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Programmed cell death pathways play an important role in innate immune responses to infection. Activation of intrinsic apoptosis promotes infected cell clearance; however, comparatively little is known about how this mode of cell death is regulated during infections and whether it can induce inflammation. Here, we identify that the pro‐survival BCL‐2 family member, A1, controls activation of the essential intrinsic apoptotic effectors BAX/BAK in macrophages and monocytes following bacterial lipopolysaccharide (LPS) sensing. We show that, due to its tight transcriptional and post‐translational regulation, A1 acts as a molecular rheostat to regulate BAX/BAK‐dependent apoptosis and the subsequent NLRP3 inflammasome‐dependent and inflammasome‐independent maturation of the inflammatory cytokine IL‐1β. Furthermore, induction of A1 expression in inflammatory monocytes limits cell death modalities and IL‐1β activation triggered by Neisseria gonorrhoeae‐derived outer membrane vesicles (NOMVs). Consequently, A1‐deficient mice exhibit heightened IL‐1β production in response to NOMV injection. These findings reveal that bacteria can induce A1 expression to delay myeloid cell death and inflammatory responses, which has implications for the development of host‐directed antimicrobial therapeutics. Synopsis: Cell death and inflammatory signalling crosstalk is critical for innate immune responses to pathogens. This study identifies the BCL‐2 family member A1 as a temporal regulator of these processes in myeloid cells upon bacterial recognition. LPS‐induced A1 expression in macrophages and monocytes delays apoptosis and NLRP3 inflammasome activation.Monocyte survival upon pathogen sensing is regulated by the short‐lived pro‐survival proteins MCL‐1 and A1.A1‐deficient myeloid cells can engage necroptotic signalling when apoptotic caspase activity is compromised.A1‐deficiency enhances IL‐1β responses to Neisseria gonorrhoeae‐derived outer membrane vesicles (NOMVs). [ABSTRACT FROM AUTHOR]

Genetic lesions in X‐linked inhibitor of apoptosis (XIAP) pre‐dispose humans to cell death–associated inflammatory diseases, although the underlying mechanisms remain unclear. Here, we report that two patients with XIAP deficiency–associated inflammatory bowel disease display increased inflammatory IL‐1β maturation as well as cell death–associated caspase‐8 and Gasdermin D (GSDMD) processing in diseased tissue, which is reduced upon patient treatment. Loss of XIAP leads to caspase‐8‐driven cell death and bioactive IL‐1β release that is only abrogated by combined deletion of the apoptotic and pyroptotic cell death machinery. Namely, extrinsic apoptotic caspase‐8 promotes pyroptotic GSDMD processing that kills macrophages lacking both inflammasome and apoptosis signalling components (caspase‐1, ‐3, ‐7, ‐11 and BID), while caspase‐8 can still cause cell death in the absence of both GSDMD and GSDME when caspase‐3 and caspase‐7 are present. Neither caspase‐3 and caspase‐7‐mediated activation of the pannexin‐1 channel, or GSDMD loss, prevented NLRP3 inflammasome assembly and consequent caspase‐1 and IL‐1β maturation downstream of XIAP inhibition and caspase‐8 activation, even though the pannexin‐1 channel was required for NLRP3 triggering upon mitochondrial apoptosis. These findings uncouple the mechanisms of cell death and NLRP3 activation resulting from extrinsic and intrinsic apoptosis signalling, reveal how XIAP loss can co‐opt dual cell death programs, and uncover strategies for targeting the cell death and inflammatory pathways that result from XIAP deficiency. Synopsis: XIAP deficiency causes hemophagocytic lymphohistiocytosis and Crohn's disease. Analysis of XIAP‐deficient patients and cellular studies show that apoptotic caspases and GSDMD act redundantly downstream of caspase‐8 to cause excess cell death and IL‐1β release. XIAP deficiency increases cleaved caspase‐8 and GSDMD in mucosal tissue and cellsCrosstalk in apoptosis and pyroptosis sensitises cells to caspase‐8‐driven death and IL‐1β activation upon XIAP inhibitionPannexin‐1 and GSDMD are dispensable for NLRP3 inflammasome formation downstream of caspase‐8Pannexin‐1 cleavage by caspase‐3 and caspase‐7 is required for efficient NLRP3 activation following intrinsic apoptosis [ABSTRACT FROM AUTHOR]

Objectives: During gastrointestinal infection, dysbiosis can result in decreased production of microbially derived short‐chain fatty acids (SCFAs). In response to the presence of intestinal pathogens, we examined whether an engineered acetate‐ or butyrate‐releasing diet can rectify the deficiency of SCFAs and lead to the resolution of enteric infection. Methods: We tested whether a high acetate‐ or butyrate‐producing diet (HAMSA or HAMSB, respectively) condition Citrobacterrodentium infection in mice and assess its impact on host‐microbiota interactions. We analysed the adaptive and innate immune responses, changes in gut microbiome function, epithelial barrier function and the molecular mechanism via metabolite sensing G protein‐coupled receptor 43 (GPR43) and IL‐22 expression. Results: HAMSA diet rectified the deficiency in acetate production and protected against enteric infection. Increased SCFAs affect the expression of pathogen virulence genes. HAMSA diet promoted compositional and functional changes in the gut microbiota during infection similar to healthy microbiota from non‐infected mice. Bacterial changes were evidenced by the production of proteins involved in acetate utilisation, starch and sugar degradation, amino acid biosynthesis, carbohydrate transport and metabolism. HAMSA diet also induced changes in host proteins critical in glycolysis, wound healing such as GPX1 and epithelial architecture such as EZR1 and PFN1. Dietary acetate assisted in rapid epithelial repair, as shown by increased colonic Muc‐2, Il‐22, and anti‐microbial peptides. We found that acetate increased numbers of colonic IL‐22 producing TCRαβ+CD8αβ+ and TCRγδ+CD8αα+ intraepithelial lymphocytes expressing GPR43. Conclusion: HAMSA diet may be an effective therapeutic approach for fighting inflammation and enteric infections and offer a safe alternative that may impact on human health. [ABSTRACT FROM AUTHOR]

The human pathogens enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC) share a unique mechanism of colonization that results from the concerted action of effector proteins translocated into the host cell by a type III secretion system (T3SS). EPEC and EHEC not only induce characteristic attaching and effacing (A/E) lesions, but also subvert multiple host cell signalling pathways during infection. Our understanding of the mechanisms by which A/E pathogens hijack host cell signalling has advanced dramatically in recent months with the identification of novel activities for many effectors. In addition to further characterization of established effectors (Tir, EspH and Map), new effectors have emerged as important mediators of virulence through activities such as mimicry of Rho guanine nucleotide exchange factors (Map and EspM), inhibition of apoptosis (NleH and NleD), interference with inflammatory signalling pathways (NleB, NleC, NleE and NleH) and phagocytosis (EspF, EspH and EspJ). The findings have highlighted the multifunctional nature of the effectors and their ability to participate in redundant, synergistic or antagonistic relationships, acting in a co-ordinated spatial and temporal manner on different host organelles and cellular pathways during infection. [ABSTRACT FROM AUTHOR]

Many bacterial pathogens utilize a type III secretion system (T3SS) to inject virulence effector proteins into host cells during infection. Previously, we found that enteropathogenic Escherichia coli (EPEC) uses the type III effector, NleE, to block the inflammatory response by inhibiting IκB degradation and nuclear translocation of the p65 subunit of NF-κB. Here we screened further effectors with unknown function for their capacity to prevent p65 nuclear translocation. We observed that ectopic expression of GFP-NleC in HeLa cells led to the degradation of p65. Delivery of NleC by the T3SS of EPEC also induced degradation of p65 in infected cells as well as other NF-κB components, c-Rel and p50. Recombinant His-NleC induced p65 and p50 cleavage in HeLa cell lysates and mutation of a consensus zinc metalloprotease motif, HEIIH, abrogated NleC proteolytic activity. NleC inhibited IL-8 production during prolonged EPEC infection of HeLa cells in a protease activity-dependent manner. A double nleE/nleC mutant was further impaired for its ability to inhibit IL-8 secretion than either a single nleE or a single nleC mutant. We conclude that NleC is a type III effector protease that degrades NF-κB thereby contributing the arsenal of bacterial effectors that inhibit innate immune activation. [ABSTRACT FROM AUTHOR]