Importance and mechanism of 'switch' function of SAP family adapters

The multifaceted role of CS1 (SLAMF7) in immunoregulation: Implications for cancer therapy and autoimmune disorders

CS1 (SLAMF7), a pivotal immune receptor, plays a dual role in modulating immune responses in autoimmune diseases and cancer. In autoimmunity, aberrant CS1 signaling contributes to the activation of autoreactive lymphocytes, driving pathologies such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). Conversely, in oncology, CS1 serves as a promising immunotherapeutic target, exemplified by the efficacy of the monoclonal antibody Elotuzumab in multiple myeloma. CS1 mediates immune cell functions through intricate signaling pathways, including interactions with EAT-2 and SAP adaptors, which influence cytotoxicity, cytokine production, and immune homeostasis. Beyond cancer and autoimmune diseases, soluble and membrane-bound forms of CS1 are emerging as biomarkers and potential therapeutic targets. Despite significant progress, gaps remain in understanding CS1\u2019s mechanisms, variability in expression, and role in other diseases. This study explores the multifaceted functions of CS1, proposing innovative strategies to leverage its therapeutic potential across diverse pathologies.

Killer instincts: natural killer cells as multifactorial cancer immunotherapy

Natural killer (NK) cells integrate heterogeneous signals for activation and inhibition using germline-encoded receptors. These receptors are stochastically co-expressed, and their concurrent engagement and signaling can adjust the sensitivity of individual cells to putative targets. Against cancers, which mutate and evolve under therapeutic and immunologic pressure, the diversity for recognition provided by NK cells may be key to comprehensive cancer control. NK cells are already being trialled as adoptive cell therapy and targets for immunotherapeutic agents. However, strategies to leverage their naturally occurring diversity and agility have not yet been developed. In this review, we discuss the receptors and signaling pathways through which signals for activation or inhibition are generated in NK cells, focusing on their roles in cancer and potential as targets for immunotherapies. Finally, we consider the impacts of receptor co-expression and the potential to engage multiple pathways of NK cell reactivity to maximize the scope and strength of antitumor activities.

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.

Human natural killer cells: Form, function, and development

Human natural killer (NK) cells are innate lymphoid cells that mediate important effector functions in the control of viral infection and malignancy. Their ability to distinguish "self" from "nonself" and lyse virally infected and tumorigenic cells through germline-encoded receptors makes them important players in maintaining human health and a powerful tool for immunotherapeutic applications and fighting disease. This review introduces our current understanding of NK cell biology, including key facets of NK cell differentiation and the acquisition and execution of NK cell effector function. Further, it addresses the clinical relevance of NK cells in both primary immunodeficiency and immunotherapy. It is intended to provide an up-to-date and comprehensive overview of this important and interesting innate immune effector cell subset.

T cell receptor (TCR) signaling in health and disease

Interaction of the T cell receptor (TCR) with an MHC-antigenic peptide complex results in changes at the molecular and cellular levels in T cells. The outside environmental cues are translated into various signal transduction pathways within the cell, which mediate the activation of various genes with the help of specific transcription factors. These signaling networks propagate with the help of various effector enzymes, such as kinases, phosphatases, and phospholipases. Integration of these disparate signal transduction pathways is done with the help of adaptor proteins that are non-enzymatic in function and that serve as a scaffold for various protein-protein interactions. This process aids in connecting the proximal to distal signaling pathways, thereby contributing to the full activation of T cells. This review provides a comprehensive snapshot of the various molecules involved in regulating T cell receptor signaling, covering both enzymes and adaptors, and will discuss their role in human disease.

The Activating Receptors of Natural Killer Cells and Their Inter-Switching Potentials

The global incidence of cancer is on the increase and researchers are prospecting for specific and non-selective therapies derived from the immune system. The killer activating receptors of NK cells are known to be involved in immunosurveillance against tumor and virally-infected cells. These receptors belong to two main categories, namely the immunoglobulin like and C-lectin like families. Though they have different signal pathways, all the killer activating receptors have similar effector functions which include direct cytotoxicity and the release of inflammatory cytokines such as IFN-gamma and TNF-alpha. To transduce signals that exceed the activation threshold for cytotoxicity, most of these receptors require synergistic effort. This review profiles 21 receptors: 13 immunoglobulin-like, 5 lectin-like, and 3 others. It critically explores their structural uniqueness, role in disease, respective transduction signal pathways and their status as current and prospective targets for cancer immunotherapy. While the native ligands of most of these receptors are known, much work is required to prospect for specific antibodies, peptides and multi-target small molecules with high binding affinities.

SLAM-SAP-Fyn: Old Players with New Roles in iNKT Cell Development and Function

Invariant natural killer T (iNKT) cells are a unique T cell lineage that develop in the thymus and emerge with a memory-like phenotype. Accordingly, following antigenic stimulation, they can rapidly produce copious amounts of Th1 and Th2 cytokines and mediate activation of several immune cells. Thus, it is not surprising that iNKT cells play diverse roles in a broad range of diseases. Given their pivotal roles in host immunity, it is crucial that we understand the mechanisms that govern iNKT cell development and effector functions. Over the last two decades, several studies have contributed to the current knowledge of iNKT cell biology and activity. Collectively, these studies reveal that the thymic development of iNKT cells, their lineage expansion, and functional properties are tightly regulated by a complex network of transcription factors and signaling molecules. While prior studies have clearly established the importance of the SLAM-SAP-Fyn signaling axis in iNKT cell ontogenesis, recent studies provide exciting mechanistic insights into the role of this signaling cascade in iNKT cell development, lineage fate decisions, and functions. Here we summarize the previous literature and discuss the more recent studies that guide our understanding of iNKT cell development and functional responses.

Mechanisms of NK Cell Activation and Clinical Activity of the Therapeutic SLAMF7 Antibody, Elotuzumab in Multiple Myeloma

Multiple myeloma (MM) is a bone marrow plasma cell neoplasm and is the second most-common hematologic malignancy. Despite advances in therapy, MM remains largely incurable. Elotuzumab is a humanized IgG1 monoclonal antibody targeting SLAMF7, which is highly expressed on myeloma cells, and the antibody is approved for the treatment of relapsed and/or refractory (RR) MM in combination with lenalidomide and dexamethasone. Elotuzumab can stimulate robust antibody-dependent cellular cytotoxicity (ADCC) through engaging with FcγRIIIA (CD16) on NK cells and antibody-dependent cellular phagocytosis (ADCP) by macrophages. Interestingly, SLAMF7 is also expressed on cytolytic NK cells, which also express the requisite adaptor protein, EAT-2, to mediate activation signaling. Accumulating evidence indicates that antibody crosslinking of SLAMF7 on human and mouse NK cells can stimulate EAT-2-dependent activation of PLCγ, ERK, and intracellular calcium mobilization. The binding of SLAMF7 by elotuzumab can directly induce signal transduction in human NK cells, including co-stimulation of the calcium signaling triggered through other surface receptors, such as NKp46 and NKG2D. In RRMM patients, elotuzumab monotherapy did not produce objective responses, but did enhance the activity of approved standard of care therapies, including lenalidomide or bortezomib, which are known to enhance anti-tumor responses by NK cells. Taken together, these preclinical results and accumulating experience in the clinic provide compelling evidence that the mechanism of action of elotuzumab in MM patients involves the activation of NK cells through both CD16-mediated ADCC and direct co-stimulation via engagement with SLAMF7, as well as promoting ADCP by macrophages. We review the current understanding of how elotuzumab utilizes multiple mechanisms to facilitate immune-mediated attack of myeloma cells, as well as outline goals for future research.

Inherited Immunodeficiencies With High Predisposition to Epstein-Barr Virus-Driven Lymphoproliferative Diseases

Epstein–Barr Virus (EBV) is a gamma-herpes virus that infects 90% of humans without any symptoms in most cases, but has an oncogenic potential, especially in immunocompromised individuals. In the past 30 years, several primary immunodeficiencies (PIDs) associated with a high risk to develop EBV-associated lymphoproliferative disorders (LPDs), essentially consisting of virus-associated hemophagocytic syndrome, non-malignant and malignant B-cell LPDs including non-Hodgkin and Hodgkin’s types of B lymphomas have been characterized. Among them are SH2D1A (SAP), XIAP, ITK, MAGT1, CD27, CD70, CTPS1, RASGRP1, and CORO1A deficiencies. Penetrance of EBV infection ranges from 50 to 100% in those PIDs. Description of large cohorts and case reports has refined the specific phenotypes associated with these PIDs helping to the diagnosis. Specific pathways required for protective immunity to EBV have emerged from studies of these PIDs. SLAM-associated protein-dependent SLAM receptors and MAGT1-dependent NKG2D pathways are important for T and NK-cell cytotoxicity toward EBV-infected B-cells, while CD27–CD70 interactions are critical to drive the expansion of EBV-specific T-cells. CTPS1 and RASGRP1 deficiencies further strengthen that T-lymphocyte expansion is a key step in the immune response to EBV. These pathways appear to be also important for the anti-tumoral immune surveillance of abnormal B cells. Monogenic PIDs should be thus considered in case of any EBV-associated LPDs.

Natural killer cell cytotoxicity and its regulation by inhibitory receptors

Natural killer (NK) cells express an array of germ-line encoded receptors that are capable of triggering cytotoxicity. NK cells tend to express many members of a given family of signalling molecules. The presence of many activating receptors and many members of a given family of signalling molecules can enable NK cells to detect different kinds of target cells, and to mount different kinds of responses. This contributes also to the robustness of NK cells responses; cytotoxic functions of NK cells often remain unaffected in the absence of selected signalling molecules. NK cells express many MHC-I-specific inhibitory receptors. Signals from MHC-I-specific inhibitory receptors tightly control NK cell cytotoxicity and, paradoxically, maintain NK cells in a state of proper responsiveness. This review provides a brief overview of the events that underlie NK cell activation, and how signals from inhibitory receptors intercept NK cell activation to prevent inappropriate triggering of cytotoxicity.

Hepatocyte SLAMF3 reduced specifically the multidrugs resistance protein MRP-1 and increases HCC cells sensitization to anti-cancer drugs

Multidrug resistance MDR proteins (MRPs) are members of the C family of a group of proteins named ATP binding cassette (ABC) transporters. MRPs can transport drugs including anticancer drugs, nucleoside analogs, antimetabolites and tyrosine kinase inhibitors. Drugs used in HCC therapy, such as tyrosine kinase inhibitor sorafenib, are substrates of uptake and/or efflux transporters. Variable expression of MRPs at the plasma membrane of tumor cells may contribute to drug resistance and subsequent clinical response. Recently, we reported that the hepatocyte SLAMF3 expression (Signaling Lymphocytic Activation Molecule Family member 3) was reduced in tumor cells from hepatocellular carcinoma (HCC) compared to its high expression in adjacent tissues. In the present study, we make a strong correlation between induced SLAMF3 overexpression and the specific loss of MRP-1 expression and its functionalities as a drugs resistance transporter. No changes were observed on expression of ABCG2 and MDR. More importantly, we highlight a strong inverse correlation between MRP-1 and SLAMF3 expression in patients with HCC. We propose that the SLAMF3 overexpression in cancerous cells could represent a potential therapeutic strategy to improve the drugs sensibility of resistant cells and thus control the therapeutic failure in HCC patients.

Beta and Gamma Human Herpesviruses: Agonistic and Antagonistic Interactions with the Host Immune System

Viruses are the most abundant and diverse biological entities in the planet. Historically, our main interest in viruses has focused on their pathogenic role, recognized by pandemics that have decimated the world population. However, viral infections have also played a major role in the evolution of cellular organisms, both through interchanging of genes with novel functions and shaping the immune system. Examples abound of infections that seriously compromise the host integrity, but evidence of plant and insect viruses mutualistic relationships have recently surfaced in which infected hosts are better suited for survival, arguing that virus-host interactions are initially parasitic but become mutualistic over years of co-evolution. A similar mutual help scenario has emerged with commensal gut bacteria. EBV is a herpesvirus that shares more than a hundred million years of co-evolution with humans, today successfully infecting close to 100% of the adult world population. Infection is usually acquired early in childhood persisting for the host lifetime mostly without apparent clinical symptoms. Disturbance of this homeostasis is rare and results in several diseases, of which the best understood are infectious mononucleosis and several EBV-associated cancers. Less understood are recently found inborn errors of the immune system that result in primary immunodeficiencies with an increased predisposition almost exclusive to EBV-associated diseases. Puzzling to these scenarios of broken homeostasis is the co-existence of immunosuppression, inflammation, autoimmunity and cancer. Homologous to EBV, HCMV, HHV-6 and HHV-7 are herpesviruses that also latently infect most individuals. Several lines of evidence support a mutualistic equilibrium between HCMV/EBV and hosts, that when altered trigger diseases in which the immune system plays a critical role. Interestingly, these beta and gamma herpesviruses persistently infect all immune lineages and early precursor cells. In this review, we will discuss the evidence of the benefits that infection of immune cells with these herpesviruses brings to the host. Also, the circumstances in which this positive relationship is broken, predisposing the host to diseases characterized by an abnormal function of the host immune system.

Immunoglobulin superfamily members encoded by viruses and their multiple roles in immune evasion

Pathogens have developed a plethora of strategies to undermine host immune defenses in order to guarantee their survival. For large DNA viruses, these immune evasion mechanisms frequently rely on the expression of genes acquired from host genomes. Horizontally transferred genes include members of the immunoglobulin superfamily, whose products constitute the most diverse group of proteins of vertebrate genomes. Their promiscuous immunoglobulin domains, which comprise the building blocks of these molecules, are involved in a large variety of functions mediated by ligand-binding interactions. The flexible structural nature of the immunoglobulin domains makes them appealing targets for viral capture due to their capacity to generate high functional diversity. Here, we present an up-to-date review of immunoglobulin superfamily gene homologs encoded by herpesviruses, poxviruses, and adenoviruses, that include CD200, CD47, Fc receptors, interleukin-1 receptor 2, interleukin-18 binding protein, CD80, carcinoembryonic antigen-related cell adhesion molecules, and signaling lymphocyte activation molecules. We discuss their distinct structural attributes, binding properties, and functions, shaped by evolutionary pressures to disarm specific immune pathways. We include several novel genes identified from extensive genome database surveys. An understanding of the properties and modes of action of these viral proteins may guide the development of novel immune-modulatory therapeutic tools.

Signals that drive T follicular helper cell formation

T follicular helper (Tfh) cells are a distinct type of CD4⁺ T cell specialized in providing help to B cells during the germinal centre (GC) reaction. As such, they are critical determinants of the quality of an antibody response following antigen challenge. Excessive production of Tfh cells can result in autoimmunity whereas too few can result in inadequate protection from infection. Hence, their differentiation and maintenance must be tightly regulated to ensure appropriate but limited help to B cells. Unlike the majority of other CD4⁺ T-cell subsets, Tfh cell differentiation occurs in three phases defined by their anatomical location. During each phase of differentiation the emerging Tfh cells express distinct patterns of co-receptors, which work together with the T-cell receptor (TCR) to drive Tfh differentiation. These signals provided by both TCR and co-receptors during Tfh differentiation alter proliferation, survival, metabolism, cytokine production and transcription factor expression. This review will discuss how engagement of TCR and co-receptors work together to shape the formation and function of Tfh cells.

Dissection of SAP-dependent and SAP-independent SLAM family signaling in NKT cell development and humoral immunity

Chen et al. dissect SAP-dependent and SAP-independent SLAM family signaling in the regulation of NKT cell development and follicular T helper cell differentiation using a novel mouse model lacking all seven SLAM family receptors.

Signaling lymphocytic activation molecule (SLAM)–associated protein (SAP) mutations in X-linked lymphoproliferative disease (XLP) lead to defective NKT cell development and impaired humoral immunity. Because of the redundancy of SLAM family receptors (SFRs) and the complexity of SAP actions, how SFRs and SAP mediate these processes remains elusive. Here, we examined NKT cell development and humoral immunity in mice completely deficient in SFR. We found that SFR deficiency severely impaired NKT cell development. In contrast to SAP deficiency, SFR deficiency caused no apparent defect in follicular helper T (T_FH) cell differentiation. Intriguingly, the deletion of SFRs completely rescued the severe defect in T_FH cell generation caused by SAP deficiency, whereas SFR deletion had a minimal effect on the defective NKT cell development in SAP-deficient mice. These findings suggest that SAP-dependent activating SFR signaling is essential for NKT cell selection; however, SFR signaling is inhibitory in SAP-deficient T_FH cells. Thus, our current study revises our understanding of the mechanisms underlying T cell defects in patients with XLP.

Mining the Complex Family of Protein Tyrosine Phosphatases for Checkpoint Regulators in Immunity

The family of protein tyrosine phosphatases (PTPs) includes 107 genes in humans that are diverse in their structures and expression profiles. The majority are present in immune cells and play various roles in either inhibiting or promoting the duration and amplitude of signaling cascades. Several PTPs, including TC-PTP (PTPN2) and SHP-1 (PTPN6), have been recognized as being crucial for maintaining proper immune response and self-tolerance, and have gained recognition as true immune system checkpoint modulators. This chapter details the most recent literature on PTPs and immunity by examining their known functions in regulating signaling from either established checkpoint inhibitors or by their intrinsic properties, as modulators of the immune response. Notably, we review PTP regulatory properties in macrophages, antigen-presenting dendritic cells, and T cells. Overall, we present the PTP gene family as a remarkable source of novel checkpoint inhibitors wherein lies a great number of new targets for immunotherapies.

Dephosphorylation of the adaptor LAT and phospholipase C-γ by SHP-1 inhibits natural killer cell cytotoxicity

Natural killer (NK) cells discriminate between healthy cells and virally infected or transformed self-cells by tuning activating and inhibitory signals received through cell surface receptors. Inhibitory receptors inhibit NK cell function by recruiting and activating the tyrosine phosphatase Src homology 2 (SH2) domain-containing protein tyrosine phosphatase-1 (SHP-1) to the plasma membrane. However, to date, the guanine nucleotide exchange factor VAV1 is the only direct SHP-1 substrate identified in NK cells. We reveal that the adaptor protein linker for activation of T cells (LAT) as well as phospholipase C-γ1 (PLC-γ1) and PLC-γ2 are SHP-1 substrates. Dephosphorylation of Tyr(132) in LAT by SHP-1 in NK cells abrogated the recruitment of PLC-γ1 and PLC-γ2 to the immunological synapse between the NK cell and a cancer cell target, which reduced NK cell degranulation and target cell killing. Furthermore, the ubiquitylation of LAT by the E3 ubiquitin ligases c-Cbl and Cbl-b, which was induced by LAT phosphorylation, led to the degradation of LAT in response to the engagement of inhibitory receptors on NK cells, which abrogated NK cell cytotoxicity. Knockdown of the Cbl proteins blocked LAT ubiquitylation, which promoted NK cell function. Expression of a ubiquitylation-resistant mutant LAT blocked inhibitory receptor signaling, enabling cells to become activated. Together, these data identify previously uncharacterized SHP-1 substrates and inhibitory mechanisms that determine the response of NK cells.

MicroRNA-30c promotes natural killer cell cytotoxicity via up-regulating the expression level of NKG2D

AIMS

Natural killer (NK) cells play critical roles in antitumor immunity. Our previous study showed that over-expression of miR-30c-1* enhanced NKL cell cytotoxicity through up-regulation of tumor necrosis factor-α via directly targeting transcription factor homeobox containing 1. MiR-30c, the complimentary microRNA of miR-30c-1*, has been found to exert regulatory effect on T cell function. However, the effect of miR-30c on NK cells is unknown. Therefore, this study aimed to investigate whether miR-30c could play a role to enhance NK cell activation and cytotoxicity.

MAIN METHODS

Chemosynthesis exogenous miR-30c mimics and miR-30c inhibitor were transfected into NKL cells and isolated human peripheral blood NK cells, respectively. The expression levels of NK group 2, member D (NKG2D), CD107a and FasL on cell surface and cytotoxic ability of miRNAs transfected NKL cells against SMMC-7721 cells were evaluated.

KEY FINDINGS

MiR-30c could increase the expression of NKG2D and CD107a on NKL cells, and enhance cytotoxic ability of NKL cells to kill SMMC-7721 cells. Moreover, miR-30c could up-regulate the expression of FasL on both NKL cells and human peripheral blood NK cells. However, the peripheral blood NK cells from only four in ten healthy donors appeared high expression levels of NKG2D and CD107a after miR-30c transfection.

SIGNIFICANCE

MiR-30c could promote the cytotoxicity of NKL cells in vitro by up-regulating the expression levels of NKG2D, CD107a and FasL. However, the effect of miR-30c on ex vivo NK cells from different human individuals is diverse, indicating that miR-30c may play complicate and fine adjustment in immune system.

RB/PLK1-dependent induced pathway by SLAMF3 expression inhibits mitosis and control hepatocarcinoma cell proliferation

Polo-like kinase PLK1 is a cell cycle protein that plays multiple roles in promoting cell cycle progression. Among the many roles, the most prominent role of PLK1 is to regulate the mitotic spindle formation checkpoint at the M-phase. Recently we reported the expression of SLAMF3 in Hepatocytes and show that it is down regulated in tumor cells of hepatocellular carcinoma (HCC). We also show that the forced high expression level of SLAMF3 in HCC cells controls proliferation by inhibiting the MAPK ERK/JNK and the mTOR pathways. In the present study, we provide evidence that the inhibitory effect of SLAMF3 on HCC proliferation occurs through Retinoblastoma (RB) factor and PLK1-dependent pathway. In addition to the inhibition of MAPK ERK/JNK and the mTOR pathways, expression of SLAMF3 in HCC retains RB factor in its hypophosphorylated active form, which in turn inactivates E2F transcription factor, thereby repressing the expression and activation of PLK1. A clear inverse correlation was also observed between SLAMF3 and PLK expression in patients with HCC. In conclusion, the results presented here suggest that the tumor suppressor potential of SLAMF3 occurs through activation of RB that represses PLK1. We propose that the induction of a high expression level of SLAMF3 in cancerous cells could control cellular mitosis and block tumor progression.

Signaling Lymphocytic Activation Molecule Family Receptor Homologs in New World Monkey Cytomegaloviruses

Throughout evolution, large DNA viruses have been usurping genes from their hosts to equip themselves with proteins that restrain host immune defenses. Signaling lymphocytic activation molecule (SLAM) family (SLAMF) receptors are involved in the regulation of both innate and adaptive immunity, which occurs upon engagement with their ligands via homotypic or heterotypic interactions. Here we report a total of seven SLAMF genes encoded by the genomes of two cytomegalovirus (CMV) species, squirrel monkey CMV (SMCMV) and owl monkey CMV (OMCMV), that infect New World monkeys. Our results indicate that host genes were captured by retrotranscription at different stages of the CMV-host coevolution. The most recent acquisition led to S1 in SMCMV. S1 is a SLAMF6 homolog with an amino acid sequence identity of 97% to SLAMF6 in its ligand-binding N-terminal Ig domain. We demonstrate that S1 is a cell surface glycoprotein capable of binding to host SLAMF6. Furthermore, the OMCMV genome encodes A33, an LY9 (SLAMF3) homolog, and A43, a CD48 (SLAMF2) homolog, two soluble glycoproteins which recognize their respective cellular counterreceptors and thus are likely to be viral SLAMF decoy receptors. In addition, distinct copies of further divergent CD48 homologs were found to be encoded by both CMV genomes. Remarkably, all these molecules display a number of unique features, including cytoplasmic tails lacking characteristic SLAMF signaling motifs. Taken together, our findings indicate a novel immune evasion mechanism in which incorporation of host SLAMF receptors that retain their ligand-binding properties enables viruses to interfere with SLAMF functions and to supply themselves with convenient structural molds for expanding their immunomodulatory repertoires.

IMPORTANCE

The way in which viruses shape their genomes under the continual selective pressure exerted by the host immune system is central for their survival. Here, we report that New World monkey cytomegaloviruses have broadly captured and duplicated immune cell receptors of the signaling lymphocyte activation molecule (SLAM) family during host-virus coevolution. Notably, we demonstrate that several of these viral SLAMs exhibit exceptional preservation of their N-terminal immunoglobulin domains, which results in maintenance of their ligand-binding capacities. At the same time, these molecules present distinctive structural properties which include soluble forms and the absence of typical SLAM signaling motifs in their cytoplasmic domains, likely reflecting the evolutionary adaptation undergone to efficiently interfere with host SLAM family activities. The observation that the genomes of other large DNA viruses might bear SLAM family homologs further underscores the importance of these molecules as a novel class of immune regulators and as convenient scaffolds for viral evolution.

Molecular and functional characterization of porcine Siglec-3/CD33 and analysis of its expression in blood and tissues

A cDNA clone encoding a 380 a-a type 1 transmembrane protein with homology to human Siglec-3/CD33 was obtained from a swine small intestine library. An analysis of protein sequence identified two immunoglobulin-like domains, a transmembrane region, and a carboxi-terminal tail with two tyrosine-based signalling motifs. Binding assays of Siglec-3 transfected CHO cells to polyacrylamide glycoconjugates showed a preference for α2-6-linked sialic acids. Using mAbs raised against a fragment containing the two Ig-like domains, porcine Siglec-3 was found to be expressed on monocytes and granulocytes, and their bone marrow precursors. It was also detected in lymph node, splenic and alveolar macrophages. MAbs immunoprecipitated, from granulocyte lysates, a protein of 51-60 kDa under both non-reducing and reducing conditions. MAbs were also used to analyse functional activity of Siglec-3 on bone marrow and blood cells. Engagement of Siglec-3 by mAb had no apparent effect on cell proliferation or cytokine production.

Negative Regulation of Humoral Immunity Due to Interplay between the SLAMF1, SLAMF5, and SLAMF6 Receptors

Whereas the SLAMF-associated protein (SAP) is involved in differentiation of T follicular helper (Tfh) cells and antibody responses, the precise requirements of SLAMF receptors in humoral immune responses are incompletely understood. By analyzing mice with targeted disruptions of the Slamf1, Slamf5, and Slamf6 genes, we found that both T-dependent and T-independent antibody responses were twofold higher compared to those in single knockout mice. These data suggest a suppressive synergy of SLAMF1, SLAMF5, and SLAMF6 in humoral immunity, which contrasts the decreased antibody responses resulting from a defective GC reaction in the absence of the adapter SAP. In adoptive co-transfer assays, both [Slamf1 + 5 + 6]^−/− B and T cells were capable of inducing enhanced antibody responses, but more pronounced enhancement was observed after adoptive transfer of [Slamf1 + 5 + 6]^−/− B cells compared to that of [Slamf1 + 5 + 6]^−/− T cells. In support of [Slamf1 + 5 + 6]^−/− B cell intrinsic activity, [Slamf1 + 5 + 6]^−/− mice also mounted significantly higher antibody responses to T-independent type 2 antigen. Furthermore, treatment of mice with anti-SLAMF6 monoclonal antibody results in severe inhibition of the development of Tfh cells and GC B cells, confirming a suppressive effect of SLAMF6. Taken together, these results establish SLAMF1, SLAMF5, and SLAMF6 as important negative regulators of humoral immune response, consistent with the notion that SLAM family receptors have dual functions in immune responses.

SAP-regulated T Cell-APC adhesion and ligation-dependent and -independent Ly108-CD3ζ interactions

The germinal center response requires cooperation between Ag-specific T and B lymphocytes, which takes the form of long-lasting cell-cell conjugation in vivo. Signaling lymphocytic activation molecule (SLAM)-associated protein (SAP) is required for stable cognate T-B cell conjugation, whereas SLAM family transmembrane (TM) receptor Ly108 may negatively regulate this process. We show that, other than phosphotyrosine-binding, SAP does not harbor motifs that recruit additional signaling intermediates to stabilize T-B adhesion. Ly108 dampens T cell adhesion to not only Ag-presenting B cells, but also dendritic cells by inhibiting CD3ζ phosphorylation through two levels of regulated Ly108-CD3ζ interactions. Constitutively associated with Src homology 2 domain-containing tyrosine phosphatase-1 even in SAP-competent cells, Ly108 is codistributed with the CD3 complex within a length scale of 100-200 nm on quiescent cells and can reduce CD3ζ phosphorylation in the absence of overt TCR stimulation or Ly108 ligation. When Ly108 is engaged in trans during cell-cell interactions, Ly108-CD3ζ interactions are promoted in a manner that uniquely depends on Ly108 TM domain, leading to more efficient CD3ζ dephosphorylation. Whereas replacement of the Ly108 TM domain still allows the constitutive, colocalization-dependent inhibition of CD3ζ phosphorylation, it abrogates the ligation-dependent Ly108-CD3ζ interactions and CD3ζ dephosphorylation, and it abolishes the suppression on Ag-triggered T-B adhesion. These results offer new insights into how SAP and Ly108 antagonistically modulate the strength of proximal TCR signaling and thereby control cognate T cell-APC interactions.

X-linked lymphoproliferative syndromes and related autosomal recessive disorders

PURPOSE OF REVIEW

X-linked lymphoproliferative (XLP) syndromes and related autosomal disorders are severe primary immune deficiencies triggered by infection with Epstein-Barr virus (EBV), the causative agent of infectious mononucleosis. Recent findings reviewed herein provided key new insights into the genetic and immunological basis of these diseases. They also improved our comprehension of the immunological mechanisms controlling EBV infection.

RECENT FINDINGS

Mutations of an X-linked gene, SH2D1A, which encodes the signaling lymphocytic activation molecule (SLAM)-associated protein (SAP), are responsible for most cases of XLP disorders. More recently, other genetic causes for XLP syndromes and autosomal recessive variants of this disease were elucidated. Mutations in genes such as XIAP, ITK, and CD27 were identified. The clinical manifestations and immunological defects seen in these patients were characterized.

SUMMARY

The similarities and differences in immunological defects and clinical manifestations between XLP syndromes and related autosomal recessive disorders enabled important new insights into the pathogenesis of these diseases. They also helped our comprehension of the mechanisms implicated in the control of EBV infection. They suggested that CD8+ T cells, natural killer (NK) cells, and NKT cells are critically involved.

SAP facilitates recruitment and activation of LCK at NTB-A receptors during restimulation-induced cell death

Upon TCR restimulation, activated, cycling T cells can undergo a self-regulatory form of apoptosis known as restimulation-induced cell death (RICD). We previously demonstrated that RICD is impaired in T cells from patients with X-linked lymphoproliferative disease, which lack SLAM-associated protein (SAP) expression. Both SAP and the specific SLAM receptor NK, T, and B cell Ag (NTB-A) are required for RICD, but the mechanism by which these molecules promote a strong, proapoptotic signal through the TCR remains unclear. In this article, we show that the Src-family kinase LCK, but not FYN, associates with NTB-A in activated human T cells. This association increased after TCR restimulation in a SAP-dependent manner, requiring both immunoreceptor tyrosine-based switch motifs in the NTB-A cytoplasmic tail. Both NTB-A-associated LCK phosphorylation and kinase activity were enhanced in restimulated T cells, amplifying proximal TCR signaling. In contrast, TCR-induced LCK association with NTB-A, as well as phosphorylation and kinase activity, was reduced in T cells from patients with X-linked lymphoproliferative disease or normal T cells transfected with SAP-specific small interfering RNA, consistent with RICD resistance. Collectively, our data reveal how SAP nucleates a previously unknown signaling complex involving NTB-A and LCK to potentiate RICD of activated human T cells.

Cytomegalovirus m154 hinders CD48 cell-surface expression and promotes viral escape from host natural killer cell control

Receptors of the signalling lymphocyte-activation molecules (SLAM) family are involved in the functional regulation of a variety of immune cells upon engagement through homotypic or heterotypic interactions amongst them. Here we show that murine cytomegalovirus (MCMV) dampens the surface expression of several SLAM receptors during the course of the infection of macrophages. By screening a panel of MCMV deletion mutants, we identified m154 as an immunoevasin that effectively reduces the cell-surface expression of the SLAM family member CD48, a high-affinity ligand for natural killer (NK) and cytotoxic T cell receptor CD244. m154 is a mucin-like protein, expressed with early kinetics, which can be found at the cell surface of the infected cell. During infection, m154 leads to proteolytic degradation of CD48. This viral protein interferes with the NK cell cytotoxicity triggered by MCMV-infected macrophages. In addition, we demonstrate that an MCMV mutant virus lacking m154 expression results in an attenuated phenotype in vivo, which can be substantially restored after NK cell depletion in mice. This is the first description of a viral gene capable of downregulating CD48. Our novel findings define m154 as an important player in MCMV innate immune regulation.

Manipulation of EAT-2 expression promotes induction of multiple beneficial regulatory and effector functions of the human innate immune system as a novel immunomodulatory strategy

The signaling lymphocytic activation molecule (SLAM) receptor-associated adaptor Ewing's sarcoma-associated transcript-2 (EAT-2) is primarily expressed in innate immune cells including dendritic cells (DCs), macrophages and NK cells. A recent human HIV vaccine study confirmed that EAT-2 expression was associated with the enhanced immunogenicity induced by the MRKAd5/HIV vaccine. We previously harnessed the capability of EAT-2 to modulate signaling mediated by SLAM receptors and demonstrated that by incorporating EAT-2 expression into vaccines, one could enhance innate and adaptive immune responses in mice, even in the face of pre-existing immunity to the vaccine vectors. Herein, we investigated the innate immune responses of human cells exposed to EAT-2-over-expressing vaccines. Our results demonstrate that EAT-2 over-expression can significantly alter the kinetics of critical pro-inflammatory cytokine and chemokine responses elaborated by human PBMCs. In addition, enhanced DC maturation and increased monocyte phagocytosis were observed in EAT-2-transduced human cells. We also found that EAT-2 over-expression improved antigen presentation by human cells. Moreover, EAT-2 over-expression increased the anti-tumor activity of human NK cells against K562 tumor cell targets. Many of these responses were extinguished with use of an EAT-2 variant carrying a mutant SH2 domain (R31Q), suggesting a critical role for the interaction between EAT-2 and SLAM receptors in mediating these responses. In conclusion, these results provide evidence that EAT-2 interacts with key components of multiple arms of the human innate immune system, and that this role highlights the potential for targeting EAT-2 functions so as to improve a number of human immunotherapeutic approaches, including vaccine development.

Identification of SLAMF3 (CD229) as an inhibitor of hepatocellular carcinoma cell proliferation and tumour progression

Although hepatocellular carcinoma (HCC) is one of the most common malignancies and constitutes the third leading cause of cancer-related deaths, the underlying molecular mechanisms are not fully understood. In the present study, we demonstrate for the first time that hepatocytes express signalling lymphocytic activation molecule family member 3 (SLAMF3/CD229) but not other SLAMF members. We provide evidence to show that SLAMF3 is involved in the control of hepatocyte proliferation and in hepatocellular carcinogenesis. SLAMF3 expression is significantly lower in primary human HCC samples and HCC cell lines than in human healthy primary hepatocytes. In HCC cell lines, the restoration of high levels of SLAMF3 expression inhibited cell proliferation and migration and enhanced apoptosis. Furthermore, SLAMF3 expression was associated with inhibition of HCC xenograft progression in the nude mouse model. The restoration of SLAMF3 expression levels also decreased the phosphorylation of MAPK ERK1/2, JNK and mTOR. In samples from resected HCC patients, SLAMF3 expression levels were significantly lower in tumorous tissues than in peritumoral tissues. Our results identify SLAMF3 as a specific marker of normal hepatocytes and provide evidence for its potential role in the control of proliferation of HCC cells.

Chicken NK cell receptors

Natural killer cells are innate immune cells that destroy virally infected or transformed cells. They recognize these altered cells by a plethora of diverse receptors and thereby differ from other lymphocytes that use clonally distributed antigen receptors. To date, several receptor families that play a role in either activating or inhibiting NK cells have been identified in mammals. In the chicken, NK cells have been functionally and morphologically defined, however, a conclusive analysis of receptors involved in NK cell mediated functions has not been available. This is partly due to the low frequencies of NK cells in blood or spleen that has hampered their intensive characterization. Here we will review recent progress regarding the diverse NK cell receptor families, with special emphasis on novel families identified in the chicken genome with potential as chicken NK cell receptors.

Primary immunodeficiencies predisposed to Epstein-Barr virus-driven haematological diseases

Epstein-Barr virus (EBV), a ubiquitous human herpesvirus, maintains lifelong subclinical persistent infections in humans. In the circulation, EBV primarily infects the B cells, and protective immunity is mediated by EBV-specific cytotoxic T cells (CTLs) and natural killer (NK) cells. However, EBV has been linked to several devastating diseases, such as haemophagocytic lymphohistiocytosis (HLH) and lymphoproliferative diseases in the immunocompromised host. Some types of primary immunodeficiencies (PIDs) are characterized by the development of EBV-associated complications as their predominant clinical feature. The study of such genetic diseases presents an ideal opportunity for a better understanding of the biology of the immune responses against EBV. Here, we summarize the range of PIDs that are predisposed to EBV-associated haematological diseases, describing their clinical picture and pathogenetic mechanisms.

Controlling natural killer cell responses: integration of signals for activation and inhibition

Understanding how signals are integrated to control natural killer (NK) cell responsiveness in the absence of antigen-specific receptors has been a challenge, but recent work has revealed some underlying principles that govern NK cell responses. NK cells use an array of innate receptors to sense their environment and respond to alterations caused by infections, cellular stress, and transformation. No single activation receptor dominates; instead, synergistic signals from combinations of receptors are integrated to activate natural cytotoxicity and cytokine production. Inhibitory receptors for major histocompatibility complex class I (MHC-I) have a critical role in controlling NK cell responses and, paradoxically, in maintaining NK cells in a state of responsiveness to subsequent activation events, a process referred to as licensing. MHC-I-specific inhibitory receptors both block activation signals and trigger signals to phosphorylate and inactivate the small adaptor Crk. These different facets of inhibitory signaling are incorporated into a revocable license model for the reversible tuning of NK cell responsiveness.

Evolving role of 2B4/CD244 in T and NK cell responses during virus infection

The signaling lymphocyte activation molecule (SLAM) family receptor, 2B4/CD244, was first implicated in anti-viral immunity by the discovery that mutations of the SLAM-associated protein, SAP/SH2D1A, impaired 2B4-dependent stimulation of T and natural killer (NK) cell anti-viral functions in X-linked lymphoproliferative syndrome patients with uncontrolled Epstein-Barr virus infections. Engagement of 2B4 has been variably shown to either activate or inhibit lymphocytes which express this receptor. While SAP expression is required for stimulatory functions of 2B4 on lymphocytes, it remains unclear whether inhibitory signals derived from 2B4 can predominate even in the presence of SAP. Regardless, mounting evidence suggests that 2B4 expression by NK and CD8 T cells is altered by virus infection in mice as well as in humans, and 2B4-mediated signaling may be an important determinant of effective immune control of chronic virus infections. In this review, recent findings regarding the expression and function of 2B4 as well as SAP on T and NK cells during virus infection is discussed, with a focus on the role of 2B4-CD48 interactions in crosstalk between innate and adaptive immunity.

Non-catalytic tyrosine-phosphorylated receptors

Leukocytes play a critical role in recognizing and responding to infection and cancer. Central to this function is an array of cell-surface receptors that lack sequence homology. Many of these receptors have in common the fact that their signaling involves phosphorylation of cytoplasmic domains by extrinsic tyrosine kinases. These non-catalytic tyrosine-phosphorylated receptors (NTRs) share a number of other features, including small size and optimal stimulation by surface-associated ligands. We argue here that NTRs are also likely to share the same kinetic-segregation triggering mechanism, which involves segregation of the engaged NTR from receptor tyrosine phosphatases with large ectodomains such as CD45 and CD148. NTRs signal through tyrosine-containing cytoplasmic motifs, which recruit distinct cytoplasmic signaling proteins when phosphorylated, transducing activatory or inhibitory signals. They have two features that make them uniquely well suited to their role in immune recognition of infection and cancer. Their modular structure enables the coupling of many rapidly evolving receptors with diverse ligand specificities to the same conserved signaling machinery. Their similarity in size and shared signaling machinery enables them to colocalize at cell-cell interfaces when they engage ligands, facilitating the integration of activatory and inhibitory signals from multiple receptors at the cell surface.

The adaptor protein SAP directly associates with CD3ζ chain and regulates T cell receptor signaling

Mutations altering the gene encoding the SLAM associated protein (SAP) are responsible for the X-linked lymphoproliferative disease or XLP1. Its absence is correlated with a defective NKT cells development, a decrease in B cell functions and a reduced T cells and NK cells cytotoxic activities, thus leading to an immunodeficiency syndrome. SAP is a small 128 amino-acid long protein that is almost exclusively composed of an SH2 domain. It has been shown to interact with the CD150/SLAM family of receptors, and in a non-canonical manner with SH3 containing proteins such as Fyn, βPIX, PKCθ and Nck1. It would thus play the role of a minimal adaptor protein. It has been shown that SAP plays an important function in the activation of T cells through its interaction with the SLAM family of receptors. Therefore SAP defective T cells display a reduced activation of signaling events downstream of the TCR-CD3 complex triggering. In the present work, we evidence that SAP is a direct interactor of the CD3ζ chain. This direct interaction occurs through the first ITAM of CD3ζ, proximal to the membrane. Additionally, we show that, in the context of the TCR-CD3 signaling, an Sh-RNA mediated silencing of SAP is responsible for a decrease of several canonical T cell signaling pathways including Erk, Akt and PLCγ1 and to a reduced induction of IL-2 and IL-4 mRNA. Altogether, we show that SAP plays a central function in the T cell activation processes through a direct association with the CD3 complex.

SAP and XIAP deficiency in hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is a multisystem inflammatory disorder due to cytokine overproduction from excessively activated lymphocytes and macrophages. HLH has been divided into two subgroups: primary HLH and secondary HLH. Primary HLH includes PRF1, UNC13D, STX11, STXBP2, RAB27A, LYST, SH2D1A and XIAP gene mutations; and secondary HLH is associated with infections, malignancies and autoimmune diseases. Among primary HLH-related genes, SH2D1A and XIAP are genetically responsible for X-linked lymphoproliferative syndrome (XLP) due to signaling-lymphocytic-activation-molecule-associated protein (SAP) and XIAP deficiencies, respectively. XLP is characterized by extreme vulnerability to Epstein-Barr virus infection. The major clinical manifestations of XLP consist of HLH (60%), lymphoproliferative disorder (30%) and dysgammaglobulinemia (30%). Analysis of clinical phenotypes of XLP patients suggests that XLP predominantly shows familial HLH phenotypes, whereas some XLP patients present sporadic HLH. For many decades, clinicians and investigators have been concerned with possible XLP in young boys presenting with Epstein-Barr-virus-associated HLH. This review aims to describe the new knowledge about XLP and to draw the attention of the pediatrician to XLP, which should be differentiated from other forms of HLH.

The receptor Ly108 functions as a SAP adaptor-dependent on-off switch for T cell help to B cells and NKT cell development

Humans and mice deficient in the adaptor protein SAP (Sh2d1a) have a major defect in humoral immunity, resulting from a lack of T cell help for B cells. The role of SAP in this process is incompletely understood. We found that deletion of receptor Ly108 (Slamf6) in CD4(+) T cells reversed the Sh2d1a(-/-) phenotype, eliminating the SAP requirement for germinal centers. This potent negative signaling by Ly108 required immunotyrosine switch motifs (ITSMs) and SHP-1 recruitment, resulting in high amounts of SHP-1 at the T cell:B cell synapse, limiting T cell:B cell adhesion. Ly108-negative signaling was important not only in CD4(+) T cells; we found that NKT cell differentiation was substantially restored in Slamf6(-/-)Sh2d1a(-/-) mice. The ability of SAP to regulate both positive and negative signals in T cells can explain the severity of SAP deficiency and highlights the importance of SAP and SHP-1 competition for Ly108 ITSM binding as a rheostat for the magnitude of T cell help to B cells.

Inherited defects causing hemophagocytic lymphohistiocytic syndrome

Hemophagocytic lymphohistiocytosis (HLH) manifests as the uncontrolled activation of T lymphocytes and macrophages infiltrating multiple organs. Molecular studies of individuals with HLH have demonstrated in most of these conditions a critical role of granule-dependent cytotoxic activity in the regulation of lymphocyte homeostasis, and have allowed the characterization of key effectors regulating cytotoxic granule release. The cytolytic process may now be considered a multistep process, including cell activation; the polarization of cytotoxic granules toward the conjugated target cell; the tethering, priming, and fusion of the cytotoxic granules with the plasma membrane; and the release of their contents (perforin and granzymes) into the intercellular cleft, leading to target cell death. Cytolytic cells have a second effector function involving the production of cytokines, principally γ-interferon, which is secreted independently of the exocytosis cytotoxic granule pathway. An analysis of the mechanisms underlying HLH has identified γ-interferon as a key cytokine inducing uncontrolled macrophage activation, and thus represents a potential therapeutic target.

CD84 negatively regulates IgE high-affinity receptor signaling in human mast cells

CD84 is a self-binding receptor from the CD150 (or signaling lymphocyte activation molecule [SLAM]) family that is broadly expressed in hematopoietic cells. It has been described that the adaptors SLAM-associated protein (SAP) and EWS-FLI1-activated transcript 2 (EAT-2) are critical for CD150 family members' signaling and function. We observed that human mast cells express CD84 but lack SAP or EAT-2, that CD84 is tyrosine phosphorylated upon FcεRI engagement, and that the release of granule contents is reduced when FcεRI is coengaged with CD84 in LAD2 and human CD34(+)-derived mast cells. In addition, we observed that the release of IL-8 and GM-CSF was also reduced in FcεRI/CD84-costimulated cells as compared with FcεRI/Ig control. To understand how CD84 downregulates FcεRI-mediated function, we analyzed signaling pathways affected by CD84 in human mast cells. Our results showed that CD84 dampens FcεRI-mediated calcium mobilization after its co-cross-linking with the receptor. Furthermore, FcεRI-mediated Syk-linker for activation of T cells-phospholipase C-γ1 axis activity is downregulated after CD84 stimulation, compared with FcεRI/Ig control. The inhibitory kinase Fes phosphorylates mainly the inhibitory motif for CD84. Moreover, Fes, which has been described to become phosphorylated after substrate binding, also gets phosphorylated when coexpressed with CD84. Consistently, Fes was observed to be more phosphorylated after CD84 and FcεRI co-cross-linking. The phosphorylation of the protein phosphatase Src homology region 2 domain-containing phosphatase-1 also increases after CD84 and FcεRI coengagement. Taken together, our results show that CD84 is highly expressed in mast cells and that it contributes to the regulation of FcεRI signaling in SAP- and EAT-2-independent and Fes- and Src homology region 2 domain-containing phosphatase-1-dependent mechanisms.

Molecular pathogenesis of EBV susceptibility in XLP as revealed by analysis of female carriers with heterozygous expression of SAP

X-linked lymphoproliferative disease (XLP) is a primary immunodeficiency caused by mutations in SH2D1A which encodes SAP. SAP functions in signalling pathways elicited by the SLAM family of leukocyte receptors. A defining feature of XLP is exquisite sensitivity to infection with EBV, a B-lymphotropic virus, but not other viruses. Although previous studies have identified defects in lymphocytes from XLP patients, the unique role of SAP in controlling EBV infection remains unresolved. We describe a novel approach to this question using female XLP carriers who, due to random X-inactivation, contain both SAP(+) and SAP(-) cells. This represents the human equivalent of a mixed bone marrow chimera in mice. While memory CD8(+) T cells specific for CMV and influenza were distributed across SAP(+) and SAP(-) populations, EBV-specific cells were exclusively SAP(+). The preferential recruitment of SAP(+) cells by EBV reflected the tropism of EBV for B cells, and the requirement for SAP expression in CD8(+) T cells for them to respond to Ag-presentation by B cells, but not other cell types. The inability of SAP(-) clones to respond to Ag-presenting B cells was overcome by blocking the SLAM receptors NTB-A and 2B4, while ectopic expression of NTB-A on fibroblasts inhibited cytotoxicity of SAP(-) CD8(+) T cells, thereby demonstrating that SLAM receptors acquire inhibitory function in the absence of SAP. The innovative XLP carrier model allowed us to unravel the mechanisms underlying the unique susceptibility of XLP patients to EBV infection in the absence of a relevant animal model. We found that this reflected the nature of the Ag-presenting cell, rather than EBV itself. Our data also identified a pathological signalling pathway that could be targeted to treat patients with severe EBV infection. This system may allow the study of other human diseases where heterozygous gene expression from random X-chromosome inactivation can be exploited.

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.

Expression of the SLAM family of receptors adapter EAT-2 as a novel strategy for enhancing beneficial immune responses to vaccine antigens

Recent studies have shown that activation of the signaling lymphocytic activation molecule (SLAM) family of receptors plays an important role in several aspects of immune regulation. However, translation of this knowledge into a useful clinical application has not been undertaken. One important area where SLAM-mediated immune regulation may have keen importance is in the field of vaccinology. Because SLAM signaling plays such a critical role in the innate and adaptive immunity, we endeavored to develop a strategy to improve the efficacy of vaccines by incorporation of proteins known to be important in SLAM-mediated signaling. In this study, we hypothesized that coexpression of the SLAM adapter EWS-FLI1-activated transcript 2 (EAT-2) along with a pathogen-derived Ag would facilitate induction of beneficial innate immune responses, resulting in improved induction of Ag-specific adaptive immune responses. To test this hypothesis, we used rAd5 vector-based vaccines expressing murine EAT-2, or the HIV-1-derived Ag Gag. Compared with appropriate controls, rAd5 vectors expressing EAT-2 facilitated bystander activation of NK, NKT, B, and T cells early after their administration into animals. EAT-2 overexpression also augments the expression of APC (macrophages and dendritic cells) surface markers. Indeed, this multitiered activation of the innate immune system by vaccine-mediated EAT-2 expression enhanced the induction of Ag-specific cellular immune responses. Because both mice and humans express highly conserved EAT-2 adapters, our results suggest that human vaccination strategies that specifically facilitate SLAM signaling may improve vaccine potency when targeting HIV Ags specifically, as well as numerous other vaccine targets in general.

Clinical similarities and differences of patients with X-linked lymphoproliferative syndrome type 1 (XLP-1/SAP deficiency) versus type 2 (XLP-2/XIAP deficiency)

X-linked lymphoproliferative syndromes (XLP) are primary immunodeficiencies characterized by a particular vulnerability toward Epstein-Barr virus infection, frequently resulting in hemophagocytic lymphohistiocytosis (HLH). XLP type 1 (XLP-1) is caused by mutations in the gene SH2D1A (also named SAP), whereas mutations in the gene XIAP underlie XLP type 2 (XLP-2). Here, a comparison of the clinical phenotypes associated with XLP-1 and XLP-2 was performed in cohorts of 33 and 30 patients, respectively. HLH (XLP-1, 55%; XLP-2, 76%) and hypogammaglobulinemia (XLP-1, 67%; XLP-2, 33%) occurred in both groups. Epstein-Barr virus infection in XLP-1 and XLP-2 was the common trigger of HLH (XLP-1, 92%; XLP-2, 83%). Survival rates and mean ages at the first HLH episode did not differ for both groups, but HLH was more severe with lethal outcome in XLP-1 (XLP-1, 61%; XLP-2, 23%). Although only XLP-1 patients developed lymphomas (30%), XLP-2 patients (17%) had chronic hemorrhagic colitis as documented by histopathology. Recurrent splenomegaly often associated with cytopenia and fever was preferentially observed in XLP-2 (XLP-1, 7%; XLP-2, 87%) and probably represents minimal forms of HLH as documented by histopathology. This first phenotypic comparison of XLP subtypes should help to improve the diagnosis and the care of patients with XLP conditions.

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 power and the promise of restimulation-induced cell death in human immune diseases

Controlled expansion and contraction of lymphocytes both during and after an adaptive immune response are imperative to sustain a healthy immune system. Both extrinsic and intrinsic pathways of lymphocyte apoptosis are programmed to eliminate cells at the proper time to ensure immune homeostasis. Genetic disorders of apoptosis described in mice and humans have established Fas and Bim as critical pro-apoptotic molecules responsible for T-cell death in response to T-cell receptor restimulation and cytokine withdrawal, respectively. Emerging evidence prompts revision of this classic paradigm, especially for our understanding of restimulation-induced cell death (RICD) and its physiological purpose. Recent work indicates that RICD employs both Fas and Bim for T-cell deletion, dispelling the notion that these molecules are assigned to mutually exclusive apoptotic pathways. Furthermore, new mouse model data combined with our discovery of defective RICD in X-linked lymphoproliferative disease (XLP) patient T cells suggest that RICD is essential for precluding excess T-cell accumulation and associated immunopathology during the course of certain infections. Here, we review how these advances offer a refreshing new perspective on the phenomenon of T-cell apoptosis induced through antigen restimulation, including its relevance to immune homeostasis and potential for therapeutic interventions.

SLAM-family receptors: immune regulators with or without SAP-family adaptors

The signaling lymphocytic activation molecule (SLAM) family of receptors and the SLAM-associated protein (SAP) family of intracellular adaptors are expressed in immune cells. By way of their cytoplasmic domain, SLAM-related receptors physically associate with SAP-related adaptors. Evidence is accumulating that the SLAM and SAP families play crucial roles in multiple immune cell types. Moreover, the prototype of the SAP family, that is SAP, is mutated in a human immunodeficiency, X-linked lymphoproliferative (XLP) disease. In the presence of SAP-family adaptors, the SLAM family usually mediates stimulatory signals that promote immune cell activation or differentiation. In the absence of SAP-family adaptors, though, the SLAM family undergoes a "switch-of-function," thereby mediating inhibitory signals that suppress immune cell functions. The molecular basis and significance of this mechanism are discussed herein.

X-linked lymphoproliferative syndromes: brothers or distant cousins?

X-linked lymphoproliferative disease (XLP1), described in the mid-1970s and molecularly defined in 1998, and XLP2, reported in 2006, are prematurely lethal genetic immunodeficiencies that share susceptibility to overwhelming inflammatory responses to certain infectious triggers. Signaling lymphocytic activation molecule-associated protein (SAP; encoded by SH2D1A) is mutated in XLP1, and X-linked inhibitor of apoptosis (XIAP; encoded by BIRC4) is mutated in XLP2. XLP1 is a disease with multiple and variable clinical consequences, including fatal hemophagocytic lymphohistiocytosis (HLH) triggered predominantly by Epstein-Barr virus, lymphomas, antibody deficiency, and rarer consequences of immune dysregulation. To date, XLP2 has been found to cause HLH with and without exposure to Epstein-Barr virus, and HLH is commonly recurrent in these patients. For both forms of XLP, the only curative therapy at present is allogeneic hematopoietic cell transplantation. Beyond their common X-linked locus and their requirement for normal immune responses to certain viral infections, SAP and XIAP demonstrate no obvious structural or functional similarity, are not coordinately regulated with respect to their expression, and do not appear to directly interact. In this review, we describe the genetic, clinical, and immunopathologic features of these 2 disorders and discuss current diagnostic and therapeutic strategies.