X-linked lymphoproliferative syndrome presenting with systemic lymphocytic vasculitis

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.

The Natural History of X-Linked Lymphoproliferative Disease (XLP1): Lessons from a Long-Term Survivor

X-linked lymphoproliferative disease (XLP1) is a rare primary immunodeficiency characterized by EBV-triggered immune dysregulation, lymphoproliferation, dysgammaglobulinemia, and lymphoma. Early childhood mortality from overwhelming inflammation is expected in most patients. The only curative therapy is hematopoietic stem cell transplant (HSCT); however, whether to perform HSCT on an asymptomatic patient remains debatable. This uncertainty arises because the natural history of XLP1 patients without transplantation is not clear. In this case report, we present the natural history of XLP1 in a 43-year-old male patient who did not receive HSCT. We also review the literature on untransplanted XLP1 patients who lived into mid-adulthood. Despite surviving childhood presentations that are typically fatal, we found that these rare patients remain susceptible to manifestations of XLP1 decades later.

X-linked inhibitor of apoptosis protein deficiency complicated with Crohn's disease-like enterocolitis and Takayasu arteritis: A case report

X-linked inhibitor of apoptosis protein (XIAP) deficiency results in monogenic inflammatory bowel disease. To date, no vasculitis associated with XIAP deficiency has been reported. A 10-year-old boy was diagnosed with Crohn's disease and he responded poorly to conventional treatment for Crohn's disease. He was dependent on corticosteroids and parenteral nutrition. To manage severe colitis, he underwent ileostomy followed by ileocolectomy for an ileo-sigmoid fistula. At the age of 15 years, he developed IgA vasculitis and at the age of 17 years, he developed refractory Takayasu arteritis (TAK), which was resistant to corticosteroid and immunosuppressive therapy. Whole-exome sequencing revealed a novel mutation of the splice acceptor site in XIAP (c.1057-1G > A) at the age of 19 years. Allogeneic hematopoietic stem cell transplantation was successful with subsequent withdrawal of intensive immunosuppressive therapy and clinical remission of both enterocolitis and TAK. This case suggests that patients with XIAP deficiency could develop intractable inflammatory disease involving the intestinal tract and blood vessels.

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.

Late-Onset Non-HLH Presentations of Growth Arrest, Inflammatory Arachnoiditis, and Severe Infectious Mononucleosis, in Siblings with Hypomorphic Defects in UNC13D

Bi-allelic null mutations affecting UNC13D, STXBP2, or STX11 result in defects of lymphocyte cytotoxic degranulation and commonly cause familial hemophagocytic lymphohistiocytosis (FHL) in early life. Patients with partial loss of function are increasingly being diagnosed after presenting with alternative features of this disease, or with HLH later in life. Here, we studied two sisters with lymphocyte degranulation defects secondary to compound heterozygote missense variants in UNC13D. The older sibling presented aged 11 with linear growth arrest and delayed puberty, 2 years prior to developing transient ischemic attacks secondary to neuroinflammation and hypogammaglobulinemia, but no FHL symptoms. Her geno-identical younger sister was initially asymptomatic but then presented at the same age with severe EBV-driven infectious mononucleosis, which was treated aggressively and did not progress to HLH. The sisters had similar natural killer cell degranulation; however, while cytotoxic activity was moderately reduced in the asymptomatic patient, it was completely absent in both siblings during active disease. Following allogeneic bone marrow transplantation at the age of 15, the older child has completely recovered NK cell cytotoxicity, is asymptomatic, and has experienced an exceptional compensatory growth spurt. Her younger sister was also successfully transplanted and is currently disease free. The current study reveals previously unappreciated manifestations of FHL in patients who inherited hypomorphic gene variants and also raises the important question of whether a threshold of minimum NK function can be defined that should protect a patient from serious disease manifestations such as HLH.

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.

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.

Clinical features and outcome of X-linked lymphoproliferative syndrome type 1 (SAP deficiency) in Japan identified by the combination of flow cytometric assay and genetic analysis

OBJECTIVE

X-linked lymphoproliferative syndrome (XLP) type 1 is a rare immunodeficiency, which is caused by mutations in SH2D1A gene. The prognosis of XLP is very poor, and hematopoietic stem cell transplantation (HSCT) is the only curative therapy. We characterized the clinical features and outcome of Japanese patients with XLP-1.

METHODS

We used a combination of flow cytometric analysis and genetic analysis to identify XLP-1 and reviewed the patient characteristics and survival with HSCT.

RESULTS

We identified 33 patients from 21 families with XLP-1 in Japan. Twenty-one of the patients (65%) who did not undergo a transplant died of the disease and complications. Twelve patients underwent HSCT, and 11 of these (92%) survived.

CONCLUSION

We described the clinical characteristics and outcomes of Japanese patients with XLP-1, and HSCT was the only curative therapy for XLP-1. The rapid and accurate diagnosis of XLP with the combination of flow cytometric assay and genetic analysis is important.

SAP and XIAP deficiency in hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is a multisystem inflammatory disorder due to cytokine overproduction from excessively activated lymphocytes and macrophages. HLH has been divided into two subgroups: primary HLH and secondary HLH. Primary HLH includes PRF1, UNC13D, STX11, STXBP2, RAB27A, LYST, SH2D1A and XIAP gene mutations; and secondary HLH is associated with infections, malignancies and autoimmune diseases. Among primary HLH-related genes, SH2D1A and XIAP are genetically responsible for X-linked lymphoproliferative syndrome (XLP) due to signaling-lymphocytic-activation-molecule-associated protein (SAP) and XIAP deficiencies, respectively. XLP is characterized by extreme vulnerability to Epstein-Barr virus infection. The major clinical manifestations of XLP consist of HLH (60%), lymphoproliferative disorder (30%) and dysgammaglobulinemia (30%). Analysis of clinical phenotypes of XLP patients suggests that XLP predominantly shows familial HLH phenotypes, whereas some XLP patients present sporadic HLH. For many decades, clinicians and investigators have been concerned with possible XLP in young boys presenting with Epstein-Barr-virus-associated HLH. This review aims to describe the new knowledge about XLP and to draw the attention of the pediatrician to XLP, which should be differentiated from other forms of HLH.

Familial hemophagocytic lymphohistiocytosis: a model for understanding the human machinery of cellular cytotoxicity

Cytotoxic T lymphocytes, natural killer cells, and NKT cells are effector cells able to kill infected cells. In some inherited human disorders, a defect in selected proteins involved in the cellular cytotoxicity mechanism results in specific clinical syndromes, grouped under the name of familial hemophagocytic lymphohistiocytosis. Recent advances in genetic studies of these patients has allowed the identification of different genetic subsets. Additional genetic immune deficiencies may also induce a similar clinical picture. International cooperation and prospective trials resulted in refining the diagnostic and therapeutic approach to these rare diseases with improved outcome but also with improved knowledge of the mechanisms underlying granule-mediated cellular cytotoxicity in humans.

X-linked lymphoproliferative disease due to SAP/SH2D1A deficiency: a multicenter study on the manifestations, management and outcome of the disease

X-linked lymphoproliferative disease (XLP1) is a rare immunodeficiency characterized by severe immune dysregulation and caused by mutations in the SH2D1A/SAP gene. Clinical manifestations are varied and include hemophagocytic lymphohistiocytosis (HLH), lymphoma and dysgammaglobulinemia, often triggered by Epstein-Barr virus infection. Historical data published before improved treatment regimens shows very poor outcome. We describe a large cohort of 91 genetically defined XLP1 patients collected from centers worldwide and report characteristics and outcome data for 43 patients receiving hematopoietic stem cell transplant (HSCT) and 48 untransplanted patients. The advent of better treatment strategies for HLH and malignancy has greatly reduced mortality for these patients, but HLH still remains the most severe feature of XLP1. Survival after allogeneic HSCT is 81.4% with good immune reconstitution in the large majority of patients and little evidence of posttransplant lymphoproliferative disease. However, survival falls to 50% in patients with HLH as a feature of disease. Untransplanted patients have an overall survival of 62.5% with the majority on immunoglobulin replacement therapy, but the outcome for those untransplanted after HLH is extremely poor (18.8%). HSCT should be undertaken in all patients with HLH, because outcome without transplant is extremely poor. The outcome of HSCT for other manifestations of XLP1 is very good, and if HSCT is not undertaken immediately, patients must be monitored closely for evidence of disease progression.

Using flow cytometry to screen patients for X-linked lymphoproliferative disease due to SAP deficiency and XIAP deficiency

X-linked lymphoproliferative disease is a rare congenital immunodeficiency that is most often caused by mutations in SH2D1A, the gene encoding signaling lymphocyte activation molecule (SLAM)-associated protein (SAP). XLP caused by SAP deficiency is most often characterized by fulminant mononucleosis/EBV- associated hemophagocytic lymphohistiocytosis (HLH), lymphoma, and dysgammaglobulinemia. XLP has also been found to be caused by mutations in BIRC4, the gene encoding X-linked inhibitor of apoptosis (XIAP). Patients with XIAP deficiency often present with HLH or recurrent HLH, which may or may not be associated with EBV. XLP is prematurely lethal in the majority of cases. While genetic sequencing can provide a genetic diagnosis of XLP, a more rapid means of diagnosis of XLP is desirable. Rapid diagnosis is especially important in the setting of HLH, as this may hasten the initiation of life-saving medical treatments and expedite preparations for allogeneic hematopoietic cell transplantation (HCT). Flow cytometry offers a means to quickly screen patients for XLP. Flow cytometry can be used to measure lymphocyte SAP or XIAP protein expression, and can also be used to observe lymphocyte phenotypes and functional defects that are unique to XLP. This review will give a brief overview of the clinical manifestations and molecular basis of SAP deficiency and XIAP deficiency, and will focus on the use of flow cytometry for diagnosis of XLP.

X-linked lymphoproliferative syndromes: brothers or distant cousins?

X-linked lymphoproliferative disease (XLP1), described in the mid-1970s and molecularly defined in 1998, and XLP2, reported in 2006, are prematurely lethal genetic immunodeficiencies that share susceptibility to overwhelming inflammatory responses to certain infectious triggers. Signaling lymphocytic activation molecule-associated protein (SAP; encoded by SH2D1A) is mutated in XLP1, and X-linked inhibitor of apoptosis (XIAP; encoded by BIRC4) is mutated in XLP2. XLP1 is a disease with multiple and variable clinical consequences, including fatal hemophagocytic lymphohistiocytosis (HLH) triggered predominantly by Epstein-Barr virus, lymphomas, antibody deficiency, and rarer consequences of immune dysregulation. To date, XLP2 has been found to cause HLH with and without exposure to Epstein-Barr virus, and HLH is commonly recurrent in these patients. For both forms of XLP, the only curative therapy at present is allogeneic hematopoietic cell transplantation. Beyond their common X-linked locus and their requirement for normal immune responses to certain viral infections, SAP and XIAP demonstrate no obvious structural or functional similarity, are not coordinately regulated with respect to their expression, and do not appear to directly interact. In this review, we describe the genetic, clinical, and immunopathologic features of these 2 disorders and discuss current diagnostic and therapeutic strategies.

Lymphocytic vasculitis involving the central nervous system occurs in patients with X-linked lymphoproliferative disease in the absence of Epstein-Barr virus infection

X-linked lymphoproliferative disease (XLP) is an immunodeficiency caused by defects in the adaptor molecule SAP. The manifestations of XLP generally occur following Epstein-Barr virus (EBV) infection and include fulminant mononucleosis, hypogammaglobulinemia and lymphoma. In this report, we describe two unrelated patients with fatal T-cell-mediated central nervous system vasculitis for whom repeated serologic and molecular testing for EBV was negative. In both patients, clonal T-cell populations were observed, but neither demonstrated evidence of lymphoma. Thus, loss of SAP function can lead to dysregulated immune responses characterized by the uncontrolled expansion and activation of T cells independent of EBV infection.

An atypical case of X-linked lymphoproliferative disease revealed as a late cerebral lymphoma

X-linked lymphoproliferative disease (XLP) is an inherited immunodeficiency, partially characterized by a defect in cytotoxicity to Epstein-Barr virus. This viral infection is therefore often fatal in affected boys, whilst a variety of immune disorders or proliferative diseases may occur in surviving patients. We report an atypical case of a 41year-old male who presented with a primitive B-cell cerebral lymphoma, revealing an XLP. This presentation was unusual because of its late onset, the broad spectrum of the familial characteristics, its initial presentation as a cerebral lymphoma, and the occurrence of B-cell alymphocytosis associated with a-gamma-globulinemia.

[Late onset of primary immune deficiencies]

Primary immune deficiencies (PID) are characterized by a failure of the immune system that is not explained by any infectious, neoplastic, or iatrogenic cause. The diagnosis of PID should be considered in cases of severe or recurrent infections but also in cases with granulomatosis, autoimmune diseases, hemophagocytic syndrome, lymphoproliferative disorders, or even some solid tumors. The onset of PID may be late, most often in adulthood. Nonetheless, late onset may also mean in the first years rather than months of life or in adolescence rather than early childhood. In adults, the diagnosis of PID cannot be considered before acquired immunodeficiencies--far more frequent--are ruled out. Factors affecting the late onset of PID are not known.

Follicular lymphoma in a X-linked lymphoproliferative syndrome carrier female

X-linked lymphoproliferative (XLP) syndrome is a rare primary immune-deficiency disorder caused by mutations of the SH2D1A or XIAP genes. Males with the disorder are usually in good health until contracting Epstein-Barr virus (EBV) whereupon the majority of patients die from fulminant infectious mononucleosis, lymphoma or hypogammaglobulinaemia. This report describes a female carrier with an XLP phenotype who was retrospectively identified after her grandson died from the disorder. Subsequent genetic testing identified the patient's mother and affected maternal grandmother as XLP carriers. The family's medical records were significant. The proband had lymphoma at ages 2 and 8 and made a full recovery following treatment. Both the maternal grandmother and uncle died of non-Hodgkin's lymphoma. We were concerned that the XLP carrier mother may be predisposed to lymphoma if the normal X chromosome is skewed towards inactivation. The human androgen receptor assay detected random X chromosome inactivation in the carrier mother. EBV was not detected in the lymphoma tissues of the proband and his grandmother, confirming previous findings that EBV is not always associated with lymphoma in XLP. More significantly, our study highlights the importance of identifying XLP in families with a high incidence of lymphoma.

Requirements for CD8 T-cell migration into the human arterial wall

Atherosclerotic lesions develop in the arterial intima. Among the leukocytes that accumulate in advanced atherosclerotic plaques, CD8 T cells play a quantitatively important role. They may be involved in disease progression and plaque destabilization, leading to plaque rupture or erosion. These events finally precipitate cardiovascular events. Therefore, we wished to determine the accessibility of the human arterial wall, particularly the arterial intima, for CD8-positive, cytotoxic T lymphocytes. We quantified the number of CD8-positive T cells in the arterial wall using human arterial tissue microarrays. The conditions for efficient cytotoxic T-lymphocyte migration into the arterial wall were determined in an in vitro tissue invasion assay. The invasion pattern of resting or activated cytotoxic T-lymphocyte clones was morphometrically analyzed by confocal microscopy. CD8 T cells represented up to 50% of the lymphocytes in advanced atherosclerotic lesions. Resting CD8-positive cytotoxic T lymphocytes were able to migrate into the arterial intima when it was affected by advanced lesions but not at the earliest stages of the disease. After T-cell receptor and/or proinflammatory cytokine activation, cytotoxic T lymphocytes migrated efficiently into the arterial intima, even in the healthy or mildly affected sites. This in vitro tissue invasion assay mimics conditions under which effector cytotoxic T lymphocytes migrate into the arterial wall to reach similar cell densities as observed in arterial tissue sections from autopsies. Interference with T-cell activation may be important to inhibit cytotoxic T-lymphocyte invasion into the unaffected, healthy artery but may not prevent cytotoxic T-lymphocyte invasion into arteries that are severely affected by atherosclerotic lesions.

Diagnostic des déficits immunitaires primitifs à l’âge adulte

The molecular bases of approximately one hundred primary immune deficiencies (PID) have been identified over the last 15 years. In adults, the diagnosis of PID cannot be evoked before ruling out acquired immunodeficiencies, which are far more frequent. The search for specific PIDs may be oriented by the type of agent responsible for severe and/or recurrent infection. More rarely, other clinical manifestations such as granulomatosis, autoimmune manifestations, hemophagocytic syndrome, lymphoproliferation, or solid tumors may also lead to the identification of PID.

A case of Pneumocystis jiroveci pneumonia in X-linked agammaglobulinaemia treated with immunosuppressive therapy: a lesson for immunologists

The case of a 20-year-old patient, affected by X-linked agammaglobulinaemia (XLA), who developed severe pneumonia from Pneumocystis jiroveci (formerly Pneumocystis carinii) (PCP), is reported. This infection usually affects patients with AIDS, children affected by severe combined immunodeficiency or hypogammaglobulinaemia with hyperimmunoglobulin M, or patients undergoing severe immunosuppression. The XLA patient developed PCP during therapy with steroids and cyclosporine A, carried out for several months, due to an extended skin vasculitis, accompanied by general symptoms. The pneumonia had a severe clinical course, requiring a long hospitalization. At the diagnosis of PCP, immunosuppressive therapy was suspended and the patient recovered after a long-term trimethoprim/sulfamethoxazole therapy. Immunological studies revealed an unexpected normal number of CD4+ and CD8+ T cells. The two subsets had an exclusive naïve phenotype (95% CD4+CD45RA+CD62L+ and 89% CD8+CD45RA+CD62L+ cells), with an absence of primed cells. Lymphoproliferative responses to P. carinii and recall antigens as well as to mitogens were extremely deficient. During the follow-up, memory cells appeared with recovery of the lymphoproliferative response to mitogens and maintained defective responses to antigens. This is one of the few reported XLA cases experiencing severe PCP. In this patient, the infection became clinically evident during immunosuppressive therapy. We believe that the absence of functional activities, despite a normal level of T lymphocyte counts, sustained this long-lasting infection. Thus, the CD4+ and CD8+ T cell count evaluation, without functional studies, may not be per se sufficient for predicting the risk of a severe clinical course of PCP in patients undergoing immunosuppression.

X-Linked Lymphoproliferative Disease in an Adult

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

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.