Identification and characterization of SF2000 and SF2001, two new members of the immune receptor SLAM/CD2 family

SLAM Family Receptors in B Cell Chronic Lymphoproliferative Disorders

The signaling lymphocytic activation molecule (SLAM) receptor family (SLAMF) consists of nine glycoproteins that belong to the CD2 superfamily of immunoglobulin (Ig) domain-containing molecules. SLAMF receptors modulate the differentiation and activation of a wide range of immune cells. Individual SLAMF receptors are expressed on the surface of hematopoietic stem cells, hematopoietic progenitor cells, B cells, T cells, NK cells, NKT cells, monocytes, macrophages, dendritic cells, neutrophils, and platelets. The expression of SLAMF receptors was studied during normal B cell maturation. Several SLAMF receptors were also detected in cancer cell lines of B-lymphoid origin and in pathological B cells from patients with B cell chronic lymphoproliferative disorders (B-CLPD), the most frequent hematological malignancies in adults. This review summarizes current knowledge on the expression of SLAMF receptors and their adaptor proteins SAP and EAT-2 in B-CLPD. Several SLAMF receptors could be regarded as potential diagnostic and differential diagnostic markers, prognostic factors, and targets for the development of novel drugs for patients with B-CLPD.

Role of signaling lymphocytic activation molecule family of receptors in the pathogenesis of rheumatoid arthritis: insights and application

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic inflammation and joint damage. The signaling lymphocytic activation molecule (SLAMF) family of receptors are expressed on various hematopoietic and non-hematopoietic cells and can regulate both immune cell activation and cytokine production. Altered expression of certain SLAMF receptors contributes to aberrant immune responses in RA. In RA, SLAMF1 is upregulated on T cells and may promote inflammation by participating in immune cell-mediated responses. SLAMF2 and SLAMF4 are involved in regulating monocyte tumor necrosis factor production and promoting inflammation. SLAMF7 activates multiple inflammatory pathways in macrophages to drive inflammatory gene expression. SLAMF8 inhibition can reduce inflammation in RA by blocking ERK/MMPs signaling. Of note, there are differences in SLAMF receptor (SFR) expression between normal and arthritic joint tissues, suggesting a role as potential diagnostic biomarkers. This review summarizes recent advances on the roles of SLAMF receptors 1, 2, 4, 7, and 8 in RA pathogenesis. However, further research is needed to elucidate the mechanisms of SLAMF regulation of immune cells in RA. Understanding interactions between SLAMF receptors and immune cells will help identify selective strategies for targeting SLAMF signaling without compromising normal immunity. Overall, the SLAMF gene family holds promise as a target for precision medicine in RA, but additional investigation of the underlying immunological mechanisms is needed. Targeting SLAMF receptors presents opportunities for new diagnostic and therapeutic approaches to dampen damaging immune-mediated inflammation in RA.

SLAM-family receptors come of age as a potential molecular target in cancer immunotherapy

The signaling lymphocytic activation molecule (SLAM) family receptors were discovered in immune cells for the first time. The SLAM-family receptors are a significant player in cytotoxicity, humoral immune responses, autoimmune diseases, lymphocyte development, cell survival, and cell adhesion. There is growing evidence that SLAM-family receptors have been involved in cancer progression and heralded as a novel immune checkpoint on T cells. Previous studies have reported the role of SLAMs in tumor immunity in various cancers, including chronic lymphocytic leukemia, lymphoma, multiple myeloma, acute myeloid leukemia, hepatocellular carcinoma, head and neck squamous cell carcinoma, pancreas, lung, and melanoma. Evidence has deciphered that the SLAM-family receptors may be targeted for cancer immunotherapy. However, our understanding in this regard is not complete. This review will discuss the role of SLAM-family receptors in cancer immunotherapy. It will also provide an update on recent advances in SLAM-based targeted immunotherapies.

Characterization of Ly108-H1 Signaling Reveals Ly108-3 Expression and Additional Strain-Specific Differences in Lupus Prone Mice

Ly108 (SLAMF6) is a homophilic cell surface molecule that binds SLAM-associated protein (SAP), an intracellular adapter protein that modulates humoral immune responses. Furthermore, Ly108 is crucial for the development of natural killer T (NKT) cells and CTL cytotoxicity. Significant attention has been paid towards expression and function of Ly108 since multiple isoforms were identified, i.e., Ly108-1, Ly108-2, Ly108-3, and Ly108-H1, some of which are differentially expressed in several mouse strains. Surprisingly, Ly108-H1 appeared to protect against disease in a congenic mouse model of Lupus. Here, we use cell lines to further define Ly108-H1 function in comparison with other isoforms. We show that Ly108-H1 inhibits IL-2 production while having little effect upon cell death. With a refined method, we could detect phosphorylation of Ly108-H1 and show that SAP binding is retained. We propose that Ly108-H1 may regulate signaling at two levels by retaining the capability to bind its extracellular as well as intracellular ligands, possibly inhibiting downstream pathways. In addition, we detected Ly108-3 in primary cells and show that this isoform is also differentially expressed between mouse strains. The presence of additional binding motifs and a non-synonymous SNP in Ly108-3 further extends the diversity between murine strains. This work highlights the importance of isoform awareness, as inherent homology can present a challenge when interpreting mRNA and protein expression data, especially as alternatively splicing potentially affects function.

Alternative Splicing of the Inhibitory Immune Checkpoint Receptor SLAMF6 Generates a Dominant Positive Form, Boosting T-cell Effector Functions

SLAMF6 is a homotypic receptor of the Ig-superfamily associated with progenitor-exhausted T cells. Here we show that in humans, SLAMF6 has three splice isoforms involving its V-domain. Although the canonical receptor inhibited T-cell activation through SAP recruitment, the short isoform SLAMF6^Δ17-65 had a strong agonistic effect. The costimulatory action depended on protein phosphatase SHP1 and led to a cytotoxic molecular profile mediated by the expression of TBX21 and RUNX3. Patients treated with immune checkpoint blockade showed a shift toward SLAMF6^Δ17-65 in peripheral blood T cells. We developed splice-switching antisense oligonucleotides (ASO) designed to target the relevant SLAMF6 splice junction. Our ASOs enhanced SLAMF6^Δ17-65 expression in human tumor-infiltrating lymphocytes and improved their capacity to inhibit human melanoma in mice. The yin-yang relationship of SLAMF6 splice isoforms may represent a balancing mechanism that could be exploited to improve cancer immunotherapy.

Urinary Cell Transcriptome Profiling and Identification of ITM2A, SLAMF6, and IKZF3 as Biomarkers of Acute Rejection in Human Kidney Allografts

Identification of a shared gene expression pattern between T cell–mediated rejection (TCMR) and antibody-mediated rejection (AMR) in human kidney allografts may help prioritize targets for the treatment of both types of acute rejection.

Signalling lymphocyte activation molecule family member 9 is found on select subsets of antigen-presenting cells and promotes resistance to Salmonella infection

Signalling lymphocyte activation molecule family member 9 (SLAMF9) is an orphan receptor of the CD2/SLAM family of leucocyte surface proteins. Examination of SLAMF9 expression and function indicates that SLAMF9 promotes inflammation by specialized subsets of antigen‐presenting cells. Within healthy liver and circulating mouse peripheral blood mononuclear cells, SLAMF9 is expressed on CD11b⁺, Ly6C⁻, CD11c^low, F4/80^low, MHC‐II⁺, CX₃CR1⁺ mononuclear phagocytes as well as plasmacytoid dendritic cells. In addition, SLAMF9 can be found on peritoneal B1 cells and small (F4/80^low), but not large (F4/80^high), peritoneal macrophages. Upon systemic challenge with Salmonella enterica Typhimurium, Slamf9^−/− mice were impaired in their ability to clear the infection from the liver. In humans, SLAMF9 is up‐regulated upon differentiation of monocytes into macrophages, and lipopolysaccharide stimulation of PMA‐differentiated, SLAMF9 knockdown THP‐1 cells showed an essential role of SLAMF9 in production of granulocyte–macrophage colony‐stimulating factor, tumour necrosis factor‐α, and interleukin‐1β. Taken together, these data implicate SLAMF9 in the initiation of inflammation and clearance of bacterial infection.

SLAM Family Receptor Signaling in Viral Infections: HIV and Beyond

The signaling lymphocytic activation molecule (SLAM) family of receptors are expressed on the majority of immune cells. These receptors often serve as self-ligands, and play important roles in cellular communication and adhesion, thus modulating immune responses. SLAM family receptor signaling is differentially regulated in various immune cell types, with responses generally being determined by the presence or absence of two SLAM family adaptor proteins-Ewing's sarcoma-associated transcript 2 (EAT-2) and SLAM-associated adaptor protein (SAP). In addition to serving as direct regulators of the immune system, certain SLAM family members have also been identified as direct targets for specific microbes and viruses. Here, we will discuss the known roles for these receptors in the setting of viral infection, with special emphasis placed on HIV infection. Because HIV causes such complex dysregulation of the immune system, studies of the roles for SLAM family receptors in this context are particularly exciting.

SLAMF6 as a Regulator of Exhausted CD8+ T Cells in Cancer

The tumor microenvironment in leukemia and solid tumors induces a shift of activated CD8+ cytotoxic T cells to an exhausted state, characterized by loss of proliferative capacity and impaired immunologic synapse formation. Efficient strategies and targets need to be identified to overcome T-cell exhaustion and further improve overall responses in the clinic. Here, we took advantage of the Eμ-TCL1 chronic lymphocytic leukemia (CLL) and B16 melanoma mouse models to assess the role of the homophilic cell-surface receptor SLAMF6 as an immune-checkpoint regulator. The transfer of SLAMF6+ Eμ-TCL1 cells into SLAMF6-/- recipients, in contrast to wild-type (WT) recipients, significantly induced expansion of a PD-1+ subpopulation among CD3+CD44+CD8+ T cells, which had impaired cytotoxic functions. Conversely, administering anti-SLAMF6 significantly reduced the leukemic burden in Eμ-TCL1 recipient WT mice concomitantly with a loss of PD-1+CD3+CD44+CD8+ T cells with significantly increased effector functions. Anti-SLAMF6 significantly reduced leukemic burden in the peritoneal cavity, a niche where antibody-dependent cellular cytotoxicity (ADCC) is impaired, possibly through activation of CD8+ T cells. Targeting of SLAMF6 affected tumor growth not only in B cell-related leukemia and lymphomas but also in nonhematopoietic tumors such as B16 melanoma, where SLAMF6 is not expressed. In vitro exhausted CD8+ T cells showed increased degranulation when anti-human SLAMF6 was added in culture. Taken together, anti-SLAMF6 both effectively corrected CD8+ T-cell dysfunction and had a direct effect on tumor progression. The outcomes of our studies suggest that targeting SLAMF6 is a potential therapeutic strategy.

SLAMF9 regulates pDC homeostasis and function in health and disease

SLAMF9 belongs to the conserved lymphocytic activation molecule family (SLAMF). Unlike other SLAMs, which have been extensively studied, the role of SLAMF9 in the immune system remained mostly unexplored. By generating CRISPR/Cas9 SLAMF9 knockout mice, we analyzed the role of this receptor in plasmacytoid dendritic cells (pDCs), which preferentially express the SLAMF9 transcript and protein. These cells display a unique capacity to produce type I IFN and bridge between innate and adaptive immune response. Analysis of pDCs in SLAMF9^-/- mice revealed an increase of immature pDCs in the bone marrow and enhanced accumulation of pDCs in the lymph nodes. In the periphery, SLAMF9 deficiency resulted in lower levels of the transcription factor SpiB, elevation of pDC survival, and attenuated IFN-α and TNF-α production. To define the role of SLAMF9 during inflammation, pDCs lacking SLAMF9 were followed during induced experimental autoimmune encephalomyelitis. SLAMF9^-/- mice demonstrated attenuated disease and delayed onset, accompanied by a prominent increase of immature pDCs in the lymph node, with a reduced costimulatory potential and enhanced infiltration of pDCs into the central nervous system. These results suggest the crucial role of SLAMF9 in pDC differentiation, homeostasis, and function in the steady state and during experimental autoimmune encephalomyelitis.

SLAMF receptors on normal and malignant B cells

The Signaling Lymphocyte Activation Molecule family (SLAMF) is a collection of nine surface receptors expressed mainly on hematopoietic cells, and was found to modulate the behavior of immune cells. SLAMF receptors are expressed on B cells in health and disease. Each SLAM receptor has a unique differential expression pattern during the development and activation of B cells. Furthermore, recent findings have revealed a principal role for this family of receptors in B cell malignancies, emphasizing their importance in the control of malignant cell survival, cell to cell communication within the tumor microenvironment, retention in the supporting niches and regulation of T cell anti-tumor response. This review summarizes the latest studies regarding SLAMF expression and behavior in B cells and in B cell pathologies, and discusses the therapeutic potential of these receptors.

The novel immunoglobulin super family receptor SLAMF9 identified in TAM of murine and human melanoma influences pro-inflammatory cytokine secretion and migration

Melanoma is a highly immunogenic tumor with a good response to treatment with immune checkpoint inhibitors. Tumor-associated macrophages (TAMs) play an important immunosuppressive role in such tumors and have therefore been identified as possible future therapeutic targets in oncology. The aim of this study was to identify novel immunoregulatory receptors specifically expressed on TAM. Expression of Slamf9, a member of the signaling lymphocytic-activating molecule (Slam) immunoreceptor family, was found to be upregulated in a gene expression analysis of murine bone marrow-derived macrophages (BMDM) stimulated with tumor-conditioned medium of B16F1 melanoma cells. SLAMF9⁺ macrophages were identified in human and murine melanomas by using self-generated antibodies against human and murine SLAMF9. A comprehensive immunohistochemical analysis of tissue microarrays detected SLAMF9⁺ TAM in 73.3% of human melanomas, but also in 95.5% of naevi of melanoma patients and in 50% of naevi from healthy controls. In addition, 20% of melanomas and 2.3% of naevi from melanoma patients displayed a positive SLAMF9 expression also in melanocytic cells. No SLAMF9 expression was detected in naevus cells of healthy donors. Although SLAMF9 has no intracellular signaling motif, a comprehensive functional analysis revealed that the molecule was able to significantly enhance TNF-α secretion after LPS-stimulation. In addition, SLAMF9 delayed the wound closure of RAW 264.7 cells in a scratch assay, while proliferation and cell death were not affected. Taken together, SLAMF9 is a novel type-I-transmembrane receptor with immunomodulatory properties in macrophages. Further studies are required to evaluate whether SLAMF9 classifies as a promising future therapeutic target in melanoma.

The SLAM family receptors: Potential therapeutic targets for inflammatory and autoimmune diseases

The signaling lymphocytic activation molecule (SLAM) family is comprised of nine distinct receptors (SLAMF1 through SLAMF9) that are expressed on hematopoietic cells. All of these receptors, with the exception of SLAMF4, are homotypic by nature as downstream signaling occurs when hematopoietic cells that express the same SLAM receptor interact. The SLAM family receptor function is largely controlled via SLAM associated protein (SAP) family adaptors. The SAP family adaptors consist of SAP, Ewing sarcoma associated transcript (EAT)-2, and EAT-2-related transducer (ERT). These adaptors associate with the cytoplasmic domain of the SLAM family receptors through phosphorylated tyrosines. Defects in SLAM family members and SAP adaptors have been implicated in causing immune deficiencies. This is exemplified in patients with X-linked lymphoproliferative (XLP) disease, where SAP undergoes a loss of function mutation. Furthermore, evidence has been accumulating that SLAM family members are potential targets for inflammatory and autoimmune diseases. This review will discuss the structure and function of the SLAM family receptors and SAP family adaptors, their role in immune regulation, and potential approaches to target this family of receptors therapeutically.

Roles of CD48 in regulating immunity and tolerance

CD48, a member of the signaling lymphocyte activation molecule family, participates in adhesion and activation of immune cells. Although constitutively expressed on most hematopoietic cells, CD48 is upregulated on subsets of activated cells. CD48 can have activating roles on T cells, antigen presenting cells and granulocytes, by binding to CD2 or bacterial FimH, and through cell intrinsic effects. Interactions between CD48 and its high affinity ligand CD244 are more complex, with both stimulatory and inhibitory outcomes. CD244:CD48 interactions regulate target cell lysis by NK cells and CTLs, which are important for viral clearance and regulation of effector/memory T cell generation and survival. Here we review roles of CD48 in infection, tolerance, autoimmunity, and allergy, as well as the tools used to investigate this receptor. We discuss stimulatory and regulatory roles for CD48, its potential as a therapeutic target in human disease, and current challenges to investigation of this immunoregulatory receptor.

Development and characterization of domain-specific monoclonal antibodies produced against human SLAMF9

SLAMF9 is a member of the signaling lymphocyte-activating molecule (SLAM) immunoreceptor family. The SLAM family receptors are expressed in a broad range of immune cells and play an important role in immunity. To date, SLAMF9 is the least studied member of this family. Its ligand, signaling properties, and cells on whose surface it is expressed are unknown. We generated hybridoma clones 6E11 and 7G5 secreting monoclonal antibodies specific to human SLAMF9. BALB/c mice were immunized with Escherichia coli-expressed purified SLAMF9 protein; splenocytes from these mice were fused with mouse myeloma cell line NS-1. Based on isotyping of the MAbs, clone 6E11 was referred to the IgG1 subclass, while 7G5 to IgG2b. The specificity of these MAbs was assessed by ELISA, immunoblotting, immunohistochemistry, and flow cytometry. According to the results of epitope analysis, clone 6E11 reacts with the C2-like domain, whereas 7G5 is specific to the V-like domain of the SLAMF9 molecule. The generated MAbs were demonstrated to be applicable in various immunochemical analyses. They may be useful tools in studies clarifying the expression and function of human SLAMF9.

Expansion of an osteopontin-expressing T follicular helper cell subset correlates with autoimmunity in B6.Sle1b mice and is suppressed by the H1-isoform of the Slamf6 receptor

The costimulatory receptor Slamf6 partially controls lupus-related autoimmunity in congenic Sle1b mice; for instance, the presence of the protein isoform Slamf6-H1 in Sle1b.Slamf6-H1 mice mitigates disease. Here, we report that young Sle1b mice, but not Sle1b.Slamf6-H1 or B6 mice, contain a memory T-helper cell subset identified by ]mt]2-fold increase in expression of 17 genes, chief among which is Spp1, encoding the cytokine osteopontin (OPN). These T follicular helper (TFH) cells, including OPN(+) TFH cells, expand concomitantly with severity of the disease. By contrast, Sle1b.Slamf6-H1 or Sle1b.SAP(-)/(-) mice do not develop autoantibodies and the number of T(FH) cells is 5 times lower than in age-matched Sle1b mice. By comparing Sle1b and Sle1b.OPN(-)/(-) mice, we find that the lack of OPN expression impedes early autoantibody production. Furthermore, on the adoptive transfer of Sle1b.OPN(-)/(-) CD4(+) T cells into bm12 recipients autoantibody production and germinal center formation is reduced compared to recipients of Sle1b.OPN(+/+) CD4(+) T cells. We propose a model in which OPN provides a survival signal for a precursor T(FH) cell subset, which is a key factor in autoimmunity.

Diversity of CD2 subfamily receptors in cyprinid fishes

CD2 family receptor (CD2f) is evolutionarily conserved and is widely expressed by various types of leukocytes. To elucidate the phylogenetic diversity of the CD2f, we characterized CD2f in teleosts using ginbuna crucian carp and zebrafish. The identified CD2f isoforms of the ginbuna carp (caauCD2f) exhibited high sequence similarity to the mammalian CD2 subsets CD48, CD244, and CD319, but it was difficult to classify them into their respective mammalian CD2f based on sequence similarity, the presence of an immunoreceptor tyrosine-based switch motif (ITSM), and phylogenetic tree analysis. Although the four caauCD2f isoforms share an extracellular domain with quite high identity (83-94% identity at the nucleic acid level), they differ in the number of ITSM motifs in their cytoplasmic tail. RT-PCR and in situ hybridization analyses showed that the caauCD2f isoforms are expressed by different cell populations, suggesting that they, like mammalian CD2f, have diverse roles. Interestingly, immunoglobulin (Ig) domain-like sequences with high identity to caauCD2fs are clustered close together within 0.6 Mbp on zebrafish chromosomes 1 and 2 (at least 8 and 35 sequences, respectively), and many pairs of the Ig domains share more than 90% identity at the amino acid level. Therefore, the teleost CD2fs with considerably high identity have been probably generated from a common ancestral Ig-domain gene by a very recent gene duplication event. These findings suggest that the identified CD2f acquired functional diversification through successive duplications together with the acquisition of ITSM.

SLAM family receptors and SAP adaptors in immunity

The signaling lymphocyte activation molecule (SLAM)-associated protein, SAP, was first identified as the protein affected in most cases of X-linked lymphoproliferative (XLP) syndrome, a rare genetic disorder characterized by abnormal responses to Epstein-Barr virus infection, lymphoproliferative syndromes, and dysgammaglobulinemia. SAP consists almost entirely of a single SH2 protein domain that interacts with the cytoplasmic tail of SLAM and related receptors, including 2B4, Ly108, CD84, Ly9, and potentially CRACC. SLAM family members are now recognized as important immunomodulatory receptors with roles in cytotoxicity, humoral immunity, autoimmunity, cell survival, lymphocyte development, and cell adhesion. In this review, we cover recent findings on the roles of SLAM family receptors and the SAP family of adaptors, with a focus on their regulation of the pathways involved in the pathogenesis of XLP and other immune disorders.

Surface molecule CD229 as a novel target for the diagnosis and treatment of multiple myeloma

BACKGROUND

To date, multiple myeloma remains an incurable malignancy due to the persistence of minimal residual disease in the bone marrow. In this setting, monoclonal antibodies against myeloma-specific cell surface antigens represent a promising therapeutic approach, which is however hampered by a lack of appropriate target structures expressed across all pathogenic myeloma cell populations. We, therefore, investigated functionally relevant immunoreceptors specifically associated with myeloma cells as well as their clonogenic precursors.

DESIGN AND METHODS

Potential target proteins were identified using antibody arrays against phosphorylated immunoreceptors with lysates from myeloma cell lines. CD229 expression was confirmed in primary myeloma cells by reverse transcriptase polymerase chain reaction, western blot, fluorescence-activated cell sorting, and immunohistochemistry. Apoptosis, clonogenic growth, and sensitivity to chemotherapy were determined following short-interfering RNA-mediated downregulation of CD229. Antibody-dependent cellular and complement-dependent cytotoxicity were analyzed using a monoclonal antibody against CD229 to demonstrate the antigen's immunotherapeutic potential.

RESULTS

Our screening assay identified CD229 as the most strongly over-expressed/phosphorylated immunoreceptor in myeloma cell lines. Over-expression was further demonstrated in the CD138-negative population, which has been suggested to represent myeloma precursors, as well as on primary tumor cells from myeloma patients. Accordingly, CD229 staining of patients' bone marrow samples enabled the identification of myeloma cells by flow cytometry and immunohistochemistry. Down-regulation of CD229 led to a decreased number of viable myeloma cells and clonal myeloma colonies, and enhanced the anti-tumor activity of conventional chemotherapeutics. Targeting CD229 with a monoclonal antibody resulted in complement- and cell-mediated lysis of myeloma cells.

CONCLUSIONS

Our results demonstrate that the immunoreceptor CD229 is specifically over-expressed on myeloma cells including their clonogenic precursors and contributes to their malignant phenotype. Monoclonal antibodies against this protein may represent a promising diagnostic and immunotherapeutic instrument in this disease.

A novel isoform of the Ly108 gene ameliorates murine lupus

The expression of the new Ly108 isoform H1 weakens lupus-like disease of C57BL/6.Sle1b mice.

Studies of human systemic lupus erythematosus patients and of murine congenic mouse strains associate genes in a DNA segment on chromosome 1 with a genetic predisposition for this disease. The systematic analysis of lupus-prone congenic mouse strains suggests a role for two isoforms of the Ly108 receptor in the pathogenesis of the disease. In this study, we demonstrate that Ly108 is involved in the pathogenesis of lupus-related autoimmunity in mice. More importantly, we identified a third protein isoform, Ly108-H1, which is absent in two lupus-prone congenic animals. Introduction of an Ly108-H1–expressing transgene markedly diminishes T cell–dependent autoimmunity in congenic B6.Sle1b mice. Thus, an immune response–suppressing isoform of Ly108 can regulate the pathogenesis of lupus.

The role of SAP and SLAM family molecules in the humoral immune response

Effective B cell-mediated immunity, including the formation of germinal centers and the generation of high-affinity memory B cells and long-lived plasma cells, is dependent on CD4(+) T cells. Immunodeficiencies that present with defects in the antibody response have provided insights into the molecular mechanisms of B cell responses and the provision of T cell help. One such immunodeficiency is X-linked lymphoproliferative disease (XLP), which results from mutations in SH2D1A, the gene encoding SLAM-associated protein (SAP). Patients with XLP present with humoral defects characterized by hypogammaglobulinemia. We now know that SAP, through its signaling downstream of multiple members of the signaling lymphocytic activation molecule (SLAM) family of cell surface receptors, plays a crucial role in many aspects of this immune response. Here, we discuss the role of SAP in the generation of humoral immunity, particularly T cell-dependent antibody responses and the generation of germinal centers.

The role of SLAM/CD2 polymorphisms in systemic autoimmunity

The SLAM/CD2 gene family encodes receptors that play important roles in regulating multiple cellular interactions in the adaptive and innate immune systems. Three members of this gene family, Ly108, Ly9, and CD84, exhibit polymorphisms that strongly influence susceptibility to systemic autoimmunity, notably in mice, but also in some human populations. Polymorphisms of Ly108 in mice strongly impact central tolerance in both B and T cell development, predominantly by modulating apoptosis, anergy, and cell-cycle progression. In addition, Ly108 and CD84, together with their downstream signaling adaptor SLAM-associated protein (SAP), have emerged as key players in B-T interactions during the formation of germinal centers. Interestingly, several independent lines of research have now associated variations in B-T interactions during germinal center formation with systemic autoimmunity, suggesting that susceptibility to systemic lupus erythematosus (SLE) may involve in part the impairment of this peripheral tolerance checkpoint. These new insights into the multiplicity of roles played by the SLAM/CD2 family and its potential importance in human autoimmunity positions the SLAM/CD2 family as an excellent target for immunotherapy.

SLAM family receptors and the SLAM-associated protein (SAP) modulate T cell functions

One or more of the signaling lymphocytic activation molecule (SLAM) family (SLAMF) of cell surface receptors, which consists of nine transmembrane proteins, i.e., SLAMF1-9, are expressed on most hematopoietic cells. While most SLAMF receptors serve as self-ligands, SLAMF2 and SLAMF4 use each other as counter structures. Six of the receptors carry one or more copies of a unique intracellular tyrosine-based switch motif, which has high affinity for the single SH2-domain signaling molecules SLAM-associated protein and EAT-2. Whereas SLAMF receptors are costimulatory molecules on the surface of CD4+, CD8+, and natural killer (NK) T cells, they also involved in early phases of lineage commitment during hematopoiesis. SLAMF receptors regulate T lymphocyte development and function and modulate lytic activity, cytokine production, and major histocompatibility complex-independent cell inhibition of NK cells. Furthermore, they modulate B cell activation and memory generation, neutrophil, dendritic cell, macrophage and eosinophil function, and platelet aggregation. In this review, we will discuss the role of SLAM receptors and their adapters in T cell function, and we will examine the role of these receptors and their adapters in X-linked lymphoproliferative disease and their contribution to disease susceptibility in systemic lupus erythematosus.

The SLAM and SAP gene families control innate and adaptive immune responses

The nine SLAM-family genes, SLAMF1-9, a subfamily of the immunoglobulin superfamily, encode differentially expressed cell-surface receptors of hematopoietic cells. Engagement with their ligands, which are predominantly homotypic, leads to distinct signal transduction events, for instance those that occur in the T or NK cell immune synapse. Upon phosphorylation of one or more copies of a unique tyrosine-based signaling motif in their cytoplasmic tails, six of the SLAM receptors recruit the highly specific single SH2-domain adapters SLAM-associated protein (SAP), EAT-2A, and/or EAT-2B. These adapters in turn bind to the tyrosine kinase Fyn and/or other protein tyrosine kinases connecting the receptors to signal transduction networks. Individuals deficient in the SAP gene, SH2D1A, develop an immunodeficiency syndrome: X-linked lympho-proliferative disease. In addition to operating in the immune synapse, SLAM receptors initiate or partake in multiple effector functions of hematopoietic cells, for example, neutrophil and macrophage killing and platelet aggregation. Here we discuss the current understanding of the structure and function of these recently discovered receptors and adapter molecules in the regulation of adaptive and innate immune responses.

Structure of CD84 provides insight into SLAM family function

The signaling lymphocyte activation molecule (SLAM) family includes homophilic and heterophilic receptors that modulate both adaptive and innate immune responses. These receptors share a common ectodomain organization: a membrane-proximal immunoglobulin constant domain and a membrane-distal immunoglobulin variable domain that is responsible for ligand recognition. CD84 is a homophilic family member that enhances IFN-gamma secretion in activated T cells. Our solution studies revealed that CD84 strongly self-associates with a K(d) in the submicromolar range. These data, in combination with previous reports, demonstrate that the SLAM family homophilic affinities span at least three orders of magnitude and suggest that differences in the affinities may contribute to the distinct signaling behavior exhibited by the individual family members. The 2.0 A crystal structure of the human CD84 immunoglobulin variable domain revealed an orthogonal homophilic dimer with high similarity to the recently reported homophilic dimer of the SLAM family member NTB-A. Structural and chemical differences in the homophilic interfaces provide a mechanism to prevent the formation of undesired heterodimers among the SLAM family homophilic receptors. These structural data also suggest that, like NTB-A, all SLAM family homophilic dimers adopt a highly kinked organization spanning an end-to-end distance of approximately 140 A. This common molecular dimension provides an opportunity for all two-domain SLAM family receptors to colocalize within the immunological synapse and bridge the T cell and antigen-presenting cell.

Regulation of cellular and humoral immune responses by the SLAM and SAP families of molecules

SAP (SLAM-associated protein) was identified in 1998 as an adaptor molecule involved in the intracellular signaling pathways elicited through the cell surface receptor SLAM and as the protein defective in the human immunodeficiency X-linked lymphoproliferative disease (XLP). During the past eight years, it has been established that the SLAM family of cell surface receptors (SLAM, 2B4, NTB-A, Ly9, CD84) and the SAP family of adaptors (SAP, EAT-2, ERT) play critical roles in lymphocyte development, differentiation, and acquisition of effector functions. Studies of these proteins have shown unexpected roles in cytokine production by T cells and myeloid cells, T cell-dependent humoral immune responses, NK cell-mediated cytotoxicity, and NKT cell development. This review highlights recent findings that have improved our understanding of the roles of the SLAM and SAP families of molecules in immune regulation and discusses how perturbations in the signaling pathways involving these proteins can result in different disease states.

The role of SLAM family receptors in immune cell signaling

The signaling lymphocyte-activating molecule (SLAM) family immunoreceptors are expressed in a wide array of immune cells, including both T and B lymphocytes. By virtue of their ability to transduce tyrosine phosphorylation signals through the so-called ITSM (immunoreceptor tyrosine-based switch motif) sequences, they play an important part in regulating both innate and adaptive immune responses. The critical role of the SLAM immunoreceptors in mediating normal immune reactions was highlighted in recent findings that SAP, a SLAM-associated protein, modulates the activities of various immune cells through interactions with different members of the SLAM family expressed in these cells. Importantly, mutations or deletions of the sap gene in humans result in the X-linked lymphoproliferative syndrome. In this review, we summarize current knowledge and survey the latest developments in signal transduction events triggered by the activation of SLAM family receptors in different cell types.

Human Ly9 (CD229) as novel tumor-associated antigen (TAA) in chronic lymphocytic leukemia (B-CLL) recognized by autologous CD8+ T cells

OBJECTIVE

CD229, a cell-surface molecule being involved in cell adhesion, is overexpressed in B-CLL cells. In this study we wanted to explore whether CD229 might function as B-CLL-specific tumor-associated antigen (TAA).

PATIENTS AND METHODS

Autologous, CD229-specific HLA-A2-restricted T cells were identified using IFN-gamma-ELISPOT assays and HLA-A2/dimer-peptide staining after 4 weeks of in vitro culture.

RESULTS

We were able to expand autologous T cells from 9/11 B-CLL patients using native B-CLL cells as antigen presenting cells (APCs) in 5 cases, whereas for 4 samples an autologous T-cell response could only be evoked by use of CD40L-stimulated B-CLL cells as APCs. The number of CD8+ T cells could be expanded during 4 weeks of in vitro culture with native or CD40L-activated B-CLL cells while the amount of specific T cells recognizing CD229 peptides bound to HLA-A2 dimers increased on average 12-fold (native CLL) and 13-fold (CD40L-activated CLL), respectively. Using IFN-gamma-ELISPOT assays we could demonstrate that the expanded T cells were able to secrete IFN-gamma upon recognition of the antigen. These T cells not only recognized HLA-A0201-binding CD229-derived peptides presented by T2 cells, but also CD229-overexpressing autologous B-CLL cells in an MHC-I-restricted manner.

CONCLUSION

In summary, CD229 was shown to be naturally processed and presented as TAA in primary B-CLL cells, enabling the expansion of autologous tumor-specific T cells.

Molecular Analysis of NTB-A Signaling: A Role for EAT-2 in NTB-A-Mediated Activation of Human NK Cells

Engagement of NTB-A on human NK cells by homophilic interaction with NTB-A-expressing target cells can trigger NK cell cytotoxicity, cytokine production, and proliferation. To better understand how NTB-A can activate NK cells, we analyzed the molecular mechanisms of NTB-A signaling. We show that NTB-A is tyrosine phosphorylated in unstimulated human NK cells and associates with SLAM-associated protein (SAP) and EAT-2. This phosphorylation of NTB-A is mediated by Src family kinases and is most likely a result of the homophilic interaction of NTB-A among neighboring NK cells. Stimulation of NK cells by NTB-A-positive targets results in increased NTB-A phosphorylation. The cytoplasmic tail of NTB-A contains three tyrosines, two of which are embedded within an immunoreceptor tyrosine-based switch motif. We generated a NTB-A-negative NK cell line, in which we expressed different mutants of NTB-A. Functional studies showed that the second tyrosine is sufficient and essential for NTB-A-mediated cytotoxicity. EAT-2, but not SAP, is recruited to this second tyrosine, indicating that SAP may be dispensable for this NTB-A function. To further investigate this, we silenced SAP expression in NK cell lines. Functional analysis of these cells showed that NTB-A can mediate NK cell cytotoxicity in the absence of SAP, probably via EAT-2. In contrast, NTB-A-mediated IFN-gamma production was greatly reduced in the absence of SAP, demonstrating that cytokine production and cytotoxicity are differentially dependent on SAP and possibly EAT-2.

The CD2 family of natural killer cell receptors

The CD2 family of receptors is evolutionarily conserved and widely expressed on cells within the hematopoietic compartment. In recent years several new members have been identified with important roles in the immune system. CD2 family members regulate natural killer (NK) cell lytic activity and inflammatory cytokine production when engaged by ligands on tumor cells. Furthermore, a subfamily of CD2 receptors, the CD 150-like molecules, has been implicated in the pathogenesis of X-linked lymphoproliferative disease (XLP). Many of these receptors have now been shown to bind homophilically or heterophilically to other molecules within the family. With these discoveries a novel mechanism for lymphocyte regulation has emerged: CD2 family members on NK cells engage ligands on neighboring NK cells, leading to NK cell stimulation. Moreover, heterotypic stimulatory interactions between NK cells and other leukocytes also occur. In this manner, CD2 family members may provide interlymphocyte communication that maintains organization within the hematopoietic compartment and amplifies immune responses. This review discusses these multiple roles for CD2 family members, focusing specifically on the regulation of NK cells.

2B4/CD48-mediated regulation of lymphocyte activation and function

2B4 (CD244) is a member of the CD2 subset of the Ig superfamily. This molecule is expressed on innate immune cells, including NK cells, and on subsets of T cells. The 2B4 molecule interacts with CD48, which is widely expressed on hemopoietic cells. Although earlier reports demonstrated a role for 2B4 as an activating receptor in both mice and humans, recent studies of 2B4-deficient mice have suggested that 2B4 functions predominantly as an inhibitory receptor in mice. In addition, 2B4 may also act as a costimulatory ligand for cells expressing CD48. Thus, the 2B4 molecule is more multifunctional than previously understood. In this study, we delineate the current view of 2B4-CD48 interactions among lymphocytes and other cells.

Impaired Ig class switch in mice deficient for the X-linked lymphoproliferative disease gene Sap

X-linked lymphoproliferative disease (XLP) is characterized by abnormal immune responses to Epstein-Barr virus attributed to inactivating mutations of the SAP gene. Previous studies showed immunoglobulin E (IgE) deficiency and low serum IgG levels in Sap-deficient mice before and after viral infections, which are associated with impaired CD4+ T-helper function. In the present work, we find that signaling lymphocytic activation molecule (SLAM)-associated protein (SAP) is expressed in B cells and this expression is down-regulated after stimulation with lipopolysaccharide (LPS) and interleukin 4 (IL-4). We demonstrate that B cells from Sap-deficient mice exhibit reduced IgG and IgA production in vitro. This impairment correlates with decreased circular transcript levels of Iα, Iγ2a, Iγ2b, and Iγ3 after stimulation, which indicate a defective Ig switch recombination in Sap-deficient B cells. While XLP is believed to cause defects in T, natural killer T (NKT), and natural killer (NK) cells, our results indicate that B cells are also affected. (Blood. 2005;106:2069-2075)

Molecular and cellular pathogenesis of X-linked lymphoproliferative disease

X-linked lymphoproliferative disease (XLP) is an inherited immune defect caused by mutations in the Src homology 2 domain-containing gene 1A, which encodes the adapter protein, signaling lymphocytic activation molecule (SLAM)-associated protein (SAP). SAP is expressed in T cells, natural killer (NK) cells, and NKT cells, where it binds to the cytoplasmic domain of the surface receptor SLAM (CD150) and the related receptors, 2B4 (CD244), CD84, Ly9 (CD229), NK-T-B-antigen, and CD2-like receptor-activating cytotoxic T cells. SAP also binds to the Src family tyrosine kinase Fyn and recruits it to SLAM, which leads to the generation of downstream phosphotyrosine signals. While the roles of the SLAM family receptors are only beginning to be understood, experiments suggest that these molecules regulate important aspects of lymphocyte function, such as proliferation, cytokine secretion, cytotoxicity, and antibody production. Thus, in XLP patients who lack functional SAP, the SLAM family receptors may not signal properly. This property likely contributes to the phenotypes of XLP, including fulminant infectious mononucleosis, lymphoma, and hypogammaglobulinemia. Further studies of SAP and the SLAM family receptors will provide insights into XLP and elucidate the signaling events regulating lymphocyte ontogeny and function.

Potential genetic modifiers of the cystic fibrosis intestinal inflammatory phenotype on mouse chromosomes 1, 9, and 10

Background

Although cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, the severity of disease is highly variable indicating the influence of modifier genes. The intestines of Cftr deficient mice (CF mice: Cftr^tm1Unc) are prone to obstruction by excessive mucus accumulation and are used as a model of meconium ileus and distal intestinal obstruction syndrome. This phenotype is strongly dependent on the genetic background of the mice. On the C57Bl/6 background, the majority of CF mice cannot survive on solid mouse chow, have inflammation of the small intestine, and are about 30% smaller than wild type littermates. In this work potential modifier loci of the CF intestinal phenotype were identified.

Results

CF mice on a mixed genetic background (95% C57Bl/6 and 5% 129Sv) were compared to CF mice congenic on the C57Bl/6 background for several parameters of the intestinal CF phenotype. CF mice on the mixed background exhibit significantly greater survival when fed dry mouse chow, have reduced intestinal inflammation as measured by quantitative RT-PCR for marker genes, have near normal body weight gain, and have reduced mucus accumulation in the intestinal crypts. There was an indication of a gender effect for body weight gain: males did not show a significant improvement at 4 weeks of age, but were of normal weight at 8 weeks, while females showed improvement at both 4 and 8 weeks. By a preliminary genome-wide PCR allele scanning, three regions were found to be potentially associated with the milder phenotype. One on chr.1, defined by marker D1Mit36, one on chr. 9 defined by marker D9Mit90, and one on chr. 10, defined by marker D10Mit14.

Conclusion

Potential modifier regions were found that have a positive impact on the inflammatory phenotype of the CF mouse small intestine and animal survival. Identification of polymorphisms in specific genes in these regions should provide important new information about genetic modifiers of the CF intestinal phenotype.

Cutting Edge: The SLAM Family Receptor Ly108 Controls T Cell and Neutrophil Functions

Ly108, a glycoprotein of the signaling lymphocytic activation molecule family of cell surface receptors expressed by T, B, NK, and APCs has been shown to have a role in NK cell cytotoxicity and T cell cytokine responses. In this study, we describe that CD4(+) T cells from mice with a targeted disruption of exons 2 and 3 of Ly108 (Ly108(DeltaE2+3)) produce significantly less IL-4 than wild-type CD4(+) cells, as judged by in vitro assays and by in vivo responses to cutaneous infection with Leishmania mexicana. Surprisingly, neutrophil functions are controlled by Ly108. Ly108(DeltaE2+3) mice are highly susceptible to infection with Salmonella typhimurium, bactericidal activity of Ly108(DeltaE2+3) neutrophils is defective, and their production of IL-6, IL-12, and TNF-alpha is increased. The aberrant bactericidal activity by Ly108(DeltaE2+3) neutrophils is a consequence of severely reduced production of reactive oxygen species following phagocytosis of bacteria. Thus, Ly108 serves as a regulator of both innate and adaptive immune responses.

Expression of the SH2D1A gene is regulated by a combination of transcriptional and post-transcriptional mechanisms

The SH2D1A gene, which is altered or deleted in patients with X-linked lymphoproliferative disease, encodes the small protein SAP (for SLAM-associated protein) that is expressed in T and NK cells. A 22-bp fragment in close proximity to an initiator-like site was defined as the basal promoter of mouse SH2D1A, and a highly homologous 33-bp segment was defined as the human basal promoter. When an Ets consensus site was mutated, no reporter activity was detectable. Gel mobility supershift assays revealed that the two transcription factors Ets-1 and Ets-2 bind to the human and mouse sequences. The involvement of Ets-1 and Ets-2 in expression of SH2D1A was functionally confirmed by overexpression studies of their dominant-negative forms. We also found that SH2D1A mRNA decays very rapidly in mouse T cells, and its 3' untranslated region (UTR) has RNA-destabilizing activity in transfection studies with reporter/3' UTR constructs. As judged by RNA-gel mobility shift assays, this rapid degradation of SH2D1A mRNA was due to a balance in binding of the factors AUF1 and HuR to its 3' UTR. Although the SH2D1A mRNA level decreased upon triggering of the T cell receptor (TCR), the RNA degradation rate itself was not altered by TCR engagement.

Cutting Edge: NTB-A Activates NK Cells via Homophilic Interaction

NK cells are an important component of the innate immune system. Their activity is tightly regulated by activating and inhibitory surface receptors. However, the exact functions of many activating surface receptors, as well as their ligands, still remain to be elucidated. NTB-A is a receptor on the surfaces of human NK, T, and B cells, mediating a signal whose malfunction may be involved in X-linked lymphoproliferative disease. However, the ligand of NTB-A has remained elusive so far. Using trimeric recombinant proteins, we now show that NTB-A is its own ligand. Homophilic interaction of NTB-A enhances NK cell cytotoxicity and influences NK cell proliferation and IFN-gamma secretion. We suggest that NTB-A is an interlymphocyte signaling molecule, which serves to orchestrate the activities of immune cells.

NTB-A, a New Activating Receptor in T Cells That Regulates Autoimmune Disease

The CD28 co-stimulatory pathway is well established for T cell activation; however, results from CD28 -/- mice suggest the existence of additional co-stimulatory pathways. Here we report the further characterization of a new member of the CD2 superfamily, NTB-A, important in T cell co-stimulation. NTB-A is expressed on T cells, and its expression is up-regulated on activated cells. Triggering of NTB-A with monoclonal antibodies in the absence of CD28 signals leads to T cell proliferation and interferon-gamma secretion but not interleukin-4. Cross-linking of NTB-A also induces phosphorylation of NTB-A and the association of SAP (SLAM-associated protein), the protein absent in X-linked lymphoproliferative disease. T helper cells differentiated by cross-linking NTB-A and CD3 developed predominantly into Th1 cells not Th2 cells. In vivo blocking of NTB-A interactions with its ligands by using soluble NTB-A-Fc fusion protein inhibits B cell isotype switching to IgG2a and IgG3, commonly induced by Th1-type cytokines. Most important, treatment of mice with NTB-A-Fc delays the onset of antigen-induced experimental allergic encephalomyelitis in myelin basic protein-T cell receptor transgenic mice, suggesting a role in T cell-mediated autoimmune disease. Regulation of interferon-gamma secretion, and not interleukin-4 in vitro, as well as inhibition of Th1 cell-induced isotype switching and attenuation of experimental allergic encephalomyelitis indicate that NTB-A is important for Th1 responses. The observation that cross-linking of NTB-A induces T cell activation, expansion, and Th1-type cytokine production suggests NTB-A is a novel co-stimulatory receptor. The identification of NTB-A as a regulator of T cell response paves the way to provide novel therapeutic approaches for modulation of the immune response.

Mice deficient in the X-linked lymphoproliferative disease gene sap exhibit increased susceptibility to murine gammaherpesvirus-68 and hypo-gammaglobulinemia

X-linked lymphoproliferative disease is characterized by immune dysregulation and uncontrolled lymphoproliferation on exposure to Epstein-Barr virus (EBV). This disease has been attributed to mutations in the SAP gene (also denominated as SH2D1A or DSHP). To delineate the role of SAP in the pathophysiology of X-linked lymphoproliferative disease, a strain of sap-deficient mice has been generated by deleting exon 2 of the gene. After infection with murine gammaherpesvirus-68, which is homologous to EBV, the mutant mice exhibit more vigorous CD8+ T cell proliferation and more disseminated lymphocyte infiltration compared to their wild-type littermates. Chronic tissue damage and hemophagocytosis were evident in sap-deficient mice but not in their wild-type littermates. Concordantly, murine gammaherpesvirus-68 reactivation was observed in sap-deficient mice, indicating an impaired control of the virus. Notably, IgE deficiency and decreased serum IgG level were observed in mutant mice prior to and after murine gammaherpesvirus-68 infection, which reproduces hypo-gammaglobulinemia in X-linked lymphoproliferative disease patients. This mouse model will therefore be a useful tool for dissecting the various phenotypes of X-linked lymphoproliferative disease.

The SAP and SLAM families in immune responses and X-linked lymphoproliferative disease

SAP (signalling lymphocytic activation molecule (SLAM)-associated protein) is a T- and natural killer (NK)-cell-specific protein containing a single SH2 domain encoded by a gene that is defective or absent in patients with X-linked lymphoproliferative syndrome (XLP). The SH2 domain of SAP binds with high affinity to the cytoplasmic tail of the haematopoietic cell-surface glycoprotein SLAM and five related receptors. SAP regulates signal transduction of the SLAM-family receptors by recruiting SRC kinases. Similarly, the SAP-related proteins EAT2A and EAT2B are thought to control signal transduction that is initiated by SLAM-related receptors in professional antigen-presenting cells. In this review, we discuss recent findings on the structure and function of proteins of the SAP and SLAM families.

Functional requirements for interactions between CD84 and Src homology 2 domain-containing proteins and their contribution to human T cell activation

Cell surface receptors belonging to the CD2 subset of the Ig superfamily of molecules include CD2, CD48, CD58, 2B4, signaling lymphocytic activation molecule (SLAM), Ly9, CD84, and the recently identified molecules NTB-A/Ly108/SLAM family (SF) 2000, CD84H-1/SF2001, B lymphocyte activator macrophage expressed (BLAME), and CRACC (CD2-like receptor-activating cytotoxic cells)/CS-1. Some of these receptors, such as CD2, SLAM, 2B4, CRACC, and NTB-A, contribute to the activation and effector function of T cells and NK cells. Signaling pathways elicited via some of these receptors are believed to involve the Src homology 2 (SH2) domain-containing cytoplasmic adaptor protein SLAM-associated protein (SAP), as it is recruited to SLAM, 2B4, CD84, NTB-A, and Ly-9. Importantly, mutations in SAP cause the inherited human immunodeficiency X-linked lymphoproliferative syndrome (XLP), suggesting that XLP may result from perturbed signaling via one or more of these SAP-associating receptors. We have now studied the requirements for SAP recruitment to CD84 and lymphocyte activation elicited following ligation of CD84 on primary and transformed human T cells. CD84 was found to be rapidly tyrosine phosphorylated following receptor ligation on activated T cells, an event that involved the Src kinase Lck. Phosphorylation of CD84 was indispensable for the recruitment of SAP, which was mediated by Y(262) within the cytoplasmic domain of CD84 and by R(32) within the SH2 domain of SAP. Furthermore, ligating CD84 enhanced the proliferation of anti-CD3 mAb-stimulated human T cells. Strikingly, this effect was also apparent in SAP-deficient T cells obtained from patients with XLP. These results reveal a novel function of CD84 on human lymphocytes and suggest that CD84 can activate human T cells via a SAP-independent mechanism.

Absence of SLAM mutations in EBV-associated lymphoproliferative disease patients

X-linked lymphoproliferative disease is a rare inherited immunodeficiency in which affected males present abnormal responses to Epstein-Barr virus (EBV) infection. The gene defective in X-linked lymphoproliferative disease, SH2D1A (also named SAP or DSHP), has been identified and shown to code for an adapter protein that interacts with signaling lymphocytic activation molecule (SLAM) and several other members of the CD2 superfamily. SH2D1A is mutated in no more than 60% of X-linked lymphoproliferative disease patients. It could be postulated that a certain percentage of patients without apparent maternal transmission might be caused by other gene(s) in SH2D1A-related signal transduction pathways. Being a partner of SH2D1A and having a key role in proliferation and differentiation of the T- and B-lymphocytes, SLAM was considered as a candidate gene for patients who manifest symptoms of X-linked lymphoproliferative disease but who have no mutations in SH2D1A. As a first step, SLAM mutations were screened for from cDNA of the lymphoblastoid cell line of all available patients. Then conditions for PCR, single-strand conformational polymorphism (SSCP), heteroduplex analysis, and sequencing were established in all eight exons of SLAM. A total of 31 typical and atypical patients were analysed, from which six novel nucleotide variants were identified; however, none of these variants seems to cause abnormal function of the SLAM gene. Therefore, mutations in coding regions or splicing sites of SLAM are unlikely to play a major role in the mechanism of EBV-associated lymphoproliferation.

Molecular and immunological basis of X-linked lymphoproliferative disease

X-linked lymphoproliferative (XLP) disease is a human immune dysfunction characterized primarily by an inappropriate response to Epstein-Barr virus infection. In 1998, it was discovered that XLP is caused by inactivating mutations in the SAP/SH2D1A/DSHP gene. This gene codes for an immune cell-specific polypeptide termed SAP (SLAM-associated protein) that is composed almost exclusively of an Src homology 2 (SH2) domain. By way of its SH2 domain, SAP interacts with tyrosine-based motifs located in the cytoplasmic region of members of the SLAM (signaling lymphocyte activation molecule) family of receptors. Recent findings indicate that SAP is required for the function of SLAM-related receptors, as a consequence of its capacity to promote the recruitment and activation of the Src-related protein tyrosine kinase FynT, thereby allowing SLAM receptor-mediated protein tyrosine phosphorylation signals in immune cells. Functional and genetic analyses suggest that the phenotype associated with XLP is caused in large part by defects in the functions of SLAM-related receptors due to SAP deficiency.

Measles virus infects and suppresses proliferation of T lymphocytes from transgenic mice bearing human signaling lymphocytic activation molecule

Humans are the only natural reservoir of measles virus (MV), one of the most contagious viruses known. MV infection and the profound immunosuppression it causes are currently responsible for nearly one million deaths annually. Human signaling lymphocytic activation molecule (hSLAM) was identified as a receptor for wild-type MV as well as for MV strains prepared as vaccines. To better evaluate the role of hSLAM in MV pathogenesis and MV-induced immunosuppression, we created transgenic (tg) mice that expressed the hSLAM molecule under the control of the lck proximal promoter. hSLAM was expressed on CD4(+) and CD8(+) T cells in the blood and spleen and also on CD4(+), CD8(+), CD4(+) CD8(+), and CD4(-) CD8(-) thymocytes. Wild-type MV, after limited passage on B95-8 marmoset B cells, and the Edmonston laboratory strain of MV infected hSLAM-expressing cells. There was a direct correlation between the amount of hSLAM expressed on the cells' surface and the degree of viral infection. Additionally, MV infection induced downregulation of receptor hSLAM and inhibited cell division and proliferation of hSLAM(+) but not hSLAM(-) T cells. Therefore, these tg mice provide the opportunity for analyzing and comparing MV-T cell interactions and MV pathogenesis in cells expressing only the hSLAM MV receptor with those of tg mice whose T cells selectively express another MV receptor, CD46.

Dual functional roles for the X-linked lymphoproliferative syndrome gene product SAP/SH2D1A in signaling through the signaling lymphocyte activation molecule (SLAM) family of immune receptors

The X-linked lymphoproliferative (XLP) syndrome gene encodes a protein named SAP or SH2D1A that is composed of a single Src homology 2 (SH2) domain. Two models have been proposed for its function in lymphocyte signaling. One postulates that it acts as an inhibitor of interactions between the phosphatase SHP-2 and the immune receptor SLAM. The other suggests that it functions as an adaptor to promote the recruitment of a kinase, FynT, to SLAM. Here, we provide evidence in support of both roles for SAP. Using an array of peptides derived from the SLAM family of receptors, we demonstrate that SAP binds with comparable affinities to the same sites in these receptors as do the SH2 domains of SHP-2 and SHIP, suggesting that these three proteins may compete against one another in binding to a given SLAM family receptor. Furthermore, in vitro and in vivo binding studies indicate that SAP is capable of binding directly to FynT, an interaction mediated by the FynT SH3 domain. In cells, FynT was shown to be indispensable for SLAM tyrosine phosphorylation, which, in turn, was drastically enhanced by SAP. Because SAP also blocked the recruitment of SHP-2 to SLAM in these cells, we propose a dual functional role for SAP in SLAM signaling by acting both as an adaptor for FynT and an inhibitor to SHP-2 binding. The physiological relevance of the dual functional role for SAP is underscored by the observation that disease-causing SAP mutants exhibited significantly reduced affinities to both FynT and SLAM.

The dual-function CD150 receptor subfamily: the viral attraction

The CD150 subfamily within the CD2 family is a growing group of dual-function receptors that have within their cytoplasmic tails a characteristic signaling motif. The ITSM (immunoreceptor tyrosine-based switch motif) enables these receptors to bind to and be regulated by small SH2 domain adaptor proteins, including SH2D1A (SH2-containing adaptor protein SH2 domain protein 1A) and EAT-2 (EWS-activated transcript 2). A major signaling pathway through the prototypic receptor in this subfamily, CD150, leads to the activation of interferon-gamma, a key cytokine for viral immunity. As a result, many viruses have designed strategies to usurp or alter CD150 functions. Measles virus uses CD150 as a receptor and Molluscum contagiosum virus encodes proteins that are homologous to CD150. Thus, viruses use CD150 subfamily receptors to create a favorable environment to elude detection and destruction. Understanding the CD150 subfamily may lead to new strategies for vaccine development and antiviral therapies.

Genetics of autoimmune diseases in humans and in animal models

Recent applications of the genetic characterisation of autoimmunity in humans and in animal models have allowed the further mapping of many disease loci and, in some cases, the identification of disease genes. New approaches to the analysis of mapping, characterisation and identification of susceptibility genes have also been developed.