A bacterial toxin co-opts caspase-3 to disable active gasdermin D and limit macrophage pyroptosis

During infections, host cells are exposed to pathogen-associated molecular patterns (PAMPs) and virulence factors that stimulate multiple signaling pathways that interact additively, synergistically, or antagonistically. The net effect of such higher-order interactions is a vital determinant of the outcome of host-pathogen interactions. Here, we demonstrate one such complex interplay between bacterial exotoxin- and PAMP-induced innate immune pathways. We show that two caspases activated during enterohemorrhagic Escherichia coli (EHEC) infection by lipopolysaccharide (LPS) and Shiga toxin (Stx) interact in a functionally antagonistic manner; cytosolic LPS-activated caspase-11 cleaves full-length gasdermin D (GSDMD), generating an active pore-forming N-terminal fragment (NT-GSDMD); subsequently, caspase-3 activated by EHEC Stx cleaves the caspase-11-generated NT-GSDMD to render it nonfunctional, thereby inhibiting pyroptosis and interleukin-1β maturation. Bacteria typically subvert inflammasomes by targeting upstream components such as NLR sensors or full-length GSDMD but not active NT-GSDMD. Thus, our findings uncover a distinct immune evasion strategy where a bacterial toxin disables active NT-GSDMD by co-opting caspase-3.

Protective Effect of Silibinin on Lipopolysaccharide-Induced Endotoxemia by Inhibiting Caspase-11-Dependent Cell Pyroptosis

OBJECTIVE

To explore the protective effect and the underlying mechanism of silibinin (SIB), one of the active compounds from Silybum marianum (L.) Gaertn in endotoxemia.

METHODS

Mouse peritoneal macrophage were isolated via intraperitoneally injection of BALB/c mice with thioglycolate medium. Cell viability was assessed using the cell counting kit-8, while cytotoxicity was determined through lactate dehydrogenase cytotoxicity assay. The protein expressions of interleukin (IL)-1 α, IL-1 β, and IL-18 were determined by enzyme-linked immunosorbent assay. Intracellular lipopolysaccharide (LPS) levels were measured by employing both the limulus amoebocyte lysate assay and flow cytometry. Additionally, proximity ligation assay was employed for the LPS and caspase-11 interaction. Mice were divided into 4 groups: the control, LPS, high-dose-SIB (100 mg/kg), and low-dose-SIB (100 mg/kg) groups (n=8). Zebrafish were divided into 4 groups: the control, LPS, high-dose-SIB (200 εmol/L), and low-dose-SIB (100 εmol/L) groups (n=30 for survival experiment and n=10 for gene expression analysis). The expression of caspase-11, gasdermin D (GSDMD), and N-GSDMD was determined by Western blot and the expressions of caspy2, gsdmeb, and IL-1 β were detected using quantitative real-time PCR. Histopathological observation was performed through hematoxylineosin staining, and protein levels in bronchoalveolar lavage fluid were quantified using the bicinchoninicacid protein assay.

RESULTS

SIB noticeably decreased caspase-11 and GSDMD-mediated pyroptosis and suppressed the secretion of IL-1 α, IL-1 β, and IL-18 induced by LPS (P<0.05). Moreover, SIB inhibited the translocation of LPS into the cytoplasm and the binding of caspase-11 and intracellular LPS (P<0.05). SIB also attenuated the expression of caspase-11 and N-terminal fragments of GSDMD, inhibited the relative cytokines, prolonged the survival time, and up-regulated the survival rate in the endotoxemia models (P<0.05).

CONCLUSIONS

SIB can inhibit pyroptosis in the LPS-mediated endotoxemia model, at least in part, by inhibiting the caspase-11-mediated cleavage of GSDMD. Additionally, SIB inhibits the interaction of LPS and caspase-11 and inhibits the LPS-mediated up-regulation of caspase-11 expression, which relieves caspase-11-dependent cell pyroptosis and consequently attenuates LPS-mediated lethality.

Roles of the Caspase-11 Non-Canonical Inflammasome in Rheumatic Diseases

Inflammasomes are intracellular multiprotein complexes that activate inflammatory signaling pathways. Inflammasomes comprise two major classes: canonical inflammasomes, which were discovered first and are activated in response to a variety of pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs), and non-canonical inflammasomes, which were discovered recently and are only activated in response to intracellular lipopolysaccharide (LPS). Although a larger number of studies have successfully demonstrated that canonical inflammasomes, particularly the NLRP3 inflammasome, play roles in various rheumatic diseases, including rheumatoid arthritis (RA), infectious arthritis (IR), gouty arthritis (GA), osteoarthritis (OA), systemic lupus erythematosus (SLE), psoriatic arthritis (PA), ankylosing spondylitis (AS), and Sjögren's syndrome (SjS), the regulatory roles of non-canonical inflammasomes, such as mouse caspase-11 and human caspase-4 non-canonical inflammasomes, in these diseases are still largely unknown. Interestingly, an increasing number of studies have reported possible roles for non-canonical inflammasomes in the pathogenesis of various mouse models of rheumatic disease. This review comprehensively summarizes and discusses recent emerging studies demonstrating the regulatory roles of non-canonical inflammasomes, particularly focusing on the caspase-11 non-canonical inflammasome, in the pathogenesis and progression of various types of rheumatic diseases and provides new insights into strategies for developing potential therapeutics to prevent and treat rheumatic diseases as well as associated diseases by targeting non-canonical inflammasomes.

NEDD4 lactylation promotes APAP induced liver injury through Caspase11 dependent non-canonical pyroptosis

Caspase-11 detection of intracellular lipopolysaccharide mediates non-canonical pyroptosis, which could result in inflammatory damage and organ lesions in various diseases such as sepsis. Our research found that lactate from the microenvironment of acetaminophen-induced acute liver injury increased Caspase-11 levels, enhanced gasdermin D activation and accelerated macrophage pyroptosis, which lead to exacerbation of liver injury. Further experiments unveiled that lactate inhibits Caspase-11 ubiquitination by reducing its binding to NEDD4, a negative regulator of Caspase-11. We also identified that lactates regulated NEDD4 K33 lactylation, which inhibits protein interactions between Caspase-11 and NEDD4. Moreover, restraining lactylation reduces non-canonical pyroptosis in macrophages and ameliorates liver injury. Our work links lactate to the exquisite regulation of the non-canonical inflammasome, and provides a basis for targeting lactylation signaling to combat Caspase-11-mediated non-canonical pyroptosis and acetaminophen-induced liver injury.

Catalytic activity and autoprocessing of murine caspase-11 mediate noncanonical inflammasome assembly in response to cytosolic LPS

Inflammatory caspases are cysteine protease zymogens whose activation following infection or cellular damage occurs within supramolecular organizing centers (SMOCs) known as inflammasomes. Inflammasomes recruit caspases to undergo proximity-induced autoprocessing into an enzymatically active form that cleaves downstream targets. Binding of bacterial LPS to its cytosolic sensor, caspase-11 (Casp11), promotes Casp11 aggregation within a high-molecular-weight complex known as the noncanonical inflammasome, where it is activated to cleave gasdermin D and induce pyroptosis. However, the cellular correlates of Casp11 oligomerization and whether Casp11 forms an LPS-induced SMOC within cells remain unknown. Expression of fluorescently labeled Casp11 in macrophages revealed that cytosolic LPS induced Casp11 speck formation. Unexpectedly, catalytic activity and autoprocessing were required for Casp11 to form LPS-induced specks in macrophages. Furthermore, both catalytic activity and autoprocessing were required for Casp11 speck formation in an ectopic expression system, and processing of Casp11 via ectopically expressed TEV protease was sufficient to induce Casp11 speck formation. These data reveal a previously undescribed role for Casp11 catalytic activity and autoprocessing in noncanonical inflammasome assembly, and shed new light on the molecular requirements for noncanonical inflammasome assembly in response to cytosolic LPS.

The tetrapeptide sequence of IL-18 and IL-1β regulates their recruitment and activation by inflammatory caspases

Inflammasomes are multiprotein signaling complexes that activate the innate immune system. Canonical inflammasomes recruit and activate caspase-1, which then cleaves and activates IL-1β and IL-18, as well as gasdermin D (GSDMD) to induce pyroptosis. In contrast, non-canonical inflammasomes, caspases-4/-5 (CASP4/5) in humans and caspase-11 (CASP11) in mice, are known to cleave GSDMD, but their role in direct processing of other substrates besides GSDMD has remained unknown. Here, we show that CASP4/5 but not CASP11 can directly cleave and activate IL-18. However, CASP4/5/11 can all cleave IL-1β to generate a 27-kDa fragment that deactivates IL-1β signaling. Mechanistically, we demonstrate that the sequence identity of the tetrapeptide sequence adjacent to the caspase cleavage site regulates IL-18 and IL-1β recruitment and activation. Altogether, we have identified new substrates of the non-canonical inflammasomes and reveal key mechanistic details regulating inflammation that may aid in developing new therapeutics for immune-related disorders.

Fusobacterium nucleatum infection activates the noncanonical inflammasome and exacerbates inflammatory response in DSS-induced colitis

Caspase activation results in pyroptosis, an inflammatory cell death that contributes to several inflammatory diseases by releasing inflammatory cytokines and cellular contents. Fusobacterium nucleatum is a periodontal pathogen frequently detected in human cancer and inflammatory bowel diseases. Studies have reported that F. nucleatum infection leads to NLRP3 activation and pyroptosis, but the precise activation process and disease association remain poorly understood. This study demonstrated that F. nucleatum infection exacerbates acute colitis in mice and activates pyroptosis through caspase-11-mediated gasdermin D cleavage in macrophages. Furthermore, F. nucleatum infection in colitis mice induces the enhancement of IL-1⍺ secretion from the colon, affecting weight loss and severe disease activities. Neutralization of IL-1⍺ protects F. nucleatum infected mice from severe colitis. Therefore, F. nucleatum infection facilitates inflammation in acute colitis with IL-1⍺ from colon tissue by activating noncanonical inflammasome through gasdermin D cleavage.

The Role of Caspase-11 and Pyroptosis in the Regulation of Inflammation in Peri-Implantitis

Peri-implantitis is an important cause of oral implant failure. In the past, TLR4 and TLR2 in the Toll-like family were generally considered as the key immune recognition receptors regulating peri-implantitis. However, under the guidance of this theory, there are still some unexplainable peri-implantitis symptoms. With the discovery of novel intracellular LPS receptor Caspase-11, a new understanding of inflammatory signaling and immune regulation in the development of peri-implantitis has been gained. However, the regulatory role of Caspase-11 in peri-implantitis and its crosstalk with the TLR4 pathway remain unclear. The therapeutic effect of drugs targeting Caspase-11 on peri-implantitis is still in its early stages. In view of this situation, this paper reviews the possible role of Caspase-11 in peri-implant inflammation, elaborated the entry process of LPS and the activation mechanism of Caspase-11, and analyzes the differences in Caspase-11 between commonly studied animals, mice and humans. The current research hotspots and challenges are also analyzed to provide new insights and ideas for researchers.

Tetracaine hydrochloride induces macrophage pyroptosis through caspase‑1/11‑GSDMD signaling pathways

Tetracaine hydrochloride (TTC) is a long-lasting local anesthetic commonly used for topical anesthesia. Inappropriate dosage or allergic reactions to TTC can lead to local anesthetic toxicity. TTC exerts cytotoxic effects on certain cell types by inducing apoptosis and necrosis; however, the effects of TTC on macrophages are currently unclear. In the present study, the RAW 264.7 and BV2 cell lines, and murine peritoneal macrophages, were used to evaluate the cytotoxicity of TTC. The present study demonstrated that TTC caused a decrease in cell viability according to a Cell Counting Kit-8 assay, increased lactate dehydrogenase and IL-1β secretion according to ELISA, and induced morphological changes characteristic of pyroptosis according to western blotting. Moreover, TTC-induced macrophage pyroptosis was mediated by gasdermin (GSDM)D, and the cleavage of GSDMD was modulated by both caspase-1 and caspase-11. These results were experimentally validated using caspase-1 and caspase-11 inhibitors. Furthermore, it was observed that TTC and lipopolysaccharide (LPS) exerted similar effects on macrophages. However, the mechanism of induction of pyroptosis by TTC was different from that of LPS. The present study demonstrated that TTC alone could induce macrophage pyroptosis mediated by canonical and non-canonical inflammatory caspases. Therapies targeting pyroptosis may potentially provide a promising future strategy for the prevention and treatment of local anesthetic toxicity induced by TTC.

GABAB Receptor Activation Attenuates Neuronal Pyroptosis in Post-cardiac Arrest Brain Injury

Brain injury is a major cause of death and disability after cardiac arrest (CA). Previous studies have shown that activating GABAB receptors significantly improves neurological function after CA, but the mechanism of this neuronal protection of damaged neurons remains unclear. Thus, the present study aimed to investigate whether GABAB receptor activation protects against neuronal injury and to reveal the underlying protective mechanisms. In this study, rats underwent 10 min of asphyxia to induce CA, and SH-SY5Y cells were subjected to oxygen and glucose deprivation/reoxygenation (OGD/R) to establish in vivo and in vitro models of hypoxic neuronal injury. Differential gene expression between CA rats and sham-operated rats was identified using RNA-seq. TUNEL and Nissl staining were used to evaluate cortical neuron damage, while Western blotting, qRT-PCR, and immunofluorescence assays were conducted to measure pyroptosis-related indicators. Furthermore, cellular models with high expression of caspase-11 were established to reveal the novel molecular mechanisms by which GABAB receptor activation exerts neuroprotective effects. Intriguingly, our results showed that caspase-11 and GSDMD were highly expressed in rats experiencing cardiac arrest. Specifically, GSDMD was expressed in neurons in the M1 area of the cerebral cortex. Moreover, activation of the GABAB receptor exerted a protective effect on neurons both in vivo and in vitro. Baclofen attenuated caspase-11 activation and neuronal pyroptosis after CA, and the anti-neuronal pyroptosis effect of baclofen was abolished by overexpression of caspase-11 in neuronal cells. In conclusion, GABAB receptor activation may play a neuroprotective role by alleviating neuronal pyroptosis through a mechanism involving caspase-11.

Cytosolic LPS-induced caspase-11 oligomerization and activation is regulated by extended synaptotagmin 1

Caspase-11 (Casp-11) is known to induce pyroptosis and defends against cytosol-invading bacterial pathogens, but its regulation remains poorly defined. Here, we identified extended synaptotagmin 1 (E-Syt1), an endoplasmic reticulum protein, as a key regulator of Casp-11 oligomerization and activation. Macrophages lacking E-Syt1 exhibited reduced production of interleukin-1β (IL-1β) and impaired pyroptosis upon cytosolic lipopolysaccharide (LPS) delivery and cytosol-invasive bacterial infection. Moreover, cleavage of Casp-11 and its downstream substrate gasdermin D were significantly diminished in ESyt1-/- macrophages. Upon LPS stimulation, E-Syt1 underwent oligomerization and bound to the p30 domain of Casp-11 via its synaptotagmin-like mitochondrial lipid-binding protein (SMP) domain. E-Syt1 oligomerization and its interaction with Casp-11 facilitated Casp-11 oligomerization and activation. Notably, ESyt1-/- mice were susceptible to infection by cytosol-invading bacteria Burkholderia thailandensis while being resistant to LPS-induced endotoxemia. These findings collectively suggest that E-Syt1 may serve as a platform for Casp-11 oligomerization and activation upon cytosolic LPS sensing.

A 360° view of the inflammasome: Mechanisms of activation, cell death, and diseases

Inflammasomes are critical sentinels of the innate immune system that respond to threats to the host through recognition of distinct molecules, known as pathogen- or damage-associated molecular patterns (PAMPs/DAMPs), or disruptions of cellular homeostasis, referred to as homeostasis-altering molecular processes (HAMPs) or effector-triggered immunity (ETI). Several distinct proteins nucleate inflammasomes, including NLRP1, CARD8, NLRP3, NLRP6, NLRC4/NAIP, AIM2, pyrin, and caspases-4/-5/-11. This diverse array of sensors strengthens the inflammasome response through redundancy and plasticity. Here, we present an overview of these pathways, outlining the mechanisms of inflammasome formation, subcellular regulation, and pyroptosis, and discuss the wide-reaching effects of inflammasomes in human disease.

The orphan receptor Nur77 binds cytoplasmic LPS to activate the non-canonical NLRP3 inflammasome

Intracellular sensing of lipopolysaccharide (LPS) by murine caspase-11 or human caspase-4 initiates a protease cascade, termed the non-canonical inflammasome, that results in gasdermin D (GSDMD) processing and subsequent NLRP3 inflammasome activation. In an effort aimed at identifying additional sensors for intracellular LPS by biochemical screening, we identified the nuclear orphan receptor Nur77 as an LPS-binding protein in macrophage lysates. Nr4a1^-/- macrophages exhibited impaired activation of the NLRP3 inflammasome, but not caspase-11, in response to LPS. Biochemical mapping revealed that Nur77 bound LPS directly through a domain in its C terminus. Yeast two-hybrid assays identified NLRP3 as a binding partner for Nur77. The association between Nur77 and NLRP3 required the presence of LPS and dsDNA. The source of dsDNA was the mitochondria, requiring the formation of gasdermin-D pores. In vivo, Nur77 deficiency ameliorated host response to endotoxins. Thus, Nur77 functions as an intracellular LPS sensor, binding mitochondrial DNA and LPS to activate the non-canonical NLRP3 inflammasome.

Lavender essential oil accelerates lipopolysaccharide-induced chronic wound healing by inhibiting caspase-11-mediated macrophage pyroptosis

Chronic wounds seriously affect the quality of life of the elderly, obese people, and diabetic patients. The excessive inflammatory response is a key driver of delayed chronic wound healing. Although lavender essential oil (EO [lav]) has been proven to have anti-inflammatory and accelerate wound curative effects, the specific molecular mechanism involved is still ambiguous. The results showed that the wounds treated with lipopolysaccharide (LPS) not only had delayed healing, but also the expression levels of pro-inflammatory cytokines, such as tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), interleukin-1β (IL-1β), and the inflammatory mediator protein, high-mobility group box 1 protein (HMGB-1), in the wound tissues were significantly increased. However, treatment of LPS-induced chronic wounds with EO (lav) accelerated wound healing and decreased IL-1β and HMGB-1 expression levels. It was further found that LPS induced macrophage pyroptosis to produce IL-1β. After treatment with EO (lav), the expression level of macrophage pyroptosis marker Gasdermin D (GSDMD) and pyroptosis-related cytotoxic effects were significantly reduced. Immunofluorescence results also directly indicate that EO (lav) can protect macrophages from LPS-induced pyroptosis. Moreover, EO (lav) can down-regulate expression levels of IL-1β, GSDMD, and nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) in the caspase-11-related pyroptotic signaling pathway. This study demonstrates that EO (lav) can reduce proinflammatory factor production and ameliorate inflammatory response by inhibiting macrophage pyroptosis, which accelerates LPS-induced chronic wound healing.

Caspase-11 promotes high-fat diet-induced NAFLD by increasing glycolysis, OXPHOS, and pyroptosis in macrophages

Introduction

Non-alcoholic fatty liver disease (NAFLD) has a global prevalence of 25% of the population and is a leading cause of cirrhosis and hepatocellular carcinoma. NAFLD ranges from simple steatosis (non-alcoholic fatty liver) to non-alcoholic steatohepatitis (NASH). Hepatic macrophages, specifically Kupffer cells (KCs) and monocyte-derived macrophages, act as key players in the progression of NAFLD. Caspases are a family of endoproteases that provide critical connections to cell regulatory networks that sense disease risk factors, control inflammation, and mediate inflammatory cell death (pyroptosis). Caspase-11 can cleave gasdermin D (GSDMD) to induce pyroptosis and specifically defends against bacterial pathogens that invade the cytosol. However, it's still unknown whether high fat diet (HFD)-facilitated gut microbiota-generated cytoplasmic lipopolysaccharides (LPS) activate caspase-11 and promote NAFLD.

Methods

To examine this hypothesis, we performed liver pathological analysis, RNA-seq, FACS, Western blots, Seahorse mitochondrial stress analyses of macrophages and bone marrow transplantation on HFD-induced NAFLD in WT and Casp11-/- mice.

Results and Discussion

Our results showed that 1) HFD increases body wight, liver wight, plasma cholesterol levels, liver fat deposition, and NAFLD activity score (NAS score) in wild-type (WT) mice; 2) HFD increases the expression of caspase-11, GSDMD, interleukin-1β, and guanylate-binding proteins in WT mice; 3) Caspase-11 deficiency decreases fat liver deposition and NAS score; 4) Caspase-11 deficiency decreases bone marrow monocyte-derived macrophage (MDM) pyroptosis (inflammatory cell death) and inflammatory monocyte (IM) surface GSDMD expression; 5) Caspase-11 deficiency re-programs liver transcriptomes and reduces HFD-induced NAFLD; 6) Caspase-11 deficiency decreases extracellular acidification rates (glycolysis) and oxidative phosphorylation (OXPHOS) in inflammatory fatty acid palmitic acid-stimulated macrophages, indicating that caspase-11 significantly contributes to maintain dual fuel bioenergetics-glycolysis and OXPHOS for promoting pyroptosis in macrophages. These results provide novel insights on the roles of the caspase-11-GSDMD pathway in promoting hepatic macrophage inflammation and pyroptosis and novel targets for future therapeutic interventions involving the transition of NAFLD to NASH, hyperlipidemia, type II diabetes, metabolic syndrome, metabolically healthy obesity, atherosclerotic cardiovascular diseases, autoimmune diseases, liver transplantation, and hepatic cancers.

Korean Red Ginseng Saponins Play an Anti-Inflammatory Role by Targeting Caspase-11 Non-Canonical Inflammasome in Macrophages

We previously reported that Korean red ginseng (KRG) exerts an anti-inflammatory role through inhibiting caspase-11 non-canonical inflammasome in macrophages; however, the components responsible for the anti-inflammatory role remained unclear. This study explored the anti-inflammatory activity of the KRG saponin fraction (KRGSF) in caspase-11 non-canonical inflammasome-activated macrophages. KRGSF inhibited pyroptosis, pro-inflammatory cytokine secretion, and inflammatory mediator production in caspase-11 non-canonical inflammasome-activated J774A.1 cells. A mechanism study revealed that KRGSF-induced anti-inflammatory action was mediated via suppressing the proteolytic activation of caspase-11 and gasdermin D (GSDMD) in caspase-11 non-canonical inflammasome-activated J774A.1 cells. Moreover, KRGSF increased the survival of lethal septic mice. Taken together, these results reveal KRGSF-mediated anti-inflammatory action with a novel mechanism, by inhibiting caspase-11 non-canonical inflammasome in macrophages.

Disulfiram ameliorates ischemia/reperfusion-induced acute kidney injury by suppressing the caspase-11-GSDMD pathway

Acute kidney injury (AKI) is a serious condition with high mortality. The most common cause is kidney ischemia/reperfusion (IR) injury, which is thought to be closely related to pyroptosis. Disulfiram is a well-known alcohol abuse drug, and recent studies have shown its ability to mitigate pyroptosis in mouse macrophages. This study investigated whether disulfiram could improve IR-induced AKI and elucidated the possible molecular mechanism. We generated an IR model in mouse kidneys and a hypoxia/reoxygenation (HR) injury model with murine tubular epithelial cells (MTECs). The results showed that IR caused renal dysfunction in mice and triggered pyroptosis in renal tubular epithelial cells, and disulfiram improved renal impairment after IR. The expression of proteins associated with the classical pyroptosis pathway (Nucleotide-binding oligomeric domain (NOD)-like receptor protein 3 (NLRP3), apoptosis-related specific protein (ASC), caspase-1, N-GSDMD) and nonclassical pyroptosis pathway (caspase-11, N-GSDMD) were upregulated after IR. Disulfiram blocked the upregulation of nonclassical but not all classical pyroptosis pathway proteins (NLRP3 and ASC), suggesting that disulfiram might reduce pyroptosis by inhibiting the caspase-11-GSDMD pathway. In vitro, HR increased intracellular ROS levels, the positive rate of PI staining and LDH levels in MTECs, all of which were reversed by disulfiram pretreatment. Furthermore, we performed a computer simulation of the TIR domain of TLR4 using homology modeling and identified a small molecular binding energy between disulfiram and the TIR domain. We concluded that disulfiram might inhibit pyroptosis by antagonizing TLR4 and inhibiting the caspase-11-GSDMD pathway.

TLR9 Negatively Regulates Intracellular Bacterial Killing by Pyroptosis in Burkholderia pseudomallei-Infected Mouse Macrophage Cell Line (Raw264.7)

Melioidosis is a serious infectious disease caused by Burkholderia pseudomallei. This bacterium is able to survive and multiply inside the immune cells such as macrophages. It is well established that Toll-like receptors (TLRs), particularly surface TLRs such as TLR2, TLR4, and TLR5, play an essential role in defending against this bacterial infection. However, the involvement of endosomal TLRs in the infection has not been elucidated. In this study, we demonstrated that the number of intracellular bacteria is reduced in TLR9-depleted RAW264.7 cells infected with B. pseudomallei, suggesting that TLR9 is involved in intracellular bacterial killing in macrophages. As several reports have previously demonstrated that pyroptosis is essential for restricting intracellular bacterial killing, particularly in B. pseudomallei infection, we also observed an increased release of cytosolic enzyme lactate dehydrogenase (LDH) in TLR9-depleted cells infected with B. pseudomallei, suggesting TLR9 involvement in pyroptosis in this context. Consistently, the increases in caspase-11 and gasdermind D (GSDMD) activations, which are responsible for the LDH release, were also detected. Moreover, we demonstrated that the increases in pyroptosis and bacterial killing in B. pseudomallei-infected TLR9-depleted cells were due to the augmentation of the IFN-β, one of the key cytokines known to regulate caspase-11. Altogether, this finding showed that TLR9 suppresses macrophage killing of B. pseudomallei by regulating pyroptosis. This information provides a novel mechanism of TLR9 in the regulation of intracellular bacterial killing by macrophages, which could potentially be leveraged for therapeutic intervention. IMPORTANCE Surface TLRs have been well established to play an essential role in Burkholderia pseudomallei infection. However, the role of endosomal TLRs has not been elucidated. In the present study, we demonstrated that TLR9 plays a crucial role by negatively regulating cytokine production, particularly IFN-β, a vital cytokine to control pyroptosis via caspase-11 activation. By depletion of TLR9, the percentage of pyroptosis was significantly increased, leading to suppression of intracellular survival in B. pseudomallei-infected macrophages. These findings provide a new role of TLR9 in macrophages.

Silica exposure activates non-canonical inflammasome complex in intratracheal instilled rat model

Background

Inhalation of silica crystals in occupational settings is a main cause of silicosis, a chronic irreversible pulmonary disorder. Our prior studies demonstrated the activation of inflammasome sensors AIM2 and NLRP3, effector protein caspase-1, and significant increase in IL-1β in silica exposed rats, suggesting that the canonical inflammasome activation may be associated with silica-induced tissue damage and inflammation.

Aims and Methods

In our current study using the same animal model system, we further evaluated the components of non-canonical inflammasome, including NEK7, caspase-11, and GSDMD following silica exposure.

Results

We demonstrated sustained NEK7 elevation in the rat lung epithelial cells and macrophages following 1- and 3-day exposure. Enhanced NEK7 expression was also detected in lung homogenate by western blot. Similarly, caspase-11 expression was induced by silica exposure in lung sections and homogenate. Elevated GSDMD was observed both in lung sections by immunohistochemical staining and in lung tissue homogenate by western blot.

Conclusion

In summary, our current study demonstrated increase in NEK7, caspase-11, and GSDMD in silica exposed rats, indicating activation of non-canonical inflammasome complex, thereby providing a broad inflammasome activation pathway caused by silica exposure.

Position-Specific Secondary Acylation Determines Detection of Lipid A by Murine TLR4 and Caspase-11

Immune sensing of the Gram-negative bacterial membrane glycolipid lipopolysaccharide (LPS) is both a critical component of host defense against bacterial infection and a contributor to the hyperinflammatory response, potentially leading to sepsis and death. Innate immune activation by LPS is due to the lipid A moiety, an acylated di-glucosamine molecule that can activate inflammatory responses via the extracellular sensor Toll-like receptor 4 (TLR4)/myeloid differentiation 2 (MD2) or the cytosolic sensor caspase-11 (Casp11). The number and length of acyl chains present on bacterial lipid A structures vary across bacterial species and strains, which affects the magnitude of TLR4 and Casp11 activation. TLR4 and Casp11 are thought to respond similarly to various lipid A structures, as tetra-acylated lipid A structures do not activate either sensor, whereas hexa-acylated structures activate both sensors. However, the precise features of lipid A that determine the differential activation of each receptor remain poorly defined, as direct analysis of extracellular and cytosolic responses to the same sources and preparations of LPS/lipid A structures have been limited. To address this question, we used rationally engineered lipid A isolated from a series of bacterial acyl-transferase mutants that produce novel, structurally defined molecules. Intriguingly, we found that the location of specific secondary acyl chains on lipid A resulted in differential recognition by TLR4 or Casp11, providing new insight into the structural features of lipid A required to activate either TLR4 or Casp11. Our findings indicate that TLR4 and Casp11 sense nonoverlapping areas of lipid A chemical space, thereby constraining the ability of Gram-negative pathogens to evade innate immunity.

Caspase-4 and -5 Biology in the Pathogenesis of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a chronic relapsing inflammatory disease of the gastrointestinal tract, associated with high levels of inflammatory cytokine production. Human caspases-4 and -5, and their murine ortholog caspase-11, are essential components of the innate immune pathway, capable of sensing and responding to intracellular lipopolysaccharide (LPS), a component of Gram-negative bacteria. Following their activation by LPS, these caspases initiate potent inflammation by causing pyroptosis, a lytic form of cell death. While this pathway is essential for host defence against bacterial infection, it is also negatively associated with inflammatory pathologies. Caspases-4/-5/-11 display increased intestinal expression during IBD and have been implicated in chronic IBD inflammation. This review discusses the current literature in this area, identifying links between inflammatory caspase activity and IBD in both human and murine models. Differences in the expression and functions of caspases-4, -5 and -11 are discussed, in addition to mechanisms of their activation, function and regulation, and how these mechanisms may contribute to the pathogenesis of IBD.

A TLR4-independent critical role for CD14 in intracellular LPS sensing

Intracellular lipopolysaccharide (LPS) sensing by the noncanonical inflammasome comprising caspase-4 or −11 governs antibacterial host defense. How LPS gains intracellular access in vivo is largely unknown. Here, we show that CD14—an LPS-binding protein with a well-documented role in TLR4 activation—plays a vital role in intracellular LPS sensing in vivo. By generating Cd14^−/− and Casp11^−/− mice strains on a Tlr4^−/− background, we dissociate CD14’s known role in TLR4 signaling from its role in caspase-11 activation and show a TLR4-independent role for CD14 in GSDMD activation, pyroptosis, alarmin release, and the lethality driven by cytosolic LPS. Mechanistically, CD14 enables caspase-11 activation by mediating cytosolic localization of LPS in a TLR4-independent manner. Overall, our findings attribute a critical role for CD14 in noncanonical inflammasome sensing of LPS in vivo and establish—together with previous literature—CD14 as an essential proximal component of both TLR4-based extracellular and caspase-11-based intracellular LPS surveillance.

How LPS attains cytosolic access in vivo is unclear. Vasudevan et al. define a TLR4-independent role for CD14 in the cytosolic localization of LPS, triggering noncanonical inflammasome activation and pyroptosis in vivo. This finding positions CD14 as an integral component of both extracellular and intracellular LPS surveillance pathways.

Autophagy May Allow a Cell to Forbear Pyroptosis When Confronted With Cytosol-Invasive Bacteria

Inflammatory caspases detect cytosol-invasive Gram-negative bacteria by monitoring for the presence of LPS in the cytosol. This should provide defense against the cytosol-invasive Burkholderia and Shigella species by lysing the infected cell via pyroptosis. However, recent evidence has shown caspase-11 and gasdermin D activation can result in two different outcomes: pyroptosis and autophagy. Burkholderia cepacia complex has the ability invade the cytosol but is unable to inhibit caspase-11 and gasdermin D. Yet instead of activating pyroptosis during infection with these bacteria, the autophagy pathway is stimulated through caspases and gasdermin D. In contrast, Burkholderia thailandensis can invade the cytosol where caspasae-11 and gasdermin D is activated but the result is pyroptosis of the infected cell. In this review we propose a hypothetical model to explain why autophagy would be the solution to kill one type of Burkholderia species, but another Burkholderia species is killed by pyroptosis. For pathogens with high virulence, pyroptosis is the only solution to kill bacteria. This explains why some pathogens, such as Shigella have evolved methods to inhibit caspase-11 and gasdermin D as well as autophagy. We also discuss similar regulatory steps that affect caspase-1 that may permit the cell to forbear undergoing pyroptosis after caspase-1 activates in response to bacteria with partially effective virulence factors.

Mechanisms and Consequences of Noncanonical Inflammasome-Mediated Pyroptosis

The noncanonical inflammasome, comprising inflammatory caspases 4, 5, or 11, monitors the cytosol for bacterial lipopolysaccharide (LPS). Intracellular LPS-elicited autoproteolysis of these inflammatory caspases leads to the cleavage of the pore-forming protein gasdermin D (GSDMD). GSDMD pore formation induces a lytic form of cell death known as pyroptosis and the release of inflammatory cytokines and DAMPs, thereby promoting inflammation. The noncanonical inflammasome-dependent innate sensing of cytosolic LPS plays important roles in bacterial infections and sepsis pathogenesis. Exciting studies in the recent past have significantly furthered our understanding of the biochemical and structural basis of the caspase-4/11 activation of GSDMD, caspase-4/11's substrate specificity, and the biological consequences of noncanonical inflammasome activation of GSDMD. This review will discuss these recent advances and highlight the remaining gaps in our understanding of the noncanonical inflammasome and pyroptosis.

Caspase-11 promotes NLRP3 inflammasome activation via the cleavage of pannexin1 in acute kidney disease

Ischemia/reperfusion (I/R) injury is a major cause of acute kidney injury (AKI) in clinic. The activation of NLRP3 inflammasome is associated with inflammation and renal injury in I/R-induced AKI. In the current study we explored the molecular and cellular mechanisms for NLRP3 inflammasome activation following renal I/R. Mice were subjected to I/R renal injury by clamping bilateral renal pedicles. We showed that I/R injury markedly increased caspase-11 expression and the cleavage of pannexin 1 (panx1) in the kidneys accompanied by NLRP3 inflammasome activation evidenced by the activation of caspase-1 and interlukin-1β (IL-1β) maturation. In Casp-11^-/- mice, I/R-induced panx1 cleavage, NLRP3 inflammasome activation as well as renal functional deterioration and tubular morphological changes were significantly attenuated. In cultured primary tubular cells (PTCs) and NRK-52E cells, hypoxia/reoxygenation (H/R) markedly increased caspase-11 expression, NLRP3 inflammasome activation, IL-1β maturation and panx1 cleavage. Knockdown of caspase-11 attenuated all those changes; similar effects were observed in PTCs isolated from Casp-11^-/- mice. In NRK-52E cells, overexpression of caspase-11 promoted panx1 cleavage; pretreatment with panx1 inhibitor carbenoxolone or knockdown of panx1 significantly attenuated H/R-induced intracellular ATP reduction, extracellular ATP elevation and NLRP3 inflammasome activation without apparent influence on H/R-induced caspase-11 increase; pretreatment with P2X7 receptor inhibitor AZD9056 also attenuated NLRP3 inflammasome activation. The above results demonstrate that the cleavage of panx1 by upregulated caspase-11 is involved in facilitating ATP release and then NLRP3 inflammasome activation in I/R-induced AKI. This study provides new insight into the molecular mechanism of NLRP3 inflammasome activation in AKI.

Caspase-1 Inhibitor Reduces Pyroptosis Induced by Brain Death in Kidney

Brain death (BD) induces an organ-level inflammatory response. However, the underlying mechanisms have not been fully elucidated. Here, we investigated the role of caspase-1-mediated pyroptosis in BD-induced kidney injury in rats. A BD model was established in Sprague-Dawley rats. The rats were intravenously injected with Z-YVAD-FMK 1 h before BD, and sham-operated rats served as controls. After 0, 1, 2, 4, and 6 h of BD, renal injury, and renal expression of the nod-like receptor family pyrin domain-containing 3 (NLRP3), caspase-1, caspase-11, gasdermin D (GSDMD), IL-1β, and IL-18 were assessed using quantitative reverse transcriptase-polymerase chain reaction, western blotting, and immunohistochemistry. Blood urea nitrogen and serum creatinine levels were measured. Additionally, renal tubular epithelial cells (NRK-52E) were subjected to 3 h of hypoxia followed by 6 h of reoxygenation and incubated with Z-YVAD-FMK before hypoxia and reoxygenation. Caspase-11 was knocked-down using small interfering RNA technology. Cell viability and levels of pyroptosis-associated proteins were assessed thereafter. NLRP3, caspase-1, GSDMD, IL-1β, and IL-18 expression levels were upregulated in BD rats. Treatment with Z-YVAD-FMK reduced mRNA and protein levels of caspase-1, GSDMD, IL-1β, and IL-18, improved renal function, and alleviated renal injury. Z-YVAD-FMK efficaciously reduced pyroptosis effects in kidneys in BD rats. Thus, it could be considered as a therapeutic target for BD-induced kidney injury.

Cytosolic detection of phagosomal bacteria-Mechanisms underlying PAMP exodus from the phagosome into the cytosol

The metazoan innate immune system senses bacterial infections by detecting highly conserved bacterial molecules, termed pathogen-associated molecular patterns (PAMPs). PAMPs are detected by a variety of host pattern recognition receptors (PRRs), whose function is to coordinate downstream immune responses. PRR activities are, in part, regulated by their subcellular localizations. Accordingly, professional phagocytes can detect extracellular bacteria and their PAMPs via plasma membrane-oriented PRRs. Conversely, phagocytosed bacteria and their PAMPs are detected by transmembrane PRRs oriented toward the phagosomal lumen. Even though PAMPs are unable to passively diffuse across membranes, phagocytosed bacteria are also detected by PRRs localized within the host cell cytosol. This phenomenon is explained by phagocytosis of bacteria that specialize in phagosomal escape and cytosolic residence. Contrary to this cytosolic lifestyle, most bacteria studied to date spend their entire intracellular lifestyle contained within phagosomes, yet they also stimulate cytosolic PRRs. Herein, we will review our current understanding of how phagosomal PAMPs become accessible to cytosolic PRRs, as well as highlight knowledge gaps that should inspire future investigations.

Caspase-6 promotes activation of the caspase-11-NLRP3 inflammasome during gram-negative bacterial infections

The innate immune system acts as the first line of defense against infection. One key component of the innate immune response to gram-negative bacterial infections is inflammasome activation. The caspase-11 (CASP11)-nucleotide-binding oligomerization domain-like receptor pyrin domain-containing 3 (NLRP3) inflammasome is activated by cytosolic lipopolysaccharide, a gram-negative bacterial cell wall component, to trigger pyroptosis and host defense during infection. Although several cellular signaling pathways have been shown to regulate CASP11-NLRP3 inflammasome activation in response to lipopolysaccharide, the upstream molecules regulating CASP11 activation during infection with live pathogens remain unclear. Here, we report that the understudied caspase-6 (CASP6) contributes to the activation of the CASP11-NLRP3 inflammasome in response to infections with gram-negative bacteria. Using in vitro cellular systems with bone marrow-derived macrophages and 293T cells, we found that CASP6 can directly process CASP11 by cleaving at Asp59 and Asp285, the CASP11 auto-cleavage sites, which could contribute to the activation of CASP11 during gram-negative bacterial infection. Thus, the loss of CASP6 led to impaired CASP11-NLRP3 inflammasome activation in response to gram-negative bacteria. These results demonstrate that CASP6 potentiates activation of the CASP11-NLRP3 inflammasome to produce inflammatory cytokines during gram-negative bacterial infections.

Samotolisib Attenuates Acute Liver Injury Through Inhibiting Caspase-11-Mediated Pyroptosis Via Regulating E3 Ubiquitin Ligase Nedd4

Acute liver injury (ALI) is associated with poor survival in patients with sepsis. During sepsis, the liver is the main site of bacterial endotoxin-induced inflammation. Lipopolysaccharide (LPS) promotes caspase-4/5/11 activation, leading to pyroptosis, a major sepsis driver. This study aimed to identify novel drugs that can control hepatocyte caspase-4/5/11 activation during sepsis. We performed LPS-induced caspase-11 activation and pyroptosis in RAW 264.7 cells and established an LPS-induced ALI mouse model. We identified samotolisib (ST), a novel dual phosphoinositide 3-kinase (PI3K) and mammalian target of rapamycin (mTOR) inhibitor, by screening a library of 441 pyroptosis compounds with known targets, which dose-dependently inhibited caspase-11 activation and N-terminal fragment of gasdermin D (GSDMD-NT) generation, reducing RAW 264.7 cell pyroptosis. In mice, ST preconditioning improved survival, attenuated LPS-induced serum alanine aminotransferase and aspartate aminotransferase activity, and inhibited severe liver inflammation and damage. Importantly, ST treatment activated Nedd4, which directly interacts with and mediates caspase-11 ubiquitination and degradation. This was largely abrogated by insulin-like growth factor 1. ST ameliorated LPS-induced hepatotoxicity by inhibiting caspase-11/GSDMD-NT pyroptosis signaling via regulating PI3K/AKT/mTOR/Nedd4 signaling. Hence, ST may play a key role in the prevention of liver injury in patients with sepsis.

Mechanical Ventilation Exacerbates Poly (I:C) Induced Acute Lung Injury: Central Role for Caspase-11 and Gut-Lung Axis

Background

The mechanisms by which moderate tidal volume ventilation (MTV) exacerbates preexisting lung injury are unclear. We hypothesized that systemic endotoxemia via the gut-lung axis would lead to non-canonical and canonical inflammasome activation and pyroptosis in a two-hit model involving polyinosinic-polycytidylic acid (Poly(I:C)), a synthetic analog of dsRNA and MTV and that this would associate with acute lung injury (ALI).

Methods

Anesthetized mice were administered Poly(I:C) intratracheally and then 6 h later, they were mechanically ventilated for 4 h with otherwise non-injurious MTV (10ml/kg). Changes in intestinal and alveolar capillary permeability were measured. Further documentation of ALI was assessed by evans blue albumin permeability, protein and IL-1 family concentration in bronchoalveolar lavage fluid (BALF) or plasma, and histopathology in cohorts of wildtype (WT), whole body genetically ablated caspase-11 (caspase-11-/-), caspase-1/caspase-11 double knockout (caspase-1/11-/-), gasdermin D (GSDMD)-/-, nucleotide-binding domain leucine-rich repeat-containing protein 3 (NLRP3)-/- and advanced glycosylation end product-specific receptor (RAGE) -/- mice.

Results

Non-injurious MTV exacerbated the mild lung injury associated with Poly(I:C) administration. This included the disruption of alveolar-capillary barrier and increased levels of interleukin (IL)-6, high mobility group proteins 1 (HMGB-1), IL-1β in BALF and IL-18 in plasma. Combined (Poly(I:C)-MTV) injury was associated with increase in gastrointestinal permeability and endotoxin in plasma and BALF. Poly(I:C)-MTV injury was sensitive to caspase-11 deletion with no further contribution of caspase-1 except for maturation and release of IL-18 (that itself was sensitive to deletion of NLRP3). Combined injury led to large increases in caspase-1 and caspase-11. Genetic ablation of GSDMD attenuated alveolar-capillary disruption and release of cytokines in combined injury model.

Conclusions

The previously noted exacerbation of mild Poly(I:C)-induced ALI by otherwise non-injurious MTV is associated with an increase in gut permeability resulting in systemic endotoxemia. The gut-lung axis resulted in activation of pulmonary non-canonical (cytosolic mediated caspase-11 activation) and canonical (caspase-1) inflammasome (NLRP3) mediated ALI in this two-hit model resulting in GSDMD sensitive alveolar capillary barrier disruption, pyroptosis (alveolar macrophages) and cytokine maturation and release (IL-1β; IL-18). Pharmacologic strategies aimed at disrupting communication between gut and lung, inhibition of inflammasomes or GSDMD in pyroptosis may be useful in ALI.

Hierarchical Cell Death Program Disrupts the Intracellular Niche Required for Burkholderia thailandensis Pathogenesis

Burkholderia infections can result in serious diseases with high mortality, such as melioidosis, and they are difficult to treat with antibiotics. Innate immunity is critical for cell-autonomous clearance of intracellular pathogens like Burkholderia by regulating programmed cell death. Inflammasome-dependent inflammatory cytokine release and cell death contribute to host protection against Burkholderia pseudomallei and Burkholderia thailandensis; however, the contribution of apoptosis and necroptosis to protection is not known. Here, we found that bone marrow-derived macrophages (BMDMs) lacking key components of pyroptosis died via apoptosis during infection. BMDMs lacking molecules required for pyroptosis, apoptosis, and necroptosis (PANoptosis), however, were significantly resistant to B. thailandensis-induced cell death until later stages of infection. Consequently, PANoptosis-deficient BMDMs failed to limit B. thailandensis-induced cell-cell fusion, which permits increased intercellular spread and replication compared to wild-type or pyroptosis-deficient BMDMs. Respiratory B. thailandensis infection resulted in higher mortality in PANoptosis-deficient mice than in pyroptosis-deficient mice, indicating that, in the absence of pyroptosis, apoptosis is essential for efficient control of infection in vivo. Together, these findings suggest both pyroptosis and apoptosis are necessary for host-mediated control of Burkholderia infection. IMPORTANCE Burkholderia infections result in a high degree of mortality when left untreated; therefore, understanding the host immune response required to control infection is critical. In this study, we found a hierarchical cell death program utilized by infected cells to disrupt the intracellular niche of Burkholderia thailandensis, which limits bacterial intercellular spread, host cell-cell fusion, and bacterial replication. In macrophages, combined loss of key PANoptosis components results in extensive B. thailandensis infection-induced cell-cell fusion, bacterial replication, and increased cell death at later stages of infection compared with both wild-type (WT) and pyroptosis-deficient cells. During respiratory infection, mortality was increased in PANoptosis-deficient mice compared to pyroptosis-deficient mice, identifying an essential role for multiple cell death pathways in controlling B. thailandensis infection. These findings advance our understanding of the physiological role of programmed cell death in controlling Burkholderia infection.

Alendronate Augments Lipid A-Induced IL-1α Release via Activation of ASC but Not Caspase-11

Nitrogen-containing bisphosphonates (NBPs), such as alendronate (ALN), are anti-bone-resorptive drugs that have inflammatory side effects. We previously reported that ALN augmented lipid A-induced interleukin (IL)-1β production and NOD-like receptor pyrin domain-containing-3 (NLRP3)/apoptosis-associated speck-like protein containing a CARD (ASC)-dependent cell death. The present study aimed to examine whether ALN augments lipid A-induced IL-1α release and necroptosis, which is induced by the activation of receptor-interacting protein kinase (RIPK) 3. Treatment of J774.1 cells with ALN augmented lipid A-induced IL-1α release, which was not inhibited by Ac-IETD-CHO, a caspase-8 inhibitor. ALN also activated mixed lineage kinase domain-like (MLKL), a key mediator of the necroptosis pathway, and upregulated the expression of caspase-11, a lipid A receptor. GSK'872, a RIPK3 inhibitor, suppressed the ALN-upregulated expression of caspase-11 and augmented lipid A-induced caspase-8 activation. Moreover, ALN induced the release of NLRP3 and ASC into culture supernatants. GSK'872, but not Ac-IETD-CHO, reduced the ALN-induced release of NLRP3, but not ASC, into culture supernatants, and reduced ALN-induced cell death, but not ALN-induced LDH release. Antibodies against NLRP3 and ASC upregulated caspase-11 expression in the cytosol by inhibiting ALN-induced cell death. However, pretreating cells with an antibody against ASC, but not NLRP3, before ALN addition also inhibited lipid A-induced IL-1α release. Pretreating cells with an antibody against caspase-11 before the addition of ALN or lipid A did not downregulate lipid A-induced production of IL-1α. Taken together, our findings suggest that ALN augments lipid A-induced IL-1α release via activation of ASC, but not caspase-11.

Caspase-11/4 and gasdermin D-mediated pyroptosis contributes to podocyte injury in mouse diabetic nephropathy

Diabetic nephropathy (DN) is characterized by sterile inflammation with continuous injury and loss of renal inherent parenchyma cells. Podocyte is an essential early injury target in DN. The injury and loss of podocytes are closely associated with proteinuria, the early symptom of renal injury in DN. However, the exact mechanism for podocyte injury and death in DN remains ambiguous. In this study we investigated whether pyroptosis, a newly discovered cell death pathway was involved in DN. Diabetic mice were generated by high-fat diet/STZ injections. We showed that the expression levels of caspase-11 and cleavage of gasdermin D (GSDMD-N) in podocytes were significantly elevated, accompanied by reduced expression of podocyte makers nephrin and podocin, loss and fusion in podocyte foot processes, increased inflammatory cytokines NF-κB, IL-1β, and IL-18, macrophage infiltration, glomerular matrix expansion and increased urinary albumin to creatinine ratio (UACR). All these changes in diabetic mice were blunted by knockout of caspase-11 or GSDMD. Cultured human and mouse podocytes were treated with high glucose (30 mM), which significantly increased the expression levels of caspase-11 or caspase-4 (the homolog of caspase-11 in human), GSDMD-N, NF-κB, IL-1β, and IL-18, and decreased the expression of nephrin and podocin. Either caspase-4 or GSDMD knockdown by siRNA significantly blunted these changes. In summary, our results demonstrate that caspase-11/4 and GSDMD-mediated pyroptosis is activated and involved in podocyte loss under hyperglycemia condition and the development of DN.

An 8-Hydroxy-Quinoline Derivative Protects Against Lipopolysaccharide-Induced Lethality in Endotoxemia by Inhibiting HMGB1-Mediated Caspase-11 Signaling

Sepsis, an inflammatory syndrome secondary to infection, is the leading cause of in-hospital lethality. It is evidenced that LPS, the major pathological component of the Gram-negative bacteria membrane, predominantly contributes to the pathogenesis of sepsis. Cytoplasmic lipopolysaccharide (LPS) can be sensed by the noncanonical inflammasome and triggers the oligomerization of caspase-11, resulting in pyroptosis and lethal immune responses in sepsis. A previous study has shown that hepatocyte-released high mobility group box 1 (HMGB1) mediates caspase-11–dependent lethality in sepsis by delivering extracellular LPS into the cytosol. Here, we established a phenotypic screening system using recombinant HMGB1 plus LPS in mouse peritoneal macrophages, identifying a novel 8-hydroxyquinoline derivative named 7-[phenyl (pyridin-2-ylamino) methyl] quinolin-8-ol (8-ol, NSC84094) that can specifically inhibit HMGB1-mediated caspase-11 signaling. 8-ol targets directly to HMGB1 and changes the secondary conformation, consequently disrupting the interaction between LPS and HMGB1 and inhibiting the HMGB1-mediated delivery of LPS into the cytosol. Intervention of 8-ol significantly reduced the release of IL-1α and IL-1β and protected against caspase-11–mediated organ injury and lethality in endotoxemic mice. Thus, this study clearly suggests that the HMGB1–caspase-11 pathway is a potential drug target in lethal immune disorders and might open a new avenue in the treatment of sepsis.

Neutrophil Caspase-11 Is Essential to Defend against a Cytosol-Invasive Bacterium

Either caspase-1 or caspase-11 can cleave gasdermin D to cause pyroptosis, eliminating intracellular replication niches. We previously showed that macrophages detect Burkholderia thailandensis via NLRC4, triggering the release of interleukin (IL)-18 and driving an essential interferon (IFN)-γ response that primes caspase-11. We now identify the IFN-γ-producing cells as a mixture of natural killer (NK) and T cells. Although both caspase-1 and caspase-11 can cleave gasdermin D in macrophages and neutrophils, we find that NLRC4-activated caspase-1 triggers pyroptosis in macrophages, but this pathway does not trigger pyroptosis in neutrophils. In contrast, caspase-11 triggers pyroptosis in both macrophages and neutrophils. This translates to an absolute requirement for caspase-11 in neutrophils during B. thailandensis infection in mice. We present an example of inflammasome sensors causing diverging outcomes in different cell types. Thus, cell fates are dictated not simply by the pathogen or inflammasome, but also by how the cell is wired to respond to detection events.

Kovacs et al. demonstrate that natural killer and T cells produce IFN-γ to prime caspase-11 during Burkholderia thailandensis infection. They demonstrate that in neutrophils, caspase-1 and caspase-11 activation lead to gasdermin D cleavage, but only caspase-11 activation leads to pyroptosis that is necessary for clearance of this cytosol-invasive pathogen in vivo.

Caspase-11-Gasdermin D-Mediated Pyroptosis Is Involved in the Pathogenesis of Atherosclerosis

Background: Pyroptosis is a form of cell death triggered by proinflammatory signals. Recent studies have reported that oxidized phospholipids function as caspase-11 agonists to induce noncanonical inflammasome activation in immune cells. As the levels of oxidized phospholipids derived from ox-LDL are largely elevated in atherosclerotic lesions, this study sought to determine whether oxidized lipids trigger pyroptosis and subsequent inflammation in the pathogenesis of atherosclerosis.

Methods and Results: In our current study, after integrating transcriptomic data available from the Gene Expression Omnibus with data from hyperlipidemic mice and ox-LDL-treated peritoneal macrophages, we discovered that caspase-4/11-gasdermin D-associated inflammatory signaling was significantly activated. Consistently, the mRNA expression of caspase-4 and gasdermin D was upregulated in peripheral blood mononuclear cells from patients with coronary heart disease. In particular, the expression of caspase-4 was closely associated with the severity of lesions in the coronary arteries. An in vivo study showed that caspase-11-gasdermin D activation occurred in response to a high-fat/high-cholesterol (HFHC) diet in ApoE^−/− mice, while caspase-11 deletion largely attenuated the volume and macrophage infiltration of atherosclerotic lesions. An in vitro mechanistic study showed that caspase-11-mediated inflammation occurred partly via gasdermin D-mediated pyroptosis in macrophages. Suppressing gasdermin D in HFHC-fed ApoE^−/− mice via delivery of an adeno-associated virus markedly decreased lesion volume and infiltrating macrophage numbers.

Conclusion: Caspase-11-gasdermin D-mediated pyroptosis and the subsequent proinflammatory response in macrophages are involved in the pathogenesis of atherosclerosis. Therefore, targeting the caspase 11-gasdermin D may serve as an alternative strategy for the treatment of atherosclerosis.

Hierarchical cell-type-specific functions of caspase-11 in LPS shock and antibacterial host defense

Caspase-11 sensing of intracellular lipopolysaccharide (LPS) plays critical roles during infections and sepsis. However, the key cell types that sense intracellular LPS and their contributions to the host responses at the organismal level are not completely clear. Here, we show that macrophage/monocyte-specific caspase-11 plays a dominant role in mediating the pathological manifestations of endotoxemia, including gasdermin D (GSDMD) activation, interleukin (IL)-1β, IL-18, and damage-associated molecular pattern (DAMP) release, tissue damage, and death. Surprisingly, caspase-11 expression in CD11c⁺ cells and intestinal epithelial cells (IECs) plays minor detrimental roles in LPS shock. In contrast, caspase-11 expression in neutrophils is dispensable for LPS-induced lethality. Importantly, caspase-11 sensing of intracellular LPS in LyzM⁺ myeloid cells and MRP8⁺ neutrophils, but not CD11c⁺ cells and IECs, is necessary for bacterial clearance and host survival during intracellular bacterial infection. Thus, we reveal hierarchical cell-type-specific roles of caspase-11 that govern the host-protective and host-detrimental functions of the cytosolic LPS surveillance.

Kumari et al. reveal hierarchical cell-type-specific roles of caspase-11 that govern the host-protective and host-detrimental functions of the cytosolic LPS surveillance pathway during bacterial infections and sepsis, respectively.

Evade Inflammasome-Dependent IL-1 Responses To Establish an Intracytosolic Replication Niche

Rickettsia species (spp.) are strict obligate intracellular bacteria, some of which are pathogenic in their mammalian host, including humans. One critical feature of these stealthy group of pathogens is their ability to manipulate hostile cytosolic environments to their benefits. Although our understanding of Rickettsia cell biology and pathogenesis is evolving, the mechanisms by which pathogenic Rickettsia spp. evade host innate immune detection remain elusive. Here, we show that disease severity in wild-type (WT) C57BL/6J mice infected with Rickettsia typhi (the etiologic agent of murine typhus) and Rickettsia rickettsii (the etiologic agent of Rocky Mountain spotted fever), but not with the nonpathogenic species Rickettsia montanensis, correlated with levels of bacterial burden as detected in the spleens of mice, as well as the serum concentrations of proinflammatory cytokine interleukin-1α (IL-1α) and, to a lesser extent, IL-1β. Antibody-mediated neutralization of IL-1α confirmed a key role in controlling mortality rates and bacterial burdens of rickettsia-infected WT mice. As macrophages are a primary source of both IL-1α and IL-1β cytokines, we determined the mechanism of the antirickettsial activities using bone marrow-derived macrophages. We found that pathogenic R. typhi and R. rickettsii, but not nonpathogenic R. montanensis, eluded pro-IL-1α induction and benefited predominantly from the reduced IL-1α secretion, via a caspase-11-gasdermin D (Gsdmd)-dependent pathway, to facilitate intracytosolic replication. Adoptive transfer experiments identified that IL-1α secretion by macrophages was critical for controlling rickettsiosis in WT mice. In sum, we identified a previously unappreciated pathway by which pathogenic, unlike nonpathogenic, rickettsiae preferentially target the caspase-11-Gsdmd-IL-1α signaling axis in macrophages, thus supporting their replication within the host. IMPORTANCE Currently, no vaccines are available to prevent rickettsioses, while vector-borne rickettsial infections in humans are on the rise globally. In fact, the insufficient understanding of how pathogenic Rickettsia species circumvent host immune defense mechanisms has significantly hindered the development of more effective therapeutics. Here, we identified a previously unappreciated role for the caspase-11-Gsdmd-IL-1α signaling axis in limiting the replication of pathogenic R. rickettsia and R. typhi species in murine macrophages and wild-type (WT) C57BL/6J mice. Adoptive transfer studies further identified IL-1α-secreting macrophages as critical mediators in controlling rickettsial infection in WT mice. Collectively, these findings provide insight into the potential mechanism of how pathogenic, but not nonpathogenic, Rickettsia spp. benefit from a reduction in the caspase-11-Gsdmd-mediated release of IL-1α to support host colonization.

Pyroptosis by caspase-11 inflammasome-Gasdermin D pathway in autoimmune diseases

Inflammasomes are a group of supramolecular complexes primarily comprise a sensor, adaptor protein and an effector. Among them, canonical inflammasomes are assembled by one specific pattern recognition receptor, the adaptor protein apoptosis-associated speck-like protein containing a CARD and procaspase-1. Murine caspase-11 and its human ortholog caspase-4/5 are identified as cytosolic sensors which directly responds to LPS. Once gaining access to cytosol, LPS further trigger inflammasome activation in noncanonical way. Downstream pore-forming Gasdermin D is a pyroptosis executioner. Emerging evidence announced in recent years demonstrate the vital role played by caspase-11 non-canonical inflammasome in a range of autoimmune diseases. Pharmacological ablation of caspase-11 and its related effector results in potent therapeutic effects. Though recent advances have highlighted the potential of caspase-11 as a drug target, the understanding of caspase-11 molecular activation and regulation mechanism remains to be limited and thus hampered the discovery and progression of novel inhibitors. Here in this timeline review, we explored how caspase-11 get involved in the pathogenesis of autoimmune diseases, we also collected the reported small-molecular caspase-11 inhibitors. Moreover, the clinical implications and therapeutic potential of caspase-11 inhibitors are discussed. Targeting non-canonical inflammasomes is a promising strategy for autoimmune diseases treatment, while information about the toxicity and physiological disposition of the promising caspase-11 inhibitors need to be supplemented before they can be translated from bench to bedside.

ASC-Mediated Inflammation and Pyroptosis Attenuates Brucella abortus Pathogenesis Following the Recognition of gDNA

Brucella abortus is a zoonotic pathogen that causes brucellosis. Because of Brucella's unique LPS layer and intracellular localization predominately within macrophages, it can often evade immune detection. However, pattern recognition receptors are capable of sensing Brucella pathogen-associated molecular patterns (PAMPS). For example, NOD-like receptors (NLRs) can form a multi-protein inflammasome complex to attenuate Brucella pathogenesis. The inflammasome activates IL-1β and IL-18 to drive immune cell recruitment. Alternatively, inflammasome activation also initiates inflammatory cell death, termed pyroptosis, which augments bacteria clearance. In this report, we assess canonical and non-canonical inflammasome activation following B. abortus infection. We conducted in vivo studies using Asc-/- mice and observed decreased mouse survival, immune cell recruitment, and increased bacteria load. We also conducted studies with Caspase-11-/- mice and did not observe any significant impact on B. abortus pathogenesis. Through mechanistic studies using Asc-/- macrophages, our data suggests that the protective role of ASC may result from the induction of pyroptosis through a gasdermin D-dependent mechanism in macrophages. Additionally, we show that the recognition of Brucella is facilitated by sensing the PAMP gDNA rather than the less immunogenic LPS. Together, these results refine our understanding of the role that inflammasome activation and pyroptosis plays during brucellosis.

Role of Gate-16 and Gabarap in Prevention of Caspase-11-Dependent Excess Inflammation and Lethal Endotoxic Shock

Sepsis is a life-threating multi-organ disease induced by host innate immunity to pathogen-derived endotoxins including lipopolysaccharide (LPS). Direct sensing of LPS by caspase-11 activates inflammasomes and causes lethal sepsis in mice. Inhibition of caspase-11 inflammasomes is important for the prevention of LPS-induced septic shock; however, whether a caspase-11 inflammasome-specific suppressive mechanism exists is unclear. Here we show that deficiency of GABARAP autophagy-related proteins results in over-activation of caspase-11 inflammasomes but not of canonical inflammasomes. Gate-16^−/−Gabarap^−/− macrophages exhibited elevated guanylate binding protein 2 (GBP2)-dependent caspase-11 activation and inflammatory responses. Deficiency of GABARAPs resulted in formation of GBP2-containing aggregates that promote IL-1β production. High mortality after low dose LPS challenge in Gate-16^−/−Gabarap^−/− mice primed with poly(I:C) or polymicrobial sepsis was ameliorated by compound GBP2 deficiency. These results reveal a critical function of Gate-16 and Gabarap to suppress GBP2-dependent caspase-11-induced inflammation and septic shock.

A comprehensive guide to studying inflammasome activation and cell death

Inflammasomes are multimeric heterogeneous mega-Dalton protein complexes that play key roles in the host innate immune response to infection and sterile insults. Assembly of the inflammasome complex following infection or injury begins with the oligomerization of the upstream inflammasome-forming sensor and proceeds through a multistep process of well-coordinated events and downstream effector functions. Together, these steps enable elegant experimental readouts with which to reliably assess the successful activation of the inflammasome complex and cell death. Here, we describe a comprehensive protocol that details several in vitro (in bone marrow-derived macrophages) and in vivo (in mice) strategies for activating the inflammasome and explain how to subsequently assess multiple downstream effects in parallel to unequivocally establish the activation status of the inflammasome and cell death pathways. Our workflow assesses inflammasome activation via the formation of the apoptosis-associated speck-like protein containing a CARD (ASC) speck; cleavage of caspase-1 and gasdermin D; release of IL-1β, IL-18, caspase-1, and lactate dehydrogenase from the cell; and real-time analysis of cell death by imaging. Analyses take up to ~24 h to complete. Overall, our multifaceted approach provides a comprehensive and consistent protocol for assessing inflammasome activation and cell death.

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.

Roles of the pyroptosis signaling pathway in a sepsis-associated encephalopathy cell model

Objectives

The inhibition of pyroptosis has a protective effect in sepsis-associated encephalopathy (SAE). However, the mechanisms underlying pyroptosis in SAE remain to be elucidated.

Methods

Here, we investigated the effects of the caspase inhibitors, Belnacasan (Beln) and Wedelolactone (Wede), on an induced model of SAE in P12 cells, using immunofluorescence, ELISA, western blotting, and flow cytometry.

Results

The cell viability decreased, IL-1β and IL-18 secretion increased, and the levels of the caspase cleavage products, N-terminal gasdermin D, cleaved caspase-1, and cleaved caspase-11, increased in P12 cells following combined treatment with lipopolysaccharides (LPS) and adenosine triphosphate (ATP). However, treatment with Beln or Wede ameliorated the effects induced by LPS and ATP. Neither Beln nor Wede notably affected the levels of cell apoptosis-associated proteins but these inhibitors regulated the levels of cell pyroptosis-associated proteins. Further, the combination of Beln and Wede exerted greater inhibitory effects on cell pyroptosis than either Beln or Wede alone.

Conclusions

The results demonstrated that both the canonical and non-canonical signaling pathways of cell pyroptosis are involved in LPS-induced cell damage and that the non-canonical signaling pathway may be involved to a greater extent. This suggests that the inhibition of pyroptosis may exert potential therapeutic effects on SAE.

Caspase-11 Non-canonical Inflammasomes in the Lung

The airway epithelium and underlying innate immune cells comprise the first line of host defense in the lung. They recognize pathogen-associated molecular patterns (PAMPs) using membrane-bound receptors, as well as cytosolic receptors such as inflammasomes. Inflammasomes activate inflammatory caspases, which in turn process and release the inflammatory cytokines IL-1β and IL-18. Additionally, inflammasomes trigger a form of lytic cell death termed pyroptosis. One of the most important inflammasomes at the host-pathogen interface is the non-canonical caspase-11 inflammasome that responds to LPS in the cytosol. Caspase-11 is important in defense against Gram-negative pathogens, and can drive inflammatory diseases such as LPS-induced sepsis. However, pathogens can employ evasive strategies to minimize or evade host caspase-11 detection. In this review, we present a comprehensive overview of the function of the non-canonical caspase-11 inflammasome in sensing of cytosolic LPS, and its mechanism of action with particular emphasis in the role of caspase-11 in the lung. We also explore some of the strategies pathogens use to evade caspase-11.

Pyroptotic and non-pyroptotic effector functions of caspase-11

Innate immune cells, epithelial cells, and many other cell types are capable of detecting infection or tissue injury, thus mounting regulated immune response. Inflammasomes are highly sophisticated and effective orchestrators of innate immunity. These oligomerized multiprotein complexes are at the center of various innate immune pathways, including modulation of the cytoskeleton, production and maturation of cytokines, and control of bacterial growth and cell death. Inflammasome assembly often results in caspase‐1 activation, which is an inflammatory caspase that is involved in pyroptotic cell death and release of inflammatory cytokines in response to pathogen patterns and endogenous danger stimuli. However, the nature of stimuli and inflammasome components are diverse. Caspase‐1 activation mediated release of mature IL‐1β and IL‐18 in response to canonical stimuli initiated by NOD‐like receptor (NLR), and apoptosis‐associated speck‐like protein containing a caspase recruitment domain (ASC). On the other hand, caspase‐11 delineates a non‐canonical inflammasome that promotes pyroptotic cell death and non‐pyroptotic functions in response to non‐canonical stimuli. Caspase‐11 in mice and its homologues in humans (caspase‐4/5) belong to caspase‐1 family of cysteine proteases, and play a role in inflammation. Knockout mice provided new genetic tools to study inflammatory caspases and revealed the role of caspase‐11 in mediating septic shock in response to lethal doses of lipopolysaccharide (LPS). Recognition of LPS mediates caspase‐11 activation, which promotes a myriad of downstream effects that include pyroptotic and non‐pyroptotic effector functions. Therefore, the physiological functions of caspase‐11 are much broader than its previously established roles in apoptosis and cytokine maturation. Inflammation induced by exogenous or endogenous agents can be detrimental and, if excessive, can result in organ and tissue damage. Consequently, the existence of sophisticated mechanisms that tightly regulate the specificity and sensitivity of inflammasome pathways provides a fine‐tuning balance between adequate immune response and minimal tissue damage. In this review, we summarize effector functions of caspase‐11.

Inhibition of C3a/C3aR Axis in Diverse Stages of Ulcerative Colitis Affected the Prognosis of UC by Modulating the Pyroptosis and Expression of Caspase-11

Ulcerative colitis (UC) is a serious digestive system disease. Furthermore, the activation of C3a/C3aR axis promoted the expression of caspase-11. And higher levels of caspase-11 could induce the pyroptosis and inflammation of cells. However, some studies suggested that caspase-11 could promote and suppress the inflammation during the development of UC. In addition, whether C3a/C3aR axis could affect the development of UC by modulating the expression of caspase-11 is unclear. We established the UC rat model in this study. Next, the C3aR inhibitor was used to treat these rats at diverse stages of UC. Next, the HE staining was performed to detect the intestinal damage. ELISA was performed to reveal the expression of IL-6 and TNF-α in different stages of UC. Western blotting was used to detect the expression of caspase-11 and C3aR in different stages of UC. Stimulation of C3aR inhibitor in early stage of UC promoted the expression of C3aR and caspase-11 in later stage of UC. Treatment of C3aR inhibitor in later stage of UC inhibited the expression of C3aR and caspase-11 in later stage of UC. Furthermore, application of C3aR inhibitor in early stage of UC aggravates the damage of colon tissue and enhanced the secretion of TNF-α and IL-6 in the later stage of UC. Treatment of C3aR inhibitor in later stage of UC relieved the symptoms of UC and suppressed the production of TNF-α and IL-6 in the later stage of UC. Application of C3aR inhibitor in early stage of UC induced the poor prognosis of UC by upregulating the expression of caspase-11. Treatment of C3aR inhibitor in later stage of UC relieved the symptoms of UC and lead to the favorable prognosis of UC by inhibiting the expression of caspase-11.

Extended subsite profiling of the pyroptosis effector protein gasdermin D reveals a region recognized by inflammatory caspase-11

Pyroptosis is the caspase-dependent inflammatory cell death mechanism that underpins the innate immune response against pathogens and is dysregulated in inflammatory disorders. Pyroptosis occurs via two pathways: the canonical pathway, signaled by caspase-1, and the noncanonical pathway, regulated by mouse caspase-11 and human caspase-4/5. All inflammatory caspases activate the pyroptosis effector protein gasdermin D, but caspase-1 mostly activates the inflammatory cytokine precursors prointerleukin-18 and prointerleukin-1β (pro-IL18/pro-IL1β). Here, in vitro cleavage assays with recombinant proteins confirmed that caspase-11 prefers cleaving gasdermin D over the pro-ILs. However, we found that caspase-11 recognizes protein substrates through a mechanism that is different from that of most caspases. Results of kinetics analysis with synthetic fluorogenic peptides indicated that P1'-P4', the C-terminal gasdermin D region adjacent to the cleavage site, influences gasdermin D recognition by caspase-11. Furthermore, introducing the gasdermin D P1'-P4' region into pro-IL18 enhanced catalysis by caspase-11 to levels comparable with that of gasdermin D cleavage. Pro-IL1β cleavage was only moderately enhanced by similar substitutions. We conclude that caspase-11 specificity is mediated by the P1'-P4' region in its substrate gasdermin D, and similar experiments confirmed that the substrate specificities of the human orthologs of caspase-11, i.e. caspase-4 and caspase-5, are ruled by the same mechanism. We propose that P1'-P4'-based inhibitors could be exploited to specifically target inflammatory caspases.

Increasing complexity of NLRP3 inflammasome regulation

Inflammasomes are multiprotein complexes that assemble upon detection of danger signals to activate the inflammatory enzyme caspase-1, trigger secretion of the highly proinflammatory cytokine IL-1β, and induce an inflammatory cell death called pyroptosis. Distinctiveness of the nucleotide-binding oligomerization (NOD), Leucine-rich repeat (LRR)-containing protein (NLRP3) inflammasome resides in the diversity of molecules that induce its activation, indicating a certain intricacy. Furthermore, besides the canonical activation of NLRP3 in response to various stimuli, caspase-11-dependent detection of intracellular LPS activates NLRP3 through a noncanonical pathway. Several aspects of the NLRP3 inflammasome are not characterized or remain unclear. In this review, we summarize the different modes of NLRP3 activation. We describe recent insights into post-translational and cellular regulation that confer further complexity to NLRP3 inflammasomes.

Inhibition of calpain alleviates coxsackievirus B3-induced myocarditis through suppressing the canonical NLRP3 inflammasome/caspase-1-mediated and noncanonical caspase-11-mediated pyroptosis pathways

This study aimed to verify the effects of calpain on coxsackievirus B3 (CVB3)-induced myocarditis and to further explore the underlying mechanisms. Transgenic mice overexpressing calpastatin, the endogenous calpain inhibitor, were introduced in the present study. The murine model of viral myocarditis (VMC) was established by intraperitoneal injection of CVB3 into transgenic and wild-type mice. Myocardial injury was measured by H&E staining and ELISA for cTnI. CVB3 replication was assessed via capsid protein VP1 detection and virus titration. The fibrotic factors collagen and TGF-β1 were evaluated by Masson staining and real-time PCR analysis, respectively. Moreover, the levels of NLRP3, AIM2, ASC, cleaved caspase-1, cleaved caspase-11 and the pyroptosis indicators GSDMD p30, IL-1β and HMGB1 were determined by real-time PCR, western blot or immunohistochemical analysis. In addition, peripheral IL-1β and HMGB1 were evaluated by ELISA. We observed that CVB3-infected transgenic mice had lower pathological scores, peripheral cTnI levels, viral loads and expression levels of collagen and TGF-β1 in the heart than CVB3-infected wild-type mice. Furthermore, we found decreased levels of NLRP3, ASC, cleaved caspase-1 and cleaved caspase-11 in the hearts of CVB3-infected transgenic mice. However, after CVB3 infection, the levels of AIM2 in transgenic mice and wild-type mice did not differ significantly. Additionally, calpastatin overexpression significantly reduced the levels of GSDMD p30, IL-1β and HMGB1 in the myocardium as well as peripheral IL-1β and HMGB1. Taken together, these findings indicate that calpain inhibition attenuates CVB3-induced myocarditis by suppressing the canonical NLRP3 inflammasome/caspase-1-mediated and noncanonical caspase-11-mediated pyroptosis pathways.