X-linked Inhibitor Of Apoptosis Deficiency Research Articles

Caspase-8-driven apoptotic and pyroptotic crosstalk causes cell death and IL-1β release in X-linked inhibitor of apoptosis (XIAP) deficiency.

Genetic lesions in X-linked inhibitor of apoptosis (XIAP) pre-dispose humans to cell death-associated inflammatory diseases, although the underlying mechanisms remain unclear. Here, we report that two patients with XIAP deficiency-associated inflammatory bowel disease display increased inflammatory IL-1β maturation as well as cell death-associated caspase-8 and Gasdermin D (GSDMD) processing in diseased tissue, which is reduced upon patient treatment. Loss of XIAP leads to caspase-8-driven cell death and bioactive IL-1β release that is only abrogated by combined deletion of the apoptotic and pyroptotic cell death machinery. Namely, extrinsic apoptotic caspase-8 promotes pyroptotic GSDMD processing that kills macrophages lacking both inflammasome and apoptosis signalling components (caspase-1, -3, -7, -11 and BID), while caspase-8 can still cause cell death in the absence of both GSDMD and GSDME when caspase-3 and caspase-7 are present. Neither caspase-3 and caspase-7-mediated activation of the pannexin-1 channel, or GSDMD loss, prevented NLRP3 inflammasome assembly and consequent caspase-1 and IL-1β maturation downstream of XIAP inhibition and caspase-8 activation, even though the pannexin-1 channel was required for NLRP3 triggering upon mitochondrial apoptosis. These findings uncouple the mechanisms of cell death and NLRP3 activation resulting from extrinsic and intrinsic apoptosis signalling, revealhow XIAP loss can co-opt dual cell death programs, and uncover strategies for targeting the celldeath and inflammatory pathways that result from XIAP deficiency.

X-LINKED INHIBITOR OF APOPTOSIS DEFICIENCY IN A PATIENT WITH ADULT-ONSET RECURRENT FEBRILE ILLNESSES

<h3>Introduction</h3> X-linked inhibitor of apoptosis (XIAP) deficiency is a primary immunodeficiency caused by a mutation in the gene XIAP. It is commonly associated with hemophagocytic lymphohistiocytosis (HLH), recurrent splenomegaly, and inflammatory bowel disease (IBD), as well as hypogammaglobulinemia. We report a case of an adult with late diagnosis of XIAP deficiency in the setting of recurrent febrile illnesses. <h3>Case Description</h3> An 18-year-old Caucasian male presented with fever and generalized lymphadenopathy without other localizing symptoms, which was attributed to cytomegalovirus infection. He recovered without specific treatment, but throughout adulthood he had recurrent infections including pneumonia, appendicitis, cholecystitis, epididymitis, sinusitis, and aseptic meningitis. These infections were often associated with high fever, transaminase elevation, and inflammatory marker elevation. At age 41 he demonstrated persistent hypogammaglobulinemia (IgG 610-635, reference range 767-1590 mg/dl; IgM 28-33, reference range 37-286 mg/dl), elevated antinuclear antibodies, and poor Pneumovax response (adequate response to serotypes 9V, 2, 6B). He denied family history of similar issues. Common variable immunodeficiency (CVID) was considered, but subsequent genetic testing showed a pathologic variant in XIAP gene (c.712C>T/p.Arg238*), leading to a diagnosis of XIAP deficiency. The patient was started on IVIG and Anakinra with good response to treatment. <h3>Discussion</h3> This case highlights the importance of considering genetic testing even in adults with delayed onset of symptoms. The clinical phenotype and presentation of XIAP deficiency is variable and can often mimic more common diseases such as CVID. Even in patients without a history of IBD or HLH, genetic testing to detect XIAP deficiency should still be considered given its variable presentation.

Immune-mediated inflammatory diseases with chronic excess of serum interleukin-18.

Review: Interleukin-18 (IL-18) is a proinflammatory cytokine that promotes various innate immune processes related to infection, inflammation, and autoimmunity. Patients with systemic juvenile idiopathic arthritis and adult-onset Still’s disease exhibit chronic excess of serum IL-18, which is associated with a high incidence of macrophage activation syndrome (MAS), although the mechanisms of IL-18 regulation in such diseases remain largely unknown. Similar elevation of serum IL-18 and susceptibility to MAS/hemophagocytic lymphohistiocytosis (HLH) have been reported in monogenic diseases such as X-linked inhibitor of apoptosis deficiency (i.e., X-linked lymphoproliferative syndrome type 2) and NLRC4-associated autoinflammatory disease. Recent advances in molecular and cellular biology allow the identification of other genetic defects such as defects in CDC42, PSTPIP1, and WDR1 that result in high serum IL-18 levels and hyperinflammation. Among these diseases, chronic excess of serum IL-18 appears to be linked with severe hyperinflammation and/or predisposition to MAS/HLH. In this review, we focus on recent findings in inflammatory diseases associated with and probably attributable to chronic excess of serum IL-18 and describe the clinical and therapeutical relevance of understanding the pathology of this group of diseases.

Primary Immunodeficiencies Associated With Early-Onset Inflammatory Bowel Disease in Southeast and East Asia.

BackgroundCauses of early-onset inflammatory bowel disease (IBD) vary, and primary immunodeficiency diseases (PIDs) are associated with early-onset IBD as monogenic disorders.AimThis review investigates the prevalence, clinical manifestation, genetic profile, and treatment of patients with early-onset IBD in Southeast and East Asia.MethodsA systemic review of articles reporting PID patients associated with early-onset IBD in Southeast and East Asia was conducted.ResultsThe prevalence of PID associated with IBD was higher than that reported in western nations, and the frequency of patients with bloody stools as an early symptom was relatively higher in monogenic diseases. A total 13 (12.0%) of 108 patients with early-onset IBD were diagnosed as PID by exome sequencing and targeted gene panel analysis in Japan, including four patients with XIAP, three with IL10RA, and two or one patient with other gene mutations. In addition, ten patients were reported as having IL-10 receptor alpha (IL-10RA) deficiency in China and Hong Kong. Allogeneic hematopoietic stem cell transplantation was performed in patients with X-linked inhibitor of apoptosis deficiency, IL-10RA deficiency, or other PID as a curative treatment, and the preferable outcome of reduced-intensity conditioning and complete resolution of IBD symptoms and dysbiosis were achieved.ConclusionComprehensive molecular diagnosis has been widely applied to screen for patients with PID-associated IBD in Southeast and East Asia. These results contributed to the awareness of monogenic PID in early-onset IBD patients and their differences in clinical manifestations and genetic profiles compared to the patients in western counties.

Phenotype, genotype, treatment, and survival outcomes in patients with X-linked inhibitor of apoptosis deficiency

X-linked inhibitor of apoptosis (XIAP) deficiency is a rare primary immunodeficiency disease caused by XIAP gene mutations. Abroad range of phenotype, severity, and age at onset present challenges for patient management. We sought to characterize the phenotype, treatment, and survival outcomes of XIAP deficiency and to assess parameters influencing prognosis. Data published from 2006 to 2020 were retrospectively analyzed. A total of 167 patients from 117 families with XIAP deficiency were reported with 90 different mutations. Awide spectrum of clinical features were seen, of which hemophagocytic lymphohistiocytosis (HLH) and inflammatory bowel disease were the most common. Patients frequently developed multiple features with no clear genotype-phenotype correlation. Atotal of 117 patients were managed conservatively and 50 underwent hematopoietic stem-cell transplantation (HSCT), with respective overall survival probabilities of 90% and 53% at age 16 years. The predominant indication for HSCT was early-onset HLH. Active HLH and myeloablative conditioning regimens increased HSCT-related mortality, although HSCT outcome was much better after 2015 than before. For conservatively managed patients reaching adulthood, survival probabilities were 86% at age 30 years and 37% by age 52 years, with worse outcomes for patients developing the disease before the age of 5 years or with new disease features in adulthood. Nine asymptomatic mutation carriers with a median age of 13.5 years were identified. Our study demonstrates the variable nature of XIAP deficiency, which evolves over life for individual patients. Better therapeutic strategies and prospective studies are required to reduce morbidity and mortality and improve decision making and long-term outcomes for patients with XIAP deficiency.

Reduced-Intensity/Reduced-Toxicity Conditioning Approaches Are Tolerated in XIAP Deficiency but Patients Fare Poorly with Acute GVHD.

X-linked inhibitor of apoptosis (XIAP) deficiency is an inherited primary immunodeficiency characterized by chronic inflammasome overactivity and associated with hemophagocytic lymphohistiocytosis (HLH) and inflammatory bowel disease (IBD). Allogeneic hematopoietic cell transplantation (HCT) with fully myeloablative conditioning may be curative but has been associated with poor outcomes. Reports of reduced-intensity conditioning (RIC) and reduced-toxicity conditioning (RTC) regimens suggest these approaches are well tolerated, but outcomes are not well established. Retrospective data were collected from an international cohort of 40 patients with XIAP deficiency who underwent HCT with RIC or RTC. Thirty-three (83%) patients had a history of HLH, and thirteen (33%) patients had IBD. Median age at HCT was 6.5 years. Grafts were from HLA-matched (n = 30, 75%) and HLA-mismatched (n = 10, 25%) donors. There were no cases of primary graft failure. Two (5%) patients experienced secondary graft failure, and three (8%) patients ultimately received a second HCT. Nine (23%) patients developed grade II–IV acute GVHD, and 3 (8%) developed extensive chronic GVHD. The estimated 2-year overall and event-free survival rates were 74% (CI 55–86%) and 64% (CI 46–77%), respectively. Recipient and donor HLA mismatch and grade II–IV acute GVHD were negatively associated with survival on multivariate analysis with hazard ratios of 5.8 (CI 1.5–23.3, p = 0.01) and 8.2 (CI 2.1–32.7, p < 0.01), respectively. These data suggest that XIAP patients tolerate RIC and RTC with survival rates similar to HCT of other genetic HLH disorders. Every effort should be made to prevent acute GVHD in XIAP-deficient patients who undergo allogeneic HCT.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10875-021-01103-6.

Evolution of Our Understanding of XIAP Deficiency.

X-linked inhibitor of apoptosis (XIAP) deficiency is a rare inborn error of immunity first described in 2006. XIAP deficiency is characterised by immune dysregulation and a broad spectrum of clinical manifestations, including haemophagocytic lymphohistiocytosis (HLH), inflammatory bowel disease (IBD), hypogammaglobulinemia, susceptibility to infections, splenomegaly, cytopaenias, and other less common autoinflammatory phenomena. Since the first description of the disease, many XIAP deficient patients have been identified and our understanding of the disease has grown. Over 90 disease causing mutations have been described and more inflammatory disease manifestations, such as hepatitis, arthritis, and uveitis, are now well-recognised. Recently, following the introduction of reduced intensity conditioning (RIC), outcomes of allogeneic haematopoietic stem cell transplantation (HSCT), the only curative treatment option for XIAP deficiency, have improved. The pathophysiology of XIAP deficiency is not fully understood, however it is known that XIAP plays a role in both the innate and adaptive immune response and in immune regulation, most notably through modulation of tumour necrosis factor (TNF)-receptor signalling and regulation of NLRP3 inflammasome activity. In this review we will provide an up to date overview of both the clinical aspects and pathophysiology of XIAP deficiency.

Recurrent lymphadenitis in a female XIAP/BIRC4 mutation carrier with normal lyonization

Deficiency of X-linked inhibitor of apoptosis (XIAP) caused by mutations in XIAP (also known as BIRC4) is a cause of X-linked lymphoproliferative disease,1 characterized by Epstein Barr virus (EBV)-associated hemophagocytic lymphohistiocytosis and hypogammaglobulinemia. Patients with XIAP deficiency also suffer from increased risk of inflammatory conditions such as inflammatory bowel disease (IBD), liver disease, uveitis, erythema nodosum, and arthritis.2-4 Here, we describe a female carrier of an incidentally found pathogenic XIAP mutation with normal lyonization, and her asymptomatic brother.

Haploidentical Hematopoietic Stem Cell Transplantation for XIAP Deficiency: a Single-Center Report.

X-linked inhibitor of apoptosis (XIAP) deficiency caused by mutations in the XIAP/BIRC4 gene is a rare inherited primary immunodeficiency also known as X-linked lymphoproliferative syndrome type 2 (XLP2). Hematopoietic stem cell transplantation (HSCT) is currently the only curative strategy available. However, few studies of haploidentical HSCT have been published regarding the outcomes in patients with this syndrome. We evaluated the XIAP gene analysis and clinical characteristics of four Chinese patients with XIAP who underwent haploidentical HSCT. The mutations in the two of four patients had not yet been reported in the literature. All of the patients had recurrent hemophagocytic lymphohistiocytosis but did not have a good matched donor and underwent haploidentical HSCT at BCH in China between September 2016 and December 2018. All four patients received antithymocyte globulin with fludarabine-based regimens. Two patients underwent reduced intensity conditioning (RIC), and the other two received modified myeloablative conditioning (MAC) regimens. Three of the four patients survived. Three patients experienced complications with mixed chimerism. One of the four patients who underwent RIC had early graft loss and then developed grade IV acute graft-versus-host disease (GVHD) after donor lymphocyte infusion with bone marrow. The two patients who received MAC survived with no or mild GVHD, even though one of them developed hepatic veno-occlusive disease in the early stage of transplantation. Haploidentical HSCT may be a treatment option for patients with XIAP deficiency who lack a good matched donor. More studies are needed to determine whether modified MAC with reduced toxicity is more suitable for haploidentical transplantation.

A Novel X-Linked Inhibitor of Apoptosis Deficient Variant Showing Attenuated Epstein-Barr Virus Response.

We report on 2 Asian siblings with X-linked inhibitor of apoptosis deficiency that arose from a novel deletion that presented with Epstein-Barr virus disease and hemophagocytic lymphohistiocytosis. This disease is ascribed to dysfunction in the nucleotide binding and oligomerization domain receptor pathway, tested using a modified muramyl dipeptide-mediated assay.

Role of Disease Mechanism in Hematopoietic Cell Transplantation Outcomes for Hemophagocytic Lymphohistiocytosis

Introduction: Primary hemophagocytic lymphohistiocytosis (HLH) has historically been associated with a number of genetic mutations resulting in impaired lymphocyte cytotoxicity. More recently, we have learned that defects of the inflammasome immune complex, such as in X-linked inhibitor of apoptosis (XIAP) deficiency, cause an HLH phenotype with normal cytotoxic function and that patients with XIAP deficiency have a high risk of peri-transplant HLH flare and mortality. This study aims to further elucidate the differential peri-transplant courses of HLH patients depending on their underlying genetic backgrounds. Methods: We retrospectively extracted data from medical records of patients who underwent allogeneic hematopoietic cell transplant (HCT) for HLH or an HLH-predisposing immunodeficiency at UCSF Benioff Children's Hospital between 2006-2018. Event-free survival (EFS) was defined as successful engraftment without rejection or failure of initial transplant. HLH recurrence was defined by HLH-2004 diagnostic criteria. All time ranges were measured from the date of HCT admission. Log rank tests were used for statistical testing of survival data. Results: We identified 23 subjects, median age 4 years (0.4-22.5 years). 13/23 (57%) were found to have a pathogenic variant in a non-inflammasome, HLH-associated gene ("Proven Typical HLH"), 2/23 (9%) had mutations involving the inflammasome ("Proven Atypical HLH," both with XIAP deficiency), and 8/23 (35%) had no identified genetic defect ("Possible Atypical HLH"). The latter two groups were analyzed as "Atypical HLH." The majority of patients received conditioning with busulfan, fludarabine, and alemtuzumab. With median follow up of 4.3 years, 3-year overall survival was 82% (95% CI, 65-98%). The 3-year EFS was 69% (95% CI, 50-88%). In engrafted patients (21/23, 91%), median donor myeloid chimerism (30-day, 100%; 180-day, 100%; 365-day 99%) and T cell chimerism (30-day, 76%; 180-day, 97%, 365-day 99%) were high. The cumulative incidence of grade II-IV acute GvHD in transplant recipients was 15% (95% CI, 0-30%). 3-year OS and EFS were both significantly higher in Typical HLH patients at 100% (95% CI, 75-100%) and 92% (95% CI, 78-100%), respectively, compared to Atypical HLH patients whose OS and EFS were 56% (95% CI, 23-90%) and 40% (95% CI, 10-70%), respectively (p=0.006 and 0.011). The only event in the Typical HLH group was a primary graft rejection, salvaged with second HCT. Peri-transplant mortality occurred only in patients without a known mutation (53%; 95% CI, 16-90%) compared to those with known mutations (0%; 95% CI, 0-25%; p=0.002). 3-year HLH-free survival was also significantly (p&lt;0.001) higher in Typical HLH patients (100%; 95% CI, 75-100%) compared to Atypical HLH patients (30%; 95% CI, 2-58%), as peri-transplant HLH flares occurred only in the Atypical HLH group, with half (3/6) surviving the flare. All flares occurred within 1 year of HCT admission (range 2-291 days), and all events occurred within 3 years of admission (range 5-993 days). Conclusions: HLH is a clinical phenotype with significant underlying genetic heterogeneity, manifested in differential peri-transplant outcomes. Patients with proven Typical HLH have improved survival and EFS as well as decreased risk of peri-transplant HLH compared to those with proven or possible Atypical HLH. Given limitations in current testing, it is possible that patients without a known defect in the cytotoxic pathway may have underlying inflammasome mutations. Defects in XIAP and the resulting loss of apoptosis protection appear to put patients at higher risk of chemotherapy complications. These results demonstrate the need for thorough genetic testing in all HLH patients prior to HCT, as well as close monitoring for development of a peri-HCT flare in patients without proven Typical HLH. If validated in a larger dataset, patients with proven or possible Atypical HLH may be considered for trials of anti-cytokine therapy in the peri-transplant period. Disclosures Shimano: Pfizer: Research Funding; Novartis: Research Funding; Daiichi Sankyo: Research Funding. Huang:Novartis: Research Funding. Dvorak:Alexion Inc: Consultancy; Jazz Pharmaceuticals: Consultancy.

XIAP Loss Triggers RIPK3- and Caspase-8-Driven IL-1β Activation and Cell Death as a Consequence of TLR-MyD88-Induced cIAP1-TRAF2 Degradation

X-linked Inhibitor of Apoptosis (XIAP) deficiency predisposes people to pathogen-associated hyperinflammation. Upon XIAP loss, Toll-like receptor (TLR) ligation triggers RIPK3-caspase-8-mediated IL-1β activation and death in myeloid cells. How XIAP suppresses these events remains unclear. Here, we show that TLR-MyD88 causes the proteasomal degradation of the related IAP, cIAP1, and its adaptor, TRAF2, by inducing TNF and TNF Receptor 2 (TNFR2) signaling. Genetically, we define that myeloid-specific cIAP1 loss promotes TLR-induced RIPK3-caspase-8 and IL-1β activity in the absence of XIAP. Importantly, deletion of TNFR2 in XIAP-deficient cells limited TLR-MyD88-induced cIAP1-TRAF2 degradation, cell death, and IL-1β activation. In contrast to TLR-MyD88, TLR-TRIF-induced interferon (IFN)β inhibited cIAP1 loss and consequent cell death.These data reveal how, upon XIAP deficiency, a TLR-TNF-TNFR2 axis drives cIAP1-TRAF2 degradation to allow TLR or TNFR1 activation of RIPK3-caspase-8and IL-1β. This mechanism may explain whyXIAP-deficient patients can exhibit symptoms reminiscent of patients with activating inflammasome mutations.

Successful allogeneic hematopoietic stem cell transplantation in a boy with X-linked inhibitor of apoptosis deficiency presenting with hemophagocytic lymphohistiocytosis: A case report

X-linked inhibitor of apoptosis (XIAP) deficiency, also known as X-linked lymphoproliferative syndrome type 2 (XLP2), is a rare inherited primary immunodeficiency resulting from the XIAP (also known as BIRC4) mutation. XIAP deficiency is mainly associated with familial hemophagocytic lymphohistiocytosis (HLH) phenotypes, and genetic testing is crucial in diagnosing this syndrome. Allogeneic hematopoietic stem cell transplantation (HSCT) is currently the only successful strategy for the treatment of this disease; however, a limited number of studies has been published concerning the outcomes of allogeneic HSCT in patients with XIAP deficiency. The present study reported a successful allogeneic HSCT performed to treat XIAP deficiency in a Chinese boy presenting with HLH. Polymerase chain reaction and DNA sequencing were performed to confirm the diagnosis of XIAP deficiency, and allogeneic HSCT was performed. Genetic tests revealed a two-nucleotide deletion (c.1021_1022delAA) in the patient, which was inherited from his mother, and resulted in frameshift mutation and premature stop codon (p.N341fsX348); this is considered to be a disease-causing mutation. The XIAP deficiency patient underwent allogeneic HSCT, receiving busulfan-containing reduced intensity myeloablative conditioning regimen, with a good intermediate follow-up result obtained. Therefore, genetic testing is essential to confirm the diagnosis of XIAP deficiency and detect the carrier of mutation. The present case study may promote the investigation of allogeneic HSCT in patients with XIAP deficiency.

Refractory monogenic Crohn’s disease due to X-linked inhibitor of apoptosis deficiency

Refractory monogenic Crohn’s disease due to X-linked inhibitor of apoptosis deficiency

Impaired antibacterial autophagy links granulomatous intestinal inflammation in Niemann–Pick disease type C1 and XIAP deficiency with NOD2 variants in Crohn's disease

ObjectivePatients with Niemann–Pick disease type C1 (NPC1), a lysosomal lipid storage disorder that causes neurodegeneration and liver damage, can present with IBD, but neither the significance nor the functional mechanism...

Successful Allogeneic Hematopoietic Stem Cell Transplantation in XIAP Deficiency Using Reduced‐Intensity Conditioning

Hematopoietic stem cell transplantation (HSCT) is currently the only available curative therapy for X-linked inhibitor of apoptosis (XIAP) deficiency. Myeloablative conditioning regimens are associated with high mortality rates. Reduced-intensity conditioning (RIC) is recommended in order to decrease treatment-related toxicities, but RIC regimens increase the risk for mixed donor-recipient chimerism that may progress to graft loss. We report our experience with a patient with XIAP deficiency who was successfully treated with allogeneic HSCT using a RIC protocol. Post-transplant chimerism was vigilantly monitored and maintained with donor lymphocyte infusions and a stem cell boost to a level that prevented hemophagocytic lymphohistiocytosis recurrence.

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

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

A mutation in X-linked inhibitor of apoptosis (G466X) leads to memory inflation of Epstein-Barr virus-specific T cells.

Mutations in the X-linked inhibitor of apoptosis (XIAP) gene have been associated with XLP-like disease, including recurrent Epstein-Barr virus (EBV)-related haemophagocytic lymphohystiocytosis (HLH), but the immunopathogenic bases of EBV-related disease in XIAP deficiency is unknown. We present the first analysis of EBV-specific T cell responses in functional XIAP deficiency. In a family of patients with a novel mutation in XIAP (G466X) leading to a late-truncated protein and varying clinical features, we identified gradual hypogammaglobulinaemia and large expansions of T cell subsets, including a prominent CD4(+) CD8(+) population. Extensive ex-vivo analyses showed that the expanded T cell subsets were dominated by EBV-specific cells with conserved cytotoxic, proliferative and interferon (IFN)-γ secretion capacity. The EBV load in blood fluctuated and was occasionally very high, indicating that the XIAP(G466X) mutation could impact upon EBV latency. XIAP deficiency may unravel a new immunopathogenic mechanism in EBV-associated disease.

Characterization of Crohn disease in X-linked inhibitor of apoptosis–deficient male patients and female symptomatic carriers

Crohn disease is an inflammatory bowel disease (IBD) with a complex mode of inheritance. Although nucleotide binding and oligomerization domain containing 2 (NOD2) is the strongest risk factor, the cause of Crohn disease remains unknown in the majority of the cases. X-linked inhibitor of apoptosis (XIAP) deficiency causes X-linked lymphoproliferative syndrome type 2. IBD has been reported in some XIAP-deficient patients. We characterize the IBD affecting a large cohort of patients with mutations in XIAP and examine the possible pathophysiologic mechanisms. We performed a phenotypical and histologic analysis of the IBD affecting 17 patients with hemizygous mutations in XIAP, including 3 patients identified by screening 83 patients with pediatric-onset IBD. The X chromosome inactivation was analyzed in female carriers of heterozygous XIAP mutations, including 2 adults with IBD. The functional consequences of XIAP deficiency were analyzed. Clinical presentation and histology of IBD in patients with XIAP deficiency overlapped with those of patients with Crohn disease. The age at onset was variable (from 3 months to 41 years), and IBD was severe and difficult to treat. In 2 patients hematopoietic stem cell transplantation fully restored intestinal homeostasis. Monocytes of patients had impaired NOD2-mediated IL-8 and monocyte chemoattractant protein 1 (MCP-1) production, as well as IL-10, in response to NOD2 and Toll-like receptor 2/4 costimulation. Nucleotide binding and oligomerization domain containing 1 (NOD1)-mediated IL-6 and IL-8 production was defective in fibroblasts from XIAP-deficient patients. The 2 heterozygous female carriers of XIAP mutations with IBD displayed abnormal expression of the XIAP mutated allele, resulting in impaired activation of the NOD2 pathway. IBD in patients with XIAP deficiency is similar to Crohn disease and is associated with defective NOD2 function in monocytes. Importantly, we report that it is not restricted to male patients because we identified 2 symptomatic female heterozygous carriers of XIAP mutations.

A new functional assay for the diagnosis of X-linked inhibitor of apoptosis (XIAP) deficiency

X-linked inhibitor of apoptosis (XIAP) deficiency, caused by mutations in BIRC4, is an immunodeficiency associated with immune dysregulation and a highly variable clinical presentation. Current diagnostic screening tests such as flow cytometry for XIAP expression or lymphocyte apoptosis assays have significant limitations. Based on recent evidence that XIAP is essential for nucleotide-binding and oligomerization domains (NOD)1/2 signalling, we evaluated the use of a simple flow cytometric assay assessing tumour necrosis factor (TNF) production of monocytes in response to NOD2 stimulation by muramyl dipeptides (L18-MDP) for the functional diagnosis of XIAP deficiency. We investigated 12 patients with XIAP deficiency, six female carriers and relevant disease controls. Irrespective of the diverse clinical phenotype, the extent of residual protein expression or the nature of the mutation, the TNF response was severely reduced in all patients. On average, L18-MDP induced TNF production in 25% of monocytes from healthy donors or female carriers, while fewer than 6% of monocytes responded in affected patients. Notably, the assay clearly discriminated affected patients from disease controls with other immunodeficiencies accompanied by lymphoproliferation, hypogammaglobulinaemia or inflammatory bowel disease. Functional testing of the NOD2 signalling pathway is an easy, fast and reliable assay in the diagnostic evaluation of patients with suspected XIAP deficiency.