XIAP Restricts TNF- and RIP3-Dependent Cell Death and Inflammasome Activation

Identification of Germline Non-coding Deletions in XIAP Gene Causing XIAP Deficiency Reveals a Key Promoter Sequence

PURPOSE

X-linked inhibitor of apoptosis protein (XIAP) deficiency, also known as the X-linked lymphoproliferative syndrome of type 2 (XLP-2), is a rare immunodeficiency characterized by recurrent hemophagocytic lymphohistiocytosis, splenomegaly, and inflammatory bowel disease. Variants in XIAP including missense, non-sense, frameshift, and deletions of coding exons have been reported to cause XIAP deficiency. We studied three young boys with immunodeficiency displaying XLP-2-like clinical features. No genetic variation in the coding exons of XIAP was identified by whole-exome sequencing (WES), although the patients exhibited a complete loss of XIAP expression.

METHODS

Targeted next-generation sequencing (NGS) of the entire locus of XIAP was performed on DNA samples from the three patients. Molecular investigations were assessed by gene reporter expression assays in HEK cells and CRISPR-Cas9 genome editing in primary T cells.

RESULTS

NGS of XIAP identified three distinct non-coding deletions in the patients that were predicted to be driven by repetitive DNA sequences. These deletions share a common region of 839 bp that encompassed the first non-coding exon of XIAP and contained regulatory elements and marks specific of an active promoter. Moreover, we showed that among the 839 bp, the exon was transcriptionally active. Finally, deletion of the exon by CRISPR-Cas9 in primary cells reduced XIAP protein expression.

CONCLUSIONS

These results identify a key promoter sequence contained in the first non-coding exon of XIAP. Importantly, this study highlights that sequencing of the non-coding exons that are not currently captured by WES should be considered in the genetic diagnosis when no variation is found in coding exons.

Detection of Gasdermin Activation and Lytic Cell Death During Pyroptosis and Apoptosis

Cytosolic pattern recognition receptors trigger pyroptosis by detection of danger- or pathogen-associated molecular patterns. These receptors initiate the assembly of inflammasomes, multimeric protein complexes that drive caspase-1 activation. Active caspase-1 cleaves the proinflammatory cytokines IL-1β and IL-18 and the pore-forming protein gasdermin-D (GSDMD) thereby liberating its N-terminal domain. The GSDMD N-termini form multimeric pores at the plasma membrane that allow leakage of intracellular content and ultimately trigger a type of cell death called "pyroptosis." Emerging studies have revealed that GSDMD is also processed by apoptotic caspases-8/-3/-7. In this chapter, we aim to describe methods to monitor lytic cell death and to distinguish between GSDMD processing events and the GSDMD fragments that are generated after pyroptosis or apoptosis induction. We also illustrate the difference between GSDMD pore formation, and final cell lysis, and how this affects to the release of intracellular content. Finally, we show that the activation of another pore-forming protein, gasdermin-E, does not exclusively translate into lytic cell death in bone marrow-derived macrophages.

Apoptosis, Pyroptosis, and Necroptosis-Oh My! The Many Ways a Cell Can Die

Cell death is an essential process in all living organisms and occurs through different mechanisms. The three main types of programmed cell death are apoptosis, pyroptosis, and necroptosis, and each of these pathways employs complex molecular and cellular mechanisms. Although there are mechanisms and outcomes specific to each pathway, they share common components and features. In this review, we discuss recent discoveries in these three best understood modes of cell death, highlighting their singularities, and examining the intriguing notion that common players shape different individual pathways in this highly interconnected and coordinated cell death system. Understanding the similarities and differences of these cell death processes is crucial to enable targeted strategies to manipulate these pathways for therapeutic benefit.

High-Fat, Western-Style Diet, Systemic Inflammation, and Gut Microbiota: A Narrative Review

The gut microbiota is responsible for recovering energy from food, providing hosts with vitamins, and providing a barrier function against exogenous pathogens. In addition, it is involved in maintaining the integrity of the intestinal epithelial barrier, crucial for the functional maturation of the gut immune system. The Western diet (WD)—an unhealthy diet with high consumption of fats—can be broadly characterized by overeating, frequent snacking, and a prolonged postprandial state. The term WD is commonly known and intuitively understood. However, the strict digital expression of nutrient ratios is not precisely defined. Based on the US data for 1908–1989, the calory intake available from fats increased from 32% to 45%. Besides the metabolic aspects (hyperinsulinemia, insulin resistance, dyslipidemia, sympathetic nervous system and renin-angiotensin system overstimulation, and oxidative stress), the consequences of excessive fat consumption (high-fat diet—HFD) comprise dysbiosis, gut barrier dysfunction, increased intestinal permeability, and leakage of toxic bacterial metabolites into the circulation. These can strongly contribute to the development of low-grade systemic inflammation. This narrative review highlights the most important recent advances linking HFD-driven dysbiosis and HFD-related inflammation, presents the pathomechanisms for these phenomena, and examines the possible causative relationship between pro-inflammatory status and gut microbiota changes.

XIAP restrains TNF-driven intestinal inflammation and dysbiosis by promoting innate immune responses of Paneth and dendritic cells

Deficiency in X-linked inhibitor of apoptosis protein (XIAP) is the cause for X-linked lymphoproliferative syndrome 2 (XLP2). About one-third of these patients suffer from severe and therapy-refractory inflammatory bowel disease (IBD), but the exact cause of this pathogenesis remains undefined. Here, we used XIAP-deficient mice to characterize the mechanisms underlying intestinal inflammation. In Xiap−/− mice, we observed spontaneous terminal ileitis and microbial dysbiosis characterized by a reduction of Clostridia species. We showed that in inflamed mice, both TNF receptor 1 and 2 (TNFR1/2) cooperated in promoting ileitis by targeting TLR5-expressing Paneth cells (PCs) or dendritic cells (DCs). Using intestinal organoids and in vivo modeling, we demonstrated that TLR5 signaling triggered TNF production, which induced PC dysfunction mediated by TNFR1. TNFR2 acted upon lamina propria immune cells. scRNA-seq identified a DC population expressing TLR5, in which Tnfr2 expression was also elevated. Thus, the combined activity of TLR5 and TNFR2 signaling may be responsible for DC loss in lamina propria of Xiap−/− mice. Consequently, both Tnfr1−/−Xiap−/− and Tnfr2−/−Xiap−/− mice were rescued from dysbiosis and intestinal inflammation. Furthermore, RNA-seq of ileal crypts revealed that in inflamed Xiap−/− mice, TLR5 signaling was abrogated, linking aberrant TNF responses with the development of a dysbiosis. Evidence for TNFR2 signaling driving intestinal inflammation was detected in XLP2 patient samples. Together, these data point toward a key role of XIAP in mediating resilience of TLR5-expressing PCs and intestinal DCs, allowing them to maintain tissue integrity and microbiota homeostasis.

Deficiency in X-linked inhibitor of apoptosis protein promotes susceptibility to microbial triggers of intestinal inflammation

Inflammatory bowel disease (IBD) is characterized by inappropriate immune responses to the microbiota in genetically susceptible hosts, but little is known about the pathways that link individual genetic alterations to microbiota-dependent inflammation. Here, we demonstrated that the loss of X-linked inhibitor of apoptosis protein (XIAP), a gene associated with Mendelian IBD, rendered Paneth cells sensitive to microbiota-, tumor necrosis factor (TNF)–, receptor-interacting protein kinase 1 (RIPK1)–, and RIPK3-dependent cell death. This was associated with deficiency in Paneth cell–derived antimicrobial peptides and alterations in the stratification and composition of the microbiota. Loss of XIAP was not sufficient to elicit intestinal inflammation but provided susceptibility to pathobionts able to promote granulomatous ileitis, which could be prevented by administration of a Paneth cell–derived antimicrobial peptide. These data reveal a pathway critical for host-microbial cross-talk, which is required for intestinal homeostasis and the prevention of inflammation and which is amenable to therapeutic targeting.

Inherited Autoinflammatory Syndromes

Autoinflammation describes a collection of diverse diseases caused by indiscriminate activation of the immune system in an antigen-independent manner. The rapid advancement of genetic diagnostics has allowed for the identification of a wide array of monogenic causes of autoinflammation. While the clinical picture of these syndromes is diverse, it is possible to thematically group many of these diseases under broad categories that provide insight into the mechanisms of disease and therapeutic possibilities. This review covers archetypical examples of inherited autoinflammatory diseases in five major categories: inflammasomopathy, interferonopathy, unfolded protein/cellular stress response, relopathy, and uncategorized. This framework can suggest where future work is needed to identify other genetic causes of autoinflammation, what types of diagnostics need to be developed to care for this patient population, and which options might be considered for novel therapeutic targeting. Expected final online publication date for the Annual Review of Pathology: Mechanisms of Disease, Volume 17 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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.

NLRP3 inflammasome activation and cell death

The NLRP3 inflammasome is a cytosolic multiprotein complex composed of the innate immune receptor protein NLRP3, adapter protein ASC, and inflammatory protease caspase-1 that responds to microbial infection, endogenous danger signals, and environmental stimuli. The assembled NLRP3 inflammasome can activate the protease caspase-1 to induce gasdermin D-dependent pyroptosis and facilitate the release of IL-1β and IL-18, which contribute to innate immune defense and homeostatic maintenance. However, aberrant activation of the NLRP3 inflammasome is associated with the pathogenesis of various inflammatory diseases, such as diabetes, cancer, and Alzheimer's disease. Recent studies have revealed that NLRP3 inflammasome activation contributes to not only pyroptosis but also other types of cell death, including apoptosis, necroptosis, and ferroptosis. In addition, various effectors of cell death have been reported to regulate NLRP3 inflammasome activation, suggesting that cell death is closely related to NLRP3 inflammasome activation. In this review, we summarize the inextricable link between NLRP3 inflammasome activation and cell death and discuss potential therapeutics that target cell death effectors in NLRP3 inflammasome-associated diseases.

Intestinal immunoregulation: lessons from human mendelian diseases

The mechanisms that maintain intestinal homeostasis despite constant exposure of the gut surface to multiple environmental antigens and to billions of microbes have been scrutinized over the past 20 years with the goals to gain basic knowledge, but also to elucidate the pathogenesis of inflammatory bowel diseases (IBD) and to identify therapeutic targets for these severe diseases. Considerable insight has been obtained from studies based on gene inactivation in mice as well as from genome wide screens for genetic variants predisposing to human IBD. These studies are, however, not sufficient to delineate which pathways play key nonredundant role in the human intestinal barrier and to hierarchize their respective contribution. Here, we intend to illustrate how such insight can be derived from the study of human Mendelian diseases, in which severe intestinal pathology results from single gene defects that impair epithelial and or hematopoietic immune cell functions. We suggest that these diseases offer the unique opportunity to study in depth the pathogenic mechanisms leading to perturbation of intestinal homeostasis in humans. Furthermore, molecular dissection of monogenic intestinal diseases highlights key pathways that might be druggable and therapeutically targeted in common forms of IBD.

Mechanisms underlying genetic susceptibility to multisystem inflammatory syndrome in children (MIS-C)

BACKGROUND

Multisystem inflammatory syndrome in children (MIS-C) is a pediatric complication of severe acute respiratory syndrome coronavirus 2 infection that is characterized by multiorgan inflammation and frequently by cardiovascular dysfunction. It occurs predominantly in otherwise healthy children. We previously reported haploinsufficiency of suppressor of cytokine signaling 1 (SOCS1), a negative regulator of type I and II interferons, as a genetic risk factor for MIS-C.

OBJECTIVES

We aimed to identify additional genetic mechanisms underlying susceptibility to severe acute respiratory syndrome coronavirus 2-associated MIS-C.

METHODS

In a single-center, prospective cohort study, whole exome sequencing was performed on patients with MIS-C. The impact of candidate variants was tested by using patients' PBMCs obtained at least 7 months after recovery.

RESULTS

We enrolled 18 patients with MIS-C (median age = 8 years; interquartile range = 5-12.25 years), of whom 89% had no conditions other than obesity. In 2 boys with no significant infection history, we identified and validated hemizygous deleterious defects in XIAP, encoding X-linked inhibitor of apoptosis, and CYBB, encoding cytochrome b-245, beta subunit. Including the previously reported SOCS1 haploinsufficiency, a genetic diagnosis was identified in 3 of 18 patients (17%). In contrast to patients with mild COVID-19, patients with defects in SOCS1, XIAP, or CYBB exhibit an inflammatory immune cell transcriptome with enrichment of differentially expressed genes in pathways downstream of IL-18, oncostatin M, and nuclear factor κB, even after recovery.

CONCLUSIONS

Although inflammatory disorders are rare in the general population, our cohort of patients with MIS-C was enriched for monogenic susceptibility to inflammation. Our results support the use of next-generation sequencing in previously healthy children who develop MIS-C.

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.

XIAP gene therapy effects on retinal ganglion cell structure and function in a mouse model of glaucoma

Glaucoma is a prevalent neurodegenerative disease that is characterized by progressive visual field loss. It is the leading cause of irreversible blindness in the world. The main risk factor for glaucoma is elevated intraocular pressure that results in the damage and death of retinal ganglion cells (RGCs) and their axons. The death of RGCs has been shown to be apoptotic. We tested the hypothesis that blocking the activation of apoptosis may be an effective strategy to prevent RGC death and preserve functional vision in glaucoma. In the magnetic microbead mouse model of induced ocular hypertension, inhibition of RGC apoptosis was targeted through viral-mediated ocular delivery of the X-linked inhibitor of apoptosis (XIAP) gene, a potent caspase inhibitor. Pattern electroretinograms revealed that XIAP therapy resulted in significant protection of both somal and axonal RGC function in glaucomatous eyes. Histology confirmed that the treated optic nerves showed preservation of axon counts and reduced glial cell infiltration. These results show that XIAP is able to provide both functional and structural protection of RGCs in the microbead model of glaucoma and provide important proof-of-principle for XIAP's efficacy as a neuroprotective treatment for glaucoma.

Interconnections among major forms of regulated cell death

As a basic biological phenomenon of cells, regulated cell death (RCD) has irreplaceable influence on the occurrence and development of many processes of life and diseases. RCD plays an important role in the stability of the homeostasis, the development of multiple systems and the evolution of organisms. Thus comprehensively understanding of RCD is undoubtedly helpful in the innovation of disease treatment. Recently, research on the underlying mechanisms of the major forms of RCD, such as apoptosis, autophagy, necroptosis, pyroptosis, paraptosis and neutrophils NETosis has made significant breakthroughs. In addition, the interconnections among them have attracted increasing attention from global scholars in the field of life sciences. Here, recent advances in RCD research field are discussed.

RIPK3 Activates MLKL-mediated Necroptosis and Inflammasome Signaling during Streptococcus Infection

Community-acquired pneumonia is the most common type of pneumonia and remains a leading cause of morbidity and mortality worldwide. Although many different pathogens can contribute to pneumonia, Streptococcus pneumoniae is one of the common bacterial pathogens that underlie community-acquired pneumonia. RIPK3 (receptor-interacting protein kinase 3) is widely recognized as a key modulator of inflammation and cell death. To elucidate a potential role of RIPK3 in pneumonia, we examined plasma from healthy control subjects and patients positive for streptococcal pneumonia. In human studies, RIPK3 protein concentrations were significantly elevated and were identified as a potential plasma marker of pneumococcal pneumonia. To expand these findings, we used an in vivo murine model of pneumococcal pneumonia to demonstrate that RIPK3 deficiency leads to reduced bacterial clearance, severe pathological damage, and high mortality. Our results illustrated that RIPK3 forms a complex with RIPK1, MLKL (mixed-lineage kinase domain-like protein), and MCU (mitochondrial calcium uniporter) to induce mitochondrial calcium uptake and mitochondrial reactive oxygen species(mROS) production during S. pneumoniae infection. In macrophages, RIPK3 initiated necroptosis via the mROS-mediated mitochondrial permeability transition pore opening and NLRP3 inflammasome activation via the mROS-AKT pathway to protect against S. pneumoniae. In conclusion, our study demonstrated a mechanism by which RIPK3-initiated necroptosis is essential for host defense against S. pneumoniae.

RIP3-mediated necroptosis increases neuropathic pain via microglia activation: necrostatin-1 has therapeutic potential

Neuropathic pain (NP) is a clinical symptom that accompanies many diseases. We investigated the effect of receptor‐interacting protein kinase 3 (RIP3)‐regulated necroptosis on NP and explored its relationship with microglia, in order to provide a theoretical basis for further research and provide new insights into the treatment of NP. In this study, the spared nerve injury (SNI) model was used along with intervention with necrostatin and the inhibitor of necroptosis necrostatin‐1 (Nec‐1). Pain behavior tests were performed 1 and 3 days before the nerve injury (or sham) operation, and on days 1, 3, 5, 7, 10, and 14 after the operation. The spinal cord tissues were collected for detection of RIP3 expression and distribution, changes in the number of microglia cells, activation of necroptosis, and the level of proinflammatory factors. Collected spinal cord tissues were analyzed using western blot, immunohistochemistry, immunofluorescence, immunoprecipitation assays, and ELISA, respectively. We found that, compared with the sham group, the expression of RIP3 protein in the spinal cord of rats in the SNI group increased from 3 to 14 days after surgery. Immunofluorescence staining showed that RIP3 was coexpressed with the microglia and the number of microglia increased significantly in the SNI model group. The results of immunoprecipitation assays suggested that a RIP3‐mediated necroptosis pathway promotes NP. After treatment with Nec‐1, the expression of RIP3 protein and the number of microglia were significantly reduced, and the expression levels of TNF‐α, IL‐1β, and IL‐6 in spinal dorsal horns were significantly decreased. These results indicate that RIP3 promotes necroptosis to increase the occurrence of NP via microglia.

Inflammatory cell death induced by cytotoxic lymphocytes: a dangerous but necessary liaison

Cytotoxic lymphocytes (CLs), and more specifically Tc and NK cells, are the main executors of cell death in the immune system, playing a key role during both immunosurveillance and immunotherapy. These cells induce regulated cell death (RCD) by different mechanisms, being granular exocytosis and expression of death ligands the most prominent and best characterized ones. Apoptosis, a traditionally considered low-inflammatory type of cell death, has been accepted for years as the paradigm of RCD induced by CLs. However, several recent studies have demonstrated that NK cells and Tc cells can also induce more inflammatory forms of cell death, namely, necroptosis, pyroptosis, and ferroptosis. Activation of these highly inflammatory types of cell death appears to critically contribute to the activation of a successful antitumour immune response. Additionally, the role of specific cell death pathways in immunogenic cell death is still under intense debate, especially considering the interconnections with other inflammatory forms of cell death. These evidences, together with the advent of new cancer immunotherapies, highlight the necessity to deepen our understanding of the link between the cell death triggered by CLs and inflammation. This knowledge will be instrumental to maximize the antitumour potential of immunotherapies, minimizing deleterious effects associated with these treatments. In this review, we will briefly summarize the main features of apoptosis, necroptosis, pyroptosis and ferroptosis, to subsequently discuss the most recent evidences about the role of these RCD pathways during the elimination of cancer cells mediated by CLs and its modulation to increase the efficacy of cancer immunotherapy.

Necroptosis, pyroptosis and apoptosis: an intricate game of cell death

Cell death is a fundamental physiological process in all living organisms. Its roles extend from embryonic development, organ maintenance, and aging to the coordination of immune responses and autoimmunity. In recent years, our understanding of the mechanisms orchestrating cellular death and its consequences on immunity and homeostasis has increased substantially. Different modalities of what has become known as 'programmed cell death' have been described, and some key players in these processes have been identified. We have learned more about the intricacies that fine tune the activity of common players and ultimately shape the different types of cell death. These studies have highlighted the complex mechanisms tipping the balance between different cell fates. Here, we summarize the latest discoveries in the three most well understood modalities of cell death, namely, apoptosis, necroptosis, and pyroptosis, highlighting common and unique pathways and their effect on the surrounding cells and the organism as a whole.

Targeting RIP Kinases in Chronic Inflammatory Disease

Chronic inflammatory disorders are characterised by aberrant and exaggerated inflammatory immune cell responses. Modes of extrinsic cell death, apoptosis and necroptosis, have now been shown to be potent drivers of deleterious inflammation, and mutations in core repressors of these pathways underlie many autoinflammatory disorders. The receptor-interacting protein (RIP) kinases, RIPK1 and RIPK3, are integral players in extrinsic cell death signalling by regulating the production of pro-inflammatory cytokines, such as tumour necrosis factor (TNF), and coordinating the activation of the NOD-like receptor protein 3 (NLRP3) inflammasome, which underpin pathological inflammation in numerous chronic inflammatory disorders. In this review, we firstly give an overview of the inflammatory cell death pathways regulated by RIPK1 and RIPK3. We then discuss how dysregulated signalling along these pathways can contribute to chronic inflammatory disorders of the joints, skin, and gastrointestinal tract, and discuss the emerging evidence for targeting these RIP kinases in the clinic.

The regulation of necroptosis by post-translational modifications

Necroptosis is a caspase-independent, lytic form of programmed cell death whose errant activation has been widely implicated in many pathologies. The pathway relies on the assembly of the apical protein kinases, RIPK1 and RIPK3, into a high molecular weight cytoplasmic complex, termed the necrosome, downstream of death receptor or pathogen detector ligation. The necrosome serves as a platform for RIPK3-mediated phosphorylation of the terminal effector, the MLKL pseudokinase, which induces its oligomerization, translocation to, and perturbation of, the plasma membrane to cause cell death. Over the past 10 years, knowledge of the post-translational modifications that govern RIPK1, RIPK3 and MLKL conformation, activity, interactions, stability and localization has rapidly expanded. Here, we review current knowledge of the functions of phosphorylation, ubiquitylation, GlcNAcylation, proteolytic cleavage, and disulfide bonding in regulating necroptotic signaling. Post-translational modifications serve a broad array of functions in modulating RIPK1 engagement in, or exclusion from, cell death signaling, whereas the bulk of identified RIPK3 and MLKL modifications promote their necroptotic functions. An enhanced understanding of the modifying enzymes that tune RIPK1, RIPK3, and MLKL necroptotic functions will prove valuable in efforts to therapeutically modulate necroptosis.

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.

Immune modulating effects of receptor interacting protein 2 (RIP2) in autoinflammation and immunity

Receptor-interacting protein 2 (RIP2) is a kinase that is involved in downstream signaling of nuclear oligomerization domain (NOD)-like receptors NOD1 and 2 sensing bacterial peptidoglycans. RIP2-deficiency or targeting of RIP2 by pharmaceutical inhibitors partially ameliorates inflammatory diseases by reducing pro-inflammatory signaling in response to peptidoglycans. However, RIP2 is widely expressed and interacts with several other proteins suggesting additional functions outside the NOD-signaling pathway. In this review, we discuss the immunological functions of RIP2 and its possible role in autoinflammation and immunity.

Caspase-8-dependent gasdermin D cleavage promotes antimicrobial defense but confers susceptibility to TNF-induced lethality

Gasdermin D (GSDMD) is a pore-forming protein that promotes pyroptosis and release of proinflammatory cytokines. Recent studies revealed that apoptotic caspase-8 directly cleaves GSDMD to trigger pyroptosis. However, the molecular requirements for caspase-8-dependent GSDMD cleavage and the physiological impact of this signaling axis are unresolved. Here, we report that caspase-8-dependent GSDMD cleavage confers susceptibility to tumor necrosis factor (TNF)-induced lethality independently of caspase-1 and that GSDMD activation provides host defense against Yersinia infection. We further demonstrate that GSDMD inactivation by apoptotic caspases at aspartate 88 (D88) suppresses TNF-induced lethality but promotes anti-Yersinia defense. Last, we show that caspase-8 dimerization and autoprocessing are required for GSDMD cleavage, and provide evidence that the caspase-8 autoprocessing and activity on various complexes correlate with its ability to directly cleave GSDMD. These findings reveal GSDMD as a potential therapeutic target to reduce inflammation associated with mutations in the death receptor signaling machinery.

Pediatric hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is a syndrome describing patients with severe systemic hyperinflammation. Characteristic features include unremitting fever, cytopenias, hepatosplenomegaly, and elevation of typical HLH biomarkers. Patients can develop hepatitis, coagulopathy, liver failure, central nervous system involvement, multiorgan failure, and other manifestations. The syndrome has a high mortality rate. More and more, it is recognized that while HLH can be appropriately used as a broad summary diagnosis, many pediatric patients actually suffer from an expanding spectrum of genetic diseases that can be complicated by the syndrome of HLH. Classic genetic diseases in which HLH is a typical and common manifestation include pathogenic changes in familial HLH genes (PRF1, UNC13D, STXBP2, and STX11), several granule/pigment abnormality genes (RAB27A, LYST, and AP3B1), X-linked lymphoproliferative disease genes (SH2D1A and XIAP), and others such as NLRC4, CDC42, and the Epstein-Barr virus susceptibility diseases. There are many other genetic diseases in which HLH is an infrequent complication of the disorder as opposed to a prominent manifestation of the disease caused directly by the genetic defect, including other primary immune deficiencies and inborn errors of metabolism. HLH can also occur in patients with underlying rheumatologic or autoinflammatory disorders and is usually designated macrophage activation syndrome in those settings. Additionally, HLH can develop in patients during infections or malignancies without a known (or as-yet-identified) genetic predisposition. This article will attempt to summarize current concepts in the pediatric HLH field as well as offer a practical diagnostic and treatment overview.

Plasma Membrane Pores Drive Inflammatory Cell Death

Necroptosis and pyroptosis are two forms of regulated cell death. They are executed by the proteins mixed-lineage kinase domain-like (MLKL) and gasdermin D (GSDMD), respectively. Once activated by numerous pathways, these proteins form membrane pores that allow the influx and efflux of various ions, proteins, and water, ultimately resulting in the death of the cell. These modalities of cell death are considered highly inflammatory because of the release of inflammatory cytokines and damage-associated molecular patterns, and are thereby not only deleterious for the dying cell itself, but also its environment or the entire organism. The relevance for these processes has been observed in various physiological and pathophysiological conditions, ranging from viral and bacterial infections over autoimmune and chronic inflammatory diseases to ischemic organ damage. In recent years, initial in vitro experiments have shed light on a range of connections between necroptosis and pyroptosis. Initial in vivo studies also indicate that, in many disease models, these two forms of cell death cannot be considered individually, as they demonstrate a complex interaction. In this article, we provide an overview of the currently known structure, pathways of activation, and functions of MLKL and GSDMD. With emerging evidence for an interconnection between necroptosis and pyroptosis in not only in vitro, but also in vivo models of disease, we highlight in particular the clinical relevance of the crosslinks between these two forms of inflammatory cell death and their implications for novel therapeutic strategies in a variety of diseases.

RIP-roaring inflammation: RIPK1 and RIPK3 driven NLRP3 inflammasome activation and autoinflammatory disease

Autoinflammatory syndromes comprise a spectrum of clinical disorders characterised by recurrent, inflammatory episodes, many of which result from the release of the pro-inflammatory cytokine, interleukin-1β (IL-1β). Inflammation and programmed cell death are tightly linked, and lytic forms of cell death, such as necroptosis and pyroptosis, are considered to be inflammatory due to the release of damage-associated molecular patterns (DAMPs). In contrast, apoptosis is traditionally regarded as immunologically silent. Recent studies, however, have uncovered a high degree of crosstalk between cell death and inflammatory signalling pathways, and effectively consolidated them into one interconnected network that converges on NLRP3 inflammasome-mediated activation of IL-1β. The receptor-interacting protein kinases (RIPK) 1 and 3 are central to this network, as highlighted by the fact that mutations in genes encoding repressors of RIPK1 and/or RIPK3 activity can lead to heightened inflammation, particularly via NLRP3 inflammasome activation. In this review, we give an overview of extrinsic cell death and inflammatory signalling pathways, and then highlight the growing number of autoinflammatory diseases that are associated with aberrant cell death and inflammasome activation.

Regulation of Cell Death and Immunity by XIAP

X-chromosome-linked inhibitor of apoptosis protein (XIAP) controls cell survival in several regulated cell death pathways and coordinates a range of inflammatory signaling events. Initially identified as a caspase-binding protein, it was considered to be primarily involved in blocking apoptosis from both intrinsic as well as extrinsic triggers. However, XIAP also prevents TNF-mediated, receptor-interacting protein 3 (RIPK3)-dependent cell death, by controlling RIPK1 ubiquitylation and preventing inflammatory cell death. The identification of patients with germline mutations in XIAP (termed XLP-2 syndrome) pointed toward its role in inflammatory signaling. Indeed, XIAP also mediates nucleotide-binding oligomerization domain-containing 2 (NOD2) proinflammatory signaling by promoting RIPK2 ubiquitination within the NOD2 signaling complex leading to NF-κB and MAPK activation and production of inflammatory cytokines and chemokines. Overall, XIAP is a critical regulator of multiple cell death and inflammatory pathways making it an attractive drug target in tumors and inflammatory diseases.

Essential role of Salmonella Enteritidis DNA adenine methylase in modulating inflammasome activation

Background

Salmonella Enteritidis (SE) is one of the major foodborne zoonotic pathogens of worldwide importance which can induce activation of NLRC4 and NLRP3 inflammasomes during infection. Given that the inflammasomes play an essential role in resisting bacterial infection, Salmonella has evolved various strategies to regulate activation of the inflammasome, most of which largely remain unclear.

Results

A transposon mutant library in SE strain C50336 was screened for the identification of the potential factors that regulate inflammasome activation. We found that T3SS-associated genes invC, prgH, and spaN were required for inflammasome activation in vitro. Interestingly, C50336 strains with deletion or overexpression of Dam were both defective in activation of caspase-1, secretion of IL-1β and phosphorylation of c-Jun N-terminal kinase (Jnk). Transcriptome sequencing (RNA-seq) results showed that most of the differentially expressed genes and enriched KEGG pathways between the C50336-VS-C50336Δdam and C50336-VS-C50336::dam groups overlapped, which includes multiple signaling pathways related to the inflammasome. C50336Δdam and C50336::dam were both found to be defective in suppressing the expression of several anti-inflammasome factors. Moreover, overexpression of Dam in macrophages by lentiviral infection could specifically enhance the activation of NLRP3 inflammasome independently via promoting the Jnk pathway.

Conclusions

These data indicated that Dam was essential for modulating inflammasome activation during SE infection, there were complex and dynamic interplays between Dam and the inflammasome under different conditions. New insights were provided about the battle between SE and host innate immunological mechanisms.

Cell death in chronic inflammation: breaking the cycle to treat rheumatic disease

Cell death is a vital process that occurs in billions of cells in the human body every day. This process helps maintain tissue homeostasis, supports recovery from acute injury, deals with infection and regulates immunity. Cell death can also provoke inflammatory responses, and lytic forms of cell death can incite inflammation. Loss of cell membrane integrity leads to the uncontrolled release of damage-associated molecular patterns (DAMPs), which are normally sequestered inside cells. Such DAMPs increase local inflammation and promote the production of cytokines and chemokines that modulate the innate immune response. Cell death can be both a consequence and a cause of inflammation, which can be difficult to distinguish in chronic diseases. Despite this caveat, excessive or poorly regulated cell death is increasingly recognized as a contributor to chronic inflammation in rheumatic disease and other inflammatory conditions. Drugs that inhibit cell death could, therefore, be used therapeutically for the treatment of these diseases, and programmes to develop such inhibitors are already underway. In this Review, we outline pathways for the major cell death programmes (apoptosis, necroptosis, pyroptosis and NETosis) and their potential roles in chronic inflammation. We also discuss current and developing therapies that target the cell death machinery.

XIAP Protects Retinal Ganglion Cells in the Mutant ND4 Mouse Model of Leber Hereditary Optic Neuropathy

Purpose

Leber hereditary optic neuropathy (LHON) is a genetic form of vision loss that occurs primarily owing to mutations in the nicotinamide adenine dinucleotide dehydrogenase (ND) subunits that make up complex I of the electron transport chain. LHON mutations result in the apoptotic death of retinal ganglion cells. We tested the hypothesis that gene therapy with the X-linked inhibitor of apoptosis (XIAP) would prevent retinal ganglion cell apoptosis and reduce disease progression in a vector-induced mouse model of LHON that carries the ND4 mutation.

Methods

Adeno-associated virus (AAV) encoding full length hemagglutinin-tagged XIAP (AAV2.HA-XIAP) or green fluorescent protein (AAV2.GFP) was injected into the vitreous of DBA/1J mice. Two weeks later, the LHON phenotype was induced by AAV delivery of mutant ND4 (AAV2.mND4FLAG) to the vitreous. Retinal function was assessed by pattern electroretinography. Optic nerves were harvested at 4 months, and the effects of XIAP therapy on nerve fiber layer and optic nerve integrity were evaluated using immunohistochemistry, transmission electron microscopy and magnetic resonance imaging.

Results

During LHON disease progression, retinal ganglion cell axons are lost. Apoptotic cell bodies are seen in the nuclei of astrocytes or oligodendrocytes in the optic nerve, and there is thinning of the optic nerve and the nerve fiber layer of the retina. At 4 months after disease onset, XIAP gene therapy protects the nerve fiber layer and optic nerve architecture by preserving axon health. XIAP also decreases nuclear fragmentation in resident astrocytes or oligodendrocytes and decreases glial cell infiltration.

Conclusions

XIAP therapy improves optic nerve health and delays disease progression in LHON.

Double agents of cell death: novel emerging functions of apoptotic regulators

Apoptosis is a highly regulated form of cell death that is required for many homeostatic and pathological processes. Recently, alternative cell death pathways have emerged whose regulation is dependent on proteins with canonical functions in apoptosis. Dysregulation of apoptotic signaling frequently underlies the pathogenesis of many cancers, reinforcing the need to develop therapies that initiate alternative cell death processes. This review outlines the convergence points between apoptosis and other death pathways with the purpose of identifying novel strategies for the treatment of apoptosis-refractory cancers. Apoptosis proteins can play key roles in the initiation, regulation, and execution of nonapoptotic death processes that include necroptosis, autophagy, pyroptosis, mPTP-mediated necrosis, and ferroptosis. Notably, recent evidence illustrates that dying cells can exhibit biochemical and molecular characteristics of more than one different type of regulated cell death. Thus, this review highlights the amazing complexity and interconnectivity of cell death processes and also raises the idea that a top-to-bottom approach to describing cell death mechanisms may be inadequate for fully understanding the means by which cells die.

Multiple roles of caspase-8 in cell death, inflammation, and innate immunity

Caspase-8 is an apical caspase involved in the programmed form of cell death called apoptosis that is critically important for mammalian development and immunity. Apoptosis was historically described as immunologically silent in contrast to other types of programmed cell death such as necroptosis or pyroptosis. Recent reports suggest considerable crosstalk between these different forms of cell death. It is becoming increasingly clear that caspase-8 has many non-apoptotic roles, participating in multiple processes including regulation of necroptosis (mediated by receptor-interacting serine/threonine kinases, RIPK1-RIPK3), inflammatory cytokine expression, inflammasome activation, and cleavage of IL-1β and gasdermin D, and protection against shock and microbial infection. In this review, we discuss the involvement of caspase-8 in cell death and inflammation and highlight its role in innate immune responses and in the relationship between different forms of cell death. Caspase-8 is one of the central components in this type of crosstalk.

Novel XIAP mutation causing enhanced spontaneous apoptosis and disturbed NOD2 signalling in a patient with atypical adult-onset Crohn's disease

X-linked inhibitor of apoptosis (XIAP) is the most potent human inhibitor of apoptosis, and is also involved in NOD2-dependent NFκB and MAPK signalling cascade activation. The absence or defective function of XIAP leads to the development of a rare and severe primary immunodeficiency known as X-linked lymphoproliferative syndrome type 2 (XLP-2), which is characterized by a triad of clinical manifestations, including a high incidence of haemophagocytic lymphohistiocytosis (HLH), lymphoproliferation and inflammatory bowel disease (IBD), usually with very early onset. Here, we present a novel XIAP mutation identified in a patient with atypical adult-onset IBD complicated by relapsing HLH, splenomegaly and sarcoid-like disease. The c.266delA mutation in the XIAP gene creates a premature stop codon, and causes a severe reduction in XIAP protein expression. The mutation is also associated with impaired spontaneous and staurosporine- and PMA-induced apoptosis accompanied by significantly increased expression of pro-apoptotic genes. We also confirmed the negative impact of this particular XIAP mutation on NOD2-dependent NFκB and MAPK activation, while NOD2-independent activation was found to be unaffected. Moreover, we assume that the mutation has an impact on the overproduction of IL-12 and IFNγ, the shift towards the Th1 immune response and increased numbers of central memory and effector memory CD4+ and CD8+ T cells. All these changes contribute to immune dysregulation and the clinical manifestation of XLP-2.

Cell Versus Cytokine - Directed Therapies for Hemophagocytic Lymphohistiocytosis (HLH) in Inborn Errors of Immunity

Hemophagocytic lymphohistiocytosis (HLH) is a heterogeneous hyperinflammatory syndrome with different pathways of pathogenesis resulting in similar clinical presentations. It is best defined and understood if presenting in the context of genetic immunodeficiencies associated with defects of lymphocyte cytotoxicity. In these "primary" forms of HLH, cellular and soluble immune effectors are relatively well characterized. While etoposide-based broad cell-directed therapies remain standard of care, more specific therapies targeting these effectors individually are increasingly available. Anti-CD52 as a cell-directed therapy and anti-IFN-gamma, IL-18BP, and JAK-inhibition as cytokine-directed therapies are expected to broaden the therapeutic options, but the precise role of these drugs in first-line and rescue treatment indications remains to be defined. A number of additional inborn errors of immunity are associated with episodes of immune activation fulfilling the clinical criteria of HLH. Impaired pathogen control is a key driver of hyperinflammation in some conditions, while others are characterized by a strong autoinflammatory component. This heterogeneity of disease-driving factors and the variable severity in disease progression in these conditions do not allow a simple adaptation of protocols established for "primary" HLH to HLH in the context of other inborn errors of immunity. Cytokine-directed therapies hold significant promise in these increasingly recognized disorders.

Sur-X, a novel peptide, kills colorectal cancer cells by targeting survivin-XIAP complex

Background

Survivin and XIAP are two important members of the inhibitor of apoptosis protein family and have been considered as potential targets for cancer treatment due to their overexpression in large variety of cancers including colorectal cancer. It has been reported that survivin and XIAP can synergistically inhibit apoptosis by forming survivin-XIAP complex. In this study, we aimed to design a peptide that targets the survivin-XIAP complex and elucidate its anticancer mechanisms in colorectal cancer cells.

Methods

We designed and synthetized Sur-X, the peptide targeting survivin-XIAP complex. The anticancer effects of Sur-X were evaluated both in vitro and in vivo. The underlying molecular mechanisms were also investigated.

Results

Sur-X exhibited potent inhibitory effects on four colorectal cancer cell lines HCT116, HCT15, RKO and HT29, but not on human peritoneal mesothelial cell line HMrSV5. Mechanistically, Sur-X induced Caspase 9-dependent intrinsic apoptosis in colorectal cancer cells by disrupting the survivin-XIAP complex and subsequently destabilizing survivin and XIAP. Interestingly, we found that Sur-X can also promote necroptosis. It was demonstrated that Sur-X destroyed the interaction between XIAP and TAB1 in the XIAP-TAB1-TAK1 complex, leading to the instability of TAK1, an endogenous necroptosis inhibitor. Subsequently, the accelerated degradation of TAK1 attenuated its inhibition on necroptosis in colorectal cancer cells. Moreover, knockdown of TAK1 restored the sensitivity of TAB1-overexpressing colorectal cancer cells to Sur-X-induced necroptosis. The in vivo pro-apoptotic effect of Sur-X was confirmed by the enhanced TUNEL staining and the decreased expression of survivin and XIAP in tumor tissues from xenograft mouse models. In addition, extensive necrosis and weaker MLKL expression in xenografts provided evidence for the in vivo pro-necroptotic effect of Sur-X.

Conclusions

Peptide Sur-X exhibits strong pro-apoptotic and pro-necroptotic effects in colorectal cancer cells and has a high clinical translation potential in the treatment of colorectal cancer.

Recent Insights on Inflammasomes, Gasdermin Pores, and Pyroptosis

Inflammasomes assemble in the cytosol of myeloid and epithelial cells on sensing of cellular stress and pathogen-associated molecular patterns and serve as scaffolds for recruitment and activation of inflammatory caspases. Inflammasomes play beneficial roles in host and immune responses against diverse pathogens but may also promote inflammatory tissue damage if uncontrolled. Gasdermin D (GSDMD) is a recently identified substrate of murine caspase-1 and caspase-11, and human caspases-1, -4, and -5 that mediates a regulated lytic cell death mode termed pyroptosis. Recent studies have identified pyroptosis as a critical inflammasome effector mechanism that controls inflammasome-dependent cytokine secretion and contributes to antimicrobial defense and inflammasome-mediated autoinflammatory diseases. Here, we review recent developments on inflammasome-associated effector functions with an emphasis on the emerging roles of gasdermin pores and pyroptosis.

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.

Targeting triple-negative breast cancers with the Smac-mimetic birinapant

Smac mimetics target inhibitor of apoptosis (IAP) proteins, thereby suppressing their function to facilitate tumor cell death. Here we have evaluated the efficacy of the preclinical Smac-mimetic compound A and the clinical lead birinapant on breast cancer cells. Both exhibited potent in vitro activity in triple-negative breast cancer (TNBC) cells, including those from patient-derived xenograft (PDX) models. Birinapant was further studied using in vivo PDX models of TNBC and estrogen receptor-positive (ER⁺) breast cancer. Birinapant exhibited single agent activity in all TNBC PDX models and augmented response to docetaxel, the latter through induction of TNF. Transcriptomic analysis of TCGA datasets revealed that genes encoding mediators of Smac-mimetic-induced cell death were expressed at higher levels in TNBC compared with ER⁺ breast cancer, resulting in a molecular signature associated with responsiveness to Smac mimetics. In addition, the cell death complex was preferentially formed in TNBCs versus ER⁺ cells in response to Smac mimetics. Taken together, our findings provide a rationale for prospectively selecting patients whose breast tumors contain a competent death receptor signaling pathway for the further evaluation of birinapant in the clinic.

Die Another Way: Interplay between Influenza A Virus, Inflammation and Cell Death

Influenza A virus (IAV) is a major concern to human health due to the ongoing global threat of a pandemic. Inflammatory and cell death signalling pathways play important roles in host defence against IAV infection. However, severe IAV infections in humans are characterised by excessive inflammation and tissue damage, often leading to fatal disease. While the molecular mechanisms involved in the induction of inflammation during IAV infection have been well studied, the pathways involved in IAV-induced cell death and their impact on immunopathology have not been fully elucidated. There is increasing evidence of significant crosstalk between cell death and inflammatory pathways and a greater understanding of their role in host defence and disease may facilitate the design of new treatments for IAV infection.

Targeting XIAP for Promoting Cancer Cell Death-The Story of ARTS and SMAC

Inhibitors of apoptosis (IAPs) are a family of proteins that regulate cell death and inflammation. XIAP (X-linked IAP) is the only family member that suppresses apoptosis by directly binding to and inhibiting caspases. On the other hand, cIAPs suppress the activation of the extrinsic apoptotic pathway by preventing the formation of pro-apoptotic signaling complexes. IAPs are negatively regulated by IAP-antagonist proteins such as Smac/Diablo and ARTS. ARTS can promote apoptosis by binding and degrading XIAP via the ubiquitin proteasome-system (UPS). Smac can induce the degradation of cIAPs but not XIAP. Many types of cancer overexpress IAPs, thus enabling tumor cells to evade apoptosis. Therefore, IAPs, and in particular XIAP, have become attractive targets for cancer therapy. In this review, we describe the differences in the mechanisms of action between Smac and ARTS, and we summarize efforts to develop cancer therapies based on mimicking Smac and ARTS. Several Smac-mimetic small molecules are currently under evaluation in clinical trials. Initial efforts to develop ARTS-mimetics resulted in a novel class of compounds, which bind and degrade XIAP but not cIAPs. Smac-mimetics can target tumors with high levels of cIAPs, whereas ARTS-mimetics are expected to be effective for cancers with high levels of XIAP.

Signaling pathways involved in the T-cell-mediated immunity against Epstein-Barr virus: Lessons from genetic diseases

Primary immunodeficiencies (PIDs) provide researchers with unique models to understand in vivo immune responses in general and immunity to infections in particular. In humans, impaired immune control of Epstein-Barr virus (EBV) infection is associated with the occurrence of several different immunopathologic conditions; these include non-malignant and malignant B-cell lymphoproliferative disorders, hemophagocytic lymphohistiocytosis (HLH), a severe inflammatory condition, and a chronic acute EBV infection of T cells. Studies of PIDs associated with a predisposition to develop severe, chronic EBV infections have led to the identification of key components of immunity to EBV - notably the central role of T-cell expansion and its regulation in the pathophysiology of EBV-associated diseases. On one hand, the defective expansion of EBV-specific CD8 T cells results from mutations in genes involved in T-cell activation (such as RASGRP1, MAGT1, and ITK), DNA metabolism (CTPS1) or co-stimulatory pathways (CD70, CD27, and TNFSFR9 (also known as CD137/4-1BB)) leads to impaired elimination of proliferating EBV-infected B cells and the occurrence of lymphoma. On the other hand, protracted T-cell expansion and activation after the defective killing of EBV-infected B cells is caused by genetic defects in the components of the lytic granule exocytosis pathway or in the small adapter protein SH2D1A (also known as SAP), a key activator of T- and NK cell-cytotoxicity. In this setting, the persistence of EBV-infected cells results in HLH, a condition characterized by unleashed T-cell and macrophage activation. Moreover, genetic defects causing selective vulnerability to EBV infection have highlighted the role of co-receptor molecules (CD27, CD137, and SLAM-R) selectively involved in immune responses against infected B cells via specific T-B cell interactions.

SMAC mimetics and RIPK inhibitors as therapeutics for chronic inflammatory diseases

New therapeutic approaches for chronic inflammatory diseases such as inflammatory bowel disease, rheumatoid arthritis, and psoriasis are needed because current treatments are often suboptimal in terms of both efficacy and the risks of serious adverse events. Inhibitor of apoptosis proteins (IAPs) are E3 ubiquitin ligases that inhibit cell death pathways and are themselves inhibited by second mitochondria-derived activator of caspases (SMAC). SMAC mimetics (SMs), small-molecule antagonists of IAPs, are being evaluated as cancer therapies in clinical trials. IAPs are also crucial regulators of inflammatory pathways because they influence both the activation of inflammatory genes and the induction of cell death through the receptor-interacting serine-threonine protein kinases (RIPKs), nuclear factor κB (NF-κB)-inducing kinase, and mitogen-activated protein kinases (MAPKs). Furthermore, there is an increasing interest in specifically targeting the substrates of IAP-mediated ubiquitylation, especially RIPK1, RIPK2, and RIPK3, as druggable nodes in inflammation control. Several studies have revealed an anti-inflammatory potential of RIPK inhibitors that either block inflammatory signaling or block the form of inflammatory cell death known as necroptosis. Expanding research on innate immune signaling through pattern recognition receptors that stimulate proinflammatory NF-κB and MAPK signaling may further contribute to uncovering the complex molecular roles used by IAPs and downstream RIPKs in inflammatory signaling. This may benefit and guide the development of SMs or selective RIPK inhibitors as anti-inflammatory therapeutics for various chronic inflammatory conditions.

NLRP3 Inflammasome-A Key Player in Antiviral Responses

The NACHT, LRR, and PYD domains-containing protein 3 (NLRP3) inflammasome is an oligomeric complex comprised of the NOD-like receptor NLRP3, the adaptor ASC, and caspase-1. This complex is crucial to the host's defense against microbes as it promotes IL-1β and IL-18 secretion and induces pyroptosis. NLRP3 recognizes variety of pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) generated during viral replication that triggers the NLRP3 inflammasome-dependent antiviral immune responses and facilitates viral eradication. Meanwhile, several viruses have evolved elaborate strategies to evade the immune system by targeting the NLRP3 inflammasome. In this review, we will focus on the crosstalk between the NLRP3 inflammasome and viruses, provide an overview of viral infection-induced NLRP3 inflammasome activation, and the immune escape strategies of viruses through their modulation of the NLRP3 inflammasome activity.

Anti-apoptotic proteins in the autophagic world: an update on functions of XIAP, Survivin, and BRUCE

X-linked inhibitor of apoptosis protein (XIAP), survivin, and BRUCE are members of the inhibitor-of-apoptosis protein (IAP) family known for their inhibitory effects on caspase activity and dysregulation of these molecules has widely been shown to cause embryonic defects and to promote tumorigenesis in human. Besides the anti-apoptotic functions, recent discoveries have revealed that XIAP, survivin, and BRUCE also exhibit regulatory functions for autophagy in cells. As the role of autophagy in human diseases has already been discussed extensively in different reviews; in this review, we will discuss the emerging autophagic role of XIAP, survivin, and BRUCE in cancer cells. We also provide an update on the anti-apoptotic functions and the roles in maintaining DNA integrity of these molecules. Second mitochondria-derived activator of caspases (Smac) is a pro-apoptotic protein and IAPs are the molecular targets of various Smac mimetics currently under clinical trials. Better understanding on the functions of XIAP, survivin, and BRUCE can enable us to predict possible side effects of these drugs and to design a more “patient-specific” clinical trial for Smac mimetics in the future.

Beyond inflammasomes: emerging function of gasdermins during apoptosis and NETosis

Programmed cell death is a key mechanism involved in several biological processes ranging from development and homeostasis to immunity, where it promotes the removal of stressed, damaged, malignant or infected cells. Abnormalities in the pathways leading to initiation of cell death or removal of dead cells are consequently associated with a range of human diseases including infections, autoinflammatory disease, neurodegenerative disease and cancer. Apoptosis, pyroptosis and NETosis are three well-studied modes of cell death that were traditionally believed to be independent of one another, but emerging evidence indicates that there is extensive cross-talk between them, and that all three pathways can converge onto the activation of the same cell death effector-the pore-forming protein Gasdermin D (GSDMD). In this review, we highlight recent advances in gasdermin research, with a particular focus on the role of gasdermins in pyroptosis, NETosis and apoptosis, as well as cell type-specific consequences of gasdermin activation. In addition, we discuss controversies surrounding a related gasdermin family protein, Gasdermin E (GSDME), in mediating pyroptosis and secondary necrosis following apoptosis, chemotherapy and inflammasome activation.

The interplay of autophagy and non-apoptotic cell death pathways

Autophagy, the process of macromolecular degradation through the lysosome, has been extensively studied for the past decade or two. Autophagy can regulate cell death, especially apoptosis, through selective degradation of both positive and negative apoptosis regulators. However, multiple other programmed cell death pathways exist. As knowledge of these other types of cell death expand, it has been suggested that they also interact with autophagy. In this review, we discuss the molecular mechanisms that comprise three non-apoptotic forms of cell death (necroptosis, pyroptosis and ferroptosis) focusing on how the autophagy machinery regulates these different cell death mechanisms through (i) its degradative functions, i.e., true autophagy, and (ii) other non-degradative functions of the autophagy machinery such as serving as a signaling scaffold or by participating in other autophagy-independent cellular processes.

Effects of XIAP on high fat diet-induced hepatic steatosis: a mechanism involving NLRP3 inflammasome and oxidative stress

Increasing evidence indicates that prolonged fat-rich diet (HFD) ingestion is a predisposing factor for metabolic disorder-associated system inflammation and oxidative stress injury, which contributes to the occurrence of non-alcoholic fatty liver disease (NAFLD). NACHT, LRR and PYD domains-containing protein 3 (NLRP3)-mediated inflammatory infiltration was determined to participate in NAFLD. X-linked inhibitor of apoptosis protein (XIAP) was recently confirmed as an essential regulator for apoptosis in cells. However, the role of XIAP in HFD-induced NAFLD is still not understood. Here, XIAP was characterized with respect to HFD-induced NLRP3 inflammasome activation and reactive oxygen species (ROS) generation in vivo and palmitate (PA)-treated cells in vitro. After HFD administration, hepatic injury was confirmed via histological assessment (grading and staging of NAFLD) and biochemical parameters, oxidative stress, and reduced antioxidant activity. Up-regulated hepatic dysfunction were further indicated by elevated dyslipidemia, lipid accumulation, and decreased fatty acid β-oxidation associated gene expression. Moreover, in the absence of XIAP, NLRP3 signaling activated by HFD-triggered oxidative stress was up-regulated, accompanied by reduction in antioxidants including HO-1, NQO-1, GST, SOD and Nrf2 activity. The detrimental effects of XIAP blocking on hepatic steatosis and related pathologies were also confirmed in PA-treated mouse liver cells. In contrast, overexpression of XIAP by transfection in vitro restrained PA-stimulated hepatic steatosis by suppression of oxidative stress, NLRP3 related inflammatory response, and impairment of Nrf2 activity, further alleviating abnormal metabolic disorder associated NAFLD. Taken together, the present study helped to elucidate how HFD-induced hepatic steatosis was regulated by XIAP, possibly via the inhibition of NLRP3 signaling and oxidative stress injury.

The Mitochondrial Apoptotic Effectors BAX/BAK Activate Caspase-3 and -7 to Trigger NLRP3 Inflammasome and Caspase-8 Driven IL-1β Activation

Intrinsic apoptosis resulting from BAX/BAK-mediated mitochondrial membrane damage is regarded as immunologically silent. We show here that in macrophages, BAX/BAK activation results in inhibitor of apoptosis (IAP) protein degradation to promote caspase-8-mediated activation of IL-1β. Furthermore, BAX/BAK signaling induces a parallel pathway to NLRP3 inflammasome-mediated caspase-1-dependent IL-1β maturation that requires potassium efflux. Remarkably, following BAX/BAK activation, the apoptotic executioner caspases, caspase-3 and -7, act upstream of both caspase-8 and NLRP3-induced IL-1β maturation and secretion. Conversely, the pyroptotic cell death effectors gasdermin D and gasdermin E are not essential for BAX/BAK-induced IL-1β release. These findings highlight that innate immune cells undergoing BAX/BAK-mediated apoptosis have the capacity to generate pro-inflammatory signals and provide an explanation as to why IL-1β activation is often associated with cellular stress, such as during chemotherapy.