Ribotoxic stress through p38 mitogen-activated protein kinase activates in vitro the human pyrin inflammasome

Complex regulation of alarmins S100A8/A9 and secretion via gasdermin D pores exacerbates autoinflammation in familial Mediterranean fever

BACKGROUND

Familial Mediterranean fever (FMF), caused by mutations in the pyrin-encoding MEFV gene, is characterized by uncontrolled caspase-1 activation and IL-1β secretion. A similar mechanism drives inflammation in cryopyrin-associated periodic fever syndrome (CAPS) caused by mutations in NLRP3. CAPS and FMF, however, result in largely different clinical manifestations, pointing to additional, autoinflammatory pathways involved in FMF. Another hallmark of FMF is extraordinarily high expression of S100A8 and S100A9. These alarmins are ligands of Toll-like receptor 4 and amplifiers of inflammation. However, the relevance of this inflammatory pathway for the pathogenesis of FMF is unknown.

OBJECTIVE

This study investigated whether mutations in pyrin result in specific secretion of S100A8/A9 alarmins through gasdermin D pores' amplifying FMF pathology.

METHODS

S100A8/A9 levels in FMF patients were quantified by enzyme-linked immunosorbent assay. In vitro models with knockout cell lines and specific protein inhibitors were used to unravel the S100A8/A9 secretion mechanism. The impact of S100A8/A9 to the pathophysiology of FMF was analyzed with FMF (MEFVV726A/V726A) and S100A9-/- mouse models. Pyrin-S100A8/A9 interaction was investigated by coimmunoprecipitation, immunofluorescence, and enzyme-linked immunosorbent assay studies.

RESULTS

The S100A8/A9 complexes directly interacted with pyrin. Knocking out pyrin, caspase-1, or gasdermin D inhibited the secretion of these S100 alarmins. Inflammatory S100A8/A9 dimers were inactivated by tetramer formation. Blocking this inactivation by targeted S100A9 deletion in a murine FMF model demonstrated the relevance of this novel autoinflammatory pathway in FMF.

CONCLUSION

This is the first proof that members of the S100 alarmin family are released in a pyrin/caspase-1/gasdermin D-dependent pathway and directly drive autoinflammation in vivo.

Inducing Pyroptosis Via the Pyrin Inflammasome

The pyrin inflammasome detects bacterial toxins and effectors that inhibit RhoA GTPases and triggers inflammatory cytokine release and a fast cell death termed pyroptosis. In addition, various endogenous molecules, drugs, synthetic molecules, or mutations can trigger pyrin inflammasome activation. The pyrin protein differs between humans and mice, and the repertoire of pyrin activators is also species-specific. Here, we present the various pyrin inflammasome activators, inhibitors, the kinetics of pyrin activation in response to the various activators, and their species specificity. In addition, we present different methods to monitor pyrin-mediated pyroptosis.

Alpinetin Suppresses Zika Virus-Induced Interleukin-1β Production and Secretion in Human Macrophages

Zika virus (ZIKV) infection has been recognized to cause adverse sequelae in the developing fetus. Specially, this virus activates the excessive release of IL-1β causing inflammation and altered physiological functions in multiple organs. Although many attempts have been invested to develop vaccine, antiviral, and antibody therapies, development of agents focusing on limiting ZIKV-induced IL-1β release have not gained much attention. We aimed to study the effects of alpinetin (AP) on IL-1β production in human macrophage upon exposure to ZIKV. Our study demonstrated that ZIKV stimulated IL-1β release in the culture supernatant of ZIKV-infected cells, and AP could effectively reduce the level of this cytokine. AP exhibited no virucidal activities against ZIKV nor caused alteration in viral production. Instead, AP greatly inhibited intracellular IL-1β synthesis. Surprisingly, this compound did not inhibit ZIKV-induced activation of NF-κB and its nuclear translocation. However, AP could significantly inhibit ZIKV-induced p38 MAPK activation without affecting the phosphorylation status of ERK1/2 and JNK. These observations suggest the possibility that AP may reduce IL-1β production, in part, through suppressing p38 MAPK signaling. Our current study sheds light on the possibility of using AP as an alternative agent for treating complications caused by ZIKV infection-induced IL-1β secretion.

Xenobiotic-induced ribosomal stress compromises dysbiotic gut barrier aging: A one health perspective

Upon exposure to internal or environmental insults, ribosomes stand sentinel. In particular, stress-driven dysregulation of ribosomal homeostasis is a potent trigger of adverse outcomes in mammalians. The present study assessed whether the ribosomal insult affects the aging process via the regulation of sentinel organs such as the gut. Analyses of the human aging dataset demonstrated that elevated features of ribosomal stress are inversely linked to barrier maintenance biomarkers during the aging process. Ribosome-insulted worms displayed reduced lifespan, which was associated with the disruption of gut barriers. Mechanistically, ribosomal stress-activated Sek-1/p38 signaling, a central platform of ribosomal stress responses, counteracted the gut barrier deterioration through the maintenance of the gut barrier, which was consistent with the results in a murine insult model. However, since the gut-protective p38 signaling was attenuated with aging, the ribosomal stress-induced distress was exacerbated in the gut epithelia and mucosa of the aged animals, subsequently leading to increased bacterial exposure. Moreover, the bacterial community-based evaluation predicted concomitant increases in the abundance of mucosal sugar utilizers and mucin metabolic enzymes in response to ribosomal insult in the aged host. All of the present evidence on ribosomal insulting against the gut barrier integrity from worms to mammals provides new insights into organelle-associated translational modulation of biological longevity in a one health perspective.

The therapeutic potential of colchicine in the complications of COVID19. Could the immunometabolic properties of an old and cheap drug help?

The present study analyzes the importance of the inflammasome that involves the NLRP3 complex in the state of hypercytokinemia observed in patients with COVID-19, significantly increasing IL-1β, IL18, IL-6, and TNF. Unfortunately, improving the immune response can sometimes worsen the outcome of the disease. Studies show that colchicine, among other actions, inhibits the assembly of NLRP3 complex that is responsible for generating the active form of Caspase-1 that will convert Pro-IL-1β and Pro-IL-18 into their active forms. We suggest using colchicine, a class of drugs with low-cost, extensively tested, well-tolerated medicine as a complementary treatment for patients with COVID-19, in early stages of the disease based on knowledge of its immunomodulatory properties.

The pyrin inflammasome in host-microbe interactions

Pyrin is an inflammasome sensor in phagocytes that is activated in response to bacterial toxins and effectors that modify RhoA. Pathogen effector-triggered pyrin activation is analogous to an indirect guard mechanism in plants. Pyrin activation appears to be triggered when RhoA GTPases in a host cell are prevented from binding downstream signaling proteins (transducers). RhoA transducers that control this response include PRK kinases, which negatively regulate pyrin by phosphorylation and binding of 14-3-3 proteins. Microtubules regulate pyrin at different levels and may serve as a platform for inflammasome nucleation. Pyrin increases inflammation in the lung, gut or systemically during infection or intoxication in mouse models and protects against systemic infection by decreasing bacterial loads. Pathogenic Yersinia spp. overcome this protective response using effectors that inhibit the pyrin inflammasome. Gain of function mutations in MEFV, the gene encoding pyrin, cause the autoinflammatory disease Familial Mediterranean Fever. Yersinia pestis may have selected for gain of function MEFV mutations in the human population.

Fosfomycin Protects Mice From Staphylococcus aureus Pneumonia Caused by α-Hemolysin in Extracellular Vesicles by Inhibiting MAPK-Regulated NLRP3 Inflammasomes

α-Hemolysin (Hla) is a significant virulence factor in Staphylococcus aureus (S. aureus)-caused infectious diseases such as pneumonia. Thus, to prevent the production of Hla when treating S. aureus infection, it is necessary to choose an antibiotic with good antibacterial activity and effect. In our study, we observed that Fosfomycin (FOM) at a sub-inhibitory concentration inhibited expression of Hla. Molecular dynamics demonstrated that FOM bound to the binding sites LYS 154 and ASP 108 of Hla, potentially inhibiting Hla. Furthermore, we verified that staphylococcal membrane-derived vesicles (SMVs) contain Hla and that FOM treatment significantly reduced the production of SMVs and Hla. Based on our pharmacological inhibition analysis, ERK and p38 activated NLRP3 inflammasomes. Moreover, FOM inhibited expression of MAPKs and NLRP3 inflammasome-related proteins in S. aureus as well as SMV-infected human macrophages (MΦ) and alveolar epithelial cells. In vivo, SMVs isolated from S. aureus DU1090 (an isogenic Hla deletion mutant) or the strain itself caused weaker inflammation than that of its parent strain 8325-4. FOM also significantly reduced the phosphorylation levels of ERK and P38 and expression of NLRP3 inflammasome-related proteins. In addition, FOM decreased MPO activity, pulmonary vascular permeability and edema formation in the lungs of mice with S. aureus-caused pneumonia. Taken together, these data indicate that FOM exerts protective effects against S. aureus infection in vitro and in vivo by inhibiting Hla in SMVs and blocking ERK/P38-mediated NLRP3 inflammasome activation by Hla.

Familial Mediterranean fever mutations are hypermorphic mutations that specifically decrease the activation threshold of the Pyrin inflammasome

Objectives

FMF is the most frequent autoinflammatory disease and is associated in most patients with bi-allelic MEFV mutations. MEFV encodes Pyrin, an inflammasome sensor activated following RhoGTPase inhibition. The functional consequences of MEFV mutations on the ability of Pyrin variants to act as inflammasome sensors are largely unknown. The aim of this study was to assess whether MEFV mutations affect the ability of Pyrin to detect RhoGTPase inhibition and other inflammasome stimuli.

Methods

IL-1β and IL-18 released by monocytes from healthy donors (HDs) and FMF patients were measured upon specific engagement of the Pyrin, NLRP3 and NLRC4 inflammasomes. Cell death kinetics following Pyrin activation was monitored in real time.

Results

Monocytes from FMF patients secreted significantly more IL-1β and IL-18 and died significantly faster than HD monocytes in response to low concentrations of Clostridium difficile toxin B (TcdB), a Pyrin-activating stimulus. Monocytes from patients bearing two MEFV exon 10 pathogenic variants displayed an increased Pyrin inflammasome response compared with monocytes from patients with a single exon 10 pathogenic variant indicating a gene-dosage effect. Using a short priming step, the response of monocytes from FMF patients to NLRP3- and NLRC4-activating stimuli was normal indicating that MEFV mutations trigger a specific hypersensitivity of monocytes to low doses of a Pyrin-engaging stimulus.

Conclusion

Contrary to the NLRP3 mutations described in cryopyrin-associated periodic syndrome, FMF-associated MEFV mutations do not lead to a constitutive activation of Pyrin. Rather, FMF-associated mutations are hypermorphic mutations that specifically decrease the activation threshold of the Pyrin inflammasome without affecting other canonical inflammasomes.

Alarming consequences - autoinflammatory disease spectrum due to mutations in proline-serine-threonine phosphatase-interacting protein 1

PURPOSE OF REVIEW

To give an overview about the expanding spectrum of autoinflammatory diseases due to mutations in proline-serine-threonine phosphatase-interacting protein 1 (PSTPIP1) and new insights into their pathogenesis.

RECENT FINDINGS

In addition to classical pyogenic sterile arthritis, pyoderma gangrenosum, and acne (PAPA) syndrome, PSTPIP1-associated myeloid-related proteinemia inflammatory (PAMI) syndrome has been described as a distinct clinical phenotype of PSTPIP1-associated inflammatory diseases (PAID) and other entities are emerging. In addition to dysregulation of IL-1ß release from activated PAPA monocytes that requires NLR family, pyrin domain containing 3 (NLRP3), PSTPIP1 mutations have an general impact on cellular dynamics of cells of the innate immune system. In addition, overwhelming expression and release of the alarmins myeloid-related protein (MRP) 8 and 14 by activated phagocytes and keratinocytes, which promote innate immune mechanisms in a Toll like receptor (TLR) 4-dependent manner, are a characteristic feature of these diseases and form a positive feed-back mechanism with IL-1ß.

SUMMARY

Autoinflammatory diseases due to PSTPIP1 mutations are not restricted to the classical PAPA phenotype but might present with other distinct clinical features. MRP8/14 serum levels are a hallmark of PAPA and PAMI and can be used as screening tool to initiate targeted genetic testing in suspected cases. The feedback mechanism of IL-1ß and MRP-alarmin release may offer novel targets for future therapeutic approaches.

Posttranslational Modification as a Critical Determinant of Cytoplasmic Innate Immune Recognition

Cell surface innate immune receptors can directly detect a variety of extracellular pathogens to which cytoplasmic innate immune sensors are rarely exposed. Instead, within the cytoplasm, the environment is rife with cellular machinery and signaling pathways that are indirectly perturbed by pathogenic microbes to activate intracellular sensors, such as pyrin, NLRP1, NLRP3, or NLRC4. Therefore, subtle changes in key intracellular processes such as phosphorylation, ubiquitination, and other pathways leading to posttranslational protein modification are key determinants of innate immune recognition in the cytoplasm. This concept is critical to establish the “guard hypothesis” whereby otherwise homeostatic pathways that keep innate immune sensors at bay are released in response to alterations in their posttranslational modification status. Originally identified in plants, evidence that a similar guardlike mechanism exists in humans has recently been identified, whereby a mutation that prevents phosphorylation of the innate immune sensor pyrin triggers a dominantly inherited autoinflammatory disease. It is also noteworthy that even when a cytoplasmic innate immune sensor has a direct ligand, such as bacterial peptidoglycan (NOD1 or NOD2), RNA (RIG-I or MDA5), or DNA (cGAS or IFI16), it can still be influenced by posttranslational modification to dramatically alter its response. Therefore, due to their existence in the cytoplasmic milieu, posttranslational modification is a key determinant of intracellular innate immune receptor functionality.

Bacterial secretion systems and regulation of inflammasome activation

Innate immunity is critical for host defenses against pathogens, but many bacteria display complex ways of interacting with innate immune signaling, as they may both activate and evade certain pathways. Gram-negative bacteria can exhibit specialized nanomachine secretion systems for delivery of effector proteins into mammalian cells. Bacterial types III, IV, and VI secretion systems (T3SS, T4SS, and T6SS) are known for their impact on caspase-1-activating inflammasomes, necessary for producing bioactive inflammatory cytokines IL-1β and IL-18, key participants of anti-bacterial responses. Here, we discuss how these secretion systems can mediate triggering and inhibition of inflammasome signaling. We propose that a fine balance between secretion system-mediated activation and inhibition can determine net activation of inflammasome activity and control inflammation, clearance, or spread of the infection.

A TPL2 (MAP3K8) disease-risk polymorphism increases TPL2 expression thereby leading to increased pattern recognition receptor-initiated caspase-1 and caspase-8 activation, signalling and cytokine secretion

OBJECTIVE

IBD is characterised by dysregulated intestinal immune homeostasis and cytokine secretion. In the intestine, properly regulating pattern recognition receptor (PRR)-mediated signalling and cytokines is crucial given the ongoing host-microbial interactions. TPL2 (MAP3K8, COT) contributes to PRR-initiated pathways, yet the mechanisms for TPL2 signalling contributions in primary human myeloid cells are incompletely understood and its role in intestinal myeloid cells is poorly defined. Furthermore, functional consequences for the IBD-risk locus rs1042058 in TPL2 are unknown.

METHODS

We analysed protein, cytokine and RNA expression, and signalling in human monocyte-derived macrophages (MDMs) through western blot, ELISA, real-time PCR and flow cytometry.

RESULTS

PRR-induced cytokine secretion was increased in MDMs from rs1042058 TPL2 GG risk individuals. TPL2 activation by the Crohn's disease-associated PRR nucleotide-oligomerisation domain (NOD)2 required PKC, and IKKβ, IKKα and IKKγ signalling. TPL2, in turn, significantly enhanced NOD2-induced ERK, JNK and NFκB signalling. We found that another major mechanism for the TPL2 contribution to NOD2 signalling was through ERK-dependent and JNK-dependent caspase-1 and caspase-8 activation, which in turn, led to early autocrine interleukin (IL)-1β and IL-18 secretion and amplification of long-term cytokines. Importantly, Salmonella typhimurium-induced cytokines from human intestinal myeloid-derived cells required TPL2 as well as autocrine IL-1β and IL-18. Finally, rs1042058 GG risk carrier MDMs from healthy individuals and patients with Crohn's disease had increased TPL2 expression and NOD2-initiated TPL2 phosphorylation, ERK, JNK and NFκB activation, and early autocrine IL-1β and IL-18 secretion.

CONCLUSIONS

Taken together, the rs1042058 GG IBD-risk polymorphism in TPL2 results in a gain-of-function by increasing TPL2 expression and signalling, thereby amplifying PRR-initiated outcomes.

Uncovering an Important Role for YopJ in the Inhibition of Caspase-1 in Activated Macrophages and Promoting Yersinia pseudotuberculosis Virulence

Pathogenic Yersinia species utilize a type III secretion system to translocate Yop effectors into infected host cells. Yop effectors inhibit innate immune responses in infected macrophages to promote Yersinia pathogenesis. In turn,Yersinia-infected macrophages respond to translocation of Yops by activating caspase-1, but different mechanisms of caspase-1 activation occur, depending on the bacterial genotype and the state of phagocyte activation. In macrophages activated with lipopolysaccharide (LPS) prior to Yersinia pseudotuberculosis infection, caspase-1 is activated by a rapid inflammasome-dependent mechanism that is inhibited by translocated YopM. The possibility that other effectors cooperate with YopM to inhibit caspase-1 activation in LPS-activated macrophages has not been investigated. Toward this aim, epistasis analysis was carried out in which the phenotype of aY. pseudotuberculosis yopM mutant was compared to that of a yopJ yopM, yopE yopM, yopH yopM, yopT yopM, or ypkA yopM mutant. Activation of caspase-1 was measured by cleavage of the enzyme, release of interleukin-1β (IL-1β), and pyroptosis in LPS-activated macrophages infected with wild-type or mutant Y. pseudotuberculosis strains. Results show enhanced activation of caspase-1 after infection with the yopJ yopM mutant relative to infection by any other single or double mutant. Similar results were obtained with the yopJ, yopM, and yopJ yopM mutants ofY ersinia pestis Following intravenous infection of mice, theY. pseudotuberculosis yopJ mutant was as virulent as the wild type, while the yopJ yopM mutant was significantly more attenuated than the yopM mutant. In summary, through epistasis analysis this work uncovered an important role for YopJ in inhibiting caspase-1 in activated macrophages and in promoting Yersinia virulence.

Single amino acid charge switch defines clinically distinct proline-serine-threonine phosphatase-interacting protein 1 (PSTPIP1)-associated inflammatory diseases

BACKGROUND

Hyperzincemia and hypercalprotectinemia (Hz/Hc) is a distinct autoinflammatory entity involving extremely high serum concentrations of the proinflammatory alarmin myeloid-related protein (MRP) 8/14 (S100A8/S100A9 and calprotectin).

OBJECTIVE

We sought to characterize the genetic cause and clinical spectrum of Hz/Hc.

METHODS

Proline-serine-threonine phosphatase-interacting protein 1 (PSTPIP1) gene sequencing was performed in 14 patients with Hz/Hc, and their clinical phenotype was compared with that of 11 patients with pyogenic arthritis, pyoderma gangrenosum, and acne (PAPA) syndrome. PSTPIP1-pyrin interactions were analyzed by means of immunoprecipitation and Western blotting. A structural model of the PSTPIP1 dimer was generated. Cytokine profiles were analyzed by using the multiplex immunoassay, and MRP8/14 serum concentrations were analyzed by using an ELISA.

RESULTS

Thirteen patients were heterozygous for a missense mutation in the PSTPIP1 gene, resulting in a p.E250K mutation, and 1 carried a mutation resulting in p.E257K. Both mutations substantially alter the electrostatic potential of the PSTPIP1 dimer model in a region critical for protein-protein interaction. Patients with Hz/Hc have extremely high MRP8/14 concentrations (2045 ± 1300 μg/mL) compared with those with PAPA syndrome (116 ± 74 μg/mL) and have a distinct clinical phenotype. A specific cytokine profile is associated with Hz/Hc. Hz/Hc mutations altered protein binding of PSTPIP1, increasing interaction with pyrin through phosphorylation of PSTPIP1.

CONCLUSION

Mutations resulting in charge reversal in the y-domain of PSTPIP1 (E→K) and increased interaction with pyrin cause a distinct autoinflammatory disorder defined by clinical and biochemical features not found in patients with PAPA syndrome, indicating a unique genotype-phenotype correlation for mutations in the PSTPIP1 gene. This is the first inborn autoinflammatory syndrome in which inflammation is driven by uncontrolled release of members of the alarmin family.

Curcumin suppresses NLRP3 inflammasome activation and protects against LPS-induced septic shock

SCOPE

The NLRP3 inflammasome responds to various pathogen-derived factors and danger-associated molecules, mediating IL-1β maturation, therefore is involved in multiple inflammatory diseases. Curcumin has been shown to possess strong anti-inflammatory activity, but the underlying mechanism is not fully understood. Here, we sought to investigate the role and mechanism of curcumin on the inhibition of mature IL-1β production via the regulation of NLRP3 inflammasome.

METHODS AND RESULTS

Curcumin dramatically inhibited the production of mature IL-1β in LPS-primed macrophages triggered by multiple NLRP3 inflammasome activators, and also reduced the level of cleaved caspase-1 as measured by western blot and ELISA. Curcumin prevented K(+) efflux, the common trigger for NLRP3 inflammasome activation, and attenuated lysosomes disruption and intracellular ROS formation as well. The inhibition of NLRP3 inflammasome by curcumin was in part mediated via the suppression of extracellular regulated protein kinases phosphorylation. Furthermore, administration of curcumin significantly reduced peritoneal IL-1β and HMGB-1 concentration induced by LPS and improved the survival of mice suffering from lethal endotoxic shock.

CONCLUSION

Curcumin potently inhibits the activation of NLRP3 inflammasome which may contribute to its anti-inflammatory activity. Our finding offers a mechanistic basis for the therapeutic potential of curcumin in septic shock and other NLRP3 inflammasome-driven diseases.

Colchicine--Update on mechanisms of action and therapeutic uses

OBJECTIVES

To review the literature and provide an update on the mechanisms of action and therapeutic uses of oral colchicine in arthritis and inflammatory conditions.

METHODS

We performed PubMed database searches through June 2014 for relevant studies in the English literature published since the last update of colchicine in 2008. Searches encompassed colchicine mechanisms of action and clinical applications in medical conditions. A total of 381 articles were reviewed.

RESULTS

The primary mechanism of action of colchicine is tubulin disruption. This leads to subsequent down regulation of multiple inflammatory pathways and modulation of innate immunity. Newly described mechanisms include various inhibitory effects on macrophages including the inhibition of the NACHT-LRRPYD-containing protein 3 (NALP3) inflammasome, inhibition of pore formation activated by purinergic receptors P2X7 and P2X2, and stimulation of dendritic cell maturation and antigen presentation. Colchicine also has anti-fibrotic activities and various effects on endothelial function. The therapeutic use of colchicine has extended beyond gouty arthritis and familial Mediterranean fever, to osteoarthritis, pericarditis, and atherosclerosis.

CONCLUSION

Further understanding of the mechanisms of action underlying the therapeutic efficacy of colchicine will lead to its potential use in a variety of conditions.

Crystal structure of TRIM20 C-terminal coiled-coil/B30.2 fragment: implications for the recognition of higher order oligomers

Many tripartite motif-containing (TRIM) proteins, comprising RING-finger, B-Box, and coiled-coil domains, carry additional B30.2 domains on the C-terminus of the TRIM motif and are considered to be pattern recognition receptors involved in the detection of higher order oligomers (e.g. viral capsid proteins). To investigate the spatial architecture of domains in TRIM proteins we determined the crystal structure of the TRIM20Δ413 fragment at 2.4 Å resolution. This structure comprises the central helical scaffold (CHS) and C-terminal B30.2 domains and reveals an anti-parallel arrangement of CHS domains placing the B-box domains 170 Å apart from each other. Small-angle X-ray scattering confirmed that the linker between CHS and B30.2 domains is flexible in solution. The crystal structure suggests an interaction between the B30.2 domain and an extended stretch in the CHS domain, which involves residues that are mutated in the inherited disease Familial Mediterranean Fever. Dimerization of B30.2 domains by means of the CHS domain is crucial for TRIM20 to bind pro-IL-1β in vitro. To exemplify how TRIM proteins could be involved in binding higher order oligomers we discuss three possible models for the TRIM5α/HIV-1 capsid interaction assuming different conformations of B30.2 domains.

Non-transcriptional regulation of NLRP3 inflammasome signaling by IL-4

Th2 cytokine IL-4 has been previously shown to suppress the production of proinflammatory cytokines in monocytes. However, the underlying molecular mechanism by which IL-4 signaling antagonizes proinflammatory responses is poorly characterized. In particular, whether IL-4 can modulate inflammasome signaling remains unknown. Here, we provide evidence that IL-4 suppresses NLRP3-dependent caspase-1 activation and the subsequent IL-1β secretion but does not inhibit absent in melanoma 2 (AIM2)- or NLRC4 (NOD-like receptor family, CARD domain-containing 4)-dependent caspase-1 activation in THP-1 and mouse bone marrow-derived macrophages. Upon lipopolysaccharide (LPS) or LPS/ATP stimulation, IL-4 markedly inhibited the assembly of NLRP3 inflammasome, including NLRP3-dependent ASC (apoptosis-associated speck-like protein containing a caspase recruitment domain) oligomerization, NLRP3-ASC interaction and NLRP3 speck-like oligomeric structure formation. The negative regulation of NLRP3 inflammasome by IL-4 was not due to the impaired mRNA or protein production of NLRP3 and proinflammatory cytokines. Supporting this observation, IL-4 attenuated NLRP3 inflammasome activation even in reconstituted NLRP3-expressing macrophages in which NLRP3 expression is not transcriptionally regulated by TLR-NF-κB signaling. Furthermore, the IL-4-mediated suppression of NLRP3 inflammasome was independent of STAT6-dependent transcription and mitochondrial reactive oxygen species (ROS). Instead, IL-4 inhibited subcellular redistribution of NLRP3 into mitochondria and microtubule polymerization upon NLRP3-activating stimulation. Our results collectively suggest that IL-4 could suppress NLRP3 inflammasome activation in a transcription-independent manner, thus providing an endogenous regulatory machinery to prevent excessive inflammasome activation.

Identification of multifaceted binding modes for pyrin and ASC pyrin domains gives insights into pyrin inflammasome assembly

Inflammasomes are macromolecular complexes that mediate inflammatory and cell death responses to pathogens and cellular stress signals. Dysregulated inflammasome activation is associated with autoinflammatory syndromes and several common diseases. During inflammasome assembly, oligomerized cytosolic pattern recognition receptors recruit procaspase-1 and procaspase-8 via the adaptor protein ASC. Inflammasome assembly is mediated by pyrin domains (PYDs) and caspase recruitment domains, which are protein interaction domains of the death fold superfamily. However, the molecular details of their interactions are poorly understood. We have studied the interaction between ASC and pyrin PYDs that mediates ASC recruitment to the pyrin inflammasome, which is implicated in the pathogenesis of familial Mediterranean fever. We demonstrate that both the ASC and pyrin PYDs have multifaceted binding modes, involving three sites on pyrin PYD and two sites on ASC PYD. Molecular docking of pyrin-ASC PYD complexes showed that pyrin PYD can simultaneously interact with up to three ASC PYDs. Furthermore, ASC PYD can self-associate and interact with pyrin, consistent with previous reports that pyrin promotes ASC clustering to form a proinflammatory complex. Finally, the effects of familial Mediterranean fever-associated mutations, R42W and A89T, on structural and functional properties of pyrin PYD were investigated. The R42W mutation had a significant effect on structure and increased stability. Although the R42W mutant exhibited reduced interaction with ASC, it also bound less to the pyrin B-box domain responsible for autoinhibition and hence may be constitutively active. Our data give new insights into the binding modes of PYDs and inflammasome architecture.

The inflammasome pyrin contributes to pertussis toxin-induced IL-1β synthesis, neutrophil intravascular crawling and autoimmune encephalomyelitis

Microbial agents can aggravate inflammatory diseases, such as multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE). An example is pertussis toxin (PTX), a bacterial virulence factor commonly used as an adjuvant to promote EAE, but whose mechanism of action is unclear. We have reported that PTX triggers an IL-6-mediated signaling cascade that increases the number of leukocytes that patrol the vasculature by crawling on its luminal surface. In the present study, we examined this response in mice lacking either TLR4 or inflammasome components and using enzymatically active and inactive forms of PTX. Our results indicate that PTX, through its ADP-ribosyltransferase activity, induces two series of events upstream of IL-6: 1) the activation of TLR4 signaling in myeloid cells, leading to pro-IL-1β synthesis; and 2) the formation of a pyrin-dependent inflammasome that cleaves pro-IL-1β into its active form. In turn, IL-1β stimulates nearby stromal cells to secrete IL-6, which is known to induce vascular changes required for leukocyte adhesion. Without pyrin, PTX does not induce neutrophil adhesion to cerebral capillaries and is less effective at inducing EAE in transgenic mice with encephalitogenic T lymphocytes. This study identifies the first microbial molecule that activates pyrin, a mechanism by which infections may influence MS and a potential therapeutic target for immune disorders.

Inflammasome priming by lipopolysaccharide is dependent upon ERK signaling and proteasome function

Caspase-1 activation is a central event in innate immune responses to many pathogenic infections and tissue damage. The NLRP3 inflammasome, a multiprotein scaffolding complex that assembles in response to two distinct steps, priming and activation, is required for caspase-1 activation. However, the detailed mechanisms of these steps remain poorly characterized. To investigate the process of LPS-mediated NLRP3 inflammasome priming, we used constitutively present pro-IL-18 as the caspase-1-specific substrate to allow study of the early events. We analyzed human monocyte caspase-1 activity in response to LPS priming, followed by activation with ATP. Within minutes of endotoxin priming, the NLRP3 inflammasome is licensed for ATP-induced release of processed IL-18, apoptosis-associated speck-forming complex containing CARD, and active caspase-1, independent of new mRNA or protein synthesis. Moreover, extracellular signal-regulated kinase 1 (ERK1) phosphorylation is central to the priming process. ERK inhibition and small interfering RNA-mediated ERK1 knockdown profoundly impair priming. In addition, proteasome inhibition prevents ERK phosphorylation and blocks priming. Scavenging reactive oxygen species with diphenylene iodonium also blocks both priming and ERK phosphorylation. These findings suggest that ERK1-mediated posttranslational modifications license the NLRP3 inflammasome to respond to the second signal ATP by inducing posttranslational events that are independent of new production of pro-IL-1β and NOD-like receptor components.

Ribosomal alteration-derived signals for cytokine induction in mucosal and systemic inflammation: noncanonical pathways by ribosomal inactivation

Ribosomal inactivation damages 28S ribosomal RNA by interfering with its functioning during gene translation, leading to stress responses linked to a variety of inflammatory disease processes. Although the primary effect of ribosomal inactivation in cells is the functional inhibition of global protein synthesis, early responsive gene products including proinflammatory cytokines are exclusively induced by toxic stress in highly dividing tissues such as lymphoid tissue and epithelia. In the present study, ribosomal inactivation-related modulation of cytokine production was reviewed in leukocyte and epithelial pathogenesis models to characterize mechanistic evidence of ribosome-derived cytokine induction and its implications for potent therapeutic targets of mucosal and systemic inflammatory illness, particularly those triggered by organellar dysfunctions.

The P2X7 receptor-inflammasome complex has a role in modulating the inflammatory response in primary Sjögren's syndrome

OBJECTIVE

Innate and adaptive immunity may contribute to gland dysfunction in patients with primary Sjögren's syndrome (pSS). The P2X7 receptor (P2X7 R)-NLRP3 inflammasome complex modulates the release of the inflammatory cytokines IL-1β and IL-18. The presence of P2X7 R in salivary glands suggests an interesting scenario for the initiation and amplification of the innate immune response in pSS. Therefore, the aim of this study was to assess the role of the P2X7 R-NLRP3 inflammasome in pSS.

SUBJECTS AND METHODS

Twenty-one consecutive patients with pSS according to the American-European Consensus Group criteria and 15 patients with sicca syndrome (i.e. without Sjögren's syndrome, non-SS) were enrolled in this study, together with six control (CTL) subjects. Expression of the P2X7R-NLRP3 platform and IL-18 was determined by real-time PCR and western blotting in gland specimens and peripheral lymphomonocytes; data were related to patients\x92 clinical, serological and histopathological characteristics. The presence of IL-18 was determined in gland and saliva samples.

RESULTS

P2X7 R expression was significantly higher in salivary glands from individuals with pSS than in those from non-SS and CTL subjects. Accordingly, the gene expression levels of the inflammasome components NLRP3, ASC and caspase-1 were significantly higher in pSS gland specimens, and this was paralleled by an increased expression of mature IL-18 in pSS saliva samples. The expression of both the P2X7 R and the inflammasome components was a marker of disease-related glandular involvement, being increased in patients with anti-Ro/SSA positivity and correlated with focus score.

CONCLUSION

The results of this study suggest an involvement of the P2X7 R-inflammasome-caspase-1-IL-18 axis in the development of pSS exocrinopathy. This finding provides the basis for studying the complex mechanisms underlying pSS, as well as for developing novel potential therapeutic strategies.