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Innate immune cells sense and respond to the cytoplasmic infection of bacterial pathogens through NLRP3, NLRC4 or AIM2 inflammasome depending on the unique molecular pattern of invading pathogens. The infection of flagellin- or type III secretion system (T3SS)-containing Gram-negative bacteria such as Salmonella enterica serovar Typhimurium (S. typhimurium) or Pseudomonas aeruginosa (P. aeruginosa) triggers NLRC4-dependent caspase-1 activation leading to the secretion of proinflammatory cytokines such as interleukin-1-beta (IL-1β) and IL-18. Previous studies have shown that apoptosis-associated speck-like protein containing a CARD (ASC) is also required for Salmonella-induced caspase-1 activation, but it is still unclear how ASC contributes to the activation of NLRC4 inflammasome in response to S. typhimurium infection. In this study, we demonstrate that S. typhimurium triggers the formation of ASC oligomer in a potassium depletion-independent manner as determined by in vitro crosslinking and in situ fluorescence imaging. Remarkably, inhibition of potassium efflux failed to block Salmonella-promoted caspase-1 activation and macrophage cell death. These results collectively suggest that ASC is substantially oligomerized to facilitate the activation of caspase-1 in response to S. typhimurium infection. Contrary to NLRP3 inflammasome, intracellular potassium depletion is not critical for NLRC4 inflammasome signaling by S. typhimurium.

Inflammasomes are multi-protein complexes that mediate activation of caspase-1, which promotes secretion of the proinflammatory cytokines interleukin (IL)-1β and IL-18 and pyroptosis, a form of phagocyte cell death induced by bacterial pathogens. Members of the Nod-like receptor family (including Nlrp1, Nlrp3, and Nlrc4), the DNA sensor Aim2, the adaptor ASC, and pro-caspase-1 are important components of inflammasomes. Stimulation with specific microbial and endogenous molecules leads to inflammasome assembly and caspase-1 activation. Inflammasomes are believed to mediate host defense against microbial pathogens and tissue homeostasis within the intestine, and their dysregulation might contribute to inflammatory diseases and intestinal cancer. Improving our understanding of inflammasome signaling pathways could provide insights into pathogenesis of many gastrointestinal disorders and the development of therapeutics targets and approaches to treat diseases such as inflammatory bowel diseases and GI cancers.

Intracellular pathogens and endogenous danger signals in the cytosol engage NOD-like receptors (NLRs), which assemble inflammasome complexes to activate caspase-1 and promote the release of proinflammatory cytokines IL-1β and IL-18. However, the NLRs that respond to microbial pathogens in vivo are poorly defined. We show that the NLRs NLRP3 and NLRC4 both activate caspase-1 in response to Salmonella typhimurium. Responding to distinct bacterial triggers, NLRP3 and NLRC4 recruited ASC and caspase-1 into a single cytoplasmic focus, which served as the site of pro–IL-1β processing. Consistent with an important role for both NLRP3 and NLRC4 in innate immune defense against S. typhimurium, mice lacking both NLRs were markedly more susceptible to infection. These results reveal unexpected redundancy among NLRs in host defense against intracellular pathogens in vivo.

Summary

The NLRP3 inflammasome is activated in response to a variety of signals that are indicative of damage to the host including tissue damage, metabolic stress, and infection. Upon activation, the NLRP3 inflammasome serves as a platform for activation of the cysteine protease caspase-1, which leads to the processing and secretion of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18. Dysregulated NLRP3 inflammasome activation is associated with both heritable and acquired inflammatory diseases. Here we review new insights into the mechanism of NLRP3 inflammasome activation and its role in disease pathogenesis.

Inflammasomes are cytosolic multiprotein complexes that assemble in response to infectious or noxious stimuli and activate the CASPASE-1 protease. The inflammasome containing the nucleotide binding domain-leucine-rich repeat (NBD-LRR) protein NLRC4 (interleukin-converting enzyme protease-activating factor [IPAF]) responds to the cytosolic presence of bacterial proteins such as flagellin or the inner rod component of bacterial type III secretion systems (e.g., Salmonella PrgJ). In some instances, such as infection with Legionella pneumophila, the activation of the NLRC4 inflammasome requires the presence of a second NBD-LRR protein, NAIP5. NAIP5 also is required for NLRC4 activation by the minimal C-terminal flagellin peptide, which is sufficient to activate NLRC4. However, NLRC4 activation is not always dependent upon NAIP5. In this report, we define the molecular requirements for NAIP5 in the activation of the NLRC4 inflammasome. We demonstrate that the N terminus of flagellin can relieve the requirement for NAIP5 during the activation of the NLRC4 inflammasome. We also demonstrate that NLRC4 responds to the Salmonella protein PrgJ independently of NAIP5. Our results indicate that NAIP5 regulates the apparent specificity of the NLRC4 inflammasome for distinct bacterial ligands.

Stimulation and release of proinflammatory cytokines is an essential step for the activation of an effective innate host defense, and subsequently for the modulation of adaptive immune responses. Interleukin-1β (IL-1β) and IL-18 are important proinflammatory cytokines that on the one hand activate monocytes, macropages, and neutrophils, and on the other hand induce Th1 and Th17 adaptive cellular responses. They are secreted as inactive precursors, and the processing of pro-IL-1β and pro-IL-18 depends on cleavage by proteases. One of the most important of these enzymes is caspase-1, which in turn is activated by several protein platforms called the inflammasomes. Inflammasome activation differs in various cell types, and knock-out mice defective in either caspase-1 or inflammasome components have an increased susceptibility to several types of infections. However, in other infections and in models of sterile inflammation, caspase-1 seems to be less important, and alternative mechanisms such as neutrophil-derived serine proteases or proteases released from microbial pathogens can process and activate IL-1β. In conclusion, IL-1β/IL-18 processing during infection is a complex process in which the inflammasomes are only one of several activation mechanisms.

Francisella tularensis is a pathogenic bacterium whose virulence is linked to its ability to replicate within the host cell cytosol. Entry into the macrophage cytosol activates a host-protective multimolecular complex called the inflammasome to release the proinflammatory cytokines interleukin (IL)-1β and -18 and trigger caspase-1–dependent cell death. In this study, we show that cytosolic F. tularensis subspecies novicida (F. novicida) induces a type I interferon (IFN) response that is essential for caspase-1 activation, inflammasome-mediated cell death, and release of IL-1β and -18. Extensive type I IFN–dependent cell death resulting in macrophage depletion occurs in vivo during F. novicida infection. Type I IFN is also necessary for inflammasome activation in response to cytosolic Listeria monocytogenes but not vacuole-localized Salmonella enterica serovar Typhimurium or extracellular adenosine triphosphate. These results show the specific connection between type I IFN signaling and inflammasome activation, which are two sequential events triggered by the recognition of cytosolic bacteria. To our knowledge, this is the first example of the positive regulation of inflammasome activation. This connection underscores the importance of the cytosolic recognition of pathogens and highlights how multiple innate immunity pathways interact before commitment to critical host responses.

Pattern recognition receptors (PRRs) play a crucial role in both the detection of pathogens and the activation of the innate immune system. Nod-like receptors (NLR) family members are cytosolic PRRs that sense bacterial products or endogenous danger signals. Recent evidence suggests that NLRs contribute to the detection of Salmonella through the activation of inflammasomes, molecular platforms that promotes the maturation of the proinflammatory cytokines IL-1β and IL-18. During enteric Salmonella infection the activation of caspase-1 and the production of IL-1β and IL-18 result in a protective host response. In macrophages, the activation of caspase-1 induced by Salmonella is mainly mediated by the NLR family member NLRC4 that senses cytosolic flagellin. Recent data suggest that an effective innate immune response against Salmonella requires the engagement of multiple inflammasomes in both hematopoietic and non-hematopoietic cell lineages. Further understanding of the innate immune response mediated by inflammasomes should provide new insights into the mechanisms of host defense and the pathogenesis of inflammatory diseases.

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.

Among a number of innate receptors, the nucleotide-binding domain leucine-rich repeat containing (NLR) nucleotide oligomerization domain (NOD)-like receptor families are involved in the recognition of cytosolic pathogen- or danger-associated molecules. Activation of these specific sets of receptors leads to the assembly of a multiprotein complex, the inflammasome, leading to the activation of caspase-1 and maturation of the cytokines interleukin (IL)-1β, IL-18, and IL-33. Among NLRs, NLR-related protein 3 (NLRP3) is one of the best-characterized receptors that activates the inflammasome. There is no doubt that NLRP3 inflammasome activation is important for host defense and effective pathogen clearance against fungal, bacterial, and viral infection. In addition, mounting evidence indicates that the NLRP3 inflammasome plays a role in a variety of inflammatory diseases, including gout, atherosclerosis, and type II diabetes, as well as under conditions of cellular stress or injury. Here, we review recent advances in our understanding of the role of the NLRP3 inflammasome in host defense and various inflammatory diseases.

Activation of caspase-1 is essential for the maturation and release of interleukin (IL)-1β and IL-18, and occurs in multi-protein complexes, referred to as inflammasomes. The apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) is the essential adaptor protein for recruiting pro-caspase-1 into inflammasomes, and consistently gene ablation of ASC abolishes caspase-1 activation and secretion of IL-1β and IL-18. However, distribution of endogenous ASC has not yet been examined in detail. In the present study we demonstrated that ASC localized primarily to the nucleus in resting human monocytes macrophages. Upon pathogen infection ASC rapidly redistributed to the cytosol, followed by assembly of perinuclear aggregates, containing several inflammasome components, including caspase-1 and Nod-like receptors (NLRs). Prevention of ASC cytosolic redistribution completely abolished pathogen induced inflammasome activity, which affirmed that cytosolic localization of ASC is essential for inflammasome function. Thus, our study characterized a novel mechanism of inflammasome regulation in host defense.

Interleukin-1β (IL-1β) is a potent pro-inflammatory cytokine involved in the pathogenesis of HCV, but the sensors and underlying mechanisms that facilitate HCV-induced IL-1β proteolytic activation and secretion remains unclear. In this study, we have identified a signalling pathway leading to IL-1β activation and secretion in response to HCV infection. Previous studies have shown the induction and secretion of IL-1β through the inflammasome complex in macrophages/monocytes. Here, we report for the first time the induction and assembly of the NALP3-inflammasome complex in human hepatoma cells infected with HCV (JFH-1). We demonstrate that activation of IL-1β in HCV-infected cells involves the proteolytic processing of pro-caspase-1 into mature caspase-1 in a multiprotein inflammasome complex. Next, we demonstrate that HCV is sensed by NALP3 protein, which recruits the adaptor protein ASC for the assembly of the inflammasome complex. Using a small interfering RNA approach, we further show that components of the inflammasome complex are involved in the activation of IL-1β in HCV-infected cells. Our study also demonstrates the role of reactive oxygen species in HCV-induced IL-1β secretion. Collectively, these observations provide an insight into the mechanism of IL-1β processing and secretion, which is likely to provide novel strategies for targeting the viral or cellular determinants to arrest the progression of liver disease associated with chronic HCV infection.

Summary

Nucleotide oligomerization domain (NOD)-like receptors (NLRs) are a specialized group of intracellular proteins that play a critical role in the regulation of the host innate immune response. NLRs act as scaffolding proteins that assemble signaling platforms that trigger nuclear factor-κB and mitogen-activated protein kinase signaling pathways and control the activation of inflammatory caspases. Importantly, mutations in several members of the NLR family have been linked to a variety of inflammatory diseases consistent with these molecules playing an important role in host-pathogen interactions and the inflammatory response. In this review, we focus on the role of Nod1 and Nod2 in host defense and in particular discuss recent finding regarding the role of Nlrc4, Nlpr1, and Nlrp3 inflammasomes in caspase-1 activation and subsequent release of proinflammatory cytokines such as interleukin-1β.

The innate immune system is of vital importance for protection against infectious pathogens. Inflammasome mediated caspase-1 activation and subsequent release of pro-inflammatory cytokines like IL-1β and IL-18 is an important arm of the innate immune system. Salmonella enterica subspecies 1 serovar Typhimurium (S. Typhimurium, SL1344) is an enteropathogenic bacterium causing diarrheal diseases. Different reports have shown that in macrophages, S. Typhimurium may activate caspase-1 by at least three different types of stimuli: flagellin, the type III secretion system 1 (T1) and the T1 effector protein SopE. However, the relative importance and interdependence of the different factors in caspase-1 activation is still a matter of debate. Here, we have analyzed their relative contributions to caspase-1 activation in LPS-pretreated RAW264.7 macrophages. Using flagellar mutants (fliGHI, flgK) and centrifugation to mediate pathogen-host cell contact, we show that flagellins account for a small part of the caspase-1 activation in RAW264.7 cells. In addition, functional flagella are of key importance for motility and host cell attachment which is a prerequisite for mediating caspase-1 activation via these three stimuli. Using site directed mutants lacking several T1 effector proteins and flagellin expression, we found that SopE elicits caspase-1 activation even when flagellins are absent. In contrast, disruption of essential genes of the T1 protein injection system (invG, sipB) completely abolished caspase-1 activation. However, a robust level of caspase-1 activation is retained by the T1 system (or unidentified T1 effectors) in the absence of flagellin and SopE. T1-mediated inflammasome activation is in line with recent work by others and suggests that the T1 system itself may represent the basic caspase-1 activating stimulus in RAW264.7 macrophages which is further enhanced independently by SopE and/or flagellin.

The inflammasome is a multi-protein complex that mediates activation of caspase-1 which promotes the secretion of the proinflammatory cytokines IL-1β and IL-18 as well as pyroptosis, a form of cell death induced by bacterial pathogens. Members of the Nod-like receptor family including NLRP1, NLRP3 and NLRC4 and the adaptor ASC are critical components of the inflammasome by linking microbial and endogenous danger signals to caspase-1 activation. Several diseases are associated with the dysregulated activation of caspase-1 and IL-1β secretion. Thus, understanding of inflammasome pathways may provide insights into disease pathogenesis that might serve as potential targets for therapeutic intervention.

The inflammasome is an important innate immune pathway that regulates at least two host responses protective against infections: (1) secretion of the proinflammatory cytokines IL-1β and IL-18 and (2) induction of pyroptosis, a form of cell death. Inflammasomes, of which different types have been identified, are multiprotein complexes containing pattern recognition receptors belonging to the Nod-like receptor family or the PYHIN family and the protease caspase-1. The molecular aspects involved in the activation of different inflammasomes by various pathogens are being rapidly elucidated, and their role during infections is being characterized. Production of IL-1β and IL-18 and induction of pyroptosis of the infected cell have been shown to be protective against many infectious agents. Here, we review the recent literature concerning inflammasome activation in the context of bacterial infections and identify important questions to be answered in the future.

Summary

In response to injurious or infectious agents caspase-1 activating multiprotein complexes, termed inflammasomes, assemble in the cytoplasm of cells. Activated caspase-1 cleaves the pro-forms of the interleukin-1 cytokine family members leading to their activation and secretion. The IL-1 family cytokines have multiple pro-inflammatory activities implicating them in the pathogenesis of many inflammatory diseases. While defined ligands have been identified for the NLRP1, IPAF and AIM2 inflammasomes, little is known about the activation mechanisms of the NLRP3 inflammasome. Numerous different molecular entities, such as various crystals, pore-forming toxins or extracellular ATP can trigger the NLRP3 inflammasome. Recent work proposes that NLRP3 is activated indirectly by host factors that are generated in response to NLRP3 triggers.

Activation of the inflammatory cysteine protease caspase-1 in inflammasome complexes plays a critical role in the host response to microbial infections. Inflammasome activation induces inflammation through secretion of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 and through extracellular release of the alarmin high mobility group box 1. Moreover, caspase-1 activation by inflammasomes counters bacterial replication and induces pyroptosis, a specialized cell death program that removes infected immune cells as part of the host defense system. It is thus not surprising that bacterial and viral pathogens evolved virulence factors targeting inflammasome activation and activity. Here, we provide an overview of the distinct inflammasome complexes that are activated in a pathogen-specific manner and discuss the diverse strategies employed by viruses and bacteria to modulate inflammasome function.

Nod-like receptors have emerged as an important family of sensors in host defense. These receptors are expressed in macrophages, dendritic cells and monocytes and play an important role in microbial immunity. Some Nod-like receptors form the inflammasome, a protein complex that activates caspase-1 in response to several stimuli. Caspase-1 activation leads to processing and secretion of pro-inflammatory cytokines such as interleukin (IL)-1β and IL-18. Here, we discuss recent advances in the inflammasome field with an emphasis on host defense. We also compare differential requirements for inflammasome activation in dendritic cells, macrophages and monocytes.

The inflammasome pathway functions to regulate caspase-1 activation in response to a broad range of stimuli. Caspase-1 activation is required for the maturation of the pivotal pro-inflammatory cytokines of the pro-IL-1β family. In addition, caspase-1 activation leads to a certain type of cell death known as pyroptosis. Activation of the inflammasome has been shown to play a critical role in the recognition and containment of various microbial pathogens, including the intracellularly replicating Listeria monocytogenes; however, the inflammasome pathways activated during L. monocytogenes infection are only poorly defined. Here, we demonstrate that L. monocytogenes activates both the NLRP3 and the AIM2 inflammasome, with a predominant involvement of the AIM2 inflammasome. In addition, L. monocytogenes-triggered cell death was diminished in the absence of both AIM2 and NLRP3, and is concomitant with increased intracellular replication of L. monocytogenes. Altogether, these data establish a role for DNA sensing through the AIM2 inflammasome in the detection of intracellularly replicating bacteria.

Inflammasomes are multi-protein complexes that activate Caspase-1 which subsequently leads to the maturation of the proinflammatory cytokines IL-1β and IL-18 as well as pyroptosis, a form of cell death induced by bacterial pathogens. Members of the Nod-like receptor family including NLRP1, Nlrp3 and Nlrc4 as well as the HIN-200 family member AIM2 are critical components of the inflammasome and link microbial and endogenous danger signals to Caspase-1 activation. In response to microbial infection, activation of the inflammasomes contribute to host protection by inducing immune responses that limit microbial invasion, but deregulated activation of inflammasomes is associated with autoinflammatory syndromes and other pathologies. Thus, understanding inflammasome pathways will provide insights into the host defense response system against microbes and the development of inflammatory disorders.

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.

Author Summary

The innate immune system, the first line of defense against invading pathogens, plays a key role not only in limiting microbe replications at early stages of infection, but also in initiating and orchestrating antigen-specific adaptive immune responses. The innate immune responses against viruses usually rely on recognition of viral nucleic acids by host pattern-recognition receptors such as Toll-like receptors and cytosolic helicases. In addition, recent studies have indicated that certain viruses activate the NLRP3 inflammasome, a multiprotein complex containing the intracellular pattern-recognition receptor NLRP3, which in turn induces secretion of proinflammatory cytokines. We have previously revealed the role of the NLRP3 inflammasome in innate recognition of influenza virus, in which the influenza virus proton-selective ion channel M2 protein, but not viral RNA, is required. Here, we demonstrate that another RNA virus, encephalomyocarditis virus (EMCV), also activates the NLRP3 inflammasome in a viral RNAindependent manner. Instead, the EMCV viroporin 2B, which is involved in Ca2+ flux from intracellular storages into the cytosol, activates the NLRP3 inflammasome. Our results highlight the importance of viroporins, virusencoded transmembrane pore-forming proteins, in recognition of virus infections by NLRP3.

Background

Chronic inflammation of the arterial wall is a key element in the pathogenesis of atherosclerosis, yet the factors that trigger and sustain the inflammation remain elusive. Inflammasomes are cytoplasmic caspase-1-activating protein complexes that promote maturation and secretion of the proinflammatory cytokines interleukin(IL)-1β and IL-18. The most intensively studied inflammasome, NLRP3 inflammasome, is activated by diverse substances, including crystalline and particulate materials. As cholesterol crystals are abundant in atherosclerotic lesions, and IL-1β has been linked to atherogenesis, we explored the possibility that cholesterol crystals promote inflammation by activating the inflammasome pathway.

Principal Findings

Here we show that human macrophages avidly phagocytose cholesterol crystals and store the ingested cholesterol as cholesteryl esters. Importantly, cholesterol crystals induced dose-dependent secretion of mature IL-1β from human monocytes and macrophages. The cholesterol crystal-induced secretion of IL-1β was caspase-1-dependent, suggesting the involvement of an inflammasome-mediated pathway. Silencing of the NLRP3 receptor, the crucial component in NLRP3 inflammasome, completely abolished crystal-induced IL-1β secretion, thus identifying NLRP3 inflammasome as the cholesterol crystal-responsive element in macrophages. The crystals were shown to induce leakage of the lysosomal protease cathepsin B into the cytoplasm and inhibition of this enzyme reduced cholesterol crystal-induced IL-1β secretion, suggesting that NLRP3 inflammasome activation occurred via lysosomal destabilization.

Conclusions

The cholesterol crystal-induced inflammasome activation in macrophages may represent an important link between cholesterol metabolism and inflammation in atherosclerotic lesions.

Nlrc4 is a member of the Nod-like receptors (NLRs), a family of cytosolic receptors involved in sensing bacterial molecules. NLRs are a group of proteins containing spans of leucine-rich repeats that senses bacterial factors within the eukaryotic cytosol. The recognition of bacterial factors provokes the formation of the inflammasome complex which includes specific NLRs. The inflammasome is responsible for caspase-1 activation which leads to the cleavage and maturation of inflammatory cytokines such as IL-1β and IL-18. Nlrc4 was considered to be a devoted flagellin sensor in eukaryotic cells. However, studies using a variety of pathogens such as Salmonella, Legionella, Shigella and Pseudomonas at high bacterial burdens revealed that Nlrc4 can mediate caspase-1 activation independent of bacterial flagellin. On the other hand, new reports showed that Nlrc4 can restrict bacterial infection independently of caspase-1. Therefore, Nlrc4 maybe involved in sensing more than one bacterial molecule and may participate in several immune complexes.

Inflammasomes represent molecular platforms for the activation of inflammatory caspases, and are essential for processing and secretion of the inflammatory cytokines IL-1β and IL-18. Multiple key proteins of inflammasomes contain caspase recruitment domains (CARDs) or PYRIN domains (PYDs). Dissecting CARD- and PYD-mediated interactions substantially improved our understanding of the mechanisms by which these protein platforms are activated, and emphasized their essential role during the inflammatory cytokine response. However, their precise regulation is still poorly understood. A family of small proteins that are composed of either a CARD or a PYD only, emerged as important inflammasome regulators. These CARD-only proteins (COPs) and PYD-only proteins (POPs) function as endogenous dominant-negative proteins that modulate activity of inflammasomes in response to pathogen infection and tissue destruction. Here we will summarize the most recent advances in the regulation of inflammasomes and highlight their importance for immunity and inflammatory disease.