Cathepsin B inhibition ameliorates the non-alcoholic steatohepatitis through suppressing caspase-1 activation

Non-alcoholic fatty liver disease (NAFLD) has emerged as the most common chronic liver disease. NLRP3 inflammasome activation has been widely studied in the pathogenesis of NAFLD. Cathepsin B (CTSB) is a ubiquitous cysteine cathepsin, and the role of CTSB in the progression and development of NAFLD has received extensive concern. However, the exact roles of CTSB in the NAFLD development and NLRP3 inflammasome activation are yet to be evaluated. In the present study, we used methionine choline-deficient (MCD) diet to establish mice NASH model. CTSB inhibitor (CA-074) was used to suppress the expression of CSTB. Expressions of CTSB and caspase-1 were evaluated by immunohistochemical staining. Serum IL-1β and IL-18 levels were also determined. Palmitic acid was used to stimulate Kupffer cells (KCs), and protein expressions of CTSB, NLRP3, ASC (apoptosis-associated speck-like protein containing CARD), and caspase-1 in KCs were detected. The levels of IL-1β and IL-18 in the supernatant of KCs were evaluated by enzyme-linked immunosorbent assay (ELISA). Our results showed that CTSB inhibition improved the liver function and reduced hepatic inflammation and ballooning, and the levels of pro-inflammatory cytokines IL-1β and IL-18 were decreased. The expressions of CTSB and caspase-1 in liver tissues were increased in the NASH group. In in vitro experiments, PA stimulation could increase the expressions of CTSB and NLRP3 inflammasome in KCs, and CTSB inhibition downregulated the expression of NLRP3 inflammasome in KCs, when challenged by PA. Moreover, CTSB inhibition effectively suppressed the expression and activity of caspase-1 and subsequently secretions of IL-1β and IL-18. Collectively, these results suggest that CTSB inhibition limits NLRP3 inflammasome-dependent NASH formation through regulating the expression and activity of caspase-1, thus providing a novel anti-inflammatory signal pathway for the therapy of NAFLD.     

Glucose regulates hypoxia-induced NLRP3 inflammasome activation in macrophages

Although the intimate linkage between hypoxia and inflammation is well known, the mechanism underlying this linkage has not been fully understood. Nucleotide-binding oligomerization domain-like receptor (NLR) family pyrin domain containing 3 (NLRP3) inflammasome is an intracellular multiprotein complex that regulates interleukin-1β (IL-1β) secretion and pyroptosis, and is implicated in the pathogenesis of sterile inflammatory diseases. Here, we investigated the regulatory mechanism of NLRP3 inflammasome activation in response to hypoxia in macrophages. Severe hypoxia (0.1% O₂) induced the processing of pro-IL-1β, pro-caspase-1, and gasdermin D, as well as the release of IL-1β and lactate dehydrogenase in lipopolysaccharide (LPS)-primed murine macrophages, indicating that hypoxia induces NLRP3 inflammasome-driven inflammation and pyroptosis. NLRP3 deficiency and a specific caspase-1 blockade inhibited hypoxia-induced IL-1β release. Hypoxia-induced IL-1β release and cell death were augmented under glucose deprivation, and an addition of glucose in the media negatively regulated hypoxia-induced IL-1β release. Under hypoxia and glucose deprivation, hypoxia-induced glycolysis was not driven and subsequently, the intracellular adenosine triphosphates (ATPs) were depleted. Atomic absorption spectrometry analysis showed a reduction of intracellular K⁺ concentrations, indicating the K⁺ efflux occurring under hypoxia and glucose deprivation. Furthermore, hypoxia and glucose deprivation-induced IL-1β release was significantly prevented by inhibition of K⁺ efflux and K_ATP channel blockers. In vivo experiments further revealed that IL-1β production was increased in LPS-primed mice exposed to hypoxia (9.5% O₂), which was prevented by a deficiency of NLRP3, an apoptosis-associated speck-like protein containing a caspase recruitment domain, and caspase-1. Our results demonstrate that NLRP3 inflammasome can sense intracellular energy crisis as a danger signal induced by hypoxia and glucose deprivation, and provide new insights into the mechanism underlying hypoxia-induced inflammation.     

Mechanisms of NLRP3 Inflammasome Activation: Its Role in the Treatment of Alzheimer’s Disease

Alzheimer’s disease (AD) is a common neurodegenerative disease of progressive dementia which is characterized pathologically by extracellular neuritic plaques containing aggregated amyloid beta (Aβ) and intracellular hyperphosphorylated tau protein tangles in cerebrum. It has been confirmed that microglia-specific nucleotide-binding oligomerization domain (NOD)-like receptor protein 3 (NLRP3) inflammasome-mediated chronic neuroinflammation plays a crucial role in the pathogenesis of AD. Stimulated by Aβ deposition, NLRP3 assembles and activates within microglia in the AD brain, leading to caspase-1 activation along with downstream interleukin (IL)-1β secretion, and subsequent inflammatory events. Activation of the NLRP3 inflammasome mediates microglia to exhibit inflammatory M1 phenotype, with high expression of caspase-1 and IL-1β. This leads to Aβ deposition and neuronal loss in the amyloid precursor protein (APP)/human presenilin-1 (PS1) mouse model of AD. However, NLRP3 or caspase-1 deletion in APP/PS1 mice promotes microglia to transform to an anti-inflammatory M2 phenotype, with decreased secretion of caspase-1 and IL-1β. It also results in improved cognition, enhanced Aβ clearance, and a lower cerebral inflammatory response. This result suggests that the NLRP3 inflammasome may be an appropriate target for reducing neuroinflammation and alleviating pathological processes in AD. In the present review, we summarize the generally accepted regulatory mechanisms of NLRP3 inflammasome activation, and explore its role in neuroinflammation. Furthermore, we speculate on the possible roles of microglia-specific NLRP3 activation in AD pathogenesis and consider potential therapeutic interventions targeting the NLRP3 inflammasome in AD.

     

Celastrol ameliorates Propionibacterium acnes/LPS-induced liver damage and MSU-induced gouty arthritis via inhibiting K63 deubiquitination of NLRP3

BackgroundCelastrol, a pentacyclic triterpenoid quinonemethide isolated from several spp. of Celastraceae family, exhibits anti-inflammatory activities in a variety of diseases including arthritis.PurposeThis study aims to investigate whether the inhibition of NLRP3 inflammasome is engaged in the anti-inflammatory activities of celastrol and delineate the underlying mechanism.MethodsThe influence of celastrol on NLRP3 inflammasome activation was firstly studied in lipopolysaccharide (LPS)-primed mouse bone marrow-derived macrophages (BMDMs) and phorbol 12-myristate 13-acetate (PMA)-primed THP-1 cells treated with nigericin. Reconstituted inflammasome was also established by co-transfecting NLRP3, ASC, pro-caspase-1 and pro-IL-1β in HEK293T cells. The changes of inflammasome components including NLRP3, ASC, pro-caspase-1/caspase-1 and pro-IL-1β/IL-1β were examined by enzyme-linked immunosorbent assay (ELISA), western blotting and immunofluorescence. Furthermore, Propionibacterium acnes (P. acnes)/LPS-induced liver injury and monosodium urate (MSU)-induced gouty arthritis in mice were employed in vivo to validate the inhibitory effect of celastrol on NLRP3 inflammasome.ResultsCelastrol significantly suppressed the cleavage of pro-caspase-1 and pro-IL-1β, while not affecting the protein expressions of NLRP3, ASC, pro-caspase-1 and pro-IL-1β in THP-1 cells, BMDMs and HEK293T cells. Celastrol suppressed NLRP3 inflammasome activation and alleviated P. acnes/LPS-induced liver damage and MSU-induced gouty arthritis. Mechanism study revealed that celastrol could interdict K63 deubiquitination of NLRP3, which may concern interaction of celastrol and BRCA1/BRCA2-containing complex subunit 3 (BRCC3), and thereby prohibited the formation of NLRP3, ASC and pro-caspase-1 complex to block the generation of mature IL-1β.ConclusionCelastrol suppresses NLRP3 inflammasome activation in P. acnes/LPS-induced liver damage and MSU-induced gouty arthritis via inhibiting K63 deubiquitination of NLRP3, which presents a novel insight into inhibition of celastrol on NLRP3 inflammasome and provides more evidences for its application in the therapy of inflammation-related diseases.     

Contribution of endogenously produced reactive oxygen species to the activation of podocyte NLRP3 inflammasomes in hyperhomocysteinemia

Hyperhomocysteinemia (hHcys) is an important pathogenic factor contributing to the progression of end-stage renal disease. Recent studies have demonstrated the implication of nicotinamide adenine dinucleotide phosphate oxidase-mediated NLRP3 inflammasome activation in the development of podocyte injury and glomerular sclerosis during hHcys. However, it remains unknown which reactive oxygen species (ROS) are responsible for this activation of NLRP3 inflammasomes and how such action of ROS is controlled. This study tested the contribution of common endogenous ROS including superoxide (O₂⁻), hydrogen peroxide (H₂O₂), peroxynitrite (ONOO⁻), and hydroxyl radical (OH) to the activation of NLRP3 inflammasomes in mouse podocytes and glomeruli. In vitro, confocal microscopy and size-exclusion chromatography demonstrated that dismutation of O₂⁻ by 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (Tempol) and decomposition of H₂O₂ by catalase prevented Hcys-induced aggregation of NLRP3 inflammasome proteins and inhibited Hcys-induced caspase-1 activation and IL-1β production in mouse podocytes. However, scavenging of ONOO⁻ or OH had no significant effect on either Hcys-induced NLRP3 inflammasome formation or activation. In vivo, scavenging of O₂⁻ by Tempol and removal of H₂O₂ by catalase substantially inhibited NLRP3 inflammasome formation and activation in glomeruli of hHcys mice as shown by reduced colocalization of NLRP3 with ASC or caspase-1 and inhibition of caspase-1 activation and IL-1β production. Furthermore, Tempol and catalase significantly attenuated hHcys-induced glomerular injury. In conclusion, endogenously produced O₂⁻ and H₂O₂ primarily contribute to NLRP3 inflammasome formation and activation in mouse glomeruli resulting in glomerular injury or consequent sclerosis during hHcys.     

Measles virus V protein inhibits NLRP3 inflammasome-mediated interleukin-1β secretion

Inflammasomes are cytosolic protein complexes that stimulate the activation of caspase-1, which in turn induces the secretion of the inflammatory cytokines Interleukin-1β (IL-1β) and IL-18. Recent studies have indicated that the inflammasome known as the NOD-like-receptor-family, pyrin domain-containing 3 (NLRP3) inflammasome recognizes several RNA viruses, including the influenza and encephalomyocarditis viruses, whereas the retinoic acid-inducible gene I (RIG-I) inflammasome may detect vesicular stomatitis virus. We demonstrate that measles virus (MV) infection induces caspase-1-dependent IL-1β secretion in the human macrophage-like cell line THP-1. Gene knockdown experiments indicated that IL-1β secretion in MV-infected THP-1 cells was mediated by the NLRP3 inflammasome but not the RIG-I inflammasome. MV produces the nonstructural V protein, which has been shown to antagonize host innate immune responses. The recombinant MV lacking the V protein induced more IL-1β than the parental virus. THP-1 cells stably expressing the V protein suppressed NLRP3 inflammasome-mediated IL-1β secretion. Furthermore, coimmunoprecipitation assays revealed that the V protein interacts with NLRP3 through its carboxyl-terminal domain. NLRP3 was located in cytoplasmic granular structures in THP-1 cells stably expressing the V protein, but upon inflammasome activation, NLRP3 was redistributed to the perinuclear region, where it colocalized with the V protein. These results indicate that the V protein of MV suppresses NLRP3 inflammasome-mediated IL-1β secretion by directly or indirectly interacting with NLRP3.     

MicroRNA-133a-1 regulates inflammasome activation through uncoupling protein-2

Inflammasomes are multimeric protein complexes involved in the processing of IL-1β through Caspase-1 cleavage. NLRP3 is the most widely studied inflammasome, which has been shown to respond to a large number of both endogenous and exogenous stimuli. Although studies have begun to define basic pathways for the activation of inflammasome and have been instrumental in identifying therapeutics for inflammasome related disorders; understanding the inflammasome activation at the molecular level is still incomplete. Recent functional studies indicate that microRNAs (miRs) regulate molecular pathways and can lead to diseased states when hampered or overexpressed. Mechanisms involving the miRNA regulatory network in the activation of inflammasome and IL-1β processing is yet unknown. This report investigates the involvement of miR-133a-1 in the activation of inflammasome (NLRP3) and IL-1β production. miR-133a-1 is known to target the mitochondrial uncoupling protein 2 (UCP2). The role of UCP2 in inflammasome activation has remained elusive. To understand the role of miR-133a-1 in regulating inflammasome activation, we either overexpressed or suppressed miR-133a-1 in differentiated THP1 cells that express the NLRP3 inflammasome. Levels of Caspase-1 and IL-1β were analyzed by Western blot analysis. For the first time, we showed that overexpression of miR-133a-1 increases Caspase-1 p10 and IL-1β p17 cleavage, concurrently suppressing mitochondrial uncoupling protein 2 (UCP2). Surprisingly, our results demonstrated that miR-133A-1 controls inflammasome activation without affecting the basal expression of the individual inflammasome components NLRP3 and ASC or its immediate downstream targets proIL-1β and pro-Caspase-1. To confirm the involvement of UCP2 in the regulation of inflammasome activation, Caspase-1 p10 and IL-1β p17 cleavage in UCP2 of overexpressed and silenced THP1 cells were studied. Suppression of UCP2 by siRNA enhanced the inflammasome activity stimulated by H₂O₂ and, conversely, overexpression of UCP2 decreased the inflammasome activation. Collectively, these studies suggest that miR-133a-1 suppresses inflammasome activation via the suppression of UCP2.     

Activation of the NLRP3 Inflammasome Complex is Not Required for Stress-Induced Death of Pancreatic Islets

Loss of pancreatic beta cells is a feature of type-2 diabetes. High glucose concentrations induce endoplasmic reticulum (ER) and oxidative stress-mediated apoptosis of islet cells in vitro. ER stress, oxidative stress and high glucose concentrations may also activate the NLRP3 inflammasome leading to interleukin (IL)-1β production and caspase-1 dependent pyroptosis. However, whether IL-1β or intrinsic NLRP3 inflammasome activation contributes to beta cell death is controversial. This possibility was examined in mouse islets. Exposure of islets lacking functional NLRP3 or caspase-1 to H₂O_2, rotenone or thapsigargin induced similar cell death as in wild-type islets. This suggests that oxidative or ER stress do not cause inflammasome-mediated cell death. Similarly, deficiency of NLRP3 inflammasome components did not provide any protection from glucose, ribose or gluco-lipotoxicity. Finally, genetic activation of NLRP3 specifically in beta cells did not increase IL-1β production or cell death, even in response to glucolipotoxicity. Overall, our results show that glucose-, ER stress- or oxidative stress-induced cell death in islet cells is not dependent on intrinsic activation of the NLRP3 inflammasome.     

Serum amyloid A activates the NLRP3 inflammasome via P2X7 receptor and a cathepsin B-sensitive pathway

Serum amyloid A (SAA) is an acute-phase protein, the serum levels of which can increase up to 1000-fold during inflammation. SAA has a pathogenic role in amyloid A-type amyloidosis, and increased serum levels of SAA correlate with the risk for cardiovascular diseases. IL-1β is a key proinflammatory cytokine, and its secretion is strictly controlled by the inflammasomes. We studied the role of SAA in the regulation of IL-1β production and activation of the inflammasome cascade in human and mouse macrophages, as well as in THP-1 cells. SAA could provide a signal for the induction of pro-IL-1β expression and for inflammasome activation, resulting in secretion of mature IL-1β. Blocking TLR2 and TLR4 attenuated SAA-induced expression of IL1B, whereas inhibition of caspase-1 and the ATP receptor P2X(7) abrogated the release of mature IL-1β. NLRP3 inflammasome consists of the NLRP3 receptor and the adaptor protein apoptosis-associated speck-like protein containing CARD (a caspase-recruitment domain) (ASC). SAA-mediated IL-1β secretion was markedly reduced in ASC(-/-) macrophages, and silencing NLRP3 decreased IL-1β secretion, confirming NLRP3 as the SAA-responsive inflammasome. Inflammasome activation was dependent on cathepsin B activity, but it was not associated with lysosomal destabilization. SAA also induced secretion of cathepsin B and ASC. In conclusion, SAA can induce the expression of pro-IL-1β and activation of the NLRP3 inflammasome via P2X(7) receptor and a cathepsin B-sensitive pathway. Thus, during systemic inflammation, SAA may promote the production of IL-1β in tissues. Furthermore, the SAA-induced secretion of active cathepsin B may lead to extracellular processing of SAA and, thus, potentially to the development of amyloid A amyloidosis.     

Encephalomyocarditis Virus Viroporin 2B Activates NLRP3 Inflammasome

Nod-like receptors (NLRs) comprise a large family of intracellular pattern- recognition receptors. Members of the NLR family assemble into large multiprotein complexes, termed the inflammasomes. The NLR family, pyrin domain-containing 3 (NLRP3) is triggered by a diverse set of molecules and signals, and forms the NLRP3 inflammasome. Recent studies have indicated that both DNA and RNA viruses stimulate the NLRP3 inflammasome, leading to the secretion of interleukin 1 beta (IL-1β) and IL-18 following the activation of caspase-1. We previously demonstrated that the proton-selective ion channel M2 protein of influenza virus activates the NLRP3 inflammasome. However, the precise mechanism by which NLRP3 recognizes viral infections remains to be defined. Here, we demonstrate that encephalomyocarditis virus (EMCV), a positive strand RNA virus of the family Picornaviridae, activates the NLRP3 inflammasome in mouse dendritic cells and macrophages. Although transfection with RNA from EMCV virions or EMCV-infected cells induced robust expression of type I interferons in macrophages, it failed to stimulate secretion of IL-1β. Instead, the EMCV viroporin 2B was sufficient to cause inflammasome activation in lipopolysaccharide-primed macrophages. While cells untransfected or transfected with the gene encoding the EMCV non-structural protein 2A or 2C expressed NLRP3 uniformly throughout the cytoplasm, NLRP3 was redistributed to the perinuclear space in cells transfected with the gene encoding the EMCV 2B or influenza virus M2 protein. 2B proteins of other picornaviruses, poliovirus and enterovirus 71, also caused the NLRP3 redistribution. Elevation of the intracellular Ca²⁺ level, but not mitochondrial reactive oxygen species and lysosomal cathepsin B, was important in EMCV-induced NLRP3 inflammasome activation. Chelation of extracellular Ca²⁺ did not reduce virus-induced IL-1β secretion. These results indicate that EMCV activates the NLRP3 inflammasome by stimulating Ca²⁺ flux from intracellular storages to the cytosol, and highlight the importance of viroporins, transmembrane pore-forming viral proteins, in virus-induced NLRP3 inflammasome activation.     

Leptospira interrogans infection leads to IL-1β and IL-18 secretion from a human macrophage cell line through reactive oxygen species and cathepsin B mediated-NLRP3 inflammasome activation

Leptospirosis is a worldwide zoonosis caused by spirochetes from the genus Leptospira. Although there is a large diversity of clinical signs and symptoms, a severe inflammatory response is common to all leptospirosis patients. The mechanism of IL-1β secretion during Leptospira infection has been previously studied in mouse macrophages. However, the outcome of Leptospira infection is very different in human and murine macrophages, and the mechanisms responsible for IL-1β secretion in human macrophages had not been investigated. This study therefore examines the effects of Leptospira interrogans infection on inflammasome activation and proinflammatory cytokine expression in human macrophages. Increased mRNA and protein expression of NLRP3 was observed by real time RT-PCR and flow cytometry at 1 h after co-cultivation. Enzyme-linked immunosorbent assay (ELISA) determination showed that IL-1β and IL-18 are released in the culture supernatants at 1 h after cultivation. The inhibition assay showed that glybenclamide (a K⁺ efflux inhibitor that blocks NLRP3 inflammasome activation) and N-benzyloxycarbony-Val-Ala-Asp (O-methyl)-fluoromethylketone (Z-VAD-FMK; a caspase-1 inhibitor) and NLRP3 depletion with siRNAs reduced the levels of IL-1β and IL-18 release. Moreover, the levels of IL-1β and IL-18 production decreased in CA-074 (a cathepsin B inhibitor) and NAC (an anti-oxidant) pretreated human macrophages, compared to untreated controls. This study suggests that L. interrogans infection leads to reactive oxygen species (ROS)- and cathepsin B-dependent NLRP3 inflammasome activation, which subsequently mediates caspase-1 activation and IL-1β and IL-18 release.     

Hypoxic expression of NLRP3 and VEGF in cultured retinal pigment epithelial cells: contribution of P2Y<Subscript>2</Subscript> receptor signaling

Retinal hypoxia is a major condition of the chronic inflammatory disease age-related macular degeneration. Extracellular ATP is a danger signal which is known to activate the NLRP3 inflammasome in various cell systems. We investigated in cultured human retinal pigment epithelial (RPE) cells whether hypoxia alters the expression of inflammasome-associated genes and whether purinergic receptor signaling contributes to the hypoxic expression of key inflammatory (NLRP3) and angiogenic factor (VEGF) genes. Hypoxia and chemical hypoxia were induced by a 0.2%-O₂ atmosphere and addition of CoCl₂, respectively. Gene expression was determined with real-time RT-PCR. Cytosolic NLRP3 and (pro-) IL-1β levels, and the extracellular VEGF level, were evaluated with Western blot and ELISA analyses. Cell culture in 0.2% O₂ induced expression of NLRP3 and pro-IL-1β genes but not of the pro-IL-18 gene. Hypoxia also increased the cytosolic levels of NLRP3 and (pro-) IL-1β proteins. Inflammasome activation by lysosomal destabilization decreased the cell viability under hypoxic, but not control conditions. In addition to activation of IL-1 receptors, purinergic receptor signaling mediated by a pannexin-dependent release of ATP and a release of adenosine, and activation of P2Y₂ and adenosine A₁ receptors, was required for the full hypoxic expression of the NLRP3 gene. P2Y₂ (but not A₁) receptor signaling also contributed to the hypoxic expression and secretion of VEGF. The data indicate that hypoxia induces priming and activation of the NLRP3 inflammasome in cultured RPE cells. The hypoxic NLRP3 and VEGF gene expression and the secretion of VEGF are in part mediated by P2Y₂ receptor signaling.     

Caspase-11 Activation in Response to Bacterial Secretion Systems that Access the Host Cytosol

Inflammasome activation is important for antimicrobial defense because it induces cell death and regulates the secretion of IL-1 family cytokines, which play a critical role in inflammatory responses. The inflammasome activates caspase-1 to process and secrete IL-1β. However, the mechanisms governing IL-1α release are less clear. Recently, a non-canonical inflammasome was described that activates caspase-11 and mediates pyroptosis and release of IL-1α and IL-1β. Caspase-11 activation in response to Gram-negative bacteria requires Toll-like receptor 4 (TLR4) and TIR-domain-containing adaptor-inducing interferon-β (TRIF)-dependent interferon production. Whether additional bacterial signals trigger caspase-11 activation is unknown. Many bacterial pathogens use specialized secretion systems to translocate effector proteins into the cytosol of host cells. These secretion systems can also deliver flagellin into the cytosol, which triggers caspase-1 activation and pyroptosis. However, even in the absence of flagellin, these secretion systems induce inflammasome activation and the release of IL-1α and IL-1β, but the inflammasome pathways that mediate this response are unclear. We observe rapid IL-1α and IL-1β release and cell death in response to the type IV or type III secretion systems of Legionella pneumophila and Yersinia pseudotuberculosis. Unlike IL-1β, IL-1α secretion does not require caspase-1. Instead, caspase-11 activation is required for both IL-1α secretion and cell death in response to the activity of these secretion systems. Interestingly, whereas caspase-11 promotes IL-1β release in response to the type IV secretion system through the NLRP3/ASC inflammasome, caspase-11-dependent release of IL-1α is independent of both the NAIP5/NLRC4 and NLRP3/ASC inflammasomes as well as TRIF and type I interferon signaling. Furthermore, we find both overlapping and non-redundant roles for IL-1α and IL-1β in mediating neutrophil recruitment and bacterial clearance in response to pulmonary infection by L. pneumophila. Our findings demonstrate that virulent, but not avirulent, bacteria trigger a rapid caspase-11-dependent innate immune response important for host defense.     

The NLRP3 inflammasome detects encephalomyocarditis virus and vesicular stomatitis virus infection

Inflammasomes are cytosolic protein complexes that regulate caspase-1 activation and the secretion of interleukin-1β (IL-1β) and IL-18. Several different inflammasome complexes have been identified, but the NLRP3 inflammasome is particularly notable because of its central role in diseases of inflammation. Recent work has demonstrated an essential role for the NLRP3 inflammasome in host defense against influenza virus. We show here that two other RNA viruses, encephalomyocarditis virus (EMCV) and vesicular stomatitis virus (VSV), activate the NLRP3 inflammasome in dendritic cells and macrophages through a mechanism requiring viral replication. Inflammasome activation in response to both viruses does not require MDA5 or RIG-I signaling. Despite the ability of the NLRP3 inflammasome to detect EMCV and VSV, wild-type and caspase-1-deficient mice were equally susceptible to infection with both viruses. These findings indicate that the NLRP3 inflammasome may be a common pathway for RNA virus detection, but its precise role in the host response may be variable.