Neisseria gonorrhoeae activates the proteinase cathepsin B to mediate the signaling activities of the NLRP3 and ASC-containing inflammasome

Lack of lipid A pyrophosphorylation and functional lptA reduces inflammation by Neisseria commensals

The interaction of the immune system with Neisseria commensals remains poorly understood. We have previously shown that phosphoethanolamine on the lipid A portion of lipooligosaccharide (LOS) plays an important role in Toll-like receptor 4 (TLR4) signaling. For pathogenic Neisseria, phosphoethanolamine is added to lipid A by the phosphoethanolamine transferase specific for lipid A, which is encoded by lptA. Here, we report that Southern hybridizations and bioinformatics analyses of genomic sequences from all eight commensal Neisseria species confirmed that lptA was absent in 15 of 17 strains examined but was present in N. lactamica. Mass spectrometry of lipid A and intact LOS revealed the lack of both pyrophosphorylation and phosphoethanolaminylation in lipid A of commensal species lacking lptA. Inflammatory signaling in human THP-1 monocytic cells was much greater with pathogenic than with commensal Neisseria strains that lacked lptA, and greater sensitivity to polymyxin B was consistent with the absence of phosphoethanolamine. Unlike the other commensals, whole bacteria of two N. lactamica commensal strains had low inflammatory potential, whereas their lipid A had high-level pyrophosphorylation and phosphoethanolaminylation and induced high-level inflammatory signaling, supporting previous studies indicating that this species uses mechanisms other than altering lipid A to support commensalism. A meningococcal lptA deletion mutant had reduced inflammatory potential, further illustrating the importance of lipid A pyrophosphorylation and phosphoethanolaminylation in the bioactivity of LOS. Overall, our results indicate that lack of pyrophosphorylation and phosphoethanolaminylation of lipid A contributes to the immune privilege of most commensal Neisseria strains by reducing the inflammatory potential of LOS.

Pathogen recognition in the human female reproductive tract: expression of intracellular cytosolic sensors NOD1, NOD2, RIG-1, and MDA5 and response to HIV-1 and Neisseria gonorrhea

PROBLEM

Expression patterns and regulation of cytosolic pattern recognition receptors (PRR) NOD-1, NOD-2, RIG-1, and MDA5 have not been elucidated in the human female reproductive tract (FRT).

METHOD OF STUDY

Primary epithelial cells (EC) isolated from Fallopian tube (FT), endometrium (EM), cervix (Cx), and ectocervix (Ecx) were treated with estradiol, poly(I:C), Neisseria gonorrhea (GC), and HIV-1. PRR mRNA expressions were analyzed by Real-time RT-PCR. Conditioned media were analyzed for IL-8 by ELISA.

RESULTS

EC from all FRT compartments constitutively expressed NOD1, NOD2, RIG-1, and MDA5 with highest levels expressed by FT. Stimulation with poly(I:C) resulted in upregulation of NOD2, RIG-1, and MDA5 in all FRT compartments and correlated with increased secretion of IL-8, whereas estradiol treatment had no effects. Exposure to GC and HIV-1 IIIB but not BaL resulted in selective upregulation of NOD2 and MDA5.

CONCLUSION

PRR are expressed throughout the FRT and differentially regulated by poly(I:C), GC and HIV-1.

Staphylococcus aureus Panton-Valentine leukocidin induces an inflammatory response in human phagocytes via the NLRP3 inflammasome

The Staphylococcus aureus pore-forming toxin PVL is most likely causative for life-threatening necrotizing infections, which are characterized by massive tissue inflammation and necrosis. Whereas the cytotoxic action of PVL on human neutrophils is already well established, the PVL effects on other sensitive cell types, such as monocytes and macrophages, are less clear. In this study, we used different types of human leukocytes (neutrophils, monocytes, macrophages, lymphocytes) to investigate cell-specific binding of PVL subunits and subsequent proinflammatory and cytotoxic effects. In all PVL-sensitive cells, we identified the binding of the subunit LukS-PV as the critical factor for PVL-induced cytotoxicity, which was followed by binding of LukF-PV. LukS-PV binds to monocytes, macrophages, and neutrophils but not to lymphocytes. Additionally, we showed that PVL binding to monocytes and macrophages leads to release of caspase-1-dependent proinflammatory cytokines IL-1β and IL-18. PVL activates the NLRP3 inflammasome, a signaling complex of myeloid cells that is involved in caspase-1-dependent IL-1β processing in response to pathogens and endogenous danger signals. Specific inhibition of this pathway at several steps significantly reduced inflammasome activation and subsequent pyronecrosis. Furthermore, we found that PAMPs and DAMPs derived from dying neutrophils can dramatically enhance this response by up-regulating pro-IL-1β in monocytes/macrophages. This study analyzes a specific host signaling pathway that mediates PVL-induced inflammation and cytotoxicity, which has high relevance for CA-MRSA-associated and PVL-mediated pathogenic processes, such as necrotizing infections.

Secretory leukocyte protease inhibitor binds to Neisseria gonorrhoeae outer membrane opacity protein and is bactericidal

PROBLEM

Secretory leukocyte protease inhibitor (SLPI) is an innate immune peptide present on the genitourinary tract mucosa that has antimicrobial activity. In this study, we investigated the interaction of SLPI with Neisseria gonorrhoeae.

METHOD OF STUDY

ELISA and far-Western blots were used to analyze binding of SLPI to gonococci. The binding site for SLPI was identified by tryptic digests and mass spectrometry. Antimicrobial activity of SLPI for gonococci was determined using bactericidal assays. SLPI protein levels in cell supernatants were measured by ELISA, and SLPI mRNA levels were assessed by quantitative RT-PCR.

RESULTS

SLPI bound directly to the gonococcal Opa protein and was bactericidal. Epithelial cells from the reproductive tract constitutively expressed SLPI at different levels. Gonococcal infection of cells did not affect SLPI expression.

CONCLUSION

We conclude that SLPI is bactericidal for gonococci and is expressed by reproductive tract epithelial cells and thus is likely to play a role in the pathogenesis of gonococcal infection.

Neisseria gonorrhoeae suppresses dendritic cell-induced, antigen-dependent CD4 T cell proliferation

Neisseria gonorrhoeae is the second most common sexually transmitted bacterial pathogen worldwide. Diseases associated with N. gonorrhoeae cause localized inflammation of the urethra and cervix. Despite this inflammatory response, infected individuals do not develop protective adaptive immune responses to N. gonorrhoeae. N. gonorrhoeae is a highly adapted pathogen that has acquired multiple mechanisms to evade its host's immune system, including the ability to manipulate multiple immune signaling pathways. N. gonorrhoeae has previously been shown to engage immunosuppressive signaling pathways in B and T lymphocytes. We have now found that N. gonorrhoeae also suppresses adaptive immune responses through effects on antigen presenting cells. Using primary, murine bone marrow-derived dendritic cells and lymphocytes, we show that N. gonorrhoeae-exposed dendritic cells fail to elicit antigen-induced CD4+ T lymphocyte proliferation. N. gonorrhoeae exposure leads to upregulation of a number of secreted and dendritic cell surface proteins with immunosuppressive properties, particularly Interleukin 10 (IL-10) and Programmed Death Ligand 1 (PD-L1). We also show that N. gonorrhoeae is able to inhibit dendritic cell- induced proliferation of human T-cells and that human dendritic cells upregulate similar immunosuppressive molecules. Our data suggest that, in addition to being able to directly influence host lymphocytes, N. gonorrhoeae also suppresses development of adaptive immune responses through interactions with host antigen presenting cells. These findings suggest that gonococcal factors involved in host immune suppression may be useful targets in developing vaccines that induce protective adaptive immune responses to this pathogen.

TRIF licenses caspase-11-dependent NLRP3 inflammasome activation by gram-negative bacteria

Systemic infections with Gram-negative bacteria are characterized by high mortality rates due to the "sepsis syndrome," a widespread and uncontrolled inflammatory response. Though it is well recognized that the immune response during Gram-negative bacterial infection is initiated after the recognition of endotoxin by Toll-like receptor 4, the molecular mechanisms underlying the detrimental inflammatory response during Gram-negative bacteremia remain poorly defined. Here, we identify a TRIF pathway that licenses NLRP3 inflammasome activation by all Gram-negative bacteria. By engaging TRIF, Gram-negative bacteria activate caspase-11. TRIF activates caspase-11 via type I IFN signaling, an event that is both necessary and sufficient for caspase-11 induction and autoactivation. Caspase-11 subsequently synergizes with the assembled NLRP3 inflammasome to regulate caspase-1 activation and leads to caspase-1-independent cell death. These events occur specifically during infection with Gram-negative, but not Gram-positive, bacteria. The identification of TRIF as a regulator of caspase-11 underscores the importance of TLRs as master regulators of inflammasomes during Gram-negative bacterial infection.

Biochemical regulation of the inflammasome

The extensively studied cytokine IL-1β is an important mediator of the inflammatory response. However, dysregulated release of IL-1β can be detrimental and is attributed to the progression and pathogenesis of multiple inflammatory diseases including, rhuematoid arthritis (RA), atherosclerosis, type 2 diabetes (T2D), Alzheimers disease and gout. IL-1β is encoded as a pro-protein. A multi-protein molecular scaffold termed the "Inflammasome" is responsible for the tightly controlled and coordinated processing of pro-IL-1β. The activation of several NLR (nucleotide-binding oligomerization domain (NOD)-like receptor) family members and PYHIN (pyrin and HIN domain) proteins can drive the formation of inflammasomes. However, the exact biochemical mechanisms governing their activation have been the subject of much research. Different inflammasomes have been demonstrated to respond to the same pathogen inducing a cooperative immune response accountable for the clearance of infection. Here, we review current knowledge surrounding the biochemical regulation of the NLRP1, NLRP3, NLRC4, AIM2 and IFI16 inflammasomes.

YopJ-induced caspase-1 activation in Yersinia-infected macrophages: independent of apoptosis, linked to necrosis, dispensable for innate host defense

Yersinia outer protein J (YopJ) is a type III secretion system (T3SS) effector of pathogenic Yersinia (Yersinia pestis, Yersinia enterocolitica and Yersinia pseudotuberculosis) that is secreted into host cells. YopJ inhibits survival response pathways in macrophages, causing cell death. Allelic variation of YopJ is responsible for differential cytotoxicity in Yersinia strains. YopJ isoforms in Y. enterocolitica O:8 (YopP) and Y. pestis KIM (YopJ^KIM) strains have high cytotoxic activity. In addition, YopJ^KIM-induced macrophage death is associated with caspase-1 activation and interleukin-1β (IL-1β secretion. Here, the mechanism of YopJ^KIM-induced cell death, caspase-1 activation, and IL-1β secretion in primary murine macrophages was examined. Caspase-3/7 activity was low and the caspase-3 substrate poly (ADP-ribose) polymerase (PARP) was not cleaved in Y. pestis KIM5-infected macrophages. In addition, cytotoxicity and IL-1β secretion were not reduced in the presence of a caspase-8 inhibitor, or in B-cell lymphoma 2 (Bcl-2)-associated X protein (Bax)/Bcl-2 homologous antagonist/killer (Bak) knockout macrophages, showing that YopJ^KIM-mediated cell death and caspase-1 activation occur independent of mitochondrial-directed apoptosis. KIM5-infected macrophages released high mobility group protein B1 (HMGB1), a marker of necrosis, and microscopic analysis revealed that necrotic cells contained active caspase-1, indicating that caspase-1 activation is associated with necrosis. Inhibitor studies showed that receptor interacting protein 1 (RIP1) kinase and reactive oxygen species (ROS) were not required for cytotoxicity or IL-β release in KIM5-infected macrophages. IL-1β secretion was reduced in the presence of cathepsin B inhibitors, suggesting that activation of caspase-1 requires cathepsin B activity. Ectopically-expressed YopP caused higher cytotoxicity and secretion of IL-1β in Y. pseudotuberculosis-infected macrophages than YopJ^KIM. Wild-type and congenic caspase 1 knockout C57BL/6 mice were equally susceptible to lethal infection with Y. pseudotuberculosis ectopically expressing YopP. These data suggest that YopJ-induced caspase-1 activation in Yersinia-infected macrophages is a downstream consequence of necrotic cell death and is dispensable for innate host resistance to a strain with enhanced cytotoxicity.

The Role of NLR-related Protein 3 Inflammasome in Host Defense and Inflammatory Diseases

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.

Sensing and reacting to microbes through the inflammasomes

Inflammasomes are multiprotein complexes that activate caspase-1, which leads to maturation of the proinflammatory cytokines interleukin 1β (IL-1β) and IL-18 and the induction of pyroptosis. Members of the Nod-like receptor (NLR) family, including NLRP1, NLRP3 and NLRC4, and the cytosolic receptor AIM2 are critical components of inflammasomes and link microbial and endogenous danger signals to the activation of caspase-1. In response to microbial infection, activation of the inflammasomes contributes 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 may provide insight into the mechanisms of host defense against microbes and the development of inflammatory disorders.

Regulation of inflammasome signaling

Innate immune responses have the ability to both combat infectious microbes and drive pathological inflammation. Inflammasome complexes are a central component of these processes through their regulation of interleukin 1β (IL-1β), IL-18 and pyroptosis. Inflammasomes recognize microbial products or endogenous molecules released from damaged or dying cells both through direct binding of ligands and indirect mechanisms. The potential of the IL-1 family of cytokines to cause tissue damage and chronic inflammation emphasizes the importance of regulating inflammasomes. Many regulatory mechanisms have been identified that act as checkpoints for attenuating inflammasome signaling at multiple steps. Here we discuss the various regulatory mechanisms that have evolved to keep inflammasome signaling in check to maintain immunological balance.

Neonatal autoimmune diseases: a critical review

Neonatal autoimmune diseases are distinctly rare. Most neonatal autoimmune diseases result from the transplacental transfer of maternal antibodies directed against fetal or neonatal antigens in various tissues. In neonatal lupus, the heart seems to be particularly susceptible. Primary autoimmunity in newborns, with the exception of familial autoinflammatory diseases, is virtually non-existent. The pathophysiologic basis for the development of neonatal autoimmunity is not entirely clear, but differences in the neonatal immune system compared with the adult immune system, as well as unique characteristics of target antigens in the newborn period may be important factors. Neonatal lupus is the most common presentation of autoimmunity in the newborn. But the characteristics defining neonatal lupus are not well defined and the presentation of neonatal lupus differs from that of classical lupus. Other neonatal autoimmune diseases involving the interaction between maternal antibodies and fetal/neonatal antigens include neonatal anti-phospholipid syndrome, Behcet's disease, neonatal autoimmune thyroid disease, neonatal polymyositis and dermatomyositis, neonatal scleroderma and neonatal type I diabetes mellitus. While autoantibodies have been detected in patients with neonatal autoimmune disease, the pathogenic role of autoantibodies has not been well defined. Other mechanisms may play a role in the development of neonatal autoimmunity, including fetal/maternal microchimerism and aberrant apoptosis of fetal cells. The autoinflammatory syndromes are a completely different category, but are also included in discussion of neonatal autoimmune diseases. The autoinflammatory syndromes include the cryopyrin associated periodic syndromes (CAPS) - familial cold autoinflammatory syndrome (FCAS), neonatal onset multisystem inflammatory disease (NOMID) and Muckle-Wells syndrome, which all share a common pathophysiologic mechanism.

TLR2/MyD88/NF-κB pathway, reactive oxygen species, potassium efflux activates NLRP3/ASC inflammasome during respiratory syncytial virus infection

Human respiratory syncytial virus (RSV) constitute highly pathogenic virus that cause severe respiratory diseases in newborn, children, elderly and immuno-compromised individuals. Airway inflammation is a critical regulator of disease outcome in RSV infected hosts. Although “controlled” inflammation is required for virus clearance, aberrant and exaggerated inflammation during RSV infection results in development of inflammatory diseases like pneumonia and bronchiolitis. Interleukin-1β (IL-1β) plays an important role in inflammation by orchestrating the pro-inflammatory response. IL-1β is synthesized as an immature pro-IL-1β form. It is cleaved by activated caspase-1 to yield mature IL-1β that is secreted extracellularly. Activation of caspase-1 is mediated by a multi-protein complex known as the inflammasome. Although RSV infection results in IL-1β release, the mechanism is unknown. Here in, we have characterized the mechanism of IL-1β secretion following RSV infection. Our study revealed that NLRP3/ASC inflammasome activation is crucial for IL-1β production during RSV infection. Further studies illustrated that prior to inflammasome formation; the “first signal” constitutes activation of toll-like receptor-2 (TLR2)/MyD88/NF-κB pathway. TLR2/MyD88/NF-κB signaling is required for pro-IL-1β and NLRP3 gene expression during RSV infection. Following expression of these genes, two “second signals” are essential for triggering inflammasome activation. Intracellular reactive oxygen species (ROS) and potassium (K⁺) efflux due to stimulation of ATP-sensitive ion channel promote inflammasome activation following RSV infection. Thus, our studies have underscored the requirement of TLR2/MyD88/NF-κB pathway (first signal) and ROS/potassium efflux (second signal) for NLRP3/ASC inflammasome formation, leading to caspase-1 activation and subsequent IL-1β release during RSV infection.

Many stimuli pull the necrotic trigger, an overview

The lab of Jürg Tschopp was the first to report on the crucial role of receptor-interacting protein kinase 1 (RIPK1) in caspase-independent cell death. Because of this pioneer finding, regulated necrosis and in particular RIPK1/RIPK3 kinase-mediated necrosis, referred to as necroptosis, has become an intensively studied form of regulated cell death. Although necrosis was identified initially as a backup cell death program when apoptosis is blocked, it is now recognized as a cellular defense mechanism against viral infections and as being critically involved in ischemia-reperfusion damage. The observation that RIPK3 ablation rescues embryonic lethality in mice deficient in caspase-8 or Fas-associated-protein-via-a-death-domain demonstrates the crucial role of this apoptotic platform in the negative control of necroptosis during development. Here, we review and discuss commonalities and differences of the increasing list of inducers of regulated necrosis ranging from cytokines, pathogen-associated molecular patterns, to several forms of physicochemical cellular stress. Since the discovery of the crucial role of RIPK1 and RIPK3 in necroptosis, these kinases have become potential therapeutic targets. The availability of new pharmacological inhibitors and transgenic models will allow us to further document the important role of this form of cell death in degenerative, inflammatory and infectious diseases.

Molecular mechanisms of inflammasome activation during microbial infections

The innate immune system plays a crucial role in the rapid recognition and elimination of invading microbes. Detection of microbes relies on germ-line encoded pattern recognition receptors (PRRs) that recognize essential bacterial molecules, so-called pathogen-associated molecular patterns (PAMPs). A subset of PRRs, belonging to the NOD-like receptor (NLR) and the PYHIN protein families, detects viral and bacterial pathogens in the cytosol of host cells and induces the assembly of a multi-protein signaling platform called the inflammasome. The inflammasome serves as an activation platform for the mammalian cysteine protease caspase-1, a central mediator of innate immunity. Active caspase-1 promotes the maturation and release of interleukin-1β (IL-1β) and IL-18 as well as protein involved in cytoprotection and tissue repair. In addition, caspase-1 initiates a novel form of cell death called pyroptosis. Here, we discuss latest advances and our insights on inflammasome stimulation by two model intracellular pathogens, Francisella tularensis and Salmonella typhimurium. Recent studies on these pathogens have significantly shaped our understanding of the molecular mechanisms of inflammasome activation and how microbes can evade or manipulate inflammasome activity. In addition, we review the role of the inflammasome adapter ASC in caspase-1 autoproteolysis and new insights into the structure of the inflammasome complex.

IL-33 induces Th17-mediated airway inflammation via mast cells in ovalbumin-challenged mice

Allergic asthma is characterized by infiltration of eosinophils, elevated Th2 cytokine levels, airway hyperresponsiveness, and IgE. In addition to eosinophils, mast cells, and basophils, a variety of cytokines are also involved in the development of allergic asthma. The pivotal role of eosinophils in the progression of the disease has been a subject of controversy. To determine the role of eosinophils in the progression of airway inflammation, we sensitized and challenged BALB/c wild-type (WT) mice and eosinophil-deficient ΔdblGATA mice with ovalbumin (OVA) and analyzed different aspects of inflammation. We observed increased eosinophil levels and a Th2-dominant response in OVA-challenged WT mice. In contrast, eosinophil-deficient ΔdblGATA mice displayed an increased proportion of mast cells and a Th17-biased response following OVA inhalation. Notably, the levels of IL-33, an important cytokine responsible for Th2 immune deviation, were not different between WT and eosinophil-deficient mice. We also demonstrated that mast cells induced Th17-differentiation via IL-33/ST2 stimulation in vitro. These results indicate that eosinophils are not essential for the development of allergic asthma and that mast cells can skew the immune reaction predominantly toward Th17 responses via IL-33 stimulation.

Cell death and infection: a double-edged sword for host and pathogen survival

Host cell death is an intrinsic immune defense mechanism in response to microbial infection. However, bacterial pathogens use many strategies to manipulate the host cell death and survival pathways to enhance their replication and survival. This manipulation is quite intricate, with pathogens often suppressing cell death to allow replication and then promoting it for dissemination. Frequently, these effects are exerted through modulation of the mitochondrial pro-death, NF-κB-dependent pro-survival, and inflammasome-dependent host cell death pathways during infection. Understanding the molecular details by which bacterial pathogens manipulate cell death pathways will provide insight into new therapeutic approaches to control infection.

Inflammasome activation via intracellular NLRs triggered by bacterial infection

Members of the nucleotide-binding, oligomerization domain (NOD)-like receptor (NLR) proteins assemble into a multiprotein platform, known as the inflammasome, to induce caspase-1 activation followed by the subsequent secretion of IL-1β and IL-18. In this review, we focus on the role of NLRs in inflammasome activation as part of the host defence against bacterial pathogens. One of activators of the NLRC4 inflammasome is bacterial flagellin secreted through type III or IV secretion systems, which are important for the pathogenicity of many Gram-negative bacteria. The NLRP3 inflammasome is mainly activated by a large number of bacterial pore-forming toxins. Despite our knowledge of inflammasome activation upon bacterial infection, the function of antibacterial defence under in vivo conditions remains to be elucidated. Further understanding of NLR function should provide new insights into the mechanisms of host pro-inflammatory responses and the pathogenesis of bacterial infections.

Role of the inflammasome, IL-1β, and IL-18 in bacterial infections

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.

Inflammasome activation and IL-1β/IL-18 processing are influenced by distinct pathways in microglia

Microglia are important innate immune effectors against invading CNS pathogens, such as Staphylococcus aureus (S. aureus), a common etiological agent of brain abscesses typified by widespread inflammation and necrosis. The NLRP3 inflammasome is a protein complex involved in IL-1β and IL-18 processing following exposure to both pathogen- and danger-associated molecular patterns. Although previous studies from our laboratory have established that IL-1β is a major cytokine product of S. aureus-activated microglia and is pivotal for eliciting protective anti-bacterial immunity during brain abscess development, the molecular machinery responsible for cytokine release remains to be determined. Therefore, the functional role of the NLRP3 inflammasome and its adaptor protein apoptosis-associated speck-like protein (ASC) in eliciting IL-1β and IL-18 release was examined in primary microglia. Interestingly, we found that IL-1β, but not IL-18 production, was significantly attenuated in both NLRP3 and ASC knockout microglia following exposure to live S. aureus. NLRP3 inflammasome activation was partially dependent on autocrine/paracrine ATP release and α- and γ-hemolysins produced by live bacteria. A cathepsin B inhibitor attenuated IL-β release from NLRP3 and ASC knockout microglia, demonstrating the existence of alternative inflammasome-independent mechanisms for IL-1β processing. In contrast, microglial IL-18 secretion occurred independently of cathepsin B and inflammasome action. Collectively, these results demonstrate that microglial IL-1β processing is regulated by multiple pathways and diverges from mechanisms utilized for IL-18 cleavage. Understanding the molecular events that regulate IL-1β production is important for modulating this potent proinflammatory cytokine during CNS disease.

The role of oxidative stress and innate immunity in O(3) and endotoxin-induced human allergic airway disease

Ozone (O(3)) and endotoxin are common environmental contaminants that cause asthma exacerbation. These pollutants have similar phenotype response characteristics, including induction of neutrophilic inflammation, changes in airway macrophage immunophenotypes, and ability to enhance response to inhaled allergen. Evoked phenotyping studies of volunteers exposed to O(3) and endotoxin were used to identify the response characteristics of volunteers to these pollutants. New studies support the hypotheses that similar innate immune and oxidant processes modulate response to these agents. These include TLR4 and inflammasome-mediated signaling and cytokine production. Innate immune responses are also impacted by oxidative stress. It is likely that continued discovery of common molecular processes which modulate response to these pollutants will occur. Understanding the pathways that modulate response to pollutants will also allow for discovery of genetic and epigenetic factors that regulate response to these pollutants and determine risk of disease exacerbation. Additionally, defining the mechanisms of response will allow rational selection of interventions to examine. Interventions focused on inhibition of Toll-like receptor 4 and inflammasome represent promising new approaches to preventing pollutant-induced asthma exacerbations. Such interventions include specific inhibitors of innate immunity and antioxidant therapies designed to counter the effects of pollutants on cell signaling.

Myxoma virus lacking the pyrin-like protein M013 is sensed in human myeloid cells by both NLRP3 and multiple Toll-like receptors, which independently activate the inflammasome and NF-κB innate response pathways

The myxoma virus (MYXV)-encoded pyrin domain-containing protein M013 coregulates inflammatory responses mediated by both the inflammasome and the NF-κB pathways. Infection of human THP-1 monocytic cells with a MYXV construct deleted for the M013 gene (vMyxM013-KO), but not the parental MYXV, activates both the inflammasome and NF-κB pathways and induces a spectrum of proinflammatory cytokines and chemokines, like interleukin-1β (IL-1β), tumor necrosis factor (TNF), IL-6, and monocyte chemoattractant protein 1. Here, we report that vMyxM013-KO virus-mediated activation of inflammasomes and secretion of IL-1β are dependent on the adaptor protein ASC, caspase-1, and NLRP3 receptor. However, vMyxM013-KO virus-mediated activation of NF-κB signaling, which induces TNF secretion, was independent of ASC, caspase-1, and either the NLRP3 or AIM2 inflammasome receptors. We also report that early synthesis of pro-IL-1β in response to vMyxM013-KO infection is dependent upon the components of the inflammasome complex. Activation of the NLRP3 inflammasome and secretion of IL-1β was also dependent on the release of cathepsin B and production of reactive oxygen species (ROS). By using small interfering RNA screening, we further demonstrated that, among the RIG-I-like receptors (RLRs) and Toll-like receptors (TLRs), only TLR2, TLR6, TLR7, and TLR9 contribute to the NF-κB-dependent secretion of TNF and the inflammasome-dependent secretion of IL-1β in response to vMyxM013-KO virus infection. Additionally, we demonstrate that early triggering of the mitogen-activated protein kinase pathway by vMyxM013-KO virus infection of THP-1 cells plays a critical common upstream role in the coordinate induction of both NF-κB and inflammasome pathways. We conclude that an additional cellular sensor(s)/receptor(s) in addition to the known RLRs/TLRs plays a role in the M013 knockout virus-induced activation of NF-κB pathway signaling, but the activation of inflammasomes entirely depends on sensing by the NLRP3 receptor in response to vMyxM013-KO infection of human myeloid cells.

Mycobacterial secretion systems ESX-1 and ESX-5 play distinct roles in host cell death and inflammasome activation

During infection of humans and animals, pathogenic mycobacteria manipulate the host cell causing severe diseases such as tuberculosis and leprosy. To understand the basis of mycobacterial pathogenicity, it is crucial to identify the molecular virulence mechanisms. In this study, we address the contribution of ESX-1 and ESX-5--two homologous type VII secretion systems of mycobacteria that secrete distinct sets of immune modulators--during the macrophage infection cycle. Using wild-type, ESX-1- and ESX-5-deficient mycobacterial strains, we demonstrate that these secretion systems differentially affect subcellular localization and macrophage cell responses. We show that in contrast to ESX-1, the effector proteins secreted by ESX-5 are not required for the translocation of Mycobacterium tuberculosis or Mycobacterium marinum to the cytosol of host cells. However, the M. marinum ESX-5 mutant does not induce inflammasome activation and IL-1β activation. The ESX-5 system also induces a caspase-independent cell death after translocation has taken place. Importantly, by means of inhibitory agents and small interfering RNA experiments, we reveal that cathepsin B is involved in both the induction of cell death and inflammasome activation upon infection with wild-type mycobacteria. These results reveal distinct roles for two different type VII secretion systems during infection and shed light on how virulent mycobacteria manipulate the host cell in various ways to replicate and spread.

Cross-regulation between the IL-1β/IL-18 processing inflammasome and other inflammatory cytokines

The inflammasome-forming NLRs are well characterized members of a protein complex mediating the activation of caspase-1 and the cleavage of pro-IL-1β and pro-IL-18 into their active, secreted forms. New data suggest that components of the inflammasome cascade may have roles in influencing inflammasome-independent pathways of cytokine production. These influences on other immune cytokine pathways are complemented by data suggesting that non-inflammasome cytokines can influence the activation of the inflammasome, either directly or by influencing transcription of inflammasome components. The crosstalk between these cytokine cascades may lead to increased abilities for the cell to respond to diverse pathogen threats.

Caspase-1 protein induces apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC)-mediated necrosis independently of its catalytic activity

The adaptor protein, apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), connects pathogen/danger sensors such as NLRP3 and NLRC4 with caspases and is involved in inflammation and cell death. We have found that ASC activation induced caspase-8-dependent apoptosis or CA-074Me (cathepsin B inhibitor)-inhibitable necrosis depending on the cell type. Unlike necroptosis, another necrotic cell death, ASC-mediated necrosis, was neither RIP3-dependent nor necrostatin-1-inhibitable. Although acetyl-YVAD-chloromethylketone (Ac-YVAD-CMK) (caspase-1 inhibitor) did not inhibit ASC-mediated necrosis, comprehensive gene expression analyses indicated that caspase-1 expression coincided with the necrosis type. Furthermore, caspase-1 knockdown converted necrosis-type cells to apoptosis-type cells, whereas exogenous expression of either wild-type or catalytically inactive caspase-1 did the opposite. Knockdown of caspase-1, but not Ac-YVAD-CMK, suppressed the monocyte necrosis induced by Staphylococcus and Pseudomonas infection. Thus, the catalytic activity of caspase-1 is dispensable for necrosis induction. Intriguingly, a short period of caspase-1 knockdown inhibited IL-1β production but not necrosis, although longer knockdown suppressed both responses. Possible explanations of this phenomenon are discussed.

Regulation of the antimicrobial response by NLR proteins

Nucleotide-binding, oligomerization domain (NOD)-like receptor (NLR) proteins are a family of innate immune receptors that play a pivotal role in microbial sensing, leading to the initiation of antimicrobial immune responses. Dysregulation of the function of multiple NLR family members has been linked, both in mice and humans, to a propensity for infection and autoinflammatory disease. Despite our increased understanding of NLR function and interactions, many aspects related to mechanisms of sensing, downstream signaling, and in vivo functions remain elusive. In this review, we focus on key members of the NLR family, describing their activation by diverse microbes, downstream effector functions, and interactions with each other and with other innate sensor protein families. Also discussed is the role of microbial sensing by NLR receptors leading to activation of the adaptive immune arm that collaborates in the antimicrobial defense.

The NLRP3 inflammasome in health and disease: the good, the bad and the ugly

While interleukin (IL)-1β plays an important role in combating the invading pathogen as part of the innate immune response, its dysregulation is responsible for a number of autoinflammatory disorders. Large IL-1β activating platforms, known as inflammasomes, can assemble in response to the detection of endogenous host and pathogen-associated danger molecules. Formation of these protein complexes results in the autocatalysis and activation of caspase-1, which processes precursor IL-1β into its secreted biologically active form. Inflammasome and IL-1β activity is required to efficiently control viral, bacterial and fungal pathogen infections. Conversely, excess IL-1β activity contributes to human disease, and its inhibition has proved therapeutically beneficial in the treatment of a spectrum of serious, yet relatively rare, heritable inflammasomopathies. Recently, inflammasome function has been implicated in more common human conditions, such as gout, type II diabetes and cancer. This raises the possibility that anti-IL-1 therapeutics may have broader applications than anticipated previously, and may be utilized across diverse disease states that are linked insidiously through unwanted or heightened inflammasome activity.

Human macrophages infected with a high burden of ESAT-6-expressing M. tuberculosis undergo caspase-1- and cathepsin B-independent necrosis

Mycobacterium tuberculosis (Mtb) infects lung macrophages, which instead of killing the pathogen can be manipulated by the bacilli, creating an environment suitable for intracellular replication and spread to adjacent cells. The role of host cell death during Mtb infection is debated because the bacilli have been shown to be both anti-apoptotic, keeping the host cell alive to avoid the antimicrobial effects of apoptosis, and pro-necrotic, killing the host macrophage to allow infection of neighboring cells. Since mycobacteria activate the NLRP3 inflammasome in macrophages, we investigated whether Mtb could induce one of the recently described inflammasome-linked cell death modes pyroptosis and pyronecrosis. These are mediated through caspase-1 and cathepsin-B, respectively. Human monocyte-derived macrophages were infected with virulent (H37Rv) Mtb at a multiplicity of infection (MOI) of 1 or 10. The higher MOI resulted in strongly enhanced release of IL-1β, while a low MOI gave no IL-1β response. The infected macrophages were collected and cell viability in terms of the integrity of DNA, mitochondria and the plasma membrane was determined. We found that infection with H37Rv at MOI 10, but not MOI 1, over two days led to extensive DNA fragmentation, loss of mitochondrial membrane potential, loss of plasma membrane integrity, and HMGB1 release. Although we observed plasma membrane permeabilization and IL-1β release from infected cells, the cell death induced by Mtb was not dependent on caspase-1 or cathepsin B. It was, however, dependent on mycobacterial expression of ESAT-6. We conclude that as virulent Mtb reaches a threshold number of bacilli inside the human macrophage, ESAT-6-dependent necrosis occurs, activating caspase-1 in the process.

The inflammasomes in kidney disease

Renal inflammation is a universal response to infectious and noninfectious triggers. Sensors of the innate immune system, such as Toll-like receptors or RIG-like receptors, provide danger recognition platforms on renal cells that integrate and translate the diverse triggers of renal inflammation by inducing cell activation and the secretion of proinflammatory cytokines and chemokines. As a new entry, the inflammasome-forming NLR genes integrate various danger signals into caspase-1-activating platforms that regulate the processing and secretion of pro-IL-1β and pro-IL-18 into the mature and active cytokines. Accumulating data now document a role for the NLRP3 inflammasome and IL-1β/IL-18 in many diseases, including atherosclerosis, diabetes, amyloidosis, malaria, crystal-related diseases, and other autoinflammatory disorders, identifying this innate immune pathway as an attractive therapeutic target. Here we review the current knowledge regarding inflammasome signaling and outline existing evidence on the expression and functional role of the inflammasome-caspase-1-IL-1β/IL-18 axis in kidney disease. We further provide a perspective on the potential roles of the inflammasomes in the pathogenesis of acute and chronic kidney diseases.

TLRs, NLRs and RLRs: innate sensors and their impact on allergic diseases--a current view

Charles Janeway first wrote 1989 about how important recognition of "certain characteristics or patterns common on infectious agents but absent from the host" would be for our immune response [1]. Surprisingly, it almost took 10 years before his ideas would lead to the revolutionary findings that fundamentally changed the view of the innate immune system over the past decade. Recognition of invading microorganisms belongs to the primary tasks of the innate immune system and is achieved through different families of innate immune sensors. Among these, Toll-like receptors (TLRs), nucleotide-binding domain and leucine-rich repeat containing receptors (NLRs) and Rig-I-like receptors (RLRs) have drawn major interests over the last decade. These receptor families are targeted by overlapping classes of pathogens and share functional domains and signal transduction pathways (see Fig. 1 and Table 1 for an overview of their structural organization, ligands, adaptors and activated pathways). This current view describes our present knowledge about these three main innate immune receptor families and their importance for adaptive immune responses such as asthma and allergy.

Salmonella and Caspase-1: A complex Interplay of Detection and Evasion

Salmonellae are intracellular pathogens that replicate within epithelial cells and macrophages, and are a significant public health threat in both developed and developing countries. The innate immune system detects microbes through pattern recognition receptors, which are compartmentalized on the subcellular level to detect either extracellular (e.g., TLRs) or cytosolic (e.g., NLRs) perturbations. Salmonella infection is detected by the NLRC4 and NLRP3 inflammasomes, which activate Caspase-1, resulting in reduced bacterial burdens during infection. NLRC4 responds to the SPI1 type III secretion system via detection of inadvertently translocated flagellin and rod protein. The signals for NLRP3 detection during Salmonella infection remain undefined. Salmonella have evolved evasion strategies to attenuate Caspase-1 responses. We review recent findings describing the interplay between detection and evasion of S. typhimurium infection by the inflammasome. We discuss how the interplay between detection and evasion affects Caspase-1 effector functions mediated by IL-1β secretion, IL-18 secretion, and pyroptosis.

The NLR adaptor ASC/PYCARD regulates DUSP10, mitogen-activated protein kinase (MAPK), and chemokine induction independent of the inflammasome

ASC/PYCARD is a common adaptor for a diverse set of inflammasomes that activate caspase-1, most prominently the NLR-based inflammasome. Mounting evidence indicates that ASC and these NLRs also elicit non-overlapping functions, but the molecular basis for this difference is unclear. To address this, we performed microarray and network analysis of ASC shRNA knockdown cells. In pathogen-infected cells, an ASC-dependent interactome is centered on the mitogen-activated protein kinase (MAPK) ERK and on multiple chemokines. ASC did not affect the expression of MAPK but affected its phosphorylation by pathogens and Toll-like receptor agonists via suppression of the dual-specificity phosphatase, DUSP10/MKP5. Chemokine induction, DUSP function, and MAPK phosphorylation were independent of caspase-1 and IL-1β. MAPK activation by pathogen was abrogated in Asc(-/-) but not Nlrp3(-/-), Nlrc4(-/-), or Casp1(-/-) macrophages. These results demonstrate a function for ASC that is distinct from the inflammasome in modulating MAPK activity and chemokine expression and further identify DUSP10 as a novel ASC target.

Inflammasomes: current understanding and open questions

The innate immune system relies on its capability to detect invading microbes, tissue damage, or stress via evolutionarily conserved receptors. The nucleotide-binding domain leucine-rich repeat (NLR)-containing family of pattern recognition receptors includes several proteins that drive inflammation in response to a wide variety of molecular patterns. In particular, the NLRs that participate in the formation of a molecular scaffold termed the "inflammasome" have been intensively studied in past years. Inflammasome activation by multiple types of tissue damage or by pathogen-associated signatures results in the autocatalytic cleavage of caspase-1 and ultimately leads to the processing and thus secretion of pro-inflammatory cytokines, most importantly interleukin (IL)-1β and IL-18. Here, we review the current knowledge of mechanisms leading to the activation of inflammasomes. In particular, we focus on the controversial molecular mechanisms that regulate NLRP3 signaling and highlight recent advancements in DNA sensing by the inflammasome receptor AIM2.

Lysosomal localization and mechanism of membrane penetration influence nonenveloped virus activation of the NLRP3 inflammasome

Adenovirus (Ad) endosomal membrane penetration activates the NLRP3 inflammasome by releasing lysosomal cathepsin B (catB) into the cytoplasm. We therefore examined the extent to which inflammasome activation correlates with Ad colocalization with catB-enriched lysosomes. Inflammasome activation, is greater during infections with Ad5 possessing an Ad16 fiber (Ad5F16gfp), or Ad5gfp neutralized by human serum, than Ad5gfp alone. Enhanced IL-1β release by Ad5F16gfp is partially due to increased TLR9 signaling but also correlates with greater release of catB into the cytoplasm. This increased TLR9 signaling and catB release correlates with a greater localization of Ad5F16gfp to lysosomes prior to endosomal escape. Another nonenveloped virus, reovirus, requires catB to penetrate cell membranes. However, reovirus did not release catB into the cytoplasm despite significantly greater colocalization with lysosomes compared to Ad5gfp and efficient membrane penetration. Thus, not only lysosomal localization, but the mechanism of membrane penetration influences viral activation of the NLRP3 inflammasome.

Role of the nlrp3 inflammasome in microbial infection

The intracellular Nod-like receptor Nlrp3 has emerged as the most versatile innate immune receptor because of its broad specificity in mediating immune response to a wide range of microbial or danger signals. Nlrp3 mediates assembly of the inflammasome complex in the presence of microbial components leading to the activation of caspase-1 and the processing and release of the pro-inflammatory cytokines IL-1β and IL-18. In this review, we give an update on the recent literature examining the role of Nlrp3 inflammasome in response to fungal, bacterial, and viral infections.

SR-A/MARCO–mediated ligand delivery enhances intracellular TLR and NLR function, but ligand scavenging from cell surface limits TLR4 response to pathogens

Phagocytic and pathogen sensing receptors are responsible for particle uptake and inflammation. It is unclear how these receptors' systems influence each other's function to shape an innate response. The class-A scavenger receptors SR-A (scavenger receptor A) and MARCO (macrophage receptor with collagenous structure) are 2 well-characterized phagocytic receptors that are unable to initiate inflammatory responses by themselves, yet are implicated in the pathogenesis of various inflammatory disorders. However, the mechanism for such an apparent discrepancy is still unclear. We utilized SR-A−/−, MARCO−/−, and SR-A−/−-MARCO−/− mice, along with microbe-derived, environmental, and synthetic polyanions to assess the inflammatory responses following combinatorial ligation of SR-A/MARCO and selected Toll-like receptors (TLRs) and nucleotide-binding oligomerization domain (NOD)–like receptors (NLRs) by their shared ligands. In addition to ligating SR-A and MARCO, these agonists also selectively activated the cell-surface sensor TLR4, endosomal TLR3, and the cytosolic NOD2 and NALP3 (NACHT domain–, leucine-rich repeat–, and pyrin domain–containing protein 3). We show that, following recognition of common ligands, SR-A and MARCO attenuate TLR4-mediated responses while enhancing responses by the intracellular TLR3, NOD2, and NALP3. We conclude that SR-A/MARCO-mediated rapid ligand internalization prevented sensing by surface TLRs while increasing ligand availability in intracellular compartments, thus allowing sensing and robust responses by intracellular sensors.

Recognition of cytoplasmic RNA results in cathepsin-dependent inflammasome activation and apoptosis in human macrophages

dsRNA is an important pathogen-associated molecular pattern that is primarily recognized by cytosolic pattern-recognition receptors of the innate-immune system during virus infection. This recognition results in the activation of inflammasome-associated caspase-1 and apoptosis of infected cells. In this study, we used high-throughput proteomics to identify secretome, the global pattern of secreted proteins, in human primary macrophages that had been activated through the cytoplasmic dsRNA-recognition pathway. The secretome analysis revealed cytoplasmic dsRNA-recognition pathway-induced secretion of several exosome-associated proteins, as well as basal and dsRNA-activated secretion of lysosomal protease cathepsins and cysteine protease inhibitors (cystatins). Inflammasome activation was almost completely abolished by cathepsin inhibitors in response to dsRNA stimulation, as well as encephalomyocarditis virus and vesicular stomatitis virus infections. Interestingly, Western blot analysis showed that the mature form of cathepsin D, but not cathepsin B, was secreted simultaneously with IL-18 and inflammasome components ASC and caspase-1 in cytoplasmic dsRNA-stimulated cells. Furthermore, small interfering RNA-mediated silencing experiments confirmed that cathepsin D has a role in inflammasome activation. Caspase-1 activation was followed by proteolytic processing of caspase-3, indicating that inflammasome activation precedes apoptosis in macrophages that had recognized cytoplasmic RNA. Like inflammasome activation, apoptosis triggered by dsRNA stimulation and virus infection was effectively blocked by cathepsin inhibition. In conclusion, our results emphasize the importance of cathepsins in the innate immune response to virus infection.

Chlamydial infection increases gonococcal colonization in a novel murine coinfection model

Genital tract infections caused by Neisseria gonorrhoeae and Chlamydia trachomatis serovars D to K occur at high incidence in many areas of the world. Despite high rates of coinfection with these pathogens, investigations of host-parasite interactions have focused on each pathogen individually. We describe here a coinfection model in which female BALB/c mice were first infected with the mouse Chlamydia species C. muridarum and then inoculated with N. gonorrhoeae following treatment with water-soluble 17β-estradiol to promote long-term gonococcal infection. Viable gonococci and chlamydiae were recovered for an average of 8 to 10 days, and diplococci and chlamydial inclusions were observed in lower genital tract tissue by immunohistochemical staining. Estradiol treatment reduced proinflammatory cytokine and chemokine levels in chlamydia-infected mice; however, coinfected mice had a higher percentage of vaginal neutrophils compared to mice infected with either pathogen alone. We detected no difference in pathogen-specific antibody levels due to coinfection. Interestingly, significantly more gonococci were recovered from coinfected mice compared to mice infected with N. gonorrhoeae alone. We found no evidence that C. muridarum increases gonococcal adherence to, or invasion of, immortalized murine epithelial cells. However, increased vaginal concentrations of inflammatory mediators macrophage inflammatory protein 2 and tumor necrosis factor alpha were detected in C. muridarum-infected mice prior to inoculation with N. gonorrhoeae concurrently with the downregulation of cathelicidin-related antimicrobial peptide and secretory leukocyte peptidase inhibitor genes. We conclude that female mice can be successfully infected with both C. muridarum and N. gonorrhoeae and that chlamydia-induced alterations in host innate responses may enhance gonococcal infection.

Adenovirus membrane penetration activates the NLRP3 inflammasome

Adenovirus type 5 (Ad5) infection of macrophages results in rapid secretion of interleukin-1β (IL-1β) and is dependent on the inflammasome components NLRP3 and ASC and the catalytic activity of caspase-1. Using lentivirus-expressed short hairpin RNA (shRNA) and competitive inhibitors, we show that Ad-induced IL-1β release is dependent upon Toll-like receptor 9 (TLR9) sensing of the Ad5 double-stranded DNA (dsDNA) genome in human cell lines and primary monocyte-derived macrophages but not in mouse macrophages. Additionally, a temperature-sensitive mutant of Ad5 unable to penetrate endosomal membranes, ts1, is unable to induce IL-1β release in TLR2-primed THP-1 cells, suggesting that penetration of endosomal membranes is required for IL-1β release. Disruption of lysosomal membranes and the release of cathepsin B into the cytoplasm are required for Ad-induced NLRP3 activation. Ad5 cell entry also induces reactive oxygen species (ROS) production, and inhibitors of ROS prevent Ad-induced IL-1β release. Ad5 activation of NLRP3 also induces necrotic cell death, resulting in the release of the proinflammatory molecule HMGB1. This work further defines the mechanisms of virally induced inflammasome activation.

Redundant roles for inflammasome receptors NLRP3 and NLRC4 in host defense against Salmonella

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.

Pathogenic Vibrio activate NLRP3 inflammasome via cytotoxins and TLR/nucleotide-binding oligomerization domain-mediated NF-kappa B signaling

Vibrio vulnificus and Vibrio cholerae are Gram-negative pathogens that cause serious infectious disease in humans. The beta form of pro-IL-1 is thought to be involved in inflammatory responses and disease development during infection with these pathogens, but the mechanism of beta form of pro-IL-1 production remains poorly defined. In this study, we demonstrate that infection of mouse macrophages with two pathogenic Vibrio triggers the activation of caspase-1 via the NLRP3 inflammasome. Activation of the NLRP3 inflammasome was mediated by hemolysins and multifunctional repeat-in-toxins produced by the pathogenic bacteria. NLRP3 activation in response to V. vulnificus infection required NF-kappaB activation, which was mediated via TLR signaling. V. cholerae-induced NLRP3 activation also required NF-kappaB activation but was independent of TLR stimulation. Studies with purified V. cholerae hemolysin revealed that toxin-stimulated NLRP3 activation was induced by TLR and nucleotide-binding oligomerization domain 1/2 ligand-mediated NF-kappaB activation. Our results identify the NLRP3 inflammasome as a sensor of Vibrio infections through the action of bacterial cytotoxins and differential activation of innate signaling pathways acting upstream of NF-kappaB.

Inflammasome-associated nucleotide-binding domain, leucine-rich repeat proteins and inflammatory diseases

The nucleotide-binding domain, leucine-rich repeat (NLR) proteins are a recently discovered family of intracellular pathogen and danger signal sensors. NLRs have emerged as important contributors to innate immunity in animals. The physiological impact of these genes is increasingly evident, underscored by the genetic association of variant family members with an array of inflammatory diseases. The association of mutations in NLR genes with autoinflammatory diseases indicates an important function of these genes in inflammation in vivo. This review summarizes the role of the inflammasome NLR proteins in innate immunity and inflammatory diseases and explores the possible utility of some of these NLRs as pharmacological targets.

Anti-inflammatory compounds parthenolide and Bay 11-7082 are direct inhibitors of the inflammasome

Activation of the inflammasome generates the pro-inflammatory cytokines interleukin-1 beta and -18, which are important mediators of inflammation. Abnormal activation of the inflammasome leads to many inflammatory diseases, including gout, silicosis, neurodegeneration, and genetically inherited periodic fever syndromes. Therefore, identification of small molecule inhibitors that target the inflammasome is an important step toward developing effective therapeutics for the treatment of inflammation. Here, we show that the herbal NF-kappaB inhibitory compound parthenolide inhibits the activity of multiple inflammasomes in macrophages by directly inhibiting the protease activity of caspase-1. Additional investigations of other NF-kappaB inhibitors revealed that the synthetic I kappaB kinase-beta inhibitor Bay 11-7082 and structurally related vinyl sulfone compounds selectively inhibit NLRP3 inflammasome activity in macrophages independent of their inhibitory effect on NF-kappaB activity. In vitro assays of the effect of parthenolide and Bay 11-7082 on the ATPase activity of NLRP3 demonstrated that both compounds inhibit the ATPase activity of NLRP3, suggesting that the inhibitory effect of these compounds on inflammasome activity could be mediated in part through their effect on the ATPase activity of NLRP3. Our results thus elucidate the molecular mechanism for the therapeutic anti-inflammatory activity of parthenolide and identify vinyl sulfones as a new class of potential therapeutics that target the NLRP3 inflammasome.

The inflammasomes: mechanisms of activation and function

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 proforms of the interleukin-1 cytokine family members leading to their activation and secretion. The IL-1 family cytokines have multiple proinflammatory 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.

The inflammasome in host defense

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.

Inflammasomes: too big to miss

Inflammation is the coordinated immune response to harmful stimuli that appear during infections or after tissue damage. Cells of the innate immune system are the central players in mediating inflammatory tissue responses. These cells are equipped with an array of signaling receptors that detect foreign molecular substances or altered endogenous molecules that appear under situations of stress. This review provides an overview of recent progress in elucidating the molecular mechanisms that lead to inflammatory reactions. We discuss the current knowledge of the mechanisms leading to the activation of cytoplasmic, multimolecular protein complexes, termed "inflammasomes," which regulate the activity of caspase-1 and the maturation and release of IL-1beta.

Inflammasomes bridge signaling between pathogen identification and the immune response

Microbial organisms express pathogen-associated molecular patterns (PAMPs) that can stimulate expression of proinflammatory mediators following ligation of pathogen recognition receptors. However, both commensal organisms and pathogens can express PAMPs. The immune system can distinguish between commensals and pathogens in part through secretion of the key inflammatory cytokines interleukin (IL)-1beta and IL-18. A PAMP such as lipopolysaccharide can induce production of intracellular pro-IL-1beta and pro-IL-18, but not their secretion. A second "danger signal", derived from host-cell molecules that are released from stressed or infected cells, or detected as a PAMP that is present in the cytosol, can stimulate assembly of an inflammasome that activates the protease caspase-1. Caspase-1, in turn, is responsible for processing and secretion of the mature IL-1beta and IL-18. Many diverse ligands leading to inflammasome activation have been identified, but the cell signaling pathways initiated by the ligands tend to converge on a small set of common mechanisms.