Loss of circulating CD27+ memory B cells and CCR4+ T cells occurring in association with elevated EBV loads in XLP patients surviving primary EBV infection

The genetics of macrophage activation syndrome

Macrophage activation syndrome (MAS), or secondary hemophagocytic lymphohistiocytosis (HLH), is a cytokine storm syndrome associated with multi-organ system dysfunction and high mortality rates. Laboratory and clinical features resemble primary HLH, which arises in infancy (1 in 50,000 live births) from homozygous mutations in various genes critical to the perforin-mediated cytolytic pathway employed by NK cells and cytotoxic CD8 T lymphocytes. MAS/secondary HLH is about ten times more common and typically presents beyond infancy extending into adulthood. The genetics of MAS are far less defined than for familial HLH. However, the distinction between familial HLH and MAS/secondary HLH is blurred by the finding of heterozygous perforin-pathway mutations in MAS patients, which may function as hypomorphic or partial dominant-negative alleles and contribute to disease pathogenesis. In addition, mutations in a variety of other pathogenic pathways have been noted in patients with MAS/secondary HLH. Many of these genetically disrupted pathways result in a similar cytokine storm syndrome, and can be broadly categorized as impaired viral control (e.g., SH2P1A), dysregulated inflammasome activity (e.g., NLRC4), other immune defects (e.g., IKBKG), and dysregulated metabolism (e.g., LIPA). Collectively these genetic lesions likely combine with states of chronic inflammation, as seen in various rheumatic diseases (e.g., still disease), with or without identified infections, to result in MAS pathology as explained by the threshold model of disease. This emerging paradigm may ultimately support genetic risk stratification for high-risk chronic and even acute inflammatory disorders. Moving forward, continued whole-exome and -genome sequencing will likely identify novel MAS gene associations, as well as noncoding mutations altering levels of gene expression.

T Cells Regulate Peripheral Naive Mature B Cell Survival by Cell-Cell Contact Mediated through SLAMF6 and SAP

The control of lymphoid homeostasis is the result of a very fine balance between lymphocyte production, proliferation, and apoptosis. In this study, we focused on the role of T cells in the maintenance/survival of the mature naive peripheral B cell population. We show that naive B and T cells interact via the signaling lymphocyte activation molecule (SLAM) family receptor, SLAMF6. This interaction induces cell type-specific signals in both cell types, mediated by the SLAM-associated protein (SAP) family of adaptors. This signaling results in an upregulation of the expression of the cytokine migration inhibitory factor in the T cells and augmented expression of its receptor CD74 on the B cell counterparts, consequently enhancing B cell survival. Furthermore, in X-linked lymphoproliferative disease patients, SAP deficiency reduces CD74 expression, resulting in the perturbation of B cell maintenance from the naive stage. Thus, naive T cells regulate B cell survival in a SLAMF6- and SAP-dependent manner.

Restimulation-induced T-cell death through NTB-A/SAP signaling pathway is impaired in tuberculosis patients with depressed immune responses

Production of IFN-γ contributes to host defense against Mycobacterium tuberculosis (Mtb) infection. We previously demonstrated that Signaling lymphocytic activation molecule-associated protein (SAP) expression on cells from tuberculosis (TB) patients was inversely correlated with IFN-γ production. Here we first investigated the role of NK, T- and B-cell antigen (NTB-A)/SAP pathway in the regulation of Th1 response against Mtb. Upon antigen stimulation, NTB-A phosphorylation rapidly increases and afterwards modulates IFN-γ and IL-17 secretion. To sustain a healthy immune system, controlled expansion and contraction of lymphocytes, both during and after an adaptive immune response, is essential. Besides, restimulation-induced cell death (RICD) results in an essential homeostatic mechanism for precluding excess T-cell accumulation and associated immunopathology during the course of certain infections. Accordingly, we found that the NTB-A/SAP pathway was required for RICD during active tuberculosis. In low responder (LR) TB patients, impaired RICD was associated with diminished FASL levels, IL-2 production and CD25^high expression after cell-restimulation. Interestingly, we next observed that SAP mediated the recruitment of the Src-related kinase FYNT, only in T cells from LR TB patients that were resistant to RICD. Together, we showed that the NTB-A/SAP pathway regulates T-cell activation and RICD during human TB. Moreover, the NTB-A/SAP/FYNT axis promotes polarization to an unfavorable Th2-phenotype.

Clinical association of baseline levels of conjugated dienes in low-density lipoprotein and nitric oxide with aggressive B-cell non-Hodgkin lymphoma and their relationship with immunoglobulins and Th1-to-Th2 ratio

Objective

The aim of this study was to highlight the clinical association of baseline levels of conjugated dienes in low-density lipoprotein (LDL-BCD) and nitric oxide (NO) with immunoglobulins (Igs) and T helper (Th)1/Th2 ratio in patients with newly diagnosed B-cell non-Hodgkin lymphoma (NHL).

Patients and methods

Thirty-two newly diagnosed patients with aggressive B-cell NHL and 25 age-, sex-, and body-mass-index-matched healthy controls were randomly selected for a cross-sectional case–control study conducted at the Hematology Department of Tlemcen Medical Centre University (northwest of Algeria).

Results

Circulating levels of LDL-BCD and NO and those of IgA and IgM were significantly higher in patients than in controls. The levels of Th1/Th2 ratio and plasma total antioxidant capacity were significantly lower in patients compared with controls, while malondialdehyde and protein carbonyl levels were significantly higher in patients. B-cell NHL was significantly associated with high levels of LDL-BCD from 25th to 75th percentile (25th percentile: relative risk [RR] =2.26, 95% confidence interval [CI] 1.42–3.59, P=0.014; 50th percentile: RR =2.84, 95% CI 1.72–4.68, P<0.001; 75th percentile: RR =5.43, 95% CI 2.58–11.42, P<0.001). Similarly, the disease was significantly associated with high levels of NO production from 25th to 75th percentile (25th percentile: RR =2.07, 95% CI 1.25–3.44, P=0.024; 50th percentile: RR =2.78, 95% CI 1.63–4.72, P<0.001; 75th percentile: RR =4.68, 95% CI 2.21–9.91, P<0.001). Moreover, LDL-BCD levels were positively and significantly correlated with interferon (IFN)-γ, whereas NO levels were inversely and significantly correlated with IFN-γ and Th1/Th2 ratio.

Conclusion

LDL-BCD and NO production seem to be associated with aggressive B-cell NHL and alteration of Th1/Th2 ratio. Our results have to be examined using ex vivo mechanistic studies leading to further investigations of these parameters, with an interest in the link between Epstein–Barr virus infection and NO and immunoglobulins.

CRISPR-Mediated Slamf1Δ/Δ Slamf5Δ/Δ Slamf6Δ/Δ Triple Gene Disruption Reveals NKT Cell Defects but Not T Follicular Helper Cell Defects

SAP (SH2D1A) is required intrinsically in CD4 T cells to generate germinal center responses and long-term humoral immunity. SAP binds to SLAM family receptors, including SLAM, CD84, and Ly108 to enhance cytokine secretion and sustained T cell:B cell adhesion, which both improve T follicular helper (Tfh) cell aid to germinal center (GC) B cells. To understand the overlapping roles of multiple SLAM family receptors in germinal center responses, Slamf1^Δ/ΔSlamf5^Δ/ΔSlamf6^Δ/Δ triple gene disruption (Slamf1,5,6^Δ/Δ) mice were generated using CRISPR-Cas9 gene editing to eliminate expression of SLAM (CD150), CD84, and Ly108, respectively. Gene targeting was highly efficient, with 6 of 6 alleles disrupted in 14 of 23 pups and the majority of alleles disrupted in the remaining pups. NKT cell differentiation in Slamf1,5,6^Δ/Δ mice was defective, but not completely absent. The remaining NKT cells exhibited substantially increased 2B4 (SLAMF4) expression. Surprisingly, there were no overt defects in germinal center responses to acute viral infections or protein immunizations in Slamf1,5,6^Δ/Δ mice, unlike Sh2d1a^-/- mice. Similarly, in the context of a competitive environment, SLAM family receptor expressing GC Tfh cell, GC B cell, and plasma cell responses exhibited no advantages over Slamf1,5,6^Δ/Δ cells.

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.

Phosphorylation of mitogen-activated protein kinases contributes to interferon γ production in response to Mycobacterium tuberculosis

Immune control of Mycobacterium tuberculosis depends on interferon γ (IFN-γ)-producing CD4(+) lymphocytes. Previous studies have shown that T cells from patients with tuberculosis produce less IFN-γ, compared with healthy donors, in response to mycobacterial antigens, although IFN-γ responses to mitogens are preserved. In this work, we found that M. tuberculosis-induced IFN-γ production by human T cells correlated with phosphorylation of the mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase (ERK), and p38. Moreover, the majority of IFN-γ-producing T cells expressed signaling lymphocyte activation molecule (SLAM), and SLAM activation further increased ERK phosphorylation. Interestingly, patients with tuberculosis had delayed activation of ERK and p38, and this was most marked in patients with the poorest IFN-γ responses (ie, low responders). Besides, SLAM signaling failed to phosphorylate ERK in low responders. Our findings suggest that activation of p38 and ERK, in part through SLAM, mediates T-cell IFN-γ production in response to M. tuberculosis, a pathway that is defective in patients with tuberculosis.

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.

T cell-B cell interactions in primary immunodeficiencies

Regulated interactions between cells of the immune system facilitate the generation of successful immune responses, thereby enabling efficient neutralization and clearance of pathogens and the establishment of both cell- and humoral-mediated immunological memory. The corollary of this is that impediments to efficient cell-cell interactions, normally necessary for differentiation and effector functions of immune cells, underly the clinical features and disease pathogenesis of primary immunodeficiencies. In affected individuals, these defects manifest as impaired long-term humoral immunity and susceptibility to infection by specific pathogens. In this review, we discuss the importance of, and requirements for, effective interactions between B cells and T cells during the formation of CD4(+) T follicular helper cells and the elicitation of cytotoxic function of virus-specific CD8(+) T cells, as well as how these processes are abrogated in primary immunodeficiencies due to loss-of-function mutations in defined genes.

CD4+ T Lymphocytes with follicular helper phenotype (T(FH)) in patients with SH2D1A deficiency (XLP)

Peripheral blood mononuclear cells with T(FH) phenotype from two asymptomatic XLP patients were studied. Normal/high numbers of CXCR5+, CD4+ T cells coexpressing PD-1 were demonstrated. Peripheral blood mononuclear cells (PBMC) from these patients responded to sub-optimal PHA/IL-2 stimulation upregulating ICOS and CD40L and increasing intracellular expression of IL-10, IL-21 and IL-4 by CD4+ T(FH) cells. However when compared to N, the time profile of activation and cytokine synthesis was different in XLP and N. While ICOS and CD40L expression in N decreased after 6-8 days, it continued to increase or was maintained in XLP cultures. Intracellular IL-10, IL-21 and IL-4 reached higher values in XLP than N after 8 days. Rather than the absence of T(FH) cells or their intrinsic inability to respond to stimuli, differences in the time profile of their response could contribute to impair their role as helpers of B lymphocytes.

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.

Follicular helper CD4 T cells (TFH)

T cell help to B cells is a fundamental aspect of adaptive immunity and the generation of immunological memory. Follicular helper CD4 T (T(FH)) cells are the specialized providers of B cell help. T(FH) cells depend on expression of the master regulator transcription factor Bcl6. Distinguishing features of T(FH) cells are the expression of CXCR5, PD-1, SAP (SH2D1A), IL-21, and ICOS, among other molecules, and the absence of Blimp-1 (prdm1). T(FH) cells are important for the formation of germinal centers. Once germinal centers are formed, T(FH) cells are needed to maintain them and to regulate germinal center B cell differentiation into plasma cells and memory B cells. This review covers T(FH) differentiation, T(FH) functions, and human T(FH) cells, discussing recent progress and areas of uncertainty or disagreement in the literature, and it debates the developmental relationship between T(FH) cells and other CD4 T cell subsets (Th1, Th2, Th17, iTreg).

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.

Possible recruitment of peripheral blood CXCR3+ CD27+ CD19+ B cells to the liver of chronic hepatitis C patients

It has been suggested that hepatitis C virus (HCV) infects not only hepatocytes but also immune cells, including B cells. HCV infection of B cells is the likely cause of B-cell dysregulation disorders such as mixed cryoglobulinemia, rheumatoid factor production, and B-cell lymphoproliferative disorders that may evolve into non-Hodgkin's lymphoma. To clarify the effects of chronic HCV infection on B-cell dynamics, peripheral B cells from chronic hepatitis C patients (CHC) were characterized. We found that the frequency of CD27(+) B cells, that is memory phenotype, was significantly reduced in the peripheral blood of CHC. At the same time, the amount of IFN-gamma-inducible protein-10 (IP-10), a CXCR3 ligand, was markedly elevated in the plasma of CHC. Furthermore, the CD27(+) B-cell population was found to highly express CXCR3 in CHC, thus suggesting that the CD27(+) B-cell population was recruited from peripheral blood to the inflammatory site of the liver of CHC, where IP-10 is produced. Immunohistochemical analyses of intrahepatic lymphocytes indicated that CXCR3(+) B cells were infiltrated in the liver of CHC. Our results thus offer new insight into the role of memory B cells in the HCV pathogenesis.

Germinal center T follicular helper cell IL-4 production is dependent on signaling lymphocytic activation molecule receptor (CD150)

CD4 T cell help is critical for the generation and maintenance of germinal centers (GCs), and T follicular helper (T(FH)) cells are the CD4 T cell subset required for this process. Signaling lymphocytic activation molecule (SLAM)-associated protein (SAP [SH2D1A]) expression in CD4 T cells is essential for GC development. However, SAP-deficient mice have only a moderate defect in T(FH) differentiation, as defined by common T(FH) surface markers. CXCR5(+) T(FH) cells are found within the GC, as well as along the boundary regions of T/B cell zones. In this study, we show that GC-associated T follicular helper (GC T(FH)) cells can be identified by their coexpression of CXCR5 and the GL7 epitope, allowing for phenotypic and functional analysis of T(FH) and GC T(FH) populations. GC T(FH) cells are a functionally discrete subset of further polarized T(FH) cells, with enhanced B cell help capacity and a specialized ability to produce IL-4 in a T(H)2-independent manner. Strikingly, SAP-deficient mice have an absence of the GC T(FH) cell subset and SAP(-) T(FH) cells are defective in IL-4 and IL-21 production. We further demonstrate that SLAM (Slamf1, CD150), a surface receptor that uses SAP signaling, is specifically required for IL-4 production by GC T(FH) cells. GC T(FH) cells require IL-4 and -21 production for optimal help to B cells. These data illustrate complexities of SAP-dependent SLAM family receptor signaling, revealing a prominent role for SLAM receptor ligation in IL-4 production by GC CD4 T cells but not in T(FH) cell and GC T(FH) cell differentiation.

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.

Increased lymphocyte viability after non-stimulated peripheral blood mononuclear cell (PBMC) culture in patients with X-linked lymphoproliferative disease (XLP)

Survival of lymphocytes after prolonged culture was studied in two asymptomatic XLP patients. Viability of XLP PBMC after 30 days of non-stimulated culture was higher than that of normal controls (N), mainly due to the persistence of CD8 memory lymphocytes. IFNgamma high CD8 T lymphocytes remained higher in XLP than in N after 30 days. The number of perforin+ CD8 lymphocytes was markedly reduced after 30 days in XLP and in N. Increased viability was not related to CD127, PD-1, CD27, or CD62L expression. Concerning B lymphocytes, memory CD27+ CD19+ cells prevailed over CD27- cells after 30 days in both XLP and N, with far more surviving cells in XLP. In N, few CD19+ B lymphocytes were viable after prolonged culture. In XLP, these cells were also IgD+, IgM+ and EBNA2+. These results demonstrate that IFNgamma-positive memory CD8 T cells persist in XLP after prolonged culture in association with a subset of viable memory CD27+ B cells expressing latent EBV antigens. The survival advantage of XLP cells might be related to increased frequency of extranodal lymphoma in XLP patients.

A novel ICOS-independent, but CD28- and SAP-dependent, pathway of T cell-dependent, polysaccharide-specific humoral immunity in response to intact Streptococcus pneumoniae versus pneumococcal conjugate vaccine

Polysaccharide (PS)- and protein-specific murine IgG responses to intact Streptococcus pneumoniae (Pn) are both dependent on CD4(+) T cell help, B7-dependent costimulation, and CD40/CD40 ligand interactions. However, the primary PS-specific, relative to protein-specific, IgG response terminates more rapidly, requires a shorter period of T cell help and B7-dependent costimulation, and fails to generate memory. In light of the critical role for ICOS/ICOS ligand interactions in sustaining T cell-dependent Ig responses and promoting germinal center reactions, we hypothesized that this interaction was nonessential for PS-specific IgG responses to Pn. We now demonstrate that ICOS(-/-), relative to wild-type, mice elicit a normal PS-specific IgG isotype response to Pn, despite marked inhibition of both the primary and secondary IgG anti-protein (i.e., PspA, PspC, and PsaA) response. A blocking anti-ICOS ligand mAb injected during primary Pn immunization inhibits both the primary anti-protein response and the generation of protein-specific memory, but has no effect when injected during secondary immunization. In contrast to Pn, both PS- and protein-specific IgG responses to a pneumococcal conjugate vaccine are inhibited in ICOS(-/-) mice. ICOS(-/-) mice immunized with intact Pn or conjugate exhibit nearly complete abrogation in germinal center formation. Finally, although mice that lack the adaptor molecule SAP (SLAM-associated protein) resemble ICOS(-/-) mice (and can exhibit decreased ICOS expression), we observe that the PS-specific, as well as protein-specific, IgG responses to both Pn and conjugate are markedly defective in SAP(-/-) mice. These data define a novel T cell-, SAP-, and B7-dependent, but ICOS-independent, extrafollicular pathway of Ig induction.

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.

Role of the spleen in peripheral memory B-cell homeostasis in patients with autoimmune thrombocytopenia purpura

The effect of splenectomy on circulating memory B cells in autoimmune thrombocytopenia purpura (AITP) patients has not yet been addressed. We therefore analyzed the distribution and phenotypic characteristics of B-cell subsets in non-splenectomized and splenectomized AITP patients and controls, as well as CD95 expression after B cell activation. Decreased frequencies of memory B cells in splenectomized individuals were observed, with a rapid decline of CD27+IgD+ and a slower decrease of CD27+IgD- and CD27-/IgD- cells. Similar results were noted following splenectomy in healthy donors (HD). CD95+ B cells were substantially increased in all subsets in patients with active AITP, indicating their enhanced activation status. After splenectomy, the percentage of CD95+ B cells were further increased in the CD27+IgD- post-switch memory population in AITP, but not in HD. CD95+CD27+ memory B cells largely reside in the region in the human spleen analogous to the murine marginal zone. Thus, the spleen plays a fundamental role in controlling peripheral memory B cell homeostasis in both AITP and HD and regulates activated CD95+ B cells in patients with AITP.

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.

Advanced therapeutic and prophylactic strategies for Epstein-Barr virus infection in immunocompromised patients

Epstein-Barr virus (EBV) is an ubiquitous human herpesvirus. Primary infection is generally subclinical but in certain circumstances, such as in patients with either hereditary or secondary immunodeficiency, EBV infection may cause overt disease that is often lethal. Strategies for the prophylaxis and treatment of these potentially life-threatening complications of EBV infection have advanced dramatically. They include immunological-based approaches targeted at EBV-infected cells, as well as improvement in the treatment of the underlying and predisposing disease. This review will discuss EBV biology and immune events that occur in both immunocompetent and immunocompromised individuals and introduce the novel prophylactic and therapeutic strategies for EBV-associated life-threatening diseases.

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.

Factors involved in the generation of memory CD8+ T cells in patients with X-linked lymphoproliferative disease (XLP)

We have analysed the phenotype of T lymphocytes in two X-linked lymphoproliferative disease (XLP) patients with the same SH2D1A mutation differing in initial exposure to Epstein-Barr virus (EBV) and treatment. While memory T lymphocytes (with low CCR7 and CD62L expression) prevailed in both XLP patients, in patient 9, who developed acute infectious mononucleosis (AIM) and received B cell ablative treatment, the predominant phenotype was that of late effector CD8 T cells (CD27-, CD28-, CCR7-, CD62L-, CD45 RA+, perforin+), while in patient 4 (who did not suffer AIM) the prevalent phenotype of CD8 T lymphocytes was similar to that of normal controls (N) or to that of adult individuals who recovered from

AIM

CD27+ , CD28+, CCR7-, CD62L-, CD45 RO+ and perforin-. CD57 expression (related to senescence) was also higher in CD8 T cells from patient 9 than in patient 4, AIM or N. Persistently high EBV viral load was observed in patient 9. The results obtained from this limited number of XLP patients suggest that events related to the initial EBV encounter (antigen load, treatment, cytokine environment) may have more weight than lack of SH2D1A in determining the long-term differentiation pattern of CD8 memory T cells.

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.

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 regulates T cell-mediated help for humoral immunity by a mechanism distinct from cytokine regulation

X-linked lymphoproliferative disease is caused by mutations affecting SH2D1A/SAP, an adaptor that recruits Fyn to signal lymphocyte activation molecule (SLAM)-related receptors. After infection, SLAM-associated protein (SAP)-/- mice show increased T cell activation and impaired humoral responses. Although SAP-/- mice can respond to T-independent immunization, we find impaired primary and secondary T-dependent responses, with defective B cell proliferation, germinal center formation, and antibody production. Nonetheless, transfer of wild-type but not SAP-deficient CD4 cells rescued humoral responses in reconstituted recombination activating gene 2-/- and SAP-/- mice. To investigate these T cell defects, we examined CD4 cell function in vitro and in vivo. Although SAP-deficient CD4 cells have impaired T cell receptor-mediated T helper (Th)2 cytokine production in vitro, we demonstrate that the humoral defects can be uncoupled from cytokine expression defects in vivo. Instead, SAP-deficient T cells exhibit decreased and delayed inducible costimulator (ICOS) induction and heightened CD40L expression. Notably, in contrast to Th2 cytokine defects, humoral responses, ICOS expression, and CD40L down-regulation were rescued by retroviral reconstitution with SAP-R78A, a SAP mutant that impairs Fyn binding. We further demonstrate a role for SLAM/SAP signaling in the regulation of early surface CD40L expression. Thus, SAP affects expression of key molecules required for T-B cell collaboration by mechanisms that are distinct from its role in cytokine regulation.

Immune regulation by SLAM family receptors and SAP-related adaptors

The signalling lymphocytic activation molecule (SLAM) family of receptors is expressed by a wide range of immune cells. Through their cytoplasmic domain, SLAM family receptors associate with SLAM-associated protein (SAP)-related molecules, a group of cytoplasmic adaptors composed almost exclusively of an SRC homology 2 domain. SAP, the prototype of the SAP family, is mutated in a human immunodeficiency named X-linked lymphoproliferative (XLP) disease. Recent observations indicate that SLAM family receptors, in association with SAP family adaptors, have crucial roles during normal immune reactions in innate and adaptive immune cells. The latest progress in this field is reviewed here.

Ly9 (CD229)-Deficient Mice Exhibit T Cell Defects yet Do Not Share Several Phenotypic Characteristics Associated with SLAM- and SAP-Deficient Mice

Signaling lymphocyte activation molecule (SLAM) family receptors are critically involved in modulating innate and adaptive immune responses. Several SLAM family receptors have been shown to interact with the adaptor molecule SAP; however, subsequent intracellular signaling is poorly defined. Notably, mutations in SLAM-associated protein (SAP) lead to X-linked lymphoproliferative disease, a rare but fatal immunodeficiency. Although the SLAM family member Ly9 (CD229) is known to interact with SAP, the functions of this receptor have remained elusive. Therefore, we have generated Ly9-/- mice and compared their phenotype with that of SLAM-/- and SAP-/- mice. We report that Ly9-/- T cells exhibit a mild Th2 defect associated with reduced IL-4 production after stimulation with anti-TCR and anti-CD28 in vitro. This defect is similar in magnitude to the previously reported Th2 defect in SLAM-/- mice but is more subtle than that observed in SAP-/- mice. In contrast to SLAM-/- and SAP-/- mice, T cells from Ly9-/- mice proliferate poorly and produce little IL-2 after suboptimal stimulation with anti-CD3 in vitro. We have also found that Ly9-/- macrophages exhibit no defects in cytokine production or bacterial killing as was observed in SLAM-/- macrophages. Additionally, Ly9-/- mice differ from SAP-/- mice in that they foster normal development of NKT cells and mount appropriate T and B cell responses to lymphocytic choriomeningitis virus. We have identified significant phenotypic differences between Ly-9-/- mice as compared with both SLAM-/- and SAP-/- mice. Although Ly9, SLAM, and SAP play a common role in promoting Th2 polarization, Ly-9 is uniquely involved in enhancing T cell activation.

Common variable immunodeficiency: test indications and interpretations

Common variable immunodeficiency (CVID) is a primary immunodeficiency disorder that can present with multiple phenotypes, all of which are characterized by hypogammaglobulinemia, in a person at any age. A specific genetic defect that accounts for all CVID phenotypes has not been identified, and it is likely that several distinct genetic disorders with similar clinical presentations are responsible for the observed variation. In this review, we summarize the known genetic mutations that give rise to hypogammaglobulinemia and how these gene products affect normal or abnormal B-cell development and function, with particular emphasis on CVID. Additionally, we describe specific phenotypic and genetic laboratory tests that can be used to diagnose CVID and provide guidelines for test interpretation and subsequent therapeutic intervention.

X-Linked Lymphoproliferative Disease in an Adult

X-linked lymphoproliferative disease (XLP) is an inherited immunodeficiency characterized by an extreme susceptibility to Epstein-Barr virus (EBV) infection. Patients with XLP mainly present with the 3 clinical manifestations of fulminant infectious mononucleosis, lymphoproliferative disorder, and dysgammaglobulinemia and in rare cases have aplastic anemia and lymphocytic vasculitis. The causative gene for XLP was identified as SH2D1A/DSHP/SLAM-associated protein (SAP) in 1998, and genetic analysis has been used for the definite diagnosis of XLP. Diagnosis for most patients occurs at ages younger than 10 years, and there are few adult patients. Here we describe a 23-year-old man with hypogammaglobulinemia and EBV-associated hemophagocytic lymphohistiocytosis and a diagnosis of XLP. In addition, the patient showed type 1 helper T-cell (Th1) skewing, as has been described in Sap knock-out mice. Th1/Th2 imbalance in humans, as well as in mice, may play an important role in the pathogenesis of XLP.

Molecular and cellular pathogenesis of X-linked lymphoproliferative disease

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

Persistent hypogammaglobulinemia following mononucleosis in boys is highly suggestive of X-linked lymphoproliferative disease--report of three cases

Hypogammaglobulinemia is a common symptom in different immunodeficiencies. It is, however, not usually associated with Epstein-Barr virus (EBV) infections. The X-linked lymphoproliferative disease (XLP) on the other hand shows immunological changes in response to the EBV. Here we report three previously healthy boys, all of which developed persistent hypogammaglobulinemia following severe acute infectious mononucleosis. All three patients revealed T-cell abnormalities including inverted CD4/CD8 and increased CD8(+) T-cell numbers. The number of IFN-gamma-producing T cells were markedly increased in the two patients studied so far. In addition, patient 2 showed mainly T cells, instead of B cells, to be infected with the EBV. Apart from an uncle of patient 3, who died of malignant lymphoma, family history was unremarkable in all cases. All three patients exhibited mutations in the SH2D1A gene, establishing the diagnosis of XLP. Protein expression was found on immunoblot analysis in one patient with a missense mutation. Development of persistent hypogammaglobulinemia after severe primary EBV infection seems to be a specific diagnostic sign for XLP even in males with unremarkable family history.

Impaired humoral immunity in X-linked lymphoproliferative disease is associated with defective IL-10 production by CD4+ T cells

X-linked lymphoproliferative disease (XLP) is an often-fatal immunodeficiency characterized by hypogammaglobulinemia, fulminant infectious mononucleosis, and/or lymphoma. The genetic lesion in XLP, SH2D1A, encodes the adaptor protein SAP (signaling lymphocytic activation molecule-associated [SLAM-associated] protein); however, the mechanism(s) by which mutations in SH2D1A causes hypogammaglobulinemia is unknown. Our analysis of 14 XLP patients revealed normal B cell development but a marked reduction in the number of memory B cells. The few memory cells detected were IgM(+), revealing deficient isotype switching in vivo. However, XLP B cells underwent proliferation and differentiation in vitro as efficiently as control B cells, which indicates that the block in differentiation in vivo is B cell extrinsic. This possibility is supported by the finding that XLP CD4(+) T cells did not efficiently differentiate into IL-10(+) effector cells or provide optimal B cell help in vitro. Importantly, the B cell help provided by SAP-deficient CD4(+) T cells was improved by provision of exogenous IL-10 or ectopic expression of SAP, which resulted in increased IL-10 production by T cells. XLP CD4(+) T cells also failed to efficiently upregulate expression of inducible costimulator (ICOS), a potent inducer of IL-10 production by CD4(+) T cells. Thus, insufficient IL-10 production may contribute to hypogammaglobulinemia in XLP. This finding suggests new strategies for treating this immunodeficiency.

Regulation of NKT cell development by SAP, the protein defective in XLP

The adaptor molecule SAP is expressed in T lymphocytes and natural killer (NK) cells, where it regulates cytokine production and cytotoxicity. Here, we show that SAP, encoded by the SH2D1A gene locus, also has a crucial role during the development of NKT cells, a lymphocyte subset with immunoregulatory functions in response to infection, cancer and autoimmune disease. Following stimulation with the NKT cell-specific agonist alpha-galactosyl ceramide (alphaGC), Sh2d1a-/- splenocytes did not produce cytokines or activate other lymphoid lineages in an NKT cell-dependent manner. While evaluating the abnormalities in alphaGC-induced immune responses, we observed that Sh2d1a-/- animals lacked NKT cells in the thymus and peripheral organs. The defect in NKT cell ontogeny was hematopoietic cell autonomous and could be rescued by reconstitution of SAP expression within Sh2d1a-/- bone marrow cells. Seventeen individuals with X-linked lymphoproliferative disease (XLP), who harbored germline mutations in SH2D1A, also lacked NKT cells. Furthermore, a female XLP carrier showed completely skewed X chromosome inactivation within NKT cells, but not T or B cells. Thus, SAP is a crucial regulator of NKT cell ontogeny in humans and in mice. The absence of NKT cells may contribute to the phenotypes of SAP deficiency, including abnormal antiviral and antitumor immunity and hypogammaglobulinemia.

X-linked immunodeficiencies

Recent advances in molecular genetics have allowed identification of at least seven genes involved in X-linked immunodeficiencies. This has resulted not only in improved diagnostic possibilities but also in a better understanding of the pathophysiology of these disorders. In some cases, mutations in the same gene have been shown to cause distinct clinical and immunologic phenotypes, demonstrating a strong genotype-phenotype correlation. Identification of the molecular basis of these diseases has permitted creation of disease-specific registries, with a better characterization of the clinical and immunologic features associated with the various forms of X-linked immunodeficiencies. Additionally, gene therapy has been attempted in X-linked severe combined immune deficiency (XSCID), with clear evidence of successful correction of the pathology, and the appearance of severe adverse effects.

[Classification and diagnosis of immunodeficiency syndromes]

Primary immunodeficiency diseases of the adult are rare disorders, but often lead to serious consequences. Therefore an early diagnosis is critical. The variety in the clinical presentation, the complexity of the immune system and the ongoing discovery of new defects render it a difficult area for the involved physician. Due to the often imprecise complaint of a weak immune system the primary task is the identification of patients with true immunodeficiency. Subsequently, the immune defect needs to be identified in collaboration with a center for immunodeficiency disorders. The diagnostic procedure is dependent on the pattern of infections and follows a defined series of steps. This procedure should prevent costly diagnostic evaluation when not indicated, and also prevent the delayed diagnosis of patients with manifest immunodeficiency disease.