RIPK3 cleavage is dispensable for necroptosis inhibition but restricts NLRP3 inflammasome activation

Caspase-8 activity is required to inhibit necroptosis during embryogenesis in mice. In vitro studies have suggested that caspase-8 directly cleaves RIPK1, CYLD and the key necroptotic effector kinase RIPK3 to repress necroptosis. However, recent studies have shown that mice expressing uncleavable RIPK1 die during embryogenesis due to excessive apoptosis, while uncleavable CYLD mice are viable. Therefore, these results raise important questions about the role of RIPK3 cleavage. To evaluate the physiological significance of RIPK3 cleavage, we generated Ripk3D333A/D333A mice harbouring a point mutation in the conserved caspase-8 cleavage site. These mice are viable, demonstrating that RIPK3 cleavage is not essential for blocking necroptosis during development. Furthermore, unlike RIPK1 cleavage-resistant cells, Ripk3D333A/D333A cells were not significantly more sensitive to necroptotic stimuli. Instead, we found that the cleavage of RIPK3 by caspase-8 restricts NLRP3 inflammasome activation-dependent pyroptosis and IL-1β secretion when Inhibitors of APoptosis (IAP) are limited. These results demonstrate that caspase-8 does not inhibit necroptosis by directly cleaving RIPK3 and further underscore a role for RIPK3 in regulating the NLRP3 inflammasome.

IFNγ: Priming for death

TNF signaling does not result in cell death unless multiple inhibitory signals are overcome, which can be accomplished by simultaneous signaling through IFNγ. In this issue, Deng and colleagues (http://doi.org/10.1083/jcb.202305026) dissect the mechanisms by which IFNγ signaling combines with TNF to mediate cell death through caspase-8, discussed by James E. Vince.

Mind bomb 2 limits inflammatory dermatitis in Sharpin mutant mice independently of cell death

Skin inflammation is a complex process implicated in various dermatological disorders. The chronic proliferative dermatitis (cpd) phenotype driven by the cpd mutation (cpdm) in the Sharpin gene is characterized by dermal inflammation and epidermal abnormalities. Tumour necrosis factor (TNF) and caspase-8-driven cell death causes the pathogenesis of Sharpincpdm mice; however, the role of mind bomb 2 (MIB2), a pro-survival E3 ubiquitin ligase involved in TNF signaling, in skin inflammation remains unknown. Here, we demonstrate that MIB2 antagonizes inflammatory dermatitis in the context of the cpd mutation. Surprisingly, the role of MIB2 in limiting skin inflammation is independent of its known pro-survival function and E3 ligase activity. Instead, MIB2 enhances the production of wound-healing molecules, granulocyte colony-stimulating factor, and Eotaxin, within the skin. This discovery advances our comprehension of inflammatory cytokines and chemokines associated with cpdm pathogenesis and highlights the significance of MIB2 in inflammatory skin disease that is independent of its ability to regulate TNF-induced cell death.

CD45 limits early Natural Killer cell development

The clinical development of Natural Killer (NK) cell-mediated immunotherapy marks a milestone in the development of new cancer therapies and has gained traction due to the intrinsic ability of the NK cell to target and kill tumor cells. To fully harness the tumor killing ability of NK cells, we need to improve NK cell persistence and to overcome suppression of NK cell activation in the tumor microenvironment. The trans-membrane, protein tyrosine phosphatase CD45, regulates NK cell homeostasis, with the genetic loss of CD45 in mice resulting in increased numbers of mature NK cells. This suggests that CD45-deficient NK cells might display enhanced persistence following adoptive transfer. However, we demonstrate here that adoptive transfer of CD45-deficiency did not enhance NK cell persistence in mice, and instead, the homeostatic disturbance of NK cells in CD45-deficient mice stemmed from a developmental defect in the progenitor population. The enhanced maturation within the CD45-deficient NK cell compartment was intrinsic to the NK cell lineage, and independent of the developmental defect. CD45 is not a conventional immune checkpoint candidate, as systemic loss is detrimental to T and B cell development, compromising the adaptive immune system. Nonetheless, this study suggests that inhibition of CD45 in progenitor or stem cell populations may improve the yield of in vitro generated NK cells for adoptive therapy.

A1 is induced by pathogen ligands to limit myeloid cell death and NLRP3 inflammasome activation

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.

The role of caspase-8 in inflammatory signalling and pyroptotic cell death

The programmed cell death machinery exhibits surprising flexibility, capable of crosstalk and non-apoptotic roles. Much of this complexity arises from the diverse functions of caspase-8, a cysteine-aspartic acid protease typically associated with activating caspase-3 and - 7 to induce apoptosis. However, recent research has revealed that caspase-8 also plays a role in regulating the lytic gasdermin cell death machinery, contributing to pyroptosis and immune responses in contexts such as infection, autoinflammation, and T-cell signalling. In mice, loss of caspase-8 results in embryonic lethality from unrestrained necroptotic killing, while in humans caspase-8 deficiency can lead to an autoimmune lymphoproliferative syndrome, immunodeficiency, inflammatory bowel disease or, when it can't cleave its substrate RIPK1, early onset periodic fevers. This review focuses on non-canonical caspase-8 signalling that drives immune responses, including its regulation of inflammatory gene transcription, activation within inflammasome complexes, and roles in pyroptotic cell death. Ultimately, a deeper understanding of caspase-8 function will aid in determining whether, and when, targeting caspase-8 pathways could be therapeutically beneficial in human diseases.

FBXO11 governs macrophage cell death and inflammation in response to bacterial toxins

Staphylococcus aureus causes severe infections such as pneumonia and sepsis depending on the pore-forming toxin Panton-Valentine leukocidin (PVL). PVL kills and induces inflammation in macrophages and other myeloid cells by interacting with the human cell surface receptor, complement 5a receptor 1 (C5aR1). C5aR1 expression is tighly regulated and may thus modulate PVL activity, although the mechanisms involved remain incompletely understood. Here, we used a genome-wide CRISPR/Cas9 screen and identified F-box protein 11 (FBXO11), an E3 ubiquitin ligase complex member, to promote PVL toxicity. Genetic deletion of FBXO11 reduced the expression of C5aR1 at the mRNA level, whereas ectopic expression of C5aR1 in FBXO11-/- macrophages, or priming with LPS, restored C5aR1 expression and thereby PVL toxicity. In addition to promoting PVL-mediated killing, FBXO11 dampens secretion of IL-1β after NLRP3 activation in response to bacterial toxins by reducing mRNA levels in a BCL-6-dependent and BCL-6-independent manner. Overall, these findings highlight that FBXO11 regulates C5aR1 and IL-1β expression and controls macrophage cell death and inflammation following PVL exposure.

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

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

A Streamlined Method for Detecting Inflammasome-Induced ASC Oligomerization Using Chemical Crosslinking

The apoptosis-associated speck-like protein containing a caspase activation and recruitment domain (ASC) functions as the integral adaptor protein between inflammasome sensors such as NOD-like receptor protein 3 (NLRP3) and the inflammatory caspase, caspase-1. Inflammasome sensor triggering allows recruitment of ASC and the formation of long amyloid-like ASC oligomers that enable binding and proximity-induced activation of caspase-1. The detection of ASC oligomerization thus constitutes a highly specific and direct test for inflammasome complex formation and activation. Here, we describe a simplified and streamlined method for the detection of ASC oligomers via Western blotting, using the chemical crosslinking reagent disuccinimidyl suberate.

Epithelial-to-Mesenchymal Transition Supports Ovarian Carcinosarcoma Tumorigenesis and Confers Sensitivity to Microtubule Targeting with Eribulin

Ovarian carcinosarcoma (OCS) is an aggressive and rare tumor type with limited treatment options. OCS is hypothesized to develop via the combination theory, with a single progenitor resulting in carcinomatous and sarcomatous components, or alternatively via the conversion theory, with the sarcomatous component developing from the carcinomatous component through epithelial-to-mesenchymal transition (EMT). In this study, we analyzed DNA variants from isolated carcinoma and sarcoma components to show that OCS from 18 women is monoclonal. RNA sequencing indicated that the carcinoma components were more mesenchymal when compared with pure epithelial ovarian carcinomas, supporting the conversion theory and suggesting that EMT is important in the formation of these tumors. Preclinical OCS models were used to test the efficacy of microtubule-targeting drugs, including eribulin, which has previously been shown to reverse EMT characteristics in breast cancers and induce differentiation in sarcomas. Vinorelbine and eribulin more effectively inhibited OCS growth than standard-of-care platinum-based chemotherapy, and treatment with eribulin reduced mesenchymal characteristics and N-MYC expression in OCS patient-derived xenografts. Eribulin treatment resulted in an accumulation of intracellular cholesterol in OCS cells, which triggered a downregulation of the mevalonate pathway and prevented further cholesterol biosynthesis. Finally, eribulin increased expression of genes related to immune activation and increased the intratumoral accumulation of CD8+ T cells, supporting exploration of immunotherapy combinations in the clinic. Together, these data indicate that EMT plays a key role in OCS tumorigenesis and support the conversion theory for OCS histogenesis. Targeting EMT using eribulin could help improve OCS patient outcomes.

SIGNIFICANCE

Genomic analyses and preclinical models of ovarian carcinosarcoma support the conversion theory for disease development and indicate that microtubule inhibitors could be used to suppress EMT and stimulate antitumor immunity.

The escape of Candida albicans from macrophages is enabled by the fungal toxin candidalysin and two host cell death pathways

The egress of Candida hyphae from macrophages facilitates immune evasion, but it also alerts macrophages to infection and triggers inflammation. To better define the mechanisms, here we develop an imaging assay to directly and dynamically quantify hyphal escape and correlate it to macrophage responses. The assay reveals that Candida escapes by using two pore-forming proteins to permeabilize macrophage membranes: the fungal toxin candidalysin and Nlrp3 inflammasome-activated Gasdermin D. Candidalysin plays a major role in escape, with Nlrp3 and Gasdermin D-dependent and -independent contributions. The inactivation of Nlrp3 does not reduce hyphal escape, and we identify ETosis via macrophage extracellular trap formation as an additional pathway facilitating hyphal escape. Suppressing hyphal escape does not reduce fungal loads, but it does reduce inflammatory activation. Our findings explain how Candida escapes from macrophages by using three strategies: permeabilizing macrophage membranes via candidalysin and engaging two host cell death pathways, Gasdermin D-mediated pyroptosis and ETosis.

Ubiquitylation of RIPK3 beyond-the-RHIM can limit RIPK3 activity and cell death

Pathogen recognition and TNF receptors signal via receptor interacting serine/threonine kinase-3 (RIPK3) to cause cell death, including MLKL-mediated necroptosis and caspase-8-dependent apoptosis. However, the post-translational control of RIPK3 is not fully understood. Using mass-spectrometry, we identified that RIPK3 is ubiquitylated on K469. The expression of mutant RIPK3 K469R demonstrated that RIPK3 ubiquitylation can limit both RIPK3-mediated apoptosis and necroptosis. The enhanced cell death of overexpressed RIPK3 K469R and activated endogenous RIPK3 correlated with an overall increase in RIPK3 ubiquitylation. Ripk3^K469R/K469R mice challenged with Salmonella displayed enhanced bacterial loads and reduced serum IFNγ. However, Ripk3^K469R/K469R macrophages and dermal fibroblasts were not sensitized to RIPK3-mediated apoptotic or necroptotic signaling suggesting that, in these cells, there is functional redundancy with alternate RIPK3 ubiquitin-modified sites. Consistent with this idea, the mutation of other ubiquitylated RIPK3 residues also increased RIPK3 hyper-ubiquitylation and cell death. Therefore, the targeted ubiquitylation of RIPK3 may act as either a brake or accelerator of RIPK3-dependent killing.

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RIPK3 can be ubiquitylated on K469 to limit RIPK3-induced necroptosis and apoptosis

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Ripk3^K469R/K469R mice are more susceptible to Salmonella infection

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Several ubiquitylated or surface exposed lysines can limit RIPK3-induced cell death

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Hyper-ubiquitylated RIPK3 correlates with RIPK3 signaling and cell death

CXCL11 expressing C57BL/6 mice have intact adaptive immune responses to viral infection

The chemokine receptor CXCR3 is expressed on immune cells to co‐ordinate lymphocyte activation and migration. CXCR3 binds three chemokine ligands, CXCL9, CXCL10 and CXCL11. These ligands display distinct expression patterns and ligand signaling biases; however, how each ligand functions individually and collaboratively is incompletely understood. CXCL9 and CXCL10 are considered pro‐inflammatory chemokines during viral infection, while CXCL11 may induce a tolerizing state. The investigation of the individual role of CXCL11 in vivo has been hampered as C57BL/6 mice carry several mutations that result in a null allele. Here, CRISPR/Cas9 was used to correct these mutations on a C57BL/6 background. It was validated that CXCL11^KI mice expressed CXCL11 protein in dendritic cells, spleen and lung. CXCL11^KI mice were largely phenotypically indistinguishable from C57BL/6 mice, both at steady‐state and during two models of viral infection. While CXCL11 expression did not modify acute antiviral responses, this study provides a new tool to understand the role of CXCL11 in other experimental settings.

Interferon-γ primes macrophages for pathogen ligand-induced killing via a caspase-8 and mitochondrial cell death pathway

Cell death plays an important role during pathogen infections. Here, we report that interferon-γ (IFNγ) sensitizes macrophages to Toll-like receptor (TLR)-induced death that requires macrophage-intrinsic death ligands and caspase-8 enzymatic activity, which trigger the mitochondrial apoptotic effectors, BAX and BAK. The pro-apoptotic caspase-8 substrate BID was dispensable for BAX and BAK activation. Instead, caspase-8 reduced pro-survival BCL-2 transcription and increased inducible nitric oxide synthase (iNOS), thus facilitating BAX and BAK signaling. IFNγ-primed, TLR-induced macrophage killing required iNOS, which licensed apoptotic caspase-8 activity and reduced the BAX and BAK inhibitors, A1 and MCL-1. The deletion of iNOS or caspase-8 limited SARS-CoV-2-induced disease in mice, while caspase-8 caused lethality independent of iNOS in a model of hemophagocytic lymphohistiocytosis. These findings reveal that iNOS selectively licenses programmed cell death, which may explain how nitric oxide impacts disease severity in SARS-CoV-2 infection and other iNOS-associated inflammatory conditions.

The ubiquitylation of IL-1β limits its cleavage by caspase-1 and targets it for proteasomal degradation

Interleukin-1β (IL-1β) is activated by inflammasome-associated caspase-1 in rare autoinflammatory conditions and in a variety of other inflammatory diseases. Therefore, IL-1β activity must be fine-tuned to enable anti-microbial responses whilst limiting collateral damage. Here, we show that precursor IL-1β is rapidly turned over by the proteasome and this correlates with its decoration by K11-linked, K63-linked and K48-linked ubiquitin chains. The ubiquitylation of IL-1β is not just a degradation signal triggered by inflammasome priming and activating stimuli, but also limits IL-1β cleavage by caspase-1. IL-1β K133 is modified by ubiquitin and forms a salt bridge with IL-1β D129. Loss of IL-1β K133 ubiquitylation, or disruption of the K133:D129 electrostatic interaction, stabilizes IL-1β. Accordingly, Il1b^K133R/K133R mice have increased levels of precursor IL-1β upon inflammasome priming and increased production of bioactive IL-1β, both in vitro and in response to LPS injection. These findings identify mechanisms that can limit IL-1β activity and safeguard against damaging inflammation.

Specific NLRP3 Inhibition Protects Against Diabetes-Associated Atherosclerosis

Low-grade persistent inflammation is a feature of diabetes-driven vascular complications, in particular activation of the Nod-like receptor family pyrin domain containing 3 (NLRP3) inflammasome to trigger the maturation and release of the inflammatory cytokine interleukin-1β (IL-1β). We investigated whether inhibiting the NLRP3 inflammasome, through the use of the specific small-molecule NLRP3 inhibitor MCC950, could reduce inflammation, improve vascular function, and protect against diabetes-associated atherosclerosis in the streptozotocin-induced diabetic apolipoprotein E-knockout mouse. Diabetes led to an approximately fourfold increase in atherosclerotic lesions throughout the aorta, which were significantly attenuated with MCC950 (P < 0.001). This reduction in lesions was associated with decreased monocyte-macrophage content, reduced necrotic core, attenuated inflammatory gene expression (IL-1β, tumor necrosis factor-α, intracellular adhesion molecule 1, and MCP-1; P < 0.05), and reduced oxidative stress, while maintaining fibrous cap thickness. Additionally, vascular function was improved in diabetic vessels of mice treated with MCC950 (P < 0.05). In a range of cell lines (murine bone marrow-derived macrophages, human monocytic THP-1 cells, phorbol 12-myristate 13-acetate-differentiated human macrophages, and aortic smooth muscle cells from humans with diabetes), MCC950 significantly reduced IL-1β and/or caspase-1 secretion and attenuated leukocyte-smooth muscle cell interactions under high glucose or lipopolysaccharide conditions. In summary, MCC950 reduces plaque development, promotes plaque stability, and improves vascular function, suggesting that targeting NLRP3-mediated inflammation is a novel therapeutic strategy to improve diabetes-associated vascular disease.

Deletion of IKK2 in haematopoietic cells of adult mice leads to elevated interleukin-6, neutrophilia and fatal gastrointestinal inflammation

The IκB kinase complex, consisting of IKK1, IKK2 and the regulatory subunit NEMO, is required for NF-κB signalling following the activation of several cell surface receptors, such as members of the Tumour Necrosis Factor Receptor superfamily and the Interleukin-1 Receptor. This is critical for haematopoietic cell proliferation, differentiation, survival and immune responses. To determine the role of IKK in the regulation of haematopoiesis, we used the Rosa26^Cre-ERT2 Cre/lox recombination system to achieve targeted, haematopoietic cell-restricted deletion of the genes for IKK1 or IKK2 in vivo. We found that the IKK complex plays a critical role in haematopoietic cell development and function. Deletion of IKK2, but not loss of IKK1, in haematopoietic cells led to an expansion of CD11b/Gr-1-positive myeloid cells (neutrophilia), severe anaemia and thrombocytosis, with reduced numbers of long-term haematopoietic stem cells (LT-HSCs), short-term haematopoietic stem cells (ST-HSCs) and multipotential progenitor cells (MPPs), increased circulating interleukin-6 (IL-6) and severe gastrointestinal inflammation. These findings identify distinct functions for the two IKK catalytic subunits, IKK1 and IKK2, in the haematopoietic system.

Necroptotic movers and shakers: cell types, inflammatory drivers and diseases

The necroptotic cell death pathway has received significant attention for its ability to trigger inflammatory responses and its potential involvement in related conditions. Recent insights into the essential membrane damaging necroptotic pseudokinase effector, Mixed lineage kinase domain like (MLKL), have revealed a number of diverse MLKL functions that contribute to the inflammatory nature of necroptosis. Here we review distinct MLKL signalling roles and document the immunogenic molecules released by necroptosis. We discuss specific in vivo MLKL-driven responses, the activation of inflammasome complexes and innate lymphoid cells, which have been documented to drive disease. Finally, we list necroptotic competent cell types and their involvement in MLKL-driven cell death-associated and inflammatory-associated conditions.

Flexible Usage and Interconnectivity of Diverse Cell Death Pathways Protect against Intracellular Infection

Programmed cell death contributes to host defense against pathogens. To investigate the relative importance of pyroptosis, necroptosis, and apoptosis during Salmonella infection, we infected mice and macrophages deficient for diverse combinations of caspases-1, -11, -12, and -8 and receptor interacting serine/threonine kinase 3 (RIPK3). Loss of pyroptosis, caspase-8-driven apoptosis, or necroptosis had minor impact on Salmonella control. However, combined deficiency of these cell death pathways caused loss of bacterial control in mice and their macrophages, demonstrating that host defense can employ varying components of several cell death pathways to limit intracellular infections. This flexible use of distinct cell death pathways involved extensive cross-talk between initiators and effectors of pyroptosis and apoptosis, where initiator caspases-1 and -8 also functioned as executioners when all known effectors of cell death were absent. These findings uncover a highly coordinated and flexible cell death system with in-built fail-safe processes that protect the host from intracellular infections.

Mitochondrial dysfunction caused by outer membrane vesicles from Gram-negative bacteria activates intrinsic apoptosis and inflammation

Sensing of microbes activates the innate immune system, depending on functional mitochondria. However, pathogenic bacteria inhibit mitochondrial activity by delivering toxins via outer membrane vesicles (OMVs). How macrophages respond to pathogenic microbes that target mitochondria remains unclear. Here, we show that macrophages exposed to OMVs from Neisseria gonorrhoeae, uropathogenic Escherichia coli and Pseudomonas aeruginosa induce mitochondrial apoptosis and NLRP3 inflammasome activation. OMVs and toxins that cause mitochondrial dysfunction trigger inhibition of host protein synthesis, which depletes the unstable BCL-2 family member MCL-1 and induces BAK-dependent mitochondrial apoptosis. In parallel with caspase-11-mediated pyroptosis, mitochondrial apoptosis and potassium ion efflux activate the NLRP3 inflammasome after OMV exposure in vitro. Importantly, in the in vivo setting, the activation and release of interleukin-1β in response to N. gonorrhoeae OMVs is regulated by mitochondrial apoptosis. Our data highlight how innate immune cells sense infections by monitoring mitochondrial health.

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.

The Pyroptotic Cell Death Effector Gasdermin D Is Activated by Gout-Associated Uric Acid Crystals but Is Dispensable for Cell Death and IL-1β Release

The pyroptotic cell death effector gasdermin D (GSDMD) is required for murine models of hereditary inflammasome-driven, IL-1β-dependent, autoinflammatory disease, making it an attractive therapeutic target. However, the importance of GSDMD for more common conditions mediated by pathological IL-1β activation, such as gout, remain unclear. In this study, we address whether GSDMD and the recently described GSDMD inhibitor necrosulfonamide (NSA) contribute to monosodium urate (MSU) crystal-induced cell death, IL-1β release, and autoinflammation. We demonstrate that MSU crystals, the etiological agent of gout, rapidly activate GSDMD in murine macrophages. Despite this, the genetic deletion of GSDMD or the other lytic effector implicated in MSU crystal killing, mixed lineage kinase domain-like (MLKL), did not prevent MSU crystal-induced cell death. Consequently, GSDMD or MLKL loss did not hinder MSU crystal-mediated release of bioactive IL-1β. Consistent with in vitro findings, IL-1β induction and autoinflammation in MSU crystal-induced peritonitis was not reduced in GSDMD-deficient mice. Moreover, we show that the reported GSDMD inhibitor, NSA, blocks inflammasome priming and caspase-1 activation, thereby preventing pyroptosis independent of GSDMD targeting. The inhibition of cathepsins, widely implicated in particle-induced macrophage killing, also failed to prevent MSU crystal-mediated cell death. These findings 1) demonstrate that not all IL-1β-driven autoinflammatory conditions will benefit from the therapeutic targeting of GSDMD, 2) document a unique mechanism of MSU crystal-induced macrophage cell death not rescued by pan-cathepsin inhibition, and 3) show that NSA inhibits inflammasomes upstream of GSDMD to prevent pyroptotic cell death and IL-1β release.

Ion Man: GSDMD Punches Pores to Knock Out cGAS

The pore-forming protein GSDMD promotes cytokine release and induces pyroptotic cell death. In this issue of Immunity, Banerjee et al. (2018) document how GSDMD triggers potassium efflux to inhibit cGAS-STING and prevent damaging interferon production after bacterial infection.

Activated MLKL attenuates autophagy following its translocation to intracellular membranes

Necroptosis is an inflammatory form of programmed cell death mediated by the pseudokinase mixed-lineage kinase domain-like protein (MLKL). Upon phosphorylation by receptor-interacting protein kinase-3 (RIPK3), MLKL oligomerizes, and translocates to and disrupts the plasma membrane, thereby causing necroptotic cell lysis. Herein, we show that activation of necroptosis in mouse dermal fibroblasts (MDFs) and HT-29 human colorectal cancer cells results in accumulation of the autophagic marker, lipidated LC3B (also known as MAP1LC3B), in an MLKL-dependent manner. Unexpectedly, the necroptosis-induced increase in lipidated LC3B was due to inhibition of autophagic flux, not the activation of autophagy. Inhibition of autophagy by MLKL correlated with a decrease in autophagosome and/or autolysosome function, and required the association of activated MLKL with intracellular membranes. Collectively, our findings uncover an additional role for the MLKL pseudokinase, namely to inhibit autophagy during necroptosis.

Pyroptosis versus necroptosis: similarities, differences, and crosstalk

Pyroptosis and necroptosis represent two pathways of genetically encoded necrotic cell death. Although these cell death programmes can protect the host against microbial pathogens, their dysregulation has been implicated in a variety of autoimmune and auto-inflammatory conditions. The disease-promoting potential of necroptosis and pyroptosis is likely a consequence of their ability to induce a lytic cell death. This cell suicide mechanism, distinct from apoptosis, allows the release of immunogenic cellular content, including damage-associated molecular patterns (DAMPs), and inflammatory cytokines such as interleukin-1β (IL-1β), to trigger inflammation. In this Review, we discuss recent discoveries that have advanced our understanding on the primary functions of pyroptosis and necroptosis, including evidence for the specific cytokines and DAMPs responsible for driving inflammation. We compare the similar and unique aspects of pyroptotic- and necroptotic-induced membrane damage, and explore how these may functionally impact distinct intracellular organelles and signalling pathways. We also examine studies highlighting the crosstalk that can occur between necroptosis and pyroptosis signalling, and evidence supporting the physiological significance of this convergence. Ultimately, a better understanding of the similarities, unique aspects and crosstalk of pyroptosis and necroptosis will inform as to how these cell death pathways might be manipulated for therapeutic benefit.

The pseudokinase MLKL activates PAD4-dependent NET formation in necroptotic neutrophils

Neutrophil extracellular trap (NET) formation can generate short-term, functional anucleate cytoplasts and trigger loss of cell viability. We demonstrated that the necroptotic cell death effector mixed lineage kinase domain-like (MLKL) translocated from the cytoplasm to the plasma membrane and stimulated downstream NADPH oxidase-independent ROS production, loss of cytoplasmic granules, breakdown of the nuclear membrane, chromatin decondensation, histone hypercitrullination, and extrusion of bacteriostatic NETs. This process was coordinated by receptor-interacting protein kinase-1 (RIPK1), which activated the caspase-8-dependent apoptotic or RIPK3/MLKL-dependent necroptotic death of mouse and human neutrophils. Genetic deficiency of RIPK3 and MLKL prevented NET formation but did not prevent cell death, which was because of residual caspase-8-dependent activity. Peptidylarginine deiminase 4 (PAD4) was activated downstream of RIPK1/RIPK3/MLKL and was required for maximal histone hypercitrullination and NET extrusion. This work defines a distinct signaling network that activates PAD4-dependent NET release for the control of methicillin-resistant Staphylococcus aureus (MRSA) infection.

Cutting Edge: Blockade of Inhibitor of Apoptosis Proteins Sensitizes Neutrophils to TNF- but Not Lipopolysaccharide-Mediated Cell Death and IL-1β Secretion

The mammalian inhibitor of apoptosis proteins (IAPs) are key regulators of cell death and inflammation. A major function of IAPs is to block the formation of a cell death-inducing complex, termed the ripoptosome, which can trigger caspase-8-dependent apoptosis or caspase-independent necroptosis. Recent studies report that upon TLR4 or TNF receptor 1 (TNFR1) signaling in macrophages, the ripoptosome can also induce NLRP3 inflammasome formation and IL-1β maturation. Whether neutrophils have the capacity to assemble a ripoptosome to induce cell death and inflammasome activation during TLR4 and TNFR1 signaling is unclear. In this study, we demonstrate that murine neutrophils can signal via TNFR1-driven ripoptosome assembly to induce both cell death and IL-1β maturation. However, unlike macrophages, neutrophils suppress TLR4-dependent cell death and NLRP3 inflammasome activation during IAP inhibition via deficiencies in the CD14/TRIF arm of TLR4 signaling.

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

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

Oxidative Stress and NLRP3-Inflammasome Activity as Significant Drivers of Diabetic Cardiovascular Complications: Therapeutic Implications

It is now increasingly appreciated that inflammation is not limited to the control of pathogens by the host, but rather that sterile inflammation which occurs in the absence of viral or bacterial pathogens, accompanies numerous disease states, none more so than the complications that arise as a result of hyperglycaemia. Individuals with type 1 or type 2 diabetes mellitus (T1D, T2D) are at increased risk of developing cardiac and vascular complications. Glucose and blood pressure lowering therapies have not stopped the advance of these morbidities that often lead to fatal heart attacks and/or stroke. A unifying mechanism of hyperglycemia-induced cellular damage was initially proposed to link elevated blood glucose levels with oxidative stress and the dysregulation of metabolic pathways. Pre-clinical evidence has, in most cases, supported this notion. However, therapeutic strategies to lessen oxidative stress in clinical trials has not proved efficacious, most likely due to indiscriminate targeting by antioxidants such as vitamins. Recent evidence now suggests that oxidative stress is a major driver of inflammation and vice versa, with the latest findings suggesting not only a key role for inflammatory pathways underpinning metabolic and haemodynamic dysfunction in diabetes, but furthermore that these perturbations are driven by activation of the NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome. This review will address these latest findings with an aim of highlighting the interconnectivity between oxidative stress, NLRP3 activation and inflammation as it pertains to cardiac and vascular injury sustained by diabetes. Current therapeutic strategies to lessen both oxidative stress and inflammation will be emphasized. This will be placed in the context of improving the burden of these diabetic complications.

CD52 inhibits Toll-like receptor activation of NF-κB and triggers apoptosis to suppress inflammation

Soluble CD52 is a small glycoprotein that suppresses T-cell activation, but its effect on innate immune cell function is unknown. Here we demonstrate that soluble CD52 inhibits Toll-like receptor and tumor necrosis factor receptor signaling to limit activation of NF-κB and thereby suppress the production of inflammatory cytokines by macrophages, monocytes and dendritic cells. At higher concentrations, soluble CD52 depletes the short-lived pro-survival protein MCL-1, contributing to activation of the BH3-only proteins BAX and BAK to cause intrinsic apoptotic cell death. In vivo, administration of soluble CD52 suppresses lipopolysaccharide (LPS)-induced cytokine secretion and other features of endotoxic shock, whereas genetic deletion of CD52 exacerbates LPS responses. Thus, soluble CD52 exhibits broad immune suppressive effects that signify its potential as an immunotherapeutic agent.

Caspase-1 Is an Apical Caspase Leading to Caspase-3 Cleavage in the AIM2 Inflammasome Response, Independent of Caspase-8

Canonical inflammasomes are multiprotein complexes that can activate both caspase-1 and caspase-8. Caspase-1 drives rapid lysis of cells by pyroptosis and maturation of interleukin (IL)-1β and IL-18. In caspase-1-deficient cells, inflammasome formation still leads to caspase-3 activation and slower apoptotic death, dependent on caspase-8 as an apical caspase. A role for caspase-8 directly upstream of caspase-1 has also been suggested, but here we show that caspase-8-deficient macrophages have no defect in AIM2 inflammasome-mediated caspase-1 activation, pyroptosis, and IL-1β cleavage. In investigating the inflammasome-induced apoptotic pathway, we previously demonstrated that activated caspase-8 is essential for caspase-3 cleavage and apoptosis in caspase-1-deficient cells. However, here we found that AIM2 inflammasome-initiated caspase-3 cleavage was maintained in Ripk3^-/-Casp8^-/- macrophages. Gene knockdown showed that caspase-1 was required for the caspase-3 cleavage. Thus inflammasomes activate a network of caspases that can promote both pyroptotic and apoptotic cell death. In cells where rapid pyroptosis is blocked, delayed inflammasome-dependent cell death could still occur due to both caspase-1- and caspase-8-dependent apoptosis. Initiation of redundant cell death pathways is likely to be a strategy for coping with pathogen interference in death processes.

Posttranslational Modification as a Critical Determinant of Cytoplasmic Innate Immune Recognition

Cell surface innate immune receptors can directly detect a variety of extracellular pathogens to which cytoplasmic innate immune sensors are rarely exposed. Instead, within the cytoplasm, the environment is rife with cellular machinery and signaling pathways that are indirectly perturbed by pathogenic microbes to activate intracellular sensors, such as pyrin, NLRP1, NLRP3, or NLRC4. Therefore, subtle changes in key intracellular processes such as phosphorylation, ubiquitination, and other pathways leading to posttranslational protein modification are key determinants of innate immune recognition in the cytoplasm. This concept is critical to establish the “guard hypothesis” whereby otherwise homeostatic pathways that keep innate immune sensors at bay are released in response to alterations in their posttranslational modification status. Originally identified in plants, evidence that a similar guardlike mechanism exists in humans has recently been identified, whereby a mutation that prevents phosphorylation of the innate immune sensor pyrin triggers a dominantly inherited autoinflammatory disease. It is also noteworthy that even when a cytoplasmic innate immune sensor has a direct ligand, such as bacterial peptidoglycan (NOD1 or NOD2), RNA (RIG-I or MDA5), or DNA (cGAS or IFI16), it can still be influenced by posttranslational modification to dramatically alter its response. Therefore, due to their existence in the cytoplasmic milieu, posttranslational modification is a key determinant of intracellular innate immune receptor functionality.

Legionella pneumophila Strain 130b Evades Macrophage Cell Death Independent of the Effector SidF in the Absence of Flagellin

The human pathogen Legionella pneumophila must evade host cell death signaling to enable replication in lung macrophages and to cause disease. After bacterial growth, however, L. pneumophila is thought to induce apoptosis during egress from macrophages. The bacterial effector protein, SidF, has been shown to control host cell survival and death by inhibiting pro-apoptotic BNIP3 and BCL-RAMBO signaling. Using live-cell imaging to follow the L. pneumophila-macrophage interaction, we now demonstrate that L. pneumophila evades host cell apoptosis independent of SidF. In the absence of SidF, L. pneumophila was able to replicate, cause loss of mitochondria membrane potential, kill macrophages, and establish infections in lungs of mice. Consistent with this, deletion of BNIP3 and BCL-RAMBO did not affect intracellular L. pneumophila replication, macrophage death rates, and in vivo bacterial virulence. Abrogating mitochondrial cell death by genetic deletion of the effectors of intrinsic apoptosis, BAX, and BAK, or the regulator of mitochondrial permeability transition pore formation, cyclophilin-D, did not affect bacterial growth or the initial killing of macrophages. Loss of BAX and BAK only marginally limited the ability of L. pneumophila to efficiently kill all macrophages over extended periods. L. pneumophila induced killing of macrophages was delayed in the absence of capsase-11 mediated pyroptosis. Together, our data demonstrate that L. pneumophila evades host cell death responses independently of SidF during replication and can induce pyroptosis to kill macrophages in a timely manner.

Combination of IAP antagonist and IFNγ activates novel caspase-10- and RIPK1-dependent cell death pathways

Peptido-mimetic inhibitor of apoptosis protein (IAP) antagonists (Smac mimetics (SMs)) can kill tumour cells by depleting endogenous IAPs and thereby inducing tumour necrosis factor (TNF) production. We found that interferon-γ (IFNγ) synergises with SMs to kill cancer cells independently of TNF- and other cell death receptor signalling pathways. Surprisingly, CRISPR/Cas9 HT29 cells doubly deficient for caspase-8 and the necroptotic pathway mediators RIPK3 or MLKL were still sensitive to IFNγ/SM-induced killing. Triple CRISPR/Cas9-knockout HT29 cells lacking caspase-10 in addition to caspase-8 and RIPK3 or MLKL were resistant to IFNγ/SM killing. Caspase-8 and RIPK1 deficiency was, however, sufficient to protect cells from IFNγ/SM-induced cell death, implying a role for RIPK1 in the activation of caspase-10. These data show that RIPK1 and caspase-10 mediate cell death in HT29 cells when caspase-8-mediated apoptosis and necroptosis are blocked and help to clarify how SMs operate as chemotherapeutic agents.

Caspase-8: not so silently deadly

Apoptosis is a caspase-dependent programmed form of cell death, which is commonly believed to be an immunologically silent process, required for mammalian development and maintenance of cellular homoeostasis. In contrast, lytic forms of cell death, such as RIPK3- and MLKL-driven necroptosis, and caspase-1/11-dependent pyroptosis, are postulated to be inflammatory via the release of damage associated molecular patterns (DAMPs). Recently, the function of apoptotic caspase-8 has been extended to the negative regulation of necroptosis, the cleavage of inflammatory interleukin-1β (IL-1β) to its mature bioactive form, either directly or via the NLRP3 inflammasome, and the regulation of cytokine transcriptional responses. In view of these recent advances, human autoinflammatory diseases that are caused by mutations in cell death regulatory machinery are now associated with inappropriate inflammasome activation. In this review, we discuss the emerging crosstalk between cell death and innate immune cell inflammatory signalling, particularly focusing on novel non-apoptotic functions of caspase-8. We also highlight the growing number of autoinflammatory diseases that are associated with enhanced inflammasome function.

Cell death is not essential for caspase-1-mediated interleukin-1β activation and secretion

Caspase-1 cleaves and activates the pro-inflammatory cytokine interleukin-1 beta (IL-1β), yet the mechanism of IL-1β release and its dependence on cell death remains controversial. To address this issue, we generated a novel inflammasome independent system in which we directly activate caspase-1 by dimerization. In this system, caspase-1 dimerization induced the cleavage and secretion of IL-1β, which did not require processing of caspase-1 into its p20 and p10 subunits. Moreover, direct caspase-1 dimerization allowed caspase-1 activation of IL-1β to be separated from cell death. Specifically, we demonstrate at the single cell level that IL-1β can be released from live, metabolically active, cells following caspase-1 activation. In addition, we show that dimerized or endogenous caspase-8 can also directly cleave IL-1β into its biologically active form, in the absence of canonical inflammasome components. Therefore, cell death is not obligatory for the robust secretion of bioactive IL-1β.

Evolutionary divergence of the necroptosis effector MLKL

The pseudokinase, MLKL (mixed-lineage kinase domain-like), is the most terminal obligatory component of the necroptosis cell death pathway known. Phosphorylation of the MLKL pseudokinase domain by the protein kinase, receptor interacting protein kinase-3 (RIPK3), is known to be the key step in MLKL activation. This phosphorylation event is believed to trigger a molecular switch, leading to exposure of the N-terminal four-helix bundle (4HB) domain of MLKL, its oligomerization, membrane translocation and ultimately cell death. To examine how well this process is evolutionarily conserved, we analysed the function of MLKL orthologues. Surprisingly, and unlike their mouse, horse and frog counterparts, human, chicken and stickleback 4HB domains were unable to induce cell death when expressed in murine fibroblasts. Forced dimerization of the human MLKL 4HB domain overcame this defect and triggered cell death in human and mouse cell lines. Furthermore, recombinant proteins from mouse, frog, human and chicken MLKL, all of which contained a 4HB domain, permeabilized liposomes, and were most effective on those designed to mimic plasma membrane composition. These studies demonstrate that the membrane-permeabilization function of the 4HB domain is evolutionarily conserved, but reveal that execution of necroptotic death by it relies on additional factors that are poorly conserved even among closely related species.

New insights into the regulation of innate immunity by caspase-8

Caspase-8 is required for extrinsic apoptosis, but is also central for preventing a pro-inflammatory receptor interacting protein kinase (RIPK) 3–mixed lineage kinase domain-like (MLKL)-dependent cell death pathway termed necroptosis. Despite these critical cellular functions, the impact of capase-8 deletion in the myeloid cell lineage, which forms the basis for innate immune responses, has remained unclear. In a recent article in Arthritis Research & Therapy, Cuda et al. report that myeloid cell-restricted caspase-8 loss leads to a very mild RIPK3-dependent inflammatory phenotype. The presented results suggest that inflammation does not arise exclusively because of RIPK3-mediated necroptotic death but that, in the absence of caspase-8, RIPK1 and RIPK3 enhance microbiome-driven Toll-like receptor-induced pro-inflammatory cytokine production.

IL-18 Production from the NLRP1 Inflammasome Prevents Obesity and Metabolic Syndrome

Interleukin-18 (IL-18) is activated by Caspase-1 in inflammasome complexes and has anti-obesity effects; however, it is not known which inflammasome regulates this process. We found that mice lacking the NLRP1 inflammasome phenocopy mice lacking IL-18, with spontaneous obesity due to intrinsic lipid accumulation. This is exacerbated when the mice are fed a high-fat diet (HFD) or a high-protein diet, but not when mice are fed a HFD with low energy density (high fiber). Furthermore, mice with an activating mutation in NLRP1, and hence increased IL-18, have decreased adiposity and are resistant to diet-induced metabolic dysfunction. Feeding these mice a HFD further increased plasma IL-18 concentrations and strikingly resulted in loss of adipose tissue mass and fatal cachexia, which could be prevented by genetic deletion of IL-18. Thus, NLRP1 is an innate immune sensor that functions in the context of metabolic stress to produce IL-18, preventing obesity and metabolic syndrome.

Post-translational control of RIPK3 and MLKL mediated necroptotic cell death

Several programmed lytic and necrotic-like cell death mechanisms have now been uncovered, including the recently described receptor interacting protein kinase-3 (RIPK3)-mixed lineage kinase domain-like (MLKL)-dependent necroptosis pathway. Genetic experiments have shown that programmed necrosis, including necroptosis, can play a pivotal role in regulating host-resistance against microbial infections. Alternatively, excess or unwarranted necroptosis may be pathological in autoimmune and autoinflammatory diseases. This review highlights the recent advances in our understanding of the post-translational control of RIPK3-MLKL necroptotic signaling. We discuss the critical function of phosphorylation in the execution of necroptosis, and highlight the emerging regulatory roles for several ubiquitin ligases and deubiquitinating enzymes. Finally, based on current evidence, we discuss the potential mechanisms by which the essential, and possibly terminal, necroptotic effector, MLKL, triggers the disruption of cellular membranes to cause cell lysis.

A RIPK2 inhibitor delays NOD signalling events yet prevents inflammatory cytokine production

Intracellular nucleotide binding and oligomerization domain (NOD) receptors recognize antigens including bacterial peptidoglycans and initiate immune responses by triggering the production of pro-inflammatory cytokines through activating NF-κB and MAP kinases. Receptor interacting protein kinase 2 (RIPK2) is critical for NOD-mediated NF-κB activation and cytokine production. Here we develop and characterize a selective RIPK2 kinase inhibitor, WEHI-345, which delays RIPK2 ubiquitylation and NF-κB activation downstream of NOD engagement. Despite only delaying NF-κB activation on NOD stimulation, WEHI-345 prevents cytokine production in vitro and in vivo and ameliorates experimental autoimmune encephalomyelitis in mice. Our study highlights the importance of the kinase activity of RIPK2 for proper immune responses and demonstrates the therapeutic potential of inhibiting RIPK2 in NOD-driven inflammatory diseases.

Mitochondrial apoptosis is dispensable for NLRP3 inflammasome activation but non-apoptotic caspase-8 is required for inflammasome priming

A current paradigm proposes that mitochondrial damage is a critical determinant of NLRP3 inflammasome activation. Here, we genetically assess whether mitochondrial signalling represents a unified mechanism to explain how NLRP3 is activated by divergent stimuli. Neither co-deletion of the essential executioners of mitochondrial apoptosis BAK and BAX, nor removal of the mitochondrial permeability transition pore component cyclophilin D, nor loss of the mitophagy regulator Parkin, nor deficiency in MAVS affects NLRP3 inflammasome function. In contrast, caspase-8, a caspase essential for death-receptor-mediated apoptosis, is required for efficient Toll-like-receptor-induced inflammasome priming and cytokine production. Collectively, these results demonstrate that mitochondrial apoptosis is not required for NLRP3 activation, and highlight an important non-apoptotic role for caspase-8 in regulating inflammasome activation and pro-inflammatory cytokine levels.

RIPK1 regulates RIPK3-MLKL-driven systemic inflammation and emergency hematopoiesis

Upon ligand binding, RIPK1 is recruited to tumor necrosis factor receptor superfamily (TNFRSF) and Toll-like receptor (TLR) complexes promoting prosurvival and inflammatory signaling. RIPK1 also directly regulates caspase-8-mediated apoptosis or, if caspase-8 activity is blocked, RIPK3-MLKL-dependent necroptosis. We show that C57BL/6 Ripk1(-/-) mice die at birth of systemic inflammation that was not transferable by the hematopoietic compartment. However, Ripk1(-/-) progenitors failed to engraft lethally irradiated hosts properly. Blocking TNF reversed this defect in emergency hematopoiesis but, surprisingly, Tnfr1 deficiency did not prevent inflammation in Ripk1(-/-) neonates. Deletion of Ripk3 or Mlkl, but not Casp8, prevented extracellular release of the necroptotic DAMP, IL-33, and reduced Myd88-dependent inflammation. Reduced inflammation in the Ripk1(-/-)Ripk3(-/-), Ripk1(-/-)Mlkl(-/-), and Ripk1(-/-)Myd88(-/-) mice prevented neonatal lethality, but only Ripk1(-/-)Ripk3(-/-)Casp8(-/-) mice survived past weaning. These results reveal a key function for RIPK1 in inhibiting necroptosis and, thereby, a role in limiting, not only promoting, inflammation.

Birinapant, a smac-mimetic with improved tolerability for the treatment of solid tumors and hematological malignancies

Birinapant (1) is a second-generation bivalent antagonist of IAP proteins that is currently undergoing clinical development for the treatment of cancer. Using a range of assays that evaluated cIAP1 stability and oligomeric state, we demonstrated that 1 stabilized the cIAP1-BUCR (BIR3-UBA-CARD-RING) dimer and promoted autoubiquitylation of cIAP1 in vitro. Smac-mimetic 1-induced loss of cIAPs correlated with inhibition of TNF-mediated NF-κB activation, caspase activation, and tumor cell killing. Many first-generation Smac-mimetics such as compound A (2) were poorly tolerated. Notably, animals that lack functional cIAP1, cIAP2, and XIAP are not viable, and 2 mimicked features of triple IAP knockout cells in vitro. The improved tolerability of 1 was associated with (i) decreased potency against cIAP2 and affinity for XIAP BIR3 and (ii) decreased ability to inhibit XIAP-dependent signaling pathways. The P2' position of 1 was critical to this differential activity, and this improved tolerability has allowed 1 to proceed into clinical studies.

Programmed cell death in Legionella infection

ABSTRACT: The causative agent of Legionnaires’ disease, Legionella pneumophila, resides within alveolar macrophages by exporting 295 bacterial virulence proteins (effectors) to modulate host cell processes. This leads to the formation of a unique vacuolar niche and the suppression of macrophage cell death pathways, which, in turn, promote bacterial survival and allow sufficient time for replication. However, once nutrients within the vacuole are depleted, Legionella must act to induce host cell death in order to facilitate bacterial egress and reinfect new cells. Intracellular Legionella also evade detection by the host cell’s innate immune system, which seeks to destroy invading pathogens by activating inflammasome complexes, thereby promoting proinflammatory cytokine activation and pyroptotic cell death. Understanding how different forms of programmed cell death contribute to Legionella infectivity and are manipulated by Legionella effector proteins will be important for identifying novel antibacterial therapeutic targets.

Bcl-2 overexpression ameliorates immune complex-mediated arthritis by altering FcγRIIb expression and monocyte homeostasis

RA is a chronic autoimmune disease characterized by accumulation of inflammatory cells within synovial joints. RA is associated with a failure of apoptosis of infiltrating leukocytes, thought to be a result of overexpression of prosurvival Bcl-2 proteins. Overexpression of Bcl-2 in hematopoietic cells can result in spontaneous autoimmunity. We therefore hypothesized that increased Bcl-2 in the hematopoietic compartment would reduce apoptosis and thereby, exacerbate inflammatory arthritis. Paradoxically, we found that overexpression of Bcl-2 in mice (vav-bcl-2) markedly reduced pathology in antibody-dependent models of RA (CIA and K/BxN serum transfer arthritis). No such protection was observed in a model of CD4(+) T cell-dependent, B cell-independent arthritis (mBSA/IL-1-induced arthritis). In CIA, vav-bcl-2 Tg mice had lower antibody production to CII, which might explain reduced disease. However, Bcl-2 overexpression also reduced passive K/BxN serum transfer arthritis. Overexpression of Bcl-2 caused a monocytosis, with preferential expansion of Ly6C(lo) monocytes and increased expression of the inhibitory receptor for IgG, FcγRIIb, on leukocytes. Skewing of the myeloid cell population, increases in FcγRIIb, and reduced arthritis were independent of the hypergammaglobulinemia found in vav-bcl-2 Tg mice. These data reveal selective effects of the Bcl-2-regulated apoptotic pathway on monocyte differentiation and the expression of FcRs critical for regulation of antibody/immune complex-mediated disease.