The X-linked lymphoproliferative-disease gene product SAP regulates signals induced through the co-receptor SLAM

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

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

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.

X-linked inhibitor of apoptosis protein deficiency: more than an X-linked lymphoproliferative syndrome

X-linked inhibitor of apoptosis (XIAP) deficiency (also known as X-linked lymphoproliferative syndrome type 2, XLP-2) is a rare primary immunodeficiency. Since the disease was first described in 2006, more than 70 patients suffering from XIAP-deficiency have been reported, thus extending the clinical presentations of the disease. The main clinical features of XLP-2 are (i) elevated susceptibility to hemophagocytic lymphohistiocytosis (HLH, frequently in response to infection with Epstein-Barr virus (EBV)), (ii) recurrent splenomegaly and (iii) inflammatory bowel disease (IBD) with the characteristics of Crohn's disease. XIAP deficiency is now considered to be one of the genetic causes of IBD in infancy. Although XIAP is an anti-apoptotic molecule, it is also involved in many other pathways, including the regulation of innate immunity and inflammation. XIAP is required for signaling through the Nod-like receptors NOD1 and 2, which are intracellular sensors of bacterial infection. XIAP-deficient T cells (including innate natural killer T cells and mucosal-associated invariant T cells) are overly sensitive to apoptosis. NOD2 function is impaired in XIAP-deficient monocytes. However, the physiopathological mechanisms underlying the clinical phenotypes in XIAP deficiency, notably the HLH and the EBV susceptibility, are not well understood. Here, we review the clinical aspects, molecular etiology and physiopathology of XIAP deficiency.

Reprint of: cancer "causation" by infections--individual contributions and synergistic networks

The search for infectious agents playing a role in human carcinogenesis and their identification remain important issues. This could provide clues for a broader spectrum of cancers preventable by vaccination and accessible to specific therapeutic regimens. Yet, the various ways of interacting among different factors functioning synergistically and their different modes of affecting individual cells should bring to question the validity of the term "causation". It also should put a word of caution into all attempts to summarize criteria for "causality" of infectious agents in cancer development. At least in the opinion of these authors, we would be much better off avoiding these terms, replacing "causal factor" by "risk factor" and grading them according to their contribution to an individual's cancer risk.

EBV Persistence--Introducing the Virus

Persistent infection by EBV is explained by the germinal center model (GCM) which provides a satisfying and currently the only explanation for EBVs disparate biology. Since the GCM touches on every aspect of the virus, this chapter will serve as an introduction to the subsequent chapters. EBV is B lymphotropic, and its biology closely follows that of normal mature B lymphocytes. The virus persists quiescently in resting memory B cells for the lifetime of the host in a non-pathogenic state that is also invisible to the immune response. To access this compartment, the virus infects naïve B cells in the lymphoepithelium of the tonsils and activates these cells using the growth transcription program. These cells migrate to the GC where they switch to a more limited transcription program, the default program, which helps rescue them into the memory compartment where the virus persists. For egress, the infected memory cells return to the lymphoepithelium where they occasionally differentiate into plasma cells activating viral replication. The released virus can either infect more naïve B cells or be amplified in the epithelium for shedding. This cycle of infection and the quiescent state in memory B cells allow for lifetime persistence at a very low level that is remarkably stable over time. Mathematically, this is a stable fixed point where the mechanisms regulating persistence drive the state back to equilibrium when perturbed. This is the GCM of EBV persistence. Other possible sites and mechanisms of persistence will also be discussed.

Primary Immunodeficiencies Associated with EBV Disease

Epstein-Barr virus (EBV) infects nearly all humans and usually is asymptomatic, or in the case of adolescents and young adults, it can result in infectious mononucleosis. EBV-infected B cells are controlled primarily by NK cells, iNKT cells, CD4 T cells, and CD8 T cells. While mutations in proteins important for B cell function can affect EBV infection of these cells, these mutations do not result in severe EBV infection. Some genetic disorders affecting T and NK cell function result in failure to control EBV infection, but do not result in increased susceptibility to other virus infections. These include mutations in SH2D1A, BIRC4, ITK, CD27, MAGT1, CORO1A, and LRBA. Since EBV is the only virus that induces proliferation of B cells, the study of these diseases has helped to identify proteins critical for interactions of T and/or NK cells with B cells. Mutations in three genes associated with hemophagocytic lymphohistocytosis, PRF1, STXBP2, and UNC13D, can also predispose to severe chronic active EBV disease. Severe EBV infection can be associated with immunodeficiencies that also predispose to other viral infections and in some cases other bacterial and fungal infections. These include diseases due to mutations in PIK3CD, PIK3R1, CTPS1, STK4, GATA2, MCM4, FCGR3A, CARD11, ATM, and WAS. In addition, patients with severe combined immunodeficiency, which can be due to mutations in a number of different genes, are at high risk for various infections as well as EBV B cell lymphomas. Identification of proteins important for control of EBV may help to identify new targets for immunosuppressive therapies.

Severe XLP Phenotype Caused by a Novel Intronic Mutation in the SH2D1A Gene

We describe here a novel c.137 + 5G > A intronic mutation in the SH2D1A gene of the signaling lymphocyte activation molecule (SLAM)-associated protein (SAP) in association with Epstein-Barr virus (EBV)-induced fatal infectious mononucleosis (FIM) in an 8-year-old male patient and his 3-year-old step brother. The mother and the maternal grandmother of the boys are healthy and heterozygous for this sequence variant. Genetic sequencing of blood-cell-derived cDNA in the younger patient revealed a 22 bp deletion in the SH2D1A cDNA. Immunoblot and flow cytometry analysis performed in this younger patient showed the lack of SAP protein expression in peripheral blood lymphocytes. These data suggest that the novel c.137 + 5G > A mutation results in loss of function of SAP protein and leads to typical X-linked lymphoproliferative disease phenotype. We propose that intron 1 and the c.137 + 5G may be the most frequent intronic hot spot for SH2D1A splicing mutation.

Homotypic NK cell-to-cell communication controls cytokine responsiveness of innate immune NK cells

While stationary organ cells are in continuous contact with neighboring cells, immune cells circulate throughout the body without an apparent requirement for cell-cell contact to persist in vivo. This study challenges current convention by demonstrating, both in vitro and in vivo, that innate immune NK cells can engage in homotypic NK-to-NK cell interactions for optimal survival, activation, and proliferation. Using a specialized cell-laden microwell approach, we discover that NK cells experiencing constant NK-to-NK contact exhibit a synergistic increase in activation status, cell proliferation, and anti-tumor function in response to IL-2 or IL-15. This effect is dependent on 2B4/CD48 ligation and an active cytoskeleton, resulting in amplification of IL-2 receptor signaling, enhanced CD122/CD132 colocalization, CD25 upregulation, and Stat3 activation. Conversely, 'orphan' NK cells demonstrate no such synergy and fail to persist. Therefore, our data uncover the existence of homotypic cell-to-cell communication among mobile innate lymphocytes, which promotes functional synergy within the cytokine-rich microenvironment.

The value of DNA storage and pedigree analysis in rare diseases: a 17-year-old boy with X-linked lymphoproliferative disease (XLP) caused by a de novo SH2D1A mutation

We present a case of a fatal Epstein-Barr infection in a 17-year-old male patient suspected to be caused by X-linked lymphoproliferative disease. At the time of hospitalization, DNA diagnostics was not available. The suspected diagnosis was confirmed several years later when a SH2D1A missense mutation was identified in stored patient DNA. Extended pedigree analysis showed that this mutation occurred de novo in his mother. In addition, we provide a summary of the currently listed SH2D1A mutations.

CONCLUSION

This case report underlines the importance of DNA storage, pedigree analysis, and multidisciplinary care in patients with rare diseases and their families.

Clinical and molecular characteristics of Chinese patients with X-linked lymphoproliferative syndrome type 1

BACKGROUND

X-linked lymphoproliferative syndrome type 1 (XLP1) is a rare inherited, life-threatening immunodeficiency disorder caused by mutations in SH2D1A gene. It affect approximately two to three males per million. Fewer than 10 cases with definite gene mutations have been reported in Chinese mainland and no rapid diagnosis method has been established.

PROCEDURE

We determined the clinical and molecular characteristics of five patients with XLP1. The SH2D1A gene were amplified by PCR and sequenced, the SAP expression was analyzed by flow cytometry.

RESULTS

Two patients had novel SH2D1A mutations and three had mutations that have been previously reported. Three patients presented with fulminant infectious mononucleosis or hemophagocytic lymphohistiocytosis and one presented with lymphoma. Null or decreased SAP expression on PBMCs was noted. The remaining patient presented with unique, recurrent, nonfulminant infectious mononucleosis and bimodal intracellular SAP protein expression.

CONCLUSIONS

The overall molecular characteristics and clinical phenotypes of Chinese patients with XLP1 matched previous reports. The unique bimodal intracellular SAP protein expression indicated the presence of some residual SAP-positive T cells that are able to respond to persistent Epstein-Barr virus infection and could explain the relatively mild clinical phenotype of this patient.

SLAM-SAP signaling promotes differentiation of IL-17-producing T cells and progression of experimental autoimmune encephalomyelitis

IL-17 plays critical roles in host defenses, combating bacterial and fungal infections, as well as the pathogenesis of autoimmune diseases such as experimental autoimmune encephalomyelitis (EAE). The signaling adaptor SAP is essential for normal immune homeostasis and mutations within SH2D1A, the locus encoding this protein, result in serious and sometimes fatal syndromes, including X-linked lymphoproliferative disease and severe cases of common variable immunodeficiency. However, the precise cellular basis of how SAP deficiency contributes to immune dysfunction remains incompletely understood. In this study, we found that CD4 and CD8 T cells lacking SAP had a diminished capacity to differentiate into IL-17-producing Th17 and T cytotoxic (Tc17) cells relative to wild-type lymphocytes. The use of costimulating SLAM Abs was found to augment the differentiation of IL-17-secreting effectors in wild-type but not Sh2d1a(-/-) splenic T cells under IL-17-polarizing conditions. In addition, SAP's regulation of IL-17-secreting T cells was shown to be a T cell-intrinsic role, as purified naive Sh2d1a(-/-) CD4 and CD8 T cells were inherently defective at converting into Th17 and Tc17 cells in vitro and in vivo. Furthermore, Sh2d1a(-/-) mice were protected from EAE and exhibited greatly decreased numbers of CNS-infiltrating Th17 and Tc17 effector T cells and reduced disease severity. Collectively, these results suggest that SLAM-SAP signaling drives the differentiation and function of Th17 and Tc17 cells in vitro and in vivo and contributes to the pathogenesis of autoimmunity in EAE.

Cancer "causation" by infections--individual contributions and synergistic networks

The search for infectious agents playing a role in human carcinogenesis and their identification remain important issues. This could provide clues for a broader spectrum of cancers preventable by vaccination and accessible to specific therapeutic regimens. Yet, the various ways of interacting among different factors functioning synergistically and their different modes of affecting individual cells should bring to question the validity of the term "causation". It also should put a word of caution into all attempts to summarize criteria for "causality" of infectious agents in cancer development. At least in the opinion of these authors, we would be much better off avoiding these terms, replacing "causal factor" by "risk factor" and grading them according to their contribution to an individual's cancer risk.

Hemophagocytic lymphohistiocytosis and primary immune deficiency disorders

Hemophagocytic lymphohistiocytosis (HLH) is characterized by uncontrolled immune activation and is traditionally associated with inherited gene defects or acquired causes. In addition to abnormalities in cytotoxic granules and lysosomes, various primary immune deficiency disorders (PID) have been identified among patients suffering from HLH. Our purpose was twofold: to better characterize and detail the association between PID and HLH. We found that HLH occurs infrequently among patients with PID, particularly those suffering from abnormalities that impair T cell function. The prognosis of patients suffering from PID and HLH is poor, emphasizing the need for rapid clinical and genetic diagnosis of the PID as well as initiation of appropriate management of the HLH, including allogeneic hematopoietic stem cell transplantations. The association of HLH and PID implicates abnormal T cell function as an important factor in HLH development. It also suggests that the partition of HLH into genetic versus acquired forms might be misleading.

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

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

Hemophagocytic lymphohistiocytosis: Recent progress in the pathogenesis, diagnosis and treatment

Hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory syndrome that develops as a primary (familial/hereditary) or secondary (non-familial/hereditary) disease characterized in the majority of the cases by hereditary or acquired impaired cytotoxic T-cell (CTL) and natural killer responses. The molecular mechanisms underlying impaired immune homeostasis have been clarified, particularly for primary diseases. Familial HLH (familial hemophagocytic lymphohistiocytosis type 2-5, Chediak-Higashi syndrome, Griscelli syndrome type 2, Hermansky-Pudlak syndrome type 2) develops due to a defect in lytic granule exocytosis, impairment of (signaling lymphocytic activation molecule)-associated protein, which plays a key role in CTL activity [e.g., X-linked lymphoproliferative syndrome (XLP) 1], or impairment of X-linked inhibitor of apoptosis, a potent regulator of lymphocyte homeostasis (e.g., XLP2). The development of primary HLH is often triggered by infections, but not in all. Secondary HLH develops in association with infection, autoimmune diseases/rheumatological conditions and malignancy. The molecular mechanisms involved in secondary HLH cases remain unknown and the pathophysiology is not the same as primary HLH. For either primary or secondary HLH cases, immunosuppressive therapy should be given to control the hypercytokinemia with steroids, cyclosporine A, or intravenous immune globulin, and if primary HLH is diagnosed, immunochemotherapy with a regimen containing etoposide or anti-thymocyte globulin should be started. Thereafter, allogeneic hematopoietic stem-cell transplantation is recommended for primary HLH or secondary refractory disease (especially EBV-HLH).

XLP: clinical features and molecular etiology due to mutations in SH2D1A encoding SAP

X-linked lymphoproliferative disease (XLP) is a rare primary immunodeficiency affecting approximately 1-2 per 1 million males. A key feature of XLP is the exquisite sensitivity of affected individuals to disease induced following EBV infection. However, patients can also develop hypogammaglobulinemia and B-cell lymphoma independently of exposure to EBV. XLP is caused by loss-of function mutations in SH2D1A, which encodes the intracellular adaptor molecule SAP. SAP is predominantly expressed in T cells and NK cells, and functions to regulate signal transduction pathways downstream of the SLAM family of surface receptors to control CD4+ T cell (and by extension B cells), CD8+ T cell and NK cell function, as well as the development of NKT cells. The study of XLP had shed substantial light on the requirements for lymphocyte differentiation and immune regulation, which in turn have the potential to be translated into novel treatments for not only XLP patients but individuals affected by EBV-induced disease, impaired humoral immunity and malignancy.

Diagnosing XLP1 in patients with hemophagocytic lymphohistiocytosis

BACKGROUND

Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening, heterogeneous, hyperinflammmatory disorder. Prompt identification of inherited forms resulting from mutation in genes involved in cellular cytotoxicity can be crucial. X-linked lymphoproliferative disease 1 (XLP1), due to mutations in SH2D1A (Xq25) encoding signaling lymphocyte activation molecule-associated protein (SAP), may present with HLH. Defective SAP induces paradoxical inhibitory function of the 2B4 coreceptor and impaired natural killer (NK) (and T) cell response against EBV-infected cells.

OBJECTIVE

To characterize a cohort of patients with HLH and XLP1 for SAP expression and 2B4 function in lymphocytes, proposing a rapid diagnostic screening to direct mutation analysis.

METHODS

We set up rapid assays for 2B4 function (degranulation or (51)Cr-release) to be combined with intracellular SAP expression in peripheral blood NK cells. We studied 12 patients with confirmed mutation in SH2D1A and some family members.

RESULTS

The combined phenotypic/functional assays allowed efficient and complete diagnostic evaluation of all patients with XLP1, thus directing mutation analysis and treatment. Nine cases were SAP(-), 2 expressed SAP with mean relative fluorescence intensity values below the range of healthy controls (SAP(dull)), and 1, carrying the R55L mutation, was SAP(+). NK cells from all patients showed inhibitory 2B4 function and defective killing of B-EBV cells. Carriers with SH2D1A mutations abolishing SAP expression and low percentage of SAP(+) cells showed neutral 2B4 function at the polyclonal NK cell level. Three novel SH2D1A mutations have been identified.

CONCLUSIONS

Study of SAP expression is specific but may have insufficient sensitivity for screening XLP1 as a single tool. Combination with 2B4 functional assay allows identification of all cases.

Reduced-intensity conditioning hematopoietic cell transplantation is an effective treatment for patients with SLAM-associated protein deficiency/X-linked lymphoproliferative disease type 1

X-linked lymphoproliferative disease type 1 (XLP1) is a rare immune deficiency caused by mutations in SH2D1A. Allogeneic hematopoietic cell transplantation (HCT) is often performed because of the morbidity and mortality associated with XLP1. There is limited experience using reduced-intensity conditioning (RIC) regimens for these patients. Here we report our 8-year single-center experience. Sixteen consecutive patients diagnosed with XLP1 underwent allogeneic HCT between 2006 and 2013 after a RIC regimen consisting of alemtuzumab, fludarabine, and melphalan. Patient phenotypes included hemophagocytic lymphohistiocytosis (HLH) after Epstein-Barr virus (n = 5) or human herpesvirus 6 (n = 1), macrophage activation syndrome (n = 1), interstitial pneumonitis and encephalitis (n = 1), B cell lymphoma (n = 8), and hypogammaglobulinemia (n = 2). One patient was asymptomatic. Fourteen of 16 patients received 8/8 HLA-matched unrelated or related bone marrow grafts, whereas 2 patients received mismatched unrelated grafts. Acute graft-versus-host disease (GVHD) prophylaxis consisted of methylprednisolone and cyclosporine in all but 1 patient, who additionally received methotrexate. All patients had hematopoietic recovery. There were no cases of hepatic veno-occlusive disease or pulmonary hemorrhage. One patient (6%) developed acute GVHD and later also developed chronic GVHD (6%). Five patients (31%) developed mixed chimerism. Only 1 patient with mixed chimerism (6%) experienced a decline of donor chimerism to less than 50% but returned to full donor chimerism after infusion of donor lymphocytes and a CD34(+) selected stem cell boost. Infectious complications were frequent, particularly viral reactivation. One-year survival estimated by Kaplan-Meier analysis was 80%, with long-term survival estimated at 71%. Survival was similar for patients with or without a history of HLH (86% versus 75%, respectively, P = .70). There were no occurrences of lymphoma or HLH after HCT. RIC HCT with alemtuzumab, fludarabine, and melphalan is an effective treatment for patients with XLP1, offering good survival rates regardless of prior disease manifestations, including HLH.

X-linked lymphoproliferative syndromes and related autosomal recessive disorders

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Expansion of murine gammaherpesvirus latently infected B cells requires T follicular help

X linked lymphoproliferative disease (XLP) is an inherited immunodeficiency resulting from mutations in the gene encoding the slam associated protein (SAP). One of the defining characteristics of XLP is extreme susceptibility to infection with Epstein-Barr virus (EBV), a gammaherpesvirus belonging to the genus Lymphocryptovirus, often resulting in fatal infectious mononucleosis (FIM). However, infection of SAP deficient mice with the related Murine gammaherpesvirus 68 (MHV68), a gammaherpesvirus in the genus Rhadinovirus, does not recapitulate XLP. Here we show that MHV68 inefficiently establishes latency in B cells in SAP deficient mice due to insufficient CD4 T cell help during the germinal center response. Although MHV68 infected B cells can be found in SAP-deficient mice, significantly fewer of these cells had a germinal center phenotype compared to SAP-sufficient mice. Furthermore, we show that infected germinal center B cells in SAP-deficient mice fail to proliferate. This failure to proliferate resulted in significantly lower viral loads, and likely accounts for the inability of MHV68 to induce a FIM-like syndrome. Finally, inhibiting differentiation of T follicular helper (T_FH) cells in SAP-sufficient C57Bl/6 mice resulted in decreased B cell latency, and the magnitude of the T_FH response directly correlated with the level of infection in B cells. This requirement for CD4 T cell help during the germinal center reaction by MHV68 is in contrast with EBV, which is thought to be capable of bypassing this requirement by expressing viral proteins that mimic signals provided by T_FH cells. In conclusion, the outcome of MHV68 infection in mice in the setting of loss of SAP function is distinct from that observed in SAP-deficient patients infected with EBV, and may identify a fundamental difference between the strategies employed by the rhadinoviruses and lymphocryptoviruses to expand B cell latency during the early phase of infection.

During an immune response, B cells respond to invading pathogens by undergoing massive expansion during the germinal center reaction. This proliferation requires signals from CD4 T cells, with some B cells then maturing into antibody secreting plasma cells, while others mature into memory B cells that may persist for the life of the host. Gammaherpesviruses take advantage of this immune response by infecting B cells, resulting in expansion of the pool of infected cells during the germinal center reaction. The human gammaherpesvirus Epstein-Barr virus (EBV) is thought to be able to accomplish this without the need for CD4 T cell help by expressing viral proteins that mimic signals from CD4 T cells. Here we show in a mouse model of gammaherpesvirus infection that infected B cells require signals from CD4 T cells for proliferation. Since the mouse gammaherpesvirus and EBV belong to different subgroups of gammaherpesviruses, this suggests that these subgroups utilize fundamentally different strategies to expand the pool of infected B cells during the establishment of latency. These different strategies may explain the different outcome of infection by these different subgroups of gammaherpesviruses in the context of defective germinal center responses that result from defective CD4 T cell help.

SAP-Dependent and -Independent Regulation of Innate T Cell Development Involving SLAMF Receptors

Signaling lymphocytic activation molecule (SLAM)-associated protein (SAP) plays an essential role in the immune system mediating the function of several members of the SLAM family (SLAMF) of receptors, whose expression is essential for T, NK, and B-cell responses. Additionally, the expression of SAP in double-positive thymocytes is mandatory for natural killer T (NKT) cells and, in mouse, for innate CD8⁺ T cell development. To date, only two members of the SLAMF of receptors, Slamf1 and Slamf6, have been shown to positively cooperate during NKT cell differentiation in mouse. However, it is less clear whether other members of this family may also participate in the development of these innate T cells. Here, we show that Slamf[1 + 6]^−/− and Slamf[1 + 5 + 6]^−/−B6 mice have ~70% reduction of NKT cells compared to wild-type B6 mice. Unexpectedly, the proportion of innate CD8⁺ T cells slightly increased in the Slamf[1 + 5 + 6]^−/−, but not in the Slamf[1 + 6]^−/− strain, suggesting that Slamf5 may function as a negative regulator of innate CD8⁺ T cell development. Accordingly, Slamf5^−/− B6 mice showed an exclusive expansion of innate CD8⁺ T cells, but not NKT cells. Interestingly, the SAP-independent Slamf7^−/− strain showed an expansion of both splenic innate CD8⁺ T cells and thymic NKT cells. On the other hand, and similar to what was recently shown in Slamf3^−/− BALB/c mice, the proportions of thymic promyelocytic leukemia zinc finger (PLZF^hi) NKT cells and innate CD8⁺ T cells significantly increased in the SAP-independent Slamf8^−/− BALB/c strain. In summary, these results show that NKT and innate CD8⁺ T cell development can be regulated in a SAP-dependent and -independent fashion by SLAMF receptors, in which Slamf1, Slamf6, and Slamf8 affect development of NKT cells, and that Slamf5, Slamf7, and Slamf8 affect the development of innate CD8⁺ T cells.

Familial hemophagocytic lymphohistiocytosis: when rare diseases shed light on immune system functioning

The human immune system depends on the activity of cytotoxic T lymphocytes (CTL), natural killer (NK) cells, and NKT cells in order to fight off a viral infection. Understanding the molecular mechanisms during this process and the role of individual proteins was greatly improved by the study of familial hemophagocytic lymphohistiocytosis (FHL). Since 1999, genetic sequencing is the gold standard to classify patients into different subgroups of FHL. The diagnosis, once based on a clinical constellation of abnormalities, is now strongly supported by the results of a functional flow-cytometry screening, which directs the genetic study. A few additional congenital immune deficiencies can also cause a resembling or even identical clinical picture to FHL. As in many other rare human disorders, the collection and analysis of a relatively large number of cases in registries is crucial to draw a complete picture of the disease. The conduction of prospective therapeutic trials allows investigators to increase the awareness of the disease and to speed up the diagnostic process, but also provides important functional and genetic confirmations. Children with confirmed diagnosis may undergo hematopoietic stem cell transplantation, which is the only cure known to date. Moreover, detailed characterization of these rare patients helped to understand the function of individual proteins within the exocytic machinery of CTL, NK, and NKT cells. Moreover, identification of these genotypes also provides valuable information on variant phenotypes, other than FHL, associated with biallelic and monoallelic mutations in the FHL-related genes. In this review, we describe how detailed characterization of patients with genetic hemophagocytic lymphohistiocytosis has resulted in improvement in knowledge regarding contribution of individual proteins to the functional machinery of cytotoxic T- and NK-cells. The review also details how identification of these genotypes has provided valuable information on variant phenotypes.

EAT-2, a SAP-like adaptor, controls NK cell activation through phospholipase Cγ, Ca++, and Erk, leading to granule polarization

Ewing's sarcoma-associated transcript 2 (EAT-2) is an Src homology 2 domain-containing intracellular adaptor related to signaling lymphocytic activation molecule (SLAM)-associated protein (SAP), the X-linked lymphoproliferative gene product. Both EAT-2 and SAP are expressed in natural killer (NK) cells, and their combined expression is essential for NK cells to kill abnormal hematopoietic cells. SAP mediates this function by coupling SLAM family receptors to the protein tyrosine kinase Fyn and the exchange factor Vav, thereby promoting conjugate formation between NK cells and target cells. We used a variety of genetic, biochemical, and imaging approaches to define the molecular and cellular mechanisms by which EAT-2 controls NK cell activation. We found that EAT-2 mediates its effects in NK cells by linking SLAM family receptors to phospholipase Cγ, calcium fluxes, and Erk kinase. These signals are triggered by one or two tyrosines located in the carboxyl-terminal tail of EAT-2 but not found in SAP. Unlike SAP, EAT-2 does not enhance conjugate formation. Rather, it accelerates polarization and exocytosis of cytotoxic granules toward hematopoietic target cells. Hence, EAT-2 promotes NK cell activation by molecular and cellular mechanisms distinct from those of SAP. These findings explain the cooperative and essential function of these two adaptors in NK cell activation.

Fine specificity and molecular competition in SLAM family receptor signalling

SLAM family receptors regulate activation and inhibition in immunity through recruitment of activating and inhibitory SH2 domain containing proteins to immunoreceptor tyrosine based switch motifs (ITSMs). Binding of the adaptors, SAP and EAT-2 to ITSMs in the cytoplasmic regions of SLAM family receptors is important for activation. We analysed the fine specificity of SLAM family receptor phosphorylated ITSMs and the conserved tyrosine motif in EAT-2 for SH2 domain containing signalling proteins. Consistent with the literature describing dependence of CRACC (SLAMF7) on EAT-2, CRACC bound EAT-2 (K_D = 0.003 μM) with approximately 2 orders of magnitude greater affinity than SAP (K_D = 0.44 μM). RNA interference in cytotoxicity assays in NK92 cells showed dependence of CRACC on SAP in addition to EAT-2, indicating selectivity of SAP and EAT-2 may depend on the relative concentrations of the two adaptors. The concentration of SAP was four fold higher than EAT-2 in NK92 cells. Compared with SAP, the significance of EAT-2 recruitment and its downstream effectors are not well characterised. We identified PLCγ1 and PLCγ2 as principal binding partners for the EAT-2 tail. Both PLCγ1 and PLCγ2 are functionally important for cytotoxicity in NK92 cells through CD244 (SLAMF4), NTB-A (SLAMF6) and CRACC. Comparison of the specificity of SH2 domains from activating and inhibitory signalling mediators revealed a hierarchy of affinities for CD244 (SLAMF4) ITSMs. While binding of phosphatase SH2 domains to individual ITSMs of CD244 was weak compared with SAP or EAT-2, binding of tandem SH2 domains of SHP-2 to longer peptides containing tandem phosphorylated ITSMs in human CD244 increased the affinity ten fold. The concentration of the tyrosine phosphatase, SHP-2 was in the order of a magnitude higher than the adaptors, SAP and EAT-2. These data demonstrate a mechanism for direct recruitment of phosphatases in inhibitory signalling by ITSMs, while explaining competitive dominance of SAP and EAT-2.

XLP1 inhibitory effect by 2B4 does not affect DNAM-1 and NKG2D activating pathways in NK cells

X-linked lymphoproliferative disease 1 (XLP1) is a rare congenital immunodeficiency caused by SH2D1A (Xq25) mutations resulting in lack or dysfunction of SLAM-associated protein adaptor molecule. In XLP1 patients, upon ligand (CD48) engagement, 2B4 delivers inhibitory signals that impair the cytolytic activity of NK (and T) cells. This causes the selective inability to control EBV infections and the occurrence of B-cell lymphomas. Here, we show that in the absence of SLAM-associated protein, co-engagement of 2B4 with different activating receptors, either by antibodies or specific ligands on target cells, inhibits different ITAM-dependent signaling pathways including activating killer Ig-like receptors. In XLP1 NK cells, 2B4 affected both the cytolytic and IFN-γ production capabilities, functions that were restored upon disruption of the 2B4/CD48 interactions. Notably, we provide evidence that 2B4 dysfunction does not affect the activity of DNAM-1 and NKG2D triggering receptors. Thus, while CD48(+) B-EBV and lymphoma B cells devoid of NKG2D and DNAM-1 ligands were resistant to lysis, the preferential usage of these receptors allowed XLP1 NK cells to kill lymphomas that expressed sufficient amounts of the specific ligands. The study sheds new light on the XLP1 immunological defect and on the cross-talk of inhibitory 2B4 with triggering NK (and T) receptors.

Clinical flow cytometric screening of SAP and XIAP expression accurately identifies patients with SH2D1A and XIAP/BIRC4 mutations

INTRODUCTION

X-linked lymphoproliferative disease is caused by mutations in two genes, SH2D1A and XIAP/BIRC4. Flow cytometric methods have been developed to detect the gene products, SAP and XIAP. However, there is no literature describing the accuracy of flow cytometric screening performed in a clinical lab setting.

METHODS

We reviewed the clinical flow cytometric testing results for 656 SAP and 586 XIAP samples tested during a 3-year period. Genetic testing was clinically performed as directed by the managing physician in 137 SAP (21%) and 115 XIAP (20%) samples. We included these samples for analyses of flow cytometric test accuracy.

RESULTS

SH2D1A mutations were detected in 15/137 samples. SAP expression was low in 13/15 (sensitivity 87%, CI 61-97%). Of the 122 samples with normal sequencing, SAP was normal in 109 (specificity 89%, CI 82-94%). The positive predictive values (PPVs) and the negative predictive values (NPVs) were 50% and 98%, respectively. XIAP/BIRC4 mutations were detected in 19/115 samples. XIAP expression was low in 18/19 (sensitivity 95%, CI 73-100%). Of the 96 samples with normal sequencing, 59 had normal XIAP expression (specificity 61%, CI 51-71%). The PPVs and NPVs were 33% and 98%, respectively. Receiver-operating characteristic analysis was able to improve the specificity to 75%.

CONCLUSION

Clinical flow cytometric screening tests for SAP and XIAP deficiencies offer good sensitivity and specificity for detecting genetic mutations, and are characterized by high NPVs. We recommend these tests for patients suspected of having X-linked lymphoproliferative disease type 1 (XLP1) or XLP2.

Chicken SLAMF4 (CD244, 2B4), a receptor expressed on thrombocytes, monocytes, NK cells, and subsets of αβ-, γδ- T cells and B cells binds to SLAMF2

The SLAM family of membrane receptors is involved in the regulation of immune responses by controlling cytokines production, cytotoxicity as well as cell development, differentiation and proliferation, but has only been described in chickens, recently. The aim of this study was to characterize the avian homologue to mammalian SLAMF4 (CD244, 2B4), a cell surface molecule which belongs to the SLAM family of membrane receptors. We generated a SLAMF4 specific monoclonal antibody (mab) designated 8C7 and analyzed the SLAMF4 expression on cells isolated from various lymphoid organs. Subsets of αβ and γδ T cells found in peripheral blood lymphocytes (PBL) and spleen coexpressed SLAMF4. The expression was restricted to CD8α(+) T cells, whereas CD4(+) T cells and all thymocytes showed little or no reactivity upon staining with the 8C7 mab. Blood and splenic γδ T cells could be further differentiated according to their expression levels of SLAMF4 into two and three subsets, respectively. SLAMF4 was absent from bursal and splenic B cells, however, it was expressed by a distinct fraction of circulating B cells that were characterized by high level expression of Bu1, Ig, and CD40. SLAMF4 was also present on NK cells isolated from intestine of adult chickens or embryonic splenocytes identified by their coexpression of the 28-4 NK cell marker. Moreover, SLAMF4 expression was found on thrombocytes and monocytes. The interaction of SLAMF4 with SLAMF2 was proven by a reporter assay and could be blocked with the 8C7 mab. In conclusion, the avian SLAMF4 expression markedly differs from mammals; it binds to SLAMF2 and will be an important tool to discriminate several γδ T cell subsets.

The adaptor protein SAP directly associates with PECAM-1 and regulates PECAM-1-mediated-cell adhesion in T-like cell lines

SAP is a small cytosolic adaptor protein expressed in hematopoietic lineages whose main function is to regulate intracellular signaling pathways induced by the triggering of members of the SLAM receptor family. In this paper, we have identified the adhesion molecule PECAM-1 as a new partner for SAP in a conditional yeast two-hybrid screen. PECAM-1 is an immunoglobulin-like molecule expressed by endothelial cells and leukocytes, which possesses both pro- and anti-inflammatory properties. However, little is known about PECAM-1 functions in T cells. We show that SAP directly and specifically interacts with the cytosolic tyrosine 686 of PECAM-1. We generated different T-like cell lines in which SAP or PECAM-1 are expressed or down modulated and we demonstrate that a diminished SAP expression correlates with a diminished PECAM-1-mediated adhesion. Although SAP has mainly been shown to associate with SLAM receptors, we evidence here that SAP is a new actor downstream of PECAM-1.

Identification of a new isoform of the murine Sh2d1a gene and its functional implications

Signaling lymphocytic activation molecule (SLAM)-associated protein (SAP) is a Src homology (SH) domain 2-containing intracellular adaptor protein that is predominantly expressed in the hematopoietic system by T lymphocytes and NK cells. SAP protein is encoded by the SH2D1A gene located on the X chromosome. Loss-of-function mutations in SAP cause the X-linked lymphoproliferative disease (XLP), a severe immunodeficiency characterized by heightened susceptibility to Epstein-Barr virus and impaired humoral immunity. Normal individuals express several functional and non-functional isoforms of SAP as a result of alternative splicing. In this study, we identify a cryptic exon in the murine Sh2d1a gene. At the mRNA level, the new isoform of SAP (SAP-2) that includes this new exon is widely expressed in lymphoid tissues by C57BL/6 and 129 strains of inbred mice. SAP-2 accounts for approximately 1%-3% of total SAP transcripts, and it is dynamically regulated during lymphocyte activation. At the protein level, the SAP-2 isoform is a 144 amino-acid protein. Compared to the dominant 126 aminoacid SAP-1 isoform, the additional 18 amino acids are inserted into a structural region that is critical for phosphotyrosine binding. Our functional analysis in vitro indicates that SAP-2 is a non-functional isoform due to decreased protein stability. Thus, both human and mouse have multiple SAP splice isoforms that may or may not function. Modulation of relative proportions of these isoforms is potentially a mechanism whereby cells can regulate SAP-mediated biological activities.

Signaling lymphocytic activation molecule (SLAM)/SLAM-associated protein pathway regulates human B-cell tolerance

BACKGROUND

Signaling lymphocytic activation molecule (SLAM)-associated protein (SAP) can mediate the function of SLAM molecules, which have been proposed to be involved in the development of autoimmunity in mice.

OBJECTIVE

We sought to determine whether the SLAM/SAP pathway regulates the establishment of human B-cell tolerance and what mechanisms of B-cell tolerance could be affected by SAP deficiency.

METHODS

We tested the reactivity of antibodies isolated from single B cells from SAP-deficient patients with X-linked lymphoproliferative disease (XLP). The expressions of SAP and SLAM family members were assessed in human bone marrow-developing B cells. We also analyzed regulatory T (Treg) cell function in patients with XLP and healthy control subjects.

RESULTS

We found that new emigrant/transitional B cells from patients with XLP were enriched in autoreactive clones, revealing a defective central B-cell tolerance checkpoint in the absence of functional SAP. In agreement with a B cell-intrinsic regulation of central tolerance, we identified SAP expression in a discrete subset of bone marrow immature B cells. SAP colocalized with SLAMF6 only in association with clustered B-cell receptors likely recognizing self-antigens, suggesting that SLAM/SAP regulate B-cell receptor-mediated central tolerance. In addition, patients with XLP displayed defective peripheral B-cell tolerance, which is normally controlled by Treg cells. Treg cells in patients with XLP seem functional, but SAP-deficient T cells were resistant to Treg cell-mediated suppression. Indeed, SAP-deficient T cells were hyperresponsive to T-cell receptor stimulation, which resulted in increased secretion of IL-2, IFN-γ, and TNF-α.

CONCLUSIONS

SAP expression is required for the counterselection of developing autoreactive B cells and prevents their T cell-dependent accumulation in the periphery.

Molecular mechanisms of SH2- and PTB-domain-containing proteins in receptor tyrosine kinase signaling

Intracellular signaling is mediated by reversible posttranslational modifications (PTMs) that include phosphorylation, ubiquitination, and acetylation, among others. In response to extracellular stimuli such as growth factors, receptor tyrosine kinases (RTKs) typically dimerize and initiate signaling through phosphorylation of their cytoplasmic tails and downstream scaffolds. Signaling effectors are recruited to these phosphotyrosine (pTyr) sites primarily through Src homology 2 (SH2) domains and pTyr-binding (PTB) domains. This review describes how these conserved domains specifically recognize pTyr residues and play a major role in mediating precise downstream signaling events.

Molecular control of Tfh-cell differentiation by Roquin family proteins

Post-transcriptional gene regulation by RNA-binding proteins is a fast and effective way to adapt gene expression and change cellular responses. These trans-acting factors have been involved in a number of cell fate decisions, and their mutation is often associated with the development of disease. The RNA-binding protein Roquin-1 has been found to be crucial in the maintenance of peripheral tolerance and the prevention of autoimmune disease. This review describes the molecular role of Roquin family proteins in the control of follicular T-helper cell differentiation. Here, we discuss the redundant regulation of Icos and Ox40 costimulatory receptor mRNAs by Roquin-1 and Roquin-2 proteins. A major focus is placed on the distinct activity of Roquin-1 or Roquin-2 proteins in the mouse models of conditional gene targeting. These recent data are then integrated into an interpretation of altered Roquin protein function in the sanroque mouse that expresses the Roquin-1 protein with just one amino acid substitution and, different from the Roquin-1-deficient mouse, develops lupus-like autoimmune disease.

The adaptor molecule signaling lymphocytic activation molecule (SLAM)-associated protein (SAP) is essential in mechanisms involving the Fyn tyrosine kinase for induction and progression of collagen-induced arthritis

Signaling lymphocytic activation molecule-associated protein (SAP) is an Src homology 2 domain-only adaptor involved in multiple immune cell functions. It has also been linked to immunodeficiencies and autoimmune diseases, such as systemic lupus erythematosus. Here, we examined the role and mechanism of action of SAP in autoimmunity using a mouse model of autoimmune arthritis, collagen-induced arthritis (CIA). We found that SAP was essential for development of CIA in response to collagen immunization. It was also required for production of collagen-specific antibodies, which play a key role in disease pathogenesis. These effects required SAP expression in T cells, not in B cells. In mice immunized with a high dose of collagen, the activity of SAP was nearly independent of its ability to bind the protein tyrosine kinase Fyn and correlated with the capacity of SAP to promote full differentiation of follicular T helper (TFH) cells. However, with a lower dose of collagen, the role of SAP was more dependent on Fyn binding, suggesting that additional mechanisms other than TFH cell differentiation were involved. Further studies suggested that this might be due to a role of the SAP-Fyn interaction in natural killer T cell development through the ability of SAP-Fyn to promote Vav-1 activation. We also found that removal of SAP expression during progression of CIA attenuated disease severity. However, it had no effect on disease when CIA was clinically established. Together, these results indicate that SAP plays an essential role in CIA because of Fyn-independent and Fyn-dependent effects on TFH cells and, possibly, other T cell types.

X-linked inhibitor of apoptosis (XIAP) deficiency: the spectrum of presenting manifestations beyond hemophagocytic lymphohistiocytosis

X-linked inhibitor of apoptosis (XIAP) deficiency caused by mutations in BIRC4 was initially described in patients with X-linked lymphoproliferative syndrome (XLP) who had no mutations in SH2D1A. In the initial reports, EBV-associated hemophagocytic lymphohistiocytosis (HLH) was the predominant clinical phenotype. Among 25 symptomatic patients diagnosed with XIAP deficiency, we identified 17 patients who initially presented with manifestations other than HLH. These included Crohn-like bowel disease (n=6), severe infectious mononucleosis (n=4), isolated splenomegaly (n=3), uveitis (n=1), periodic fever (n=1), fistulating skin abscesses (n=1) and severe Giardia enteritis (n=1). Subsequent manifestations included celiac-like disease, antibody deficiency, splenomegaly and partial HLH. Screening by flow cytometry identified 14 of 17 patients in our cohort. However, neither genotype nor protein expression nor results from cell death studies were clearly associated with the clinical phenotype. Only mutation analysis can reliably identify affected patients. XIAP deficiency must be considered in a wide range of clinical presentations.

The cup runneth over: lessons from the ever-expanding pool of primary immunodeficiency diseases

A recent surge in newly described primary immunodeficiencies (PIDs) has highlighted new physiological and pathophysiological pathways that affect the immune system. Furthermore, the study of individuals with PIDs has substantially improved our understanding of basic cellular and signalling pathways in host defence and immune regulation. Single-gene defects can lead to disease manifestations that range from extremely narrow infectious phenotypes to remarkably broad multisystem effects. Hypomorphic or hypermorphic gene mutations often occur in human diseases; when coupled with the fact that humans are exposed to naturally encountered antigens and pathogens, this helps to make the case that the study of immunological diseases in humans should be at the forefront of basic immunological research.

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.

Primary antibody deficiencies

Primary antibody deficiencies (PADs) are the most common inherited immunodeficiencies in humans. The use of novel approaches, such as whole-exome sequencing and mouse genetic engineering, has helped to identify new genes that are involved in the pathogenesis of PADs and has enabled the characterization of the molecular pathways that are involved in B cell development and function. Here, we review the different PADs in terms of their known or putative mechanisms, which can be B cell intrinsic, B cell extrinsic or not defined so far. We also describe the clinical manifestations (including susceptibility to infections, autoimmunity and cancer) that have been associated with the various PADs.

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

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

Hemophagocytic lymphohistiocytosis: an update on diagnosis and pathogenesis

Hemophagocytic lymphohistiocytosis (HLH) is a frequently fatal and likely underdiagnosed disease involving a final common pathway of hypercytokinemia, which can result in end-organ damage and death. Although an early diagnosis is crucial to decrease mortality, the definitive diagnosis is often challenging because of the lack of specificity of currently accepted diagnostic criteria and the absence of confirmatory gold standards. Because of the wide range of laboratory assays involved in the diagnosis of HLH, practicing pathologists from a broad spectrum of clinical specialties need to be aware of the disease so that they may appropriately flag results and convey them to their clinical counterparts. Our article summarizes these new advances in the diagnosis of HLH and includes a review of clinical findings, updated understanding of the pathogenesis, and promising new testing methods.

Semi-supervised prediction of SH2-peptide interactions from imbalanced high-throughput data

Src homology 2 (SH2) domains are the largest family of the peptide-recognition modules (PRMs) that bind to phosphotyrosine containing peptides. Knowledge about binding partners of SH2-domains is key for a deeper understanding of different cellular processes. Given the high binding specificity of SH2, in-silico ligand peptide prediction is of great interest. Currently however, only a few approaches have been published for the prediction of SH2-peptide interactions. Their main shortcomings range from limited coverage, to restrictive modeling assumptions (they are mainly based on position specific scoring matrices and do not take into consideration complex amino acids inter-dependencies) and high computational complexity. We propose a simple yet effective machine learning approach for a large set of known human SH2 domains. We used comprehensive data from micro-array and peptide-array experiments on 51 human SH2 domains. In order to deal with the high data imbalance problem and the high signal-to-noise ration, we casted the problem in a semi-supervised setting. We report competitive predictive performance w.r.t. state-of-the-art. Specifically we obtain 0.83 AUC ROC and 0.93 AUC PR in comparison to 0.71 AUC ROC and 0.87 AUC PR previously achieved by the position specific scoring matrices (PSSMs) based SMALI approach. Our work provides three main contributions. First, we showed that better models can be obtained when the information on the non-interacting peptides (negative examples) is also used. Second, we improve performance when considering high order correlations between the ligand positions employing regularization techniques to effectively avoid overfitting issues. Third, we developed an approach to tackle the data imbalance problem using a semi-supervised strategy. Finally, we performed a genome-wide prediction of human SH2-peptide binding, uncovering several findings of biological relevance. We make our models and genome-wide predictions, for all the 51 SH2-domains, freely available to the scientific community under the following URLs: http://www.bioinf.uni-freiburg.de/Software/SH2PepInt/SH2PepInt.tar.gz and http://www.bioinf.uni-freiburg.de/Software/SH2PepInt/Genome-wide-predictions.tar.gz, respectively.

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

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

The adaptor molecule SAP plays essential roles during invariant NKT cell cytotoxicity and lytic synapse formation

The adaptor molecule signaling lymphocytic activation molecule-associated protein (SAP) plays critical roles during invariant natural killer T (iNKT) cell ontogeny. As a result, SAP-deficient humans and mice lack iNKT cells. The strict developmental requirement for SAP has made it difficult to discern its possible involvement in mature iNKT cell functions. By using temporal Cre recombinase-mediated gene deletion to ablate SAP expression after completion of iNKT cell development, we demonstrate that SAP is essential for T-cell receptor (TCR)-induced iNKT cell cytotoxicity against T-cell and B-cell leukemia targets in vitro and iNKT-cell-mediated control of T-cell leukemia growth in vivo. These findings are not restricted to the murine system: silencing RNA-mediated suppression of SAP expression in human iNKT cells also significantly impairs TCR-induced cytolysis. Mechanistic studies reveal that iNKT cell killing requires the tyrosine kinase Fyn, a known SAP-binding protein. Furthermore, SAP expression is required within iNKT cells to facilitate their interaction with T-cell targets and induce reorientation of the microtubule-organizing center to the immunologic synapse (IS). Collectively, these studies highlight a novel and essential role for SAP during iNKT cell cytotoxicity and formation of a functional IS.

Development and evaluation of novel phosphotyrosine mimetic inhibitors targeting the Src homology 2 domain of signaling lymphocytic activation molecule (SLAM) associated protein

Specific interactions between Src homology 2 (SH2) domain-containing proteins and the phosphotyrosine-containing counterparts play significant role in cellular protein tyrosine kinase (PTK) signaling pathways. The SH2 domain inhibitors could potentially serve as drug candidates in treating human diseases. Here we have incorporated a novel phosphotyrosine mimetic, which is an unusual amino acid carrying a cyclosaligenyl (cycloSal) phosphodiester moiety, into dipeptides to investigate the inhibitory effect on SH2 domain-containing proteins. A plate-based assay was also established to screen for inhibitors that disrupt the interaction between a phosphopeptide of SLAM (signaling lymphocytic activation molecule) and its interacting protein SAP (SLAM-associated protein). We identified a number of inhibitors with IC50 values in the range of 17-35 μM, implying that the cycloSal phosphodiester-carrying amino acid could mimic the phosphotyrosyl residue. Our results also raise the possibility of integrating the newly developed phosphotyrosine mimetic moiety into inhibitors designed for other SH2 domain-containing proteins.