LIGAND FOR CD7, AND METHODS FOR USE THEREOF

Abstract: The invention relates to CD7 and the discovery of its cognate ligand, the K12 protein, and the identification and cloning of polynucletides that encode the murine homolog of the human K12. Also disclosed are methods of screening candidate molecules to determine potential antagonists and agonists of the interaction between CD7 and K12. The use of the antagonists and agonists, including soluble K12 proteins, as therapeutics to treat diseases modulated by CD7 are also disclosed.

Description: LIGAND FOR CD7, AND METHODS FOR USE THEREOF RELATED APPLICATION This application claims the benefit of U.S. Provisional Application Serial No. 60/136,450, filed May 28, 1999, the disclosure of which is incorporated by reference herein. FIELD OF THE INVENTION This invention relates generally to ligands for CD7 and uses thereof to treat diseases, isolated polynucleotide molecules encoding such ligands, purified ligands, and methods of screening for molecules that affect the interaction between CD7 and its cognate ligand K12. BACKGROUND OF THE INVENTION Cellular change is often triggered by the binding of an extrinsic element, such as a ligand, to the extracellular domain of a receptor protein. This binding can result in cellular changes by activating and controlling intracellular signaling cascades and cell regulatory mechanisms. As such, understanding the binding interaction between the ligand and its receptor protein can be of great interest to the scientific community. A greater understanding of the interaction would enable one skilled in the art to modulate the resulting signaling cascade governed by the ligand/receptor interaction by selecting agents for co-stimulation or inhibition of the binding of the ligand to its receptor. Furthermore, in the instances of bi-directional or reverse signaling, the interaction would not only activate the signaling cascade of the receptor but would also trigger a signaling cascade in a cell bearing the ligand (Wiley et al., J. oflmmun., 3235-39 (1996)). Thus, understanding the interaction between a receptor and ligand can lead to therapeutic treatments involving the inhibition or enhancement of either one or both of the receptor activity or ligand activity. CD7 is a 40 kDa protein found primarily on hematopoietic cells. CD7 is expressed on mature T and natural killer (NK cells), as well as on progenitors of T, B, NK, and myeloid cells (Aruffo, Embo J. 6:3313 (1987); Yoshikawa, Immunogenetics 33:352 (1991); Yoshikawa, Immunogenetics 37: 114 (1993); Yoshikawa, Immunogenetics 41: 159 (1995)) and on intestinal intraepithelial lymphocytes (Spencer, Gut 30:339 (1989); Eiras, Cytometry 34:95 (1998)). In addition, CD7 is thought to be a marker for one of the earliest stages of T cell development (reviewed in Barcena, Leuk Lymphoma 17:1 (1995) and Haynes, Semin Immunol 2:67 (1990)). Although all of the functions of the CD7 protein in the immune system are unknown, signaling through CD7 has been reported using anti-CD7 antibodies (Carrera, J.Immunol 141:1919 (1988); Rabinowich, J.Immunol Vol. 153:3504 (1994)), indicating that the cytoplasmic domain of CD7 must contain some signal tranducing elements or that it complexes with a protein containing such elements. Recombinant soluble CD7 also inhibits antigenic- and alloantigenic-induced T cell proliferation (Leta, Cell Immunol 165: 101 (1995)). Previous efforts to identify a cognate for CD7 have not been successful, although a putative ligand has reportedly been detected in serum (Leta, Cell Immunol 173:15 (1996)). The extracellular domain of CD7, expressed by either mammalian or insect cells, has been shown to interact with some specificity with Con A (Id.). This interaction is at least partly mediated through carbohydrate residues on CD7 since treatment of the extracellular domain of CD7 with glycosidases specifically reduces binding of Con A to CD7. A recent study used model systems to investigate whether CD7 deficient mice are resistant to LPS-induced shock. According to the study, CD7 deficient mice were totally resistant to low-dose LPS-induced shock syndrome and were partially resistant to high- dose LPS-induced shock syndrome. Thus, this data indicates that CD7 is involved in the LPS-induced shock pathway (Sempowski et al., J. Exp. Med. 189, 1011-1016 (1999)). Given the role CD7 plays in signal transduction (including its mediation of the LPS-induced shock pathway), there is a need in the art for the identification and understanding of the interaction of CD7 with its cognate ligand or binding partner. Further, there is a need for the development of assays and therapeutic methods using the interaction between CD7 and its binding ligand. The K12 gene (also known as SECTM1) was originally identified (Slentz-Kesler, Genomics 47:327(1998)) as being directly 5' of the locus encoding the human CD7 gene on human chromosome 17 (Osada, Cytogenet Cell Genet (1988)). The 3' end of the K12 gene is about 5 kb upstream of the start of the human CD7 gene, and both genes are transcribed in the same direction (Slentz-Kesler, 1998). The K12 gene encodes a transmembrane protein with two short regions in the extracellular domain with weak similarity to Ig-like domains. Both membrane-bound (localized to the Golgi apparatus) and secreted forms of the protein were observed (Id.). In humans, the protein is primarily expressed in spleen, prostate, testis, small intestine, and in peripheral blood leukocytes (Id). However, no known function for the K12 protein has yet been identified. SUMMARY OF THE INVENTION The invention is based, in part, on the discovery that the cognate for the K12 protein is CD7. Thus, K12 is the previously unknown ligand or binding partner for CD7. Accordingly, an aspect of the invention is the use of the K12 protein, and polynucleotides encoding the same, in methods of modulating CD7 and K12 signaling events, and in treating diseases. The invention also provides isolated polynucleotide molecules encoding novel K12 proteins, and the recombinant and/or purified K12 polypeptides encoded by such polynucleotides. Another aspect of the invention is methods screening for compounds that affect the K12/CD7 interaction. In one aspect, the method comprises the steps of forming a composition comprising a CD7 protein, a K12 protein, and the test compound; assaying for the level of interaction of the CD7 protein and the K12 protein; and comparing the level obtained in the presence of the test compound to that obtained in the absence of the test compound, such that if the level obtained differs, a compound that affects the interaction of the CD7 protein and the K12 protein is identified. At least one of the CD7 protein and the K12 protein can be labeled with a detectable moiety. Preferred test compounds are small organic molecules, antibodies, and small peptides. One of the CD7 protein or the K12 protein can be soluble, and the other can be bound, although alternative assay formats are possible and well known. The test compound can be added to the composition after addition of the CD7 protein and the K12 protein, before both proteins are added, or after one protein is added and before the other is added. In another aspect, the screening methods of the invention comprise forming a composition comprising the test compound, the K12 protein and cells expressing CD7; determining the level of biological activity of the K12 protein in the composition; and comparing the level of biological activity with that which occurs in the absence of test compound, wherein a difference in the level of biological activity indicates that the test compound affects the biological activity of a K12 protein. Biological activity of K12 can be assayed in any number of ways, for example, by determining the phosphorylation state of intracellular proteins, by determining the activation of NK cells and by determining the production of interferon gamma and or GM-CSF. In a related aspect, the cells express K12 and soluble CD7, or cells expressing CD7, is used. The present invention also provides a screening method for identifying molecules that enhance or inhibit the interaction between CD7 and K12, or that prevent or inhibit dissociation of a complex formed by CD7 and K12. This screening method involves contacting a mixture of cells which express CD7 and cells which express K12 with a candidate molecule, measuring cellular responses, and detecting the ability of the candidate molecule to inhibit or enhance the interaction between CD7 and K12 or inhibit the dissociation of the complex formed by CD7 and K12. Successful inhibition indicates that the candidate molecule is an antagonist. Increased activation of CD7 or K12 indicates that the candidate molecule is an agonist. In another aspect, the invention provides antagonists and agonists of the interaction between CD7 and K12. In yet a further aspect, the invention provides for a therapeutic use of agonists and antagonists of the interaction between CD7 and K12 in the treatment of diseases modulated by CD7 and/or K12. In still a further aspect, the invention provides for a therapeutic use of K12 in the treatment of disease modulated by CD7. In yet still a further aspect, the invention provides for a therapeutic use of CD7 in the treatment of disease modulated by K12. BRIEF DESCRIPTION OF THE SEQUENCES SEQ ID NO: 1 is the cDNA sequence of human CD7. SEQ ID NO:2 is the amino acid sequence of human CD7. SEQ ID NO:3 is the nucleic acid sequence of human K12. SEQ ID NO:4 is the amino acid sequence of human K12. SEQ ID NO:5 is the cDNA sequence of murine CD7. SEQ ID NO: 6 is the amino acid sequence of murine CD7. SEQ ID NO:7 is the nucleic acid sequence of murine K12. SEQ ID NO:8 is the amino acid sequence of murine K12. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1: Sequence comparison of the human K12 protein (SEQ ID NO:4) and its mouse homologue (SEQ ID NO: 8). Protein sequences were compared using the GCG program GAP. The arrow indicates the predicted signal peptide cleavage site in both the human and mouse proteins. Potential N-linked glycosylation sites are underlined, and the core transmembrane regions (predicted by the TRANSMEMBRANE program) as well as conserved cysteine residues are boxed. Figure 2: Effect of K12 poly-His Flag, K12 hu IgGl Fc fusion protein, soluble CD7 poly-His flag, soluble CD4 and a negative control on inhibiting syncytia. The graph demonstrates the effect of K12 poly-His Flag, K12/hu IgGl Fc fusion protein, CD7, CD4 and a negative control in the assay described in Example 10 on syncytia induction by HIV-1 MN at various concentrations. DETAILED DESCRIPTION OF THE INVENTION The invention is based, in part, on the discovery that the ligand for CD7 is K12. In brief, a soluble labeled version of a human K12 extracellular domain fused to the Fc portion of human IgGl was recombinantly produced and found, using flow cytometry experiments, to bind at high levels to both human T and NK cells. This K12-Fc fusion was used to clone the cognate binding protein from a human T cell cDNA expression library. Sequencing of two independent clones revealed that the cDNAs encoded CD7. Subsequent experiments verified that CD7 was indeed the cognate binding partner for K12. The term "CD7" includes the protein and its variants disclosed in U.S. Patent No. 5,506,126, and includes fragments of CD7, soluble forms of CD7 and homologous analogs. Full length CD7 comprises an extracellular domain, a transmembrane domain, and a cytoplasmic domain. "Fragments" of CD7 encompass truncated amino acid sequences of the CD7 protein that retain the biological ability to bind to K12. An example of such a fragment is the extracellular domain of CD7. "Soluble CD7" includes truncated proteins that lack a functional transmembrane domain of the protein but retain the biological activity of binding to K12. The soluble, extracellular domain can be used to inhibit cellular activation. "Homologous analogs" of CD7 include proteins encoded by nucleic acids that are at least about 30% identical to SEQ ID NO: l, yet which still retain the biological activity of binding to K12. Also contemplated by the term are embodiments in which the homologous analog of CD7 is encoded by a nucleic acid molecule that is at least about 40% identical, preferably at least about 50% identical, more preferably at least about 60% identical, yet more preferably at least about 70%, even more preferably at least about 80% identical, still more preferably at least about 90% identical, and most preferably at least about 95% identical to SEQ ID NO:l, wherein the encoded homologous analog of CD7 retains the biological activity of binding to a K12 protein. Further included are proteins which are at least about 35% similar, preferably at least about 55% similar, more preferably at least about 70%, yet more preferably at least about 85% similar, even more preferably at least about 95% similar, and most preferably at least about 99% similar to the CD7 protein polypeptide as described in SEQ ID NO:2, and that maintain a binding affinity to a K12 protein. By "similar" in this context is meant that the variant protein contains either the same amino acid residue at a particular relative position, or a conservative amino acid substitution at that residue position. Conserved amino acid substitutions are made on the basis of similarity in polarity, charge, solubility, hydrophobicity, hydrophilicity, and/or the amphipathic nature of the residues involved. For example, nonpolar (hydrophobic) amino acids which can be substituted for eachother include alanine, leucine, isoleucine, valine, proline, phenylalanine, tryptophan, and mefhionine; polar neutral amino acids include glycine, serine, threonine, cysteine, tyrosine, asparagine, and glutamine; positively charged (basic) amino acids include arginine, lysine, and histidine; and negatively charged (acidic) amino acids include aspartic acid and glutamic acid. Still further included are all substantially homologous analogs and allelic variations. An example of a homologous analog of CD7 is a murine homologue. The human CD7 protein differs from its mouse homologue in that it contains a "spacer" region consisting of 4 tandem repeats of 9 amino acids (XPPXASALP) in the area proximal to the membrane spanning part of the protein (Yoshikawa, Immunogenetics 37:114 (1993)). Targeted disruption of the CD7 gene in mice has been achieved by several groups (Lee, J.lmmunol. 160:5749 (1998)); Bonilla, Int. Immunol. 9: 1875 (1997)). One group found no demonstrable effect on either the function or subsets of lymphocytes, and no effect on NK cell cytotoxicity (Bonilla, 1997). However, the second group did note a transient increase in thymocyte numbers at 3 months and an alteration in antigen specific CTL effector activity (Lee, 1998.) When examined, expression of the human CD7 gene in transgenic mice had no effect on mouse thymopoiesis, even though the gene was expressed in T cells and was induced during T cell activation (Schanberg, J. Immunol. 155:2407(1995)). This result can now be explained by the fact that mouse K12-Fc cannot bind to human CD7 (see Example 5, below). Thus, human CD7 does not substitute for mouse CD7 in a mouse model. In addition, present inventors have discovered that the murine homologue of CD7 binds to the murine homologue of K12 but not its human counterpart (see below). The percent identity and percent similar can be determined by visual inspection and mathematical calculation. Alternatively, the percent identity of two nucleic acid molecules can be determined by comparing their sequences using the GAP computer program, version 6.0 described by Devereux et al. (Nucl. Acids Res. 12:387, 1984) and available from the University of Wisconsin Genetics Computer Group (UWGCG). The preferred default parameters for the GAP program include: (1) a unary comparison matrix (containing a value of 1 for identities and 0 for non-identities), and the weighted comparison matrix of Gribskov and Burgess, Nucl. Acids Res. 74:6745, 1986, as described by Schwartz and Dayhoff, eds., Atlas of Protein Sequence and Structure, National Biomedical Research Foundation, pp. 353-358, 1979; (2) a penalty of 3.0 for each gap and an additional 0.10 penalty for each symbol in each gap; and (3) no penalty for end gaps. Other programs used by one skilled in the art of sequence comparison may also be used such as the BLAST program for both nucleotide and amino acid comparisons. The term "K12" includes the protein as described above and as shown in Figure 1, including fragments of K12 that retain the ability to bind to CD7. The full-length human K12 protein comprises a signal sequence (approximately amino acids 1-28 of SEQ ID NO:4) an extracellular domain sequence (approximately amino acids 29-145 of SEQ ID NO:4), a transmembrane domain sequence (approximately amino acids 146-167 of SEQ ID NO:4), and a cytoplasmic domain sequence (approximately amino acids 168-248 of SEQ ID NO:4). The skilled artisan will recognize that the above-described boundaries of such regions of the polypeptide are approximate. To illustrate, the boundaries of the signal peptide may differ from those described above. As a guideline, the boundaries may differ by about 1 to 4 residues on either side of the above-described boundaries. "Fragments" of K12 encompass truncated amino acids of the K12 protein that retain the biological ability to bind to CD7. An example of such a fragment is the extracellular domain of K12, which binds CD7. "Soluble K12" includes truncated proteins that lack a functional transmembrane domain of the protein but retain the biological activity of binding to CD7. Naturally occurring soluble forms of human K12 are fragments of the extracellular domain of approximately 19-22 Kd. Thus, soluble K12 molecules need not contain the entire above-described extracellular domain of K12, but can be truncated by 1 to 20 amino acids at the N terminus, and/or truncated by 1 to 20 amino acids at the C terminus, as long as they retain binding affinity for CD7. Soluble forms of K12 can be fusion proteins. Fusion proteins are proteins covalently linked to heterologous amino acid sequences (i.e., amino acid sequences that are not naturally linked to the K12 protein). Particularly preferred fusion proteins contain heterologous amino acid sequences that promote aggregation of the extracellular domain of K12 into multimeric (e.g., dimer, trimer, tetramer, etc.) forms of the protein, and/or that facilitate purification and/or detection (e.g., Fc domains, leucine zipper domains and poly-His tags, to name just a few). "Homologous analogs" of K12 include proteins encoded by nucleic acids that are at least about 30% identical to SEQ ID NO:3, yet which still retain the biological ability to bind to a CD7. Also contemplated by the term are embodiments in which the homologous analog of K12 is encoded by a nucleic acid molecule that is at least about 40% identical, preferably at least about 50% identical, more preferably at least about 60% identical, yet more preferably at least about 70%, even more preferably at least about 80% identical, still more preferably at least about 90% identical, and most preferably at least about 95% identical to SEQ ID NO:3, wherein the encoded homologous analog of K12 still retains the biological ability of binding to a CD7. Further included are proteins which are at least about 35% similar, preferably at least about 55% similar, more preferably at least about 70%, yet more preferably at least about 85% similar, even more preferably at least about 95% similar, and most preferably at least about 99% similar to the CD7 protein polypeptide as described in SEQ ID NO:4, and that maintain a binding affinity to a CD7 protein. Still further included are all substantially homologous analogs and allelic variations. An example of a homologous analog of K12 is the murine homologue, which is capable of binding to the murine homologue of CD7. This novel polypeptide is described in SEQ ID NO: 8. Encoded within the cDNA of the murine K12 homologue is a 212 amino acid transmembrane protein that overall shares 36% amino acid identity over its entire length with the human K12 protein (Figure 1) and 44% amino acid identity over the extracellular regions. The full-length murine K12 protein comprises a signal sequence (approximately amino acids 1-28 of SEQ ID NO: 8) an extracellular domain sequence (approximately amino acids 29-160 of SEQ ID NO: 8), a transmembrane domain sequence (approximately amino acids 161-181 of SEQ ID NO:8), and a cytoplasmic domain sequence (approximately amino acids 182-212 of SEQ ID NO:8). As noted above, the above-described boundaries of such regions of the polypeptide are approximate and may differ from those described above by about 1 to 4 residues on either side of the above- described boundaries. Further, as noted above, the soluble extracellular domain can be truncated by 1 to 20 amino acids at the N terminus, and/or truncated by 1 to 20 amino acids at the C terminus, as long as it retains binding affinity for murine CD7. As the murine K12 homologue was previously unknown, an aspect of the invention described herein is these novel K12 proteins, and polynucleotides encoding the same. In one embodiment, the invention provides an isolated polynucleotide molecule that encodes a soluble K12 protein, wherein the polynucleotide molecule or its complement hybridizes to nucleotides 279 to 554 of SEQ ID NO:7. In another aspect, the invention provides an isolated polynucleotide molecule that encodes an intracellular domain of a murine K12 protein, wherein the polynucleotide molecule or its complement hybridizes to nucleotides 677 to 770 of SEQ ID NO:7. Preferably, the polynucleotide hybridizes under conditions of moderate stringency, such as, for example, 50% formamide and 6X SSC, at 42°C with washing conditions of 60°C 0.5X SSC, 0.1% SDS. Even more preferably, the polynucleotide hybridizes under high stringency conditions, such as, for example, the hybridization conditions given above, but with washing at approximately 68°C, 0.2X SSC, 0.1% SDS. A preferred example is a polynucleotide molecule that encodes a protein comprising amino acid residues 49 to 140 of SEQ ID NO:8. The polynucleotides of the invention can be used to recombinantly produce the encoded proteins. Recombinantly produced proteins can be prepared in greater quantities and at increased purity over that of proteins isolated from a naturally occurring source. In addition, recombinant production of proteins can ensure that no undesirable contamination is present from, from example, pathogenic organisms such as viruses and bacteria. The invention also encompasses host cells that contain the above polynucleotides, and host cells genetically engineered to express the polynucleotide molecules of the invention under the control of a heterologous promoter. Host cells can be bacterial, fungal, animal, or plant cells, but preferably are mammalian cells such as, for example, cells grown in culture. Vectors, including expression vectors, and processes for transforming or transfecting host cells with such vectors are well known in the art (see Sambrook et al., Molecular Cloning: A Laboratory Manual, 2ed. Vol. 1, pp. 1.101-104, Cold Spring Harbor Laboratory Press, (1989)). If the host cell contains an endogenous K12 gene, the host cells can also be induced to express, or express increased quantities, of the K12 gene product by the techniques of homologous recombination and gene activation (see U.S. Patent No. 5,272,071). The invention also encompasses the purified and/or recombinant proteins encoded by, inter alia, the above polynucleotides. Purified protein is protein that is substantially free of other cellular constituents (e.g., greater than 50%, preferably greater than 90%) of the protein in the composition is the desired protein). For example, the invention encompasses a protein that has the sequence of SEQ ID NO: 8, or a protein that comprises amino acid residues 49 to 140 of SEQ ID NO:8 yet is soluble and binds to a CD7 protein, or a protein that comprises amino acid residues 182 to 212 of SEQ ID NO: 8 (the intracellular domain of murine K12). The soluble forms of murine K12 that bind to CD7 can be used in the methods described herein. The intracellular domain of murine K12 protein is useful for screening for the specific intracellular targets of K12, and for determining the cellular machinery responsible for targeting of proteins, in general, to the Golgi apparatus (where K12 has been described by other groups as localizing) and/or the cell surface (where K12 was found to accumulate in the experiments described herein). For example, the intracellular domain likely contains a specific recognition signal for coated vesicle transport. The term "CD7/K12 complex" refers to the protein unit formed by the binding interaction of CD7 to K12. The term "CD7/K12 fragment complex" includes the protein units formed in which at least one binding partner is either a fragment of CD7 or K12 (e.g. the binding interaction of a CD7 fragment to K12, CD7 to a K12 fragment, or a CD7 fragment to a K12 fragment) or a homologous analog of CD7 or K12. In one embodiment of the invention, the 36 amino acid "spacer" region (4 repeats of 9 amino acids (XPPXASALP) in the area proximal to the membrane spanning part of the protein) within the human CD7 protein was removed as an internal deletion. Human K12-Fc still bound to the truncated (HuCD7-TR) form of human CD7, though the level of binding was lower (approximately 10 fold lower) than that which was observed to the full length human CD7 protein. Thus, there are different affinities of human K12-Fc for the normal vs. truncated versions of human CD7 or differential cell surface expression of the normal vs. truncated CD7 proteins in the transfected cells. In addition, knowing that the cognate ligand for CD7 is K12, one skilled in the art can easily determine which soluble forms of the CD7 receptor can be used as antagonists of the cell associated CD7 receptor. The term "biological activity" includes the binding of CD7 to K12 or fragments thereof and the biological effects that result therefrom. A K12 protein has the biological activity of binding to a CD7 protein if the binding affinity (K a ) of the complex formed between the K12 protein and the CD7 protein is within about 2 orders of magnitude of that of the complex formed between the naturally occurring CD7 and K12 proteins. In preliminary experiments, the K a of human K12-Fc for human CD7 was estimated to be in the range of 1 x 10 8 M "1 on both Jurkat (a human T cell leukemia cell line) and KG-1 cells (a human myelogenous leukemia cell line). Because of the species specificity of the interaction between CD7 and K12, care should be taken to choose a K12 protein, or antagonist thereof, that is appropriate for the particular CD7/K12 pair that is to be modulated in the methods of the invention. The term "biological effects" includes the cellular changes or effects that result from interaction of a CD7 protein with a K12 protein. The interaction between CD7 and K12 is, at least in part, a protein-protein interaction. Protein-protein interactions can be observed and measured in binding assays using a variety of detection methodologies that include, but are not limited to, surface plasmon resonance (Biacore), radioimmune based assays, and fluorescence polarization binding assays. When performed in the presence of a test compound, the ability of the test compound to modulate (e.g., enhance or inhibit) the protein-protein binding affinity is measured. CD7 and K12 are very similar in size (248 amino acids for human K12; 240 amino acids for human CD7) so neither fits the classical definition of a receptor, which is usually significantly larger than its ligand. Although one group reported that significant amounts of the full length, membrane-bound K12 do not accumulate on the cell surface (Slentz-Kesler, 1998), the instant invention demonstrates that membrane-bound K12 does accumulate on the cell surface, at least in transfected cells (see below). Human CD7-Fc bound to the surface of cells transfected with a full length human K12 cDNA, and mouse CD7-Fc bound to cells transfected with a full length mouse K12 cDNA. Therefore, the CD7 protein could trigger some signaling pathway in cells expressing K12 on the cell surface. Accordingly, soluble forms of CD7 could be used to trigger K12 signaling. Knowing that K12 binds to CD7 indicates several uses for exploiting the interaction between CD7 and K12, the K12 protein, and/or molecules derived thereof. As stated above, signaling through CD7 has been reported using anti-CD7 antibodies. Antibody-induced ligation of CD7 on the surface of either T cells or NK cells lead to the phosphorylation of intracellular proteins in those cells (Rabinowich, J. Immunol. Vol. 153:3504 (1994)). Crosslinking of CD7 on T cells has been shown to increase adhesion of the cells to fibronectin, ICAM-1, and V-CAM1 (Shimizu, J Exp Med 175:577 (1992)), and cross-linking of CD7 on NK cells has been shown to induce adhesion to fibronectin (Rabinowich, J. Immunol. Vol. 152:517 (1994)). Triggering CD7 has also been shown to regulate the functional activity of beta 1 integrins on NK cells (Rabinowich, J. Immunol. Vol. 152:517 (1994)). The interaction between CD7 and anti-CD7 antibodies inhibits proliferation in allogeneic and autologous mixed lymphocyte reactions (Lazarovits, Transplant Proc 20: 1253(1988)), stimulates NK cell proliferation, enhances the cytotoxicity of the cells, and induces interferon gamma production (Rabinowich, J. Immunol. Vol. 152:517 (1994)). Furthermore, the effects of the interaction between CD7 and anti-CD7 antibodies are not limited to T and NK cells as GM-CSF production by several myeloid cell lines is also stimulated (Hou, Blood 88: 124 (1996)). K12, including immobilized soluble forms of K12, also stimulates NK cell activation (see below). Accordingly, as a binding protein of CD7, K12 can have the same biological activities and effects on CD7-expressing cells as the agonistic anti-CD7 antibodies, indeed, the use of soluble forms of K12 to stimulate the activity of CD7 has a number of advantages over the use of an antibody to CD7. For example, soluble forms of K12 can be less immunogenic than a heterologous antibody, and using K12 to stimulate CD7 will more closely mimic the biological events that normally activate CD7. Thus, the invention provides methods of inhibiting T cell proliferation and/or activating NK cell proliferation and or inducing interferon gamma production in a mammal comprising administering to the mammal an effective amount of a soluble K12 protein. In a related aspect, the invention provides a method of stimulating the intracellular signaling of CD7 comprising contacting a cell that expresses CD7 with an effective amount of a recombinant K12 protein. A recombinant K12 protein is a protein that is produced using genetic engineering techniques such that it is essentially free of the contaminating proteins normally associated with the protein in its naturally occurring state, e.g., for human K12, other human proteins. CD7 has been suggested to be involved in both HIV- 1 infection and syncytia formation since anti-CD7 antibodies block both of these processes (Sato, J. Immunol 152:5142 (1994)). A K12 protein, preferably a soluble form of K12, can also be used to block HIV infection and syncytia formation. In fact, as shown herein below, soluble forms of K12 were able to block syncytia formation. Thus, soluble forms of K12 can be used to treat diseases caused by HIV-1, as well as other viruses whose transmission is mediated by CD7 and/or K 12. A monoclonal antibody directed against CD7 has also been shown to inhibit T cell proliferation in the allogeneic mixed lymphocyte reaction (Lazarovits, Transplant Proc. 20: 1253 (1988)). This study has led to a clinical trial of an anti-CD7 antibody for the prophylaxis of kidney transplant rejection (Lazarovits, J.Immunol. 150:5163 (1993)). K12, such as soluble K12 protein, could be used in a similar fashion. K12 is involved in the LPS-induced shock pathway. As si in Sempowski (1999), transgenic mice that do not express CD7 are resistant to LPS-induced shock. Thus the interaction between CD7 and K12 plays a role in the LPS-induced shock pathway and associated syndromes such as sepsis. Neutralizing antibodies to K12 can be used to block the interaction between K12 and CD7, prevent CD7 activation, and thus treat sepsis. Since CD7 can be a marker for T cell leukemias (reviewed in Haynes, Immunol Today 10:87 (1989)), immunotoxins have been created by fusing anti-human CD7 monoclonal antibodies to toxins such as ricin (Frankel, Leuk Lymphoma 26:287 (1997); Pauza, J.Immunol 158:3259 (1997)) or saporin (Flavell, Br J Cancer 15: 1035 (1997)). One can conjugate toxins to the extracellular domain of K12 as well; these conjugates can be less immunogenic than antibody-based conjugates and/or can have a longer half-life. Conjugation of the anti-CD7 antibodies with toxins may not even be required, since anti- CD7 antibodies alone have been effective anti-tumor agents in a xenografted human T cell ALL model (Baum, Br. J. Haemotol. 95:327(1996)). K12-Fc should have a similar effect in this model, and thus can be used against T cell leukemias. Screening Assays The interaction between CD7 and K12 can also be exploited in screening assays for proteins and small molecules. Specific screening methods are known in the art and many are extensively incorporated in high throughput test systems so that large numbers of test compounds can be screened within a short amount of time. The assays can be performed in a variety of formats, including protein-protein binding assays, biochemical screening assays, immunoassays, cell based assays, etc. These assay formats are well known in the art. The screening assays of the present invention are amenable to screening of chemical libraries and are suitable for the identification of small molecule drug candidates, antibodies, peptides, etc. By observing the effect that candidate molecules have on the interaction between CD7 and K12 in various binding assays, on CD7/K12-mediated activity in functional tests, and in cell based screens, molecules that are potential therapeutics because they can modulate the interaction between CD7 and K12 are identified. Such molecules can promote the biological activity of the CD7/K12 complex, prevent the formation of the CD7/K12 complex or inhibit dissociation of the CD7/K12 complex already formed. Molecules preventing the interaction of CD7 and K12, and hence the activation of CD7, can be useful as immunosuppressants or anti-inflammatory agents. Molecules that promote the interaction of CD7 and K12 can be useful when enhancement of the immune system is desired. A preferred method that can be used to screen test compounds for their ability to affect the interaction of CD7 with K12 comprises the steps of forming a composition comprising a CD7 protein, a K12 protein, and the test compound; assaying for the level of interaction of the CD7 protein and the K12 protein; and comparing the level obtained in the presence of the test compound to that obtained in the absence of the test compound, such that if the level obtained differs, a compound that affects the interaction of the CD7 protein and the K12 protein is identified. Preferably, at least one of the CD7 protein and the K12 protein can be labeled with a detectable moiety. One of the CD7 protein or the K12 protein can be soluble, and the other can be bound, although alternative assay formats are possible and well known. The test compound can be added to the composition after addition of the CD7 protein and the K12 protein, before both proteins are added, or after one protein is added and before the other is added. In another aspect, the screening methods of the invention comprise forming a composition comprising the test compound, the K12 protein and cells expressing CD7; determining the level of biological activity of the K12 protein in the composition; and comparing the level of biological activity with that which occurs in the absence of test compound, wherein a difference in the level of biological activity indicates that the test compound affects the biological activity of a K12 protein. Biological activity of K12 can be assayed in any number of ways, for example, by determining the phosphorylation state of intracellular proteins (activation of CD7 by agonist antibodies is known to induce phosphorylation of intracellular proteins), by determining the activation of NK cells and by determining the production of interferon gamma and or GM-CSF. In a related aspect, the cells express K12 and soluble CD7, or cells expressing CD7, is used. A particular example of an assay for the identification of potential CD7 or K12 antagonists is a competitive assay, which combines K12 and a candidate molecule with CD7 under the appropriate conditions for a competitive assay. Either CD7 or K12 can be labeled so that the binding can be measured and the effectiveness of the antagonist judged. The label allows for detection by direct or indirect means. Direct means include, but are not limited to luminescence, radioactivity, optical or electron density. Indirect means include but are not limited to an enzyme or epitope tag. Molecules that inhibit or prevent the dissociation of the CD7/K12 complex can be identified by forming the complex in the absence of a candidate molecule, then adding the candidate molecule to the mixture, and changing the conditions so that, but for the presence of the candidate molecule, CD7 would be released from the complex. The concentration of the free or bound CD7 can then be measured and the dissociation constant of the complex could be determined and compared to a control. Another method by which molecules can be identified that affect (either inhibit or promote) the interaction between CD7 and K12 is the solid phase method, in which CD7 is bound and placed in a medium with labeled K12. The amount of signal produced by the interaction between CD7 and K12 is measured in the presence and in the absence of a candidate molecule. Diminished levels of signal, in comparison to a control, indicate that the candidate molecule inhibited the interaction between CD7 and K12. Increased levels of signal, in comparison to a control, indicate that the candidate molecule promotes the interaction between CD7 and K12. In alternative embodiments, K12 could be bound and CD7 labeled. The CD7 and/or K12 proteins can be directly or indirectly labeled. For example, if the protein is recombinantly produced, one can engineer fusion proteins that can facilitate solubility, labeling, immobilization and/or detection. Fusion proteins which facilitate these processes can include, but are not limited to soluble Ig-tailed fusion proteins and His-tagged proteins. Methods for engineering such soluble Ig-tailed fusion proteins are well known to those of skill in the art. See, for example, U.S. Pat. No. 5,116,964, and the illustrative embodiments described below. Indirect labeling involves the use of a protein, such as a labeled antibody, which specifically binds to a component of the assay. Thus, the present invention encompasses methods of screening candidate molecules for their ability to modulate the interaction of CD7 and K12, and their ability to modulate activities mediated by the interaction of CD7 and K12. By observing the effect that the candidate molecule has on the known binding characteristics of CD7, K12 or fragments thereof, compounds that inhibit or enhance the interaction of CD7 and K12 can be identified. Typical test compounds are small molecules, antibodies, or peptides and can be part of extensive small molecule libraries developed for use in screening methods. The term "small peptides" includes peptidomimetics. In this context, the identification of small molecules which interact with the CD7 protein and/or the K12 ligand can be used to develop drugs that modulate the activation pathway and can allow physicians to treat distinct immune conditions without the negative side effects present in current therapies. Indications and Formulations Thus, in one aspect, the invention provides methods of treating diseases mediated by CD7and/or K12, the method comprising administering to a mammal in need thereof an effective amount of a K12 protein and small molecules identified as being agonists of the interaction of CD7 and K12. The K12 protein can be a soluble form of the protein, and preferably can be a multimeric fusion protein such as, for example, a Fc fusion. Diseases that are mediated by the interaction between CD7 and K12 and that can be treated using K12 protein include HIV-1 infection, cancers (especially T cell lineage neoplasias, e.g., T cell leukemia, acute lymphomic leukemia, cutaneous T cell lymphoma), and infections including bacterial and viral infections (e.g., CMV and EBV infections). In an alternative aspect, the invention provides a method of treating a disease mediated by CD7 and/or K12, the method comprising administering to a mammal in need thereof an effective amount of a K12 antagonist. An example of a K12 antagonist is a neutralizing antibody to K12. The generation of antibodies to extracellular portions of K12 molecules are well known to those skilled in the art. Antibodies can be polyclonal or monoclonal, and full length or fragments (e.g., Fab and Fab2 fragments). Preferably, for use in humans, the antibodies are human or humanized; techniques for creating such human or humanized antibodies are also well known and are commercially available from, for example, Medarex Inc. (Princeton, NJ) and Abgennix Inc. (Fremont, CA). Antibodies can be screened for their ability to neutralize the biological effects of K12 using the screening assays described herein. Examples of diseases that can be mediated by CD7 and that can be treated with antagonists of K12 include sepsis, graft vs. host disease due to transplantation, autoimmune diseases including multiple sclerosis, arthritis such as, for example, rhumatoid arthritis and psoriatic arthritis, scleroderma, lupus, psoriasis, atopic dermatitis, type I diabetes mellitus, Hashimoto's thyroiditis, pernicious anemia, Addison's disease, myasthenia gravis, uveitis, psoriasis, Guillain-Barre Syndrome, Grave's disease, systemic lupus erythematosus and dermatomyositis, asthma, eczema, atopical dermatitis, contact dermatitis, other eczematous dermatitides, seborrheic dermatitis, and allergic rhinitis. The terms "treat", "treating", and "treatment" used herein includes curative, preventative (e.g., prophylactic) and palliative treatment. For such therapeutic uses, the identified agonists or antagonists of the interaction between CD7 and K12, CD7, and/or K12 can be administered to the mammal in need through well-known means, including oral, parenterally (e.g., subcutaneous, intramuscular, intravenous, intradermal, etc. injection), buccal, rectal, topically, or via inhalation and/or insufflation. Compounds are usually formulated with a suitable carrier. Formulations suitable for administration include aqueous and non-aqueous sterile injection solutions which may contain anti-oxidants, buffers, bacteriostats and solutes which render the formulation isotonic with the blood of the recipient; and aqueous and non-aqueous sterile suspensions which may include suspending agents or thickening agents. The identified compounds that affect the interaction between CD7 and K12 can be administered to a patient at therapeutically effective doses to treat or ameliorate diseases associated with the activity of CD7 and/or K12. A therapeutically effective dose refers to that amount of the compound sufficient to result in amelioration of symptoms of the disease. The dosage will depend on the specific activity of the agonist or antagonist and can be readily determined by routine experimentation. For example, the therapeutically effective dose can be estimated initially from cell culture assays. A dose may be formulated in animal models to achieve a circulating plasma concentration range that includes the IC 50 (i.e., the concentration of the test compound which achieves a half- maximal inhibition of symptoms) as determined in cell culture, while minimizing toxicities. Such information can be used to more accurately determine useful doses in humans. The amount and timing of compound administered will be dependent upon the subject being treated, on the severity of the affliction, on the manner of administration and upon the judgment of the prescribing physician. The following examples are offered by way of illustration, and not by way of limitation. Those skilled in the art will recognize that variations of the invention embodied in the examples can be made, especially in light of the teachings of the various references cited herein, the disclosures of which are incorporated by reference in their entirety. Example 1 Cloning of the human and mouse K12 and CD7 genes The human K12 protein was cloned based on the published sequence (Slentz- Kesler, 1998) using RT-PCR from mRNA prepared from the K562 erythroleukemia cell line. A mouse protein related to the human K12 sequence was identified as an EST (AA734402) from a proximal colon cDNA library. The EST was purchased and sequenced in its entirety. After the correct sequence was determined, it was found that the cDNA encoded a 212 amino acid transmembrane protein that shares 36% overall amino acid identity over its entire length with the human K12 protein. Homology over the extracellular portion was slightly higher. The mouse CD7 gene was cloned using PCR from an EL4.6 λ zap library. All cDNA clones were sequenced on both strands to confirm that no amino acid changes had been introduced by PCR into the published CD7 sequence (Yoshikawa, Immunogenetics 27: 114 (1993); Yoshikawa, Immunogenetics 41: 159 (1995)). Example 2 Generation of K12-Fc fusion proteins Both the human and mouse K12-Fc fusion proteins were made by using Sew-PCR to attach the Fc portion of human IgGl (see U.S. Patent No. 5,457,035 for a description of the Fc portion) to that part of the gene encoding the extracellular domain of human K12 (amino acids 1-145 in the human clone (SEQ ID NO:4) (Slentz-Kesler, 1998), and amino acids 1-160 in the murine clone (SEQ ID NO:8)). The fusion proteins were transiently expressed in CV-1/EBNA cells and purified from the conditioned medium using protein A Sepaharose (Pharmacia, Piscataway, NJ) and standard techniques. Because of the fusion to Fc sequences, these fusion proteins exist as dimers in solution. Example 3 Precipitation of a K12-Fc binding protein from NK cells Primary human NK cells (1 x 10 6 cells/ml) were radiolabeled overnight with 50 uCi/ml [ 35 S]-cysteine/methionine) (ProMix, Amersham, Arlington, IL). Radiolabeled cells were lysed with 1 ml RIPA E lysis buffer (PBS, 1% Triton). 150 ul of lysate were incubated with 1 ug of HuK12-Fc or a control Fc fusion protein for 1 hour at 4°. Precipitated proteins were collected onto Protein A Sepharose and loaded separated onto a 4-20% acrylamide gradient Tris-Glycine gel (Novex, San Diego, CA) under denaturing, reducing conditions. The gel was fixed, treated with Amplify (Amersham, Arlington, IL), dried, and exposed to XAR-5 film. A single protein band of approximately 40 kDa was precipitated from the NK cells, indicating that expression cloning might be a viable way to identify the Kl 2 cognate. Example 4 Cloning of the K12 cognate from a human PBT cDNA expression library The human peripheral blood T cell library cDNA expression library was constructed in the pDC409 vector using methods previously described (McMahan, EMBO J. Vol. 10:2821 (1991)) and contained about 0.5 x 10 6 cDNA clones. Approximately 78% of clones in the library contain inserts, and the average insert size is about 1.2 Kb. The K12-Fc fusion protein was used to screen the expression library and fifty pools of approximately 1600 cDNAs each were transfected into CV-1/EBNA cells. Two days later the transfected cells were tested for their capacity to bind the K12-Fc fusion protein. Two positive pools were identified. These cDNA pools were subsequently subdivided into smaller and smaller groups and assayed until single positive cDNA clones were isolated from each original pool. Sequencing of these cDNAs and comparison with public DNA databases revealed that the cDNA from each positive pool encoded a full length clone of the human CD7 gene. This result explained why the size of the protein precipitated from NK cell lysates in Example 3 (40 kDa) was identical to the reported size of human CD7 (Haynes, Immunol. Rev. 57: 127 (1981); Aruffo, Embo J. 6:3313 (1987)). Example 5 K12/CD7 binding studies COS-1 cells were transfected with an expression vector encoding full length HuCD7 (human CD7), HuCD7-TR (human CD7 containing an internal deletion of the 36 amino acid tandem repeat), MuCD7 and vector only cDNA using DEAE-Dextran. However, any of a large number of expression vectors that are commercially available from numerous companies (e.g., Clontech Inc., Palo Alto, CA) could also be used to express the desired cDNAs. Two days post transfection the cells were assayed for the capacity to bind HuK12-Fc as previously described for other Fc fusion proteins (Ly an et /.,1993, Cell 75: 1157-1167) with the following modifications. The binding media (RPMI 1640, 1%FBS, 0.02% NaAzide, 20mM HEPES, pH7.2) was modified to include ImM MnCl 2 . In some experiments, the transfected cells were incubated with zero, 20 ug/ml or 2 mg/ml of Con A in BM/MM prior to binding with the Fc proteins. Following incubation for 30 minutes at room temperature and the continued presence of Con A, lug/ml of either HuK12-Fc or MuK12-Fc was then added to the appropriate slides. Binding of the Fc protein was detected with [ ~ I] mouse anti-Human Fc antibody as previously described (Lyman, 1993). After binding the iodinated antibody, the cells were washed and then the radioactivity was quantitated by Storm Phosphorimager. The results demonstrated that cells transfected with the full length HuCD7 construct bound HuK12-Fc best; when the cells were transfected with the truncated HuCD7-TR construct, relative binding of the labeled HuK12-Fc was about 10 fold less than to cells expressing the full length HuCD7 construct. However, binding of HuK12-Fc to the cells expressing murine CD7 was approximately 4 fold less than binding to cells expressing the full length HuCD7. Binding to the cells transfected with vector alone was, as expected, negligible. The complementary experiments were also performed to assay the binding of either human or murine CD7-Fc fusion proteins to cells transfected with either human or murine K12 expression plasmids (full length). The binding assays were done as described above. Human CD7-Fc bound to the surface of cells transfected with a full length human K12 cDNA expression vector, and murine CD7-Fc bound to cells transfected with a full length murine K12 cDNA expression vector. However, only background binding (indistinguishable from negative controls) was observed when human CD7-Fc fusion protein was assayed against cells transfected with a full length murine K12 cDNA, or when murine CD7-Fc fusion protein was assayed against cells transfected with a full length human K12 cDNA. Example 6 Monoclonal antibodies to human CD7 block binding of K12-Fc Six commercial antibodies to the extracellular domain of human CD7 were tested for their capacity to block the binding of K12-Fc to Jurkat cells, which express high levels of CD7. Monoclonal antibodies directed against human CD7 thai were tested were the following: Clone M-T701 (Becton Dickinson, Franklin Lakes, NJ); Clone 8118.1 (Immunotech, Westbrook, ME); Clone 4H9 (Pharmingen, San Diego, CA); and Clones RFT-2a, WM31, and CLB-3A1 (Research Diagnostics, Inc., Flanders, NJ). The calculated results are shown in the tables below. TABLE 1 shows the reductions in the percentage of positive cells and mean fluorescence intensity that occur when Jurkat cells are preincubated with human K12-Fc fusion protein prior to staining the cells with the antibodies. TABLE 1 TABLE 2 shows that, in a similar fashion, preincubation of the Jurkat cells with the six anti-human CD7 antibodies blocks the binding (to varying degrees) of human K12-Fc to the cells. TABLE 2 Thus, the K12-Fc fusion protein blocked the binding of each of the antibodies to Jurkat cells to varying degrees, and each member of the panel of monoclonal antibodies could interfere with the ability of K12-Fc to bind to CD7 on Jurkat cells. Example 7 Chromosomal location of the mouse K12 gene The chromosomal location of the mouse K12 protein was determined using the Jackson Laboratory radiation hybrid panel mapping resource (Flaherty, Mamm. Genome 9:417-418 (1998)). The murine K12 protein maps on mouse chromosome 11, proximal to CD7 locus. Example 8 K12-Fc blocks Con A. but not anti-TcR- induced cell proliferation The mouse K12-Fc fusion protein was tested for its capacity to inhibit the proliferation of lymph node cells (from BALB/c mice) that had been stimulated with either soluble Con A or immobilized anti-TcR α β. The K12-Fc fusion protein inhibited Con A induced proliferation of the cells in a dose-dependent manner, but had no effect on anti-TcR induced cell proliferation. Since Con A is known to bind to a number of proteins, including CD7, the inhibition of Con A-induced T cell proliferation by K12-Fc could simply be due to K12 blocking Con A binding to CD7. Whether Con A could inhibit K12-Fc binding to COS-1 cells transfected with cDNAs encoding CD7 was then examined. No blocking of either human K12-Fc (added at a concentration of 1 ug/ml) to human CD7 or mouse K12-Fc (added at a concentration of 1 ug/ml) to mouse CD7 was seen when the cells were preincubated with 20 ug/ml of Con A. When the amount of Con A in the medium was raised to 2 mg/ml, a 30% inhibition of human K12-Fc binding and about 90% inhibition of mouse K12-Fc binding was observed. Given that the concentration of ConA used in the cell proliferation experiment was 1 μg/ml, it seems unlikely that the inhibitory effect of K12-Fc on the proliferation of lymph node cells was due to the blocking of ConA binding to the cells. Thus. K12-Fc inhibits ConA-induced proliferation of lymph node cells. Example 9 Human K12-Fc mediated human NK cell activation The capacity of K12 to induce human NK cell activation through its interaction with CD7 was analyzed by examining the expression of surface molecules associated with cellular activation. Human NK cells were prepared from normal donor peripheral blood. PBMC were isolated by standard techniques from 200ml of donor 362 heparinized whole blood. Prior to NK enrichment macrocytes and monocytes were depleted from PBMC by plastic adherence. NK cells were then enriched by negative selection. CD3 + , CD19 + and MHC class II + cells were removed using biotinylated mAb specific for these markers followed by incubation with streptavidin-coated magnetic beads. This procedure was completed twice resulting in an enriched population of human CD56 + NK cells. For in vitro NK cell activation assays, enriched NK cells were added to tissue culture wells that had been treated with mu/hu IgG, huCD7 mAb or huK12-Fc that had been immobilized onto the plastic well at a concentration of 2.5 ug/ml. These cultures were incubated for 20 hours at 37°C and then expression of CD25 and CD69, and the adhesion molecule CD54, evaluated by flow cytometry. Increased expression of the early activation marker CD69 and of the IL-2 receptor alpha chain (CD25) indicates that the NK cells have received an activation signal. The results of two experiments using the NK preparation from donor 362, which was about 92% CD56 + . are shown in TABLE 3 below. These results demonstrate that immobilized huK12-Fc and immobilized huCD7 mAb induce activation of human NK cells. TABLE 3 Thus, in two separate experiments, immobilized huK12-Fc, and to a lesser extent immobilized huCD7 mAb, induced activation of a preparation of human NK cells from donor 362. Addition of anti-CD7 antibodies or K12-Fc in solution (not immobilized) to NK cultures did not result in enhancement of cell surface molecule expression. In other experiments, two different NK cell donors did not demonstrate significant activation induction by huK12-Fc, indicating either that activation of fresh NK cells through CD7 demonstrates donor variability or that the kinetics of activation of different donors may be different. This lack of effect in different donors may be due to examining at a 20 hour time point when optimal activation induced by huK12 may be 24 hours or even later. Example 10 Human K12 Inhibits Syncytia Formation In this experiment, a fusion inhibition assay was performed to test the antiviral activity of soluble forms of CD7 and K12. H9 cells infected with HIV-1 MN virus and SupTl cells were harvested at log phase (5xl0 4 /ml) by centrifuging cells for 8 minutes at 1,500 rpm. The cells were resuspended in 20 ml of R10 and centrifuged again at 1500 rpm. The cells were counted to determine cell number and viability. The cells were resuspended at concentrations of 30,000 cells/25 μl (SupTl) or 10,000/25 μl (H9/MN). A mixture of 10,000 cells of H9/MN and 30,000 SupTl were distributed on 96 well tissue culture plates. Soluble K12 poly-His Flag (hu K12 HF), soluble K12/hu IgGl Fc fusion protein (hu K12:Fc), and soluble '• 7 poly-His Flag at varied concentrations were added to duplicate wells. Twenty four hours later, each of the wells was examined under an inverted microscope. The syncytia were counted in each well by dividing the well into three equal fields. Two controls were also examined: human CD4 as a positive control (AIDS Research and Reference Reagent Program Catalog No. 1813) and wild type Rev protein as a negative control (AIDS Reference and Reagent Program Catalog No. 1457). Any cell with the appearance of multiple syncytia which was at least three times the size of a single control cell, or any cell or group of cells which was associated with a balloon/bubble-like structure, was considered a syncytium. The different treatment groups at each concentration were compared to each other and to control groups in terms of statistical significance. Figure 2 is a composite graph illustrating the effect of soluble K12 poly-His Flag (hu K12 HF), soluble K12/hu IgGl Fc fusion protein (hu K12:Fc), and soluble CD7 poly His flag at varied concentrations on syncytia numbers. All three compounds have inhibitory activity against HIV-1 when measured by the ability to inhibit multinucleated syncytia formation between HIV-1 infected H9 and SupTl cells. Specifically, the three compounds were equally effective as the positive control CD4 in inhibiting syncytia formation at concentrations of 16, 4, 1, and 0.25 μg/ml. At 64 μg/ml, CD4 appears to be more effective than CD7 or K12 compounds at inhibiting syncytia formation. CD7 and K12 compounds were more effective than the negative control at inhibiting syncytia formation. These results demonstrate that, like CD4, soluble forms of CD7 and/or K12 can be used to treat HIV-1, and should be effective in inhibiting the transmission of HIV-1 between cells and between individuals, and at reducing the viral load in infected individuals.

Claims: CLAIMS What is claimed is: 1. A method of stimulating the intracellular signaling of CD7 comprising contacting a cell that expresses CD7 with an effective amount of a recombinant K12 protein. 2. The method of claim 1 wherein the K12 protein is a soluble protein comprising approximately amino acid residues 49 to 125 of SEQ ID NO:4. 3. The method of claim 2 wherein the soluble K12 protein further comprises a Fc portion of human IgGl . 4. The method of claim 1 wherein the K12 protein is a multimeric protein. 5. A method of inhibiting T cell proliferation and/or activating NK cell proliferation and/or inducing NK cytotoxicity in a mammal comprising administering to the mammal an effective amount of a K12 protein. 6. A method of treating a disease mediated by CD7, the method comprising administering to a mammal in need thereof an effective amount of a K12 protein. 7. The method of claim 6 wherein the K12 protein is a soluble K12 protein. 8. The method of claim 7 wherein the soluble K12 protein comprises approximately amino acid residues 49 to 165 of SEQ ID NO:4. 9. The method of claim 7 wherein the soluble K12 protein further comprises a Fc portion of human IgGl. 10. The method of claim 7 wherein the soluble K12 protein is a multimeric protein. 11. The method of claim 6 wherein the disease is selected from the group consisting of HIV-1 infection, cancer (e.g., T cell leukemia, acute lymphomic leukemia, cutaneous T cell lymphoma), bacterial infections and viral infections. 12. The method of claim 11 wherein the disease is HIV-1 infection. 13. The method of claim 11 wherein the disease is T cell leukemia and the K12 protein is a soluble form of K12 covalently attached to a toxin. 14. An immunotoxin consisting of a soluble form of K12 covalently attached to a toxin. 15. The immunotoxin of claim 14, wherein said toxin is selected from the group consisting of ricin and saponin. 16. A method of treating a disease mediated by CD7, the method comprising administering to a mammal in need thereof an effective amount of a K12 antagonist. 17. The method of claim 16, wherein the K12 antagonist is a neutralizing antibody to K12. 18. The method of claim 16, wherein the disease is selected from the group consisting of sepsis, graft vs. host disease due to transplantation, autoimmune diseases, multiple sclerosis, arthritis, rhumatoid arthritis, psoriatic arthritis, scleroderma, lupus, psoriasis, atopic dermatitis, type I diabetes mellitus, Hashimoto's thyroiditis, pernicious anemia, Addison's disease, myasthenia gravis, uveitis, psoriasis, Guillain-Barre Syndrome, Grave's disease, systemic lupus erythematosus and dermatomyositis, asthma, eczema, atopical dermatitis, contact . dermatitis, other eczematous dermatitides, seborrheic dermatitis, and rhinitis. 19. A method of screening a test compound to identify its ability to affect the interaction of CD7 with K12, the method comprising the steps of: a. forming a composition comprising a CD7 protein, a K12 protein, and the test compound; b. assaying for the level of interaction of the CD7 protein and the K12 protein; and c. comparing the level obtained in (b) to that obtained in the absence of the test compound, such that if the level obtained in (b) differs from that obtained in the absence of test compound, a compound that affects the interaction of the CD7 protein and the K12 protein is identified. 20. The method of claim 19 wherein at least one of the CD7 protein and the K12 protein is labeled with a detectable moiety. 21. The method of claim 19 wherein the test compound is selected from the group consisting of a small organic molecule, an antibody, or a small peptide. 22. The method of 19, wherein at least one of the CD7 protein or the K12 protein is soluble. 23. The method of claim 19 wherein the test compound is added to the composition after addition of the K12 protein. 24. A method of screening a test compound to identify its ability to affect the biological activity of a K12 protein, the method comprising: a. forming a composition comprising the test compound, the K12 protein and cells expressing CD7; b. determining a biological activity of the K12 protein in the composition; and c. comparing the level of the biological activity of step (b) with that which occurs in the absence of test compound, wherein a difference in the level of biological activity indicates that the test compound affects the biological activity of a K12 protein. 25. The method of claim 24 wherein the K12 protein is a soluble protein. 26. The method of claim 24 wherein the K12 protein is present on a cell that has been genetically engineered to express K12. 27. The method of claim 24 wherein the biological activity of the K12 protein is assayed by determining the phosphorylation state of intracellular proteins, and/or determining the activation of NK cells and/or determining the production of interferon gamma and/or GM-CSF. 28. An isolated polynucleotide molecule that encodes a soluble K12 protein, wherein the polynucleotide molecule or its complement hybridizes under moderately stringent conditions to nucleotides 279 to 554 of SEQ ID NO:7. 29. The polynucleotide molecule of claim 29 that encodes a protein consisting essentially of amino acid residues 29 to 160 of SEQ ID NO:8. 30. A host cell genetically engineered to express the polynucleotide molecule of claim 28 under the control of a heterologous promoter. 31. A purified murine Kl 2 protein. 32. The protein of claim 31 that has the sequence of SEQ ID NO:8. 33. The protein of claim 31 that is soluble. 34. The protein of claim 33 comprising amino acid residues 49 to 140 of SEQ ID NO:8. 35. A purified protein comprising amino acid residues 182 to 212 of SEQ ID NO:8.

Inventors: FANSLOW WILLIAM C, LYMAN STEWART D, FANSLOW, WILLIAM, C., LYMAN, STEWART, D.
Applicants: FANSLOW WILLIAM C, IMMUNEX CORP, LYMAN STEWART D, FANSLOW, WILLIAM, C., IMMUNEX CORPORATION, LYMAN, STEWART, D.
Application year: 2000
Main classification: C07K14
IPC codes: A61K38, A61K47, C07K14