Pyroptotic cell death defends against intracellular pathogens

Melatonin delays photoreceptor degeneration in a mouse model of autosomal recessive retinitis pigmentosa

Retinitis pigmentosa (RP) comprises a group of incurable inherited retinal degenerations. Targeting common processes, instead of mutation-specific treatment, has proven to be an innovative strategy to combat debilitating retinal degeneration. Growing evidence indicates that melatonin possesses a potent activity against neurodegenerative disorders by mitigating cell damage associated with apoptosis and inflammation. Given the pleiotropic role of melatonin in central nervous system, the aim of the present study was to investigate whether melatonin would afford protection against retinal degeneration in autosomal recessive RP (arRP). Rd10, a well-characterized murine model of human arRP, received daily intraperitoneal injection of melatonin (15 mg/kg) between postnatal day (P) 13 and P30. Retinas treated with melatonin or vehicle were harvested for analysis at P30 and P45, respectively. The findings showed that melatonin could dampen the photoreceptors death and delay consequent retinal degeneration. We also observed that melatonin weakened the expression of glial fibrillary acidic protein (GFAP) in Müller cells. Additionally, melatonin could alleviate retinal inflammatory response visualized by IBA1 staining, which was further corroborated by downregulation of inflammation-related genes, such as tumor necrosis factor alpha (Tnf-α), chemokine (C-C motif) ligand 2 (Ccl2), and chemokine (C-X-C motif) ligand 10 (Cxcl10). These data revealed that melatonin could ameliorate retinal degeneration through potentially attenuating apoptosis, reactive gliosis, and microglial activation in rd10 mice. Moreover, these results suggest melatonin as a promising agent improving photoreceptors survival in human RP.

Comparative effects of schisandrin A, B, and C on Propionibacterium acnes-induced, NLRP3 inflammasome activation-mediated IL-1β secretion and pyroptosis

Propionibacterium acnes, a common pathogen associated with acne, is also responsible for various surgical infections. Schisandrin A, schisandrin B and schisandrin C, the representative lignans of Schisandra chinensis (Turcz.) Baill. extract, inhibit P. acnes-induced inflammation. However, their effects on P. acnes-induced IL-1β secretion and pyroptosis mediated by NLRP3 inflammasome activation remain unknown. In this study, we compared the effects of schisandrin A, B, and C (Sch A, B, and C) on IL-1β secretion and pyroptosis in P. acnes-infected THP-1 cells. As NLRP3 plays important roles in P. acnes-mediated inflammation and pyroptosis, we also investigated the effects of Schs on P. acnes-induced NLRP3 inflammasome activation by measuring the levels of NLRP3, active caspase-1, and mature IL-1β, and activity of caspase-1. Our results showed that Sch A, B, and C suppressed P. acnes-induced pyroptosis. Further, the three lignans significantly suppressed NLRP3 inflammasome activation, with the following potency: Sch C > Sch B > Sch A. Three lignans also inhibited the production of mitochondrial ROS and ATP release. Additionally, Sch B and C almost completely prevented the efflux of K^+., whereas Sch A had a relatively weak effect. Collectively, our novel findings showed that Sch A, B, and C effectively suppressed IL-1β secretion and pyroptosis by inhibiting NLRP3 inflammasome activation in P. acnes-infected THP-1 cells. Thus, Schs may be promising agents for the treatment of P. acnes-related infections.

NLRP3 Phosphorylation Is an Essential Priming Event for Inflammasome Activation

Many infections and stress signals can rapidly activate the NLRP3 inflammasome to elicit robust inflammatory responses. This activation requires a priming step, which is thought to be mainly for upregulating NLRP3 transcription. However, recent studies report that the NLRP3 inflammasome can be activated independently of transcription, suggesting that the priming process has unknown essential regulatory steps. Here, we report that JNK1-mediated NLRP3 phosphorylation at S194 is a critical priming event and is essential for NLRP3 inflammasome activation. We show that NLRP3 inflammasome activation is disrupted in NLRP3-S194A knockin mice. JNK1-mediated NLRP3 S194 phosphorylation is critical for NLRP3 deubiquitination and facilitates its self-association and the subsequent inflammasome assembly. Importantly, we demonstrate that blocking S194 phosphorylation prevents NLRP3 inflammasome activation in cryopyrin-associated periodic syndromes (CAPS). Thus, our study reveals a key priming molecular event that is a prerequisite for NLRP3 inflammasome activation. Inhibiting NLRP3 phosphorylation could be an effective treatment for NLRP3-related diseases.

The Role of Txnip in Mitophagy Dysregulation and Inflammasome Activation in Diabetic Retinopathy: A New Perspective

Mitochondria are responsible for bioenergetics, metabolism and apoptosis signals in health and disease. The retina being a part of the central nervous system consumes large amounts of glucose and oxygen to generate ATP via the mitochondrial oxidative phosphorylation for its phototransduction and visual function. During ATP generation, electrons leak from the mitochondrial electron transport chain, which is captured by molecular oxygen to produce reactive oxygen species (ROS). These mtROS damage mitochondrial proteins, mtDNA, and membrane lipids and release them in the cytosol. Mitochondrial components are recognized as danger-associated molecular patterns (DAMPS) by cytosolic pattern recognition receptors such as NOD-like receptors, NLRP3 inflammasomes. They process pro-caspase-1 to active caspase-1, which cleaves pro-inflammatory IL-1β o mature IL-1β causing inflammation and cell death by pyroptosis. To counter the damaging action of mtROS and inflammasomes in fully differentiated cells in the retina, the removal of the damaged and dysfunctional mitochondria by a double-membrane autophagic process via lysosomal degradation called mitophagy is critical for mitochondrial homeostasis and cell survival. Nonetheless, under chronic diseases including diabetic retinopathy (DR), mitophagy dysregulation and NLRP3 inflammasome activation exist, which cause premature cell death and disease progression. Recently, the thioredoxin-interacting protein TXNIP, which is strongly induced by diabetes and inhibits anti-oxidant function of thioredoxin, has been implicated in mitochondrial dysfunction, mitophagic dysregulation and NLRP3 inflammasome activation in DR. Therefore, TXNIP silencing or pharmacological inhibition may normalize mitophagic flux and NLRP3 inflammasome activation, which will prevent or slow down the progression of DR.

Gasdermins: Effectors of Pyroptosis

Pyroptosis is a form of lytic programmed cell death initiated by inflammasomes, which detect cytosolic contamination or perturbation. This drives activation of caspase-1 or caspase-11/4/5, which cleave gasdermin D, separating its N-terminal pore-forming domain (PFD) from the C-terminal repressor domain (RD). The PFD oligomerizes to form large pores in the membrane that drive swelling and membrane rupture. Gasdermin D is one of six (in humans) gasdermin family members; several other gasdermins have also been shown to form pores that cause pyroptosis after cleavage to activate their PFDs. One of these, gasdermin E, is activated by caspase-3 cleavage. We review our current understanding of pyroptosis as well as current knowledge of the gasdermin family.

Metabolic Remodeling, Inflammasome Activation, and Pyroptosis in Macrophages Stimulated by Porphyromonas gingivalis and Its Outer Membrane Vesicles

Porphyromonas gingivalis is one of the bacterial species most closely associated with periodontitis and can shed large numbers of outer membrane vesicles (OMVs), which are increasingly thought to play a significant role in bacterial virulence and pathogenicity. Macrophages are amongst the first immune cells to respond to bacteria and their products, so we sought to directly compare the response of macrophages to P. gingivalis or its purified OMVs. Macrophages stimulated with OMVs produced large amounts of TNFα, IL-12p70, IL-6, IL-10, IFNβ, and nitric oxide compared to cells infected with P. gingivalis, which produced very low levels of these mediators. Both P. gingivalis and OMVs induced a shift in macrophage metabolism from oxidative phosphorylation (OXPHOS) to glycolysis, which was supported by enhanced lactate release, decreased mitochondrial oxygen consumption with reduced spare respiratory capacity, as well as increased mitochondrial reactive oxygen species (ROS) production. Corresponding to this metabolic shift, gene expression analysis of macrophages infected with P. gingivalis or stimulated with OMVs revealed a broad transcriptional upregulation of genes critical to glycolysis and a downregulation of genes associated with the TCA cycle. Upon examination of inflammasome signaling and pyroptosis it was found that P. gingivalis did not activate the inflammasome in macrophages as the mature forms of caspase-1, IL-1β, and IL-18 were not detected and there was no extracellular release of lactate dehydrogenase (LDH) or 7-AAD staining. In comparison, macrophages stimulated with OMVs potently activated caspase-1, produced large amounts of IL-1β, IL-18, released LDH, and were positive for 7-AAD indicative of pyroptotic cell death. These data directly quantitate the distinct effects of P. gingivalis and its OMVs on macrophage inflammatory phenotype, mitochondrial function, inflammasome activation, and pyroptotic cell death that may have potential implications for their roles in chronic periodontitis.

Melatonin alleviates inflammasome-induced pyroptosis through inhibiting NF-κB/GSDMD signal in mice adipose tissue

Pyroptosis is a proinflammatory form of cell death that is associated with pathogenesis of many chronic inflammatory diseases. Melatonin is substantially reported to possess anti-inflammatory properties by inhibiting inflammasome activation. However, the effects of melatonin on inflammasome-induced pyroptosis in adipocytes remain elusive. Here, we demonstrated that melatonin alleviated lipopolysaccharides (LPS)-induced inflammation and NLRP3 inflammasome formation in mice adipose tissue. The NLRP3 inflammasome-mediated pyroptosis was also inhibited by melatonin in adipocytes. Further analysis revealed that gasdermin D (GSDMD), the key executioner of pyroptosis, was the target for melatonin inhibition of adipocyte pyroptosis. Importantly, we determined that nuclear factor κB (NF-κB) signal was required for the GSDMD-mediated pyroptosis in adipocytes. We also confirmed that melatonin alleviated adipocyte pyroptosis by transcriptional suppression of GSDMD. Moreover, GSDMD physically interacted with interferon regulatory factor 7 (IRF7) and subsequently formed a complex to promote adipocyte pyroptosis. Melatonin also attenuated NLRP3 inflammasome activation and pyroptosis, which was induced by LPS or obesity. In summary, our results demonstrate that melatonin alleviates inflammasome-induced pyroptosis by blocking NF-κB/GSDMD signal in mice adipose tissue. Our data reveal a novel function of melatonin on adipocyte pyroptosis, suggesting a new potential therapy for melatonin to prevent and treat obesity caused systemic inflammatory response.

Programmed Cell Death and Inflammation: Winter Is Coming

The life of an organism requires the assistance of an unlikely process: programmed cell death. Both development and the maintenance of homeostasis result in the production of superfluous cells that must eventually be disposed of. Furthermore, programmed cell death can also represent a defense mechanism; for example, by depriving pathogens of a replication niche. The responsibility of handling these dead cells falls on phagocytes of the immune system, which surveil their surroundings for dying or dead cells and efficiently clear them in a quiescent manner. This process, termed efferocytosis, depends on cooperation between the phagocyte and the dying cell. In this review we explore different types of programmed cell death and their impact on innate immune responses.

Caspases control antiviral innate immunity

Caspases are a family of cysteine proteases whose functions have been scrutinized intensively in recent years. Beyond their established roles in programmed cell death and inflammatory response, some caspases are also fundamental players in antiviral immunity by fine-tuning the levels of antiviral signaling adapters and cytokines, such as type I interferons, which serves as a major, sophisticated weapon against viruses. Viral infections can result in inflammasome activation and the initiation of cell death, including apoptosis and pyroptosis, and multiple caspases are significantly involved in these processes. This review will focus on the cutting-edge discoveries regarding the multifaceted roles of caspases in antiviral innate immunity.

Clostridium difficile Toxin Biology

Clostridium difficile is the cause of antibiotics-associated diarrhea and pseudomembranous colitis. The pathogen produces three protein toxins: C. difficile toxins A (TcdA) and B (TcdB), and C. difficile transferase toxin (CDT). The single-chain toxins TcdA and TcdB are the main virulence factors. They bind to cell membrane receptors and are internalized. The N-terminal glucosyltransferase and autoprotease domains of the toxins translocate from low-pH endosomes into the cytosol. After activation by inositol hexakisphosphate (InsP6), the autoprotease cleaves and releases the glucosyltransferase domain into the cytosol, where GTP-binding proteins of the Rho/Ras family are mono-O-glucosylated and, thereby, inactivated. Inactivation of Rho proteins disturbs the organization of the cytoskeleton and affects multiple Rho-dependent cellular processes, including loss of epithelial barrier functions, induction of apoptosis, and inflammation. CDT, the third C. difficile toxin, is a binary actin-ADP-ribosylating toxin that causes depolymerization of actin, thereby inducing formation of the microtubule-based protrusions. Recent progress in understanding of the toxins' actions include insights into the toxin structures, their interaction with host cells, and functional consequences of their actions.

Molecular basis of mycobacterial survival in macrophages

Macrophages play an essential role in the immune system by ingesting and degrading invading pathogens, initiating an inflammatory response and instructing adaptive immune cells, and resolving inflammation to restore homeostasis. More interesting is the fact that some bacteria have evolved to use macrophages as a natural habitat and tools of spread in the host, e.g., Mycobacterium tuberculosis (Mtb) and some non-tuberculous mycobacteria (NTM). Mtb is considered one of humanity's most successful pathogens and is the causal agent of tuberculosis, while NTMs cause opportunistic infections all of which are of significant public health concern. Here, we describe mechanisms by which intracellular pathogens, with an emphasis on mycobacteria, manipulate macrophage functions to circumvent killing and live inside these cells even under considerable immunological pressure. Such macrophage functions include the selective evasion or engagement of pattern recognition receptors, production of cytokines, reactive oxygen and nitrogen species, phagosome maturation, as well as other killing mechanisms like autophagy and cell death. A clear understanding of host responses elicited by a specific pathogen and strategies employed by the microbe to evade or exploit these is of significant importance for the development of effective vaccines and targeted immunotherapy against persistent intracellular infections like tuberculosis.

Chemotherapy drugs induce pyroptosis through caspase-3 cleavage of a gasdermin

Pyroptosis is a form of cell death that is critical for immunity. It can be induced by the canonical caspase-1 inflammasomes or by activation of caspase-4, -5 and -11 by cytosolic lipopolysaccharide. The caspases cleave gasdermin D (GSDMD) in its middle linker to release autoinhibition on its gasdermin-N domain, which executes pyroptosis via its pore-forming activity. GSDMD belongs to a gasdermin family that shares the pore-forming domain. The functions and mechanisms of activation of other gasdermins are unknown. Here we show that GSDME, which was originally identified as DFNA5 (deafness, autosomal dominant 5), can switch caspase-3-mediated apoptosis induced by TNF or chemotherapy drugs to pyroptosis. GSDME was specifically cleaved by caspase-3 in its linker, generating a GSDME-N fragment that perforates membranes and thereby induces pyroptosis. After chemotherapy, cleavage of GSDME by caspase-3 induced pyroptosis in certain GSDME-expressing cancer cells. GSDME was silenced in most cancer cells but expressed in many normal tissues. Human primary cells exhibited GSDME-dependent pyroptosis upon activation of caspase-3 by chemotherapy drugs. Gsdme^-/- (also known as Dfna5^-/-) mice were protected from chemotherapy-induced tissue damage and weight loss. These findings suggest that caspase-3 activation can trigger necrosis by cleaving GSDME and offer new insights into cancer chemotherapy.

Pore-forming toxin-mediated ion dysregulation leads to death receptor-independent necroptosis of lung epithelial cells during bacterial pneumonia

We report that pore-forming toxins (PFTs) induce respiratory epithelial cell necroptosis independently of death receptor signaling during bacterial pneumonia. Instead, necroptosis was activated as a result of ion dysregulation arising from membrane permeabilization. PFT-induced necroptosis required RIP1, RIP3 and MLKL, and could be induced in the absence or inhibition of TNFR1, TNFR2 and TLR4 signaling. We detected activated MLKL in the lungs from mice and nonhuman primates experiencing Serratia marcescens and Streptococcus pneumoniae pneumonia, respectively. We subsequently identified calcium influx and potassium efflux as the key initiating signals responsible for necroptosis; also that mitochondrial damage was not required for necroptosis activation but was exacerbated by MLKL activation. PFT-induced necroptosis in respiratory epithelial cells did not involve CamKII or reactive oxygen species. KO mice deficient in MLKL or RIP3 had increased survival and reduced pulmonary injury during S. marcescens pneumonia. Our results establish necroptosis as a major cell death pathway active during bacterial pneumonia and that necroptosis can occur without death receptor signaling.

Papilloma-pseudovirus eradicates intestinal tumours and triples the lifespan of ApcMin/+ mice

Inducing tumour-specific adaptive immunity, such as cytotoxic T lymphocyte (CTL) response, can result in promising antitumour effect against several human malignancies, especially in combination with immune checkpoint blockade strategies. However, little is known whether activation of innate immunity can lead to direct tumoricidal effect. Here, we develop a papilloma pseudovirus-based oral immunotherapeutic approach that shows strong tumoricidal effects in the gut, resulting in an almost tripled lifespan of Apc^Min/+ mice (an animal model of human intestinal tumorigenesis). Mechanistically, these pseudoviruses activate the NLRP3 and AIM2 inflammasomes, leading to caspase-1-mediated tumour regression that is dependent on neither cytotoxic T lymphocytes nor humoral immune response. Blocking caspase-1 activation abrogated the therapeutic effects of the pseudoviruses. Thus, targeting innate immune sensors in tumours by the pseudoviruses might represent a strategy to treat intestinal tumours.

Inflammasomes in Inflammation-Induced Cancer

The inflammasome is an important multiprotein complex that functions during inflammatory immune responses. The activation of inflammasome will lead to the autoactivation of caspase-1 and subsequent cleavage of proIL-1β and proIL-18, which are key sources of inflammatory manifestations. Recently, the roles of inflammasomes in cancers have been extensively explored, especially in inflammation-induced cancers. In different and specific contexts, inflammasomes exhibit distinct and even contrasting effects in cancer development. In some cases, inflammasomes initiate carcinogenesis through the extrinsic pathway and maintain the malignant cancer microenvironment through the intrinsic pathway. On the contrary, inflammasomes also exert anticancer effects by specialized programmed cell death called pyroptosis and immune regulatory functions. The phases and compartments in which inflammasomes are activated strongly influence the final immune effects. We systemically summarize the functions of inflammasomes in inflammation-induced cancers, especially in gastrointestinal and skin cancers. Besides, information about the current therapeutic use of inflammasome-related products and potential future developing directions are also introduced.

A bioluminescent caspase-1 activity assay rapidly monitors inflammasome activation in cells

Inflammasomes are protein complexes induced by diverse inflammatory stimuli that activate caspase-1, resulting in the processing and release of cytokines, IL-1β and IL-18, and pyroptosis, an immunogenic form of cell death. To provide a homogeneous method for detecting caspase-1 activity, we developed a bioluminescent, plate-based assay that combines a substrate, Z-WEHD-aminoluciferin, with a thermostable luciferase in an optimized lytic reagent added directly to cultured cells. Assay specificity for caspase-1 is conferred by inclusion of a proteasome inhibitor in the lytic reagent and by use of a caspase-1 inhibitor to confirm activity. This approach enables a specific and rapid determination of caspase-1 activation. Caspase-1 activity is stable in the reagent thereby providing assay convenience and flexibility. Using this assay system, caspase-1 activation has been determined in THP-1 cells following treatment with α-hemolysin, LPS, nigericin, gramicidin, MSU, R848, Pam3CSK4, and flagellin. Caspase-1 activation has also been demonstrated in treated J774A.1 mouse macrophages, bone marrow-derived macrophages (BMDMs) from mice, as well as in human primary monocytes. Caspase-1 activity was not detected in treated BMDMs derived from Casp1^-/- mice, further confirming the specificity of the assay. Caspase-1 activity can be measured directly in cultured cells using the lytic reagent, or caspase-1 activity released into medium can be monitored by assay of transferred supernatant. The caspase-1 assay can be multiplexed with other assays to monitor additional parameters from the same cells, such as IL-1β release or cell death. The caspase-1 assay in combination with a sensitive real-time monitor of cell death allows one to accurately establish pyroptosis. This assay system provides a rapid, convenient, and flexible method to specifically and quantitatively monitor caspase-1 activation in cells in a plate-based format. This will allow a more efficient and effective assessment of inflammasome activation as well as enable high-throughput screening for inflammasome modulators.

The NLRP3 inflammasome contributes to host protection during Sporothrix schenckii infection

Sporotrichosis is a mycosis caused by fungi from the Sporothrix schenckii species complex, whose prototypical member is Sporothrix schenckii sensu stricto. Pattern recognition receptors (PRRs) recognize and respond to pathogen-associated molecular patterns (PAMPs) and shape the following adaptive immune response. A family of PRRs most frequently associated with fungal recognition is the nucleotide-binding oligomerization domain-like receptor (NLR). After PAMP recognition, NLR family pyrin domain-containing 3 (NLRP3) binds to apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) and caspase-1 to form the NLRP3 inflammasome. When activated, this complex promotes the maturation of the pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18 and cell death through pyroptosis. In this study, we aimed to evaluate the importance of the NLRP3 inflammasome in the outcome of S. schenckii infection using the following three different knockout (KO) mice: NLRP3^-/- , ASC^-/- and caspase-1^-/- . All KO mice were more susceptible to infection than the wild-type, suggesting that NLRP3-triggered responses contribute to host protection during S. schenckii infection. Furthermore, the NLRP3 inflammasome appeared to be critical for the ex vivo release of IL-1β, IL-18 and IL-17 but not interferon-γ. Additionally, a role for the inflammasome in shaping the adaptive immune response was suggested by the lower frequencies of type 17 helper T (Th17) cells and Th1/Th17 but not Th1 cells in S. schenckii-infected KO mice. Overall, our results indicate that the NLRP3 inflammasome links the innate recognition of S. schenckii to the adaptive immune response, so contributing to protection against this infection.

'Hints' in the killer protein gasdermin D: unveiling the secrets of gasdermins driving cell death

Pyroptosis is a lytic form of cell death distinguished from apoptosis, ferroptosis, necrosis, necroptosis, NETosis, oncosis, pyronecrosis and autophagy. Proinflammatory caspases cleave a gasdermin D (GSDMD) protein to generate a 31 kDa N-terminal domain. The cleavage relieves the intramolecular inhibition on the gasdermin-N domain, which then moves to the plasma membrane to exhibit pore-forming activity. Thus, GSDMD acts as the final and direct executor of pyroptotic cell death. Owing to the selective targeting of the inner leaflet of the plasma membrane with the pore-forming that determines pyroptotic cell death, GSDMD could be a potential target to control cell death or extracellular bacterial infections. Intriguingly, other gasdermin family members also share similar N-terminal domains, but they present different cell death programs. Herein, we summarize features and functions of the novel player proteins in cell death, including GSDMD triggering pyroptosis, Gsdma3/GSDMA initiating autophagy/apoptosis and DFNA5 inducing apoptosis/secondary necrosis. The gasdermin N terminus appears to be a novel pore-forming protein. This provides novel insight into the underlying roles and mechanisms of lytic or nonlytic forms of programmed cell death, as well as their potential applications in inflammation-associated diseases.

Recent Insights into the Molecular Mechanisms Underlying Pyroptosis and Gasdermin Family Functions

Pyroptosis is an inflammatory form of cell death that not only protects multicellular organisms from invading pathogenic bacteria and microbial infections, but can also lead to sepsis and lethal septic shock if overactivated. Here, we present an overview of recent developments within the pyroptosis field, beginning with the discovery of Gasdermin D (GSDMD) as a substrate of caspase-1 and caspase-11 upon detection of cytosolic lipopolysaccharide (LPS). Cleavage releases the N-terminal domain of GSDMD, causing it to form cytotoxic pores in the plasma membrane of cells. We further discuss the implications for the rest of the gasdermin (GSDM) family, which are emerging as mediators of programmed cell death in a variety of processes that regulate cellular differentiation and proliferation.

Analyses of caspase-1-regulated transcriptomes in various tissues lead to identification of novel IL-1β-, IL-18- and sirtuin-1-independent pathways

BACKGROUND

It is well established that caspase-1 exerts its biological activities through its downstream targets such as IL-1β, IL-18, and Sirt-1. The microarray datasets derived from various caspase-1 knockout tissues indicated that caspase-1 can significantly impact the transcriptome. However, it is not known whether all the effects exerted by caspase-1 on transcriptome are mediated only by its well-known substrates. Therefore, we hypothesized that the effects of caspase-1 on transcriptome may be partially independent from IL-1β, IL-18, and Sirt-1.

METHODS

To determine new global and tissue-specific gene regulatory effects of caspase-1, we took novel microarray data analysis approaches including Venn analysis, cooperation analysis, and meta-analysis methods. We used these statistical methods to integrate different microarray datasets conducted on different caspase-1 knockout tissues and datasets where caspase-1 downstream targets were manipulated.

RESULTS

We made the following important findings: (1) Caspase-1 exerts its regulatory effects on the majority of genes in a tissue-specific manner; (2) Caspase-1 regulatory genes partially cooperates with genes regulated by sirtuin-1 during organ injury and inflammation in adipose tissue but not in the liver; (3) Caspase-1 cooperates with IL-1β in regulating less than half of the genes involved in cardiovascular disease, organismal injury, and cancer in mouse liver; (4) The meta-analysis identifies 40 caspase-1 globally regulated genes across tissues, suggesting that caspase-1 globally regulates many novel pathways; and (5) The meta-analysis identified new cooperatively and non-cooperatively regulated genes in caspase-1, IL-1β, IL-18, and Sirt-1 pathways.

CONCLUSIONS

Our findings suggest that caspase-1 regulates many new signaling pathways potentially via its known substrates and also via transcription factors and other proteins that are yet to be identified.

Epigenetic regulation of nitric oxide synthase 2, inducible (Nos2) by NLRC4 inflammasomes involves PARP1 cleavage

Nitric oxide synthase 2, inducible (Nos2) expression is necessary for the microbicidal activity of macrophages. However, NOS2 over-activation causes multiple inflammatory disorders, suggesting a tight gene regulation is necessary. Using cytosolic flagellin as a model for inflammasome-dependent NOS2 activation, we discovered a surprising new role for NLRC4/caspase-1 axis in regulating chromatin accessibility of the Nos2 promoter. We found that activation of two independent mechanisms is necessary for NOS2 expression by cytosolic flagellin: caspase-1 and NF-κB activation. NF-κB activation was necessary, but not sufficient, for NOS2 expression. Conversely, caspase-1 was necessary for NOS2 expression, but dispensable for NF-κB activation, indicating that this protease acts downstream NF-κB activation. We demonstrated that epigenetic regulation of Nos2 by caspase-1 involves cleavage of the chromatin regulator PARP1 (also known as ARTD1) and chromatin accessibility of the NF-κB binding sites located at the Nos2 promoter. Remarkably, caspase-1-mediated Nos2 transcription and NO production contribute to the resistance of macrophages to Salmonella typhimurium infection. Our results uncover the molecular mechanism behind the constricted regulation of Nos2 expression and open new therapeutic opportunities based on epigenetic activities of caspase-1 against infectious and inflammatory diseases.

Programmed cell death as a defence against infection

Eukaryotic cells can die from physical trauma, which results in necrosis. Alternatively, they can die through programmed cell death upon the stimulation of specific signalling pathways. In this Review, we discuss the role of different cell death pathways in innate immune defence against bacterial and viral infection: apoptosis, necroptosis, pyroptosis and NETosis. We describe the interactions that interweave different programmed cell death pathways, which create complex signalling networks that cross-guard each other in the evolutionary 'arms race' with pathogens. Finally, we describe how the resulting cell corpses - apoptotic bodies, pore-induced intracellular traps (PITs) and neutrophil extracellular traps (NETs) - promote the clearance of infection.

Phagosomal Neutralization by the Fungal Pathogen Candida albicans Induces Macrophage Pyroptosis

The interaction of Candida albicans with the innate immune system is the key determinant of the pathogen/commensal balance and has selected for adaptations that facilitate the utilization of nutrients commonly found within the host, including proteins and amino acids; many of the catabolic pathways needed to assimilate these compounds are required for persistence in the host. We have shown that C. albicans co-opts amino acid catabolism to generate and excrete ammonia, which raises the extracellular pH, both in vitro and in vivo and induces hyphal morphogenesis. Mutants defective in the uptake or utilization of amino acids, such as those lacking STP2, a transcription factor that regulates the expression of amino acid permeases, are impaired in multiple aspects of fungus-macrophage interactions resulting from an inability to neutralize the phagosome. Here we identified a novel role in amino acid utilization for Ahr1p, a transcription factor previously implicated in regulation of adherence and hyphal morphogenesis. Mutants lacking AHR1 were defective in growth, alkalinization, and ammonia release on amino acid-rich media, similar to stp2Δ and ahr1Δ stp2Δ cells, and occupied more acidic phagosomes. Notably, ahr1Δ and stp2Δ strains did not induce pyroptosis, as measured by caspase-1-dependent interleukin-1β release, though this phenotype could be suppressed by pharmacological neutralization of the phagosome. Altogether, we show that C. albicans-driven neutralization of the phagosome promotes hyphal morphogenesis, sufficient for induction of caspase-1-mediated macrophage lysis.

Beginning to Understand the Role of the Type IV Secretion System Effector Proteins in Coxiella burnetii Pathogenesis

Coxiella burnetii is the etiological agent of the zoonotic disease Q fever, which manifests in severe outbreaks and is associated with important health and economic burden. Moreover, C. burnetii belongs to the list of class B bioterrorism organisms, as it is an airborne and highly infective pathogen with remarkable resistance to environmental stresses. Detailed study of the host-pathogen interaction during C. burnetii infection has been hampered due to the obligate intracellular nature of this pathogen. However, the development of an axenic culture medium, together with the implementation of bioinformatics tools and high-content screening approaches, have significantly progressed C. burnetii research in the last decade. This has facilitated identification of the Dot/Icm type IV secretion system (T4SS) as an essential virulence factor. T4SS is used to deliver an arsenal of effector proteins into the cytoplasm of the host cell. These effectors mediate the survival of the host cell and the development of very large replicative compartments called Coxiella-containing vacuoles (CCVs). Biogenesis of the CCV relies on T4SS-dependent re-routing of numerous intracellular trafficking pathways to deliver membranes and nutrients that are essential for bacterial replication. This review aims to illustrate the key milestones that have contributed to ascribe C. burnetii as a model organism for the study of host/pathogen interactions as well as presenting an up-to-date description of our knowledge of the cell biology of C. burnetii infections.

Innate immune signaling and regulation in cancer immunotherapy

A pre-existing T cell-inflamed tumor microenvironment has prognostic utility and also can be predictive for response to contemporary cancer immunotherapies. The generation of a spontaneous T cell response against tumor-associated antigens depends on innate immune activation, which drives type I interferon (IFN) production. Recent work has revealed a major role for the STING pathway of cytosolic DNA sensing in this process. This cascade of events contributes to the activation of Batf3-lineage dendritic cells (DCs), which appear to be central to anti-tumor immunity. Non-T cell-inflamed tumors lack chemokines for Batf3 DC recruitment, have few Batf3 DCs, and lack a type I IFN gene signature, suggesting that failed innate immune activation may be the ultimate cause for lack of spontaneous T cell activation and accumulation. With this information in hand, new strategies for triggering innate immune activation and Batf3 DC recruitment are being developed, including novel STING agonists for de novo immune priming. Ultimately, the successful development of effective innate immune activators should expand the fraction of patients that can respond to immunotherapies, such as with checkpoint blockade antibodies.

Efferocytosis of Pathogen-Infected Cells

The prompt and efficient clearance of unwanted and abnormal cells by phagocytes is termed efferocytosis and is crucial for organism development, maintenance of tissue homeostasis, and regulation of the immune system. Dying cells are recognized by phagocytes through pathways initiated via "find me" signals, recognition via "eat me" signals and down-modulation of regulatory "don't eat me" signals. Pathogen infection may trigger cell death that drives phagocytic clearance in an immunologically silent, or pro-inflammatory manner, depending on the mode of cell death. In many cases, efferocytosis is a mechanism for eliminating pathogens and pathogen-infected cells; however, some pathogens have subverted this process and use efferocytic mechanisms to avoid innate immune detection and assist phagocyte infection. In parallel, phagocytes can integrate signals received from infected dying cells to elicit the most appropriate effector response against the infecting pathogen. This review focuses on pathogen-induced cell death signals that drive infected cell recognition and uptake by phagocytes, and the outcomes for the infected target cell, the phagocyte, the pathogen and the host.

Flow signaling and atherosclerosis

Atherosclerosis rarely develops in the region of arteries exposed to undisturbed flow (u-flow, unidirectional flow). Instead, atherogenesis occurs in the area exposed to disturbed flow (d-flow, multidirectional flow). Based on these general pathohistological observations, u-flow is considered to be athero-protective, while d-flow is atherogenic. The fact that u-flow and d-flow induce such clearly different biological responses in the wall of large arteries indicates that these two types of flow activate each distinct intracellular signaling cascade in vascular endothelial cells (ECs), which are directly exposed to blood flow. The ability of ECs to differentially respond to the two types of flow provides an opportunity to identify molecular events that lead to endothelial dysfunction and atherosclerosis. In this review, we will focus on various molecular events, which are differentially regulated by these two flow types. We will discuss how various kinases, ER stress, inflammasome, SUMOylation, and DNA methylation play roles in the differential flow response, endothelial dysfunction, and atherosclerosis. We will also discuss the interplay among the molecular events and how they coordinately regulate flow-dependent signaling and cellular responses. It is hoped that clear understanding of the way how the two flow types beget each unique phenotype in ECs will lead us to possible points of intervention against endothelial dysfunction and cardiovascular diseases.

Non-Canonical Cell Death Induced by p53

Programmed cell death is a vital biological process for multicellular organisms to maintain cellular homeostasis, which is regulated in a complex manner. Over the past several years, apart from apoptosis, which is the principal mechanism of caspase-dependent cell death, research on non-apoptotic forms of programmed cell death has gained momentum. p53 is a well characterized tumor suppressor that controls cell proliferation and apoptosis and has also been linked to non-apoptotic, non-canonical cell death mechanisms. p53 impacts these non-canonical forms of cell death through transcriptional regulation of its downstream targets, as well as direct interactions with key players involved in these mechanisms, in a cell type- or tissue context-dependent manner. In this review article, we summarize and discuss the involvement of p53 in several non-canonical modes of cell death, including caspase-independent apoptosis (CIA), ferroptosis, necroptosis, autophagic cell death, mitotic catastrophe, paraptosis, and pyroptosis, as well as its role in efferocytosis which is the process of clearing dead or dying cells.

Extracellular histones, cell-free DNA, or nucleosomes: differences in immunostimulation

In inflammation, extensive cell death may occur, which results in the release of chromatin components into the extracellular environment. Individually, the purified chromatin components double stranded (ds)DNA and histones have been demonstrated, both in vitro and in vivo, to display various immunostimulatory effects, for example, histones induce cytotoxicity and proinflammatory signaling through toll-like receptor (TLR)2 and 4, while DNA induces signaling through TLR9 and intracellular nucleic acid sensing mechanisms. However, DNA and histones are organized in nucleosomes in the nucleus, and evidence suggests that nucleosomes are released as such in inflammation. The cytotoxicity and proinflammatory signaling induced by nucleosomes have not been studied as extensively as the separate effects brought about by histones and dsDNA, and there appear to be some marked differences. Remarkably, little distinction between the different forms in which histones circulate has been made throughout literature. This is partly due to the limitations of existing techniques to differentiate between histones in their free or DNA-bound form. Here we review the current understanding of immunostimulation induced by extracellular histones, dsDNA and nucleosomes, and discuss the importance of techniques that in their detection differentiate between these different chromatin components.

Pyroptosis: Gasdermin-Mediated Programmed Necrotic Cell Death

Pyroptosis was long regarded as caspase-1-mediated monocyte death in response to certain bacterial insults. Caspase-1 is activated upon various infectious and immunological challenges through different inflammasomes. The discovery of caspase-11/4/5 function in sensing intracellular lipopolysaccharide expands the spectrum of pyroptosis mediators and also reveals that pyroptosis is not cell type specific. Recent studies identified the pyroptosis executioner, gasdermin D (GSDMD), a substrate of both caspase-1 and caspase-11/4/5. GSDMD represents a large gasdermin family bearing a novel membrane pore-forming activity. Thus, pyroptosis is redefined as gasdermin-mediated programmed necrosis. Gasdermins are associated with various genetic diseases, but their cellular function and mechanism of activation (except for GSDMD) are unknown. The gasdermin family suggests a new area of research on pyroptosis function in immunity, disease, and beyond.

Coxiella burnetii Avirulent Nine Mile Phase II Induces Caspase-1-Dependent Pyroptosis in Murine Peritoneal B1a B Cells

Our recent study demonstrated that virulent Coxiella burnetii Nine Mile phase I (NMI) is capable of infecting and replicating within peritoneal B1a cells and that B1a cells play an important role in host defense against C. burnetii infection in mice. However, it remains unknown if avirulent Nine Mile phase II (NMII) can infect and replicate in B1a cells and whether NMI and NMII can differentially interact with B1a cells. In this study, we examined if NMI and NMII can differentially modulate host cell apoptotic signaling in B1a cells. The results showed that NMII induced dose-dependent cell death in murine peritoneal B1a cells but NMI did not, suggesting that NMI and NMII may differentially activate host cell apoptotic signaling in B1a cells. Western blotting indicated that NMII-induced B1a cell death was not dependent on either caspase-3 or PARP-1 cleavage, but cleavage of caspase-1 was detected in NMII-infected B1a cells. In addition, inhibition or deficiency of caspase-1 activity blocked NMII-induced B1a cell death. These results suggest that NMII induces a caspase-1-dependent pyroptosis in murine peritoneal B1a cells. We also found that heat-killed NMII and type 4 secretion system (T4SS) mutant NMII were unable to induce B1a cell death and that NMII infection did not induce cell death in peritoneal B1a cells from Toll-like receptor 2 (TLR-2)- or NLRP3 inflammasome-deficient mice. These data suggest that NMII-induced caspase-1-dependent pyroptosis may require its T4SS and activation of the TLR-2 and NLRP3 signaling pathways.

Pyroptosis of Salmonella Typhimurium-infected macrophages was suppressed and elimination of intracellular bacteria from macrophages was promoted by blocking QseC

QseC is a membrane-bound histidine sensor kinase found in Gram-negative pathogens and is involved in the regulation of bacterial virulence. LED209, a QseC-specific inhibitor, significantly inhibits the virulence of several pathogens and partially protects infected mice from death by blocking QseC. However, the mechanism of its antibacterial effects remains unclear. In this experiment, a Salmonella Typhimurium (S. Typhimurium) and macrophage co-culture system was utilized to investigate possible mechanisms underlying the antimicrobial effects of the QseC inhibitor. QseC blockade inhibited the expression of QseC-dependent virulence genes, including flhDC, sifA, and sopB, in S. Typhimurium, leading to inhibition of swimming motility, invasion capacity, and replication capacity of the pathogens. Release of lactate dehydrogenase (LDH) from S. Typhimurium-infected macrophages was significantly inhibited by blocking QseC. Activated caspase-1 and IL-1β levels were suppressed, and intracellular bacterial count was reduced in infected macrophages. QseC blockade effectively reduced the virulence of S. Typhimurium, inhibited S. Typhimurium-induced pyroptosis of macrophages, and promoted elimination of intracellular bacteria from infected macrophages. Thus, the antibacterial effects of QseC inhibitor are mediated via enhancement of intracellular killing of S. Typhimurium in macrophages.

The Listeria monocytogenes PASTA Kinase PrkA and Its Substrate YvcK Are Required for Cell Wall Homeostasis, Metabolism, and Virulence

Obstacles to bacterial survival and replication in the cytosol of host cells, and the mechanisms used by bacterial pathogens to adapt to this niche are not well understood. Listeria monocytogenes is a well-studied Gram-positive foodborne pathogen that has evolved to invade and replicate within the host cell cytosol; yet the mechanisms by which it senses and responds to stress to survive in the cytosol are largely unknown. To assess the role of the L. monocytogenespenicillin-binding-protein and serine/threonine associated (PASTA) kinase PrkA in stress responses, cytosolic survival and virulence, we constructed a ΔprkA deletion mutant. PrkA was required for resistance to cell wall stress, growth on cytosolic carbon sources, intracellular replication, cytosolic survival, inflammasome avoidance and ultimately virulence in a murine model of Listeriosis. In Bacillus subtilis and Mycobacterium tuberculosis, homologues of PrkA phosphorylate a highly conserved protein of unknown function, YvcK. We found that, similar to PrkA, YvcK is also required for cell wall stress responses, metabolism of glycerol, cytosolic survival, inflammasome avoidance and virulence. We further demonstrate that similar to other organisms, YvcK is directly phosphorylated by PrkA, although the specific site(s) of phosphorylation are not highly conserved. Finally, analysis of phosphoablative and phosphomimetic mutants of YvcK in vitro and in vivo demonstrate that while phosphorylation of YvcK is irrelevant to metabolism and cell wall stress responses, surprisingly, a phosphomimetic, nonreversible negative charge of YvcK is detrimental to cytosolic survival and virulence in vivo. Taken together our data identify two novel virulence factors essential for cytosolic survival and virulence of L. monocytogenes. Furthermore, our data demonstrate that regulation of YvcK phosphorylation is tightly controlled and is critical for virulence. Finally, our data suggest that yet to be identified substrates of PrkA are essential for cytosolic survival and virulence of L. monocytogenes and illustrate the importance of studying protein phosphorylation in the context of infection.

Infection with intracellular pathogens causes a majority of the global infectious disease associated mortality. A number of intracellular pathogens must directly access the host cytosol in order to cause disease; however, non-cytosol adapted bacteria do not survive or replicate upon access to the cytosol. The mechanisms cytosolic pathogens use to adapt to this niche are largely unknown. The model cytosolic bacterial pathogen Listeria monocytogenes contains a single penicillin-binding-protein and serine/threonine associated (PASTA) kinase, PrkA. In other bacteria, PASTA kinases bind cell wall fragments and phosphorylate downstream effectors involved in cell wall synthesis, central metabolism, virulence, cell division, and biofilm formation. We demonstrate that in L. monocytogenes, PrkA is required for cell wall homeostasis, growth under nutrient limiting conditions, survival and replication in host cells, and virulence in vivo. Furthermore, we identify a highly conserved protein of unknown function, YvcK, as a PrkA substrate. We demonstrate that L. monocytogenes YvcK is similarly required for cell wall stress responses, growth on glycerol, cytosolic survival and virulence in vivo. Surprisingly, a phosphomimetic, nonreversible negative charge at the phosphorylation sites on YvcK inactivates functions of the protein related to intracellular survival and virulence, suggesting that the identification of PASTA kinase substrates phosphorylated during infection will be critical to our understanding of this central regulator metabolism, cell wall homeostasis and ultimately virulence.

Saturated fatty acids activate caspase-4/5 in human monocytes, triggering IL-1β and IL-18 release

Obesity is associated with metabolic tissue infiltration by monocyte-derived macrophages. Saturated fatty acids contribute to proinflammatory gene induction in tissue-embedded immune cells. However, it is unknown how circulating monocytes, the macrophage precursors, react to high-fat environments. In macrophages, saturated fatty acids activate inflammatory pathways and, notably, prime caspase-associated inflammasomes. Inflammasome-activated IL-1β contributes to type 2 diabetes. We hypothesized that 1) human monocytes from obese patients show caspase activation, and 2) fatty acids trigger this response and consequent release of IL-1β/IL-18. Human peripheral blood monocytes were sorted by flow cytometry, and caspase activity was measured with a FLICA dye-based assay. Blood monocytes from obese individuals exhibited elevated caspase activity. To explore the nature and consequence of this activity, human THP1 monocytes were exposed to saturated or unsaturated fatty acids. Caspase activity was revealed by isoform-specific cleavage and enzymatic activity; cytokine expression/release was measured by qPCR and ELISA. Palmitate, but not palmitoleate, increased caspase activity in parallel to the release of IL-1β and IL-18. Palmitate induced eventual monocyte cell death with features of pyroptosis (an inflammation-linked cell death program involving caspase-4/5), scored through LDH release, vital dye influx, cell volume changes, and nuclear morphology. Notably, selective gene silencing or inhibition of caspase-4/5 reduced palmitate-induced release of IL-1β and IL-18. In summary, monocytes from obese individuals present elevated caspase activity. Mechanistically, palmitate activates a pyroptotic program in monocytes through caspase-4/5, causing inflammatory cytokine release, additional to inflammasomes. These caspases represent potential, novel, therapeutic targets to taper obesity-associated inflammation.

Heat Shock Protein 90 Facilitates Latent HIV Reactivation through Maintaining the Function of Positive Transcriptional Elongation Factor b (p-TEFb) under Proteasome Inhibition

The persistence of HIV in resting memory CD4⁺ T cells at a latent state is considered as the major barrier on the path to achieve a cure for HIV. Proteasome inhibitors (PIs) were previously reported as latency reversing agents (LRAs) but the mechanism underlying this function is yet unclear. Here we demonstrate that PIs reactivate latent HIV ex vivo without global T cell activation, and may facilitate host innate immune responses. Mechanistically, latent HIV reactivation induced by PIs is mediated by heat shock factor 1 (HSF1) via the recruitment of the heat shock protein (HSP) 90-positive transcriptional elongation factor b (p-TEFb) complex. Specifically, HSP90 downstream HSF1 gives positive feedback to the reactivation process through binding to cyclin-dependent kinase 9 (CDK9) and preventing it from undergoing degradation by the proteasome. Overall, these findings suggest proteasome inhibitors as potential latency reversing agents. In addition, HSF1/HSP90 involved in HIV transcription elongation, may serve as therapeutic targets in HIV eradication.

Pyroptosis triggers pore-induced intracellular traps (PITs) that capture bacteria and lead to their clearance by efferocytosis

When inflammasomes trigger pyroptosis, the plasma membrane tears release-soluble cytosolic contents but retains organelles and intracellular bacteria. Neutrophils then efferocytose the pyroptotic cell debris and kill the entrapped bacteria.

Inflammasomes activate caspase-1 in response to cytosolic contamination or perturbation. This inflammatory caspase triggers the opening of the GSDMD pore in the plasma membrane, resulting in lytic cell death called pyroptosis. We had previously assumed that pyroptosis releases intracellular bacteria to the extracellular space. Here, we find that viable bacteria instead remain trapped within the cellular debris of pyroptotic macrophages. This trapping appears to be an inevitable consequence of how osmotic lysis ruptures the plasma membrane, and may also apply to necroptosis and some forms of nonprogrammed necrosis. Although membrane tears release soluble cytosolic contents, they are small enough to retain organelles and bacteria. We call this structure the pore-induced intracellular trap (PIT), which is conceptually parallel to the neutrophil extracellular trap (NET). The PIT coordinates innate immune responses via complement and scavenger receptors to drive recruitment of and efferocytosis by neutrophils. Ultimately, this secondary phagocyte kills the bacteria. Hence, caspase-1–driven pore-induced cell death triggers a multifaceted defense against intracellular bacteria facilitated by trapping the pathogen within the cellular debris. Bona fide intracellular bacterial pathogens, such as Salmonella, must prevent or delay pyroptosis to avoid being trapped in the PIT and subsequently killed by neutrophils.

The ubiquitin system: a critical regulator of innate immunity and pathogen-host interactions

The ubiquitin system comprises enzymes that are responsible for ubiquitination and deubiquitination, as well as ubiquitin receptors that are capable of recognizing and deciphering the ubiquitin code, which act in coordination to regulate almost all host cellular processes, including host-pathogen interactions. In response to pathogen infection, the host innate immune system launches an array of distinct antimicrobial activities encompassing inflammatory signaling, phagosomal maturation, autophagy and apoptosis, all of which are fine-tuned by the ubiquitin system to eradicate the invading pathogens and to reduce concomitant host damage. By contrast, pathogens have evolved a cohort of exquisite strategies to evade host innate immunity by usurping the ubiquitin system for their own benefits. Here, we present recent advances regarding the ubiquitin system-mediated modulation of host-pathogen interplay, with a specific focus on host innate immune defenses and bacterial pathogen immune evasion.

ZBP1/DAI is an innate sensor of influenza virus triggering the NLRP3 inflammasome and programmed cell death pathways

The interferon-inducible protein Z-DNA binding protein 1 (ZBP1, also known as DNA-dependent activator of IFN-regulatory factors (DAI) and DLM-1) was identified as a dsDNA sensor, which instigates innate immune responses. However, this classification has been disputed and whether ZBP1 functions as a pathogen sensor during an infection has remained unknown. Herein, we demonstrated ZBP1-mediated sensing of the influenza A virus (IAV) proteins NP and PB1, triggering cell death and inflammatory responses via the RIPK1-RIPK3-Caspase-8 axis. ZBP1 regulates NLRP3 inflammasome activation as well as induction of apoptosis, necroptosis and pyroptosis in IAV-infected cells. Importantly, ZBP1 deficiency protected mice from mortality during IAV infection owing to reduced inflammatory responses and epithelial damage. Overall, these findings indicate that ZBP1 is an innate immune sensor of IAV and highlight its importance in the pathogenesis of IAV infection.

Cell-Cycle Regulators and Cell Death in Immunity

Various cell death mechanisms are integral to host defense in both plants and mammals. Plant defense against biotrophic pathogens is associated with programmed cell death (PCD) of the infected cell. This effector-triggered PCD is partly analogous to pyroptosis, an inflammatory host cell death process that plays a crucial role in defense against microbial infections in mammals. Plant effector-triggered PCD also shares with mammalian apoptosis the involvement of cell-cycle regulators as signaling components. Here we explore the similarities between these different cell death programs as they relate to host defense and their relationship to the cell cycle.

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.

Active Caspase-1 Induces Plasma Membrane Pores That Precede Pyroptotic Lysis and Are Blocked by Lanthanides

Canonical inflammasome activation induces a caspase-1/gasdermin D (Gsdmd)-dependent lytic cell death called pyroptosis that promotes antimicrobial host defense but may contribute to sepsis. The nature of the caspase-1-dependent change in plasma membrane (PM) permeability during pyroptotic progression remains incompletely defined. We assayed propidium(2+) (Pro(2+)) influx kinetics during NLRP3 or Pyrin inflammasome activation in murine bone marrow-derived macrophages (BMDMs) as an indicator of this PM permeabilization. BMDMs were characterized by rapid Pro(2+) influx after initiation of NLRP3 or Pyrin inflammasomes by nigericin (NG) or Clostridium difficile toxin B (TcdB), respectively. No Pro(2+) uptake in response to NG or TcdB was observed in Casp1(-/-) or Asc(-/-) BMDMs. The cytoprotectant glycine profoundly suppressed NG and TcdB-induced lysis but not Pro(2+) influx. The absence of Gsdmd expression resulted in suppression of NG-stimulated Pro(2+) influx and pyroptotic lysis. Extracellular La(3+) and Gd(3+) rapidly and reversibly blocked the induced Pro(2+) influx and markedly delayed pyroptotic lysis without limiting upstream inflammasome assembly and caspase-1 activation. Thus, caspase-1-driven pyroptosis requires induction of initial prelytic pores in the PM that are dependent on Gsdmd expression. These PM pores also facilitated the efflux of cytosolic ATP and influx of extracellular Ca(2+) Although lanthanides and Gsdmd deletion both suppressed PM pore activity and pyroptotic lysis, robust IL-1β release was observed in lanthanide-treated BMDMs but not in Gsdmd-deficient cells. This suggests roles for Gsdmd in both passive IL-1β release secondary to pyroptotic lysis and in nonlytic/nonclassical IL-1β export.

Regulation and Function of the Nucleotide Binding Domain Leucine-Rich Repeat-Containing Receptor, Pyrin Domain-Containing-3 Inflammasome in Lung Disease

Inflammasomes are specialized inflammatory signaling platforms that govern the maturation and secretion of proinflammatory cytokines, such as IL-1β and IL-18, through the regulation of caspase-1-dependent proteolytic processing. Several nucleotide binding domain leucine-rich repeat-containing receptor (NLR) family members (i.e., NLR family, pyrin domain containing [NLRP] 1, NLRP3, and NLR family, caspase recruitment domain containing-4 [NLRC4]) as well as the pyrin and hemopoietic expression, interferon-inducibility, nuclear localization domain-containing family member, absent in melanoma 2, can form inflammasome complexes in human cells. In particular, the NLRP3 inflammasome is activated in response to cellular stresses through a two-component pathway, involving Toll-like receptor 4-ligand interaction (priming) followed by a second signal, such as ATP-dependent P2X purinoreceptor 7 receptor activation. Emerging studies suggest that the NLRP3 inflammasome can exert pleiotropic effects in human diseases with potentially both pro- and antipathogenic sequelae. Whereas NLRP3 inflammasome activation can serve as a vital component of host defense against invading bacteria and pathogens, excessive activation of the inflammasome can lead to inflammation-associated tissue injury in the setting of chronic disease. In addition, pyroptosis, an inflammasome-associated mode of cell death, contributes to host defense. Recent research has described the regulation and function of the NLRP3 inflammasome in various pulmonary diseases, including acute lung injury and acute respiratory distress syndrome, sepsis, respiratory infections, chronic obstructive pulmonary disease, asthma, pulmonary hypertension, cystic fibrosis, and idiopathic pulmonary fibrosis. The NLRP3 and related inflammasomes, and their regulated cytokines or receptors, may represent novel diagnostic or therapeutic targets in pulmonary diseases and other diseases in which inflammation contributes to pathogenesis.

Pore-forming activity and structural autoinhibition of the gasdermin family

Inflammatory caspases cleave the gasdermin D (GSDMD) protein to trigger pyroptosis, a lytic form of cell death that is crucial for immune defences and diseases. GSDMD contains a functionally important gasdermin-N domain that is shared in the gasdermin family. The functional mechanism of action of gasdermin proteins is unknown. Here we show that the gasdermin-N domains of the gasdermin proteins GSDMD, GSDMA3 and GSDMA can bind membrane lipids, phosphoinositides and cardiolipin, and exhibit membrane-disrupting cytotoxicity in mammalian cells and artificially transformed bacteria. Gasdermin-N moved to the plasma membrane during pyroptosis. Purified gasdermin-N efficiently lysed phosphoinositide/cardiolipin-containing liposomes and formed pores on membranes made of artificial or natural phospholipid mixtures. Most gasdermin pores had an inner diameter of 10–14 nm and contained 16 symmetric protomers. The crystal structure of GSDMA3 showed an autoinhibited two-domain architecture that is conserved in the gasdermin family. Structure-guided mutagenesis demonstrated that the liposome-leakage and pore-forming activities of the gasdermin-N domain are required for pyroptosis. These findings reveal the mechanism for pyroptosis and provide insights into the roles of the gasdermin family in necrosis, immunity and diseases.

An endogenous caspase-11 ligand elicits interleukin-1 release from living dendritic cells

Dendritic cells (DCs) use pattern recognition receptors to detect microorganisms and activate protective immunity. These cells and receptors are thought to operate in an all-or-nothing manner, existing in an immunologically active or inactive state. Here, we report that encounters with microbial products and self-encoded oxidized phospholipids (oxPAPC) induce an enhanced DC activation state, which we call "hyperactive." Hyperactive DCs induce potent adaptive immune responses and are elicited by caspase-11, an enzyme that binds oxPAPC and bacterial lipopolysaccharide (LPS). oxPAPC and LPS bind caspase-11 via distinct domains and elicit different inflammasome-dependent activities. Both lipids induce caspase-11-dependent interleukin-1 release, but only LPS induces pyroptosis. The cells and receptors of the innate immune system can therefore achieve different activation states, which may permit context-dependent responses to infection.

Matters of life and death. How neutrophils die or survive along NET release and is "NETosis" = necroptosis?

Neutrophil extracellular trap (NET) formation is a hallmark of many disorders that involve neutrophil recruitment, tissue damage, and inflammation. As NET formation is often associated with neutrophil death, the term "NETosis" has become popular. Upon discovery that neutrophils may survive NET release, apparent misnomers, such as "vital NETosis," have been proposed. Meanwhile, it has become obvious that certain stimuli can trigger neutrophil necroptosis, a process associated with NET-like chromatin release. Here, we discuss the relationship between NET release and neutrophil death in view highlighting that many assays used in the field do not properly distinguish between the two. An updated nomenclature is needed replacing the term "NETosis" to meet the growing variety of settings leading to chromatin release with and without neutrophil death. Dissecting which triggers of NET release involve which signaling pathway will help to define drugable molecular targets that inhibit NET release and/or neutrophil necrosis in specific disorders.