XIAP regulates cytosol-specific innate immunity to Listeria infection

Murine Models of Familial Cytokine Storm Syndromes

Hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory disease caused by mutations in effectors and regulators of cytotoxicity in cytotoxic T cells (CTL) and natural killer (NK) cells. The complexity of the immune system means that in vivo models are needed to efficiently study diseases like HLH. Mice with defects in the genes known to cause primary HLH (pHLH) are available. However, these mice only develop the characteristic features of HLH after the induction of an immune response (typically through infection with lymphocytic choriomeningitis virus). Nevertheless, murine models have been invaluable for understanding the mechanisms that lead to HLH. For example, the cytotoxic machinery (e.g., the transport of cytotoxic vesicles and the release of granzymes and perforin after membrane fusion) was first characterized in the mouse. Experiments in murine models of pHLH have emphasized the importance of cytotoxic cells, antigen-presenting cells (APC), and cytokines in hyperinflammatory positive feedback loops (e.g., cytokine storms). This knowledge has facilitated the development of treatments for human HLH, some of which are now being tested in the clinic.

Structure shows that the BIR2 domain of E3 ligase XIAP binds across the RIPK2 kinase dimer interface

RIPK2 is an essential adaptor for NOD signalling and its kinase domain is a drug target for NOD-related diseases, such as inflammatory bowel disease. However, recent work indicates that the phosphorylation activity of RIPK2 is dispensable for signalling and that inhibitors of both RIPK2 activity and RIPK2 ubiquitination prevent the essential interaction between RIPK2 and the BIR2 domain of XIAP, the key RIPK2 ubiquitin E3 ligase. Moreover, XIAP BIR2 antagonists also block this interaction. To reveal the molecular mechanisms involved, we combined native mass spectrometry, NMR, and cryo-electron microscopy to determine the structure of the RIPK2 kinase BIR2 domain complex and validated the interface with in cellulo assays. The structure shows that BIR2 binds across the RIPK2 kinase antiparallel dimer and provides an explanation for both inhibitory mechanisms. It also highlights why phosphorylation of the kinase activation loop is dispensable for signalling while revealing the structural role of RIPK2-K209 residue in the RIPK2-XIAP BIR2 interaction. Our results clarify the features of the RIPK2 conformation essential for its role as a scaffold protein for ubiquitination.

XIAP deletion sensitizes mice to TNF-induced and RIP1-mediated death

XIAP is a caspase-inhibitory protein that blocks several cell death pathways, and mediates proper activation of inflammatory NOD2-RIP2 signaling. XIAP deficiency in patients with inflammatory diseases such as Crohn's disease, or those needing allogeneic hematopoietic cell transplantation, is associated with a worse prognosis. In this study, we show that XIAP absence sensitizes cells and mice to LPS- and TNF-mediated cell death without affecting LPS- or TNF-induced NF-κB and MAPK signaling. In XIAP deficient mice, RIP1 inhibition effectively blocks TNF-stimulated cell death, hypothermia, lethality, cytokine/chemokine release, intestinal tissue damage and granulocyte migration. By contrast, inhibition of the related kinase RIP2 does not affect TNF-stimulated events, suggesting a lack of involvement for the RIP2-NOD2 signaling pathway. Overall, our data indicate that in XIAP's absence RIP1 is a critical component of TNF-mediated inflammation, suggesting that RIP1 inhibition could be an attractive option for patients with XIAP deficiency.

Lentiviral Gene Transfer Corrects Immune Abnormalities in XIAP Deficiency

BACKGROUND

X-linked inhibitor of apoptosis protein (XIAP) deficiency is a severe immunodeficiency with clinical features including hemophagocytic lymphohistiocytosis (HLH) and inflammatory bowel disease (IBD) due to defective NOD2 responses. Management includes immunomodulatory therapies and hematopoietic stem cell transplant (HSCT). However, this cohort is particularly susceptible to the chemotherapeutic regimens and acutely affected by graft-vs-host disease (GvHD), driving poor long-term survival in transplanted patients. Autologous HSC gene therapy could offer an alternative treatment option and would abrogate the risks of alloreactivity.

METHODS

Hematopoietic progenitor (Lin-ve) cells from XIAPy/- mice were transduced with a lentiviral vector encoding human XIAP cDNA before transplantation into irradiated XIAP y/- recipients. After 12 weeks animals were challenged with the dectin-1 ligand curdlan and recovery of innate immune function was evaluated though analysis of inflammatory cytokines, body weight, and splenomegaly. XIAP patient-derived CD14+ monocytes were transduced with the same vector and functional recovery was demonstrated using in vitro L18-MDP/NOD2 assays.

RESULTS

In treated XIAPy/- mice, ~40% engraftment of gene-corrected Lin-ve cells led to significant recovery of weight loss, splenomegaly, and inflammatory cytokine responses to curdlan, comparable to wild-type mice. Serum IL-6, IL-10, MCP-1, and TNF were significantly reduced 2-h post-curdlan administration in non-corrected XIAPy/- mice compared to wild-type and gene-corrected animals. Appropriate reduction of inflammatory responses was observed in gene-corrected mice, whereas non-corrected mice developed an inflammatory profile 9 days post-curdlan challenge. In gene-corrected patient CD14+ monocytes, TNF responses were restored following NOD2 activation with L18-MDP.

CONCLUSION

Gene correction of HSCs recovers XIAP-dependent immune defects and could offer a treatment option for patients with XIAP deficiency.

Epstein Barr virus-mediated transformation of B cells from XIAP-deficient patients leads to increased expression of the tumor suppressor CADM1

X-linked lymphoproliferative disease (XLP) is either caused by loss of the SLAM-associated protein (SAP; XLP-1) or the X-linked inhibitor of apoptosis (XIAP; XLP-2). In both instances, infection with the oncogenic human Epstein Barr virus (EBV) leads to pathology, but EBV-associated lymphomas only emerge in XLP-1 patients. Therefore, we investigated the role of XIAP during B cell transformation by EBV. Using humanized mice, IAP inhibition in EBV-infected mice led to a loss of B cells and a tendency to lower viral titers and lymphomagenesis. Loss of memory B cells was also observed in four newly described patients with XIAP deficiency. EBV was able to transform their B cells into lymphoblastoid cell lines (LCLs) with similar growth characteristics to patient mothers' LCLs in vitro and in vivo. Gene expression analysis revealed modest elevated lytic EBV gene transcription as well as the expression of the tumor suppressor cell adhesion molecule 1 (CADM1). CADM1 expression on EBV-infected B cells might therefore inhibit EBV-associated lymphomagenesis in patients and result in the absence of EBV-associated malignancies in XLP-2 patients.

XIAP promotes melanoma growth by inducing tumour neutrophil infiltration

Elevated expression of the X‐linked inhibitor of apoptosis protein (XIAP) has been frequently reported in malignant melanoma suggesting that XIAP renders apoptosis resistance and thereby supports melanoma progression. Independent of its anti‐apoptotic function, XIAP mediates cellular inflammatory signalling and promotes immunity against bacterial infection. The pro‐inflammatory function of XIAP has not yet been considered in cancer. By providing detailed in vitro analyses, utilising two independent mouse melanoma models and including human melanoma samples, we show here that XIAP is an important mediator of melanoma neutrophil infiltration. Neutrophils represent a major driver of melanoma progression and are increasingly considered as a valuable therapeutic target in solid cancer. Our data reveal that XIAP ubiquitylates RIPK2, involve TAB1/RIPK2 complex and induce the transcriptional up‐regulation and secretion of chemokines such as IL8, that are responsible for intra‐tumour neutrophil accumulation. Alteration of the XIAP‐RIPK2‐TAB1 inflammatory axis or the depletion of neutrophils in mice reduced melanoma growth. Our data shed new light on how XIAP contributes to tumour growth and provides important insights for novel XIAP targeting strategies in cancer.

The Role of E3 Ubiquitin Ligases and Deubiquitinases in Inflammatory Bowel Disease: Friend or Foe?

Inflammatory bowel disease (IBD), which include Crohn’s disease (CD) and ulcerative colitis (UC), exhibits a complex multifactorial pathogenesis involving genetic susceptibility, imbalance of gut microbiota, mucosal immune disorder and environmental factors. Recent studies reported associations between ubiquitination and deubiquitination and the occurrence and development of inflammatory bowel disease. Ubiquitination modification, one of the most important types of post-translational modifications, is a multi-step enzymatic process involved in the regulation of various physiological processes of cells, including cell cycle progression, cell differentiation, apoptosis, and innate and adaptive immune responses. Alterations in ubiquitination and deubiquitination can lead to various diseases, including IBD. Here, we review the role of E3 ubiquitin ligases and deubiquitinases (DUBs) and their mediated ubiquitination and deubiquitination modifications in the pathogenesis of IBD. We highlight the importance of this type of posttranslational modification in the development of inflammation, and provide guidance for the future development of targeted therapeutics in IBD.

A solid-phase approach for the synthesis of muramyl dipeptide conjugates for detection of NOD2

Proper recognition of invading pathogens and prompt initiation of host defense mechanisms are instrumental for the maintenance of organismal homeostasis. Nucleotide-binding oligomerization domain-containing (NOD)-like receptors (NLRs) serve as pathogen-recognition receptors that specifically recognize bacterial peptidoglycans. NOD2 detects muramyl dipeptide (MDP) through its carboxy-terminal leucine rich repeats (LRRs), which enables the activation of downstream inflammatory signaling. Synthesis of MDP conjugates based on solution phase chemistry have been previously reported. Our solid phase approach synthetically provides a facile approach for the conjugation of biological probes to MDP, with the advantage of minimal functional/protecting group manipulation, and reduction in the laborious process of intermediate purification and isolation. MDP conjugates that we generated using solid phase synthesis allow detection of NOD2 is cell lysates and NOD2 subcellular localization by immunofluorescence microscopy. MDP-PEG6-Cyanine5.5 conjugate selectively colocalized with WT NOD2 but not NOD2 variant found in Crohn's disease, which lacks carboxy-terminal end and cannot bind MDP. Overall, these data indicate that distinct solid phase-produced MDP conjugates can be used to examine biological properties of NOD2 and could potentially facilitate further development of NOD2 targeting agents.

Infection-induced inflammation from specific inborn errors of immunity to COVID-19

Inborn errors of immunity (IEIs) are a group of genetically defined disorders leading to defective immunity. Some IEIs have been linked to mutations of immune receptors or signaling molecules, resulting in defective signaling of respective cascades essential for combating specific pathogens. However, it remains incompletely understood why in selected IEIs, such as X-linked lymphoproliferative syndrome type 2 (XLP-2), hypo-immune response to specific pathogens results in persistent inflammation. Moreover, mechanisms underlying the generation of anticytokine autoantibodies are mostly unknown. Recently, IEIs have been associated with coronavirus disease 2019 (COVID-19), with a small proportion of patients that contract severe COVID-19 displaying loss-of-function mutations in genes associated with type I interferons (IFNs). Moreover, approximately 10% of patients with severe COVID-19 possess anti-type I IFN-neutralizing autoantibodies. Apart from IEIs that impair immune responses to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), SARS-CoV-2 encodes several proteins that suppress early type I IFN production. One primary consequence of the lack of type I IFNs during early SARS-CoV-2 infection is the increased inflammation associated with COVID-19. In XLP-2, resolution of inflammation rescued experimental subjects from infection-induced mortality. Recent studies also indicate that targeting inflammation could alleviate COVID-19. In this review, we discuss infection-induced inflammation in IEIs, using XLP-2 and COVID-19 as examples. We suggest that resolving inflammation may represent an effective therapeutic approach to these diseases.

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.

A regulatory region on RIPK2 is required for XIAP binding and NOD signaling activity

Signaling via the intracellular pathogen receptors nucleotide-binding oligomerization domain-containing proteins NOD1 and NOD2 requires receptor interacting kinase 2 (RIPK2), an adaptor kinase that can be targeted for the treatment of various inflammatory diseases. However, the molecular mechanisms of how RIPK2 contributes to NOD signaling are not completely understood. We generated FLAG-tagged RIPK2 knock-in mice using CRISPR/Cas9 technology to study NOD signaling mechanisms at the endogenous level. Using cells from these mice, we were able to generate a detailed map of post-translational modifications on RIPK2. Similar to other reports, we did not detect ubiquitination of RIPK2 lysine 209 during NOD2 signaling. However, using site-directed mutagenesis we identified a new regulatory region on RIPK2, which dictates the crucial interaction with the E3 ligase XIAP and downstream signaling outcomes.

IAP inhibitor, Embelin increases VCAM-1 levels on the endothelium, producing lymphocytic infiltration and antitumor immunity

There is an increasing unmet need for successful immunotherapeutic interventions. Lymphocyte extravasation via tumor tissue endothelial cells (TECs) is required for lymphocyte infiltration into tumor sites. This study aimed to investigate the clinical significance of dysfunctional TECs in pancreatic ductal adenocarcinoma (PDAC) and identify chemical compounds that boost tumor-infiltrating lymphocyte (TIL) numbers. We performed immunohistochemical detection and clinicopathological analysis of VCAM-1 on TECs, which is essential for lymphocyte trafficking. We characterized the gene expression profiles of TECs from fresh PDAC tissues. We isolated compounds that upregulated VCAM-1 and E-selectin expression in TECs and examined their biological activities. Compared to endothelial cells from chronic pancreatitis tissue, TECs showed significantly lower VCAM-1 and E-selectin expression and significant weaknesses in lymphocyte adhesion and transmigration, resulting in decreased T cell infiltration around vessels. Patients with a relatively high percentage of VCAM-1⁺ vessels among all vessels in PDAC tissue had an improved prognosis. A bioinformatics survey demonstrated that TNFR1 signaling was involved in abnormal VCAM-1 and E-selectin expression in TECs. We screened compounds affecting TNFR1 signaling, and the IAP inhibitor, Embelin, induced these molecules on TECs and enhanced T cell adhesion to TECs and transmigration. Furthermore, in vivo, Embelin enhanced tumor-infiltrating T cell numbers, leading to an antitumor immune response. Embelin accelerates TIL infiltration and the antitumor immune response by recovering VCAM-1 expression in TECs. Our strategy may be a therapeutic approach for accelerating the immunotherapeutic response in immune-quiescent tumors, leading to clinical trials’ success.

IAP-Mediated Protein Ubiquitination in Regulating Cell Signaling

Over the last decade, the E3-ubiquitine ligases from IAP (Inhibitor of Apoptosis) family have emerged as potent regulators of immune response. In immune cells, they control signaling pathways driving differentiation and inflammation in response to stimulation of tumor necrosis factor receptor (TNFR) family, pattern-recognition receptors (PRRs), and some cytokine receptors. They are able to control the activity, the cellular fate, or the stability of actors of signaling pathways, acting at different levels from components of receptor-associated multiprotein complexes to signaling effectors and transcription factors, as well as cytoskeleton regulators. Much less is known about ubiquitination substrates involved in non-immune signaling pathways. This review aimed to present IAP ubiquitination substrates and the role of IAP-mediated ubiquitination in regulating signaling pathways.

Recent advances in understanding inhibitor of apoptosis proteins

The inhibitor of apoptosis proteins (IAPs) are a family of proteins that were chiefly known for their ability to inhibit apoptosis by blocking caspase activation or activity. Recent research has shown that cellular IAP1 (cIAP1), cIAP2, and X-linked IAP (XIAP) also regulate signaling by receptors of the innate immune system by ubiquitylating their substrates. These IAPs thereby act at the intersection of pathways leading to cell death and inflammation. Mutation of IAP genes can impair tissue homeostasis and is linked to several human diseases. Small-molecule IAP antagonists have been developed to treat certain malignant, infectious, and inflammatory diseases. Here, we will discuss recent advances in our understanding of the functions of cIAP1, cIAP2, and XIAP; the consequences of their mutation or dysregulation; and the therapeutic potential of IAP antagonist drugs.

Small molecule inhibitors reveal an indispensable scaffolding role of RIPK2 in NOD2 signaling

RIPK2 mediates inflammatory signaling by the bacteria‐sensing receptors NOD1 and NOD2. Kinase inhibitors targeting RIPK2 are a proposed strategy to ameliorate NOD‐mediated pathologies. Here, we reveal that RIPK2 kinase activity is dispensable for NOD2 inflammatory signaling and show that RIPK2 inhibitors function instead by antagonizing XIAP‐binding and XIAP‐mediated ubiquitination of RIPK2. We map the XIAP binding site on RIPK2 to the loop between β2 and β3 of the N‐lobe of the kinase, which is in close proximity to the ATP‐binding pocket. Through characterization of a new series of ATP pocket‐binding RIPK2 inhibitors, we identify the molecular features that determine their inhibition of both the RIPK2‐XIAP interaction, and of cellular and in vivoNOD2 signaling. Our study exemplifies how targeting of the ATP‐binding pocket in RIPK2 can be exploited to interfere with the RIPK2‐XIAP interaction for modulation of NOD signaling.

IL-6 receptor blockade corrects defects of XIAP-deficient regulatory T cells

X-linked lymphoproliferative syndrome type-2 (XLP-2) is a primary immunodeficiency disease attributed to XIAP mutation and is triggered by infection. Here, we show that mouse Xiap^−/− regulatory T (Treg) cells and human XIAP-deficient Treg cells are defective in suppressive function. The Xiap^−/− Treg cell defect is linked partly to decreased SOCS1 expression. XIAP binds SOCS1 and promotes SOCS1 stabilization. Foxp3 stability is reduced in Xiap^−/− Treg cells. In addition, Xiap^−/− Treg cells are prone to IFN-γ secretion. Transfer of wild-type Treg cells partly rescues infection-induced inflammation in Xiap^−/− mice. Notably, inflammation-induced reprogramming of Xiap^−/− Treg cells can be prevented by blockade of the IL-6 receptor (IL-6R), and a combination of anti-IL-6R and Xiap^−/− Treg cells confers survival to inflammatory infection in Xiap^−/− mice. Our results suggest that XLP-2 can be corrected by combination treatment with autologous iTreg (induced Treg) cells and anti-IL-6R antibody, bypassing the necessity to transduce Treg cells with XIAP.

XLP-2 syndrome is caused by XIAP mutation. Here the authors show that mouse and human XIAP-deficient regulatory T cells have defective suppressive function as a result of conversion to proinflammatory cytokine producing cells, an effect that can be prevented by blocking the IL-6 receptor.

S100A4 contributes to colitis development by increasing the adherence of Citrobacter rodentium in intestinal epithelial cells

S100A4 has been implicated in cancer and several inflammatory diseases, but its role in inflammatory bowel disease has not been well investigated. Here, upon infection with Citrobacter rodentium, a model for enteropathogenic Escherichia coli infection in humans, induced the infiltration of a large number of S100A4⁺ cells into the colon in wild type (WT) mice. Deficiency of S100A4 reduced weight loss, bacterial colonization and colonic pathology. Furthermore, the expression of inflammatory cytokines and the recruitment of macrophages and neutrophils also decreased significantly in S100A4 knock out (S100A4 ^-/-) mice. In vitro, soluble S100A4 directly up-regulated expression of integrin β-1 in intestinal epithelial cells and significantly increased the adherence of C. rodentium to intestinal epithelial cells. Additionally, the effects of S100A4 on the adherence of C. rodentium to epithelial cells could be abolished by a receptor for advanced glycation end products (RAGE)-specific inhibitor (FPS-ZM1). Therefore, these data indicate a novel mechanism for S100A4 that promotes colitis development by enhancing host adhesion and colonization of Citrobacter rodentium through the S100A4-mediated host inflammatory responses.

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

OBJECTIVE

Patients 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 of this association is clear. We studied bacterial handling and antibacterial autophagy in patients with NPC1.

DESIGN

We characterised intestinal inflammation in 14 patients with NPC1 who developed IBD. We investigated bacterial handling and cytokine production of NPC1 monocytes or macrophages in vitro and compared NPC1-associated functional defects to those caused by IBD-associated nucleotide-binding oligomerization domain-containing protein 2 (NOD2) variants or mutations in X-linked inhibitor of apoptosis (XIAP).

RESULTS

Patients with the lysosomal lipid storage disorder NPC1 have increased susceptibility to early-onset fistulising colitis with granuloma formation, reminiscent of Crohn's disease (CD). Mutations in NPC1 cause impaired autophagy due to defective autophagosome function that abolishes NOD2-mediated bacterial handling in vitro similar to variants in NOD2 or XIAP deficiency. In contrast to genetic NOD2 and XIAP variants, NPC1 mutations do not impair NOD2-receptor-interacting kinase 2 (RIPK2)-XIAP-dependent cytokine production. Pharmacological activation of autophagy can rescue bacterial clearance in macrophages in vitro by increasing the autophagic flux and bypassing defects in NPC1.

CONCLUSIONS

NPC1 confers increased risk of early-onset severe CD. Our data support the concept that genetic defects at different checkpoints of selective autophagy cause a shared outcome of CD-like immunopathology linking monogenic and polygenic forms of IBD. Muramyl dipeptide-driven cytokine responses and antibacterial autophagy induction are parallel and independent signalling cascades downstream of the NOD2-RIPK2-XIAP complex.

Nucleotide-binding oligomerization domain (NOD) signaling defects and cell death susceptibility cannot be uncoupled in X-linked inhibitor of apoptosis (XIAP)-driven inflammatory disease

The X-linked inhibitor of apoptosis (XIAP) protein has been identified as a key genetic driver of two distinct inflammatory disorders, X-linked lymphoproliferative syndrome 2 (XLP-2) and very-early-onset inflammatory bowel disease (VEO-IBD). Molecularly, the role of XIAP mutations in the pathogenesis of these disorders is unclear. Recent work has consistently shown XIAP to be critical for signaling downstream of the Crohn's disease susceptibility protein nucleotide-binding oligomerization domain-containing 2 (NOD2); however, the reported effects of XLP-2 and VEO-IBD XIAP mutations on cell death have been inconsistent. In this manuscript, we describe a CRISPR-mediated genetic system for cells of the myeloid lineage in which XIAP alleles can be replaced with disease-associated XIAP variants expressed at endogenous levels to simultaneously study inflammation-related cell death and NOD2 signaling. We show that, consistent with previous studies, NOD2 signaling is critically dependent on the BIR2 domain of XIAP. We further used this system to reconcile the aforementioned inconsistent XIAP cell death data to show that XLP-2 and VEO-IBD XIAP mutations that exhibit a loss-of-function NOD2 phenotype also lower the threshold for inflammatory cell death. Last, we identified and studied three novel patient XIAP mutations and used this system to characterize NOD2 and cell death phenotypes driven by XIAP. The results of this work support the role of XIAP in mediating NOD2 signaling while reconciling the role of XLP-2 and VEO-IBD XIAP mutations in inflammatory cell death and provide a set of tools and framework to rapidly test newly discovered XIAP variants.

Regulating the balance between necroptosis, apoptosis and inflammation by inhibitors of apoptosis proteins

Understanding how inhibitor of apoptosis proteins (IAPs) regulate apoptosis and necroptosis has been fast-forwarded by the use of Smac mimetics (SMs) to deplete or inhibit the IAPs, specifically cIAP1, cIAP2 and XIAP. The loss or inhibition of cIAP1, cIAP2 and XIAP causes the majority of cells to be sensitized to death receptor induced cell death, such as with tumour necrosis factor (TNF). Mouse genetics shows that there is some functional redundancy and the use of SMs has allowed us to understand how changing the composition of proteins recruited to TNF receptor 1 on TNF ligation can alter protein complex formation and activation of apoptosis or necroptosis, particularly when caspases are inhibited. Determining when or how caspase inhibition occurs physiologically combined with the loss of IAPs will be the next challenge in understanding the ability of IAPs to prevent cell death and/or limit inflammation.

XIAP deficiency and MEFV variants resulting in an autoinflammatory lymphoproliferative syndrome

A 16-year-old boy of Caucasian ethnicity was evaluated for recurrent febrile episodes occurring during most of his life without establishment of any microbial aetiology. During febrile episodes he developed extensive splenomegaly, lymphadenopathy, anaemia, severe abdominal pain and general malaise. Lymph node biopsies demonstrated inflammation and sinus histiocytosis but no malignancy or granuloma. The patient underwent seroconversion for Epstein-Barr virus (EBV) infection during the hospitalisation. Genetic testing identified a hemizygous frameshift mutation in the X linked inhibitor of apoptosis (XIAP)-gene as well as variants in the MEFV gene indicating Familial Mediterranean Fever (FMF). XIAP expression was markedly reduced in the patient, while a functional assay assessing tumour necrosis factor (TNF)α production of monocytes in response to NOD2 stimulation displayed reduced activity. We suggest that the heterozygous MEFV variants and the hemizygous XIAP variant in combination triggered the prolonged and pathological inflammatory response to EBV infection.

Intact Regulatory T-Cell Function but Defective Generation of IL-17A-Producing CD4+ T Cells in XIAP Deficiency

OBJECTIVE

X-linked inhibitor of apoptosis (xIAP) deficiency is a primary immune deficiency disorder associated with hemophagocytic lymphohistiocytosis. About 17% of xIAP-deficient patients present with very early onset severe colitis with high mortality. We hypothesized that xIAP deficiency leads to defective generation and/or survival of T regulatory cells (Treg) through its involvement in transforming growth factor-β signaling.

METHODS AND RESULTS

We used a T-cell transfer model of chronic colitis and observed a mild increase in colitis severity induced by naïve CD4 T cells from xIAP mice compared with colitis induced by naïve CD4 T cells from WT mice. We did not observe any significant difference in the induction of Treg cells in these studies. We next tested whether xIAP is required for Treg cell function by co-transferring xIAP or WT Treg cells with naïve WT CD4 cells in this model. We demonstrate that XIAP-deficient Treg cells were able to prevent disease similarly to WT Treg cells. In these experiments we, however, found a significantly decreased percentage of IL-17A-producing CD4 T cells in mice receiving Tregs from xIAP mice.

CONCLUSIONS

xIAP appears dispensable for the generation of induced Treg cells as well as function of natural Treg cells. There appeared to be a role of xIAP in generation of IL-17-producing cells from either naïve CD4 T cells or Treg cells. Further research is needed to explore the role of xIAP in generation of IL-17-producing cells.

CYLD Limits Lys63- and Met1-Linked Ubiquitin at Receptor Complexes to Regulate Innate Immune Signaling

Innate immune signaling relies on the deposition of non-degradative polyubiquitin at receptor-signaling complexes, but how these ubiquitin modifications are regulated by deubiquitinases remains incompletely understood. Met1-linked ubiquitin (Met1-Ub) is assembled by the linear ubiquitin assembly complex (LUBAC), and this is counteracted by the Met1-Ub-specific deubiquitinase OTULIN, which binds to the catalytic LUBAC subunit HOIP. In this study, we report that HOIP also interacts with the deubiquitinase CYLD but that CYLD does not regulate ubiquitination of LUBAC components. Instead, CYLD limits extension of Lys63-Ub and Met1-Ub conjugated to RIPK2 to restrict signaling and cytokine production. Accordingly, Met1-Ub and Lys63-Ub were individually required for productive NOD2 signaling. Our study thus suggests that LUBAC, through its associated deubiquitinases, coordinates the deposition of not only Met1-Ub but also Lys63-Ub to ensure an appropriate response to innate immune receptor activation.

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CYLD associates with LUBAC via HOIP and limits signaling by NOD2

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RIPK2 ubiquitination is regulated by CYLD and OTULIN

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CYLD trims Lys63 and Met1 linkages conjugated to RIPK2

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Productive NOD2 signaling requires Lys63 and Met1 linkages

Signaling cascades and inflammasome activation in microbial infections

Recognition of extracellular pathogenassociated molecular patterns (PAMPs) by pattern recognition receptors (PRRs) results in activation of host defense signaling pathways. Some virulent microbes can attenuate and escape antimicrobial immunity by manipulating these signaling pathways. However, impairment of the primary innate response may potentiate the activation of secondary defense program, centered around Nucleotide-binding domain and Leucine-rich repeat containing Receptor (NLRs) for inflammasome formation and IL-1β production. This review analyzes the current knowledge regarding association of innate immune signaling pathways with inflammasome activation in response to bacterial infection.

Ubiquitin-Mediated Regulation of Cell Death, Inflammation, and Defense of Homeostasis

Cell death and inflammation are ancient processes of fundamental biological importance in both normal physiology and human disease pathologies. The recent observation that apoptosis regulatory components have dual roles in cell death and inflammation suggests that these proteins function, not primarily to kill, but to coordinate tissue repair and remodeling. This perspective unifies cell death components as positive regulators of tissue repair that replaces malfunctioning or damaged tissues and enhances the resilience of epithelia to insult. It is now recognized that cells that die by apoptosis do not do so silently, but release a variety of paracrine signals to communicate with their cellular environment to ensure tissue regeneration, and wound healing. Moreover, inflammatory signaling pathways, such as those emanating from the TNF receptor or Toll-related receptors, take part in cell competition to eliminate developmentally aberrant clones. Ubiquitylation has emerged as crucial mediator of signal transduction in cell death and inflammation. Here, we focus on recent advances on ubiquitin-mediated regulation of cell death and inflammation, and how this is used to regulate the defense of homeostasis.

Regulation of Cell Death by IAPs and Their Antagonists

Inhibitors of apoptosis (IAPs) family of genes encode baculovirus IAP-repeat domain-containing proteins with antiapoptotic function. These proteins also contain RING or UBC domains and act by binding to major proapoptotic factors and ubiquitylating them. High levels of IAPs inhibit caspase-mediated apoptosis. For these cells to undergo apoptosis, IAP function must be neutralized by IAP-antagonists. Mammalian IAP knockouts do not exhibit obvious developmental phenotypes, but the cells are more sensitized to apoptosis in response to injury. Loss of the mammalian IAP-antagonist ARTS results in reduced stem cell apoptosis. In addition to the antiapoptotic properties, IAPs regulate the innate immune response, and the loss of IAP function in humans is associated with immunodeficiency. The roles of IAPs in Drosophila apoptosis regulation are more apparent, where the loss of IAP1, or the expression of IAP-antagonists in Drosophila cells, is sufficient to trigger apoptosis. In this organism, apoptosis as a fate is conferred by the transcriptional induction of the IAP-antagonists. Many signaling pathways often converge on shared enhancer regions of IAP-antagonists. Cell death sensitivity is further regulated by posttranscriptional mechanisms, including those regulated by kinases, miRs, and ubiquitin ligases. These mechanisms are employed to eliminate damaged or virus-infected cells, limit neuroblast (neural stem cell) numbers, generate neuronal diversity, and sculpt tissue morphogenesis.

The X-Linked Inhibitor of Apoptosis Protein Inhibitor Embelin Suppresses Inflammation and Bone Erosion in Collagen Antibody Induced Arthritis Mice

Objective. To investigate the effect of Embelin, an inhibitor of X-Linked Inhibitor of Apoptosis Protein (XIAP), on inflammation and bone erosion in a collagen antibody induced arthritis (CAIA) in mice. Methods. Four groups of mice (n = 6 per group) were allocated: CAIA untreated mice, CAIA treated with Prednisolone (10 mg/kg/day), CAIA treated with low dose Embelin (30 mg/kg/day), and CAIA treated with high dose Embelin (50 mg/kg/day). Joint inflammation was evaluated using clinical paw score and histological assessments. Bone erosion was assessed using micro-CT, tartrate resistant acid phosphatase (TRAP) staining, and serum carboxy-terminal collagen crosslinks (CTX-1) ELISA. Immunohistochemistry was used to detect XIAP protein. TUNEL was performed to identify apoptotic cells. Results. Low dose, but not high dose Embelin, suppressed inflammation as reflected by lower paw scores (P < 0.05) and lower histological scores for inflammation. Low dose Embelin reduced serum CTX-1 (P < 0.05) and demonstrated lower histological score and TRAP counting, and slightly higher bone volume as compared to CAIA untreated mice. XIAP expression was not reduced but TUNEL positive cells were more abundant in Embelin treated CAIA mice. Conclusion. Low dose Embelin suppressed inflammation and serum CTX-1 in CAIA mice, indicating a potential use for Embelin to treat pathological bone loss.

A potential role of X-linked inhibitor of apoptosis protein in mitochondrial membrane permeabilization and its implication in cancer therapy

X-chromosome-linked inhibitor of apoptosis protein (XIAP) has an important regulatory role in programmed cell death by inhibiting the caspase cascade. Activation of XIAP-dependent signaling culminates into regulation of multiple cellular processes including apoptosis, innate immunity, epithelial-to-mesenchymal transition, cell migration, invasion, metastasis and differentiation. Although XIAP localizes to the cytosolic compartment, XIAP-mediated cellular signaling encompasses mitochondrial and post-mitochondrial levels. Recent findings demonstrate that XIAP also localizes to mitochondria and regulates mitochondria functions. XIAP acts upstream of mitochondrial cytochrome c release and modulates caspase-dependent apoptosis. The new function of XIAP has potential to enhance mitochondrial membrane permeabilization and other cellular functions controlling cytochrome c release. These findings could exploit the overexpression of XIAP in human tumors for therapeutic benefits.

Employing XIAP to enhance the duration of antigen expression and immunity against an avian influenza H5 DNA vaccine

DNA vaccine represents a powerful approach for prevention of avian H5N1 influenza infection. Yet, DNA vaccine-induced immune responses might be limited by the short duration of antigen expression. As a strategy to enhance adaptive immune responses elicited by a hemagglutinin 5 (H5) DNA vaccine, we explored the effect of co-administration of a DNA encoding X-linked inhibitor of apoptosis protein (XIAP) as a modulator of apoptosis and a stimulator of inflammatory signaling. In cultured cells as early as 24 hours (h), we found that the DNA vaccine encoded H5 antigen was a potent stimulator of apoptosis, and the H5 pro-apoptotic activity was significantly suppressed by the co-expression of full-length XIAP or mutant XIAP (ΔRING). However, full-length XIAP showed a higher potency than mutant XIAP (ΔRING) in the inhibition of H5-induced apoptosis. We also compared the immunizing ability of transmembrane and secretory forms of H5. Mice vaccinated (twice with 3-week intervals) with the secretory form of H5 showed higher hemagglutination inhibition (HI) antibody titers than mice vaccinated with the transmembrane form of H5. Furthermore, co-administration of XIAP with the secretory form of H5 resulted into a stronger antibody response than the transmembrane form of H5. Our findings suggest that in the design of DNA vaccines for a given pro-apoptotic antigen, using an anti-apoptotic molecular adjuvant and the secretory form of antigen may be a greater stimulus to induce immune responses.

WNT-inflammasome signaling mediates NOD2-induced development of acute arthritis in mice

In addition to its role in innate immunity, the intracellular pathogen sensor nucleotide-binding oligomerization domain 2 (NOD2) has been implicated in various inflammatory disorders, including the development of acute arthritis. However, the molecular mechanisms involved in the development of NOD2-responsive acute arthritis are not clear. In this study, we demonstrate that NOD2 signals to a cellular protein, Ly6/PLAUR domain-containing protein 6, in a receptor-interacting protein kinase 2-TGF-β-activated kinase 1-independent manner to activate the WNT signaling cascade. Gain- or loss-of-function of the WNT signaling pathway in an in vivo experimental mouse arthritis model or in vitro systems established the role for WNT-responsive X-linked inhibitor of apoptosis during the development of acute arthritis. Importantly, WNT-stimulated X-linked inhibitor of apoptosis mediates the activation of inflammasomes. The subsequent caspase-1 activation and IL-1β secretion together contribute to the phenotypic character of the inflammatory condition of acute arthritis. Thus, identification of a role for WNT-mediated inflammasome activation during NOD2 stimulation serves as a paradigm to understand NOD2-associated inflammatory disorders and develop novel therapeutics.

XIAP variants in male Crohn's disease

OBJECTIVE

The genetic basis of inflammatory bowel disease (IBD) is incompletely understood. The aim of this study was to identify rare genetic variants involved in the pathogenesis of IBD.

DESIGN

Exome sequencing and immunological profiling were performed in a patient with early onset Crohn's disease (CD). The coding region of the gene encoding X-linked inhibitor of apoptosis protein (XIAP) was sequenced in samples of 275 paediatric IBD and 1047 adult-onset CD patients. XIAP genotyping was performed in samples of 2680 IBD patients and 2864 healthy controls. Functional effects of the variants identified were investigated in primary cells and cultured cell lines.

RESULTS

Our results demonstrate the frequent occurrence of private variants in XIAP in about four percent of male patients with paediatric-onset CD. While XIAP mutations are known to be associated with the primary immunodeficiency (PID) X-linked lymphoproliferative disease type 2 (XLP2), CD patients described here exhibited intestinal inflammation in the absence of XLP2 and harboured a spectrum of mutations partially distinct from that observed in XLP2. The majority of XIAP variants identified was associated with a selective defect in NOD1/2 signalling, impaired NOD1/2-mediated activation of NF-κB, and altered NF-κB-dependent cytokine production.

CONCLUSIONS

This study reveals the unanticipated, frequent occurrence of XIAP variants in male paediatric-onset CD. The link between XIAP and NOD1/2, and the association of XIAP variants with XLP2, support the concept of PID in a subset of IBD patients. Moreover, these studies provide a rationale for the implementation of XIAP sequencing in clinical diagnostics in male patients with severe CD.

Inhibitors of apoptosis (IAPs) regulate intestinal immunity and inflammatory bowel disease (IBD) inflammation

The inhibitor of apoptosis (IAP) family members, notably cIAP1, cIAP2, and XIAP, are critical and universal regulators of tumor necrosis factor (TNF) mediated survival, inflammatory, and death signaling pathways. Furthermore, IAPs mediate the signaling of nucleotide-binding oligomerization domain (NOD)1/NOD2 and other intracellular NOD-like receptors in response to bacterial pathogens. These pathways are important to the pathogenesis and treatment of inflammatory bowel disease (IBD). Inactivating mutations in the X-chromosome-linked IAP (XIAP) gene causes an immunodeficiency syndrome, X-linked lymphoproliferative disease type 2 (XLP2), in which 20% of patients develop severe intestinal inflammation. In addition, 4% of males with early-onset IBD also have inactivating mutations in XIAP. Therefore, the IAPs play a greater role in gut homeostasis, immunity and IBD development than previously suspected, and may have therapeutic potential.

Inability to resolve specific infection generates innate immunodeficiency syndrome in Xiap-/- mice

Emerging evidence indicates that innate immunodeficiency syndromes are linked to mutations in innate receptors and to specific infections. X-linked lymphoproliferative syndrome type-2 (XLP-2) is associated with deficiency in X-linked inhibitor of apoptosis protein (XIAP), with poorly understood molecular mechanisms. Here we showed that XIAP deficiency selectively impaired B-cell chronic lymphocytic leukemia/lymphoma 10 (BCL10)-mediated innate responses to dectin-1 ligands but did not affect responses to various Toll-like receptor agonists. Consequently, Xiap(-/-) mice became highly vulnerable on Candida albicans infection. The compromised early innate responses led to the persistent presence of C albicans and inflammatory cytokines in Xiap(-/-) mice. Furthermore, priming of Xiap(-/-) mice with the dectin-1 ligand curdlan alone resulted in XLP-2-like syndromes. Restoration of dectin-1-induced Rac1 activation and phagocytosis by resolvin D1, but not up-regulation of nuclear factor-κB, rescued Xiap(-/-) mice from C albicans lethal infection. Therefore, development of XLP-2 in XIAP-deficient patients could be partly due to sustained inflammation as a consequence of defective BCL10-dependent innate immunity toward specific pathogens. Importantly, our results suggest the potential therapeutic value of resolvin D1 in the treatment of XLP-2 and innate immunodeficiency syndromes.

A near death experience: Shigella manipulates host death machinery to silence innate immunity

Release of mitochondrial contents often triggers inflammation and cell death, and modulating this process can be advantageous to invading pathogens. In this issue of The EMBO Journal, Andree and colleagues reveal new findings that an intracellular bacterial pathogen exploits apoptotic machinery to suppress host immune signaling, yet avoids cell death. This study emphasizes the need to expand our understanding of the roles played by pro‐apoptotic proteins in non‐death scenarios.

BID-dependent release of mitochondrial SMAC dampens XIAP-mediated immunity against Shigella

The X-linked inhibitor of apoptosis protein (XIAP) is a potent caspase inhibitor, best known for its anti-apoptotic function in cancer. During apoptosis, XIAP is antagonized by SMAC, which is released from the mitochondria upon caspase-mediated activation of BID. Recent studies suggest that XIAP is involved in immune signaling. Here, we explore XIAP as an important mediator of an immune response against the enteroinvasive bacterium Shigella flexneri, both in vitro and in vivo. Our data demonstrate for the first time that Shigella evades the XIAP-mediated immune response by inducing the BID-dependent release of SMAC from the mitochondria. Unlike apoptotic stimuli, Shigella activates the calpain-dependent cleavage of BID to trigger the release of SMAC, which antagonizes the inflammatory action of XIAP without inducing apoptosis. Our results demonstrate how the cellular death machinery can be subverted by an invasive pathogen to ensure bacterial colonization.

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.

IAPs, regulators of innate immunity and inflammation

As indicated by their name, members of the Inhibitor of APoptosis (IAP) family were first believed to be functionally restricted to apoptosis inhibition. It is now clear that IAPs have a much wider spectrum of action, and recent studies even suggest that some of its members primarily regulate inflammatory responses. Inflammation, the first response of the immune system to infection or tissue injury, is highly regulated by ubiquitylation - a posttranslational modification of proteins with various consequences. In this review, we focus on the recently reported functions of XIAP, cIAP1 and cIAP2 as ubiquitin ligases regulating innate immunity and inflammation.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSION

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

cIAPs and XIAP regulate myelopoiesis through cytokine production in an RIPK1- and RIPK3-dependent manner

Loss of inhibitor of apoptosis proteins (IAPs), particularly cIAP1, can promote production of tumor necrosis factor (TNF) and sensitize cancer cell lines to TNF-induced necroptosis by promoting formation of a death-inducing signaling complex containing receptor-interacting serine/threonine-protein kinase (RIPK) 1 and 3. To define the role of IAPs in myelopoiesis, we generated a mouse with cIAP1, cIAP2, and XIAP deleted in the myeloid lineage. Loss of cIAPs and XIAP in the myeloid lineage caused overproduction of many proinflammatory cytokines, resulting in granulocytosis and severe sterile inflammation. In vitro differentiation of macrophages from bone marrow in the absence of cIAPs and XIAP led to detectable levels of TNF and resulted in reduced numbers of mature macrophages. The cytokine production and consequent cell death caused by IAP depletion was attenuated by loss or inhibition of TNF or TNF receptor 1. The loss of RIPK1 or RIPK3, but not the RIPK3 substrate mixed lineage kinase domain-like protein, attenuated TNF secretion and thereby prevented apoptotic cell death and not necrosis. Our results demonstrate that cIAPs and XIAP together restrain RIPK1- and RIPK3-dependent cytokine production in myeloid cells to critically regulate myeloid homeostasis.

Inhibitors of apoptosis proteins (IAPs) are required for effective T-cell expansion/survival during antiviral immunity in mice

IAPs are required for survival and expansion of activated T cells. IAP antagonists sensitize to tumor necrosis factor (TNF)-induced cell death of activated T cells during viral infection.

Ubiquitin in the immune system

Ubiquitination is a post-translational modification process that has been implicated in the regulation of innate and adaptive immune responses. There is increasing evidence that both ubiquitination and its reversal, deubiquitination, play crucial roles not only during the development of the immune system but also in the orchestration of an immune response by ensuring the proper functioning of the different cell types that constitute the immune system. Here, we provide an overview of the latest discoveries in this field and discuss how they impact our understanding of the ubiquitin system in host defence mechanisms as well as self-tolerance.