Negative regulation of NLRP3 inflammasome signaling

Abstract

Inflammasomes are multiprotein complexes that serve as a platform for caspase-1 activation and interleukin-1β (IL-1β) maturation as well as pyroptosis. Though a number of inflammasomes have been described, the NLRP3 inflammasome is the most extensively studied. NLRP3 inflammasome is triggered by a variety of stimuli, including infection, tissue damage and metabolic dysregulation, and then activated through an integrated cellular signal. Many regulatory mechanisms have been identified to attenuate NLRP3 inflammasome signaling at multiple steps. Here, we review the developments in the negative regulation of NLRP3 inflammasome that protect host from inflammatory damage.

Keywords

inflammasome / NLRP3 / negative regulation / inflammation

INTRODUCTION OF INFLAMMSOMES

The innate immune system is a sophisticated system for sensing signals of "danger" such as pathogenic microbes or host-derived signals of cellular stress. Host innate immune system is activated by the engagement of germline-encoded pattern-recognition receptors (PRRs) to detect invariant motifs, which are the presence of unique microbial components called pathogen-associated molecular patterns (PAMPs) or endogenous danger-associated molecular patterns (DAMPs) generated in the setting of cellular injury or tissue damage (Takeuchi and Akira, 2010). PRRs are expressed by cells at the first line of defense against infection, including macrophages, dendritic cells (DCs), monocytes, neutrophils, epithelial cells, and also some cells of the adaptive immune system. There are four kinds of PRRs: Toll-like receptors (TLRs), NOD-like receptors (NLRs), RIG-like helicases (RLHs) and C-type lectins (CTLs). TLRs and CTLs are membrane-bound, which detect the extracellular and endosomal compartments for PAMPs, whereas RLHs are intracellular PRRs that sense the cytosolic RNA. NLRs, the latest found, are also intracellular PRRs that detect a series of materials, including microbial components, endogenous stress and damage signals (Schroder and Tschopp, 2010).

To date, 23 human genes and 34 mouse genes are found for the NLR family, which is characterized by the presence of a central nucleotide-binding and oligomerization (NACHT) domain, C-terminal leucine-rich repeats (LRRs) and N-terminal caspase recruitment (CARD) or pyrin (PYD) domains. Phylogenetic analysis of NACHT domains reveals 3 distinct subfamilies within the NLR family: the NODs (NOD1-2, NOD3/NLRC3, NOD4/NLRC5, NOD5/NLRX1, CIITA), the NLRPs (NLRP1-14, also called NALPs) and the IPAF subfamily, consisting of IPAF (NLRC4) and NAIPs (Martinon et al., 2009; Schroder and Tschopp, 2010). The accepted theory posits that activation signals are sensed by the LRR domain of these NLRs, which can autoregulate the PYD/CARD and NACHT domains. The NACHT domain, the only domain common to all NLR family members, enables activation of the signaling complex via ATP-dependent oligomerization. The CARD and PYD domains mediate homotypic protein-protein interactions for downstream signaling (Tschopp and Schroder, 2010).

Recognition of a diverse range of microbial, stress and damage signals by NLRs results in inflammasomes formation. The inflammasome is a multiprotein complex composed of a sensor NLR protein, the PYD-and CARD-domain containing adapter protein ASC, and the inflammatory protease caspase-1 (Martinon et al., 2002). Once activated, NLR changes conformation to recruit ASC and assemble the inflammasome platform, which in turn promotes the activation of caspase-1. Caspase-1 subsequently cleaves pro-IL-1β and pro-IL-18 and renders maturation and secretion of these potent inflammatory cytokines, meanwhile, a form of cell death called pyroptosis is also resulted from the caspase-1 activation (De Nardo and Latz, 2011; Lamkanfi, 2011).

So far, four inflammasomes have been identified: NLRP1, NLRP3, NLRC4 and AIM2 (Schroder and Tschopp, 2010; Gross et al., 2011). Formation of the NLRP1, NLRC4, and AIM2 inflammasomes is promoted by specific stimuli. NLRP1, the first inflammasome to be described, is activated mainly by the lethal toxin from Bacillus anthracis (Martinon et al., 2002; Lamkanfi and Dixit, 2009). AIM2 senses dsDNA of viral or bacterial origin (Burckstummer et al., 2009; Fernandes-Alnemri et al., 2009; Hornung et al., 2009; Roberts et al., 2009) while NLRC4 responds to bacterial flagellin and PrgJ, a component of the type III secretion system (T3SS) (Franchi et al., 2006; Miao et al., 2006; Zhao et al., 2011; Gong and Shao, 2012). The NLRP3 inflammasome is currently the most fully characterized inflammasome, which responds to numerous physically and chemically diverse stimuli, and leads to a series of diseases when out of control. In this review, we focus on the negative regulation of NLRP3 inflammasome activation.

NLRP3 INFLAMMASOME AND ITS ACTIVATION

The activators of NLRP3 are quite varied and include environmental irritants, endogenous danger signals, pathogens, and distinct PAMPs. In detail, the NLRP3 inflammasome is activated by pathogens such as Sendai virus, adenovirus, influenza virus (Gram et al., 2012), Candida albicans (Gross et al., 2009), Saccharomyces cerevisiae (Kumar et al., 2009), Listeria monocytogenes (Meixenberger et al., 2010) and Staphylococcus aureus (Munoz-Planillo et al., 2009), and also by endogenous stimuli and particulate matter, including extracellular ATP (Mariathasan et al., 2006; Sutterwala et al., 2006), extracellular glucose (Masters et al., 2010; Zhou et al., 2010), monosodium urate (MSU) crystals (Martinon et al., 2006), cholesterol (Duewell et al., 2010), silica (Dostert et al., 2008), aluminum salts (Eisenbarth et al., 2008), asbestos (Dostert et al., 2008), malarial hemozoin (Shio et al., 2009), amyloid deposits (Halle et al., 2008) and fatty acids (Wen et al., 2011).

NLRP3 inflammasome is triggered by a variety of stimuli, thus it has close relations with many diseases as proposed. Firstly, gain-of-function mutations within the NACHT domain of NLRP3 were found associated with three autoinflammatory disorders characterized by skin rashes and prolonged episodes of fever in the absence of any apparent infection. Then these three hereditary periodic fever syndromes, Muckle–Wells syndrome (MWS), familial cold autoinflammatory syndrome (FACS), and neonatal-onset multisystem inflammatory disease (NOMID), are collectively referred to as the cryopyrin NLRP3-associated periodic syndromes (CAPS) (Martinon and Tschopp, 2004; Lamkanfi and Dixit, 2009). Furthermore, mononuclear cells from these CAPS patients spontaneously secrete IL-1β and IL-18, and IL-1 receptor antagonists have been proved to be an effective treatment for these autoinflammatory syndromes (Hoffman et al., 2004; Rosengren et al., 2007). Recently, increasing evidence indicates that dysregulated production of IL-1β are implicated in the pathophysiology of several common diseases, including atherosclerosis (Duewell et al., 2010), osteoarthritis (Denoble et al., 2011), metabolic syndrome (Strowig et al., 2012), and type 2 diabetes (Zhou et al., 2010).

Activation of the NLRP3 inflammasome requires two signals in mouse macrophages. The first priming signal is provided by microbial or endogenous molecules that activate NF-κB to induce the expression of not only pro-IL-1β, but also NLRP3, thus controls the threshold of inflammasome activation. The second signal directly activates NLRP3 and is provided by ATP, bacterial toxins or particulate matter (De Nardo and Latz, 2011). But in human monocytes and microglia cells, the situation may be different, as TLR ligands alone can induce the release of IL-1β (Ferrari et al., 1997; Kahlenberg and Dubyak, 2004; Netea et al., 2009).

Given the chemical and structural diversity of the NLRP3 activators, it has been hypothesized that NLRP3 does not interact directly with its activators. Instead, the actual triggering of NLRP3 is an integrated and intermediate cellular signal elicited by all these activators. A number of theories have been proposed for the identity of the cellular signal responsible for NLRP3 activation, including potassium (K⁺) efflux, pore formation in cell membranes, lysosomes damage, the elevation of reactive oxygen species (ROS) and damage in the mitochondria. The NLRP3 agonists, including ATP, the antibiotic nigericin and pore-forming toxins, lead to lower intracellular concentration of K⁺, and a high extracellular concentration of K⁺ inhibits activation of the NLRP3 inflammasome as supposed (Mariathasan et al., 2006; Gross et al., 2011). The activation of NLRP3 by crystals requires endocytosis, which leads to lysosomal rupture. The NLRP3 inflammasome senses lysosomal content in the cytoplasm such as cathepsin B as a direct NLRP3 ligand. This hypothesis is supported by the efficacy of endocytosis inhibitors in blocking inflammasome activation by particulates. Furthermore, inhibitors of cathepsin B can prevent the activation of caspase-1 induced by certain microbes (Martinon et al., 2006; Hornung et al., 2008; Duewell et al., 2010). The production of ROS has also been suggested to act as a common cellular signal upstream of NLRP3 activation. When treated with ROS scavengers and NAPDH-oxidase inhibitors, the NLRP3 activation is blocked (Allen et al., 2009; Zhou et al., 2011). However, a recent study showed that the ROS inhibitors interfered with LPS mediated priming instead caspase-1 activation from ATP pulse (Bauernfeind et al., 2011). Additional study found that, thioredoxin-interaction protein (TXNIP) binds an activated NLRP3 after the production of ROS by NLRP3 activators (Zhou et al., 2010). Moreover, by using inhibitors of the respiratory chain, the source of ROS that mediates activation of the NLRP3 inflammasome is suggested to be mitochondria (Zhou et al., 2011). Furthermore, the release of mitochondrial DNA by damaged mitochondria is proved to activate the NLRP3 inflammasome (Nakahira et al., 2011; Shimada et al., 2012). Although none of the aforementioned studies provides a satisfactory explanation for the mechanism of NLRP3 activation, we propose that NLRP3 activation requires integrate and complicated signals that indicate cellular damage or stress.

NEGATIVE REGULATION OF NLRP3 INFLAMMASOME

The maturation and release of IL-1β and IL-18, as well as the pyroptosis that followed with inflammasome activation, have the potential to cause tissue damage in the host. Misregulation of inflammasome might lead to autoinflammatory such as in the cases of familial Mediterranean fever (FMF) or CAPS. Thus to control injury effectively, the inflammasome has to be quickly and efficiently engaged and then must be tightly controlled and switched off once the stimuli are no longer present, or the adaptive immune response has been initiated to avoid unnecessary collateral damage. To this end, both priming and activation of inflammasome are regulated tightly. During this process, several proteins, both of endogenous as well as of microbial origin, have been studied.

Autophagy

Autophagy, a lysosome-mediated cytoprotective process, is a cellular response to starvation, can deliver damaged organelles and long-lived proteins engulfed by autophagosomes from the cytoplasm to lysosomes for degradation. Recent studies have disclosed multiple roles of autophagy in the regulation of cell death, differentiation, antigen presentation and anti-microbial response (Levine et al., 2011). By studies of autophagocytosis-deficient Atg16L1^−/− (autophagy-related gene 16-like 1) mice, autophagy is demonstrated as an inhibitor of NLRP3 inflammasome. Blocking autophagy by genetic deletion of Atg16L1 potentiates inflammasome activity induced by TLR4 signaling. Mice lacking Atg16L1 suffer severe dextran sulfate sodium–induced colitis, which can be alleviated by neutralization of IL-18 and IL-1β (Saitoh et al., 2008). The autophagy inhibits IL-1β activation indirectly by lowering the endogenous sources of inflammasome activation and directly via autophagic degradation of inflammasome components. For indirect inhibition, autophagy can suppress inflammasome activity by diminishing the generation of ROS. The ROS, generated from damaged mitochondria by blocking autophagy, activate NLRP3 inflammasomes as mentioned above. Conversely, suppressing mitochondrial activity inhibits activation of the NLRP3 inflammasome due to less generation of ROS (Zhou et al., 2011). Two other autophagic proteins, LC3B and beclin1, are thought involved in NLRP3 inflammasome activation by regulating the release of mitochondrial DNA (mtDNA) besides of ROS. Depletion of LC3B and beclin1 enhances the activation of caspase-1 and secretion of IL-1β and IL-18 as a result of promoting the accumulation of dysfunctional mitochondria and cytosolic translocation of mtDNA in response to LPS and ATP in macrophages. Moreover, LC3B-deficient mice produced more caspase-1-dependent cytokines in sepsis models and were susceptible to LPS-induced mortality (Nakahira et al., 2011). mtDNA, precisely oxidized mtDNA, later is proved binding to NLRP3 directly to induce the NLRP3 inflammasome activation (Shimada et al., 2012). For direct inhibition, additional evidence indicates that the induction of inflammasomes is accompanied by autophagosome formation, which limits inflammasome activity by degrading inflammasome via ASC ubiquitination and the recruitment of p62 and LC3 in turn (Shi et al., 2012). There is also an inflammasome-independent manner in which autophagy acts to regulate IL-1β responses; autophagy sequesters pro-IL-1β and targets it for lysosomal degradation to limit the substrate necessary for caspase-1 activation (Harris et al., 2011).

Type I interferons

Type I interferons (IFNs) inhibit IL-1β production through two distinct mechanisms. Type I IFNs suppress the activity of the NLRP1 and NLRP3 inflammasomes to reduce caspase-1-dependent IL-1β maturation dependent on the transcription factor STAT1 (Guarda et al., 2011). But how STAT1 blocks the activation of caspase-1 exactly is unclear. In addition, Type I IFNs diminish the abundance of intracellular pro-IL-1β by inducing production of the anti-inflammatory cytokine IL-10 in a STAT1-dependent manner. IL-10 then inhibits the synthesis of pro-IL-1β and pro-IL-1α via the STAT3 signaling pathway. In line with the inhibitory role for type I IFNs on the inflammasome in vitro, poly(I:C)-induced type I IFNs suppress NLRP3 activation by alum and C. albicans, thus increasing susceptibility to this fungal pathogen. Importantly, the NLRP3 inflammasome activation in monocytes from patients with multiple sclerosis that are being treated with IFN-β is repressed as decreased IL-1β production than that from healthy donors (Guarda et al., 2011). Another paper has indicated the importance of type I IFNs in suppressing IL-1 in both inflammatory monocyte-macrophage and DC populations and in vivo during infection with M. tuberculosis, in which IL-1α and IL-1β are critically required for host resistance (Mayer-Barber et al., 2011).

T cell

The adaptive immune system can exert a profound inhibitory effect on the innate immune system. Effector and memory CD4⁺T cells abolish inflammasome-mediated caspase-1 activation and IL-1β release in macrophages and DCs. This inhibitory effect requires cell-to-cell contact and mediates by CD40L, OX40L and RANKL, which are ligands of the TNF superfamily. But it is still unclear how interactions between ligands of the TNF family and their receptors mediate the inhibition of caspase-1-dependent production of IL-1β (Guarda et al., 2009). Additionally, the other study found the anti-inflammatory role for CD4⁺T cell-derived IFN-γ in dampening IL-1 expression by monocyte-macrophages. But the effect of IFN-γ is controversial: IFN-γ potentiates IL-1β release from human cells, but transiently inhibits the production of IL-1β from mouse cells. In mice, the inhibition of IFN-γ on IL-1β production is regulated strictly by suppressor of cytokine signaling 1 (SOCS1) (Mayer-Barber et al., 2011). In conclusion, as a feedback loop, effector and memory T cells suppress potentially damaging inflammation, yet leave the primary inflammatory response intact, which is crucial for the onset of immunity. IFN-γ is the signature cytokine of T helper type 1 and CD8⁺T cell responses, which may also act as a feedback regulator of inflammasome responses once the adaptive immune response has been elicited. Such an inhibitory pathway is likely to aid in "switching off" innate immune responses once the adaptive arm of the immune system is engaged.

The TRIM family of proteins

Emerging evidence indicates that the TRIM family proteins also modulate the activation of inflammasome. The initial evidence was provided by studies that missense mutations in the C-terminal B30.2 domain of TRIM20 (pyrin or MEFV) cause FMF, the most common Mendelian autoinflammatory disease. Targeted disruption of Pyrin in mice causes increased endotoxin sensitivity and enhanced caspase-1 activation (Chae et al., 2003). The PYD of Pyrin interacts with the PYD of ASC, suggesting that it may be involved in blocking the recruitment of ASC and inflammasome formation. Later study has demonstrated that a gain-of-function mutant of TRIM20 constitutively forms inflammasome complexes containing ASC and caspase-1 in mice (Chae et al., 2011).

Previous work in our laboratory has demonstrated that TRIM30, a RING domain-containing TRIM protein which has been known as a negative regulator of TLR signaling, specifically suppresses the activation of the NLRP3 inflammasome. Expression of TRIM30 is induced by TLR stimulation and in turn restrict NLRP3 inflammasome activation by inhibiting ROS production, in response to a variety of ligands, including ATP, nigericin, silica and monosodium urate in vitro and in vivo (Hu et al., 2010).

Regulation of inflammasome activation by microRNA

microRNA (miRNA)-mediated posttranscriptional regulation plays an important role in controlling gene expression. miRNAs are endogenous noncoding RNAs that are 20–23 nt in length and exert regulatory functions through complementary base pairing to the 3′ untranslated regions (3′ UTRs) of protein-coding mRNAs (Bartel, 2009). Although miRNA has been found involved in many cell signalings, and its role in regulating innate immune responses has primarily been addressed for TLR signal-transduction pathways, there has not been observed in inflammasome. Recently, miR-223 is identified as the regulator of NLRP3 inflammasome, by targeting the NLRP3 3′-UTR and preventing accumulation of NLRP3 protein. In addition, the study also showed that EBV miR-BART15 targeted the same site in the NLRP3 3′-UTR to dampen inflammasome activation (Bauernfeind et al., 2012; Haneklaus et al., 2012).

POPs and COPs

Pyrin-only proteins (POPs) and CARD-only proteins (COPs) are regulators of the inflammasomes mainly in humans, because except caspase-12, neither POPs nor COPs is encoded in the mouse genome. The POP regulators include POP1, POP2, and viral PYDs (vPYDs), which are believed to interfere with PYD-PYD interaction between NLRPs and the adaptor ASC. Poxviruses deficient in vPYD produce enhanced activation of caspase-1 and secretion of IL-1β (Johnston et al., 2005). POP1 interacts with ASC and sequesters it from other PYD-containing proteins, such as the NLRs, because of its higher degree of homology with the PYD of ASC (64%), which ultimately results in the inhibition of inflammasome activation. Otherwise, POP2 binds NLRs and suppresses inflammasome activation by resembling the PYD of NLRP2 and NLRP7 (Dorfleutner et al., 2007; Stehlik and Dorfleutner, 2007; Martinon et al., 2009).

In contrast to the POPs, the COPs have similarity to the CARD of caspase-1 and function as decoy inhibitors of caspase-1 via CARD interaction, thereby sequestering it from the association with activating adaptors. To date, five proteins qualify to belong to the COP protein family: Iceberg, COP1/Pseudo-ICE, INCA, caspase-12s, and Nod2-S. Iceberg, the first demonstrated as COP, is highly similar to the CARD of caspase-1 and might function as a negative feedback regulator to prevent systemic inflammation because of elevating expression during inflammation (Humke et al., 2000). COP1/Pseudo-ICE and INCA, additional COPs found later, interact with the CARD of caspase-1 to prevent its activation similar to Iceberg (Druilhe et al., 2001; Lamkanfi et al., 2004). Nod2-S, a short variant of Nod2, encodes only the first CARD. Nod2-S does not interact with caspase-1 but competes with Nod2 for Rip2 binding, resulting in impaired caspase-1 activation (Rosenstiel et al., 2006). The best studied COP, caspase-12, has been explored in detail in mice. The pro-domain (a CARD) is sufficient for causing reduced cytokine secretion. In mice, caspase-12 deficiency confers resistance to sepsis, and the presence of caspase-12 exerts a dominant-negative suppressive effect on caspase-1 (Saleh et al., 2004; Saleh et al., 2006).

Nitric oxide

Nitric oxide (NO) is a small molecule synthesized by many cell types in various tissues and is involved in multiple physiological and pathological responses, including circulation, blood pressure, platelet function, host defense and neurotransmission in the central nervous system and peripheral nerves (Bogdan et al., 2000). In immune responses, NO also plays many roles (Bogdan, 2001), including the control of infection and the regulation of signaling cascades, transcription factors, vascular responses, leukocyte rolling, migration, cytokine production and T-cell differentiation (Clancy et al., 1998; Rawlingson, 2003; Niedbala et al., 2011). Our recent work shows that NO inhibits the activation of the NLRP3 inflammasome, thus preventing ASC pyroptosome formation, caspase-1 activation and IL-1β secretion in myeloid cells from both mice and humans. In contrast, in iNOS-deficient macrophages, NLRP3 inflammasome activation was enhanced by the accumulation of dysfunctional mitochondria. In vivo, iNOS deficiency or pharmacological inhibition of NO production enhanced NLRP3-dependent cytokine production, increasing mortality from LPS-induced sepsis in mice, which was prevented by NLRP3 deficiency. NO is known as an immunomodulatory molecule, although the mechanisms underlying this immunomodulation are poorly understood. In the study, NO is found as a critical negative regulator of the NLRP3 inflammasome via the stabilization of mitochondria (Mao et al., 2013). At the same time, two other studies also found that NO induced by IFN-γ specifically inhibited assembly of the NLRP3 inflammasome via S-nitrosylation of NLRP3 (Hernandez-Cuellar et al., 2012; Mishra et al., 2012).

Strategy of virus

Viruses inhibit activation of the NLRP3 inflammasome by several means to the benefit of their own infection. Mice lacking components of the NLRP3 inflammasome, such as NLRP3, ASC, or caspase-1, exhibited dramatically increased mortality and a viral clearance defect after exposure to the influenza virus (Allen et al., 2009; Ichinohe et al., 2009). In addition to POPs and miRNA mentioned above, some other viruses encode specific proteins to suppress host innate immune responses. Orf63 is a viral homolog of human NLRP1 encoded by KSHV. Orf63 blocks NLRP1-dependent innate immune responses, including caspase-1 activation and processing of IL-1β and IL-18. Inhibition of Orf63 expression resulted in increased expression of IL-1β during the KSHV life cycle. Although Orf63 does not demonstrate significant similarity to NLRP3, it interacts with NLRP3 and blocks NLRP3 activity (Gregory et al., 2011). Another virus, measles virus (MV), also encodes the nonstructural V protein to antagonize host innate immune responses. The recombinant MV lacking the V protein induces more IL-1β than the parental virus. On the contrary, THP-1 cells stably expressing the V protein suppress NLRP3 inflammasome-mediated IL-1β secretion. Furthermore, the study indicates that the V protein of MV interacts with NLRP3 through its C-terminal domain, thereby inhibiting NLRP3 inflammasome-mediated IL-1β secretion (Komune et al., 2011).

CONCLUSION

In the past decades, great advances have been made in our understanding of molecular mechanisms in innate immune system activation, particularly about inflammasomes. The activation and regulation mechanisms of NLRP1, NLRP3, NLRC4, and AIM2 inflammasomes are beginning to be characterized in detail, among which NLRP3 inflammasome is the best indentified, several activation models are proposed and parts of the negative regulatory mechanisms are proved. Pathogens inhibit activation of the NLRP3 inflammasome by evading recognition or disrupting the formation and signaling of it. On the other hand, the host keeps inflammasome under control through autophagy, T cell modulation, endogenous host molecules such as Trims and NO, microRNA and cytokines of innate or adaptive origin. Such multiple and nonredundant negative regulators of the NLRP3 inflammasome provide multiple checkpoints to ensure appropriate immune responses by concerted effort. But other regulators not found yet as well as the relative importance of these regulators need to be investigated in future.

References

[1] Allen, I.C., Scull, M.A., Moore, C.B., Holl, E.K., McElvania-TeKippe, E., Taxman, D.J., Guthrie, E.H., Pickles, R.J., and Ting, J.P. (2009a). The NLRP3 inflammasome mediates in vivo innate immunity to influenza A virus through recognition of viral RNA. Immunity 30, 556-565.

[2] Bartel, D.P. (2009). MicroRNAs: target recognition and regulatory functions. Cell 136, 215-233.

[3] Bauernfeind, F., Bartok, E., Rieger, A., Franchi, L., Nunez, G., and Hornung, V. (2011). Cutting edge: reactive oxygen species inhibitors block priming, but not activation, of the NLRP3 inflammasome. J Immunol 187, 613-617.

[4] Bauernfeind, F., Rieger, A., Schildberg, F.A., Knolle, P.A., Schmid-Burgk, J.L., and Hornung, V. (2012). NLRP3 Inflammasome Activity Is Negatively Controlled by miR-223. J Immunol 189, 4175-4181.

[5] Bogdan, C. (2001). Nitric oxide and the immune response. Nat Immunol 2, 907-916.

[6] Bogdan, C., Rollinghoff, M., and Diefenbach, A. (2000). The role of nitric oxide in innate immunity. Immunol Rev 173, 17-26.

[7] Burckstummer, T., Baumann, C., Bluml, S., Dixit, E., Durnberger, G., Jahn, H., Planyavsky, M., Bilban, M., Colinge, J., Bennett, K.L., et al. (2009). An orthogonal proteomic-genomic screen identifies AIM2 as a cytoplasmic DNA sensor for the inflammasome. Nat Immunol 10, 266-272.

[8] Chae, J.J., Cho, Y.H., Lee, G.S., Cheng, J., Liu, P.P., Feigenbaum, L., Katz, S.I., and Kastner, D.L. (2011). Gain-of-function Pyrin mutations induce NLRP3 protein-independent interleukin-1beta activation and severe autoinflammation in mice. Immunity 34, 755-768.

[9] Chae, J.J., Komarow, H.D., Cheng, J., Wood, G., Raben, N., Liu, P.P., and Kastner, D.L. (2003). Targeted disruption of pyrin, the FMF protein, causes heightened sensitivity to endotoxin and a defect in macrophage apoptosis. Mol Cell 11, 591-604.

[10] Clancy, R.M., Amin, A.R., and Abramson, S.B. (1998). The role of nitric oxide in inflammation and immunity. Arthritis Rheum 41, 1141-1151.

[11] De Nardo, D., and Latz, E. (2011). NLRP3 inflammasomes link inflammation and metabolic disease. Trends Immunol 32, 373-379.

[12] Denoble, A.E., Huffman, K.M., Stabler, T.V., Kelly, S.J., Hershfield, M.S., McDaniel, G.E., Coleman, R.E., and Kraus, V.B. (2011). Uric acid is a danger signal of increasing risk for osteoarthritis through inflammasome activation. Proc Natl Acad Sci U S A 108, 2088-2093.

[13] Dorfleutner, A., Bryan, N.B., Talbott, S.J., Funya, K.N., Rellick, S.L., Reed, J.C., Shi, X., Rojanasakul, Y., Flynn, D.C., and Stehlik, C. (2007). Cellular pyrin domain-only protein 2 is a candidate regulator of inflammasome activation. Infect Immun 75, 1484-1492.

[14] Dostert, C., Petrilli, V., Van Bruggen, R., Steele, C., Mossman, B.T., and Tschopp, J. (2008). Innate immune activation through Nalp3 inflammasome sensing of asbestos and silica. Science 320, 674-677.

[15] Druilhe, A., Srinivasula, S.M., Razmara, M., Ahmad, M., and Alnemri, E.S. (2001). Regulation of IL-1beta generation by Pseudo-ICE and ICEBERG, two dominant negative caspase recruitment domain proteins. Cell Death Differ 8, 649-657.

[16] Duewell, P., Kono, H., Rayner, K.J., Sirois, C.M., Vladimer, G., Bauernfeind, F.G., Abela, G.S., Franchi, L., Nunez, G., Schnurr, M., et al. (2010). NLRP3 inflammasomes are required for atherogenesis and activated by cholesterol crystals. Nature 464, 1357-1361.

[17] Eisenbarth, S.C., Colegio, O.R., O′Connor, W., Sutterwala, F.S., and Flavell, R.A. (2008). Crucial role for the Nalp3 inflammasome in the immunostimulatory properties of aluminium adjuvants. Nature 453, 1122-1126.

[18] Fernandes-Alnemri, T., Yu, J.W., Datta, P., Wu, J., and Alnemri, E.S. (2009). AIM2 activates the inflammasome and cell death in response to cytoplasmic DNA. Nature 458, 509-513.

[19] Ferrari, D., Chiozzi, P., Falzoni, S., Hanau, S., and Di Virgilio, F. (1997). Purinergic modulation of interleukin-1 beta release from microglial cells stimulated with bacterial endotoxin. J Exp Med 185, 579-582.

[20] Franchi, L., Amer, A., Body-Malapel, M., Kanneganti, T.D., Ozoren, N., Jagirdar, R., Inohara, N., Vandenabeele, P., Bertin, J., Coyle, A., et al. (2006). Cytosolic flagellin requires Ipaf for activation of caspase-1 and interleukin 1beta in salmonella-infected macrophages. Nat Immunol 7, 576-582.

[21] Gong, Y.N., and Shao, F. (2012). Sensing bacterial infections by NAIP receptors in NLRC4 inflammasome activation. Protein Cell 3, 98-105.

[22] Gram, A.M., Frenkel, J., and Ressing, M.E. (2012). Inflammasomes and viruses: cellular defence versus viral offence. J Gen Virol 93, 2063-2075.

[23] Gregory, S.M., Davis, B.K., West, J.A., Taxman, D.J., Matsuzawa, S., Reed, J.C., Ting, J.P., and Damania, B. (2011). Discovery of a viral NLR homolog that inhibits the inflammasome. Science 331, 330-334.

[24] Gross, O., Poeck, H., Bscheider, M., Dostert, C., Hannesschlager, N., Endres, S., Hartmann, G., Tardivel, A., Schweighoffer, E., Tybulewicz, V., et al. (2009). Syk kinase signalling couples to the Nlrp3 inflammasome for anti-fungal host defence. Nature 459, 433-436.

[25] Gross, O., Thomas, C.J., Guarda, G., and Tschopp, J. (2011). The inflammasome: an integrated view. Immunol Rev 243, 136-151.

[26] Guarda, G., Braun, M., Staehli, F., Tardivel, A., Mattmann, C., Forster, I., Farlik, M., Decker, T., Du Pasquier, R.A., Romero, P., et al. (2011). Type I interferon inhibits interleukin-1 production and inflammasome activation. Immunity 34, 213-223.

[27] Guarda, G., Dostert, C., Staehli, F., Cabalzar, K., Castillo, R., Tardivel, A., Schneider, P., and Tschopp, J. (2009). T cells dampen innate immune responses through inhibition of NLRP1 and NLRP3 inflammasomes. Nature 460, 269-273.

[28] Halle, A., Hornung, V., Petzold, G.C., Stewart, C.R., Monks, B.G., Reinheckel, T., Fitzgerald, K.A., Latz, E., Moore, K.J., and Golenbock, D.T. (2008). The NALP3 inflammasome is involved in the innate immune response to amyloid-beta. Nat Immunol 9, 857-865.

[29] Haneklaus, M., Gerlic, M., Kurowska-Stolarska, M., Rainey, A.A., Pich, D., McInnes, I.B., Hammerschmidt, W., O′Neill, L.A., and Masters, S.L. (2012). Cutting Edge: miR-223 and EBV miR-BART15 Regulate the NLRP3 Inflammasome and IL-1beta Production. J Immunol 189, 3795-3799.

[30] Harris, J., Hartman, M., Roche, C., Zeng, S.G., O′Shea, A., Sharp, F.A., Lambe, E.M., Creagh, E.M., Golenbock, D.T., Tschopp, J., et al. (2011). Autophagy controls IL-1beta secretion by targeting pro-IL-1beta for degradation. J Biol Chem 286, 9587-9597.

[31] Hernandez-Cuellar, E., Tsuchiya, K., Hara, H., Fang, R., Sakai, S., Kawamura, I., Akira, S., and Mitsuyama, M. (2012). Cutting Edge: Nitric Oxide Inhibits the NLRP3 Inflammasome. The Journal of Immunology 189, 5113-5117.

[32] Hoffman, H.M., Rosengren, S., Boyle, D.L., Cho, J.Y., Nayar, J., Mueller, J.L., Anderson, J.P., Wanderer, A.A., and Firestein, G.S. (2004). Prevention of cold-associated acute inflammation in familial cold autoinflammatory syndrome by interleukin-1 receptor antagonist. Lancet 364, 1779-1785.

[33] Hornung, V., Ablasser, A., Charrel-Dennis, M., Bauernfeind, F., Horvath, G., Caffrey, D.R., Latz, E., and Fitzgerald, K.A. (2009). AIM2 recognizes cytosolic dsDNA and forms a caspase-1-activating inflammasome with ASC. Nature 458, 514-518.

[34] Hornung, V., Bauernfeind, F., Halle, A., Samstad, E.O., Kono, H., Rock, K.L., Fitzgerald, K.A., and Latz, E. (2008). Silica crystals and aluminum salts activate the NALP3 inflammasome through phagosomal destabilization. Nat Immunol 9, 847-856.

[35] Hu, Y., Mao, K., Zeng, Y., Chen, S., Tao, Z., Yang, C., Sun, S., Wu, X., Meng, G., and Sun, B. (2010). Tripartite-motif protein 30 negatively regulates NLRP3 inflammasome activation by modulating reactive oxygen species production. J Immunol 185, 7699-7705.

[36] Humke, E.W., Shriver, S.K., Starovasnik, M.A., Fairbrother, W.J., and Dixit, V.M. (2000). ICEBERG: a novel inhibitor of interleukin-1beta generation. Cell 103, 99-111.

[37] Ichinohe, T., Lee, H.K., Ogura, Y., Flavell, R., and Iwasaki, A. (2009). Inflammasome recognition of influenza virus is essential for adaptive immune responses. J Exp Med 206, 79-87.

[38] Johnston, J.B., Barrett, J.W., Nazarian, S.H., Goodwin, M., Ricciuto, D., Wang, G., and McFadden, G. (2005). A poxvirus-encoded pyrin domain protein interacts with ASC-1 to inhibit host inflammatory and apoptotic responses to infection. Immunity 23, 587-598.

[39] Kahlenberg, J.M., and Dubyak, G.R. (2004). Differing caspase-1 activation states in monocyte versus macrophage models of IL-1beta processing and release. J Leukoc Biol 76, 676-684.

[40] Komune, N., Ichinohe, T., Ito, M., and Yanagi, Y. (2011). Measles virus V protein inhibits NLRP3 inflammasome-mediated interleukin-1beta secretion. J Virol 85, 13019-13026.

[41] Kumar, H., Kumagai, Y., Tsuchida, T., Koenig, P.A., Satoh, T., Guo, Z., Jang, M.H., Saitoh, T., Akira, S., and Kawai, T. (2009). Involvement of the NLRP3 inflammasome in innate and humoral adaptive immune responses to fungal beta-glucan. J Immunol 183, 8061-8067.

[42] Lamkanfi, M. (2011). Emerging inflammasome effector mechanisms. Nat Rev Immunol 11, 213-220.

[43] Lamkanfi, M., Denecker, G., Kalai, M., D′Hondt, K., Meeus, A., Declercq, W., Saelens, X., and Vandenabeele, P. (2004). INCA, a novel human caspase recruitment domain protein that inhibits interleukin-1beta generation. J Biol Chem 279, 51729-51738.

[44] Lamkanfi, M., and Dixit, V.M. (2009). Inflammasomes: guardians of cytosolic sanctity. Immunol Rev 227, 95-105.

[45] Levine, B., Mizushima, N., and Virgin, H.W. (2011). Autophagy in immunity and inflammation. Nature 469, 323-335.

[46] Mao, K., Chen, S., Chen, M., Ma, Y., Wang, Y., Huang, B., He, Z., Zeng, Y., Hu, Y., Sun, S., et al. (2013). Nitric oxide suppresses NLRP3 inflammasome activation and protects against LPS-induced septic shock. Cell Research 23, 201-212.

[47] Mariathasan, S., Weiss, D.S., Newton, K., McBride, J., O′Rourke, K., Roose-Girma, M., Lee, W.P., Weinrauch, Y., Monack, D.M., and Dixit, V.M. (2006). Cryopyrin activates the inflammasome in response to toxins and ATP. Nature 440, 228-232.

[48] Martinon, F., Burns, K., and Tschopp, J. (2002). The inflammasome: a molecular platform triggering activation of inflammatory caspases and processing of proIL-beta. Mol Cell 10, 417-426.

[49] Martinon, F., Mayor, A., and Tschopp, J. (2009). The inflammasomes: guardians of the body. Annu Rev Immunol 27, 229-265.

[50] Martinon, F., Petrilli, V., Mayor, A., Tardivel, A., and Tschopp, J. (2006). Gout-associated uric acid crystals activate the NALP3 inflammasome. Nature 440, 237-241.

[51] Martinon, F., and Tschopp, J. (2004). Inflammatory caspases: linking an intracellular innate immune system to autoinflammatory diseases. Cell 117, 561-574.

[52] Masters, S.L., Dunne, A., Subramanian, S.L., Hull, R.L., Tannahill, G.M., Sharp, F.A., Becker, C., Franchi, L., Yoshihara, E., Chen, Z., et al. (2010). Activation of the NLRP3 inflammasome by islet amyloid polypeptide provides a mechanism for enhanced IL-1beta in type 2 diabetes. Nat Immunol 11, 897-904.

[53] Mayer-Barber, K.D., Andrade, B.B., Barber, D.L., Hieny, S., Feng, C.G., Caspar, P., Oland, S., Gordon, S., and Sher, A. (2011). Innate and adaptive interferons suppress IL-1alpha and IL-1beta production by distinct pulmonary myeloid subsets during Mycobacterium tuberculosis infection. Immunity 35, 1023-1034.

[54] Meixenberger, K., Pache, F., Eitel, J., Schmeck, B., Hippenstiel, S., Slevogt, H., N′Guessan, P., Witzenrath, M., Netea, M.G., Chakraborty, T., et al. (2010). Listeria monocytogenes-infected human peripheral blood mononuclear cells produce IL-1beta, depending on listeriolysin O and NLRP3. J Immunol 184, 922-930.

[55] Miao, E.A., Alpuche-Aranda, C.M., Dors, M., Clark, A.E., Bader, M.W., Miller, S.I., and Aderem, A. (2006). Cytoplasmic flagellin activates caspase-1 and secretion of interleukin 1beta via Ipaf. Nat Immunol 7, 569-575.

[56] Mishra, B.B., Rathinam, V.A., Martens, G.W., Martinot, A.J., Kornfeld, H., Fitzgerald, K.A., and Sassetti, C.M. (2012). Nitric oxide controls the immunopathology of tuberculosis by inhibiting NLRP3 inflammasome-dependent processing of IL-1beta. Nat Immunol 14, 52-60.

[57] Munoz-Planillo, R., Franchi, L., Miller, L.S., and Nunez, G. (2009). A critical role for hemolysins and bacterial lipoproteins in Staphylococcus aureus-induced activation of the Nlrp3 inflammasome. J Immunol 183, 3942-3948.

[58] Nakahira, K., Haspel, J.A., Rathinam, V.A., Lee, S.J., Dolinay, T., Lam, H.C., Englert, J.A., Rabinovitch, M., Cernadas, M., Kim, H.P., et al. (2011). Autophagy proteins regulate innate immune responses by inhibiting the release of mitochondrial DNA mediated by the NALP3 inflammasome. Nat Immunol 12, 222-230.

[59] Netea, M.G., Nold-Petry, C.A., Nold, M.F., Joosten, L.A., Opitz, B., van der Meer, J.H., van de Veerdonk, F.L., Ferwerda, G., Heinhuis, B., Devesa, I., et al. (2009). Differential requirement for the activation of the inflammasome for processing and release of IL-1beta in monocytes and macrophages. Blood 113, 2324-2335.

[60] Niedbala, W., Alves-Filho, J.C., Fukada, S.Y., Vieira, S.M., Mitani, A., Sonego, F., Mirchandani, A., Nascimento, D.C., Cunha, F.Q., and Liew, F.Y. (2011). Regulation of type 17 helper T-cell function by nitric oxide during inflammation. Proc Natl Acad Sci U S A 108, 9220-9225.

[61] Rawlingson, A. (2003). Nitric oxide, inflammation and acute burn injury. Burns 29, 631-640.

[62] Roberts, T.L., Idris, A., Dunn, J.A., Kelly, G.M., Burnton, C.M., Hodgson, S., Hardy, L.L., Garceau, V., Sweet, M.J., Ross, I.L., et al. (2009). HIN-200 proteins regulate caspase activation in response to foreign cytoplasmic DNA. Science 323, 1057-1060.

[63] Rosengren, S., Mueller, J.L., Anderson, J.P., Niehaus, B.L., Misaghi, A., Anderson, S., Boyle, D.L., and Hoffman, H.M. (2007). Monocytes from familial cold autoinflammatory syndrome patients are activated by mild hypothermia. J Allergy Clin Immunol 119, 991-996.

[64] Rosenstiel, P., Huse, K., Till, A., Hampe, J., Hellmig, S., Sina, C., Billmann, S., von Kampen, O., Waetzig, G.H., Platzer, M., et al. (2006). A short isoform of NOD2/CARD15, NOD2-S, is an endogenous inhibitor of NOD2/receptor-interacting protein kinase 2-induced signaling pathways. Proc Natl Acad Sci U S A 103, 3280-3285.

[65] Saitoh, T., Fujita, N., Jang, M.H., Uematsu, S., Yang, B.G., Satoh, T., Omori, H., Noda, T., Yamamoto, N., Komatsu, M., et al. (2008). Loss of the autophagy protein Atg16L1 enhances endotoxin-induced IL-1beta production. Nature 456, 264-268.

[66] Saleh, M., Mathison, J.C., Wolinski, M.K., Bensinger, S.J., Fitzgerald, P., Droin, N., Ulevitch, R.J., Green, D.R., and Nicholson, D.W. (2006). Enhanced bacterial clearance and sepsis resistance in caspase-12-deficient mice. Nature 440, 1064-1068.

[67] Saleh, M., Vaillancourt, J.P., Graham, R.K., Huyck, M., Srinivasula, S.M., Alnemri, E.S., Steinberg, M.H., Nolan, V., Baldwin, C.T., Hotchkiss, R.S., et al. (2004). Differential modulation of endotoxin responsiveness by human caspase-12 polymorphisms. Nature 429, 75-79.

[68] Schroder, K., and Tschopp, J. (2010). The inflammasomes. Cell 140, 821-832.

[69] Shi, C.S., Shenderov, K., Huang, N.N., Kabat, J., Abu-Asab, M., Fitzgerald, K.A., Sher, A., and Kehrl, J.H. (2012). Activation of autophagy by inflammatory signals limits IL-1beta production by targeting ubiquitinated inflammasomes for destruction. Nat Immunol 13, 255-263.

[70] Shimada, K., Crother, T.R., Karlin, J., Dagvadorj, J., Chiba, N., Chen, S., Ramanujan, V.K., Wolf, A.J., Vergnes, L., Ojcius, D.M., et al. (2012). Oxidized mitochondrial DNA activates the NLRP3 inflammasome during apoptosis. Immunity 36, 401-414.

[71] Shio, M.T., Eisenbarth, S.C., Savaria, M., Vinet, A.F., Bellemare, M.J., Harder, K.W., Sutterwala, F.S., Bohle, D.S., Descoteaux, A., Flavell, R.A., et al. (2009). Malarial hemozoin activates the NLRP3 inflammasome through Lyn and Syk kinases. PLoS Pathog 5, e1000559.

[72] Stehlik, C., and Dorfleutner, A. (2007). COPs and POPs: modulators of inflammasome activity. J Immunol 179, 7993-7998.

[73] Strowig, T., Henao-Mejia, J., Elinav, E., and Flavell, R. (2012). Inflammasomes in health and disease. Nature 481, 278-286.

[74] Sutterwala, F.S., Ogura, Y., Szczepanik, M., Lara-Tejero, M., Lichtenberger, G.S., Grant, E.P., Bertin, J., Coyle, A.J., Galan, J.E., Askenase, P.W., et al. (2006). Critical role for NALP3/CIAS1/Cryopyrin in innate and adaptive immunity through its regulation of caspase-1. Immunity 24, 317-327.

[75] Takeuchi, O., and Akira, S. (2010). Pattern recognition receptors and inflammation. Cell 140, 805-820.

[76] Tschopp, J., and Schroder, K. (2010). NLRP3 inflammasome activation: The convergence of multiple signalling pathways on ROS production? Nat Rev Immunol 10, 210-215.

[77] Wen, H., Gris, D., Lei, Y., Jha, S., Zhang, L., Huang, M.T., Brickey, W.J., and Ting, J.P. (2011). Fatty acid-induced NLRP3-ASC inflammasome activation interferes with insulin signaling. Nat Immunol 12, 408-415.

[78] Zhao, Y., Yang, J., Shi, J., Gong, Y.N., Lu, Q., Xu, H., Liu, L., and Shao, F. (2011). The NLRC4 inflammasome receptors for bacterial flagellin and type III secretion apparatus. Nature 477, 596-600.

[79] Zhou, R., Tardivel, A., Thorens, B., Choi, I., and Tschopp, J. (2010). Thioredoxin-interacting protein links oxidative stress to inflammasome activation. Nat Immunol 11, 136-140.

[80] Zhou, R., Yazdi, A.S., Menu, P., and Tschopp, J. (2011). A role for mitochondria in NLRP3 inflammasome activation. Nature 469, 221-225.

Rights & permissions

Higher Education Press and Springer-Verlag Berlin Heidelberg 2013