MOLECULES DESIGNATED B7L-1

Abstract: The invention is directed to B7L-1 as a purified and isolated protein, the DNA encoding the B7L-1, host cells transfected with cDNAs encoding B7L-1 and processes for preparing B7L-1 polypeptides.

Description: MOLECULES DESIGNATED B7L-1 FIELD OF THE INVENTION The present invention relates to a novel polypeptide, designated B7L-1. B7L-1 has weak homology to a number of proteins including B7-1, (CD80) and is a binding protein for LDCAM. The invention includes B7L-1 molecules, DNA encoding B7L-1 molecules, processes for production of recombinant B7L-1 polypeptides, and pharmaceutical compositions containing such B7L-1 polypeptides. BACKGROUND OF THE INVENTION B7-1 (CD80) is a T cell costimulatory molecule that is found on the surface of antigen presenting cells (APCs). Originally described as a cell adhesion molecule, it is now known that B7-1 sends important costimulatory signals through its two T cell surface receptors, CD28 and CTLA4 (CD152). B7-1 interacts with CD28 to signal cytokine production, cell proliferation, and the generation of effector and memory T cells. If the signal through CD28 is blocked T cell, anergy or immune deviation can occur, resulting in severely depressed or altered immune response. For example, when B7-1 and (B7-2) interaction with CD28 (and CTLA40 is blocked with a soluble CTLA4Ig, allograft tolerance and resistance to autoimmune diseases have been observed. B7-1 also interacts with the T cell CTLA4 receptor. Its signaling is complex, but one component provides a negative feedback signal, causing the T cell to attenuate the CD28 signal. In the absence of this signal for a long period of time, rampant T cell proliferation and effector cell activation continues. However, shorter term intervention can be beneficial by leading to a more vigorous immune response. For example, when the interaction of B7-1 (and B7-2) is blocked with antibodies to CTLA4 increased rejection of tumors has been found. When this feedback regulation malfunctions, autoimmune diseases and lymphoproliferation (refs) can result. For example, when the CTLA4 and B7-1 interaction is blocked with a soluble CTLA4Ig, allograft tolerance and resistance to autoimmune diseases have been observed. In addition to B7-1, other molecules are known to send costimulatory signals to T cells. For example, B7-2 (CD86), which is expressed on different cells and at different stages of APC activation from that of B7-1, also delivers its costimulatory signal to T cells through CD28 and CTLA4. The B7-2 signal can lead to immune responses that are identical to (refs), or different (refs) from the immune responses resulting from B7-1 signaling. The nature of the B7-2 signaling depends upon the cellular context and the timing of the costimulation. Some evidence suggests that additional molecules bind CTLA4 (ref). Evidence also exists that other molecules are involved in sending important CD28-independent costimulatory signals to T cells (ref). Even though they bind to the same cellular receptors, B7-1 and B7-2 are only weakly related at the amino acid level. Both, however, are members of the extended immunoglobulin domain containing superfamily and much of their shared sequence homology is due to the particular residues shared by their common Ig domains, which are characteristic of the Ig-domain subfamily. Clearly, costimulatory signaling through T cell surface receptors plays an important role in maintaining balance in the immune system. Systems with a predominance of activatory signals, such as the costimulatory signaling between CD28 and B7-1, can lead to autoimmunity and inflammation. Immune systems with a predominance of inhibitory signals, such as the costimulatory signaling between CTLA4 and are less able to challenge infected cells or cancer cells. Isolating new molecules involved in costimulatory signaling is highly desirable for studying the biological signal(s) transduced via the receptor. Additionally, identifying such molecules provides a means of regulating and treating diseased states associated with autoimmunity, inflammation and infection. For example, engaging a molecule that stimulates inhibitory or negative signaling with an agonistic antibody or signaling partner can be used to downregulate a cell function in disease states in which the immune system is overactive and excessive inflammation or immunopathology is present. On the other hand, using an antagonistic antibody specific for a molecule that stimulates negative signaling, or using a soluble form of the molecule to block signaling, can activate the specific immune function in disease states associated with suppressed immune function. Conversely, engaging a molecule that stimulates positive signaling with an agonistic antibody can be used to upregulate the effect of that molecule's signaling. In view of the evidence that undefined T cell costimulatory molecules exist and further in view of the continuing search for new therapeutics for treating infection, autoimmune diseases, and inflammation, it would be desirable to identify additional T- cell costimulatory molecules. In particular there is a need for additional molecules that alter T cell costimulation during an in vivo immune response. SUMMARY OF THE INVENTION The present invention provides mammalian B7L1 polypeptides as isolated or homogeneous proteins. The present invention further includes isolated DNAs encoding B7L1 and expression vectors comprising DNA encoding mammalian B7L1. Within the scope of this invention are host cells that have been transfected or transformed with expression vectors that comprise a DNA encoding B7L1, and processes for producing B7L1 by culturing such host cells under conditions conducive to expression of B7L1. Further within the present invention are pharmaceutical composition comprising soluble forms B7L-1 molecules. DETAILED DESCRIPTION OF THE INVENTION Novel proteins, designated B7L-1, and DNA the encodes B7L-1 proteins are provided herein. The B7L-1 polypeptides of the present invention share a weak homology with B7-1 and is a binding protein for LDCAM, a novel polypeptides, described in copending application S/N 60/095,672 filed August 7, 1998. The human and murine LDCAM nucleotide sequence is disclosed in SEQ ID NO: 19 and SEQ ID NO:21, respectively. The amino acid sequences encoded by SEQ ID NO: 19 and SEQ LDNO:21 are shown in SEQ LD NO:20 and SEQ LD NO:22, respectively. Mammalian B7L-1 proteins exist as different splice forms, designated long extracellular and short extracellular forms. Example 1 describes identifying and isolating a full length human clone, designated here-in as long extracellular B7L-1. The nucleotide sequence of human long extracellular B7L-1 DNA, isolated as described in Example 1, is presented in SEQ ID NO:l, and the amino acid sequence encoded thereby is presented in SEQ ID NO:2. The encoded long extracellular human B7L-1 amino acid sequence (SEQ LD NO:2) has a predicted extracellular domain of 364 amino acids (1-364) including a leader sequence of 20 amino acids (1-20); a transmembrane domain of 21 amino acids (365-385) and a cytoplasmic domain of 47 amino acids (386-432). Example 3 describes isolating a murine B7L-1 DNA with a shorter extracellular region. This DNA is disclosed in SEQ ID NO:3. The amino acid sequence encoded by the nucleotide sequence of SEQ ID NO:3 is disclosed in SEQ LD NO:4. The encoded short extracellular murine B7L-1 amino acid sequence (SEQ LD NO:4) has a predicted extracellular domain of 330 amino acids (1-330), including a leader sequence of 20 amino acids (1-20); a transmembrane domain of 21 amino acids (331-351); and a cytoplasmic domain of 47 amino acids (352-398). The leader sequence of SEQ LD NO:4 includes the first 8 amino acids of the isolated human B7L-1 long molecule. Example 3 also describes a "short extracellular" form of human B7L-1 DNA that is though to be an alternatively spliced B7L-1 variant. The nucleotide sequence for the "short extracellular" form is disclosed in SEQ LD NO:5 and the amino acid sequence encoded by the sequence of SEQ LD NO:5 is described in SEQ LD NO:6. The encoded short extracellular human B7L-1 amino acid sequence (SEQ LD NO: 6) has a predicted extracellular domain of 330 amino acids including a leader sequence of 20 amino acids; a transmembrane domain of 21 amino acids 331-351; and a cytoplasmic domain of 47 amino acids 352-398. The sequences described in SEQ LD NO:5 and SEQ ED NO:6 were obtained by isolating a clone from human cDNA with primers designed to flank the potential alternative splice between "long" and "short" forms and then comparing a resulting cloned fragment of SEQ ED NO:5 (nucleotides 193-358), the murine short extracellular form described in SEQ ED NO:3 and SEQ ED NO:4 and the human long extracellular form described in SEQ ED NO:l and SEQ ED NO:2. The comparison confirmed the existence of a human short extracellular form and provided a basis for the sequences of SEQ ED NOS:5 and 6. The purified mammalian B7L-1 molecules described herein are Type I transmembrane proteins having limited homology to B7-1, poliovirus receptors, and thymocyte activation and development protein. For these and many other weakly homologous proteins, the homology lies in their Ig domains. As described below, B7L-1 proteins are expressed on brain tissue, dendritic cells, dendritic cell subsets and CD40 ligand-activated B cells. The discovery of the DNA sequences disclosed in SEQ ED NOs:l, 3 and 5 enables construction of expression vectors comprising DNAs encoding human and mouse B7L-1 proteins; host cells transfected or transformed with the expression vectors; biologically active B7L-1 as homogeneous proteins; and antibodies immunoreactive with B7L-1. Since B7L-1 is found in bone marrow-derived and peripheral blood-monocyte derived dendritic cells, these molecules may be used to regulate inflammation in a therapeutic setting. The binding study results described in Example 13 show B7L-1 binding on tumor cell lines.. Thus, biological signaling mediated by B7L1 could mediate functional anti tumor effects on these types of tumors. As used herein, the term "B7L-1" refers to a genus of polypeptides that are binding proteins for LDCAM, novel polypeptides described in copending application S/N 60/095,672 filed August 7, 1998, and complex structures found in variety of cell lines including, but not limited to, lung epithelial cells, B lymphoblastoid cells and B cells. The term B7L-1 encompasses polypeptides having the amino acid sequence 1-432 of SEQ ED NO:2, the amino acid sequence 1-398 of SEQ ED NO:4; and amino acids 1-398 of SEQ LD NO:6. In addition, B7L-1 encompasses polypeptides that have a high degree of similarity or a high degree of identity with the amino acid sequence of SEQ ED NO:2, the amino acid sequence of SEQ ED NO:4, and amino acid sequence of SEQ ID NO:6, and which polypeptides are biologically active and bind their counterstructure, LDCAM. The term "murine B7L-1" refers to biologically active gene products of the DNA of SEQ ED NO: 3 and the term "human B7L-1" refers to biologically active gene products of the DNA of SEQ ED NO:l and SEQ ED NO:5. Further encompassed by the term "B7L-1" are soluble or truncated proteins that include the binding portion of the protein and retain biological activity. Specific examples of such soluble proteins are those comprising the sequence of amino acids 1-364 of SEQ ED NO:2; those comprising the sequence of amino acids 1-330 of SEQ ED NO:4; and 1-330 of SEQ ID NO:6. Alternatively, such soluble proteins can exclude a leader sequence and thus encompass amino acids 21-364 of SEQ ED NO:2; amino acids 21-330 of SEQ ED NO:4; and amino acids 21-330 of SEQ LD NO:6. The term "biologically active" as it refers to B7L-1, means that the B7L-1 is capable of binding to LDCAM, described in copending U.S. Patent Application S/N 60/095,672 filed August 7, 1998. LDCAM and B7L-1 are termed counterstructures because B7L-1 is a binding protein for LDCAM. "Isolated" means that B7L-1 is free of association with other proteins or polypeptides, for example, as a purification product of recombinant host cell culture or as a purified extract. A "B7L-1 variant" as referred to herein, means a polypeptide substantially homologous to native B7L-1, but which has an amino acid sequence different from that of native B7L-1 (human, murine or other mammalian species) because of one or more deletions, insertions or substitutions. The variant amino acid sequence preferably is at least 80% identical to a native B7L-1 amino acid sequence, most preferably at least 90% identical. The percent identity may be determined, for example, by comparing sequence information 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) for nucleotides, and the weighted comparison matrix of Gribskov and Burgess, Nucl. Acids Res. 14:6745, 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. Variants may comprise conservatively substituted sequences, meaning that a given amino acid residue is replaced by a residue having similar physiochemical characteristics. Examples of conservative substitutions include substitution of one aliphatic residue for another, such as He, Val, Leu, or Ala for one another, or substitutions of one polar residue for another, such as between Lys and Arg; Glu and Asp; or Gin and Asn. Other such conservative substitutions, for example, substitutions of entire regions having similar hydrophobicity characteristics, are well known. Naturally occurring B7L-1 variants or alleles are also encompassed by the invention. Examples of such variants are proteins that result from alternate mRNA splicing events or from proteolytic cleavage of the B7L-1 protein, wherein the B7L-1 binding property is retained. Alternate splicing of mRNA may yield a truncated but biologically active B7L-1 protein, such as a naturally occurring soluble form of the protein, for example. Variations attributable to proteolysis include, for example, differences in the N- or C-termini upon expression in different types of host cells, due to proteolytic removal of one or more terminal amino acids from the B7L-1 protein (generally from 1-5 terminal amino acids). As mentioned above, Example 1 describes identifying and isolating the complete coding region of human long extracellular B7L-1 DNA. This process involved searching a nucleotide sequence databank using a human B7-1 nucleotide sequence as the query sequence. Two expressed sequence tag (EST) files, GenBank accession numbers T08949 EST06841 and T32071 EST 43348, were identified has having homology with a portion of human B7-1. The GenBank record does not disclose a coding region for polypeptides encoded by these ESTs. Example 5 describes the construction of a novel human B7L-1/Fc fusion protein that may be utilized in screening cell lines for binding to B7L-1 and in studying biological characteristics of B7L-1. Other antibody Fc regions may be substituted for the human IgGl Fc region described in the Example. Other suitable Fc regions, are those that can bind with high affinity to protein A or protein G, and include fragments of the human or murine IgGl Fc region, e.g., fragments comprising at least the hinge region so that interchain disulfide bonds will form. In addition, the Fc region may be altered or mutated to a form having lower Fc receptor binding characteristics. The B7L-1/Fc fusion protein offers the advantage of being easily purified. Another advantage is the formation of disulfide bonds between the Fc regions of two separate fusion protein chains, thus creating dimers. As described supra, an aspect of the invention is soluble B7L-1 polypeptides. Soluble B7L-1 polypeptides comprise all or part of the extracellular domain of a native B7L-1 but lack the signal that would cause retention of the polypeptide on a cell membrane. Soluble B7L-1 polypeptides advantageously comprise the native (or a heterologous) signal peptide when initially synthesized to promote secretion, but the signal peptide is cleaved upon secretion of B7L-1 from the cell. Soluble B7L-1 polypeptides encompassed by the invention retain at least one functional characteristic and in one embodiment are capable of binding a counterstructure described in copending application S/N 60/095,672 filed August 7, 1998. Indeed, soluble B7L-1 may also include part of the signal or part of the cytoplasmic domain or other sequences, provided that the soluble B7L-1 protein can be secreted. Soluble B7L-1 may be identified (and distinguished from its non-soluble membrane-bound counterparts) by separating intact cells which express the desired protein from the culture medium, e.g., by centrifugation, and assaying the medium or supernatant for the presence of the desired protein. The presence of B7L-1 in the medium indicates that the protein was secreted from the cells and thus is a soluble form of the desired protein. Soluble forms of B7L-1 possess many advantages over the native bound B7L-1 protein. Purification of the proteins from recombinant host cells is feasible, since the soluble proteins are secreted from the cells. Further, soluble proteins are generally more suitable for intravenous administration. Examples of soluble B7L-1 polypeptides include those comprising a substantial portion of the extracellular domain of a native B7L-1 protein. An example of a soluble B7L-1 protein comprises amino acids 1-364 of SEQ ED NO:2 and amino acids 1-330 of SEQ ED NO:4, and 1-330 of SEQ ED NO:6. In addition, truncated soluble B7L-1 proteins comprising less than the entire extracellular domain are included in the invention. When initially expressed within a host cell, soluble B7L-1 may additionally comprise one of the heterologous signal peptides described below that is functional within the host cells employed. Alternatively, the protein may comprise the native signal peptide. In one embodiment of the invention, soluble B7L-1 can be expressed as a fusion protein comprising (from N- to C-terminus) the yeast α-factor signal peptide, a FLAG® peptide described below and in U.S. Patent No. 5,011,912, and soluble B7L-1 consisting of amino acids 21-364 of SEQ ED NO:2 or 21-330 of SEQ ED NO:4, or 21-330 of SEQ ED NO:6. This recombinant fusion protein is expressed in and secreted from yeast cells. The FLAG® peptide facilitates purification of the protein, and subsequently may be cleaved from the soluble B7L-1 using bovine mucosal enterokinase. Isolated DNA sequences encoding soluble B7L-1 proteins are encompassed by the invention. Truncated B7L-1, including soluble polypeptides, may be prepared by any of a number of conventional techniques. A desired DNA sequence may be chemically synthesized using techniques known per se. DNA fragments also may be produced by restriction endonuclease digestion of a full length cloned DNA sequence, and isolated by electrophoresis on agarose gels. Linkers containing restriction endonuclease cleavage site(s) may be employed to insert the desired DNA fragment into an expression vector, or the fragment may be digested at cleavage sites naturally present therein. The well known polymerase chain reaction procedure also may be employed to amplify a DNA sequence encoding a desired protein fragment. As a further alternative, known mutagenesis techniques may be employed to insert a stop codon at a desired point, e.g., immediately downstream of the codon for the last amino acid of the receptor-binding domain. As stated above, the invention provides isolated or homogeneous B7L-1 polypeptides, both recombinant and non-recombinant. Variants and derivatives of native B7L-1 proteins that retain the desired biological activity (e.g., the ability to bind LDCAM) may be obtained by mutations of nucleotide sequences coding for native B7L-1 polypeptides. Alterations of the native amino acid sequence may be accomplished by any of a number of conventional methods. Mutations can be introduced at particular loci by synthesizing oligonucleotides containing a mutant sequence, flanked by restriction sites enabling ligation to fragments of the native sequence. Following ligation, the resulting reconstructed sequence encodes an analog having the desired amino acid insertion, substitution, or deletion. Alternatively, oligonucleotide-directed site-specific mutagenesis procedures can be employed to provide an altered gene wherein predetermined codons can be altered by substitution, deletion or insertion. Exemplary methods of making the alterations set forth above are disclosed by Walder et al. (Gene 42:133, 1986); Bauer et al. (Gene 37:73, 1985); Craik (BioTechniques, January 1985, 12-19); Smith et al. (Genetic Engineering: Principles and Methods, Plenum Press, 1981); Kunkel (Proc. Natl. Acad. Sci. USA 82:488, 1985); Kunkel et al. (Methods in Enz mol. 154:367, 1987); and U.S. Patent Nos. 4,518,584 and 4,737,462 all of which are incorporated by reference. B7L-1 may be modified to create B7L-1 derivatives by forming covalent or aggregative conjugates with other chemical moieties, such as glycosyl groups, lipids, phosphate, acetyl groups and the like. Covalent derivatives of B7L-1 may be prepared by linking the chemical moieties to functional groups on B7L-1 amino acid side chains or at the N-terminus or C-terminus of a B7L-1 polypeptide or the extracellular domain thereof. Other derivatives of B7L-1 within the scope of this invention include covalent or aggregative conjugates of B7L-1 or its fragments with other proteins or polypeptides, such as by synthesis in recombinant culture as N-terminal or C-terminal fusions. For example, the conjugate may comprise a signal or leader polypeptide sequence (e.g. the α- factor leader of Saccharomyces) at the N-terminus of a B7L-1 polypeptide. The signal or leader peptide co-translationally or post-translationally directs transfer of the conjugate from its site of synthesis to a site inside or outside of the cell membrane or cell wall. B7L-1 polypeptide fusions can comprise peptides added to facilitate purification and identification of B7L-1. Such peptides include, for example, poly-His or the antigenic identification peptides described in U.S. Patent No. 5,011,912 and in Hopp et al., Bio/Technology 6:1204, 1988. The invention further includes B7L-1 polypeptides with or without associated native-pattern glycosylation. B7L-1 expressed in yeast or mammalian expression systems (e.g., COS-7 cells) may be similar to or significantly different from a native B7L-1 polypeptide in molecular weight and glycosylation pattern, depending upon the choice of expression system. Expression of B7L-1 polypeptides in bacterial expression systems, such as E. coli, provides non-glycosylated molecules. Equivalent DNA constructs that encode various additions or substitutions of amino acid residues or sequences, or deletions of terminal or internal residues or sequences not needed for biological activity or binding are encompassed by the invention. For example, N-glycosylation sites in the B7L-1 extracellular domain can be modified to preclude glycosylation, allowing expression of a reduced carbohydrate analog in mammalian and yeast expression systems. N-glycosylation sites in eukaryotic polypeptides are characterized by an amino acid triplet Asn-X-Y, wherein X is any amino acid except Pro and Y is Ser or Thr. The murine B7L-1 and human B7L-1 proteins comprise two such triplets. In the human long extracellular B7L-1, glycosylation sites occur at amino acids 25-27 and at amino acids 324-326. In the murine short extracellular B7L-1 and the human short extracellular as shown in SEQ ED NO:6, glycosylation sites occur at amino acids 25-27 and at amino acids 290-292. Appropriate substitutions, additions or deletions to the nucleotide sequence encoding these triplets will result in prevention of attachment of carbohydrate residues at the Asn side chain. Alteration of a single nucleotide, chosen so that Asn is replaced by a different amino acid, for example, is sufficient to inactivate an N-glycosylation site. Known procedures for inactivating N- glycosylation sites in proteins include those described in U.S. Patent 5,071,972 and EP 276,846, hereby incorporated by reference. In another example, sequences encoding Cys residues that are not essential for biological activity can be altered to cause the Cys residues to be deleted or replaced with other amino acids, preventing formation of incorrect intramolecular disulfide bridges upon renaturation. Other equivalents are prepared by modification of adjacent dibasic amino acid residues to enhance expression in yeast systems in which KEX2 protease activity is present. EP 212,914 discloses the use of site-specific mutagenesis to inactivate KEX2 protease processing sites in a protein. KEX2 protease processing sites are inactivated by deleting, adding or substituting residues to alter Arg-Arg, Arg-Lys, and Lys-Arg pairs to eliminate the occurrence of these adjacent basic residues. Lys-Lys pairings are considerably less susceptible to KEX2 cleavage, and conversion of Arg-Lys or Lys-Arg to Lys-Lys represents a conservative and preferred approach to inactivating KEX2 sites. The human B7L-1 and murine B7L-1 contain one KEX2 protease processing site at amino acids 109-110 and 200-201 of SEQ LD NO:2 and amino acids 75-76 and 166-167 of SEQ LD NO:4 and SEQ ED NO:6. Nucleic acid sequences within the scope of the invention include isolated DNA and RNA sequences that hybridize to the B7L-1 nucleotide sequences disclosed herein under conditions of moderate or severe stringency, and that encode biologically active B7L-1. Conditions of moderate stringency, as defined by Sambrook et al. Molecular Cloning: A Laboratory Manual, 2 ed. Vol. 1, pp. 101-104, Cold Spring Harbor Laboratory Press, (1989), include use of a prewashing solution of 5 X SSC, 0.5% SDS, 1.0 mM EDTA (pH 8.0) and hybridization conditions of about 55 « C, 5 X SSC, overnight. Conditions of severe stringency include higher temperatures of hybridization and washing. The skilled artisan will recognize that the temperature and wash solution salt concentration may be adjusted as necessary according to factors such as the length of the nucleic acid molecule. Due to the known degeneracy of the genetic code wherein more than one codon can encode the same amino acid, a DNA sequence may vary from that shown in SEQ ED NO:l, SEQ ED NO:3 and SEQ ED NO:5 and still encode a B7L-1 protein having the amino acid sequence of SEQ LD NO:2, SEQ ED NO:4 and SEQ ED NO:6, respectively. Such variant DNA sequences may result from silent mutations (e.g., occurring during PCR amplification), or may be the product of deliberate mutagenesis of a native sequence. The invention provides equivalent isolated DNA sequences encoding biologically active B7L-1, selected from: (a) cDNA comprising the nucleotide sequence presented in SEQ LD NO:l, SEQ ED NO:3, or SEQ ED NO:5; (b) DNA capable of hybridization to a DNA of (a) under moderately stringent conditions and that encodes biologically active B7L-1; and, (c) DNA that is degenerate as a result of the genetic code to a DNA defined in (a), or (b) and that encodes biologically active B7L-1. B7L-1 proteins encoded by such DNA equivalent sequences are encompassed by the invention. DNAs that are equivalents to the DNA sequence of SEQ LD NO:l, SEQ LD NO:3, or SEQ ED NO: 5 will hybridize under moderately and severely stringent conditions to DNA sequences that encode polypeptides comprising SEQ LD NO:2, SEQ LD NO:4, SEQ ED NO:6 or fragments and variants of SEQ ED NO:2, 4, and 6. Examples of B7L-1 proteins encoded by such DNA, include, but are not limited to, B7L-1 fragments (soluble or membrane bound) and B7L-1 proteins comprising inactivated N-glycosylation site(s), inactivated KEX2 protease processing site(s), or conservative amino acid substitution(s), as described above. B7L-1 proteins encoded by DNA derived from other mammalian species, wherein the DNA will hybridize to the cDNA of SEQ ED NO:l, SEQ ED NO:3, or SEQ ED NO:5 are also encompassed by the present invention. Variants possessing the ability to bind B7L-1 counterstructures, or binding partners, e.g. LDCAM may be identified by any suitable assay. Biological activity of B7L-1 may be determined, for example, by competition for binding to the ligand binding domain of LDCAM (i.e. competitive binding assays). One type of a competitive binding assay for a B7L-1 polypeptide uses a radiolabeled, soluble B7L-1 and intact cells expressing a B7L-1 counterstructure, or cells expressing LDCAM. Instead of intact cells, one could substitute soluble B7L-1 counterstructure Fc fusion proteins (such as a LDCAM/Fc fusion protein) bound to a solid phase through the interaction of a Protein A, Protein G or an antibody to the counterstructure or Fc portions of the molecule, with the Fc region of the fusion protein. Another type of competitive binding assay utilizes radiolabeled soluble B7L-1 binding proteins and intact cells expressing B7L-1. Competitive binding assays can be performed following conventional methodology. For example, radiolabeled B7L-1 can be used to compete with a putative B7L-1 homologue to assay for binding activity against a surface-bound B7L-1 binding protein or a binding counterstructure, e.g. LDCAM. Qualitative results can be obtained by competitive autoradiographic plate binding assays, or Scatchard plots may be utilized to generate quantitative results. Binding proteins for a B7L-1 counterstructure, such as B7L-1 itself and anti-B7L- 1 ligand antibodies, can be bound to a solid phase such as a column chromatography matrix or a similar substrate suitable for identifying, separating or purifying cells that express the B7L-1 binding protein on their surface. Binding of a B7L-1 counterstructure binding protein to a solid phase contacting surface can be accomplished by using a number of different techniques. For example, a B7L-1/Fc fusion protein can be constructed and then attached to the solid phase through the interaction of Protein A or Protein G. Various other means for fixing proteins to a solid phase are well known in the art and are suitable arrogant for use in the present invention. For example, magnetic microspheres can be coated with B7L-1 and held in the incubation vessel through a magnetic field. Suspensions of cell mixtures containing a B7L-1 counterstructure- expressing cells are contacted with the solid phase that has B7L-1 polypeptides thereon. Cells having B7L-1 counterstructure on their surface bind to the fixed B7L-1 and unbound cells then are washed away. This affinity-binding method is useful for purifying, screening or separating such B7L-1 counterstructure-expressing cells from solution. In particular, this method is useful for separating cells expressing LDCAM from cells that do not express a B7L-1 binding protein or B7L-1 counter structure. Methods of releasing positively selected cells from the solid phase are known in the art and encompass, for example, the use of enzymes. Such enzymes are preferably non-toxic and non-injurious to the cells and are preferably directed to cleaving the cell- surface binding partner. In the case of LDCAM-B7L-1 interactions, the enzyme preferably would cleave the LDCAM thereby freeing the resulting cell suspension from the "foreign" B7L-1 material. Alternatively, mixtures of cells suspected of containing LDCAM+ cells first can be incubated with biotinylated B7L-1. Incubation periods are typically at least one hour in duration to ensure sufficient binding to B7L-1 The resulting mixture then is passed through a column packed with avidin-coated beads, whereby the high affinity of biotin for avidin provides the binding of the cell to the beads. Use of avidin-coated beads is known in the art. See Berenson, et al. J. Cell. Biochem., 10D:239 (1986). Wash of unbound material and the release of the bound cells is performed using conventional methods. As described above, B7L-1 can be used to separate cells expressing a protein to which it binds. In an alternative method, B7L-1 or an extracellular domain or a fragment thereof can be conjugated to a detectable moiety such as 125 I to detect cells expressing a B7L-1 binding protein. Radiolabeling with 125 I can be performed by any of several standard methodologies that yield a functional 125 I-B7L-1 molecule labeled to high specific activity. Or an iodinated or biotinylated antibody against B7L-1 region or the Fc region of the molecule could be used. Another detectable moiety such as an enzyme that can catalyze a colorimetric or fluorometric reaction, biotin or avidin may be used. For example, cells to be tested for LDCAM expression can be contacted with labeled B7L-1. After incubation, unbound labeled B7L-1 is removed and binding is measured using the detectable moiety. The binding characteristics of B7L-1, B7L-1 fragments and B7L-1 variants may also be determined using the a labeled B7L-1 binding protein (for example, 125 I- LDCAM:Fc) in competition assays similar to those described above. In this case, however, intact cells expressing LDCAM bound to a solid substrate, are used to measure the extent to which a sample containing a putative B7L-1 variant competes for binding with a B7L-1 binding protein. Other means of assaying for B7L-1 include the use of anti-B7L-l antibodies, cell lines that proliferate in response to B7L-1, or recombinant cell lines that express LDCAM and proliferate in the presence of B7L- 1. The B7L-1 proteins disclosed herein also may be employed to measure the biological activity of LDCAM proteins in terms of their binding affinity for B7L-1. As one example, B7L-1 may be used in determining whether biological activity is retained after modification of a LDCAM (e.g., chemical modification, truncation, mutation, etc.). The biological activity of a LDCAM protein thus can be ascertained before it is used in a research study, or possibly in the clinic, for example. B7L-1 proteins find use as reagents that may be employed by those conducting "quality assurance" studies, e.g., to monitor shelf life and stability of proteins to which B7L-1 binds under different conditions. To illustrate, B7L-1 may be employed in a binding affinity study to measure the biological activity of its binding protein that has been stored at different temperatures, or produced in different cell types. The binding affinity of the modified protein for B7L-1 is compared to that of an unmodified protein to detect any adverse impact of the modifications on biological activity of B7L-1 binding protein. B7L-1 polypeptides also find use as carriers for delivering agents attached thereto to cells bearing its counter structure, LDCAM or other cell surface receptor to which B7L-1 binds. For example, soluble forms of B7L-1 can be conjugated to agents such as toxins, inhibitors, or antigens and the resulting conjugated agent can be delivered to cells carrying the B7L-1 counterstructure (LDCAM). Such cells include lymphoid dendritic cells that are known product EL- 12 during an immune response and can inhibit T cell cytokine production. Thus, these cells are a target for induction of antigen tolerance and B7L-1 conjugates can be used to block, enhance or modify lymphoid dendritic cell activity. Diagnostic and therapeutic agents that may be attached to a B7L-1 polypeptide include, but are not limited to, drugs, toxins, radionuclides, chromophores, enzymes that catalyze a colorimetric or fluorometric reaction, and the like, with the particular agent being chosen according to the intended application. Examples of drugs include those used in treating various forms of cancer, e.g., nitrogen mustards such as L-phenylalanine nitrogen mustard or cyclophosphamide, intercalating agents such as cis- diaminodichloroplatinum, antimetabolites such as 5-fluorouracil, vinca alkaloids such as vincristine, and antibiotics such as bleomycin, doxorubicin, daunorubicin, and derivatives thereof. Among the toxins are ricin, abrin, diptheria toxin, Pseudomonas aeruginosa exotoxin A, ribosomal inactivating proteins, mycotoxins such as trichothecenes, and derivatives and fragments (e.g., single chains) thereof. Radionuclides suitable for diagnostic use include, but are not limited to, 123 I, 131 I, 99mτ c> l l lι n , and 76 Br. Radionuclides suitable for therapeutic use include, but are not limited to, 131 I, 211 At, 77 Br , 186 Re , 188 Re5 212pb 5 212 B i, 109p d; 64 Cu> and 67 Cu . Such agents may be attached to the B7L-1 by any suitable conventional procedure. B7L-1, being a protein, comprises functional groups on amino acid side chains that can be reacted with functional groups on a desired agent to form covalent bonds, for example. Alternatively, the protein or agent may be derivatized to generate or attach a desired reactive functional group. The derivatization may involve attachment of one of the bifunctional coupling reagents available for attaching various molecules to proteins (Pierce Chemical Company, Rockford, Illinois). A number of techniques for radiolabeling proteins are known. Radionuclide metals may be attached to B7L-1 by using a suitable bifunctional chelating agent, for example. Conjugates comprising B7L-1 and a suitable diagnostic or therapeutic agent (preferably covalently linked) are thus prepared. The conjugates are administered or otherwise employed in an amount appropriate for the particular application. Another use of the B7L-1 of the present invention is as a research tool for studying the role that B7L-1, in conjunction with LDCAM, may play in T cell signaling and proliferation . The B7L-1 polypeptides of the present invention also may be employed in in vitro assays for detection of LDCAM or B7L-1 or the interactions thereof. As discussed above, when various tissues were analyzed for mRNA for B7L-1, transcripts were detected in human bone marrow derived CD34+ derived dendritic cells and peripheral blood derived dendritic cells, B cells after stimulation with CD40L, brain and mouse splenic dendritic cells CD40L stimulated splenic B cells and brain. Because of the restricted expression pattern of B7L-1, antibodies to B7L-1 can be used to identify, isolate, and purify potent antigen presenting cells, including dendritic cells and CD40 ligand activated B cells. Additionally, the presence and level of mRNA for B7L-1 can be exploited to determine the purity of bone marrow derived and blood derived dendritic cell preparations. Other uses of antibodies to B7L-1 molecules include targeting antigens to myeloid dendritic cells or eliminating myeloid dendritic cells with anti-B7L-l antibody mediated depletion or with an conjugate of a toxin and the antibody. Soluble fragments of B7L-1, including, but not restricted to the extracellular domains of SEQ ED NO:2, SEQ ED NO:4, and SEQ ED NO:6 can be used to enhance or inhibit the activity of lymphoid dendritic cells and/or B cells activated for presentation by CD40-L. One embodiment of the present invention is directed to a method of treating disorders mediated by the interaction of B7L-1 and a binding partner and involves administering B7L-1 to a mammal having the disorder. B7L-1 polypeptides of the invention can be formulated according to known methods used to prepare pharmaceutically useful compositions. B7L-1 can be combined in admixture, either as the sole active material or with other known active materials, with pharmaceutically suitable diluents (e.g., Tris-HCl, acetate, phosphate), preservatives (e.g., Thimerosal, benzyl alcohol, parabens), emulsifiers, solubilizers, adjuvants and/or carriers. Suitable carriers and their formulations are described in Remington's Pharmaceutical Sciences, 16th ed. 1980, Mack Publishing Co. In addition, such compositions can contain B7L-1 complexed with polyethylene glycol (PEG), metal ions, or incorporated into polymeric compounds such as polyacetic acid, polyglycolic acid, hydrogels, etc., or incorporated into liposomes, microemulsions, micelles, unilamellar or multilamellar vesicles, erythrocyte ghosts or spheroblasts. Such compositions will influence the physical state, solubility, stability, rate of in vivo release, and rate of in vivo clearance of B7L-1. B7L-1 can also be conjugated to antibodies against tissue-specific receptors, ligands or antigens, or coupled to ligands of tissue-specific receptors. For tumor cells on which LDCAM is found, B7L-1 may be conjugated to a toxin whereby B7L-1 is used to deliver the toxin to the specific cell site. B7L-1 can be administered topically, parenterally, or by inhalation. The term "parenteral" includes subcutaneous injections, intravenous, intramuscular, intracisternal injection, or infusion techniques. These compositions will typically contain an effective amount of the B7L-1, alone or in combination with an effective amount of any other active material. Such dosages and desired drug concentrations contained in the compositions may vary depending upon many factors, including the intended use, patient's body weight and age, and route of administration. Preliminary doses can be determined according to animal tests, and the scaling of dosages for human administration can be performed according to art-accepted practices. B7L-1 polypeptides may exist as oligomers, such as covalently-linked or non- covalently-linked dimers or trimers. Oligomers may be linked by disulfide bonds formed between cysteine residues on different B7L-1 polypeptides. In one embodiment of the invention, a B7L-1 dimer is created by fusing B7L-1 to the Fc region of an antibody (e.g., IgGl) in a manner that does not interfere with binding of B7L-1 to the B7L-1 ligand- binding domain. The Fc polypeptide preferably is fused to the C-terminus of a soluble B7L-1 (comprising only the receptor-binding). General preparation of fusion proteins comprising heterologous polypeptides fused to various portions of antibody-derived polypeptides (including the Fc domain) has been described, e.g., by Ashkenazi et al. (PNAS USA 88:10535, 1991) and Byrn et al. (Nature 344:677, 1990), hereby incorporated by reference. A gene fusion encoding the B7L-l:Fc fusion protein is inserted into an appropriate expression vector. B7L-l:Fc fusion proteins are allowed to assemble much like antibody molecules, whereupon interchain disulfide bonds form between Fc polypeptides, yielding divalent B7L-1. If fusion proteins are made with both heavy and light chains of an antibody, it is possible to form a B7L-1 oligomer with as many as four B7L-1 extracellular regions. Alternatively, one can link two soluble B7L-1 domains with a peptide linker. Suitable host cells for expression of B7L-1 polypeptides include prokaryotes, yeast or higher eukaryotic cells. Appropriate cloning and expression vectors for use with bacterial, fungal, yeast, and mammalian cellular hosts are described, for example, in Pouwels et al. Cloning Vectors: A Laboratory Manual, Elsevier, New York, (1985). Cell-free translation systems could also be employed to produce B7L-1 polypeptides using RNAs derived from DNA constructs disclosed herein. Prokaryotes include gram negative or gram positive organisms, for example, E. coli or Bacilli. Suitable prokaryotic host cells for transformation include, for example, E. coli, Bacillus subtilis, Salmonella typhimurium, and various other species within the genera Pseudomonas, Streptomyces, and Staphylococcus. In a prokaryotic host cell, such as E. coli, a B7L-1 polypeptide may include an N-terminal methionine residue to facilitate expression of the recombinant polypeptide in the prokaryotic host cell. The N- terminal Met may be cleaved from the expressed recombinant B7L-1 polypeptide. B7L-1 polypeptides may be expressed in yeast host cells, preferably from the Saccharomyces genus (e.g., S. cerevisiae). Other genera of yeast, such as Pichia , K. lactis or Kluyveromyces, may also be employed. Yeast vectors will often contain an origin of replication sequence from a 2μ yeast plasmid, an autonomously replicating sequence (ARS), a promoter region, sequences for polyadenylation, sequences for transcription termination, and a selectable marker gene. Suitable promoter sequences for yeast vectors include, among others, promoters for metallothionein, 3-phosphoglycerate kinase (Hitzeman et al., J. Biol. Chem. 255:2073, 1980) or other glycolytic enzymes (Hess et al., J. Adv. Enzyme Reg. 7:149, 1968; and Holland et al., Biochem. 17:4900, 1978), such as enolase, glyceraldehyde-3-phosphate dehydrogenase, hexokinase, pyruvate decarboxylase, phosphofructokinase, glucose-6-phosphate isomerase, 3-phosphoglycerate mutase, pyruvate kinase, triosephosphate isomerase, phosphoglucose isomerase, and glucokinase. Other suitable vectors and promoters for use in yeast expression are further described in Hitzeman, EPA-73,657 or in Fleer et. al., Gene, 107:285-195 (1991); and van den Berg et. al., Bio/Technology, 8:135-139 (1990). Another alternative is the glucose-repressible ADH2 promoter described by Russell et al. (J. Biol. Chem. 258:2674, 1982) and Beier et al. (Nature 300:724, 1982). Shuttle vectors replicable in both yeast and E. coli may be constructed by inserting DNA sequences from pBR322 for selection and replication in E. coli (Amp r gene and origin of replication) into the above-described yeast vectors. The yeast α-f actor leader sequence may be employed to direct secretion of the B7L-1 polypeptide. The α-factor leader sequence is often inserted between the promoter sequence and the structural gene sequence. See, e.g., Kurjan et al., Cell 30:933, 1982; Bitter et al., Proc. Natl. Acad. Sci. USA 81:5330, 1984; U. S. Patent 4,546,082; and EP 324,274. Other leader sequences suitable for facilitating secretion of recombinant polypeptides from yeast hosts are known to those of skill in the art. A leader sequence may be modified near its 3' end to contain one or more restriction sites. This will facilitate fusion of the leader sequence to the structural gene. Yeast transformation protocols are known to those of skill in the art. One such protocol is described by Hinnen et al, Proc. Natl. Acad. Sci. USA 75:1929, 1978. The Hinnen et al. protocol selects for Trp + transformants in a selective medium, wherein the selective medium consists of 0.67% yeast nitrogen base, 0.5% casamino acids, 2% glucose, 10 μg/ml adenine and 20 μg/ml uracil. Yeast host cells transformed by vectors containing ADH2 promoter sequence may be grown for inducing expression in a "rich" medium. An example of a rich medium is one consisting of 1% yeast extract, 2% peptone, and 1% glucose supplemented with 80 μg/ml adenine and 80 μg/ml uracil. Derepression of the ADH2 promoter occurs when glucose is exhausted from the medium. Mammalian or insect host cell culture systems could also be employed to express recombinant B7L-1 polypeptides. Baculovirus systems for production of heterologous proteins in insect cells are reviewed by Luckow and Summers, Bio/Technology 6:47 (1988). Established cell lines of mammalian origin also may be employed. Examples of suitable mammalian host cell lines include the COS-7 line of monkey kidney cells (ATCC CRL 1651) (Gluzman et al., Cell 23:175, 1981), L cells, C127 cells, 3T3 cells (ATCC CCL 163), Chinese hamster ovary (CHO) cells, HeLa cells, and BHK (ATCC CRL 10) cell lines, and the CV-l/EBNA-1 cell line derived from the African green monkey kidney cell line CVI (ATCC CCL 70) as described by McMahan et al. (EMBO J. 10: 2821, 1991). Transcriptional and translational control sequences for mammalian host cell expression vectors may be excised from viral genomes. Commonly used promoter sequences and enhancer sequences are derived from Polyoma virus, Adenovirus 2, Simian Virus 40 (SV40), and human cytomegalovirus. DNA sequences derived from the SV40 viral genome, for example, SV40 origin, early and late promoter, enhancer, splice, and polyadenylation sites may be used to provide other genetic elements for expression of a structural gene sequence in a mammalian host cell. Viral early and late promoters are particularly useful because both are easily obtained from a viral genome as a fragment which may also contain a viral origin of replication (Fiers et al., Nature 273:113, 1978). Smaller or larger SV40 fragments may also be used, provided the approximately 250 bp sequence extending from the Hind III site toward the Bgl I site located in the SV40 viral origin of replication site is included. Exemplary expression vectors for use in mammalian host cells can be constructed as disclosed by Okayama and Berg (Mol. Cell. Biol. 3:280, 1983). A useful system for stable high level expression of mammalian cDNAs in C 127 murine mammary epithelial cells can be constructed substantially as described by Cosman et al. (Mol. Immunol. 23:935, 1986). A useful high expression vector, PMLSV N1/N4, described by Cosman et al., Nature 312:768, 1984 has been deposited as ATCC 39890. Additional useful mammalian expression vectors are described in EP-A-0367566, and in U.S. Patent Application Serial No. 07/701,415, filed May 16, 1991, incorporated by reference herein. The vectors may be derived from retroviruses. In place of the native signal sequence, and in addition to an initiator methionine, a heterologous signal sequence may be added, such as the signal sequence for LL-7 described in United States Patent 4,965,195; the signal sequence for IL-2 receptor described in Cosman et al., Nature 312:768 (1984); the LL-4 signal peptide described in EP 367,566; the type I EL-l receptor signal peptide described in U.S. Patent 4,968,607; and the type II EL-l receptor signal peptide described in EP 460,846. B7L-1 as an isolated, purified or homogeneous protein according to the invention may be produced by recombinant expression systems as described above or purified from naturally occurring cells. B7L-1 can be purified to substantial homogeneity, as indicated by a single protein band upon analysis by SDS-polyacrylamide gel electrophoresis (SDS- PAGE). One process for producing B7L-1 comprises culturing a host cell transformed with an expression vector comprising a DNA sequence that encodes B7L-1 under conditions sufficient to promote expression of B7L-1. B7L-1 is then recovered from culture medium or cell extracts, depending upon the expression system employed. As is known to the skilled artisan, procedures for purifying a recombinant protein will vary according to such factors as the type of host cells employed and whether or not the recombinant protein is secreted into the culture medium. For example, when expression systems that secrete the recombinant protein are employed, the culture medium first may be concentrated using a commercially available protein concentration filter, for example, an Amicon or Millipore Pellicon ultrafiltration unit. Following the concentration step, the concentrate can be applied to a purification matrix such as a gel filtration medium. Alternatively, an anion exchange resin can be employed, for example, a matrix or substrate having pendant diethylaminoethyl (DEAE) groups. The matrices can be acrylamide, agarose, dextran, cellulose or other types commonly employed in protein purification. Alternatively, a cation exchange step can be employed. Suitable cation exchangers include various insoluble matrices comprising sulfopropyl or carboxymethyl groups. Sulfopropyl groups are preferred. Finally, one or more reversed-phase high performance liquid chromatography (RP-HPLC) steps employing hydrophobic RP-HPLC media, (e.g., silica gel having pendant methyl or other aliphatic groups) can be employed to further purify B7L-1. Some or all of the foregoing purification steps, in various combinations, are well known and can be employed to provide a substantially homogeneous recombinant protein. It is possible to utilize an affinity column comprising the B7L-1 binding domain of a protein to which B7L-1 binds, such as LDCAM, to affinity-purify expressed B7L-1 polypeptides. B7L-1 polypeptides can be removed from an affinity column using conventional techniques, e.g., in a high salt elution buffer and then dialyzed into a lower salt buffer for use or by changing pH or other components depending on the affinity matrix utilized. Alternatively, the affinity column may comprise an antibody that binds B7L-1. Example 5 describes a procedure for employing B7L-1 of the invention to generate monoclonal antibodies directed against B7L-1. Recombinant protein produced in bacterial culture can be isolated by initial disruption of the host cells, centrifugation, extraction from cell pellets if an insoluble polypeptide, or from the supernatant fluid if a soluble polypeptide, followed by one or more concentration, salting-out, ion exchange, affinity purification or size exclusion chromatography steps. Finally, RP-HPLC can be employed for final purification steps. Microbial cells can be disrupted by any convenient method, including freeze-thaw cycling, sonication, mechanical disruption, or use of cell lysing agents. Transformed yeast host cells are preferably employed to express B7L-1 as a secreted polypeptide in order to simplify purification. Secreted recombinant polypeptide from a yeast host cell fermentation can be purified by methods analogous to those disclosed by Urdal et al. (J. Chromatog. 296:171, 1984). Urdal et al. describe two sequential, reversed-phase HPLC steps for purification of recombinant human LL-2 on a preparative HPLC column. Useful fragments of the B7L-1 nucleic acids include antisense or sense oligonucleotides comprising a single-stranded nucleic acid sequence (either RNA or DNA) capable of binding to target B7L-1 mRNA (sense) or B7L-1 DNA (antisense) sequences. Antisense or sense oligonucleotides, according to the present invention, comprise a fragment of the coding region of B7L-1 cDNA. Such a fragment generally comprises at least about 14 nucleotides, preferably from about 14 to about 30 nucleotides. The ability to derive an antisense or a sense oligonucleotide, based upon a cDNA sequence encoding a given protein is described in, for example, Stein and Cohen (Cancer Res. 48:2659, 1988) and van der Krol et al. (BioTechniques 6:958, 1988). Binding of antisense or sense oligonucleotides to target nucleic acid sequences results in the formation of duplexes that block transcription or translation of the target sequence by one of several means, including enhanced degradation of the duplexes, premature termination of transcription or translation, or by other means. The antisense oligonucleotides thus may be used to block expression of B7L-1 proteins. Antisense or sense oligonucleotides further comprise oligonucleotides having modified sugar- phosphodiester backbones (or other sugar linkages, such as those described in WO91/06629) and wherein such sugar linkages are resistant to endogenous nucleases. Such oligonucleotides with resistant sugar linkages are stable in vivo (i.e., capable of resisting enzymatic degradation) but retain sequence specificity to be able to bind to target nucleotide sequences. Other examples of sense or antisense oligonucleotides include those oligonucleotides which are covalently linked to organic moieties, such as those described in WO 90/10448, and other moieties that increases affinity of the oligonucleotide for a target nucleic acid sequence, such as poly-(L-lysine). Further still, intercalating agents, such as ellipticine, and alkylating agents or metal complexes may be attached to sense or antisense oligonucleotides to modify binding specificities of the antisense or sense oligonucleotide for the target nucleotide sequence. Antisense or sense oligonucleotides may be introduced into a cell containing the target nucleic acid sequence by any gene transfer method, including, for example, CaPO4-mediated DNA transfection, electroporation, or by using gene transfer vectors such as Epstein-Barr virus. Antisense or sense oligonucleotides are preferably introduced into a cell containing the target nucleic acid sequence by insertion of the antisense or sense oligonucleotide into a suitable retroviral vector, then contacting the cell with the retrovirus vector containing the inserted sequence, either in vivo or ex vivo. Suitable retroviral vectors include, but are not limited to, those derived from the murine retrovirus M-MuLV, N2 (a retrovirus derived from M-MuLV), or the double copy vectors designated DCT5A, DCT5B and DCT5C (see PCT Application US 90/02656). Sense or antisense oligonucleotides also may be introduced into a cell containing the target nucleotide sequence by formation of a conjugate with a ligand binding molecule, as described in WO 91/04753. Suitable ligand binding molecules include, but are not limited to, cell surface receptors, growth factors, other cytokines, or other ligands that bind to cell surface receptors. Preferably, conjugation of the ligand binding molecule does not substantially interfere with the ability of the ligand binding molecule to bind to its corresponding molecule or receptor, or block entry of the sense or antisense oligonucleotide or its conjugated version into the cell. Alternatively, a sense or an antisense oligonucleotide may be introduced into a cell containing the target nucleic acid sequence by formation of an oligonucleotide-lipid complex, as described in WO 90/10448. The sense or antisense oligonucleotide-lipid complex is preferably dissociated within the cell by an endogenous lipase. In addition to the above, the following examples are provided to illustrate particular embodiments and not to limit the scope of the invention. EXAMPLE 1 Identifying human B7L-1 Long Extracellular Form The DNA encoding human B7-1, a T cell costimulatory molecule and a SMARTLIST protein, was used in a BLAST sequence analysis to identify ESTs having homology to B7-1. This BLAST analysis resulted in the identification of two GENBANK ESTs, File No. T08949 (EST06841) and File No. T32071 (EST 43348), TIGR (?) having low but significant homology to a portion of the B7-1 molecule. The two GENBANK EST sequences were used to design PCR primers for probing cDNA libraries in order to identify a cDNA source for the ESTs. The primer sequences were as follows: 5' AGGGCGAGTACACCTG 3' (SEQ ID NO:7) (sense bases 22-37 of EST T32071) 5' GTGGATCTGTCAGCTCC 3' (SEQ ID NO:8) (anti-sense bases 376-360 of EST T32071) Oligonucleotide primers identified in SEQ ED NO:7 and SEQ ED NO:8 were used to screen cDNA libraries by PCR. Of the 16 cDNA libraries examined, PCR product was obtained only from a human brain lambda library (purchased from Clonetech HL3002b). The product was cloned into bacteria and sequenced to verify that the product included an open reading frame. The cloned EST sequence was radioactively labeled and used to probe the brain lambda library using standard probing techniques, in order to isolate clones from the brain lambda library that included the EST derived sequence. One clone, designated 32071-2, extended the EST sequence to the 5' end by 140 bases. A subsequent BLAST analysis of the GENBANK EST database using the extended EST sequence as the query sequence lead to the identification of an overlapping EST (HI 5268) derived from IMAGE Consortium clone #44904. The IMAGE Consortium clone was obtained (Research Genetics, 07002) and fully sequenced to reveal an open reading frame and a new full length human cDNA sequence encoding B7L-1. SEQ ED NO: 1 provides the complete cDNA of human B7L-1 and SEQ ID NO:2 provides the amino acid sequence encoded by the cDNA. The encoded full length protein has a predicted extracellular region of amino acid 364 amino acids (1-364), including a leader sequence of 20 amino acids (1-20); a transmembrane domain of 21 amino acids (365-385) and a cytoplasmic domain of 47 amino acids (386- 432). EXAMPLE 2 Expressing Human Long Extracellular B7L-1 To prepare a vector construct for expressing human long extracellular B7L-1, the entire coding region of SEQ ED NO:l was obtained from clone #49904. First, the B7L-1 insert was excised from the clone using the HindlLI and Notl sites on the clone. Then the oligonucleotides identified in SEQ LD NO:9 and SEQ ED NO: 10 were used as adapters to change the HindlLI cohesive end to a Sail cohesive end by annealing and ligating the oligonucleotides to the excised insert containing nucleotide residues 1-1820 of SEQ ED NO:l. The resulting construct was ligated into a pDC409 expression vector that had been cut with Sail and Notl. The expression vector construct was then transfected in CV1/EBNA cells and B7L-1 was expressed using techniques described in McMahan et al., EMBO J. 70:2821,1991. After the cells were shocked and incubated for several days, cell supernatants containing any soluble form of the protein were collected and the B7L-1 protein was recovered using HPLC techniques. To recover forms of B7L-1 that are membrane bound, the transfected cells were harvested, fixed in 1% paraformaldehyde, washed and used in their intact form. EXAMPLE 3 Isolating Murine and Human Short Extracellular B7L-1 To identify a cDNA source of murine B7L-1, sequence information obtained from the human B7L-1 identified in EXAMPLE 1 above was used to design PCR primers, one of which is the oligonucleotides of SEQ LD NO: 12; the second of which is disclosed in SEQ LD NO: 13. These PCR primers were used to identify cDNA libraries that give PCR products when used as templates in PCR reactions. PCR product was identified in PCR reactions using mouse brain lambda cDNA library (Clonetech ML3000a). The mouse brain lambda cDNA library was screened and a clone was identified and sequenced using standard techniques. The sequenced clone lacked the 5 'end of the coding region as determined by comparing the clone with the human B7L-1. RT-PCR off of mouse brain RNA using the lambda gtlO vector entry oligonucleotide and the human B7L-1 specific oligonucleotide of SEQ ED NO:7 extended the sequence from the 3' to the end of the sequence and 5' to nearly full length. The clone encodes an open reading frame that begins at a position analogous to amino acid residue 9 of human B7L-1 (SEQ ED NO:2) and terminates at a position that is analogous to the terminal amino acid of the human B7L-1. The cloned murine B7L-1 cDNA having a composite murine/human leader is provided in SEQ ED NO:3 and its encoded polypeptide is provided in SEQ ED NO:4. The composite murine/human leader sequence includes 7 amino acids of the human sequence and 12 amino acids of the murine sequence. The murine B7L-1 clone is 95% identical to the human B7L-1 of SEQ LD NO:l as determined by the GCG GAP program.. The murine clone has a single gap and represents a shorter splice variant of B7L-1. To investigate the existence of a human shorter splice variant of B7L-1, the oligonucleotide primers disclosed in SEQ LD NO: 14 and SEQ ED NO: 15 were used in RT-PCR reactions to probe human brain RNA. Using standard ethidium bromide agarose gel and Southern Blot analyses methodologies, a shorter splice form was shown to exist and predominate. The product of the RT-PCR reaction was cloned and subjected to standard dideoxynucleotide terminator sequence analysis. SEQ ED NO:5 provides the nucleotide sequence of the human short extracellular form of B7L-1 and SEQ ED NO:6 provides the encoded amino acid sequence. The results of this work indicate that there are at least 2 different splice forms of B7L-1 and the predominant form is the short form which was first identified while cloning the murine B7L-1 homologue. EXAMPLE 4 Expressing Murine Short Extracellular Form B7L-1 Polypeptide The following describes methods for expressing a soluble fragment and the full length membrane bound murine short extracellular form of B7L-1. To prepare a vector construct for expressing the full length membrane bound murine short extracellular form of B7L-1 the coding region of SEQ ED NO: 3 was prepared using a PCR SOEing technique. The oligonucleotides used are described in SEQ ED NO:16, SEQ ED NO:17 and SEQ LD NO:18. The inner 5' oligonucleotide (SEQ LD NO: 16) included bases to code for the initiator Met and 5 additional amino acids that form the first 6 residues of the long extracellular human B7L-1 signal peptide. The outer 5 'oligonucleotide (SEQ LD NO 17) was present in a 9 fold excess to the inner 5' oligonucleotide and included a Sail restriction site. The 3' oligonucleotide, described in SEQ ED NO: 18, included a Notl restriction site. The PCR SOEing product was subjected to a restriction enzyme digest with Sail and Notl and then ligated into a pDC412 expression vector. The expression vector was then transfected in DH10B E coli by electroporation. B7L-1 was expressed using techniques described in McMahan et al., EMBO J. i 0:2821, 1991. To recover forms of B7L-1 that are membrane bound, the transfected cells were harvested, fixed in 1% paraformaldehyde, washed and used in their intact form. To prepare a vector construct for expressing a soluble murine short extracellular B7L-1 polypeptide, the extracellular coding region of SEQ ED NO:3 was prepared using a PCR SOEing technique. The oligonucleotides used were identical to those used to prepare the vector construct for the murine full length membrane bound polypeptide except that the oligonucleotide of SEQ LD NO: 12 was the 3' oligonucleotide, thus replacing SEQ ED NO: 18. The PCR SOEing product was subjected to a restriction enzyme digest with the Sail and Bglll sites and then ligated into a Bluescript SK vector. This clone fusion was excised with a SalI//BglII double digestion and ligated into a Sall/Bglll digested pDC412 expression vector. The expression vector was then transfected in DH10B E coli by electroporation and the soluble murine B7L-1 polypeptide was expressed as described above for the full length murine B7L-1 protein. EXAMPLE 5 Preparing B7L-1/Fc Fusion Protein The following describes generating a human B7L-1/Fc protein which was used to study binding characteristics of B7L-1. The fusion protein includes the predicted extracellular region of human B7L-1 and the mutein human Fc region. To isolate the nucleotides that encode the extracellular domain of SEQ LD NO:2 (nucleotides 108-1249 of SEQ LD NO:l), oligonucleotides that flank the extracellular region of B7L-1 (SEQ ED NO: 11 and SEQ ED NO: 12) were used as primers in a PCR reaction to obtain a PCR product from clone #44904 which was the template in the reaction. The resulting PCR product was digested with Sail and Bglll restriction enzymes at the Sail and Bglll sites incorporated by the primers. The resulting fragment was ligated into an expression vector (pDC409) containing the human IgGl Fc region mutated to lower Fc receptor binding. The resulting DNA construct was transfected into the monkey kidney cell lines CV-1/EBNA (with co-transfection of psv3neo). After 7 days of culture in medium containing 0.5% low immunoglobulin bovine serum, a solution of 0.2% azide was added to the supernatant and the supernatant was filtered through a 0.22 μm filter. Then approximately 1L of culture supernatant was passed through a BioCad Protein A HPLC protein purification system using a 4.6 x 100 mm Protein A column (POROS 20A from PerSeptive Biosystems) at 10 mL/min. The Protein A column binds the Fc Portion of the fusion protein in the supernatant, immobilizing the fusion protein and allowing other components of the supernatant to pass through the column. The column was washed with 30 mL of PBS solution and bound fusion protein was eluted from the HPLC column with citric acid adjusted to pH 3.0. Eluted purified fusion protein was neutralized as it eluted using 1M HEPES solution at pH 7.4. EXAMPLE 6 Preparing Murine Short Extracellular B7L-1 Fc Fusion Protein The following describes preparing a murine Fc fusion protein that included the soluble extracellular portion of the murine short extracellular B7L-1 and the mutein Fc peptide described above in Example 5. The extracellular domain coding region of the murine extracellular short B7L-1 was excised from the vector described in Example 4 using Sail and Bglll restriction enzymes. The excised fragment was ligated into a pDC412 expression vector that included the human IgGlFc region. The resulting DNA construct was transfected into the monkey kidney cell lines CV-1/EBNA. The cells were cultured and the fusion protein collected and purified as described in Example 5. EXAMPLE 7 Preparing B7L-l/polyHis Fusion Protein The following describes preparing a human B7L-l/polyHis fusion protein (B7L- 1/polyHis). The process included preparing a DNA construct that encodes the fusion protein, transfecting a cell line with the DNA construct, and harvesting supernatants from the transfected cells. The oligonucleotide primers described in SEQ ED NO: 12 and SEQ LD NO: 13, containing a Spel restriction site, were used to isolate the nucleotides encoding amino acids 1-364 of SEQ ID NO:2 from the IMAGE Consortium clone (H15268, clone #49904). The PCR product was digested with Spel and Pstl restriction enzymes, the Pstl enzyme cutting the PCR product at a site within the B7L-1 coding region. The excised product was ligated into a Spel/Pstl digested Bluescript based vector containing a CMV viral leader upstream and in-frame with the Spel site. The viral leader and the B7L-1 encoding cDNA construct was excised from the vector using Sail and Pstl restriction enzyme digestions and the excised construct was then ligated in a three way ligation with a Pstl/Notl fragment containing the remainder of the human B7L-1 cDNA and a pDC409 expression vector (McMahon et al., EMBO J. 10:2821,1991). The polyHis fusion construct was prepared using an oligonucleotide primer that primes upstream in the vector prepared as described above (?) and a primer which includes l)nucleotides complementary to those present in human B7L-1 cDNA that are positioned just before the transmembrane domain; 2)nucleotides complementary to the polyHis nucleotides; and, a Notl site. The polyHis containing fragment was digested with Sail and Notl and then ligated into a similarly digested pDC409 vector. The resulting DNA fusion construct was transiently transfected into the monkey cell line COS-1 (ATCC CRL-1650). Following a 7 day culture in medium containing 0.5% low immunoglobulin bovine serum, cell supernatants were harvested and a solution of 0.2% sodium azide was added to the supernatants. The supernatants were filtered through a 0.22 μm filter, concentrated 10 fold with a prep scale concentrator (Millipore; Bedford, MA) and purified on a BioCad HPLC protein purification equipped with a Nickel NTA Superflow self pack resin column (Qiagen, Santa Clarita, CA). After the supernatant passed through the column, the column was washed with Buffer A (20mM NaPO4, pH7.4; 300mMNaCl; 50mM Imidazole). Bound protein was then eluted from the column using a gradient elution techniques. Fractions containing protein were collected and analyzed on a 4-20% SDS-PAGE reducing gel. Fractions containing soluble B7L-l/polyHis fusion protein were pooled, concentrated 2 fold, and then dialyzed in PBS. The resulting soluble B7L-l/polyHis fusion protein was then filtered through a 0.22μm sterile filter. EXAMPLE 8 Screening Cell Lines for Binding to B7L-1 The B7L-1/Fc fusion protein prepared as described in Example 5 was used to screen cell lines for binding using quantitative binding studies according to standard flow cytometry methodologies. For each cell line screened, the procedure involved incubating approximately 250,000 to 1,000,000 of the cells blocked with 2% FCS (fetal calf serum), 5% normal goat serum and 5% rabbit serum in PBS for 1 hour. Then the blocked cells were incubated with 5 μg/mL of B7L-1/Fc fusion protein in 2% FCS, 5% goat serum and 5% rabbit serum in PBS. Following the incubation the sample was washed 2 times with FACS buffer (2% FCS in PBS) and then treated with mouse anti human Fc/biotin (purchased from Jackson Research) and SAPE (streptavidin-phycoerythrin purchased from Molecular Probes). This treatment causes the antihuman Fc/biotin to bind to any bound B7L-1/Fc and the SAPE to bind to the anti- human Fc biotin resulting in a fluorescent identifying label on B7L-1 Fc which is bound to cells. The cells were analyzed for any bound protein using fluorescent detection flow cytometry. The results indicated that human B7L-1 binds well to human lung epithelial line (WI-26), human B lymphoblastoid lines (Daudi and PAE8LBM1, human fresh tonsillar B cells, murine CD8 + dendritic cells from spleens/lymph nodes of flt3-L treated animals and murine T cell lymphoma (S49.1). EXAMPLE 9 Preparing Monoclonal Antibodies to B7L-1 This example illustrates a method for preparing monoclonal antibodies to B7L-1. Purified B7L-1, a fragment thereof such as the extracellular domain, synthetic peptides or cells that express B7L-1 can be used to generate monoclonal antibodies against B7L-1 using conventional techniques, for example, those techniques described in U.S. Patent 4,411,993. Briefly, rodents are immunized with B7L-1 as an immunogen emulsified in complete Freund's adjuvant, and injected in amounts ranging from 10-100 μg subcutaneously or intraperitoneally. Ten to twelve days later, the immunized animals are boosted with additional B7L-1 emulsified in incomplete Freund's adjuvant. The animals are periodically boosted thereafter on a weekly to bi-weekly immunization schedule. Serum samples are periodically taken by retro-orbital bleeding or tail-tip excision to test for B7L-1 antibodies by dot blot assay, ELISA (Enzyme-Linked Immunosorbent Assay), immunoprecipitation, or other suitable assays, including FACS analysis. Following detection of an appropriate antibody titer, positive animals are provided one last intravenous injection of B7L-1 in saline. Three to four days later, the animals are sacrificed, spleen cells harvested, and spleen cells are fused to a murine myeloma cell line, e.g., NS1 or preferably P3x63Ag8.653 (ATCC CRL 1580). Fusions generate hybridoma cells, which are plated in multiple microtiter plates in a HAT (hypoxanthine, aminopterin and thymidine) selective medium to inhibit proliferation of non-fused cells, myeloma hybrids, and spleen cell hybrids. The hybridoma cells are screened by ELISA for reactivity against purified B7L-1 by adaptations of the techniques disclosed in Engvall et al., Immunochem. 8:871, 1971 and in U.S. Patent 4,703,004. A preferred screening technique is the antibody capture technique described in Beckmann et al., (J. Immunol. 144:4212, 1990) Positive hybridoma cells can be injected intraperitoneally into syngeneic BALB/c mice to produce ascites containing high concentrations of anti-B7L-l-L monoclonal antibodies. Alternatively, hybridoma cells can be grown in vitro in flasks or roller bottles by various techniques. Monoclonal antibodies produced in mouse ascites can be purified by ammonium sulfate precipitation, followed by gel exclusion chromatography. Alternatively, affinity chromatography based upon binding of antibody to protein A or protein G can also be used, as can affinity chromatography based upon binding to B7L-1. EXAMPLE 10 Determining Tissue and Cells That Express Human and Murine B7L-1 The following describes RT-PCR and Northern Blot experiments that were carried out to identify tissue and cell types that express human and murine B7L-1 polypeptides of the present invention. The RT-PCR process involved the reverse transcription of about lμg of total RNA from various human tissue and cell sources to make a first strand cDNA using the Pharmacia, First Strand cDNA Synthesis Kit following the manufacturer's instructions. Cell lines from which total RNA was transcribed included dendritic cells derived from human bone marrow; CD34+ cells and CD34- cells; human peripheral blood B cells cultured in LL-4, SAC or CD40L; human monocyte derived dendritic cells; human monocytes cultured in LFN gamma; human and mouse brain; mouse splenic B cells cultured +/- CD40L; mouse splenic T cells +/- ConA stimulation. The RT-PCR results indicated that B7L-1 is expressed by human bone marrow CD34+ derived and peripheral blood derived dendritic cells and human peripheral blood B cells after stimulation with CD40L. Additionally mRNA was found in brain and a weak PCR signal was found in CD34- bone marrow cells. The results showed that murine B7L-1 is expressed in murine splenic dendritic cells, CD40L stimulated splenic B cells, and in murine brain. Northern Blot analysis was performed by fractionating 5 μg or 10 μg total RNA on 1.2% agarose gels containing formaldehyde. The RNA was then blotted onto Hybond Nylon membranes using standard blotting techniques. Poly A+ multiple tissue blots containing 1 μg of mouse mRNA from a number of different sources were purchased from Clonetech. The purchased blots were prehybridized according to manufacturer's instructions for at least 6 hours at 68°C. Riboprobes, containing the coding region of murine B7L-1, were generated using Promega's Riboprobe Combination Kit and T7 RNA Polymerase according to the manufacturer's instructions. Standard Northern Blot generating procedures as described in Maniatis, (Molecular Cloning: a Laboratory Manual, Cold Spring Harbor Lab. Press, 1989) were used. The results of probing the Northern blots with the riboprobes and visualizing the resulting x-ray film for positively binding probes show that hybridizing RNA was detected in brain, liver, skeletal muscle and heart. In the brain, two band sizes were observed. One RNA was approximately 4.0kB and the other approximately 2.7 kB. In the liver the predominate band indicated hybridizing RNA of mostly 2.7 kB and in the skeletal muscle and heart only the 2.7 kB RNA band was observed. EXAMPLE 11 Cell Binding Studies In order to study binding to murine NK cells, spleens were removed from LL-15 treated CB-17/SCLD mice and used as a source for highly enriched and activated murine NK cells. Spleen cells isolated from IL-15 treated SCLD mice are 60-80% DX-5 positive. DX-5 is a pan NK marker than is expressed on NK cells from many different strains of mice. Flow cytometric analysis was performed to detect B7L1 and LDCAM binding to DX-5+ in vivo LL-15 activated murine NK cells. Table I gives the results of a murine NK cell binding study. Table I Fc molecule DX-5+ NK cell %+ / MFI control Fc 8% / 88 B7LlFc 19% / 265 LDCAM Fc 38% / 432 LDCAM and B7L1 binding can be detected on in vivo activated murine NK cells. Results of experiments directed at studying B7L1 and LDCAM binding to human endothelial cells demonstrated no binding on human umbilical vein endothelial cells (HUVEC) from different donors. However, for a HUVEC from one donor, B7L1 did induce low levels of CD62E and CD106 compared to control Fc. EXAMPLE 12 Isolating A Counterstructure that Binds B7L-1 Based upon the results of the binding experiments described in Example 8, cDNA pools from a WI-26 cell line expression library were screened for binding to the purified B7L-1/Fc fusion protein prepared as described in Example 5. The expression library was prepared using standard methodologies. The cDNA pools were transfected into CV1/EBNA cells and then incubated for 2 days with lμg/mL of B7L-1/Fc fusion protein. Following the B7L-1/Fc incubation period the cells were incubated with 125 I-labeled anti- human F(AB) 2 . Autoradiographs were obtained and examined visually for positive cDNA pools having bound B7L-1/Fc. The positively identified pools were subdivided and rescreened. A single clone was identified that when retransfected into CV1 EBNA cells specifically bound the B7L-1/Fc fusion protein. The clone, designated LDCAM, is described more fully in copending patent application S/N 60/095,672 filed August 7, 1998, which is incorporated herein by reference. EXAMPLE 13 Immune System Cell Binding Studies The following describes FACS cell binding experiments that demonstrate binding characteristics of B7L-1 and a protein for which it is a binding partner, LDCAM. Cells studied included murine T cells, human T cells, murine B cells, murine NK cells, human endothelial cells, and human tumor cell lines. To study murine T cell binding, BALB/c murine lymph node (LN) cells were cultured in culture medium alone and in the presence of different stimuli for 18-20 hours. The cultured cells were harvested and prepared for binding studies using B7L1/Fc fusion protein, LDCAM/Fc fusion protein and a control Fc protein. Following an overnight culture BALB/c murine LN cells are typically >90% CD3+. Bound protein was detected using flow cytometric analysis. The results shown in Table I indicate observed binding expressed as mean fluorescence intensity units (MFI) on unstimulated T cells (medium) and on stimulated T cells (by stimuli). Table I Fc medium Con A TCR mAb PHA control Fc 12.7 10.4 14.5 14.2 B7LlFc 11.7 14.3 24.0 12.6 LDCAM Fc 18.7 51.7 230.0 91.4 When analyzed by T cell subsets, 75-80 % of LN CD4+ murine T cells displayed detectable LDCAM binding after anti-TCR stimulation in vitro. About 50% of LN CD8+ murine T cells display detectable binding. In addition, CD4+ T cells exhibit higher levels of LDCAM binding than do CD8+ murine T cells. The results demonstrate that LDCAM/Fc binds at low levels to naive T cells. However, after an overnight activation with polyclonal stimuli binding increased 5-20 fold depending on the stimuli. Of the stimuli studied PMA induces the least LDCAM binding to murine T cells, and anti-TCR induces the highest binding. To study human T cells binding to LDCAM and its counterstructure, B7L1, human peripheral blood (PB) T cells were cultured in culture medium only or in the presence of different stimuli for 18-20 hours. The cultured cells were harvested and prepared for binding studies using either B7L/lFc fusion protein, LDCAM/Fc fusion protein and a control Fc protein. Bound protein on the human PB T cells was determined by flow cytometric analysis. Table II details results observed, expressed as MFI, on unstimulated T cells (medium) and on stimulated T cells( by stimuli). Table II Fc medium Con A PMA PHA control Fc 4.7 4.8 3.5 4.3 B7LlFc 6.3 7.5 4.5 5.7 LDCAM Fc 22.3 42.8 61.9 38.8 The results show that, PMA induces greater LDCAM binding on human T cells than it does on murine T cells. The presence of specific binding of LDCAM to both murine and human T cells in the absence of B7L1 binding suggests that LDCAM is binding to B7L1, or a different molecule and not to itself. Because studies indicate that T cells express little or no B7L1, LDCAM may have another binding partner. Studies similar to those described above were performed to evaluate LDCAM and B7L1 binding to murine splenic B cells. Neither B7L1 nor LDCAM binding was detected on unstimulated murine B cells . Culturing murine splenic B cells with muCD40L or LPS induced low levels of LDCAM binding but no appreciable level of B7L1 binding was detected. In order to study binding to murine NK cells, spleens were removed from LL-15 treated CB-17/SCLD mice and used as a source for highly enriched and activated murine NK cells. Spleen cells isolated from LL-15 treated SCLD mice are 60-80% DX-5 positive. DX-5 is a pan NK marker than is expressed on NK cells from many different strains of mice. Flow cytometric analysis was performed as described above to detect B7L1 and LDCAM binding to DX-5+ in vivo LL-15 activated murine NK cells. Table II gives the results of a binding murine NK cell binding study. Table LU Fc molecule DX-5+ NK cell %+ / MFI control Fc 8% / 88 B7LlFc 19% / 265 LDCAM Fc 38% / 432 In contrast to that which was observed on murine and human T cells, LDCAM and B7L1 binding can be detected on in vivo activated murine NK cells. Results of experiments directed at studying B7L1 and LDCAM binding to human endothelial cells demonstrated no binding on human umbilical vein endothelial cells (HUVEC) from different donors. However, one HUVEC from one donor B7L1 did induce low levels of CD62E and CD106 compared to control Fc. Table IV details the results of experiments directed at evaluating B7L1 and LDCAM binding to human tumor cell lines. The results are expressed as percentage of cells binding LDCAM or B7L1. Table IV Cell line Cell type LDCAMFc (%+)** B7L1FC (%+)** U937 monocytic leukemia 10 7 K562 erythroblastic 7 5 leukemia Jurkat acute T cell leukemia 10 7 MP-1 B-cell LCL 46 10 DAUDI-hi B-cell Burkitt's 8 6 RPMI 8866 B-cell lymphoma 0 0 #88EBV B-cell LCL 4 3 #33EBV B-cell LCL 0 0 Tonsil G EBV B-cell LCL 25 13 MDA231 breast 8 9 adenocarcinoma OVCAR-3 ovarian carcinoma 48 30 H2126M1 lung adenocarcinoma 0 0 **binding of control Fc has been subtracted out so this is net %+ cells binding over background The results show significant LDCAM binding on ovarian carcinoma cell line and 2 of the human B-cell tumor lines (MP-1 and Tonsil G). B7L1 also binds to these three tumor cell lines but a much lower levels. These results demonstrate that LDCAM is a marker for certain types of B cell lymphomas or different types of carcinomas. In addition, biological signaling mediated by LDCAM or B7L1 could mediate functional anti tumor effects on these types of tumors.

Claims: 1. An isolated DNA sequence encoding a polypeptide that is at least 80% identical to the sequence of amino acid residues selected from the group consisting of amino acids 1- 432 of SEQ LD NO:2, amino acids 1-398 of SEQ LD NO:4, and amino acids 1-398 of SEQ ED NO:6, the polypeptide being capable of binding to a LDCAM polypeptide. 2. An isolated DNA sequence encoding a polypeptide having an amino acid sequence selected from the group SEQ JD NO:2, SEQ ED NO:4, and SEQ ED NO:6. 3. An isolated DNA encoding a soluble polypeptide wherein said soluble polypeptide comprises an amino acid sequence that is at least 90% identical to a sequence selected from the group consisting of: a) amino acids x, to 364 of SEQ ED NO:2, wherein x, is amino acid 1 or 21; b) amino acids x,'to 330 of SEQ ED NO:4, where x,' is amino acid 1 or 21; c) amino acids y, to 330 of SEQ LD NO:6, wherein y, is amino acid 1 or 21; and d) a fragment of the sequences of a) b) or c), wherein the soluble polypeptide is capable of binding to a LDCAM polypeptide. 4. An isolated DNA encoding a soluble polypeptide wherein said soluble polypeptide comprises an amino acid sequence selected from the group consisting of: a) amino acids x : to 364 of SEQ ED NO:2, wherein x, is amino acid 1 or 21; b) amino acids x,'to 330 of SEQ LD NO:4, where x,' is amino acid 1 or 21; c) amino acids y, to 330 of SEQ ED NO:6, wherein y, is amino acid 1 or 21; and d) a fragment of the sequences of a) b) or c), 5. DNA selected from the group consisting of: a) nucleic acids x, to 1452 of SEQ ID NO:l, wherein x l is nucleic acid 157 or 217; b) nucleic acids x,' to 1206 of SEQ ED NO:3, wherein x is nucleic acid 13 or 73; c) nucleic acids y j to 1350 of SEQ ID NO: 5, wherein y, is nucleic acid 157 or 217; d) DNA sequences that hybridize under moderately stringent conditions to the DNA of a) b) or c); and which DNA sequences encode a polypeptide that binds itself; and e) DNA complementary to the DNA of a), b) c) and d).. f) DNA sequences that, due to the degeneracy of the genetic code, encode B7L-1 polypeptide having the amino acid sequence of the polypeptide encoded by the DNA sequences of a), b), c) d) or e). 6. A polypeptide encoded by DNA selected from the group consisting of: a) nucleic acids x, to 1452 of SEQ ED NO:l, wherein x, is nucleic acid 157 or 217; b) nucleic acids x,' to 1206 of SEQ LD NO:3, wherein x,' is nucleic acid 13 or 73; c) nucleic acids y, to 1350 of SEQ ED NO:5, wherein y l is nucleic acid 157 or 217; d) DNA sequences that hybridize under moderately stringent conditions to the DNA of a) b) or c); and which DNA sequences encode a polypeptide that binds itself; and e) DNA complementary to the DNA of a), b) c) and d).. f) DNA sequences that, due to the degeneracy of the genetic code, encode B7L-1 polypeptide having the amino acid sequence of the polypeptide encoded by the DNA sequences of a), b), c) d) or e). 7. A polypeptide comprising an amino acid sequence that is at least 80% identical to an amino acid sequence selected from the group consisting of SEQ ED NO:2, SEQ LD NO:4 and SEQ LD NO:6, the polypeptide being capable of binding to a LDCAM polypeptide. 8. A soluble polypeptide comprising an amino acid sequence selected from the group consisting of: a) amino acids x, to 364 of SEQ ED NO:2, wherein x l is amino acid 1 or 21; b) amino acids x,'to 330 of SEQ LD NO:4, where x,' is amino acid 1 or 21; c) amino acids y, to 330 of SEQ ED NO:6, wherein y, is amino acid 1 or 21; and d) a fragment of the sequences of a) b) or c), wherein the fragment is capable of binding to a LDCAM polypeptide. 9. A soluble polypeptide comprising an amino acid sequence that is at least 90% identical to an amino acids sequence selected from the group consisting of: a) amino acids x, to 364 of SEQ ED NO:2, wherein x, is amino acid 1 or 21; b) amino acids x,'to 330 of SEQ ED NO:4, where x,' is amino acid 1 or 21; c) amino acids y, to 330 of SEQ ED NO:6, wherein y j is amino acid 1 or 21; and d) a fragment of the sequences of a) b) or c) wherein the soluble polypeptide is capable of binding to a LDCAM polypeptide. 10. A fusion protein comprising an amino acid selected from the group consisting of: a) amino acids x, to 364 of SEQ ED NO:2, wherein x, is amino acid 1 or 21; b) amino acids x,'to 330 of SEQ LD NO:4, where x,' is amino acid 1 or 21; c) amino acids y, to 330 of SEQ ED NO:6, wherein y, is amino acid 1 or 21; and d) a fragment of the sequences of a) b) or c), wherein the fragment is capable of binding to a LDCAM polypeptide 11. A recombinant expression vector comprising DNA of claim 1. 12. A process for preparing a polypeptide, the process comprising culturing a host cell transformed with an expression vector of claim 11 under conditions that promote expression of the polypeptide, and recovering the polypeptide. 13. A composition comprising a suitable carrier and a polypeptide of claim 7. 14. A recombinant expression vector comprising DNA of claim 3. 15. A process for preparing a polypeptide, the process comprising culturing a host cell transformed with an expression vector of claim 14 under conditions that promote expression of the polypeptide, and recovering the polypeptide.

Inventors: BAUM PETER ROBERT, BAUM, PETER, ROBERT
Applicants: BAUM PETER ROBERT, IMMUNEX CORP, BAUM, PETER, ROBERT, IMMUNEX CORPORATION
Application year: 1999
Main classification: C07K14
IPC codes: A61K38, C07K14