A spectrum of mutations in SH2D1A that causes X-linked lymphoproliferative disease and other Epstein-Barr virus-associated illnesses

Potential pathogenic mechanism of type 1 X-linked lymphoproliferative syndrome caused by a mutation of SH2D1A gene in an infant: A case report

BACKGROUND

X-linked lymphoproliferative syndrome (XLP) is a rare X-linked recessive inborn errors of immunity. The pathogenesis of XLP might be related to phophatidylinositol-3-kinase (PI3K)-associated pathways but insight details remain unclear. This study was to study an infant XLP-1 case caused by a mutation in SH2D1A gene, investigate the structural and functional alteration of mutant SAP protein, and explore the potential role of PI3K-associated pathways in the progression of XLP-1.

METHODS

The proband's condition was monitored by laboratory and imagological examinations. Whole exome sequencing and Sanger sequencing were performed to detect the genetic disorder. Bioinformatics tools including PolyPhen-2, SWISS-MODEL and SWISS-PDB Viewer were used to predict the pathogenicity and estimate structural change of mutant protein. Flow cytometry was used to investigate expression of SAP and PI3K-associated proteins.

RESULTS

The proband was diagnosed with XLP-1 caused by a hemizygous mutation c.96G > T in SH2D1A gene resulting in a missense substitution of Arginine to Serine at the site of amino acid 32 (p.R32S). The mutant protein contained a hydrogen bond turnover at the site of mutation and was predicted to be highly pathogenic. Expression of SH2D1A encoded protein SAP was downregulated in proband. The PI3K-AKT-mTOR signaling pathway was fully activated in XLP-1 patients, but it was inactive or only partially activated in healthy people or HLH patients.

CONCLUSIONS

The mutation c.96G > T in SH2D1A gene caused structural and functional changes in the SAP protein, resulting in XLP-1. The PI3K-AKT-mTOR signaling pathway may play a role in XLP-1 pathogenesis.

Exon skipping caused by a complex structural variation in SH2D1A resulted in X-linked lymphoproliferative syndrome type 1

Background

X‐linked lymphoproliferative syndrome type 1 (XLP1) is a rare primary immunodeficiency disorder characterized by severe immune dysregulation often after viral infection. It is caused by hemizygous mutations in the X‐linked SH2D1A gene. People with XLP1 have complex and variable phenotype manifestations as EBV‐driven severe or fulminant mononucleosis, hemophagocytic lymphohistiocytosis (EBV‐HLH), dysgammaglobulinemia, and B‐cell lymphoma.

Methods

Immunological analyses, clinical laboratory testing, and whole exome sequencing (WES) were performed to help the disease diagnosis for the patient with severe immune dysregulation. Routine and extended WES analysis pipelines were applied to explore candidates. A complex genomic structural variation in SH2D1A was detected and verified by Inverse‐PCR, Gap‐PCR, and RT‐PCR.

Results

Here we reported that a five‐year‐old male patient manifested with EBV‐HLH, recurrent infection by severe immune dysregulation, and successfully managed with HSCT. He finally established precise disease diagnosis as XLP1 caused by a complex genomic structural variation in SH2D1A (NC_000023.11:g. [124,350,560_124365777del; 124,365,777_124365917inv; 124,365,911_124365916del]). The mother and grandmother of the proband were confirmed to be carriers. The complex variant resulted in the exon 2 skipping and was predicted to generate a prematurely truncated protein.

Conclusion

The complex structural variant combined with paracentric inversion and large size deletions was first reported in XLP1 cases. It is considered to be pathogenic based on the truncation of the mRNA sequence and cosegregation with the disease in three‐generation pedigree analysis. This finding has expanded the known XLP‐related mutation spectrum in Chinese patients and indicated remarkable effects on the early diagnosis and therapeutic implication using proper molecular testing techniques.

Chromosome X-wide Analysis of Positive Selection in Human Populations: Common and Private Signals of Selection and its Impact on Inactivated Genes and Enhancers

The ability of detecting adaptive (positive) selection in the genome has opened the possibility of understanding the genetic basis of population-specific adaptations genome-wide. Here, we present the analysis of recent selective sweeps, specifically in the X chromosome, in human populations from the third phase of the 1,000 Genomes Project using three different haplotype-based statistics. We describe instances of recent positive selection that fit the criteria of hard or soft sweeps, and detect a higher number of events among sub-Saharan Africans than non-Africans (Europe and East Asia). A global enrichment of neural-related processes is observed and numerous genes related to fertility appear among the top candidates, reflecting the importance of reproduction in human evolution. Commonalities with previously reported genes under positive selection are found, while particularly strong new signals are reported in specific populations or shared across different continental groups. We report an enrichment of signals in genes that escape X chromosome inactivation, which may contribute to the differentiation between sexes. We also provide evidence of a widespread presence of soft-sweep-like signatures across the chromosome and a global enrichment of highly scoring regions that overlap potential regulatory elements. Among these, enhancers-like signatures seem to present putative signals of positive selection which might be in concordance with selection in their target genes. Also, particularly strong signals appear in regulatory regions that show differential activities, which might point to population-specific regulatory adaptations.

Preimplantation Genetic Testing for a Chinese Family With X-Linked Lymphoproliferative Syndrome Type 1

Background

X-linked lymphoproliferative disease (XLP) is a rare primary immunodeficiency disorder. We performed experiments based on two strategies of preimplantation genetic testing (PGT) for a family with XLP caused by a mutation in SH2D1A (c.191G > A).

Methods

First, a single-cell polymerase chain reaction (PCR) protocol was established using single lymphocytes. A nested PCR experiment was performed with direct sequencing after whole genome amplification of single cells to assess the accuracy of the genetic diagnosis. Embryos obtained after intracytoplasmic sperm injection were biopsied on day 3 and detected using the established single-cell PCR protocol. In the second PGT cycle, targeted next generation sequencing (NGS) was performed and the single nucleotide polymorphism (SNP) markers flanking SH2D1A were selected to determine the disease-carrying haplotype phase in each embryo.

Result

In the first PGT cycle, six embryos were biopsied. Discounting an embryo from a single failed PCR experiment, five embryos were identified, including three unaffected and two hemizygous. After PCR, one normal embryo was transferred when it was developing into an early blastocyst. Although the ultrasound images indicated a viable singleton pregnancy, the implantation was on the cesarean scar. Therefore, an artificial abortion was performed. In the haplotyping cycle, six embryos were identified to have inherited a haplotype without pathogenic mutations. After the embryo implantation process failed twice, a successful singleton pregnancy was established, and subsequently, a healthy female child was born.

Conclusion

Targeted NGS with haplotyping analysis circumvents the laborious process of multiplex PCR and is more likely to ensure diagnostic accuracy. However, when a genetic recombination occurs close to the site of mutation, confirmed identification using selected SNP markers can be challenging.

Neonatal Hemophagocytic Lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is extremely rare in the neonatal period. The incidence of neonatal HLH is not confirmed and may range from 1 in 50,000 to 150,000. The incidence varies based on ethnicity, particularly in populations in which consanguinity is common. HLH is associated with a high fatality rate and poor prognosis, making it important to recognize and diagnose it early. This review will concentrate primarily on the diagnosis and management of neonatal HLH.

Rare cause of Hemophagocytic Lymphohistiocytosis due to mutation in PRF1 and SH2D1A genes in two children - a case report with a review

Background

Hemophagocytic Lymphohistiocytosis (HLH) is a rare, complex, life-threatening hyper-inflammatory condition due to over activation of lymphocytes mediated secretory cytokines in the body. It occurs as a primary HLH due to genetic defect that mostly occurs in the childhood and associated with early neonatal death. Secondary HLH is triggered by secondary to infection and can occur at any age.

Case presentation

The current report presents two cases of HLH. Case 1, three-months-old boy born to second degree consanguineous parents was clinically suspected with HLH. A pathogenic variant in exon 2 of PRF1 gene [c.386G > C (p.Trp129Ser); FLH-type2] was detected. The parents and the fetus under investigation were shown to be heterozygous carriers, while Case-1 was homozygous for the said variant. Case 2, a one and half-year old male child referred for work-up was born to non-consanguineous young parents. His HLH suspicion was in accordance with HLH-2004 Revised diagnostic guidelines (fulfilling 5/8 criteria). Molecular study revealed hemizygous likely pathogenic variant c.138-3C > G in intron 1 of SH2D1A gene. Both the mother and younger sister were confirmed to be the carrier of the same variant.

Conclusion

This study has represented two rare cases of HLH carrying missense variant in PRF1 and splice site variant in SH2D1A gene. Detailed molecular analysis has helped the families with precise genetic counselling and prenatal diagnosis during subsequent pregnancy. It is advocated that male patients presenting with EBV-associated HLH may be screened for XLP that may lead to early diagnosis and therapeutic implication if any.

Electronic supplementary material

The online version of this article (10.1186/s12887-019-1444-4) contains supplementary material, which is available to authorized users.

2B4 dysfunction in XLP1 NK cells: More than inability to control EBV infection

X-linked lymphoproliferative disease 1 (XLP1) is a monogenic disorder caused by mutations in SH2D1A, resulting in the absence/dysfunction of the signaling lymphocyte activation molecule (SLAM)-associated protein (SAP). Consequently, SLAM receptors as 2B4 (CD244) and NTB-A (SLAMF6), upon ligand engagement, exert inhibitory instead of activating function. This causes an immune dysfunction that is worsened by the selective inability of NK and T cells to kill EBV-infected B cells with dramatic clinical sequelae (e.g. fulminant mononucleosis, hyperinflammation, lymphoma). Here we outline recent findings on the interplay between inhibitory 2B4 and the various activating receptors in NK cells. 2B4 engagement selectively blocks ITAM-dependent activating receptors as NCR and CD16, while it does not affect NKG2D and DNAM-1. Furthermore, inhibitory 2B4 participates to NK cell education, as highlighted by the existence in XLP1 patients of a large subset of fully functional NK cells that lack self-HLA specific inhibitory receptors and exert autoreactivity against mature dendritic cells.

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

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

How Viruses Contribute to the Pathogenesis of Hemophagocytic Lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening, hyperinflammatory syndrome, characterized by the uncontrolled activation of macrophages and T cells, eliciting key symptoms such as persistent fever, hepatosplenomegaly, pancytopenia, hemophagocytosis, hyperferritinemia, and coagulopathy. Viral infections are frequently implicated in the onset of active HLH episodes, both in primary, genetic HLH as in the secondary, acquired form. Infections with herpesviruses such as Epstein-Barr virus and cytomegalovirus are the most common. In autoimmune diseases, a link between viral infections and autoreactive immune responses has been recognized for a considerable time. However, the mechanisms by which viruses contribute to HLH pathogenesis remain to be clarified. In this viewpoint, different factors that may come into play are discussed. Viruses, particularly larger DNA viruses such as herpesviruses, are potent modulators of the immune response. By evading immune recognition, interfering with cytokine balances and inhibiting apoptotic pathways, viruses may increase the host's susceptibility to HLH development. In particular cases, a direct connection between the viral infection and inhibition of natural killer cell or T cell cytotoxicity was reported, indicating that viruses may create immunological deficiencies reminiscent of primary HLH.

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.

Hemophagocytic lymphohistiocytosis (HLH): A heterogeneous spectrum of cytokine-driven immune disorders

Hemophagocytic lymphohistiocytosis (HLH) comprises a group of life-threatening immune disorders classified into primary or secondary HLH. The former is caused by mutations in genes involved in granule-mediated cytotoxicity, the latter occurs in a context of infections, malignancies or autoimmune/autoinflammatory disorders. Both are characterized by systemic inflammation, severe cytokine storms and immune-mediated organ damage. Despite recent advances, the pathogenesis of HLH remains incompletely understood. Animal models resembling different subtypes of HLH are therefore of great value to study this disease and to uncover novel treatment strategies. In this review, all known animal models of HLH will be discussed, highlighting findings on cell types, cytokines and signaling pathways involved in disease pathogenesis and extrapolating therapeutic implications for the human situation.

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

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

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.

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.

HHV-8-related hemophagocytic lymphohistiocytosis in a boy with XLP phenotype

We report a 2.5-year-old boy with an X-linked lymphoproliferative disease (XLP) phenotype who presented with human herpes virus-8 (HHV-8)-related hemophagocytic lymphohistiocytosis (HLH). XLP is a rare primary immunodeficiency characterized by extreme susceptibility to herpes viruses, mainly Epstein-Barr virus (EBV). Approximately 60% of patients with XLP present with fulminant mononucleosis associated with HLH, whereas remaining patients present with hypogammaglobulinemia or lymphoproliferative disease. Most commonly, one of the XLP phenotypes appears after exposure to EBV, but at least 12% of affected individuals developed symptoms without an evidence of EBV infection. Rarely, patients with XLP may present with central nervous system vasculitis or aplastic anemia. HHV-8 is lymphotrophic and it is associated with lymphoproliferative disorders and Kaposi sarcoma in immunodeficient hosts. Kaposi sarcoma rarely occurs in children with well-defined primary immunodeficiency. Also, HHV-8-related HLH was previously reported in 2 siblings with a perforin gene deficiency. Recently, it became evident that besides EBV, other viruses may trigger the symptoms in XLP. We report for the first time HHV-8-related HLH in EBV-negative pediatric patient with an XLP phenotype.

Screening the PRF1, UNC13D, STX11, SH2D1A, XIAP, and ITK gene mutations in Chinese children with Epstein-Barr virus-associated hemophagocytic lymphohistiocytosis

BACKGROUND

This study aimed to investigate the prevalence of mutations in the PRF1, UNC13D, STX11, SH2D1A, XIAP, and ITK in Chinese pediatric patients with EBV-HLH.

METHODS

Sixty-seven pediatric patients diagnosed with EBV-HLH in Beijing Children's Hospital were recruited. Nucleotide sequences of all exons and their flanking intronic sequences of PRF1, UNC13D, STX11, SH2D1A, XIAP, and ITK were amplified by PCR followed by direct sequencing.

RESULTS

Eight patients were identified with heterozygous, compound heterozygous, or homozygous mutations in PFR1, UNC13D, and XIAP. Three missense mutations (c.83G>A, c.503G>A, c.632C>T) were found in PRF1 of two males and two females. Compound heterozygous c.93C>G and c.1066C>T were found in PRF1 of a 2.5-year-old female. Four different mutations were found in UNC13D of two patients: compound nonsense heterozygous mutations c.766C>T and c.1215C>G were found in one male and two splicing mutations c.1596+1G>C and c.2709+1G>A were found in another male. A heterozygous mutation c.1099+2T>C in XIAP was found in a 4-year-old male. No detrimental mutations were identified in STX11, SH2D1A, or ITK. NK cell activity did not differ between the eight FHL patients and the remaining patients. There was no statistical difference in clinical features and laboratory data for these two subgroups with biallelic and heterozygous mutations.

CONCLUSIONS

Seven novel mutations in PRF1, UNC13D, and XIAP were identified in EBV-HLH patients. Only a fraction of the Chinese children with EBV-HLH have genetic defects in PRF1, UNC13D, and XIAP. There were no gene mutations of PRF1/UNC13D/STX11/SH2D1A/XIAP/ITK in the majority of Chinese child patients with EBV-HLH.

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.

Of mice and men: Different functions of the murine and human 2B4 (CD244) receptor on NK cells

2B4 was initially discovered on murine NK cells and T cells displaying non-MHC dependent cytotoxicity. Human 2B4 was cloned based on sequence homology with mouse 2B4. Recent evidence suggests that the function of this receptor might be different in the two species. Human 2B4 activates NK cell cytotoxicity and interferon gamma production when engaged by CD48, its ligand, on target cells. This activating function of human 2B4 requires recruitment of the SH2 domain containing molecule, SLAM-associated protein or SAP. In the absence of SAP in human NK cells, as occurs in immature NK cells or NK cells from X-linked lymphoproliferative disorder (XLPD) patients, human 2B4 acts as an inhibitory receptor. In contrast, in vitro and in vivo studies using 2B4-deficient mice suggest that the major function of mouse 2B4 is to inhibit murine NK cell functions when triggered by CD48 on target cells, although there are reports of activating function of murine 2B4. This inhibitory function of murine 2B4 is mediated by EAT-2, ERT and possibly other phosphatases like SHP-1 and SHIP. 2B4-SAP interaction in mouse NK cells might be a low affinity one and might not be physiologically relevant considering the inhibitory function of 2B4. This suggests that mouse and human 2B4 diverged functionally with the evolution of greater affinity between 2B4 and SAP in the human species. We speculate that evolutionary pressure from viral infections, possibly EBV, might have led to the emergence of this association and activating function of 2B4 in humans.

Signal transduction in natural killer cells

Tolerance of natural killer (NK) cells toward normal cells is mediated through their expression of inhibitory receptors that detect the normal expression of self in the form of class I major histocompatibility complex (MHC-I) molecules on target cells. These MHC-I-binding inhibitory receptors recruit tyrosine phosphatases, which are believed to counteract activating receptor-stimulated tyrosine kinases. The perpetual balance between signals derived from inhibitory and activating receptors controls NK cell responsiveness and provides an interesting paradigm of signaling cross talk. This review summarizes our knowledge of the intracellular mechanisms by which cell surface receptors influence biological responses by NK cells. Special emphasis focuses on the dynamic signaling events at the NK immune synapse and the unique signaling characteristics of specific receptors, such as NKG2D, 2B4, and KIR2DL4.

Rapid detection of intracellular SH2D1A protein in cytotoxic lymphocytes from patients with X-linked lymphoproliferative disease and their family members

Mutations in the SH2D1A gene have been described in most patients with the clinical syndrome of X-linked lymphoproliferative disease (XLP). The diagnosis of XLP is still difficult given its clinical heterogeneity and the lack of a readily available rapid diagnostic laboratory test, particularly in patients without a family history of XLP. XLP should always be a consideration in males with Epstein-Barr virus-associated hemophagocytic lymphohistiocytosis (EBV-HLH). Four-color flow cytometric analysis was used to establish normal patterns of SH2D1A protein expression in lymphocyte subsets for healthy subjects. Three of 4 patients with XLP, as confirmed by the detection of mutations in the SH2D1A gene, had minimal intracellular SH2D1A protein in all cytotoxic cell types. The remaining patient lacked intracellular SH2D1A protein in CD56+ natural killer (NK) and T lymphocytes and had an abnormal bimodal pattern in CD8+ T cells. Carriers of SH2D1A mutations had decreased SH2D1A protein staining patterns compared with healthy controls. Eleven males with clinical syndromes consistent with XLP, predominantly EBV-HLH, had patterns of SH2D1A protein expression similar to those of healthy controls. Four-color flow cytometry provides diagnostic information that may speed the identification of this fatal disease, differentiating it from other causes of EBV-HLH.

Mice deficient in the X-linked lymphoproliferative disease gene sap exhibit increased susceptibility to murine gammaherpesvirus-68 and hypo-gammaglobulinemia

X-linked lymphoproliferative disease is characterized by immune dysregulation and uncontrolled lymphoproliferation on exposure to Epstein-Barr virus (EBV). This disease has been attributed to mutations in the SAP gene (also denominated as SH2D1A or DSHP). To delineate the role of SAP in the pathophysiology of X-linked lymphoproliferative disease, a strain of sap-deficient mice has been generated by deleting exon 2 of the gene. After infection with murine gammaherpesvirus-68, which is homologous to EBV, the mutant mice exhibit more vigorous CD8+ T cell proliferation and more disseminated lymphocyte infiltration compared to their wild-type littermates. Chronic tissue damage and hemophagocytosis were evident in sap-deficient mice but not in their wild-type littermates. Concordantly, murine gammaherpesvirus-68 reactivation was observed in sap-deficient mice, indicating an impaired control of the virus. Notably, IgE deficiency and decreased serum IgG level were observed in mutant mice prior to and after murine gammaherpesvirus-68 infection, which reproduces hypo-gammaglobulinemia in X-linked lymphoproliferative disease patients. This mouse model will therefore be a useful tool for dissecting the various phenotypes of X-linked lymphoproliferative disease.

The SAP and SLAM families in immune responses and X-linked lymphoproliferative disease

SAP (signalling lymphocytic activation molecule (SLAM)-associated protein) is a T- and natural killer (NK)-cell-specific protein containing a single SH2 domain encoded by a gene that is defective or absent in patients with X-linked lymphoproliferative syndrome (XLP). The SH2 domain of SAP binds with high affinity to the cytoplasmic tail of the haematopoietic cell-surface glycoprotein SLAM and five related receptors. SAP regulates signal transduction of the SLAM-family receptors by recruiting SRC kinases. Similarly, the SAP-related proteins EAT2A and EAT2B are thought to control signal transduction that is initiated by SLAM-related receptors in professional antigen-presenting cells. In this review, we discuss recent findings on the structure and function of proteins of the SAP and SLAM families.