Defective B cell responses in the absence of SH2D1A

T-B Collaboration in Autoimmunity, Infection, and Transplantation

We have attempted here to provide an up-to-date review of the collaboration between helper T cells and B cells in response to protein and glycoprotein antigens. This collaboration is essential as it not only protects from many pathogens but also contributes to a litany of autoimmune and immune-mediated diseases.

The landscape of PBMC methylome in canine mammary tumors reveals the epigenetic regulation of immune marker genes and its potential application in predicting tumor malignancy

BACKGROUND

Genome-wide dysregulation of CpG methylation accompanies tumor progression and characteristic states of cancer cells, prompting a rationale for biomarker development. Understanding how the archetypic epigenetic modification determines systemic contributions of immune cell types is the key to further clinical benefits.

RESULTS

In this study, we characterized the differential DNA methylome landscapes of peripheral blood mononuclear cells (PBMCs) from 76 canines using methylated CpG-binding domain sequencing (MBD-seq). Through gene set enrichment analysis, we discovered that genes involved in the growth and differentiation of T- and B-cells are highly methylated in tumor PBMCs. We also revealed the increased methylation at single CpG resolution and reversed expression in representative marker genes regulating immune cell proliferation (BACH2, SH2D1A, TXK, UHRF1). Furthermore, we utilized the PBMC methylome to effectively differentiate between benign and malignant tumors and the presence of mammary gland tumors through a machine-learning approach.

CONCLUSIONS

This research contributes to a better knowledge of the comprehensive epigenetic regulation of circulating immune cells responding to tumors and suggests a new framework for identifying benign and malignant cancers using genome-wide methylome.

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.

The immunoreceptor SLAMF8 promotes the differentiation of follicular dendritic cell-dependent monocytic cells with B cell-activating ability

Follicular dendritic cells (FDCs) play a crucial role in generating high-affinity antibody-producing B cells during the germinal center (GC) reaction. Herein, we analyzed the altered gene expression profile of a mouse FDC line, FL-Y, following lymphotoxin β receptor stimulation, and observed increased Slam-family member 8 (Slamf8) mRNA expression. Forced Slamf8 expression and SLAMF8-Fc addition enhanced the ability of FL-Y cells to induce FDC-induced monocytic cell (FDMC) differentiation. FDMCs accelerated GC-phenotype proliferation in cultured B cells, suggesting that they are capable of promoting GC responses. Furthermore, a pulldown assay showed that SLAMF8-Fc could bind to SLAMF8-His. Overall, the homophilic interaction of SLAMF8 promotes FDMC differentiation and SLAMF8 might act as a novel regulator of GC responses by regulating FDMC differentiation.

Sex differences in neuro(auto)immunity and chronic sciatic nerve pain

Chronic pain occurs with greater frequency in women, with a parallel sexually dimorphic trend reported in sufferers of many autoimmune diseases. There is a need to continue examining neuro-immune-endocrine crosstalk in the context of sexual dimorphisms in chronic pain. Several phenomena in particular need to be further explored. In patients, autoantibodies to neural antigens have been associated with sensory pathway hyper-excitability, and the role of self-antigens released by damaged nerves remains to be defined. In addition, specific immune cells release pro-nociceptive cytokines that directly influence neural firing, while T lymphocytes activated by specific antigens secrete factors that either support nerve repair or exacerbate the damage. Modulating specific immune cell populations could therefore be a means to promote nerve recovery, with sex-specific outcomes. Understanding biological sex differences that maintain, or fail to maintain, neuroimmune homeostasis may inform the selection of sex-specific treatment regimens, improving chronic pain management by rebalancing neuroimmune feedback. Given the significance of interactions between nerves and immune cells in the generation and maintenance of neuropathic pain, this review focuses on sex differences and possible links with persistent autoimmune activity using sciatica as an example.

Tfh Cells in Health and Immunity: Potential Targets for Systems Biology Approaches to Vaccination

T follicular helper (Tfh) cells are a specialised subset of CD4+ T cells that play a significant role in the adaptive immune response, providing critical help to B cells within the germinal centres (GC) of secondary lymphoid organs. The B cell receptors of GC B cells undergo multiple rounds of somatic hypermutation and affinity maturation within the GC response, a process dependent on cognate interactions with Tfh cells. B cells that receive sufficient help from Tfh cells form antibody-producing long-lived plasma and memory B cells that provide the basis of decades of effective and efficient protection and are considered the gold standard in correlates of protection post-vaccination. However, the T cell response to vaccination has been understudied, and over the last 10 years, exponential improvements in the technological underpinnings of sampling techniques, experimental and analytical tools have allowed multidisciplinary characterisation of the role of T cells and the immune system as a whole. Of particular interest to the field of vaccinology are GCs and Tfh cells, representing a unique target for improving immunisation strategies. Here, we discuss recent insights into the unique journey of Tfh cells from thymus to lymph node during differentiation and their role in the production of high-quality antibody responses as well as their journey back to the periphery as a population of memory cells. Further, we explore their function in health and disease and the power of next-generation sequencing techniques to uncover their potential as modulators of vaccine-induced immunity.

Regulation of MHC class I-independent NK cell education by SLAM family receptors

Seven members of signaling lymphocytic activation molecule (SLAM) family receptors (SFRs) are ubiquitously expressed on hematopoietic cells and they play critical roles in immune cell differentiation and activation. The engagement of these receptors transmits intracellular signaling mainly by recruiting SLAM-associated protein (SAP) and its related adaptors, EWS-FLI1-activated transcript-2 (EAT-2) and EAT-2-related transducer (ERT). The critical roles of SFRs and SAP-family adaptors are highlighted by the discovery that SAP is mutated in human X-linked lymphoproliferative (XLP1) disease in which the contact between T and B cells in germinal center and cytotoxic lymphocytes (NK cells and CD8⁺ T cells) function are severely compromised. These immune defects are closely associated with the defective antibody production and the high incidence of lymphoma in the patients with XLP1. In addition to these well-known functions, SLAM-SAP family is involved in NK cell education, a process describing NK cell functional competence. In this chapter, we will mainly discuss these unappreciated roles of SAP-dependent and SAP-independent SFR signaling in regulating MHC-I-independent NK cell education.

Genes known to escape X chromosome inactivation predict co-morbid chronic musculoskeletal pain and posttraumatic stress symptom development in women following trauma exposure

Co-morbid chronic musculoskeletal pain (CMSP) and posttraumatic stress symptoms (PTSS) are frequent sequelae of motor vehicle collision, are associated with greater disability than either outcome alone, and are more prevalent in women than men. In the current study we assessed for evidence that gene transcripts originating from the X chromosome contribute to sex differences in vulnerability to CMSP and PTSS after motor vehicle collision. Nested samples were drawn from a longitudinal study of African American individuals, and CMSP (0-10 numeric rating scale) and PTSS (impact of events scale, revised) outcomes were assessed 6 months following motor vehicle collision. Blood RNA were sequenced (n = 101) and the relationship between X chromosome mRNA expression levels and co-morbid CMSP and PTSS outcomes was evaluated using logistic regression analyses. A disproportionate number of peritraumatic X chromosome mRNA predicting CMSP and PTSS in women were genes previously found to escape X chromosome inactivation (11/40, z = -2.9, p = .004). Secondary analyses assessing gene ontology relationships between these genes identified an enrichment in genes known to influence neuronal plasticity. Further, the relationship of expression of two critical regulators of X chromosome inactivation, X-inactive specific transcript (XIST) and Yin Yang 1 (YY1), was different in women developing CMSP and PTSS. Together, these data suggest that X chromosome genes that escape inactivation may contribute to sex differences in vulnerability to CMSP and PTSS after motor vehicle collision.

X-Linked Lymphoproliferative Disease Type 1: A Clinical and Molecular Perspective

X-linked lymphoproliferative disease (XLP) was first described in the 1970s as a fatal lymphoproliferative syndrome associated with infection with Epstein–Barr virus (EBV). Features include hemophagocytic lymphohistiocytosis (HLH), lymphomas, and dysgammaglobulinemias. Molecular cloning of the causative gene, SH2D1A, has provided insight into the nature of disease, as well as helped characterize multiple features of normal immune cell function. Although XLP type 1 (XLP1) provides an example of a primary immunodeficiency in which patients have problems clearing primarily one infectious agent, it is clear that XLP1 is also a disease of severe immune dysregulation, even independent of EBV infection. Here, we describe clinical features of XLP1, how molecular and biological studies of the gene product, SAP, and the associated signaling lymphocyte activation molecule family receptors have provided insight into disease pathogenesis including specific immune cell defects, and current therapeutic approaches including the potential use of gene therapy. Together, these studies have helped change the outcome of this once almost uniformly fatal disease.

Follicular Helper T Cells are Essential for the Elimination of Plasmodium Infection

CD4⁺ follicular helper T (Tfh) cells have been shown to be critical for the activation of germinal center (GC) B-cell responses. Similar to other infections, Plasmodium infection activates both GC as well as non-GC B cell responses. Here, we sought to explore whether Tfh cells and GC B cells are required to eliminate a Plasmodium infection. A CD4 T cell-targeted deletion of the gene that encodes Bcl6, the master transcription factor for the Tfh program, resulted in complete disruption of the Tfh response to Plasmodium chabaudi in C57BL/6 mice and consequent disruption of GC responses and IgG responses and the inability to eliminate the otherwise self-resolving chronic P. chabaudi infection. On the other hand, and contrary to previous observations in immunization and viral infection models, Signaling Lymphocyte Activation Molecule (SLAM)-Associated Protein (SAP)-deficient mice were able to activate Tfh cells, GC B cells, and IgG responses to the parasite. This study demonstrates the critical role for Tfh cells in controlling this systemic infection, and highlights differences in the signals required to activate GC B cell responses to this complex parasite compared with those of protein immunizations and viral infections. Therefore, these data are highly pertinent for designing malaria vaccines able to activate broadly protective B-cell responses.

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Chronic Plasmodium infection cannot be eliminated in the absence of Tfh cell responses.

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SAP-deficient mice are able to activate GC Tfh and GC B-cell responses to Plasmodium infection.

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There is a hierarchical requirement for the control of chronic Plasmodium infection following IL-21R > Tfh cells > SAP.

Successful vaccines work through activation of protective B-cell responses. Malaria, caused by Plasmodium infection transmitted by mosquito bites, remains a global threat. Despite substantial efforts, a vaccine able to bring about high levels of protection from Plasmodium infection remains elusive. Here, using an experimental malaria model including natural mosquito transmission, we demonstrate that proper activation of follicular helper CD4⁺ T cells is essential for the control and eradication of chronic Plasmodium infection through protective B-cell responses. Thus, it is strongly advisable for novel vaccine efforts to monitor the robust activation of this important immune compartment.

Decreased SAP Expression in T Cells from Patients with Systemic Lupus Erythematosus Contributes to Early Signaling Abnormalities and Reduced IL-2 Production

T cells from patients with systemic lupus erythematosus (SLE) display a number of abnormalities, including increased early signaling events following engagement of the TCR. Signaling lymphocytic activation molecule family cell surface receptors and the X-chromosome-defined signaling lymphocytic activation molecule-associated protein (SAP) adaptor are important in the development of several immunocyte lineages and modulating the immune response. We present evidence that SAP protein levels are decreased in T cells and in their main subsets isolated from 32 women and three men with SLE, independent of disease activity. In SLE T cells, SAP protein is also subject to increased degradation by caspase-3. Forced expression of SAP in SLE T cells normalized IL-2 production, calcium (Ca(2+)) responses, and tyrosine phosphorylation of a number of proteins. Exposure of normal T cells to SLE serum IgG, known to contain anti-CD3/TCR Abs, resulted in SAP downregulation. We conclude that SLE T cells display reduced levels of the adaptor protein SAP, probably as a result of continuous T cell activation and degradation by caspase-3. Restoration of SAP levels in SLE T cells corrects the overexcitable lupus T cell phenotype.

Responses to Microbial Challenges by SLAMF Receptors

The SLAMF family (SLAMF) of cell surface glycoproteins is comprised of nine glycoproteins and while SLAMF1, 3, 5, 6, 7, 8, and 9 are self-ligand receptors, SLAMF2 and SLAMF4 interact with each other. Their interactions induce signal transduction networks in trans, thereby shaping immune cell-cell communications. Collectively, these receptors modulate a wide range of functions, such as myeloid cell and lymphocyte development, and T and B cell responses to microbes and parasites. In addition, several SLAMF receptors serve as microbial sensors, which either positively or negatively modulate the function of macrophages, dendritic cells, neutrophils, and NK cells in response to microbial challenges. The SLAMF receptor-microbe interactions contribute both to intracellular microbicidal activity as well as to migration of phagocytes to the site of inflammation. In this review, we describe the current knowledge on how the SLAMF receptors and their specific adapters SLAM-associated protein and EAT-2 regulate innate and adaptive immune responses to microbes.

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.

Staphylococcal Enterotoxin B (SEB) Induces Memory CD4 T Cell Anergy in vivo and Impairs Recall Immunity to Unrelated Antigens

INTRODUCTION

Naïve and memory T cells can utilize unique regulatory pathways to promote protection but prevent self-reactivity. A bacterial superantigen SEB exploits unique TCR proximal signaling processes in memory CD4 T cells to induce clonal anergy. The aim of this study was to determine if SEB could antagonize memory CD4 T cells in vivo and whether there would be consequences on recall immune responses. We evaluated Ab responses to a T-dependent antigen as a measurement of memory T cell helper function.

METHOD

BALB/c mice were primed with TNP-RGG to elicit memory B cells and also immunized with an ovalbumin peptide to elicit memory helper T cells. Another group of TNP-RGG immunized mice were used as adoptive transfer recipients of exogenous DO11.10 memory T cells. Mice were challenged with TNP-OVA with or without prior administration of SEB. B cells secreting IgM or IgG TNP-specific Ab were enumerated by ELISPOT as indicators of primary versus secondary humoral immunity.

RESULTS

Comparing the SEB and non-SEB-treated groups, the SEB-treated group failed to produce TNP-specific IgG in response to challenge with TNP-OVA, even if they were previously immunized with OVA. All groups produced IgM, indicating that the primary Ab responses and naïve helper T cells were not impacted by SEB. SEB had no negative impact when DO11.10 × Fyn^-/- memory T cells were used as donor cells.

CONCLUSION

The present study indicated that SEB selectively targeted memory CD4 T cells in vivo and prevented helper function. Consequently, recall humoral immunity was lost. The data are most consistent with in vivo T cell anergy as opposed to indirect suppression as elimination of Fyn kinase restored helper function. These data suggest that bacterial superantigens can impair post-vaccination memory cell responses to unrelated antigens via their ability to target Vb families and antagonize memory cell activation.

Differential transcriptomic response in the spleen and head kidney following vaccination and infection of Asian seabass with Streptococcus iniae

Vaccination is an important strategy in the protection of aquaculture species from major diseases. However, we still do not have a good understanding of the mechanisms underlying vaccine-induced disease resistance. This is further complicated by the presence of several lymphoid organs that play different roles when mounting an immune response. In this study, we attempt to elucidate some of these mechanisms using a microarray-based approach. Asian seabass (Lates calcarifer) were vaccinated against Streptococcus iniae and the transcriptomic changes within the spleen and head kidney at one and seven days post-vaccination were profiled. We subsequently challenged the seabass at three weeks post-vaccination with live S. iniae and similarly profiled the transcriptomes of the two organs after the challenge. We found that vaccination induced an early, but transient transcriptomic change in the spleens and a delayed response in the head kidneys, which became more similar to one another compared to un-vaccinated ones. When challenged with the pathogen, the spleen, but not the head kidneys, responded transcriptomically at 25-29 hours post-challenge. A unique set of genes, in particular those involved in the activation of NF-κB signaling, was up-regulated in the vaccinated spleens upon pathogen challenge but not in the un-vaccinated spleens. A semi-quantitative PCR detection of S. iniae using metagenomic DNA extracted from the water containing the seabass also revealed that vaccination resulted in reduction of pathogen shedding. This result indicated that vaccination not only led to a successful immune defense against the infection, but also reduced the chances for horizontal transmission of the pathogen. In conclusion, we have provided a transcriptomic analysis of how the teleost spleen and head kidneys responded to vaccination and subsequent infection. The different responses from the two organs are suggestive of their unique roles in establishing a vaccine-induced disease resistance.

SAP modulates B cell functions in a genetic background-dependent manner

Mutations affecting the SLAM-associated protein (SAP) are responsible for the X-linked lympho-proliferative syndrome (XLP), a severe primary immunodeficiency syndrome with disease manifestations that include fatal mononucleosis, B cell lymphoma and dysgammaglobulinemia. It is well accepted that insufficient help by SAP-/- CD4+ T cells, in particular during the germinal center reaction, is a component of dysgammaglobulinemia in XLP patients and SAP-/- animals. It is however not well understood whether in XLP patients and SAP-/- mice B cell functions are affected, even though B cells themselves do not express SAP. Here we report that B cell intrinsic responses to haptenated protein antigens are impaired in SAP-/- mice and in Rag-/- mice into which B cells derived from SAP-/- mice together with wt CD4+ T cells had been transferred. This impaired B cells functions are in part depending on the genetic background of the SAP-/- mouse, which affects B cell homeostasis. Surprisingly, stimulation with an agonistic anti-CD40 causes strong in vivo and in vitro B cell responses in SAP-/- mice. Taken together, the data demonstrate that genetic factors play an important role in the SAP-related B cell functions. The finding that anti-CD40 can in part restore impaired B cell responses in SAP-/- mice, suggests potentially novel therapeutic interventions in subsets of XLP patients.

SAP expression in invariant NKT cells is required for cognate help to support B-cell responses

One of the manifestations of X-linked lymphoproliferative disease (XLP) is progressive agammaglobulinemia, caused by the absence of a functional signaling lymphocyte activation molecule (SLAM)-associated protein (SAP) in T, invariant natural killer T (NKT) cells and NK cells. Here we report that α-galactosylceramide (αGalCer) activated NKT cells positively regulate antibody responses to haptenated protein antigens at multiple checkpoints, including germinal center formation and affinity maturation. Whereas NKT cell-dependent B cell responses were absent in SAP(-/-).B6 mice that completely lack NKT cells, the small number of SAP-deficient NKT cells in SAP(-/-).BALB/c mice adjuvated antibody production, but not the germinal center reaction. To test the hypothesis that SAP-deficient NKT cells can facilitate humoral immunity, SAP was deleted after development in SAP(fl/fl).tgCreERT2.B6 mice. We find that NKT cell intrinsic expression of SAP is dispensable for noncognate helper functions, but is critical for providing cognate help to antigen-specific B cells. These results demonstrate that SLAM-family receptor-regulated cell-cell interactions are not limited to T-B cell conjugates. We conclude that in the absence of SAP, several routes of NKT cell-mediated antibody production are still accessible. The latter suggests that residual NKT cells in XLP patients might contribute to variations in dysgammaglobulinemia.

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.

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.

Biochemical and genetic evidence for a SAP-PKC-theta interaction contributing to IL-4 regulation

Signaling lymphocytic activation molecule-associated protein (SAP), an adaptor molecule that recruits Fyn to the signaling lymphocytic activation molecule (SLAM) family of immunomodulatory receptors, is mutated in X-linked lymphoproliferative disease. CD4(+) T cells from SAP-deficient mice have defective TCR-induced and follicular Th cell IL-4 production and impaired T cell-mediated help for germinal center formation; however, the downstream intermediates contributing to these defects remain unclear. We previously found that SAP-deficient CD4(+) T cells exhibit decreased protein kinase C (PKC)-theta recruitment upon TCR stimulation. We demonstrate in this paper using GST pulldowns and coimmunoprecipitation studies that SAP constitutively associates with PKC- in T cells. SAP-PKC-theta interactions required R78 of SAP, a residue previously implicated in Fyn recruitment, yet SAP's interactions with PKC-theta occurred independent of phosphotyrosine binding and Fyn. Overexpression of SAP in T cells increased and sustained PKC-theta recruitment to the immune synapse and elevated IL-4 production in response to TCR plus SLAM-mediated stimulation. Moreover, PKC-theta, like SAP, was required for SLAM-mediated increases in IL-4 production, and, conversely, membrane-targeted PKC-theta mutants rescued IL-4 expression in SAP(-/-) CD4(+) T cells, providing genetic evidence that PKC-theta is a critical component of SLAM/SAP-mediated pathways that influence TCR-driven IL-4 production.

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

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

T follicular helper cells during immunity and tolerance

Helper T cells are required for the generation of a potent immune response to foreign antigens. Amongst them, T follicular helper (Tfh) cells are specialized in promoting protective, long-lived antibody responses that arise from germinal centers. Within these structures, the specificity of B cell receptors may change, due to the process of random somatic hypermutation aimed at increasing the overall affinity of the antibody response. The danger of emerging self-reactive specificities is offset by a stringent selection mechanism delegated in great part to Tfh cells. Only those B cells receiving survival signals from Tfh cells can exit the germinal centers to join the long-lived pools of memory B cells and bone marrow-homing plasma cells. Thus, a crucial immune tolerance checkpoint to prevent long-term autoantibody production lies in the ability to tolerize Tfh cells and to control positive and negative selection signals delivered by this subset. This review tackles the known mechanisms that ensure Tfh tolerance, many of them shared by other T helper subsets during thymic development and priming, but others unique to Tfh cells. Amongst the latter are checkpoints at the stages of Tfh differentiation, follicular migration, growth, longevity, and quality control of selection signals. Finally, we also discuss the consequences of a breakdown in Tfh tolerance.

Short telomeres are sufficient to cause the degenerative defects associated with aging

Telomerase function is critical for telomere maintenance. Mutations in telomerase components lead to telomere shortening and progressive bone marrow failure in the premature aging syndrome dyskeratosis congenita. Short telomeres are also acquired with aging, yet the role that they play in mediating age-related disease is not fully known. We generated wild-type mice that have short telomeres. In these mice, we identified hematopoietic and immune defects that resembled those present in dyskeratosis congenita patients. When mice with short telomeres were interbred, telomere length was only incrementally restored, and even several generations later, wild-type mice with short telomeres still displayed degenerative defects. Our findings implicate telomere length as a unique heritable trait that, when short, is sufficient to mediate the degenerative defects of aging, even when telomerase is wild-type.

X-linked lymphoproliferative disease (XLP): a model of impaired anti-viral, anti-tumor and humoral immune responses

A major focus of our research is to understand the molecular and cellular basis of X-linked lymphoproliferative disease (XLP), a rare and often fatal immunodeficiency caused by mutations in the SH2D1A gene, which encodes the adaptor molecule SAP. Recently, we observed that SAP is essential for the development of natural killer T (NKT) cells, a lymphocyte population that participates in protection against certain tumors, infections, and autoimmune states. In this review, we describe the approaches that we are taking to understand the role of SAP in immune cells, including NKT cells. By using SAP as the focal point of our studies, we hope to identify novel signaling pathways that could be targeted to improve the treatment for patients with XLP as well as more common disorders, such as autoimmunity and cancer.

Follicular helper T cells are required for systemic autoimmunity

Production of high-affinity pathogenic autoantibodies appears to be central to the pathogenesis of lupus. Because normal high-affinity antibodies arise from germinal centers (GCs), aberrant selection of GC B cells, caused by either failure of negative selection or enhanced positive selection by follicular helper T (T(FH)) cells, is a plausible explanation for these autoantibodies. Mice homozygous for the san allele of Roquin, which encodes a RING-type ubiquitin ligase, develop GCs in the absence of foreign antigen, excessive T(FH) cell numbers, and features of lupus. We postulated a positive selection defect in GCs to account for autoantibodies. We first demonstrate that autoimmunity in Roquin(san/san) (sanroque) mice is GC dependent: deletion of one allele of Bcl6 specifically reduces the number of GC cells, ameliorating pathology. We show that Roquin(san) acts autonomously to cause accumulation of T(FH) cells. Introduction of a null allele of the signaling lymphocyte activation molecule family adaptor Sap into the sanroque background resulted in a substantial and selective reduction in sanroque T(FH) cells, and abrogated formation of GCs, autoantibody formation, and renal pathology. In contrast, adoptive transfer of sanroque T(FH) cells led to spontaneous GC formation. These findings identify T(FH) dysfunction within GCs and aberrant positive selection as a pathway to systemic autoimmunity.

SLAM receptors and SAP influence lymphocyte interactions, development and function

Mutations that affect the adaptor molecule SLAM-associated protein (SAP) underlie the primary immunodeficiency disease X-linked lymphoproliferative syndrome. SAP is required for mediating signals from members of the signalling lymphocytic activation molecule (SLAM) family of immunomodulatory receptors. Recent data have highlighted a role for SAP in the development of innate-like T-cell lineages, including natural killer T cells, and in the regulation of the interactions between B cells and T cells that are required for germinal-centre formation and long-term humoral immunity. These data have revealed that SLAM family members and SAP have crucial roles in regulating lymphocyte interactions and adhesion, which are required for the normal development, homeostasis and function of the immune system.

SAP binds to CD22 and regulates B cell inhibitory signaling and calcium flux

The signaling lymphocyte activation molecule (SLAM)-associated protein (SAP or SH2D1A) is an important regulator of immune function which, when mutated or deleted, causes the X-linked lymphoproliferative syndrome (XLP). Because B cell lymphoma is a major phenotype of XLP, it is important to understand the function of SAP in B cells. Here we report that SAP is expressed endogenously in mouse splenic B cells, is inducibly expressed in the human BJAB cells, and co-localizes and interacts with CD22. We also show that SAP binding to the inhibitory immunoreceptor CD22 regulates calcium mobilization in B cells. Moreover, forced expression of SAP leads to constitutive CD22 tyrosine phosphorylation and decreased Ca(2+) response in B cells. Biochemical analysis reveals that, in response to IgM cross-linking, the phosphorylation of Syk, Blnk, or PLCgamma2 and their interactions with one another were either diminished or completely abolished in SAP-expressing cells compared to cells that lack SAP. Collectively our work identifies a novel role for SAP in B cells and extends its function to inhibitory immunoreceptor signaling and calcium mobilization.

SAP enables T cells to help B cells by a mechanism distinct from Th cell programming or CD40 ligand regulation

Genetic mutations disrupting the function of signaling lymphocytic activation molecule-associated protein (SAP) lead to T cell intrinsic defects in T cell-dependent Ab responses. To better understand how SAP enables Th cells to help B cells, we first assessed whether molecules important for B cell help are dysregulated in SAP-deficient (SAP knockout (KO)) mice. CD40 ligand (CD40L) expression was enhanced on unpolarized SAP KO T cells; however, Th2 polarization returned their CD40L expression to wild-type levels without rescuing their ability to help B cells. CD40L also localized normally to the site of contact between SAP KO T cells and Ag-bearing B cells. Finally, CD40L-deficient Th cells and SAP KO Th cells differed in their abilities to help B cells in vitro. These data argue that Ab defects caused by SAP deficiency do not result from a loss of CD40L regulation or CD40L function on CD4 T cells. SAP KO Th cells additionally displayed normal patterns of migration and expression of ICOS and CXCR5. Global gene expression was remarkably similar in activated SAP KO vs wild-type T cells, prompting us to investigate whether SAP is necessary for "programming" T cells to become B cell helpers. By restricting SAP expression during differentiation, we determined that SAP is not required during the first 5 days of T cell activation/differentiation to generate Th cells capable of helping B cells. Instead, SAP is necessary for very late stages of differentiation or, most likely, for allowing Th cells to communicate during cognate T:B interactions.

SAP-controlled T-B cell interactions underlie germinal centre formation

Generation of long-term antibody-mediated immunity depends on the germinal centre (GC) reaction, which requires cooperation between antigen-specific T and B lymphocytes. In the human X-linked lymphoproliferative disease and its gene-targeted mouse model, loss-of-function mutations in signalling lymphocyte activation molecule-associated protein (SAP, encoded by SH2D1a) cause a profound defect in GC formation by an as yet unknown mechanism. Using two-photon intravital imaging, here we show that SAP deficiency selectively impairs the ability of CD4⁺ T cells to stably interact with cognate B cells but not antigen-presenting dendritic cells. This selective defect results in a failure of antigen-specific B cells to receive adequate levels of contact-dependent T cell help to expand normally, despite sap^−/− T cells exhibiting the known characteristics of otherwise competent helper T cells. Furthermore, lack of stable interactions with B cells renders sap^−/− T cells unable to be efficiently recruited to and retained in a nascent GC to sustain the GC reaction. These results offer a compelling explanation for the GC defect due to SAP deficiency and provide novel insights into the bi-directional communication between cognate T and B cells in vivo.

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.

The X-linked lymphoproliferative syndrome gene product SAP regulates B cell function through the FcgammaRIIB receptor

X-linked lympho-proliferative (XLP) is an immunodeficiency condition caused by mutation or deletion of the gene encoding the adaptor protein SAP/SH2D1A. Besides defects in T cell and NK cell function and an absence of NKT cells, XLP can also manifest as lymphomas resulting primarily from uncontrolled B cell proliferation upon acute infection by Epstein-Barr virus. While it has been demonstrated that SAP regulates the functions of T cells and NK cells through the SLAM family of immunoreceptors, its role in B cells has not been defined. Here we show that SAP forms a ternary complex with the kinase Lyn and the inhibitory IgG Fc receptor FcgammaRIIB to regulate B cell proliferation and survival. SAP binds directly and simultaneously to the Lyn SH3 domain and an Immuno-receptor Tyrosine-based Inhibitory Motif (ITIM) in FcgammaRIIB, resulting in the activation of the latter. Moreover, SAP associates with FcgammaRIIB in mouse splenic B cells and promotes its tyrosine phosphorylation. Expression of SAP in the A20 B cell line led to a marked reduction in Blnk phosphorylation, a decrease in Akt activation, and a near-complete ablation of phosphorylation of the MAP kinases Erk1/2, p38 and JNK upon colligation of FcgammaRIIB with the B cell receptor (BCR). In contrast, an XLP-causing SAP mutant was much less efficient in eliciting these effects in B cells. Furthermore, compared to A20 cells, SAP transfectants displayed a significantly reduced rate of proliferation and an increased sensitivity to activation-induced cell death. Collectively these data identify an intrinsic function for SAP in inhibitory signaling in B cells and suggests that SAP may play an important role in balancing positive versus negative immune responses.

SAP expression in T cells, not in B cells, is required for humoral immunity

SAP (also named SH2D1A) is an intracellular adaptor molecule expressed in T cells, natural killer (NK) cells, and some B cells. The SAP gene is mutated in X-linked lymphoproliferative (XLP) disease, a human immunodeficiency characterized by a faulty immune response to Epstein-Barr virus infection. Previous reports documented severe defects in antibody production and germinal center (GC) formation in SAP-deficient humans and mice genetically engineered to lack SAP expression. However, in vitro studies and adoptive transfer experiments provided conflicting data as to whether this phenotype is caused by a functional defect resulting from SAP deficiency in T cells, B cells, or both. Here, we ascertained which cell types are responsible for this humoral immunity defect by using a conditional gene targeting approach. We also thoroughly examined the expression pattern of SAP in normal immune cells by using intracellular flow cytometry. The results showed that expression of SAP in T cells, but not in B cells or NK cells, is required and sufficient for SAP-dependent antibody production and GC formation. These data provide a critical insight into the mechanism by which SAP regulates humoral immunity. They also help elucidate the basis of a severe human immunodeficiency.

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.

Increased proliferation of CD8+ T cells in SAP-deficient mice is associated with impaired activation-induced cell death

Defective signaling lymphocyte activation molecule (SLAM)-associated protein (SAP) is responsible for the human X-linked lymphoproliferative syndrome. Defects in T helper 2, natural killer, natural killer T and B cells have been demonstrated in SAP-deficient humans and mice, and increased proliferation of CD8+ T cells has been observed. In the current study, we investigated the properties of CD8+ T cell proliferation and activation-induced cell death (AICD), using OT-I T cell receptor (TCR)-transgenic mice on either wild-type (WT) or SAP-/- background. Interestingly, we found that ovalbumin peptide-activated SAP-/- CD8+ T cells have lower AICD compared to their WT counterparts. Furthermore, the induction of p73, a key mediator of TCR-induced apoptosis through the mitochondrial apoptotic pathway, was significantly reduced at both the mRNA and protein levels in the activated mutant cells. Meanwhile, a reduced level of activated caspase 9 was observed in the mutant cells. We conclude that reduced AICD in activated SAP-/- CD8+ T cells is associated with impaired p73 induction, indicating that the initiation of the mitochondrial apoptotic pathway might be impaired. Our data demonstrate an intrinsic defect in SAP-/- CD8+ T cells and shed light on the increased responsiveness of CD8+ T cells in SAP-/- mice.

Perturbation of B cell activation in SLAM-associated protein-deficient mice is associated with changes in gammaherpesvirus latency reservoirs

Signaling lymphocyte activation molecule (SLAM)-associated protein (SAP)) interactions with SLAM family proteins play important roles in immune function. SAP-deficient mice have defective B cell function, including impairment of germinal center formation, production of class-switched Ig, and development of memory B cells. B cells are the major reservoir of latency for both EBV and the homologous murine gammaherpesvirus, gammaherpesvirus 68. There is a strong association between the B cell life cycle and viral latency in that the virus preferentially establishes latency in activated germinal center B cells, which provides access to memory B cells, a major reservoir of long-term latency. In the current studies, we have analyzed the establishment and maintenance of gammaHV68 latency in wild-type and SAP-deficient mice. The results show that, despite SAP-associated defects in germinal center and memory B cell formation, latency was established and maintained in memory B cells at comparable frequencies to wild-type mice, although the paucity of memory B cells translated into a 10-fold reduction in latent load. Furthermore, there were defects in normal latency reservoirs within the germinal center cells and IgD(+)"naive" B cells in SAP-deficient mice, showing a profound effect of the SAP mutation on latency reservoirs.

SAP regulation of follicular helper CD4 T cell development and humoral immunity is independent of SLAM and Fyn kinase

Mutations in SH2D1A resulting in lack of SLAM-associated protein (SAP) expression cause the human genetic immunodeficiency X-linked lymphoproliferative disease. A severe block in germinal center development and lack of long-term humoral immunity is one of the most prominent phenotypes of SAP(-) mice. We show, in this study, that the germinal center block is due to an essential requirement for SAP expression in Ag-specific CD4 T cells to develop appropriate follicular helper T cell functions. It is unknown what signaling molecules are involved in regulation of SAP-dependent CD4 T cell help functions. SAP binds to the cytoplasmic tail of SLAM, and we show that SLAM is expressed on resting and activated CD4 T cells, as well as germinal center B cells. In addition, SAP can recruit Fyn kinase to SLAM. We have now examined the role(s) of the SLAM-SAP-Fyn signaling axis in in vivo CD4 T cell function and germinal center development. We observed normal germinal center development, long-lived plasma cell development, and Ab responses in SLAM(-/-) mice after a viral infection (lymphocytic choriomeningitis virus). In a separate series of experiments, we show that SAP is absolutely required in CD4 T cells to drive germinal center development, and that requirement does not depend on SAP-Fyn interactions, because CD4 T cells expressing SAP R78A are capable of supporting normal germinal center development. Therefore, a distinct SAP signaling pathway regulates follicular helper CD4 T cell differentiation, separate from the SLAM-SAP-Fyn signaling pathway regulating Th1/Th2 differentiation.

SAP is required for Th cell function and for immunity to influenza

Ab is a crucial component of protective immunity to infection, but Ab responses do not proceed normally when defects occur in a protein called signaling lymphocytic activation molecule-associated protein (SAP). To explain this Ab defect, we analyzed B cell and plasma cell responses under conditions of SAP deficiency. Our results demonstrate that SAP-deficient (SAP knockout (KO)) mice have a profound CD4 T cell-intrinsic defect in generating Ag-specific plasma cells following challenge with model Ags or influenza virus, resulting in low Ag-specific Ab titers. We also show that SAP is required in CD4 T cells for normal division and expansion of B cells. These B cell and plasma cell defects were observed during the expansion phase of the primary immune response, indicating early defects in Th cell activity. In fact, additional experiments revealed a nearly complete lack of T cell help for B cells in SAP KO mice. Our work suggests that the ability of SAP to promote T-dependent humoral immune responses is important for antiviral immunity because mice lacking SAP are unable to prevent high dose secondary influenza infection, and because passive transfer of IgG in immune serum from wild-type, but not SAP KO mice can protect mice from an otherwise lethal influenza infection. Overall, our results demonstrate that SAP is required in CD4 T cells for their ability to help B cell responses and promote influenza-specific immunity.

SLAM and CD31: Signaling molecules involved in cytokine secretion during the development of innate and adaptive immune responses

Immune cells are modulated through the crosslinking of receptors named "immunoreceptors". Ligation of immunoreceptors by their ligands induces a tyrosine-phosphorylation signal that is essential for cell activation or inhibition. Physiologically, immunoreceptor triggering is not enough for cell activation, and stimulation of co-receptors is necessary for antigen-evoked cytokine production. Thus, signal transduction pathways mediated by proteins that regulate cytokine secretion are critical to achieve an effective immune response of the host, where the balance between positive and negative signaling allows effective immune responses, preventing tolerance and autoimmunity. This review deals with recent studies based on the role of the receptor signaling lymphocytic activation molecule (SLAM), a signaling protein that modulates cytokine secretion by immune cells, and the transmembrane glycoprotein CD31, which plays multiple roles in cellular signaling events by modulating the balance between inhibitory and stimulatory signals to immune cells. Recent studies have shed light on the ability of these molecules to transmit different signals that regulate the ability of innate and adaptive immune cells to synthesize stimulatory and inhibitory cytokines.

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.

Hypogammaglobulinemia and exacerbated CD8 T-cell–mediated immunopathology in SAP-deficient mice with chronic LCMV infection mimics human XLP disease

The human genetic disease X-linked lymphoproliferative disease (XLP), which is caused by mutations in SH2D1A/SAP that encode SLAM-associated protein (SAP), is characterized by an inability to control Epstein-Barr virus (EBV) and hypogammaglobulinemia. It is unclear which aspects of XLP disease are specific to herpesvirus infection and which reflect general immunologic functions performed by SAP. We examined SAP– mice during a chronic LCMV infection, specifically to address the following question: Which SAP deficiency immunologic problems are general, and which are EBV specific? Illness, weight loss, and prolonged viral replication were much more severe in SAP– mice. Aggressive immunopathology was observed. This inability to control chronic LCMV was associated with both CD8 T-cell and B-cell response defects. Importantly, we demonstrate that SAP– CD8 T cells are the primary cause of the immunopathology and clinical illness, because depletion of CD8 T cells blocked disease. This is the first direct demonstration of SAP– CD8 T-cell–mediated immunopathology, confirming 30 years of XLP clinical observations and indirect experimentation. In addition, germinal center formation was extremely defective in chronically infected SAP– animals, and hypogammaglobulinemia was observed. These findings in a chronic viral infection mouse model recapitulate key features of human XLP and clarify SAP's critical role regulating both cellular and humoral immunity.

SAP Deficiency Results in a Striking Alteration of the Protein Profile in Activated CD4 T Cells

Deficiency in a protein called signaling lymphocytic activation molecule-associated protein (SAP) causes X-linked lymphoproliferative disease (XLP) and helper T cell-dependent antibody defects. To identify proteins regulated by SAP, we performed proteomic analyses of SAP deficient vs wild type T cells. Our results reveal protein species whose abundances are profoundly altered by SAP. Our work therefore identifies candidate cellular mediators of SAP-dependent T cell help.