed, for example, by Ellison et al., Nucl. Acid Res. 10, 4071-4079 (1982) for IgG 1 or Huck et al., Nucl. Acid Res. 14, 1779-1789 (1986) for IgG 3 . The isolated DNA sequence encoding the soluble fragment of human IL-12 beta2 receptor protein may be fused in frame, by known methods [Sambrook et al., "Molecular Cloning", 2nd ed., Cold Spring Harbor Laboratory Press (1989)], to the isolated DNA sequence encoding all domains of a human Ig heavy chain (preferably IgG) except the first domain of the constant region. The resulting chimeric polynucleotide has located at its 5' end the isolated DNA sequence encoding the soluble fragment of human IL-12 beta2 receptor protein and at its 3' end the isolated DNA sequence encoding all domains of the human Ig heavy chain except the first domain of the constant region. Similarly, the isolated DNA sequence encoding the soluble fragment of human IL-12 beta1 receptor protein may be fused in frame, by known methods [Sambrook et al., "Molecular Cloning", 2nd ed., Cold Spring Harbor Laboratory Press (1989)], to the isolated DNA sequence encoding all domains of a human Ig heavy chain (preferably IgG) except the first domain of the constant region. The resulting chimeric polynucleotide has located at its 5' end the isolated DNA sequence encoding the soluble fragment of human IL-12 beta1 receptor protein and at its 3' end the isolated DNA sequence encoding all domains of a human Ig heavy chain except the first domain of the constant region. The chimeric polynucleotides can then be integrated using known methods [Sambrook et al., "Molecular Cloning", 2nd ed., Cold Spring Harbor Laboratory Press (1989)] into suitable expression vectors for expression in a non-human mammalian cell, such as a CHO cell. In order to make the homodimeric protein of the invention, the chimeric polynucleotide having located at its 5' end the isolated DNA sequence encoding the soluble fragment of human IL-12 beta2 receptor protein is integrated into a suitable expression vector. In order to make the heterodimeric protein of the invention, the chimeric polynucleotide having located at its 5' end the isolated DNA sequence encoding the soluble fragment of human IL-12 beta2 receptor protein and the chimeric polynucleotide having located at its 5' end the isolated DNA sequence encoding the soluble fragment of human IL-12 beta1 receptor protein are integrated into a single suitable expression vector, or two separate suitable expression vectors. Preferably, the chimeric polynucleotide(s) is/are co-transfected together with a selectable marker, for example neomycin, hygromycin, dihydrofolate reductase (dhfr) or hypoxanthin guanine phosphoribosyl transferase (hgpt) using methods which are known in the art. The DNA sequence stably incorporated in the chromosome can subsequently be amplified. A suitable selection marker for this is, for example, dhfr. Mammalian cells, for example, CHO cells, which contain no intact dhfr gene, are thereby incubated with increasing amounts of methotrexate after transfection has been performed. In this manner, cell lines which contain a higher number of the desired DNA sequence than the unamplified cells can be obtained. The baculovirus expression system can also be used for the expression of recombinant proteins in insect cells. Postranslational modifications performed by insect cells are very similar to those occurring in mammalian cells. For the production of a recombinant baculovirus which expresses the desired protein a transfer vector is used. A transfer vector is a plasmid which contains the chimeric polynucleotide(s) under the control of a strong promoter, for example, that of the polyhedron gene, surrounded on both sides by viral sequences. The transfer vector is then transfected into the insect cells together with the DNA sequence of the wild type baculovirus. The recombinant viruses which result in the cells by homologous recombination can then be identified and isolated according to known methods. When using the baculovirus expression system, DNA sequences encoding the immunoglobulin part have to be in the form of cDNA. The expressed recombinant protein may be purified, for example, by known methods. For example, protein G affinity chromatography may be used to purify the homodimeric protein of the invention. Column chromatography, or any other method that enables differentiation between homodimeric proteins and heterodimeric proteins, may be used to purify the heterodimeric protein of the invention. Expression of human IL-12 receptor protein having high binding affinity to human IL-12: The cDNA of cells where the IL-12 receptor is known to be found is incorporated by conventional methods into a bacterial host to establish a cDNA library. PHA-activated PBMC and cells from the Kit 225/K6 cell line are examples of cell sources for the cDNA. RNA from the cells is extracted, characterised, and transcribed into single stranded cDNA by conventional methods. The single stranded cDNA is converted into double stranded cDNA by conventional methods. The double stranded cDNA is incorporated by conventional techniques into an expression vector, such as pEF-BOS. The plasmid DNA from the expression vector is then incorporated into a bacterial host by conventional methods to form a library of recombinants. The cDNA library is screened by conventional expression screening methods, as described by Hara and Mijayima, 1992, EMBO, 11:1875, for cDNA's which when expressed with cDNA's for the human IL-12 beta1 receptor protein, give rise to a high affinity human IL-12 receptor. A small number of clones from the library are grown in pools. DNA is extracted by conventional methods from the pools of clones. The DNA extracted from a pool of clones is then transfected by conventional methods, along with a small amount of DNA from a plasmid containing the cDNA encoding the human IL-12 beta1 receptor protein, into non-human host cells. The non-human host cells are preferably mammalian, such as a COS cell. Labeled recombinant human IL-12 is then added to the non-human host cells previously transfected as described above and the binding signal of the pool is determined. This process is repeated for each pool. The pools showing a positive binding signal for IL-12 may then be subsequently broken down into smaller pools and reassayed in the above manner until a single clone is selected which shows a positive binding signal. The plasmid DNA from the selected clone is sequenced on both strands using conventional methods, such as an ABI automated DNA sequencer in conjunction with a thermostable DNA polymerase and dye-labeled dideoxynucleotides as terminators. Amino acid sequence alignments may be run as described by M. O. Dayhoff et al., Methods Enzymology 91:524 (1983) with the mutation data matrix, a break penalty of 6 and 100 random runs. The DNA from the selected clone is then co-transfected by conventional methods with DNA from a plasmid containing the cDNA encoding the human IL-12 beta1 receptor protein into a non-human host cell, preferably a non-human mammalian cell such as a COS cell or a Ba/F3 cell. Alternatively, by conventional recombinant methods, a plasmid may be engineered which contains transcription units (promoter, cDNA, and polyA regions) for both human IL-12 beta1 receptor protein and human IL-12 beta2 receptor protein. Plasmid DNA is transfected by conventional methods into a non-human host cell, preferably a non-human mammalian cell such as a COS cell or a Ba/F3 cell. In accordance with this invention, DNA may be isolated which encodes human IL-12 beta2 receptor protein, or a fragment thereof, which fragment (1) has low binding affinity for human IL-12 and (2) when complexed with human IL-12 beta1 receptor protein, forms a complex having high binding affinity for human IL-12. An isolated nucleic acid sequence refers to a nucleic acid polymer, in the form of a separate fragment or as a component of a larger nucleic acid construct, which has been derived from nucleic acid isolated at least once in substantially pure form, that is, free of contaminating endogenous materials and in a quantity or concentration enabling identification, manipulation, and recovery of the sequence and its component nucleotide sequences by standard biochemical methods, for example, using a cloning vector. Such sequences, e.g. DNA, are preferably provided in the form of an open reading frame uninterrupted by internal nontranslated sequences, or introns, which are typically present in eukaryotic genes. Genomic DNA containing the relevant sequences could also be used as a source of coding sequences. Sequences of non-translated DNA may be present 5' or 3' from the open reading frame, where the same do not interfere with manipulation or expression of the coding regions. In accordance with this invention, a mammalian cell having the human IL-12 beta2 receptor protein or the complex expressed on its surface and which proliferates in response to human IL-12 is useful for determining IL-12 bioactivity. For example, such cells are useful for determining whether a given compound inhibits biological activity of human IL-12 or is an IL-12 agonist. In addition, through the ability to express the human IL-12 beta2 receptor protein on a non-human mammalian cell surface, we can also express fragments of the human IL-12 beta2 receptor protein, and can determine whether these fragments, when complexed with the beta1 subunit, or an active fragment thereof, have the same properties and high binding affinity for IL-12 as the intact complex. Isolated DNA encoding the human IL-12 beta2 receptor protein may be used to make a purified, recombinant protein which is soluble, and which binds to IL-12 with the same affinity as human IL-12 beta2 receptor protein. The isolated DNA encoding the human IL-12 beta2 receptor protein may also be used to make a purified, recombinant protein which is soluble, and which binds to IL-12 with the same affinity as the recombinant human IL-12 receptor complex of the beta1 receptor protein with the beta2 receptor protein [See, for example, Charnow, S. M. et al., Trends in Biotechnology, Vol. 14, 52-60 (1996)]. Such purified, recombinant proteins, which bind to human IL-12, are useful for preventing or treating pathological conditions caused by excess or inappropriate activity of cells possessing IL-12 receptors, by inhibiting binding of IL-12 to such cells. Pathological conditions caused by excess activity of cells possessing IL-12 receptors include autoimmune dysfunctions, such as without limitation rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis. A purified, recombinant protein which is soluble, and which binds to IL-12 with the same affinity as human IL-12 beta2 receptor protein is the fusion of a soluble fragment of human IL-12 beta2 receptor protein and a human Ig heavy chain (such as IgG, IgM or IgE, preferably IgG) having all domains except the first domain of the constant region. This recombinant protein, which is homodimeric, is encoded by a chimeric polynucleotide which has 2 DNA subsequences fused in frame. The first DNA subsequence, at the 5' end of the chimeric polynucleotide, is an isolated DNA sequence encoding a soluble fragment of human IL-12 beta2 receptor protein. The second DNA subsequence, located at the 3' end of the chimeric polynucleotide, is an isolated DNA sequence encoding all domains of a human heavy chain Ig (preferably IgG) except the first domain of the constant region. The desired recombinant protein can be generated by transfection of the chimeric polynucleotide into a non-human mammalian cell, such as a chinese hamster ovary (CHO) cell. The expressed recombinant protein can be purified, for example, by protein G affinity chromatography. A purified, recombinant protein which is soluble, and which binds to IL-12 with the same affinity as the recombinant human IL-12 receptor complex of the beta1 receptor with the beta2 receptor is encoded by two chimeric polynucleotides which each have two DNA subsequences fused in frame. The first DNA subsequence of the first chimeric polynucleotide, located at the 5' end, is an isolated DNA sequence encoding a soluble fragment of human IL-12 beta2 receptor protein. The second DNA subsequence of the first chimeric polynucleotide, located at the 3' end, is an isolated DNA sequence encoding all domains of a human Ig heavy chain (for example, IgG, IgM, IgE, preferably IgG) except the first domain of the constant region. The first DNA subsequence of the second chimeric polynucleotide, located at the 5' end, is an isolated DNA sequence encoding a soluble fragment of human IL-12 beta1 receptor protein. The second DNA subsequence of the second chimeric polynucleotide, located at the 3' end, is an isolated DNA sequence encoding all domains of a human Ig heavy chain (for example, IgG, IgM, IgE, preferably IgG) except the first domain of the constant region. The desired recombinant protein may be generated by cotransfection of the two chimeric polynucleotides into a non human mammalian cell, such as a CHO cell. The expressed protein can be purified, for example, by any method that enables differentiation of homodimeric proteins from heterodimeric proteins, such as, for example, column chromatography. In addition, the invention also relates to a process for the preparation of a protein mentioned above comprising the expression of an above mentioned nucleic acid in a suitable host cell. Purified, recombinant proteins are useful for preventing or treating pathological conditions caused by excess or inappropriate activity of cells possessing IL-12 receptors by inhibiting binding of IL-12 to such cells. "Purified", as used to define the purity of a recombinant protein encoded by the combined DNA sequences described above, or protein compositions thereof, means that the protein or protein composition is substantially free of other proteins of natural or endogenous origin and contains less than about 1% by mass of protein contaminants residual of production processes. Such compositions, however, can contain other proteins added as stabilizers, carders, excipients or co-therapeutics. A protein is purified if it is detectable, for example, as a single protein band in a polyacrylamide gel by silver staining. Purified recombinant proteins as described above can be administered in clinical treatment of autoimmune dysfunctions, such as without limitation rheumatoid arthritis, inflammatory bowel disease and multiple sclerosis. The purified recombinant proteins described above can be used in combination with other cytokine antagonists such as antibodies to the IL-2 receptor, soluble TNF (tumor necrosis factor) receptor, the IL-1 antagonist, and the like to treat or prevent the above disorders or conditions. In addition, the invention relates to pharmaceutical compositions comprising a protein mentioned above and a pharmaceutically acceptable carrier. The pharmaceutical compositions may comprise a therapeutically effective amount of one or more cytokine antagonists. Further, the invention relates to the use of a protein mentioned above for the preparation of a medicament. These compounds are especially useful for the treatment of autoimmune dysfunction. The dose ranges for the administration of the purified, recombinant proteins described above may be determined by those of ordinary skill in the art without undue experimentation. In general, appropriate dosages are those which are large enough to produce the desired effect, for example, blocking the binding of endogenous IL-12 to its natural receptor. The dosage should not be so large as to cause adverse side effects, such as unwanted cross-reactions, anaphylactic reactions, and the like. Generally, the dosage will vary with the age, condition, sex and extent of disease in the patient, counter indications, if any, immune tolerance and other such variables, to be adjusted by the individual physician. The purified, recombinant proteins described above can be administered parenterally by injection or by gradual perfusion over time. They can be administered intravenously, intraperitoneally, intramuscularly, or subcutaneously. The dose ranges for the administration of the IL-12 receptor proteins and fragments thereof may be determined by those of ordinary skill in the art without undue experimentation. In general, appropriate dosages are those which are large enough to produce the desired effect, for example, blocking the binding of endogenous IL-12 to its natural receptor. The dosage should not be so large as to cause adverse side effects, such as unwanted cross-reactions, anaphylactic reactions, and the like. Generally, the dosage will vary with the age, condition, sex and extent of disease in the patient, counter indications, if any, immune tolerance and other such variables, to be adjusted by the individual physician. The expected dose range is about 1 ng/kg/day to about 10 mg/kg/day. The IL-12 receptor proteins and fragments thereof can be administered parenterally by injection or by gradual perfusion over time. They can be administered intravenously, intraperitoneally, intramuscularly, or subcutaneously. Preparations for parenteral administration include sterile aqueous or non-aqueous solutions, suspensions and emulsions. Examples of non-aqueous solvents are propylene glycol, polyethylene glycol, vegetable oils such as olive oil, and injectable organic esters such as ethyl oleate. Aqueous carriers include water, alcohol/aqueous solutions, emulsions or suspensions, including saline and buffered media. Parenteral vehicles include sodium chloride solution, Ringer's dextrose, dextrose and sodium chloride, lactated Ringer's, or fixed oils. Intravenous vehicles include fluid and nutrient replenishes, electrolyte replinishes, such as those based on Ringer's dextrose, and the like. Preservatives and other additives may also be present, such as, for example, anti-micorbials, anti-oxidants, chelating agents, inert gases and the like. See, generally, Remington's Pharmaceutical Science , 16th Ed., Mack Eds., 1980. Assays for determining whether a given compound blocks IL-12 activity: An aspect of the invention is the use of either the human IL-12 beta2 receptor protein or the complex of this invention as a screening agent for pharmaceuticals. In accordance with this invention, we can determine whether a given compound blocks human IL-12 activity or acts as an agonist of IL-12. A biological activity of human IL-12 is the stimulation of the proliferation of activated T- and NK-cells. Proliferation of activated T-cells causes alloantigen-induced immune responses, such as allograft rejection (such as skin, kidney, and heart transplants) and graft-versus-host reaction in patients who have received bone marrow transplants. This biological activity of human IL-12 is mediated by the binding of the human IL-12 molecules to cell surface receptors on the activated T-cells. A compound that blocks human IL-12 activity would, therefore, inhibit the proliferation of activated T-cells and would be useful to treat or prevent alloantigen induced immune responses. In order to determine if a compound blocks human IL-12 activity, first, a plurality of cells having expressed on their surface either the human IL-12 beta2 receptor protein or a fragment thereof, or the complex of the invention, which cells proliferate in the presence of human IL-12, is provided. The human IL-12 beta2 receptor protein or a fragment thereof binds to human IL-12 with low binding affinity, but when complexed with human beta1 receptor protein forms a complex having high binding affinity for human IL-12. The complex of the invention binds to human IL-12 with high binding affinity and comprises a complex of (1) human IL-12 beta2 receptor protein, or a fragment thereof which when complexed with a human IL-12 beta1 receptor protein forms a complex having high binding affinity to human IL-12, and (2) human IL-12 beta1 receptor protein, or a fragment thereof which when complexed with a human IL-12 beta2 receptor protein forms a complex having high binding affinity to human IL-12. Second, the cells are contacted with human IL-12 and the given compound. Third, it is determined whether the presence of the given compound inhibits proliferation of the cells. In order to determine if a compound is an agonist of human IL-12, first, a plurality of cells having expressed on their surface either the IL-12 beta2 receptor protein or a fragment thereof, or the complex of the invention, and which cells proliferate in the presence of human IL-12, is provided. The human IL-12 beta2 receptor protein or a fragment thereof binds to human IL-12 with low binding affinity, but when complexed with human beta1 receptor protein forms a complex having high binding affinity for human IL-12. The complex of the invention binds to human IL-12 with high binding affinity and comprises a complex of (1) human IL-12 beta2 receptor protein, or a fragment thereof which when complexed with a human IL-12 beta1 receptor protein forms a complex having high binding affinity to human IL-12, and (2) human IL-12 beta1 receptor protein, or a fragment thereof which when complexed with a human IL-12 beta2 receptor protein forms a complex having high binding affinity to human IL-12. Second, the cells are contacted with human IL-12 or the given compound. Third, it is determined whether the presence of the given compound stimulates proliferation of the cells. Examples of cells capable of expressing on their surface the complex, which cells proliferate in the presence of human IL-12 include, without limitation, PHA-activated PBMC, Kit 225/K6 cells, and Ba/F3 cells transfected with cDNA for both human IL-12 beta1 receptor protein and human IL-12 beta2 receptor protein. Examples of cells capable of expressing on their surface the human IL-12 beta2 receptor protein, or a fragment thereof, which cells proliferate in the presence of human IL-12 include, without limitation, Ba/F3 cells transfected with cDNA for human IL-12 beta2 receptor protein. In order to determine whether the presence of the given compound inhibits proliferation of the cells, the following procedure may be carried out. The human IL-12 responsive cells, having expressed on their surface the human IL-12 beta2 receptor protein, or a fragment thereof, or the human IL-12 receptor complex of the invention, are plated into wells of a microtiter plate. Human IL-12 is then added to some wells of the microtiter plate (standard wells) and allowed to react with the cells. The compound to be tested is added either before or simultaneously with human IL-12 to different wells of the microtiter plate (sample wells) and allowed to react with the cells. Any solvent used must be non-toxic to the cell. The proliferation of the cells is then measured by known methods, for example, labeling the cells after contact with human IL-12 and the compound (such as by incorporation of tritiated thymidine into the replicating DNA), measuring the accumulation of cellular metabolites (such as lactic acid), and the like. The proliferation of the cells of the standard wells is compared to proliferation of the cells of the sample wells. If the cells of the sample wells proliferate significantly less than the cells of the standard wells, the compound blocks IL-12 activity. In order to determine whether the presence of the given compound simulates proliferation of the cells, the following procedure may be carried out. The human IL-12 responsive cells having expressed on their surface the human IL-12 beta2 receptor protein, or a fragment thereof, or the human IL-12 receptor complex of the invention are plated into wells of a microtiter plate. Human IL-12 is then added to some wells of the microtiter plate (standard wells) and allowed to react with the cells. The compound to be tested is added to different wells of the microtiter plate (sample wells) and allowed to react with the cells. Any solvent used must be non-toxic to the cell. The proliferation of the cells is then measured by known methods, for example, labeling the cells after contact with the compound (such as by incorporation of tritiated thymidine into the replicating DNA), measuring the accumulation of cellular metabolites (such as lactic acid), and the like. The proliferation of the cells of the standard wells is compared to proliferation of the cells of the sample wells. If the cells of the sample wells proliferate significantly more than cells that were not exposed to human IL-12, the compound is an agonist of human IL-12. Accordingly, the present invention relates to a method for screening of compounds useful for inhibition of IL-12 activity or compounds useful as agonists of IL-12 activity, comprising contacting a compound suspected of inhibiting IL-12 activity or of being an agonist of IL-12 activity, to a protein mentioned above, followed by detection of the biological effect. The following examples are offered by way of illustration, not by limitation. EXAMPLES MATERIALS AND METHODS: 1. Proteins, Plasmids and Strains Recombinant human IL-12 (U. Gubler et al., 1991, Proc. Natl. Acad. Sci. USA., 88:4143) was obtained as described therein. Recombinant human IL-2 (H.W. Lahm et al., 1985, J. Chromatog, 326:357) was obtained as described therein. The plasmid pEF-BOS is based on a pUC 119 backbone and contains the elongation factor 1 alpha promoter to drive expression of genes inserted at the BstXI site (S. Mizushima and S. Nagata, Nucl. Acids Res., 1990, 18:5322). The human IL-12 receptor beta1 cDNA in the plasmid pEF-BOS was obtained as described in A. Chua et al., 1994, J. Immunology 153:128 and in European Patent Application Publication No. 0638644. Electrocompetent E . coli DH-10B (S. Grant et al., 1990, Proc. Natl. Acad. Sci USA 87:4645) was obtained from Bethesda Research Laboratory (Bethesda, Maryland). 2. Labeling of Human IL-12 with 125 I Recombinant human IL-12 was labeled with 125 I as follows. Iodogen was dissolved in chloroform. 0.05 mg aliquots of Iodogen were dried in 12 x 150 mm borosilicate glass tubes. For radiolabeling, 1.0 mCi Na[ 125 I] was added to the Iodogen-coated borosilicate glass tube, which also contained 0.05 ml of Tris-iodination buffer (25 mM Tris-HCL pH 7.5, 0.4 M NaCl and 1 mM EDTA) to form a 125 I solution. The 125 I solution was activated by incubating for 6 minutes at room temperature. The activated 125 I solution was transferred to a tube containing 0.05 to 0.1 ml recombinant human IL-12 (31.5 mg) in Tris-iodination buffer. The resulting mixture of the activated 125 I solution and the recombinant human IL-12 was incubated for 6 minutes at room temperature. At the end of the incubation, 0.05 ml of Iodogen stop buffer (10 mg/ml tyrosine, 10% glycerol in Dulbecco's phosphate buffered saline (PBS), pH 7.40) was added and reacted for 3 minutes. The resulting mixture was then diluted with 1.0 ml Tris-iodination buffer containing 0.25% bovine serum albumin (BSA), and applied to a Bio-Gel P10DG desalting column for chromatography. The column was eluted with Tris-iodination buffer containing 0.25% BSA. 1 ml fractions containing the eluted peak amounts of labeled recombinant human IL-12 were combined. The combined fractions were diluted to 1x10 8 cpm/ml with 1% BSA in Tris-iodination buffer. Incorporation of 125 I into recombinant human IL-12 was monitored by precipitation with trichloroacetic acid (TCA). The TCA precipitable radioactivity (10% TCA final concentration) was typically in excess of 95% of the total radioactivity. The radiospecific activity of the labeled recombinant human IL-12 was typically 1000 to 2000 cpm/fmole. Example 1 Preparation of Human PHA-activated Lymphoblasts Human peripheral blood mononuclear cells (PBMC) were isolated from blood collected from healthy donors as described in Gately et al., J. Natl. Cancer Inst. 69 , 1245 (1982). The blood was collected into heparinized syringes, diluted with an equal volume of Hank's balanced salt solution and layered over lymphocyte separation medium (LSM® obtained from Organon Teknika Corporation, Durham, North Carolina) in tubes. The tubes were spun at 2000 rpm for 20 minutes at room temperature. PBMC at the interface of the aqueous blood solution and the lymphocyte separation medium were collected. Collected PBMC were pelleted at 1500 rpm for 10 minutes through a 15 ml cushion of 20% sucrose in Hank's balanced salt solution. Pelleted PBMC were resuspended in tissue culture medium (1:1 mixture of RPMI 1640 and Dulbecco's modified Eagle's medium, supplemented with 0.1 mM nonessential amino acids, 60 mg/ml arginine HCl, 10 mM Hepes buffer, 2 mM L-glutamine, 100 U/ml penicillin, 100 mg/ml streptomycin, 0.05 mM 2-mercaptoethanol, and 1 mg/ml dextrose) (TCM) plus 5% human serum and washed twice in TCM. The PBMC were then activated to form lymphoblasts. In particular, 0.5 - 1x10 6 cells/ml in TCM plus 5% human serum plus 0.1% (v/v) PHA-P (Difco, Detroit, MI) were cultured for 3 days at 37°C in a 5% CO 2 atmosphere. After three days, cell cultures were split 1:1 by volume in TCM plus 5% human serum and 50 U/ml recombinant human IL-2 to yield >95% T-cells. These cells were utilized for preparation of a cDNA library. Example 2 Extraction and Characterization of RNA PBMC isolated as in Example 1, activated with PHA for 2-3 days, were harvested and total RNA was extracted using Guanidine Isothiocyanate/Phenol as described by P. Chomczynski and N. Sacchi, Anal. Biochem., 162:156, 1987. PolyA + RNA was isolated from the total RNA by one batch adsorption to oligo dT latex beads as described (K. Kuribayashi et al., Nucl. Acids Res. Symposium Series 19:61, 1988). The mass yield of this purification was about 4% of polyA+ RNA. Example 3 cDNA Library From the above polyA + RNA, a cDNA library was established in the mammalian expression vector pEF-BOS as follows. 3 mg of polyA + RNA were reverse transcribed into single stranded cDNAs using RNaseH minus reverse transcriptase in the presence of a- 32 P-dCTP. The resulting single stranded cDNAs were converted into blunt ended double stranded cDNAs as described by U. Gubler and A. Chua, Essential Molecular Biology Volume II, T.A. Brown, editor, pp. 39-56, IRL Press 1991. BstXI linkers (A. Aruffo and B. Seed, Proc. Natl. Acad. Sci (USA) 84, 8573, 1987) were ligated to the resulting double stranded cDNAs. cDNA molecules having a size of greater than 800 base pairs (bp) were selected by size exclusion chromatography as follows. A Sephacryl SF 500 column (0.8 x 29 cm) was packed by gravity in 10 mM Tris-HCl pH 7.8 - 1 mM EDTA - 100 mM NaAcetate. The radioactive cDNA with added BstXI linkers was applied to the column and 0.5 ml fractions were collected. The size distribution of radioactive cDNA was determined by performing electrophoresis on a small aliquot of each fraction on a 1% agarose gel, drying the gel, and visualizing the size by exposure of the gel to X-ray film. cDNA molecules larger than 800 bp were size selected in this fashion. The selected cDNA molecules were pooled and concentrated by ethanol precipitation. The pooled and concentrated selected cDNA molecules were subsequently ligated to the plasmid pEF-BOS as follows. The plasmid had been restricted with BstXI and purified over two consecutive 1% agarose gels. 300 ng of the restricted and purified plasmid DNA were ligated to 30 ng of size selected cDNA in 60 ml of ligation buffer (50 mM Tris-HCl pH 7.8 - 10 mM MgCl 2 - 10 mM DTT - 1 mM rATP - 25 mg/ml BSA) at 15°C overnight. The following day, the plasmid ligated with the size selected cDNA was extracted with phenol. 6 mg of mussel glycogen were added to the resulting extract, and the nucleic acids were precipitated by ethanol. The resulting precipitate was dissolved in water and the nucleic acids again were precipitated by ethanol, followed by a wash with 80% ethanol. A pellet was formed from the precipitated and washed nucleic acids. The pellet was dissolved in 6 ml of water. 1 ml aliquots of the dissolved pellet were subsequently electroporated into E . Coli strain DH-10B. Upon electroporation of 5 parallel aliquots, a library of about 10 million recombinants was generated. Example 4 Expression Screening for cDNAs Encoding High Affinity IL-12 Receptors The library was screened according to the general expression screening method described by Hara and Miyajima, 1992, EMBO, 11:1875. Pools of about 100 E.coli clones from the above library were grown and the plasmid DNA was extracted from the pools by conventional methods. 2 x 10 5 COS cells were plated per 35 mm culture well. COS cells were transfected with a transfection cocktail using the standard DEAE dextran technique described in "Molecular Cloning, a Laboratory Manual", 2nd Ed., J. Sambrook et al., Cold Spring Harbor Laboratory Press, 1989 ("Molecular Cloning"). The transfection cocktail contained (1) 1 mg of plasmid DNA extracted from the E . Coli clone pools derived from the above library, and (2) 0.1 mg of pEF-BOS plasmid DNA containing the human IL-12 receptor beta1 cDNA. 3 days after transfection, the wells of COS cells were incubated with 10 pM labeled human recombinant IL-12 (specific activity = 1000-2000 cpm/fmole) for 90 minutes at room temperature. The labeled human recombinant IL-12 was removed, and the COS cell monolayer was washed for one hour three times with binding buffer (RPMI 1640, 5% fetal bovine serum (FBS), 25 mM HEPES pH 7) to further select for COS cells expressing high affinity IL-12 receptors only (the binding of the IL-12 ligand to the low affinity sites was further reduced because the low affinity sites have a higher dissociation rate). Subsequently, the cell monolayers were lysed and counted in a gamma counter. After screening 440 pools (representing about 44,000 clones), one pool consistently showed a positive binding signal (300 cpm over 100 cpm background). From this pool, a single clone was subsequently isolated by sib-selection. This single clone (B5-10) contained a cDNA insert of about 3 kb that was completely sequenced. The cDNA insert of clone B5-10 was incomplete with regard to the protein coding region because it did not contain an in-frame stop codon. The cDNA library of Example 3 was rescreened by conventional DNA hybridization techniques with the cDNA insert from clone B5-10, as described in Molecular Cloning and by Grunstein and Hogness, 1975, Proc. Nat. Acad. Sci. USA., 72:3961. Additional clones were thus isolated and then partially sequenced. The nucleotide sequence of one clone (No. 3) was found to (i) overlap with the 3' end of the nucleotide sequence of clone B5-10, (ii) extend beyond the nucleotide sequence of clone B5-10, and (iii) contain an in-frame stop codon. This composite DNA sequence is shown in Figure 1 (SEQ ID NO:1). The deduced amino acid sequence for the encoded receptor protein is shown in Figure 2. Based on the previously suggested nomenclature of Stahl and Yancopolous, 1993, Cell 74:587, we call this newly isolated human IL-12 receptor chain the beta2 chain. Example 5 Binding Assays COS cells (4-5x10 7 ) were transfected by electroporation using a BioRad Gene Pulser (250 mF, 250 volts) with either (1) 25 mg of the B5-10 plasmid DNA expressing recombinant human IL-12 beta2 receptor protein, (2) 25 mg of the pEF-BOS plasmid DNA expressing recombinant human IL-12 beta1 receptor protein, or (3) a mixture of 12.5 mg of the B5-10 plasmid DNA expressing recombinant human IL-12 beta2 receptor protein and 12.5 mg of the pEF-BOS plasmid DNA expressing recombinant human IL-12 beta1 receptor protein. The electroporated cells were plated in a 600 cm 2 culture plate, harvested after 72 hours by scraping, washed and resuspended in binding buffer. The cells were assayed to determine affinities of the expressed IL-12 receptors for human IL-12. In particular, equilibrium binding of labeled recombinant human IL-12 to the cells was performed and analyzed as described by R. Chizzonite, et al., 1992, J. Immunol., 148:3117. Electroporated cells (8x10 4 ) were incubated with increasing concentrations of 125 I-labeled recombinant human IL-12 at room temperature for 2 hours. Incubations were carried out in duplicate or triplicate. Cell bound radioactivity was separated from free labeled 125 I-IL-12 by centrifugation of the mixture of electroporated cells and 125 I-labeled recombinant human IL-12 through 0.1 ml of an oil mixture (1:2 mixture of Thomas Silicone Fluid 6428-R15 {A.H. Thomas} and Silicone Oil AR 200 {Gallard-Schlessinger}) at 4°C for 90 seconds at 10,000 x g to form a cell pellet in a tube. The cell pellet was excised from the tip of the tube in which it was formed, and cell bound radioactivity was determined in a gamma counter. Receptor binding data were analyzed and the affinities were calculated according to Scatchard using the method described by McPherson, J., 1985, Pharmacol. Methods, 14:213. Example 6 Production of IL-12 Responsive Cell Line Wild-type Ba/F3 cells, an IL-3-dependent mouse pro-B cell (Palacios, R. et al., 1985, Cell 41:727) and Ba/F3 cells expressing human IL-12 beta1 receptor protein (Chua, A., et al., 1994, J. Immunology 153:128) were cotransfected with (1) 80 mg of pEF-BOS plasmid DNA expressing recombinant human IL-12 beta2 receptor protein and (2) 8 mg of a plasmid expressing a hygromycin resistance gene (Giordano, T.J., et al., 1990, Gene 88:285) by electroporation using a BioRad Gene Pulser (960 mF, 400 volts). All cells were resuspended at a density of 2 x 10 5 viable cells/ml in a growth medium of RPMI 1640, 10% FBS, glutamine (2mM), penicillin G (100 U/ml), streptomycin (100 mg/ml), and 10% conditioned medium from the WEHI-3 cell line (ATCC No. TIB 68, American Type Culture Collection, Rockville, Maryland). The WEHI-3 cell line is a source of IL-3. The resuspended cells were then incubated at 37°C under 5% CO 2 for 120 hours. Cells were selected by their ability to grow in (1) the above growth medium in the presence of 1 mg/ml hygromycin or (2) an IL-12 containing growth medium of RPMI 1640, 10% FBS, glutamine (2mM), penicillin G (100 U/ml), streptomycin (100 mg/ml), and various concentrations (10, 50 or 250 ng/ml) of human IL-12. Ba/F3 cells expressing human IL-12 beta1 receptor protein transfected with pEF-BOS plasmid DNA expressing recombinant human IL-12 beta2 receptor protein grew in the IL-12 containing growth medium, demonstrating that coexpression of human IL-12 beta1 receptor protein and human IL-12 beta2 receptor protein conferred human IL-12 responsiveness to the Ba/F3 cells. Additionally, Ba/F3 cells expressing human IL-12 beta2 receptor protein grow in the IL-12 containing growth medium, demonstrating that expression of human IL-12 beta2 receptor protein conferred human IL-12 responsiveness to the Ba/F3 cells. Example 7 Effect of Human IL-12 on Transfected Ba/F3 Cell Lines Ba/F3 cells (1) expressing human IL-12 beta1 receptor protein, (2) expressing human IL-12 beta2 receptor protein, or (3) coexpressing human IL-12 beta1 receptor protein and human IL-12 beta2 receptor protein were cultured in RPMI-1640 medium supplemented with 10% FBS, 100 U/ml penicillin G, 100 mg/ml streptomycin, and 2 mM L-glutamine at 2 x 10 4 cells/well in Costar 3596 flat-bottom microplates for 24 hours. Various dilutions of human IL-12, as shown in Figure 6, were then added to the microplates and the cells were incubated for 42 hours at 37°C in a humidified atmosphere of 5% CO 2 in air. 50 ml of 3 H-thymidine, 10 mCi/ml in culture medium, was then added to each well. The cultures were further incubated for 6 hours at 37°C. Subsequently, the culture contents were harvested onto glass fiber filters by means of a cell harvester. 3 H-thymidine incorporation was measured by use of a liquid scintillation counter. All samples were assayed in quadruplicate. Example 8 Sequence Analysis of IL-12 Receptor cDNA Clones and Encoded IL-12 Receptor Protein. The IL-12 beta2 receptor protein, composed of 862 amino acids and a calculated molecular weight of 97231, had the following features: N-terminal signal peptide, extracellular domain, transmembrane domain and cytoplasmic tail. The classical hydrophobic N-terminal signal peptide is predicted to be 23 amino acids in length. Signal peptide cleavage occurs mostly after the amino acids Ala, Ser, Gly, Cys, Thr, Gln (von Heijne, G., 1986, Nucl. Acids Research, 14:4683). For the IL-12 receptor, the cleavage could thus take place after Ala23 in the sequence shown in Figure 2, leaving a mature protein of 839 amino acids based on cleavage at Ala23. The extracellular domain of the receptor is predicted to encompass the region from the C-terminus of the signal peptide to amino acid No. 622 in the sequence shown in Figure 2. Hydrophobicity analysis shows the area from amino acid No. 623 to 646 to be hydrophobic, as would be expected for a transmembrane anchor region. Charged transfer stop residues can be found at the N- as well as the C-terminus of this predicted transmembrane area. The extracellular domain of the receptor is thus 599 amino acids long and contains 9 predicted N-linked glycosylation sites. The cytoplasmic portion is 215 amino acids long (amino acid residue nos. 647 to 862). Further analysis of the amino acid sequence shown in Figure 2 shows the human IL-12 beta2 receptor protein is a member of the cytokine receptor superfamily, by virtue of the sequence motifs [Cys132 --- Cys143TW] and [W305SKWS]. Comparing the sequence shown in Figure 2 to all the members of the superfamily by running the ALIGN program shows that the human IL-12 beta2 receptor protein has the highest homology to human gp130. The cytoplasmic region of the IL-12 receptor beta2 chain contains the box 1 and 2 motifs found in other cytokine receptor superfamily members, as well as three tyrosine residues. Phosphorylation of tyrosines is commonly associated with cytokine receptor signalling; the presence of these tyrosine residues underscores the importance of the IL-12 receptor beta2 chain in the formation of a functional IL-12 receptor. The IL-12 receptor beta1 chain does not contain any tyrosine residues in its cytoplasmic tail. Example 9 Analysis of the Binding Assays The results of the binding assays are shown in Figure 5. As shown in Figures 5A and 5B, human IL-12 binds to recombinant IL-12 receptor beta1 or beta2 alone with an apparent affinity of about 2-5 nM. The binding data was described by a single site receptor model, corresponding to the low affinity component of the functional IL-12 receptor found on PHA-activated PBMC (R. Chizzonite et al., 1992, J. Immunol., 148:3117; B. Desai et al., 1992, J. Immunol., 148:3125). In contrast to these results, as shown in Figure 5C, both high and low affinity IL-12 binding sites were generated upon cotransfection of COS cells with IL-12 receptor beta1 and beta2 plasmids. In this case, the binding data were described by a two receptor site model, with affinities of 50 pM and 5 nM. Example 10 Effect of Human IL-12 on Transfected Ba/F3 Cell Lines The results of the proliferation assay for the effect of human IL-12 on Ba/F3 cells (1) expressing human IL-12 beta1 receptor protein, (2) expressing human IL-12 beta2 receptor protein, and (3) coexpressing human IL-12 beta1 receptor protein and human IL-12 beta2 receptor protein are shown in Figure 6. Cells that are transfected with cDNAs for both human IL-12 beta1 receptor protein and human IL-12 beta2 receptor protein respond to stimulation by human IL-12 by proliferating in a dose-dependent manner. Additionally, cells that are transfected with cDNAs for human IL-12 beta2 receptor protein respond to stimulation by human IL-12 by proliferating in a dose-dependent manner. Consequently, isolated cDNA (clone No. B5-10, SEQ.ID. No:1) coding for a type I transmembrane protein represents a second component of the IL-12 receptor (IL-12R beta2) found on normal human T-cells. The beta1 and beta2 chains each alone bind IL-12 only with low affinity (Kd= 2-5 nM). Upon coexpression of beta1 and beta2, two affinity sites are observed, with Kd values of 50 pM and 5 nM. Ba/F3 cells expressing human IL-12 beta2 receptor protein or coexpressing human IL-12 beta1 receptor protein and human IL-12 beta2 receptor protein are responsive to human IL-12. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof.

Claims: A low binding affinity interleukin-12 (IL-12) beta2 receptor protein, or a fragment thereof which (a) has a binding affinity for IL-12 of from about 1 to about 10 nM and (b) when complexed with a IL-12 beta1 receptor protein forms a complex having a binding affinity to IL-12 of from about 5 to about 100 pM, wherein the IL-12 beta2 receptor protein is encoded by the nucleic acid sequence SEQ ID NO:1 or a nucleic acid having a sequence that hybridises under stringent conditions to the complementary strain of nucleic acid sequence SEQ ID NO:1; and the IL-12 beta1 receptor protein is encoded by the nucleic acid sequence SEQ ID NO:3 or a nucleic acid having a sequence that hybridises under stringent conditions to the complementary strain of nucleic acid sequence SEQ ID NO:3. The protein of claim 1 which shares at least 80% sequence homology with the polypeptide which is encoded by the nucleic acid sequence SEQ ID NO:1. The protein of claim 2, wherein the IL-12 beta2 receptor protein has SEQ ID NO:2 or allelic forms or variants thereof. A protein encoded by a nucleic acid which comprises two subsequences, wherein one of said subsequences encodes a fragment of an interleukin-12 (IL-12) beta2receptor protein as defined in any one of the preceding claims which is soluble, and the other of said subsequences encodes all of the domains of the constant region of the heavy chain of Ig other than the first domain of said constant region. A complex capable of binding to IL-12 with high affinity, comprising interleukin-12 (IL-12) beta2 receptor protein, or a fragment thereof as defined in any of claims 1 - 4 complexed with IL-12 beta1 receptor protein as defined in claim 1, or a fragment thereof wich (a) has a binding affinity for IL-12 of from about 1 to about 10 nM, and (b) when complexed with an IL-12 beta2 receptor protein forms a complex having a binding affinity to IL-12 of from about 5 to about 100 pM. The complex of claim 5, wherein the IL-12 beta1 receptor protein is encoded either by the nucleic acid sequence SEQ ID NO:3 or by a nucleic acid having a sequence that hybridises under stringent conditions to the complementary strain of nucleic acid sequence SEQ ID NO:3. The complex of claim 6 wherein the IL- 2 beta1 receptor protein shares at least 80% sequence homology with the polypeptide which is encoded by SEQ ID NO:3. The complex of claim 7, wherein the IL-12 beta1 receptor protein has SEQ ID NO:4 or allelic forms or variants thereof. A protein encoded by a first and a second nucleic acid, wherein the first nucleic acid comprises two subsequences, wherein one of said subsequences encodes a fragment of an interleukin-12 (IL-12)beta2 receptor protein as defined in any one of claims 1 to 3 which is soluble and the other of said subsequences encodes all of the domains of the constant region of the heavy chain of human Ig other than the first domain of said constant region, and the second nucleic acid comprises two subsequences wherein one of said subsequences encodes a fragment of the IL-12 beta1 receptor protein as defined in any of claims 6 to 8 which is soluble and the other of said subsequences encodes all of the domains of the constant region of the heavy chain of human Ig other than the first domain of said constant region. A protein or complex of any one of claims 1 to 9 which is soluble. Nucleic acids which encode a low binding affinity interleukin-12 (IL-12) beta2 receptor protein or a fragment thereof according to claim 1. The nucleic acid of claim 11, wherein the nucleic acid encodes human IL-12 beta2 receptor protein having the SEQ ID NO:1. A vector comprising a nucleic acid of any one of claims 11 to 12. An expression vector comprising a nucleic acid of any one of claims 11 - 12 operably linked to control sequences recognised by a host cell. A host cell transformed with a nucleic acid of any one of claims 11 to 14. The host cell of claim 15 wherein the protein or complex is expressed on its surface. The host cell of claim 17 wherein the host cell proliferates in the presence of IL-12. A host cell transformed with a first vector comprising a nucleic acid encoding the protein as defined in claim 1 and a second vector comprising a nucleic acid encoding the IL-12 beta1 receptor protein as defined in claim 6 or 7 or with a single vector comprising a nucleic acid encoding the protein as defined in claim 1 and a nucleic acid encoding the IL-12 beta1 receptor protein as defined in claim 6 or 7. A process for the preparation of a protein or complex of any of claims 1 to 10 which comprises the expression of a nucleic acid of any one of claims 11 to 12 and optionally the nucleic acid sequence SEQ ID NO:3 or a nucleic acid having a sequence that hybridises under stringent conditions to the complementary strain of nucleic acid sequence SEQ ID NO:3 in a suitable host cell. A pharmaceutical composition comprising a protein or complex as claimed in any one of claims 1 to 10 or as obtained by the process of claim 19 and a pharmaceutically acceptable carrier. The pharmaceutical composition of claim 20 which further comprises a therapeutically effective amount of one or more cytokine antagonists. A protein or complex as claimed in any one of claims 1 to 10 or as obtained by the process of claim 19 for use in medicine. The use of a protein or complex of any one of claims 1 to 10 or as obtained by the process of claim 19 for the preparation of a medicament for the treatment of autoimmune dysfunction. A method for screening compounds useful for inhibition ofIL-12 activity, comprising a) contacting a compound suspected of inhibiting IL-12 activity to a protein or complex of any one of claims 1 to 10 or as obtained by the process of claim 19, and b) detection of the inhibiting effect. A method for screening compounds useful as agonists of IL-12, comprising a) contacting a compound suspected of being an IL-12 agonist to a protein or complex of any one of claims 1 to 10 or as obtained by the process of claim 19, and b) detection of an agonist effect.

Inventors: GUBLER ULRICH ANDREAS, PRESKY DAVID HOWARD, GUBLER, ULRICH ANDREAS, PRESKY, DAVID HOWARD
Applicants: HOFFMANN LA ROCHE, F. HOFFMANN-LA ROCHE AG
Application year: 1996
Main classification: C12N15
IPC codes: A61P37, C12N5, G01N33, C07K16, C12N15, C07K14, A61K38, C07K19, A61K39, C12P21, A61K31, C07H21, C12R1