Inflammasome activation and IL-1β and IL-18 processing during infection. Trends Immunol. 2011;32:110-116. [PMID: 21333600 DOI: 10.1016/j.it.2011.01.003]

U1-small nuclear ribonucleoprotein activates the NLRP3 inflammasome in human monocytes

The NOD-like receptor family, pyrin domain-containing 3 (NLRP3) inflammasome is a caspase-1-containing cytosolic protein complex that is essential for processing and secretion of IL-1β. The U1-small nuclear ribonucleoprotein (U1-snRNP) that includes U1-small nuclear RNA is a highly conserved intranuclear molecular complex involved in splicing pre-mRNA. Abs against this self nuclear molecule are characteristically found in autoimmune diseases like systemic lupus erythematosus, suggesting a potential role of U1-snRNP in autoimmunity. Although endogenous DNA and microbial nucleic acids are known to activate the inflammasomes, it is unknown whether endogenous RNA-containing U1-snRNP could activate this molecular complex. In this study, we show that U1-snRNP activates the NLRP3 inflammasome in CD14(+) human monocytes dependently of anti-U1-snRNP Abs, leading to IL-1β production. Reactive oxygen species and K(+) efflux were responsible for this activation. Knocking down the NLRP3 or inhibiting caspase-1 or TLR7/8 pathway decreased IL-1β production from monocytes treated with U1-snRNP in the presence of anti-U1-snRNP Abs. Our findings indicate that endogenous RNA-containing U1-snRNP could be a signal that activates the NLRP3 inflammasome in autoimmune diseases like systemic lupus erythematosus where anti-U1-snRNP Abs are present.

Caspase-1 has both proinflammatory and regulatory properties in Helicobacter infections, which are differentially mediated by its substrates IL-1β and IL-18

The proinflammatory cysteine protease caspase-1 is autocatalytically activated upon cytosolic sensing of a variety of pathogen-associated molecular patterns by Nod-like receptors. Active caspase-1 processes pro-IL-1β and pro-IL-18 to generate the bioactive cytokines and to initiate pathogen-specific immune responses. Little information is available on caspase-1 and inflammasome activation during infection with the gastric bacterial pathogen Helicobacter pylori. In this study, we addressed a possible role for caspase-1 and its cytokine substrates in the spontaneous and vaccine-induced control of Helicobacter infection, as well as the development of gastritis and gastric cancer precursor lesions, using a variety of experimental infection, vaccine-induced protection, and gastric disease models. We show that caspase-1 is activated and IL-1β and IL-18 are processed in vitro and in vivo as a consequence of Helicobacter infection. Caspase-1 activation and IL-1 signaling are absolutely required for the efficient control of Helicobacter infection in vaccinated mice. IL-1R(-/-) mice fail to develop protective immunity but are protected against Helicobacter-associated gastritis and gastric preneoplasia as a result of their inability to generate Helicobacter-specific Th1 and Th17 responses. In contrast, IL-18 is dispensable for vaccine-induced protective immunity but essential for preventing excessive T cell-driven immunopathology. IL-18(-/-) animals develop strongly accelerated pathology that is accompanied by unrestricted Th17 responses. In conclusion, we show in this study that the processing and release of a regulatory caspase-1 substrate, IL-18, counteracts the proinflammatory activities of another caspase-1 substrate, IL-1β, thereby balancing control of the infection with the prevention of excessive gastric immunopathology.

Immunology in clinic review series; focus on autoinflammatory diseases: role of inflammasomes in autoinflammatory syndromes

OTHER THEMES PUBLISHED IN THIS IMMUNOLOGY IN THE CLINIC REVIEW SERIES Allergy, Host Responses, Cancer, Type 1 diabetes and viruses, Metabolic diseases.

SUMMARY

Autoinflammatory syndromes are disorders characterized by the hyperactivation of the innate immune system in the absence of microbial infection or autoantibody production. Some autoinflammatory syndromes are associated with recurrent episodes of fever and systemic inflammation that are caused by dysregulated activation of inflammasomes, molecular platforms responsible for the activation of caspase-1 and the production of interleukin (IL)-1β. In this review we will discuss the role of IL-1β and the inflammasomes in host defence and how mutations of two genes, NLRP3 and PYRIN, leads to the autoinflammatory syndromes, cryopyrin-associated periodic syndromes (CAPS) and familial Mediterranean fever (FMF). Both CAPS and FMF are characterized by increased inflammasome activity and overproduction of IL-1β which is ultimately responsible for disease manifestations. Importantly, understanding the molecular mechanisms of these syndromes has led to effective treatment for these rare diseases with biological drugs that target IL-1β-mediated signalling.

[Clinical aspects of Familial Mediterranean fever]

Familial Mediterranean fever (FMF) is a hereditary autoinflammatory disease characterized by recurrent and short duration (1-3 days) of fever, and serositis. Based on the nationwide survey of FMF in Japan, the estimated number of Japanese FMF patients is about three hundred. High grade fever was observed in 95.5%, chest pain in 35.8% abdominal pain in 62.7% and arthritis in 31.3% among Japanese FMF patients. AA amyloidosis was confirmed in 5 patients (3.7%). Colchicine was effective in 91.8% of Japanese FMF patients. A significant number of FMF patients exist in Japan, and early diagnosis and treatments should be required to prevent AA amyloidosis.

The double-stranded RNA analogue polyinosinic-polycytidylic acid induces keratinocyte pyroptosis and release of IL-36γ

IL-36 is the common name for the three IL-1 family members IL-36α, IL-36β, and IL-36γ, formerly known as IL-1F6, IL-1F8, and IL-1F9, respectively. IL-36 appears to have pro-inflammatory activities; however, the physiological function of these cytokines remains unknown. Expression of IL-36 by keratinocytes implies its possible involvement in innate immune responses in the skin. We observed that, of the three IL-36 isoforms, human keratinocytes express high levels of IL-36γ. IL-36γ mRNA expression was dramatically induced by the Toll-like receptor ligands polyinosinic-polycytidylic acid (poly(I:C)) and flagellin. Surprisingly, the IL-36γ protein was released by cells treated with poly(I:C), but remained intracellular in cells treated with flagellin only. poly(I:C), but not flagellin, induced cell death and caspase-3/7 activation. Inhibition of caspase-3/7 and caspase-1 blocked extracellular release of IL-36γ from poly(I:C)-treated cells. Furthermore, caspase-1 inhibition prevented poly(I:C)-induced caspase-3/7 activation. Interestingly, transcription of the gene IL36G was dependent on caspase-1, but not caspase-3/7, activation. This demonstrates that the pathways leading to IL36G transcription and caspase-3/7 activation branch after caspase-1. This divergence of the pathways allows the cells to enter a state of de novo protein synthesis before committing to pyroptosis. Overall, our observations suggest that IL-36γ may be an alarmin that signals the cause, e.g., viral infection, of cell death.

IL-33 is processed into mature bioactive forms by neutrophil elastase and cathepsin G

Interleukin-33 (IL-33) (NF-HEV) is a chromatin-associated nuclear cytokine from the IL-1 family, which has been linked to important diseases, including asthma, rheumatoid arthritis, ulcerative colitis, and cardiovascular diseases. IL-33 signals through the ST2 receptor and drives cytokine production in type 2 innate lymphoid cells (ILCs) (natural helper cells, nuocytes), T-helper (Th)2 lymphocytes, mast cells, basophils, eosinophils, invariant natural killer T (iNKT), and natural killer (NK) cells. We and others recently reported that, unlike IL-1β and IL-18, full-length IL-33 is biologically active independently of caspase-1 cleavage and that processing by caspases results in IL-33 inactivation. We suggested that IL-33, which is released upon cellular damage, may function as an endogenous danger signal or alarmin, similar to IL-1α or high-mobility group box 1 protein (HMGB1). Here, we investigated the possibility that IL-33 activity may be regulated by proteases released during inflammation. Using a combination of in vitro and in vivo approaches, we demonstrate that neutrophil serine proteases cathepsin G and elastase can cleave full-length human IL-33(1-270) and generate mature forms IL-33(95-270), IL-33(99-270), and IL-33(109-270). These forms are produced by activated human neutrophils ex vivo, are biologically active in vivo, and have a ~10-fold higher activity than full-length IL-33 in cellular assays. Murine IL-33 is also cleaved by neutrophil cathepsin G and elastase, and both full-length and cleaved endogenous IL-33 could be detected in the bronchoalveolar lavage fluid in an in vivo model of acute lung injury associated with neutrophil infiltration. We propose that the inflammatory microenvironment may exacerbate disease-associated functions of IL-33 through the generation of highly active mature forms.

DNA-responsive inflammasomes and their regulators in autoimmunity

Upon sensing microbial and self-derived DNA, DNA sensors initiate innate immune responses. These sensors include the interferon (IFN)-inducible Toll-like receptor 9 (TLR9) and PYHIN proteins. Upon sensing DNA, cytosolic (murine Aim2 and human AIM2) and nuclear (IFI16) PYHIN proteins recruit an adaptor protein (ASC) and pro-caspase-1 to form an inflammasome, which activates caspase-1. The activated caspase-1 cleaves pro-IL-1β and pro-IL-18 to generate active forms. However, upon sensing cytosolic DNA, the IFI16 protein recruits STING to induce the expression of type I IFN. Recognition of self DNA by innate immune cells contributes to the production of increased levels of type I IFN. Given that the type I IFNs modulate the expression of inflammasome proteins and that the IFN-inducible proteins inhibit the activity of DNA-responsive inflammasomes, an improved understanding of the molecular mechanisms that regulate the activity of DNA-responsive inflammasomes is likely to identify new therapeutic targets to treat autoimmune diseases.

Caspase work model during pathogen infection

Caspases are an evolutionarily conserved family of aspartate-specific cystein-dependent proteases with essential functions in apoptosis and normally exist in cells as inactive proenzymes. In addition to the inflammatory caspases, the initiator and effector caspases have been shown to have an important role in regulating the immune response, but are involved in different ways. We give a brief introduction on the benefit of apoptosis on the clearance of invasive pathogens, and the caspase functions involved in the immune response. Then we construct a working model of caspases during pathogen invasion. A detailed description of the three modes is given in the discussion. These three modes are regulated by different inhibitors, and there may be a novel way to treat intracellular pathogen and autoimmune diseases based on the specific inhibitors.

Inflammasomes in intestinal inflammation and cancer

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

Inflammasome activation of IL-18 results in endothelial progenitor cell dysfunction in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease with heterogeneous manifestations including severe organ damage and vascular dysfunction leading to premature atherosclerosis. IFN-α has been proposed to have an important role in the development of lupus and lupus-related cardiovascular disease, partly by repression of IL-1 pathways leading to impairments in vascular repair induced by endothelial progenitor cells (EPCs) and circulating angiogenic cells (CACs). Counterintuitively, SLE patients also display transcriptional upregulation of the IL-1β/IL-18 processing machinery, the inflammasome. To understand this dichotomy and its impact on SLE-related cardiovascular disease, we examined cultures of human and murine control or lupus EPC/CACs to determine the role of the inflammasome in endothelial differentiation. We show that caspase-1 inhibition improves dysfunctional SLE EPC/CAC differentiation into mature endothelial cells and blocks IFN-α-mediated repression of this differentiation, implicating inflammasome activation as a crucial downstream pathway leading to aberrant vasculogenesis. Furthermore, serum IL-18 levels are elevated in SLE and correlate with EPC/CAC dysfunction. Exogenous IL-18 inhibits endothelial differentiation in control EPC/CACs and neutralization of IL-18 in SLE EPC/CAC cultures restores their capacity to differentiate into mature endothelial cells, supporting a deleterious effect of IL-18 on vascular repair in vivo. Upregulation of the inflammasome machinery was operational in vivo, as evidenced by gene array analysis of lupus nephritis biopsies. Thus, the effects of IFN-α are complex and contribute to an elevated risk of cardiovascular disease by suppression of IL-1β pathways and by upregulation of the inflammasome machinery and potentiation of IL-18 activation.

Psoriasis

Psoriasis is a common relapsing and remitting immune-mediated inflammatory disease that affects the skin and joints. This review focuses on current immunogenetic concepts, key cellular players, and axes of cytokines that are thought to contribute to disease pathogenesis. We highlight potential therapeutic targets and give an overview of the currently used immune-targeted therapies.

Role of Toll interleukin-1 receptor (IL-1R) 8, a negative regulator of IL-1R/Toll-like receptor signaling, in resistance to acute Pseudomonas aeruginosa lung infection

Toll interleukin-1 receptor (IL-1R) 8 (TIR8), also known as single Ig IL-1 receptor (IL-R)-related molecule, or SIGIRR, is a member of the IL-1R-like family, primarily expressed by epithelial cells. Current evidence suggests that TIR8 plays a nonredundant role as a negative regulator in vivo under different inflammatory conditions that are dependent on IL-R and Toll-like receptor (TLR) activation. In the present study, we examined the role of TIR8 in innate resistance to acute lung infections caused by Pseudomonas aeruginosa, a Gram-negative pathogen responsible for life-threatening infections in immunocompromised individuals and cystic fibrosis patients. We show that Tir8 deficiency in mice was associated with increased susceptibility to acute P. aeruginosa infection, in terms of mortality and bacterial load, and to exacerbated local and systemic production of proinflammatory cytokines (gamma interferon [IFN-γ], tumor necrosis factor alpha [TNF-α], IL-1β, and IL-6) and chemokines (CXCL1, CXCL2, and CCL2). It has been reported that host defense against P. aeruginosa acute lung infection can be improved by blocking IL-1 since exaggerated IL-1β production may be harmful for the host in this infection. In agreement with these data, IL-1RI deficiency rescues the phenotype observed in Tir8-deficient mice: in Tir8-/- IL-1RI-/- double knockout mice we observed higher survival rates, enhanced bacterial clearance, and reduced levels of local and systemic cytokine and chemokine levels than in Tir8-deficient mice. These results suggest that TIR8 has a nonredundant effect in modulating the inflammation caused by P. aeruginosa, in particular, by negatively regulating IL-1RI signaling, which plays a major role in the pathogenesis of this infectious disease.

IL-1 receptor/Toll-like receptor signaling in infection, inflammation, stress and neurodegeneration couples hyperexcitability and seizures

Increasing evidence supports the involvement of immune and inflammatory processes in the etiopathogenesis of seizures. In particular, activation of innate immune mechanisms and the subsequent inflammatory responses, that are induced in the brain by infection, febrile seizures, neurotrauma, stroke are well documented conditions associated with acute symptomatic seizures and with a high risk of developing epilepsy. A decade ago, pharmacological experiments showed that elevated brain levels of the anti-inflammatory molecule IL-1 receptor antagonist reduced seizures in epilepsy models. This observation, together with the evidence of in situ induction of inflammatory mediators and their receptors in experimental and human epileptogenic brain tissue, established the proof-of-concept evidence that the activation of innate immunity and inflammation in the brain are intrinsic features of the pathologic hyperexcitable tissue. Recent breakthroughs in understanding the molecular organization of the innate immune system first in macrophages, then in the different cell types of the CNS, together with pharmacological and genetic studies in epilepsy models, showed that the activation of IL-1 receptor/Toll-like receptor (IL-1R/TLR) signaling significantly contributes to seizures. IL-1R/TLR mediated pro-excitatory actions are elicited in the brain either by mimicking bacterial or viral infections and inflammatory responses, or via the action of endogenous ligands. These ligands include proinflammatory cytokines, such as IL-1beta, or danger signals, such as HMGB1, released from activated or injured cells. The IL-1R/TLR signaling mediates rapid post-translational changes in voltage- and ligand-gated ion channels that increase excitability, and transcriptional changes in genes involved in neurotransmission and synaptic plasticity that contribute to lower seizure thresholds chronically. The anticonvulsant effects of inhibitors of the IL-1R/TLR signaling in various seizures models suggest that this system could be targeted to inhibit seizures in presently pharmaco-resistant epilepsies.

Interleukin-1 type 1 receptor/Toll-like receptor signalling in epilepsy: the importance of IL-1beta and high-mobility group box 1

Inflammatory processes in brain tissue have been described in human epilepsy of various aetiologies and in experimental models of seizures. This, together with the anticonvulsant properties of anti-inflammatory therapies both in clinical and in experimental settings, highlights the important role of brain inflammation in the aetiopathogenesis of seizures. Preclinical investigations in experimental models using pharmacological and genetic tools have identified a significant contribution of interleukin-1 (IL-1) type 1 receptor/Toll-like receptor (IL-1R/TLR) signalling to seizure activity. This signalling can be activated by ligands associated with infections (pathogen-associated molecular patterns) or by endogenous molecules, such as proinflammatory cytokines (e.g. IL-1beta) or danger signals [damage-associated molecular patterns, e.g. high-mobility group box 1 (HMGB1)]. IL-1beta and HMGB1 are synthesized and released by astrocytes and microglia in the rodent brain during seizures. Notably, a rapid release of HMGB1 from neurons appears to be triggered by proconvulsant drugs even before seizure occurrence and is involved in their precipitation of seizures. The activation of IL-1R/TLR signalling mediates rapid post-translational changes in N-methyl-d-aspartate-gated ion channels in neurons. A long-term decrease in seizure threshold has also been observed, possibly mediated by transcriptional activation of genes contributing to molecular and cellular plasticity. This emerging evidence identifies specific targets with potential anticonvulsant effects in drug-resistant forms of epilepsy.

Pathogenesis of Y. enterocolitica and Y. pseudotuberculosis in Human Yersiniosis

Yersiniosis is a food-borne illness that has become more prevalent in recent years due to human transmission via the fecal-oral route and prevalence in farm animals. Yersiniosis is primarily caused by Yersinia enterocolitica and less frequently by Yersinia pseudotuberculosis. Infection is usually characterized by a self-limiting acute infection beginning in the intestine and spreading to the mesenteric lymph nodes. However, more serious infections and chronic conditions can also occur, particularly in immunocompromised individuals. Y. enterocolitica and Y. pseudotuberculosis are both heterogeneous organisms that vary considerably in their degrees of pathogenicity, although some generalizations can be ascribed to pathogenic variants. Adhesion molecules and a type III secretion system are critical for the establishment and progression of infection. Additionally, host innate and adaptive immune responses are both required for yersiniae clearance. Despite the ubiquity of enteric Yersinia species and their association as important causes of food poisoning world-wide, few national enteric pathogen surveillance programs include the yersiniae as notifiable pathogens. Moreover, no standard exists whereby identification and reporting systems can be effectively compared and global trends developed. This review discusses yersinial virulence factors, mechanisms of infection, and host responses in addition to the current state of surveillance, detection, and prevention of yersiniosis.

Autophagy: cellular defense to excessive inflammation

Autophagy can orchestrate a variety of cellular responses to dangerous stimuli. Our understanding of the physiologic roles of autophagy has recently expanded; in addition to its other roles, autophagy now appears to play an essential role in regulating inflammatory responses. This review describes recent findings concerning the roles and mechanisms of autophagy in controlling excessive inflammation.

A functional role for Nlrp6 in intestinal inflammation and tumorigenesis

The nucleotide-binding oligomerization domain-like receptor (NLR) family member, Nlrp6, has been implicated in inflammasome signaling to activate caspase-1, which is essential for the production of mature IL-1β and IL-18. However, a function for Nlrp6 in vivo has never been demonstrated. Due to the relative high expression of Nlrp6 in intestinal tissue, we hypothesized that Nlrp6 has a role in intestinal homeostasis. Indeed, Nlrp6-deficient mice are more susceptible to chemically induced colitis as well as colitis-induced tumorigenesis than wild-type (WT) mice. Nlrp6-deficient mice exhibited significantly more inflammation within the colon than WT mice after dextran sulfate sodium treatment. Their inability to resolve inflammation and repair damaged epithelium as efficiently as WT mice resulted in prolonged increases in epithelial proliferative activity that likely underlie the increased propensity for tumors in these mice during chronic inflammation. We further show that the activity of Nlrp6 in hematopoietic cells is critical for protection against inflammation-related colon tumorigenesis. This study highlights the importance of NLR function in maintaining intestinal homeostasis to prevent the development of aberrant inflammation and tumor development within the colon.