2B4 (CD244) and CS1: novel members of the CD2 subset of the immunoglobulin superfamily molecules expressed on natural killer cells and other leukocytes.

2B4 is a member of the CD2 subset of the immunoglobulin superfamily molecules expressed on natural killer (NK) cells and other leukocytes. It is the high affinity ligand for CD48. Engagement of 2B4 on NK-cell surfaces with specific antibodies or CD48 can trigger cell-mediated cytotoxicity, interferon-gamma secretion, phosphoinositol turnover and NK-cell invasiveness. The function of 2B4 in CD8+ T cells and myeloid cells remains unknown. The cytoplasmic domain of 2B4 contains unique tyrosine motifs (TxYxxV/I) that associate with src homology 2 domain-containing protein or signaling lymphocyte activation molecule (SLAM)-associated protein, whose mutation is the underlying genetic defect in the X-linked lymphoproliferative disease (XLPD). Impaired signaling via 2B4 and SLAM is implicated in the immunopathogenesis of XLPD. CS1 is a novel member of the CD2 subset that contains two of the unique tyrosine motifs present in 2B4 and SLAM. Signaling through 2B4, CS1 and other members of the CD2 subset may play a major role in the regulation of NK cells and other leukocyte functions.

Potential pathways for regulation of NK and T cell responses: differential X-linked lymphoproliferative syndrome gene product SAP interactions with SLAM and 2B4.

SAP, the gene that is altered or absent in the X-linked lymphoproliferative syndrome (XLP), encodes a small protein that comprises a single SH2 domain and binds to the cell-surface protein SLAM which is present on activated or memory T and B cells. Because defective NK cell activity also has been reported in XLP patients, we studied the SAP gene in NK cells. SAP was induced upon viral infection of SCID mice and shown to be expressed in NK cells by in vitro culturing in the presence of IL-2. Moreover, SAP was expressed in the NK cell lines YT and RNK 16. Because SLAM, the cell-surface protein with which SAP interacts, and 2B4, a membrane protein having sequence homologies with SLAM, also were found to be expressed on the surfaces of activated NK and T cell populations, they may access SAP functions in these populations. Whereas we found that 2B4 also binds SAP, 2B4-SAP interactions occurred only upon tyrosine phosphorylation of 2B4. By contrast, SLAM-SAP interactions were independent of phosphorylation of Y281 and Y327 on SLAM. As CD48, the ligand for 2B4, is expressed on the surface of Epstein-Barr virus (EBV)-infected B cells, it is likely that SAP regulates signal transduction through this pair of cell-surface molecules. These data support the hypothesis that XLP is a result of both defective NK and T lymphocyte responses to EBV. The altered responses may be due to aberrant control of the signaling cascades which are initiated by the SLAM-SLAM and 2B4-CD48 interactions.

Molecular cloning of transmembrane and soluble forms of a novel rat natural killer cell receptor related to 2B4.

Natural killer (NK)-cell recognition of target cells and cytolytic function are controlled by multiple receptor-ligand interactions. These receptors can transmit either positive or negative signals and belong to the lectin superfamily or immunoglobulin superfamily (IgSF). One member of the IgSF, 2B4, is expressed on the surface of all mouse and human NK cells and the subset of T cells that mediate NK-like killing. In both mouse and human, 2B4 is a transmembrane protein and is the counter-receptor for CD48. Northern blot analysis had indicated the existence of 2B4-related genes. Here we report the cloning of novel cDNAs (r2B4R) closely related to the rat 2B4. Unlike 2B4, rat NK cells express mRNA corresponding to both transmembrane (r2B4R-tm) and soluble (r2B4R-se) forms of r2B4R. r2B4R-tm contains an open reading frame encoding a polypeptide of 311 amino acid residues. The encoded protein has characteristics of type I transmembrane proteins with a 20-amino acid leader sequence, a 203-amino acid extracellular domain, a 23-amino acid transmembrane domain, and a 65-amino acid cytoplasmic domain. r2B4R-se encodes a protein of 205 amino acid residues without a putative transmembrane domain. Northern blot analysis and reverse transcriptase-PCR analysis revealed that both transmembrane and soluble forms of r2B4R are expressed in interleukin-2-activated NK cells.

Molecular characterization of the rat NK cell receptor 2B4.

2B4 (CD244) is a cell surface glycoprotein of the immunoglobulin superfamily involved in the regulation of natural killer and T lymphocyte function. It is the high affinity counter-receptor for CD48. In mouse and human NK cells, crosslinking of 2B4 with a specific monoclonal antibody or with CD48 can trigger cell-mediated cytotoxicity, IFN-gamma secretion, phosphoinositol turnover and NK cell invasiveness. Recent reports of defective 2B4 signaling and NK cell function in X-linked lymphoproliferative syndrome suggest that this may contribute to the progression of this human disease. Here we describe the molecular characterization of the rat 2B4 gene. The cDNA encodes a protein of 395 amino acid residues that contain two Ig domains in the extracellular region and three unique tyrosine motifs (TxYxxV/I/A) in the cytoplasmic region. The predicted protein has 81 and 68% similarity with mouse 2B4 and human 2B4, respectively. Additionally, it has 94 and 89% similarity at the protein level with the recently reported rat 2B4 related genes, r2B4R-tm and r2B4R-se respectively. Northern blot analysis indicated the presence of multiple transcripts in rat LAK cells and RNK-16 cells. Immunoprecipitation and deglycosylation studies showed that rat 2B4 is glycosylated to similar extent as that of mouse and human 2B4. The cloning of r2B4 in the light of the availability of rat NK cell lines should facilitate in vitro and in vivo experiments to decipher the functional role of 2B4 in NK cell biology.

Perforin gene defects in familial hemophagocytic lymphohistiocytosis.

Familial hemophagocytic lymphohistiocytosis (FHL) is a rare, rapidly fatal, autosomal recessive immune disorder characterized by uncontrolled activation of T cells and macrophages and overproduction of inflammatory cytokines. Linkage analyses indicate that FHL is genetically heterogeneous and linked to 9q21.3-22, 10q21-22, or another as yet undefined locus. Sequencing of the coding regions of the perforin gene of eight unrelated 10q21-22-linked FHL patients revealed homozygous nonsense mutations in four patients and missense mutations in the other four patients. Cultured lymphocytes from patients had defective cytotoxic activity, and immunostaining revealed little or no perforin in the granules. Thus, defects in perforin are responsible for 10q21-22-linked FHL. Perforin-based effector systems are, therefore, involved not only in the lysis of abnormal cells but also in the down-regulation of cellular immune activation.

Molecular cloning and characterization of the promoter region of murine natural killer cell receptor 2B4.

Natural killer (NK) cells are bone marrow-derived lymphocytes that have the ability to kill certain tumor cells and virally infected cells. The activation of NK cells is mediated by a balance of negative and positive signals from cell-cell interactions and from responses to cytokines. However, the molecular basis of NK cell activation and recognition of target cells is poorly understood. We have previously identified, cloned and characterized a receptor, 2B4, expressed on murine NK cells. 2B4 is not only expressed on all NK cells, but also on a subset of T-cells which have NK-like killing properties. Structural analysis indicated that 2B4 belongs to the CD2 subset of immunoglobulin superfamily. We have also shown 2B4 to interact with CD48 with nine times more affinity than that of CD2-CD48 interaction. In order to understand the transcriptional regulation as well as the mechanisms controlling the restricted expression of the 2B4 gene, we obtained a genomic 2B4 clone including the sequence of the 5'-flanking region. To define the start site of transcription, we performed primer extension and 5'-RACE assays and found that the 2B4 gene may be initiated at multiple start sites and driven by a TATA-less promoter. Transient transfections of nested 5'-fragments of the 2B4 promoter to drive CAT expression revealed tissue specific expression in CTLL-2 cells, a mouse T-cell line. A promoter fragment of 348 bases upstream from the first base of the mouse 2B4 cDNA clone p2B4.8 produced maximal CAT activity in CTLL-2 cells. The presence of the region -653 to -540 on the other hand, drastically reduced transcription. Sequence analysis of this promoter region has identified potential recognition motifs for a number of lymphocyte-restricted in addition to ubiquitous transcription factors, which may play a role in the transcriptional regulation of the mouse 2B4 gene.

Gene structure of the murine NK cell receptor 2B4: presence of two alternatively spliced isoforms with distinct cytoplasmic domains.

The NK cell receptor 2B4 is expressed on the surface of all murine NK cells and a subset of T cells. Ligation of 2B4 with monoclonal antibodies increases target cell lysis and IFN-gamma production. 2B4 is the high-affinity counter-receptor for CD48 in mice and humans. 2B4-L is a member of the CD2 subgroup of the immunoglobulin supergene family, which includes CD48, LFA-3, CD84, Ly9 and SLAM. Here we describe 2B4-S, a second 2B4 isoform, and the genomic structure of the 2B4 gene. 2B4-S is identical to the 5' end of 2B4-L, differing only at the 3' end, corresponding to a portion of the cytoplasmic domain and the 3' untranslated sequence. Both 2B4-L and 2B4-S are expressed on IL-2-activated NK cells. The genomic clone of 2B4 reveals that the two cDNA clones are products of alternative splicing. Since they differ only in a portion of the cytoplasmic domain, it is likely that they transduce different signals.

Molecular characterization of a novel human natural killer cell receptor homologous to mouse 2B4.

Natural killer (NK) cells spontaneously detect and kill cancerous and virally infected cells through receptors that transduce either activating or inhibiting signals. The majority of well studied NK receptors are involved in inhibitory signaling. However, we have previously described an activating receptor, 2B4, expressed on all murine NK cells and a subset of T cells that mediate non-major histocompatibility complex (MHC) restricted killing. Anti-2B4 monoclonal antibodies directed against IL-2-activated NK cells enhanced their destruction of tumor cells. Recently, we determined binding of 2B4 to CD48 with a much higher affinity than CD2 to CD48. Here we describe the molecular characterization of a cDNA clone homologous to mouse 2B4, isolated from a human NK cell library. The cDNA clone contained an open reading frame encoding a polypeptide chain of 365 amino acid residues. The predicted protein sequence showed 70% similarity to murine 2B4. Additionally, it has 48, 45, and 43% similarity to human CD84, CDw150 (SLAM), and CD48, respectively. RNA blot analysis indicates the presence of 3 kb and 5 kb transcripts in T- and NK-cell lines. A single transcript of 3 kb is identified in poly(A)+ RNA from human spleen, peripheral blood leukocytes, and lymph node, whereas, the level of expression in bone marrow and fetal liver was indeterminate. Preliminary functional data suggests that NK-cell interaction with target cells via 2B4 modulates human NK-cell cytolytic activity.

Characterization of inhibitory and stimulatory forms of the murine natural killer cell receptor 2B4.

The receptor 2B4 belongs to the Ig superfamily and is found on the surface of all murine natural killer (NK) cells as well as T cells displaying non-MHC-restricted cytotoxicity. Previous studies have suggested that 2B4 is an activating molecule because cross-linking of this receptor results in increased cytotoxicity and gamma-interferon secretion as well as granule exocytosis. However, it was recently shown that the gene for 2B4 encodes two different products that arise by alternative splicing. These gene products differ solely in their cytoplasmic domains. One form has a cytoplasmic tail of 150 amino acids (2B4L) and the other has a tail of 93 amino acids (2B4S). To determine the function of each receptor, cDNAs for 2B4S and 2B4L were transfected into the rat NK cell line RNK-16. Interestingly, the two forms of 2B4 had opposing functions. 2B4S was able to mediate redirected lysis of P815 tumor targets, suggesting that this form represents an activating receptor. However, 2B4L expression led to an inhibition of redirected lysis of P815 targets when the mAb 3.2.3 (specific for rat NKRP1) was used. In addition, 2B4L constitutively inhibits lysis of YAC-1 tumor targets. 2B4L is a tyrosine phosphoprotein, and removal of domains containing these residues abrogates its inhibitory function. Like other inhibitory receptors, 2B4L associates with the tyrosine phosphatase SHP-2. Thus, 2B4L is an inhibitory receptor belonging to the Ig superfamily.