The X-linked lymphoproliferative disease: from autopsy toward cloning the gene 1975-1990

Vasculitis and vasculopathy associated with inborn errors of immunity: an overview

Systemic autoinflammatory diseases (SAIDs) are disorders of innate immunity, which are characterized by unprovoked recurrent flares of systemic inflammation often characterized by fever associated with clinical manifestations mainly involving the musculoskeletal, mucocutaneous, gastrointestinal, and nervous systems. Several conditions also present with varied, sometimes prominent, involvement of the vascular system, with features of vasculitis characterized by variable target vessel involvement and organ damage. Here, we report a systematic review of vasculitis and vasculopathy associated with inborn errors of immunity.

Clinical Case of X-Linked Lymphoproliferative Syndrome Burdened with Hemophagocytic Lymphohistiocytosis and Crohn's Disease

Background. X-linked lymphoproliferative (XLP) syndrome is hereditary disease with the incidence of 1-3 per 1 million born boys. This clinical case demonstrates a rare picture of XLP type 2 manifestation without prior Epstein-Barr virus.

Clinical case description. Boy D., 15 years old, was admitted to Morozovskaya Children's City Hospital with complaints on fever, abdominal pain, loose stools, weight loss. The past medical history included hemophagocytic syndrome (remission) and acute erythema nodosum. We have performed several studies: abdominal ultrasound (hepatomegaly, dynamic changes in the intestine: parts of the small intestine were enlarged and walls were thickened, mass peristalsis, walls of transverse colon and descending colon are thickened up to 5 mm, mesenteric lymphadenopathy), rectosigmoidoscopy (high-activity ulcerative proctosigmoiditis corresponds to Crohn's disease), biochemical and clinical blood tests (active hemophagocytic syndrome), coagulogram (secondary hypocoagulation), myelogram (no data on hemoblastosis or aplastic condition). Virological blood tests (CMV, EBV, HHV-VI): negative. Laboratory and instrumental tests have revealed recurrence of hemophagocytic syndrome and Crohn's disease. The child was consulted by rheumatologist, hematologist, gastroenterologist, geneticist, neurologist, and clinical pharmacologist. The primary immune deficiency disease was suspected in this patient due to his medical history. Molecular genetic study was performed (deletion including the XIAP gene was revealed) and the diagnosis of primary immune deficiency was verified: X-linked lymphoproliferative syndrome type 2. Thus, allogeneic haematopoietic stem cell transplantation (HSCT) was performed.

Conclusion. XLP diagnosis and management require multidisciplinary approach. The early diagnosis is crucial due to the high risk of secondary complications development that can significantly worsen the disease's prognosis. Allogeneic HSCT is the only effective treatment for the disease. 

Targeted sequencing identifies a novel SH2D1A pathogenic variant in a Chinese family: Carrier screening and prenatal genetic testing

X-linked lymphoproliferative disease type 1 (XLP1) is a rare primary immunodeficiency characterized by a clinical triad consisting of severe EBV-induced hemophagocytic lymphohistiocytosis, B-cell lymphoma, and dysgammaglobulinemia. Mutations in SH2D1A gene have been revealed as the cause of XLP1. In this study, a pregnant woman with recurrence history of birthing immunodeficiency was screened for pathogenic variant because the proband sample was unavailable. We aimed to clarify the genetic diagnosis and provide prenatal testing for the family. Next-generation sequencing (NGS)-based multigene panel was used in carrier screening of the pregnant woman. Variants of immunodeficiency related genes were analyzed and prioritized. Candidate variant was verified by using Sanger sequencing. The possible influence of the identified variant was evaluated through RNA assay. Amniocentesis, karyotyping, and Sanger sequencing were performed for prenatal testing. We identified a novel de novo frameshift SH2D1A pathogenic variant (c.251_255delTTTCA) in the pregnant carrier. Peripheral blood RNA assay indicated that the mutant transcript could escape nonsense-mediated mRNA decay (NMD) and might encode a C-terminal truncated protein. Information of the variant led to success prenatal diagnosis of the fetus. In conclusion, our study clarified the genetic diagnosis and altered disease prevention for a pregnant carrier of XLP1.

Variable clinical phenotypes of X-linked lymphoproliferative syndrome in China: Report of five cases with three novel mutations and review of the literature

BACKGROUND

X-linked lymphoproliferative disease (XLP) is a rare life-threatening syndrome. Rapid recognition and definitive diagnosis are critical to improve the prognosis and survival of patients with XLP. Nowadays, little is known about patients with XLP in China.

METHODS

We report the characterization of five Chinese XLP patients with three novel mutations and review the literature related to this syndrome. Male patients with fulminant infectious mononucleosis (FIM), Epstein-Barr virus (EBV)-associated hemophagocytic lymphohistiocytosis (HLH) or persistent EBV viraemia were enrolled in this study. The patients' clinical features were assessed by retrieval of data from medical records. Immunological function included analysis of lymphocyte subsets and the detection of immunoglobulins G, A, M and/or E were evaluated by flow cytometry and nephelometry. Direct sequencing was used to detect SH2D1A/XIAP gene mutations.

RESULTS

Twenty-two male patients with FIM, EBV-associated HLH or persistent EBV viraemia were evaluated among 421 PID patients in our centre. Four patients had SH2D1A mutations, and one patient had an XIAP mutation. The onset age of the 5 patients range from 1month to 4years which was earlier than that in the western world. The diagnosis age was between 16months and 9years with a long diagnosis lag (1-97months). Two of them had positive family history. The clinical phenotypes varied in different patients among which two patients with FHLH and hypogammaglobulinaemia, one with hypogammaglobulinaemia, lymphoma and aplastic anaemia (AA) which is the first case with AA in China, one with hypogammaglobulinaemia only and the other one with FHLH. For immunological function, three exhibited reduced CD4/CD8 ratios. Arg55stop mutations as well as splice mutation in intron 1 were most frequently found and exon 2 was the hottest exon in China. Two patients died at the time of diagnosis for severe infection or hepatic coma. Three were alive and waiting for haematopoietic stem cell transplantation (HSCT).

CONCLUSION

For patients with severe EBV-associated HLH, hypogammaglobulinaemia, lymphoma and aplastic anaemia, possibility of XLP should be considered and if confirmed, HSCT should be performed as soon as possible.

Cerebral Vasculitis in X-linked Lymphoproliferative Disease Cured by Matched Unrelated Cord Blood Transplant

Vasculitis occurs rarely in association with X-linked lymphoproliferative disease (XLP). There are four published cases of non-EBV XLP-associated cerebral vasculitis reported, none of whom have survived without major cognitive impairment.

CASE

A 9-year old boy initially presented aged 5 years with a restrictive joint disease. He subsequently developed dysgammaglobulinemia, episodic severe pneumonitis, aplastic anaemia, gastritis and cerebral vasculitis. A diagnosis of XLP was made, based on flow cytometric analysis and the identification of a novel mutation in SH2D1A, c.96G>C. No peripheral blood lymphocyte clonal proliferation was identified and he was EBV negative, although human herpes virus-7 (HHV7) was detected repeatedly in his cerebrospinal fluid. He underwent a reduced intensity unrelated umbilical cord blood transplant, but failed to engraft. A second 5/6 matched cord gave 100 % donor engraftment. Complications included BK virus-associated haemorrhagic cystitis, a possible NK-cell mediated immune reconstitution syndrome and post-transplant anti-glomerular basement membrane disease, the latter treated with cyclophosphamide and rituximab. At +450 days post-transplant he is in remission from his vasculitis and anti-glomerular basement membrane disease, and HHV-7 has remained undetectable.

CONCLUSION

This is the second published description of joint disease in XLP, and only the fourth case of non-EBV associated cerebral vasculitis in XLP, as well as being the first to be successfully treated for this manifestation. This case raises specific questions about vasculitis in XLP, in particular the potential relevance of HHV-7 to the pathogenesis.

Girls homozygous for an IL-2-inducible T cell kinase mutation that leads to protein deficiency develop fatal EBV-associated lymphoproliferation

The fatal immune dysregulation that sometimes follows EBV infection in boys has been linked to mutations in two X chromosome-encoded genes, SLAM-associated protein (SAP) and X-linked inhibitor of apoptosis (XIAP). In this study we describe 2 girls from a consanguineous Turkish family who died after developing severe immune dysregulation and therapy-resistant EBV-positive B cell proliferation following EBV infection. SNP array-based genome-wide linkage analysis revealed IL-2-inducible T cell kinase (ITK) as a candidate gene for this immunodeficiency syndrome. Both girls harbored a homozygous missense mutation that led to substitution of a highly conserved residue (R335W) in the SH2 domain of ITK. Characteristics of ITK deficiency in mouse models, such as absence of NKT cells and high levels of eomesodermin in CD8+ cells, were seen in either one or both of the girls. Two lines of evidence suggested that R335W caused instability of the ITK protein. First, in silico modeling of the mutant protein predicted destabilization of the SH2 domain. Additionally, Western blot analysis revealed that, unlike wild-type ITK, the R335W mutant was nearly undetectable when expressed in 293 T cells. Our results suggest that ITK deficiency causes what we believe to be a novel immunodeficiency syndrome that leads to a fatal inadequate immune response to EBV. Because ITK deficiency resembles EBV-associated lymphoproliferative disorders in boys, we suggest that this molecular cause should be considered during diagnosis and treatment.

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

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

Familial and acquired hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening condition of severe hyperinflammation caused by the uncontrolled proliferation of activated lymphocytes and histiocytes secreting high amounts of inflammatory cytokines. Cardinal signs and symptoms are prolonged fever, hepatosplenomegaly and pancytopenia. Characteristic biochemical markers include elevated triglycerides, ferritin and low fibrinogen. HLH occurs on the basis of various inherited or acquired immune deficiencies. Impaired function of natural killer (NK) cells and cytotoxic T-cells (CTL) is shared by all forms of HLH. Genetic HLH occurs in familial forms (FHLH) in which HLH is the primary and only manifestation, and in association with the immune deficiencies Chédiak-Higashi syndrome 1 (CHS 1), Griscelli syndrome 2 (GS 2) and x-linked lymphoproliferative syndrome (XLP), in which HLH is a sporadic event. Most patients with acquired HLH have no known underlying immune deficiency. Both acquired and genetic forms are triggered by infections, mostly viral, or other stimuli. HLH also occurs as a complication of rheumatic diseases (macrophage activation syndrome) and of malignancies. Several genetic defects causing FHLH have recently been discovered and have elucidated the pathophysiology of HLH. The immediate aim of therapy in genetic and acquired HLH is suppression of the severe hyperinflammation, which can be achieved with immunosuppressive/immunomodulatary agents and cytostatic drugs. Patients with genetic forms have to undergo stem cell transplantation to exchange the defective immune system with normally functioning immune effector cells. In conclusion, awareness of the clinical symptoms and of the diagnostic criteria of HLH is crucial in order not to overlook HLH and to start life-saving therapy in time.

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

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

Signaling Lymphocyte Activation Molecule-Associated Protein Is a Negative Regulator of the CD8 T Cell Response in Mice

The primary manifestation of X-linked lymphoproliferative syndrome, caused by a dysfunctional adapter protein, signaling lymphocyte activation molecule-associated protein (SAP), is an excessive T cell response upon EBV infection. Using the SAP-/- mouse as a model system for the human disease, we compared the response of CD8+ T cells from wild-type (wt) and mutant mice to various stimuli. First, we observed that CD8+ T cells from SAP-/- mice proliferate more vigorously than those from wt mice upon CD3/CD28 cross-linking in vitro. Second, we analyzed the consequence of SAP deficiency on CTL effector function and homeostasis. For this purpose, SAP-/- and wt mice were infected with the murine gamma-herpesvirus 68 (MHV-68). At 2 wk postinfection, the level of viral-specific CTL was much higher in mutant than in wt mice, measured both ex vivo and in vivo. In addition, we established that throughout 45 days of MHV-68 infection the frequency of virus-specific CD8+ T cells producing IFN-gamma was significantly higher in SAP-/- mice. Consequently, the level of latent infection by MHV-68 was considerably lower in SAP-/- mice, which indicates that SAP-/- CTL control this infection more efficiently than wt CTL. Finally, we found that the Vbeta4-specific CD8+ T cell expansion triggered by MHV-68 infection is also enhanced and prolonged in SAP-/- mice. Taken together, our data indicate that SAP functions as a negative regulator of CD8+ T cell activation.

Primary hemophagocytic syndromes point to a direct link between lymphocyte cytotoxicity and homeostasis

Hemophagocytic syndrome (HS) is a severe and often fatal syndrome resulting from potent and uncontrolled activation and proliferation of T-lymphocytes, leading to excessive macrophage activation and multiple deleterious effects. The onset of HS characterizes several inherited disorders in humans. In each condition, the molecular defect impairs the granule-dependent cytotoxic activity of lymphocytes, thus highlighting the determinant role of this function in driving the immune system to a state of equilibrium following infection. It has also been shown that some of the proteins required for lytic granule secretion are required for melanocyte function, leading to associated hypopigmentation in these conditions. This review focuses on several effectors of this secretory pathway, recently identified, because their defects cause these disorders, and discusses their role and molecular interactions in granule-dependent cytotoxic activity.

Signaling lymphocytic activation molecule-associated protein controls NKT cell functions

X-linked lymphoproliferative disease (XLP) is a fatal immunological disorder that typically manifests following EBV infection. XLP patients exhibit a number of immune defects including abnormal T, B, and NK lymphocyte function. These defects have been attributed to mutations of Src homology 2 domain-containing gene 1A (SH2D1A), the gene encoding signaling lymphocytic activation molecule-associated protein (SAP), an intracellular adaptor molecule expressed in lymphocytes. We have observed that SAP knockout (SAPKO) mice and humans with XLP have a complete lack of CD1d-restricted NKT cells. As expected, SAPKO mice injected with the NKT cell agonist, alpha-galactosylceramide failed to generate NKT cell IFN-gamma or IL-4. Furthermore, in contrast to wild-type littermates, SAPKO mice coinjected with OVA and alpha-galactosylceramide failed to mount OVA-specific CTL responses. These data suggest that an absence of NKT cells may underlie part of the immune dysregulation seen in SAPKO mice and in XLP patients.

X-linked lymphoproliferative syndrome presenting with systemic lymphocytic vasculitis

X-linked lymphoproliferative syndrome (XLP) is a rare, often fatal, primary immunodeficiency disease characterized by an abnormal response to Epstein-Barr virus (EBV) infection. The gene responsible for XLP has been identified as SH2D1A/DSHP/SLAM-associated protein (SAP). The major clinical manifestations include fulminant infectious mononucleosis, lymphoproliferative disorder, and dysgammaglobulinemia. Affected males uncommonly present with lymphocytic vasculitis in addition to aplastic anemia. In this study, we describe a Japanese XLP patient who presented with hypogammaglobulinemia following acute EBV-induced infectious mononucleosis in the infancy and later had systemic lymphocytic vasculitis and hemophagocytic lymphohistiocytosis in the adulthood, which resolved by steroid pulse therapy. The patient's SAP gene was found to harbor a missense mutation (His8Asp), presumably resulting in defective expression of SAP in T cells. Biopsy specimens of lung and skin disclosed that CD8+ T cells predominantly infiltrated vascular vessels. However, immunohistochemical examination showed that EBV-infected cells were not identifiable in the vessels. We propose that T-cell-mediated immune dysregulation in XLP can cause vasculitis by EBV infection-unrelated mechanism.

X-linked lymphoproliferative disease: genetic lesions and clinical consequences

X-linked lymphoproliferative disorder (XLP) was first described almost 30 years ago; remarkably, the three major manifestations of XLP, fulminant infectious mononucleosis (FIM), lymphoma, and dysgammaglobulinemia, are all described in the report of the initial kindred. Subsequent establishment of an XLP registry has led to recognition of more unusual phenotypes in affected males; concurrently, much progress has been made in caring for boys with XLP, including treatment for the three major phenotypes, and curative bone marrow transplantation (BMT). The immunologic and genetic mechanisms resulting in XLP have also been intensively studied. Several years ago, the gene defective in XLP was identified as SAP (SLAM-associated protein), and recent data suggest that SAP plays a broad role in immune signaling. Here, we review the clinical manifestations and therapy of XLP, and briefly summarize recent research into the structure and function of SAP.

Distinct interactions of the X-linked lymphoproliferative syndrome gene product SAP with cytoplasmic domains of members of the CD2 receptor family

X-linked lymphoproliferative syndrome (XLP; Duncan's disease) is a primary immunodeficiency disease that manifests as an inability to regulate the immune response to Epstein-Barr virus (EBV) infection. Here we examine the ability of the product of the gene defective in XLP, SAP (DSHP/SH2D1A), to associate with the cytoplasmic domains of several members of the CD2 subfamily of cell surface receptors, including SLAM, 2B4, and CD84. While recruitment of SAP to SLAM occurred in a phosphorylation-independent manner, SAP was found to bind preferentially to tyrosine-phosphorylated cytoplasmic domains within 2B4 and CD84. Missense or nonsense mutations in the SAP open reading frame were identified in five of seven clinically diagnosed XLP patients from different kindreds. Four of these variants retained the ability to bind to the cytoplasmic tails of SLAM and CD84. While ectopic expression of wild-type SAP was observed to block the binding of SHP-2 to SLAM, mutant SAP derivatives that retained the ability to bind SLAM did not inhibit recruitment of SHP-2 to SLAM. In contrast, SAP binding to CD84 had no effect on the ability of CD84 to recruit SHP-2, but instead displaced SHP-1 from the cytoplasmic tail of CD84. These results suggest that mutations in the gene encoding the XLP protein SAP lead to functional defects in the protein that include receptor binding and SHP-1 and SHP-2 displacement and that SAP utilizes different mechanisms to regulate signaling through the CD2 family of receptors.

SAP controls T cell responses to virus and terminal differentiation of TH2 cells

SH2D1A, which encodes signaling lymphocyte activation molecule (SLAM)-associated protein (SAP), is altered in patients with X-linked lymphoproliferative disease (XLP), a primary immunodeficiency. SAP-deficient mice infected with lymphocytic choriomeningitis virus had greatly increased numbers of CD8+ and CD4+ interferon-gamma-producing spleen and liver cells compared to wild-type mice. The immune responses of SAP-deficient mice to infection with Leishmania major together with in vitro studies showed that activated SAP-deficient T cells had an impaired ability to differentiate into T helper 2 cells. The aberrant immune responses in SAP-deficient mice show that SAP controls several distinct key T cell signal transduction pathways, which explains in part the complexity of the XLP phenotypes.

Lymphocytic vasculitis in X-linked lymphoproliferative disease

Systemic vasculitis is an uncommon manifestation of X-linked lymphoproliferative disease (XLP), a disorder in which there is a selective immune deficiency to Epstein-Barr virus (EBV). The molecular basis for XLP has recently been ascribed to mutations within SLAM-associated protein (SAP), an SH2 domain-containing protein expressed primarily in T cells. The authors describe a patient who died as a result of chronic systemic vasculitis and fulfilled clinical criteria for the diagnosis of XLP. Sequencing of this patient's SAP gene uncovered a novel point mutation affecting the SH2 domain. The patient presented with virus-associated hemophagocytic syndrome (VAHS) and later had chorioretinitis, bronchiectasis, and hypogammaglobulinemia develop. He further developed mononeuritis and fatal respiratory failure. Evidence of widespread small and medium vessel vasculitis was noted at autopsy with involvement of retinal, cerebral, and coronary arteries as well as the segmental vessels of the kidneys, testes, and pancreas. Immunohistochemical analysis using antibodies to CD20, CD45RO, and CD8 revealed that the vessel wall infiltrates consisted primarily of CD8(+) T cells, implying a cytotoxic T-lymphocyte response to antigen. EBV DNA was detected by polymerase chain reaction (PCR) in arterial wall tissue microdissected from infiltrated vessels further suggesting that the CD8(+) T cells were targeting EBV antigens within the endothelium. The authors propose that functional inactivation of the SAP protein can impair the immunologic response to EBV, resulting in systemic vasculitis.

Recurrent B-cell non-Hodgkin's lymphoma in two brothers with X-linked lymphoproliferative disease without evidence for Epstein-Barr virus infection

We present two male siblings suffering from recurrent manifestations of B-cell non-Hodgkin's lymphoma (NHL) and recurrent infections of the lower respiratory tract associated with bronchiectasis. Immunodeficiency could not be demonstrated by any laboratory investigation. In both patients, lymphomas developed without evidence for Epstein-Barr virus (EBV) infection, i.e. no antibody response to EBV-specific antigens, negative EBV-PCR (polymerase chain reaction) in peripheral blood cells, and absence of latent membrane protein (LMP) and EBV-encoded RNA (EBER) in lymphoma cells. Molecular analysis of the SH2D1A, the gene for X-linked lymphoproliferative disease (XLP) led to the identification of a deletion in the first exon in both patients. Therefore, we postulate that the genetic defect and the following dysregulation of the B-/T-cell interaction rendered these patients susceptible to the early onset of B-cell NHL and that EBV infection is not an obligate prerequisite.

Novel mode of ligand binding by the SH2 domain of the human XLP disease gene product SAP/SH2D1A

BACKGROUND

The Src homology 2 (SH2) domains of cytoplasmic signaling proteins generally bind phosphotyrosine (pTyr) sites in the context of carboxy-terminal residues. SAP (also known as SH2D1A or DSHP), the product of the gene that is mutated in human X-linked lymphoproliferative (XLP) disease, comprises almost exclusively a single SH2 domain, which may modulate T-cell signaling by engaging T-cell co-activators such as SLAM, thereby blocking binding of other signaling proteins that contain SH2 domains. The SAP-SLAM interaction can occur in a phosphorylation-independent manner.

RESULTS

To characterize the interaction between SAP and SLAM, we synthesized peptides corresponding to the SAP-binding site at residue Y281 in SLAM. Both phosphorylated and non-phosphorylated versions of an 11-residue SLAM peptide bound SAP, with dissociation constants of 150 nM and 330 nM, respectively. SLAM phosphopeptides that were truncated either at the amino or carboxyl terminus bound with high affinity to SAP, suggesting that the SAP SH2 domain recognizes both amino-terminal and carboxy-terminal sequences relative to the pTyr residue. These results were confirmed by nuclear magnetic resonance (NMR) studies on (15)N- and (13)C-labeled SAP complexed with three SLAM peptides: an amino-terminally truncated phosphopeptide, a carboxy-terminally truncated phosphopeptide and a non-phosphorylated Tyr-containing full-length peptide.

CONCLUSIONS

The SAP SH2 domain has a unique specificity. Not only does it bind peptides in a phosphorylation-independent manner, it also recognizes a pTyr residue either preceded by amino-terminal residues or followed by carboxy-terminal residues. We propose that the three 'prongs' of a peptide ligand (the amino and carboxyl termini and the pTyr) can engage the SAP SH2 domain, accounting for its unusual properties. These data point to the flexibility of modular protein-interaction domains.

Inactivating mutations in an SH2 domain-encoding gene in X-linked lymphoproliferative syndrome

X-linked lymphoproliferative syndrome (XLP) is an inherited immunodeficiency characterized by increased susceptibility to Epstein-Barr virus (EBV). In affected males, primary EBV infection leads to the uncontrolled proliferation of virus-containing B cells and reactive cytotoxic T cells, often culminating in the development of high-grade lymphoma. The XLP gene has been mapped to chromosome band Xq25 through linkage analysis and the discovery of patients harboring large constitutional genomic deletions. We describe here the presence of small deletions and intragenic mutations that specifically disrupt a gene named DSHP in 6 of 10 unrelated patients with XLP. This gene encodes a predicted protein of 128 amino acids composing a single SH2 domain with extensive homology to the SH2 domain of SHIP, an inositol polyphosphate 5-phosphatase that functions as a negative regulator of lymphocyte activation. DSHP is expressed in transformed T cell lines and is induced following in vitro activation of peripheral blood T lymphocytes. Expression of DSHP is restricted in vivo to lymphoid tissues, and RNA in situ hybridization demonstrates DSHP expression in activated T and B cell regions of reactive lymph nodes and in both T and B cell neoplasms. These observations confirm the identity of DSHP as the gene responsible for XLP, and suggest a role in the regulation of lymphocyte activation and proliferation. Induction of DSHP may sustain the immune response by interfering with SHIP-mediated inhibition of lymphocyte activation, while its inactivation in XLP patients results in a selective immunodeficiency to EBV.

Host response to EBV infection in X-linked lymphoproliferative disease results from mutations in an SH2-domain encoding gene

X-linked lymphoproliferative syndrome (XLP or Duncan disease) is characterized by extreme sensitivity to Epstein-Barr virus (EBV), resulting in a complex phenotype manifested by severe or fatal infectious mononucleosis, acquired hypogammaglobulinemia and malignant lymphoma. We have identified a gene, SH2D1A, that is mutated in XLP patients and encodes a novel protein composed of a single SH2 domain. SH2D1A is expressed in many tissues involved in the immune system. The identification of SH2D1A will allow the determination of its mechanism of action as a possible regulator of the EBV-induced immune response.

Epstein-Barr virus-associated hemophagocytic syndrome and fatal infectious mononucleosis

Virus-associated hemophagocytic syndrome (VAHS) has been thought to be a distinct clinical entity, characterized by intermittent fever, enlarged liver and spleen, and the appearance of hemophagocytosis. Hemopoietic cells are actively ingested by monocytes/macrophages in various organs, including lymph nodes, bone marrow, liver, and spleen. Epstein-Barr virus (EBV) is now thought to be one of the major causes for the development of this unique syndrome. Additionally, VAHS is often associated with fatal infectious mononucleosis (IM). The relationship between EBV-associated VAHS and fatal IM is discussed in this concise review.

Retinal necrosis in X-linked lymphoproliferative disease

BACKGROUND

X-linked lymphoproliferative disease is a hereditary disease that renders the males susceptible to fatal infectious mononucleosis, aplastic anemia, malignant lymphoma, and hypogammaglobulinemia after infection by the Epstein-Barr virus.

METHODS

The authors examined the clinical and pathologic findings in an 8-year-old boy with bilateral retinal necrosis who died with aplastic anemia as a complication of X-linked lymphoproliferative disease.

RESULTS

Results of histopathologic examination of the eyes disclosed retinal necrosis, and examination using the polymerase chain reaction technique showed Epstein-Barr virus genomic DNA in the left eye.

CONCLUSION

Retinal necrosis may be part of the expanding spectrum of X-linked lymphoproliferative disease. It is unknown if the retinal necrosis is due directly to Epstein-Barr virus infection or a host inflammatory response.

Evaluation of families wherein a single male manifests a phenotype of X-linked lymphoproliferative disease (XLP)

The Epstein-Barr virus (EBV)-induced diseases of males with X-linked lymphoproliferative disease (XLP) include fatal infectious mononucleosis (IM), non-Hodgkin lymphoma (ML), agammaglobulinemia, and aplastic anemia. These phenotypes also occur as sporadic cases in families, and EBV seronegative males in these families must be considered at risk for XLP until they seroconvert normally to EBV. Given that 50% of males inheriting the defective XLP gene die following primary EBV infection, it is vital that they be identified pre-EBV infection. Here we report results using molecular genetic techniques to provide information as to the relative risks of EBV negative males and potential carrier females in ten families wherein a single male had died of IM.