B cell induction of IL-13 expression in NK cells: role of CD244 and SLAM-associated protein

B-cell performance in chemotherapy: Unravelling the mystery of B-cell therapeutic potential

BACKGROUND AND MAIN BODY

The anti-tumour and tumour-promoting roles of B cells in the tumour microenvironment (TME) have gained considerable attention in recent years. As essential orchestrators of humoral immunity, B cells potentially play a crucial role in anti-tumour therapies. Chemotherapy, a mainstay in cancer treatment, influences the proliferation and function of diverse B-cell subsets and their crosstalk with the TME. Modulating B-cell function by targeting B cells or their associated cells may enhance chemotherapy efficacy, presenting a promising avenue for future targeted therapy investigations.

CONCLUSION

This review explores the intricate interplay between chemotherapy and B cells, underscoring the pivotal role of B cells in chemotherapy treatment. We summarise promising B-cell-related therapeutic targets, illustrating the immense potential of B cells in anti-tumour therapy. Our work lays a theoretical foundation for harnessing B cells in chemotherapy and combination strategies for cancer treatment.

KEY POINTS

Chemotherapy can inhibit B-cell proliferation and alter subset distributions and functions, including factor secretion, receptor signalling, and costimulation. Chemotherapy can modulate complex B-cell-T-cell interactions with variable effects on anti-tumour immunity. Targeting B-cell surface markers or signalling improves chemotherapy responses, blocks immune evasion and inhibits tumour growth. Critical knowledge gaps remain regarding B-cell interactions in TME, B-cell chemoresistance mechanisms, TLS biology, heterogeneity, spatial distributions, chemotherapy drug selection and B-cell targets that future studies should address.

Applications of CRISPR technology in cellular immunotherapy

CRISPR technology has transformed multiple fields, including cancer and immunology. CRISPR-based gene editing and screening empowers direct genomic manipulation of immune cells, opening doors to unbiased functional genetic screens. These screens aid in the discovery of novel factors that regulate and reprogram immune responses, offering novel drug targets. The engineering of immune cells using CRISPR has sparked a transformation in the cellular immunotherapy field, resulting in a multitude of ongoing clinical trials. In this review, we discuss the development and applications of CRISPR and related gene editing technologies in immune cells, focusing on functional genomics screening, gene editing-based cell therapies, as well as future directions in this rapidly advancing field.

Neutrophilic granulocyte-derived B-cell activating factor supports B cells in skin lesions in hidradenitis suppurativa

BACKGROUND

Hidradenitis suppurativa (HS) is a chronic inflammatory disease characterized by painful inflamed nodules, abscesses, and pus-draining tunnels appearing in axillary, inguinal, and perianal skin areas. HS lesions contain various types of immigrated immune cells.

OBJECTIVE

This study aimed to characterize mediators that support lesional B/plasma cell persistence in HS.

METHODS

Skin samples from several cohorts of HS patients and control cohorts were assessed by mRNA sequencing, quantitative PCR on reverse-transcribed RNA, flow cytometry, and immunohistofluorescence. Blood plasma and cultured skin biopsy samples, keratinocytes, dermal fibroblasts, neutrophilic granulocytes (neutrophils), monocytes, and B cells were analyzed. Complex systems biology approaches were used to evaluate bulk and single-cell RNA sequencing data.

RESULTS

Proportions of B/plasma cells, neutrophils, CD8+ T cells, and M0 and M1 macrophages were elevated in HS lesions compared to skin of healthy and perilesional intertriginous areas. There was an association between B/plasma cells, neutrophils, and B-cell activating factor (BAFF, aka TNFSF13B). BAFF was abundant in HS lesions, particularly in nodules and abscesses. Among the cell types present in HS lesions, myeloid cells were the main BAFF producers. Mechanistically, granulocyte colony-stimulating factor in the presence of bacterial products was the major stimulus for neutrophils' BAFF secretion. Lesional upregulation of BAFF receptors was attributed to B cells (TNFRSF13C/BAFFR and TNFRSF13B/TACI) and plasma cells (TNFRSF17/BCMA). Characterization of the lesional BAFF pathway revealed molecules involved in migration/adhesion (eg, CXCR4, CD37, CD53, SELL), proliferation/survival (eg, BST2), activation (eg, KLF2, PRKCB), and reactive oxygen species production (eg, NCF1, CYBC1) of B/plasma cells.

CONCLUSION

Neutrophil-derived BAFF supports B/plasma cell persistence and function in HS lesions.

Perturbomics of tumor-infiltrating NK cells

Natural killer (NK) cells are an innate immune cell type that serves at the first level of defense against pathogens and cancer. NK cells have clinical potential, however, multiple current limitations exist that naturally hinder the successful implementation of NK cell therapy against cancer, including their effector function, persistence, and tumor infiltration. To unbiasedly reveal the functional genetic landscape underlying critical NK cell characteristics against cancer, we perform perturbomics mapping of tumor infiltrating NK cells by joint in vivo AAV-CRISPR screens and single cell sequencing. We establish a strategy with AAV-SleepingBeauty(SB)- CRISPR screening leveraging a custom high-density sgRNA library targeting cell surface genes, and perform four independent in vivo tumor infiltration screens in mouse models of melanoma, breast cancer, pancreatic cancer, and glioblastoma. In parallel, we characterize single-cell transcriptomic landscapes of tumor-infiltrating NK cells, which identifies previously unexplored sub-populations of NK cells with distinct expression profiles, a shift from immature to mature NK (mNK) cells in the tumor microenvironment (TME), and decreased expression of mature marker genes in mNK cells. CALHM2, a calcium homeostasis modulator that emerges from both screen and single cell analyses, shows both in vitro and in vivo efficacy enhancement when perturbed in chimeric antigen receptor (CAR)-NK cells. Differential gene expression analysis reveals that CALHM2 knockout reshapes cytokine production, cell adhesion, and signaling pathways in CAR- NKs. These data directly and systematically map out endogenous factors that naturally limit NK cell function in the TME to offer a broad range of cellular genetic checkpoints as candidates for future engineering to enhance NK cell-based immunotherapies.

Downregulation of zinc finger protein 71 expression in oral squamous cell carcinoma tissues and its underlying molecular mechanism

BACKGROUND

Patients with oral squamous cell carcinoma (OSCC) have poor prognoses due to a limited understanding of the pathogenesis of OSCC. Zinc finger protein (ZNF) is the largest transcription factor family in the human genome and exert diverse and important functions. Nevertheless, the exact expression status and molecular mechanism of ZNF71 have not been described in OSCC. Therefore, this study aimed to identify the specific expression level of ZNF71 in OSCC tissues and to further interpret the potential molecular mechanism of ZNF71 in the pathogenesis of OSCC.

METHODS

In-house immunohistochemical staining of 116 OSCC samples and 29 non-OSCC samples was employed to detect the expression status of ZNF71 at the protein level of OSCC tissues. Single-cell RNA sequencing data from 7 OSCC samples was used to explore the expression landscape of ZNF71 in different cell types from OSCC tissues. High-throughput RNA sequencing data and gene chips data from 893 OSCC samples and 301 non-OSCC samples were utilized to identify the specific expression level of ZNF71 at the bulk mRNA level of OSCC tissues. Here, standardized mean difference (SMD) value was applied to calculate the expression differences between OSCC group and non-OSCC group. Multiple datasets were included; hence, the results were considered to be more reliable. Sensitivity analysis was conducted to evaluate the stability of the results. Enrichment analysis and immune infiltration analysis were used to explore the underlying molecular mechanism of ZNF71 in OSCC.

RESULTS

ZNF71 was significantly downregulated in OSCC tissues at the protein level (SMD = -1.96, 95 % confidence interval [95 % CI]: -2.43 to -1.50). ZNF71 was absent in various cell types from OSCC tissues including cancerous epithelial cells and tumor-infiltrating immune cells. ZNF71 was downregulated in OSCC tissues at the bulk mRNA level (SMD = -0.38, 95 % CI: -0.75 to -0.02). Enrichment analysis showed that positively and differentially co-expressed genes mainly concentrated on "herpes simplex virus 1 infection" and "regulation of plasma membrane bounded cell projection organization", and negatively and differentially co-expressed genes mainly participated in "cell cycle" and "DNA metabolic process". Moreover, the putative target genes of ZNF71 mainly participated in "cellular respiration" and "protein catabolic process". Finally, immune infiltration analysis revealed that ZNF71 expression was positively correlated with multiple immune cells including activated B cells, memory B cells, and natural killer (NK) cells, and negatively correlated with various immune cells, including CD56 bright NK cells, neutrophil, and immature dendritic cells.

CONCLUSION

The downregulation of ZNF71 may influence the initiation and promotion of OSCC by reducing immune infiltration, accelerating cell cycle progression, and affecting metabolic process, and this requires further research.

Natural Killer Cell Regulation of B Cell Responses in the Context of Viral Infection

Secretion of both neutralizing and nonneutralizing virus-specific antibodies by B cells is a key component of immune control of many virus infections and a critical benchmark of successful preventative vaccines. Natural killer (NK) cells also play a vital role in antiviral immune defense via cytolytic elimination of infected cells and production of proinflammatory antiviral cytokines. Accumulating evidence points to multifaceted crosstalk between NK cells and antiviral B cell responses that can determine virus elimination, pathogenesis of infection, and efficacy of vaccine-elicited protection. These outcomes are a result of both positive and negative influences of NK cells on the B cell responses, as well as canonical antiviral killing of infected B cells. On one hand, NK cell-derived cytokines such as interferon-gamma (IFN-γ) may promote B cell activation and enhance immunoglobulin production. In contrast, NK cell immunoregulatory killing of CD4 T cells can limit affinity maturation in germinal centers resulting in weak infection or vaccine induction of antiviral neutralizing antibodies. In this review, we will discuss these and other dueling contributions of NK cells to B cell responses during virus infection or vaccination.

Natural Killer Cells: Development, Maturation, and Clinical Utilization

Natural killer (NK) cells are the predominant innate lymphocyte subsets that mediate anti-tumor and anti-viral responses, and therefore possess promising clinical utilization. NK cells do not express polymorphic clonotypic receptors and utilize inhibitory receptors (killer immunoglobulin-like receptor and Ly49) to develop, mature, and recognize “self” from “non-self.” The essential roles of common gamma cytokines such as interleukin (IL)-2, IL-7, and IL-15 in the commitment and development of NK cells are well established. However, the critical functions of pro-inflammatory cytokines IL-12, IL-18, IL-27, and IL-35 in the transcriptional-priming of NK cells are only starting to emerge. Recent studies have highlighted multiple shared characteristics between NK cells the adaptive immune lymphocytes. NK cells utilize unique signaling pathways that offer exclusive ways to genetically manipulate to improve their effector functions. Here, we summarize the recent advances made in the understanding of how NK cells develop, mature, and their potential translational use in the clinic.

CD244 is expressed on dendritic cells and regulates their functions

Signaling lymphocytic activation molecule (SLAM) receptors have an important role in the development of immune responses because of their roles, for exampe, in NK cell cytotoxicity and cytokine production by NK, T cells and myeloid cells. The SLAM receptor CD244 (2B4, SLAMf4) is expressed on a variety of immune cell types but most of its functions have been examined on NK and T cells. In the present study, we investigated expression and function of CD244 in murine subsets of dendritic cells (DCs). We report that all subsets of murine DCs examined expressed CD244, although the expression levels of CD244 varied between subsets. Splenic and resident mesenteric lymph node (MLN) DCs from CD244(-/-) mice expressed lower levels of CD86 and MHC class II compared with wild-type mice. Upon Toll-like receptor (TLR) stimulation, no differences in surface expression of these molecules were observed between DCs from CD244(-/-) and wild-type mice. However, splenic DCs from CD244(-/-) mice upon stimulation with TLR binding ligands lipopolysaccharide (LPS) and CpG produced significantly higher levels of pro-inflammatory cytokines. In addition, DCs from CD244(-/-) mice elicited increased NK cell activation in vitro. These data add CD244 to a growing list of immuno-modulatory receptors found on DCs.

Combined immune checkpoint protein blockade and low dose whole body irradiation as immunotherapy for myeloma

BACKGROUND

Multiple myeloma is characterized by the presence of transformed neoplastic plasma cells in the bone marrow and is generally considered to be an incurable disease. Successful treatments will likely require multi-faceted approaches incorporating conventional drug therapies, immunotherapy and other novel treatments. Our lab previously showed that a combination of transient lymphodepletion (sublethal whole body irradiation) and PD-1/PD-L1 blockade generated anti-myeloma T cell reactivity capable of eliminating established disease. We hypothesized that blocking a combination of checkpoint receptors in the context of low-dose, lymphodepleting whole body radiation would boost anti-tumor immunity.

METHODS

To test our central hypothesis, we utilized a 5T33 murine multiple myeloma model. Myeloma-bearing mice were treated with a low dose of whole body irradiation and combinations of blocking antibodies to PD-L1, LAG-3, TIM-3, CD48 (the ligand for 2B4) and CTLA4.

RESULTS

Temporal phenotypic analysis of bone marrow from myeloma-bearing mice demonstrated that elevated percentages of PD-1, 2B4, LAG-3 and TIM-3 proteins were expressed on T cells. When PD-L1 blockade was combined with blocking antibodies to LAG-3, TIM-3 or CTLA4, synergistic or additive increases in survival were observed (survival rates improved from ~30% to >80%). The increased survival rates correlated with increased frequencies of tumor-reactive CD8 and CD4 T cells. When stimulated in vitro with myeloma cells, CD8 T cells from treated mice produced elevated levels proinflammatory cytokines. Cytokines were spontaneously released from CD4 T cells isolated from mice treated with PD-L1 plus CTLA4 blocking antibodies.

CONCLUSIONS

These data indicate that blocking PD-1/PD-L1 interactions in conjunction with other immune checkpoint proteins provides synergistic anti-tumor efficacy following lymphodepletive doses of whole body irradiation. This strategy is a promising combination strategy for myeloma and other hematologic malignancies.

Enhancement of antigen-specific immunoglobulin G responses by anti-CD48

CD48 is a glycosylphosphatidylinositol-anchored protein expressed ubiquitously on many cell types. Despite the poor ability to signal on its own, CD48 can activate cells via interaction with its counter receptors CD2 and CD244 as well as influence the function of other cell surface molecules by costimulatory activities. We show, herein, that injection of anti-CD48 antibodies into mice can augment the antibody response to a T-independent antigen, NP-Ficoll, that is representative of antigenic determinants expressed on the surface of various pathogens, such as Streptococcus pneumoniae. In C57BL/6 mice, enhancement of the response is dependent on natural killer (NK) cells as well as on the presence of CD2 and CD244, ligands for CD48, suggesting a requirement for direct interaction between NK and B cells. Interestingly, in this case, despite a similar augmentation by anti-CD48 in BALB/C mice, the response is independent of NK or T cells, suggesting that help for this response can be derived from other innate cell types. These results provide a pathway by which CD48, when appropriately activated, can influence the course of an antigen-specific antibody response via the innate system.

IFN type I and type II independent enhancement of B cell TLR7 expression by natural killer cells

The PRR TLR7 plays a key role in the activation of autoantigen-reactive B cells. This response is increased markedly by IFN-α, produced by accessory cells, as a result of the up-regulation of TLR7. We report herein an alternative pathway by which TLR7 expression can be augmented. This finding was derived from continuation of ongoing studies to uncover interactions between NK and B cells. Here, we have compared gene expression profiles by microarray analysis of B cells before and after their interaction with purified NK cells. The most outstanding alteration of genes transcribed in B cells is a significant increase in the expression of many members of the ISG family, among which is TLR7. Further analysis revealed that the enhancement of TLR7 on B cells is not mediated via type I or type II IFN but by another cytokine, IL-28, a type III IFN, which acts in concert with contact-mediated interactions with NK cells. This increased expression allows B cells to respond more readily upon stimulation by its ligand and may increase in vivo responses to other TLR7 ligands, such as autoantigens, prior to or jointly with stimulation by other cytokines.

The role of NK cells in the development of autoantibodies

The systemic lupus erythematosus (Sle1) interval from the NZM2410 mouse strain has been shown to be responsible for high levels of autoantibody production against antinuclear antibodies (ANA) when transferred into C57BL/6 mice. B cells derived from the B6.Sle1 strain are required for the production but help from both T-dependent and independent sources have been documented. Using radiation chimeras constructed in a strain of mice that is chronically depleted of Natural killer (NK) cells, but not NKT cells, we have examined the role of NK cells in the development of ANA in this context. Our results show that in the presence of intact T cell help depletion of NK cells does not affect ANA production. However, when T cell help is compromised, the prevalence of animals producing ANA is significantly decreased suggesting that NK cells can provide help for the T-independent production of ANA. Further experiments provide a possible mechanism for the NK-cell dependence.

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.

Mechanism of induction of NK activation by 2B4 (CD244) via its cognate ligand

We have previously shown that coincubation of purified B cells with IL-2-propagated NK cells can result in the induction of IL-13 mRNA and that the induction requires the presence of CD48 on B cells and 2B4 on NK cells. Because both of these molecules are expressed on NK cells, it is surprising that very little IL-13 mRNA can be detected in the absence of B cells. We have now found that incubation of NK cells on plates containing immobilized anti-CD48 Abs results in the clustering of CD48 and colocalization with 2B4 on the same cell. This colocalization, together with the requirement for SAP, the signal transducer for 2B4, is necessary for the induction of IL-13 mRNA expression. Activation of NK cell via CD48 on another cell may require a similar ability to alter the configuration of 2B4 to activate downstream signaling. By the use of double CD2/2B4 knockout mice, we have also shown that the induction of NK cell activation by anti-CD48 or by B cells is not due to the release of inhibitory effects of 2B4.

Regulatory role of natural killer (NK) cells on antibody responses to Brucella abortus

Our previous studies have indicated an important regulatory role for natural killer (NK) cells, a major constituent of the innate immune system in modulating antigen-specific responses. Herein, we have investigated the possible participation of these cells in regulating the polyclonal response as well. For these studies we have utilized heat-killed Brucella abortus (HKBA). Brucella abortus is a facultative intracellular bacterium that is pathogenic for both humans and animals. An outstanding feature of the infectious process is the rapid production of polyclonal antibodies, particularly of the IgG2c subclass, that bypasses the requirement for clonally specific antigen recognition. We report here that NK-cell depletion profoundly reduced the production of these polyclonal antibodies suggesting that activation of B cells by HKBA requires help from NK cells. This help may not be solely derived from NK-cell amplification of the cytokine circuit initiated by HKBA but may involve direct NK-B-cell interactions as suggested by results of in vitro analyses of NK induction of γ2a mRNA by B cells. These findings have therapeutic implications in that the induction of polyclonal Ig production may be more important for altering the chronic phase rather than the acute stage of infection by B. abortus.

Phenotypic differences between mice deficient in XIAP and SAP, two factors targeted in X-linked lymphoproliferative syndrome (XLP)

Mutations in the X-linked inhibitor of apoptosis (XIAP) have recently been identified in patients with the rare genetic disease, X-linked lymphoproliferative syndrome (XLP), which was previously thought to be solely attributable to mutations in a distinct gene, SAP. To further understand the roles of these two factors in the pathogenesis of XLP, we have compared mice deficient in Xiap with known phenotypes of Sap-null mice. We show here that in contrast to Sap-deficient mice, animals lacking Xiap have apparently normal NKT cell development and no apparent defect in humoral responses to T cell-dependent antigens. However, Xiap-deficient cells were more susceptible to death upon infection with the murine herpesvirus MHV-68 and gave rise to more infectious virus. These differences could be rescued by restoration of XIAP. These data provide insight into the differing roles of XIAP and SAP in the pathogenesis of XLP.

Antigen-specific responses and ANA production in B6.Sle1b mice: a role for SAP

B6.Sle1b mice, which contain the Sle1b gene interval derived from lupus prone NZM2410 mice on a C57BL/6 background, present with gender-biased, highly penetrant anti-nuclear antibody (ANA) production. To obtain some insight into the possible induction mechanism of autoantibodies in these mice we compared antigen-specific T dependent (TD) and T independent (TI-II) responses between B6.Sle1b and B6 mice before the development of high ANA titers. Our results show that B6.Sle1b mice mount enhanced responses to a TI-II antigen. Additionally, the memory T cell response generated by a TD antigen also increased. This enhancement correlates with the greater ability of B cells from B6.Sle1b mice to present antigen to T cells. The SLAM Associated Protein (SAP) is critical for signaling of many of the molecules encoded by the SLAM/CD2 gene cluster, candidates for mediating the Sle1b phenotype; therefore, we also investigated the effect of sap deletion in these strains on the TD and TI-II responses as well as on ANA production. The results of these studies of responses to non-self-antigens provide further insight into the mechanism by which responses to self-antigens might be initiated in the context of specific genetic alterations.

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.

Requirements for the natural killer cell-mediated induction of IgG1 and IgG2a expression in B lymphocytes

Upon interaction with resting B lymphocytes, IL-2-propagated NK cells can initiate the process of Ig constant region switch recombination (CSR) by inducing germ line transcripts for gamma2a (Igamma2a) as well as increased levels of mRNA for activation-induced cytidine deaminase enzyme. Whereas both these processes are necessary for CSR, they are not sufficient because the cells do not proceed to the expression of mature mRNA for gamma2a (VDJCgamma2a). In addition, NK cells can also upregulate mRNA for the T-box transcription factor (T-bet) in B cells without being able to induce further differentiation. Using transgenic B cells with B cell receptor specificity for nitrophenol (NP), we have now shown that NP-Ficoll-stimulated B cells can be induced by NK cells to express IgG2a as well as IgG1 presumably due to the completion of the process of switch recombination. The inductive ability of NK cells does not require IFN-gamma but does require signals transmitted via CD48 by direct cell contact. In addition, NP-Ficoll on its own can induce proliferation of antigen-specific B cells as well as germ line transcripts of gamma1; however, expression of VDJCgamma1 mRNA also requires NK cell interaction with B lymphocytes. Therefore, in the presence of antigen, NK cells can provide a necessary signal that substitutes for cytokines in the induction of IgG2a as well as IgG1 expression. This in vitro analysis provides a mechanistic basis for understanding the documented NK cell effects on T-independent B cell responses in vivo.

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.

NK cell regulation by SLAM family receptors and SAP-related adapters

Signaling lymphocytic activating molecule (SLAM) family receptors and SLAM-associated protein (SAP)-related adapters play several important roles in the immune system. Natural killer (NK) cells express at least three members of the SLAM family. They are 2B4, NK, T- and B-cell antigen (NTB-A), and CD2-like receptor-activating cytotoxic cells (CRACC), which recognize their respective ligands CD48, NTB-A, and CRACC on target cells and possibly on other NK cells. In mature human NK cells, SLAM family receptors appear to have activating functions. In mature mouse NK cells, however, the only available information is for 2B4, which reportedly has the capacity to either stimulate or inhibit NK cell activation. The ability of SLAM family receptors to regulate NK cell functions seems to be largely dependent on their capacity to associate, by way of their cytoplasmic domain, with members of the SAP family of adapters, including SAP, Ewing's sarcoma-activated transcript-2 (EAT-2), and EAT-2-related transducer (ERT). By binding to SAP, SLAM family receptors are coupled to the Src kinase FynT, thereby evoking protein tyrosine phosphorylation signals. In human NK cells, SAP is likely to be crucial for the activating function of 2B4 and NTB-A but not of CRACC and also crucial for the activating function of 2B4 in mouse NK cells. EAT-2. SAP is ERT link SLAM family receptors to distinct, albeit poorly understood, signals. These two SAP-related adapters may be implicated in the inhibitory function of 2B4 observed in mouse NK cells. While much work remains to be carried out to fully understand the roles and mechanisms of action of the SLAM and SAP families in human and mouse NK cells, the published findings clearly establish that these molecules have important functions in NK cell biology.