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

Targeting the inflammasome in rheumatic diseases

Activation of the inflammasome, a protein complex responsible for many cellular functions, including the activation of the proinflammatory cytokines interleukin (IL)-1β and IL-18, has been identified as a key participant in many rheumatic diseases including autoimmune, inflammatory, and autoinflammatory syndromes. This review will discuss the recent advances in understanding the role of this complex in various rheumatic diseases. Furthermore, it will focus on available therapies, which directly and indirectly target the inflammasome and its downstream cytokines to quiet inflammation and possibly dampen autoimmune processes.

NLRP3 and ASC suppress lupus-like autoimmunity by driving the immunosuppressive effects of TGF-β receptor signalling

OBJECTIVES

The NLRP3/ASC inflammasome drives host defence and autoinflammatory disorders by activating caspase-1 to trigger the secretion of mature interleukin (IL)-1β/IL-18, but its potential role in autoimmunity is speculative.

METHODS

We generated and phenotyped Nlrp3-deficient, Asc-deficient, Il-1r-deficient and Il-18-deficient C57BL/6-lpr/lpr mice, the latter being a mild model of spontaneous lupus-like autoimmunity.

RESULTS

While lack of IL-1R or IL-18 did not affect the C57BL/6-lpr/lpr phenotype, lack of NLRP3 or ASC triggered massive lymphoproliferation, lung T cell infiltrates and severe proliferative lupus nephritis within 6 months, which were all absent in age-matched C57BL/6-lpr/lpr controls. Lack of NLRP3 or ASC increased dendritic cell and macrophage activation, the expression of numerous proinflammatory mediators, lymphocyte necrosis and the expansion of most T cell and B cell subsets. In contrast, plasma cells and autoantibody production were hardly affected. This unexpected immunosuppressive effect of NLRP3 and ASC may relate to their known role in SMAD2/3 phosphorylation during tumour growth factor (TGF)-β receptor signalling, for example, Nlrp3-deficiency and Asc-deficiency significantly suppressed the expression of numerous TGF-β target genes in C57BL/6-lpr/lpr mice and partially recapitulated the known autoimmune phenotype of Tgf-β1-deficient mice.

CONCLUSIONS

These data identify a novel non-canonical immunoregulatory function of NLRP3 and ASC in autoimmunity.

Epidermal injury promotes nephritis flare in lupus-prone mice

Systemic lupus erythematosus is clinically characterized by episodes of flare and remission. In patients, cutaneous exposure to ultraviolet light has been proposed as a flare trigger. However, induction of flare secondary to cutaneous exposure has been difficult to emulate in many murine lupus models. Here, we describe a system in which epidermal injury is able to trigger the development of a lupus nephritis flare in New Zealand Mixed (NZM) 2328 mice. 20-week old NZM2328 female mice underwent removal of the stratum corneum via duct tape, which resulted in rapid onset of proteinuria and death when compared to sham-stripped littermate control NZM2328 mice. This was coupled with a drop in serum C3 concentrations and dsDNA antibody levels and enhanced immune complex deposition in the glomeruli. Recruitment of CD11b(+)CD11c(+)F4/80(high) macrophages and CD11b(+)CD11c(+)F4/80(low) dendritic cells was noted prior to the onset of proteinuria in injured mice. Transcriptional changes within the kidney suggest a burst of type I IFN-mediated and inflammatory signaling which is followed by upregulation of CXCL13 following epidermal injury. Thus, we propose that tape stripping of lupus-prone NZM2328 mice is a novel model of lupus flare induction that will allow for the study of the role of cutaneous inflammation in lupus development and how crosstalk between dermal and systemic immune systems can lead to lupus flare.

Lupus nephritis: glycogen synthase kinase 3β promotion of renal damage through activation of the NLRP3 inflammasome in lupus-prone mice

OBJECTIVE

Glycogen synthase kinase 3β (GSK-3β) has been demonstrated to be involved in immune and inflammatory responses via multiple signaling pathways, leading to the production of proinflammatory cytokines. The purpose of this study was to investigate the role of GSK-3β in the pathogenesis of lupus nephritis in 2 mouse models.

METHODS

Thiadiazolidinone 8 (TDZD-8), a selective inhibitor of GSK-3β, was administered intraperitoneally to 12-week-old MRL/lpr mice for 8 weeks or to 22-week-old (NZB × NZW)F1 mice for 12 weeks. The expression of GSK-3β and NLRP3 inflammasome components was analyzed. Proteinuria, biochemical parameters, proinflammatory cytokines, anti-double-stranded DNA (anti-dsDNA) antibody levels, and renal pathology were examined. In vitro, the effect of GSK-3β-directed small interfering RNA (siRNA) on NLRP3 inflammasome activation was evaluated in bone marrow-derived macrophages (BMMs) from the mice and in the J774A.1 macrophage cell line.

RESULTS

The incidence of severe proteinuria and renal inflammation was significantly attenuated in both models, with a significant reduction in anti-dsDNA antibody production, immune complex deposition in the kidney, and circulating proinflammatory cytokine levels. TDZD-8 inhibited the activation of GSK-3β and caspase 1, with a concomitant decrease in interleukin-1β (IL-1β) synthesis. In vitro, GSK-3β siRNA transfection of mouse BMMs and the J774A.1 cell line with GSK-3β siRNA inhibited the expression of GSK-3β, the activation of caspase 1, and the production of IL-1β.

CONCLUSION

These results show that GSK-3β promotes lupus nephritis at least partly by activating the NLRP3/IL-1β pathway. The linking of GSK-3β to the NLRP3/IL-1β pathway is a novel observation in our study. Our results suggest that the GSK-3β/NLRP3/IL-1β pathway may be a potential therapeutic target for lupus in humans.

The inflammasome and lupus: another innate immune mechanism contributing to disease pathogenesis?

PURPOSE OF REVIEW

The role of innate immunity in systemic lupus erythematosus (SLE) has been a rapidly expanding area of research over the last decade. Included in this rubric is the concept that activation of the inflammasome, a molecular complex that activates caspase-1 and in turn the cytokines IL-1β and IL-18, is important in lupus pathogenesis. This review will summarize the recent discoveries exploring the role of the inflammasome machinery in SLE.

RECENT FINDINGS

Immune complexes can activate the NLRP3 inflammasome, and SLE-derived macrophages are hyper-responsive to innate immune stimuli, leading to enhanced activation of the inflammasome and production of inflammatory cytokines. Work in several murine models suggests an important role for the NLRP3 inflammasome in mediating lupus nephritis. Caspase-1, the central enzyme of the inflammasome, is essential for the development of type I interferon responses, autoantibody production, and nephritis in the pristane model of lupus. The absence of melanoma 2 inflammasome may have protective and pathogenic roles in SLE.

SUMMARY

Recent evidence suggests that the inflammasome machinery is dysregulated in SLE, plays an important role in promotion of organ damage, and may mediate cross-talk between environmental triggers and the development of lupus. Further research should focus on whether inhibition of inflammasome components may serve as a viable target for therapeutic development in SLE.

High-mobility group box 1 inhibition alleviates lupus-like disease in BXSB mice

High-mobility group box 1 protein (HMGB1), a ubiquitous nuclear DNA-binding protein, functions as a potent proinflammatory factor. In this study, we evaluated the effects of HMGB1 inhibition on murine lupus using the lupus-prone model. We treated male BXSB mice with neutralizing anti-HMGB1 monoclonal antibody (HMGB1 mAb) from age 16 weeks to 26 weeks. The control group received the same amount of control IgG. Lupus-prone male BXSB mice treated with HMGB1mAb showed attenuated proteinuria, glomerulonephritis, circulating anti-dsDNA and immune complex deposition. Levels of serum IL-1β, IL-6, IL-17 and IL-18 were also significantly decreased by administration of HMGB1mAb in lupus-prone BXSB mice. HMGB1mAb treatment also decreased the caspase-1 activity in the kidneys of BXSB mice and reduced the mouse mortality. Our study supports that HMGB1 inhibition alleviates lupus-like disease in BXSB mice and might be a potential treatment option for human SLE.

Human monocytes have increased IFN-γ-mediated IL-15 production with age alongside altered IFN-γ receptor signaling

IL-15 is involved in regulating host defense and inflammation. Monocytes produce the biologically active cell surface IL-15 in response to IFN-γ. Although aging can alter the immune system, little is known about whether and how aging affects IFN-γ-mediated IL-15 production in human monocytes. We showed that monocytes of healthy older adults (age ≥ 65) had increased cell surface IL-15 expression in response to IFN-γ compared to those of healthy young adults (age ≤ 40). This finding stems in part from increased IFN-γ receptor (R)1/2 expression on monocytes in older adults, leading to enhanced STAT1 activation and interferon regulatory factor 1 synthesis with increased IL15 gene expression. Our study suggests that with aging the IFN-γ-mediated IL-15 production pathway in human monocytes is uncompromised, but rather augmented, and could be considered as a therapeutic target point to modulate host defense and inflammation in older adults.

Autoimmunity in 2012

The initiation and perpetuation of autoimmunity recognize numerous checkpoints, from the genomic susceptibility to the breakdown of tolerance. This latter phenomenon includes the loss of B cell anergy and T regulatory cell failure, as well as the production of autoantibodies and autoreactive T cells. These mechanisms ultimately lead to tissue injury via different mechanisms that span from the production of proinflammatory cytokines to the chemotaxis of immune cells to the target sites. The pathways to autoimmunity have been widely investigated over the past year and resulted in a number of articles in peer-reviewed journals that has increased by nearly 10 % compared to 2011. We herein follow on the attempt to provide a brief discussion of the majority of articles on autoimmune diseases that were published in the major immunology journals in the previous solar year. The selection is necessarily arbitrary and may thus not be seen as comprehensive but reflects current research trends. Indeed, 2012 articles were mostly dedicated to define new and old mechanisms with potential therapeutic implications in autoimmunity in general, though based on specific clinical conditions or animal models. As paradigmatic examples, the environmental influence on autoimmunity, Th17 changes modulating the autoimmune response, serum autoantibodies and B cell changes as biomarkers and therapeutic targets were major issues addressed by experimental articles in 2012. Further, a growing number of studies investigated the sex bias of autoimmunity and supported different working hypotheses to explain the female predominance, including sex chromosome changes and reproductive life factors. In conclusion, the resulting scenario illustrates that common factors may underlie different autoimmune diseases and this is well represented by the observed alterations in interferon-α and TGFβ or by the shared signaling pathways.

An essential role of caspase 1 in the induction of murine lupus and its associated vascular damage

OBJECTIVE

Systemic lupus erythematosus (SLE) is a systemic autoimmune syndrome associated with organ damage and an elevated risk of cardiovascular disease resulting from activation of both innate and adaptive immune pathways. Recently, increased activation of the inflammasome machinery in SLE has been described. Using the mouse model of pristane-induced lupus, we undertook this study to explore whether caspase 1, the central enzyme of the inflammasome, plays a role in the development of SLE and its associated vascular dysfunction.

METHODS

Eight-week-old wild-type (WT) or caspase 1(-/-) mice were injected intraperitoneally with phosphate buffered saline or pristane. Six months after injection, mice were euthanized, and the development of a lupus phenotype and vascular dysfunction was assessed.

RESULTS

While WT mice exposed to pristane developed autoantibodies and a strong type I interferon response, mice lacking caspase 1 were significantly protected against these features as well as against pristane-induced vascular dysfunction. Further, the development of immune complex glomerulonephritis, which was prominent after pristane exposure in WT mice, was significantly abrogated in caspase 1(-/-) mice.

CONCLUSION

These results indicate that caspase 1 is an essential component in the development of lupus and its associated vascular dysfunction and that it may play an important role in the cross-talk between environmental exposures and autoimmunity development, thus identifying a novel pathway for therapeutic targeting.

P2X7 blockade attenuates murine lupus nephritis by inhibiting activation of the NLRP3/ASC/caspase 1 pathway

OBJECTIVE

The NLRP3 inflammasome plays key roles in inflammation and autoimmunity, and purinergic receptor P2X7 has been proposed to be upstream of NLRP3 activation. The aim of the present study, using murine models, was to investigate whether the P2X7 /NLRP3 inflammasome pathway contributes to the pathogenesis of lupus nephritis (LN).

METHODS

MRL/lpr mice were treated with the selective P2X7 antagonist brilliant blue G (BBG) for 8 weeks. Following treatment, the severity of renal lesions, production of anti-double-stranded DNA (anti-dsDNA) antibodies, rate of survival, activation of the NLRP3/ASC/caspase 1 inflammasome pathway, and ratio of Th17 cells to Treg cells were evaluated. P2X7 -targeted small interfering RNA (siRNA) was also used for in vivo intervention. Similar evaluations were carried out in NZM2328 mice, a model of LN in which the disease was accelerated by administration of adenovirus-expressing interferon-α (AdIFNα).

RESULTS

Significant up-regulation of P2X7 /NLRP3 inflammasome signaling molecules was detected in the kidneys of MLR/lpr mice as compared with normal control mice. Blockade of P2X7 activation by BBG suppressed NLRP3/ASC/caspase 1 assembly and the subsequent release of interleukin-1β (IL-1β), resulting in a significant reduction in the severity of nephritis and circulating anti-dsDNA antibodies. The lifespan of the treated mice was significantly prolonged. BBG treatment reduced the serum levels of IL-1β and IL-17 and the Th17:Treg cell ratio. Similar results were obtained by specific siRNA silencing of P2X7 in vivo. The effectiveness of BBG treatment in modulating LN was confirmed in NZM2328 mice with AdIFNα-accelerated disease.

CONCLUSION

Activation of the P2X7 signaling pathway accelerates murine LN by activating the NLRP3/ASC/caspase 1 inflammasome, resulting in increased IL-1β production and enhanced Th17 cell polarization. Thus, targeting of the P2X7 /NLRP3 pathway should be considered as a novel therapeutic strategy in patients with lupus.

Canonical and non-canonical effects of the NLRP3 inflammasome in kidney inflammation and fibrosis

NLRP-3 inflammasome is one of several intracellular danger recognition platforms that integrates infectious or non-infectious types of danger into the expression of pro-inflammatory cytokines to set-up inflammation for danger control. NLRP3 activation induces three types of caspase-1-mediated responses: secretion of IL-1beta, secretion of IL-18 and a programmed form of cell death, referred to as pyroptosis. Similar to the well-documented impact of Toll-like receptor-driven danger signalling in kidney disease, evolving data now suggest a similar involvement of the NLRP3 inflammasome in renal inflammation. Here, we discuss the accumulating data on NLRP3 in the kidney: its IL-1beta and IL-18-dependent 'canonical' effects and the current evidence for its 'non-canonical' effects, e.g. in tumor growth factor (TGF)-beta signalling, epithelial-mesenchymal transition and fibrosis. Research in this area will certainly uncover yet unknown aspects of danger signalling in the kidney and how it drives renal inflammation and immunopathology.

The inflammasome: pathways linking psychological stress, depression, and systemic illnesses

Stress is a common occurrence in everyday life and repeated or traumatic stress can be a precipitating factor for illnesses of the central nervous system, as well as peripheral organ systems. For example, severe or long-term psychological stress can not only induce depression, a leading illness worldwide, but can also cause psychosomatic diseases such as asthma and rheumatoid arthritis. Related key questions include how psychological stress influences both brain and peripheral systems, and what detection mechanisms underlie these effects? A clue is provided by the discovery of the pathways underlying the responses to host "danger" substances that cause systemic diseases, but can also contribute to depression. The inflammasome is a protein complex that can detect diverse danger signals and produce the accompanying immune-inflammatory reactions. Interestingly, the inflammasome can detect not only pathogen-associated molecules, but also cell damage-associated molecules such as ATP. Here, we propose a new inflammasome hypothesis of depression and related comorbid systemic illnesses. According to this hypothesis, the inflammasome is a central mediator by which psychological and physical stressors can contribute to the development of depression, and as well as a bridge to systemic diseases. This hypothesis includes an explanation for how psychological stress can influence systemic diseases, and conversely how systemic diseases can lead to psychiatric illnesses. The evidence suggests that the inflammasome may be a new target for the development of treatments for depression, as well as psychosomatic and somato-psycho diseases.

NLRP3 inflammasome activation induced by engineered nanomaterials

Engineered nanomaterials (ENMs) continue to attract significant attention because they have novel physicochemical properties that can improve the functions of products that will benefit human lives. However, the physicochemical properties that make ENMs attractive could interact with biological systems and induce cascades of events that cause toxicological effects. Recently, there have been more studies suggesting inflammasome activation may play an important role in ENM-induced biological responses. Inflammasomes are a family of multiprotein complexes that are increasingly recognized as major mediators of the host immune system. Among these, NLRP3 inflammasome is the most studied that could directly interact with ENMs to generate inflammatory responses. In this review, the ENM physicochemical properties are linked to NLRP3 inflammasome activation. An understanding of the mechanisms of ENM-NLRP3 inflammasome interactions will provide us with strategies for safer nanomaterial design and therapy.

Self double-stranded (ds)DNA induces IL-1β production from human monocytes by activating NLRP3 inflammasome in the presence of anti-dsDNA antibodies

The pathogenic hallmark of systemic lupus erythematosus is the autoimmune response against self nuclear Ags, including dsDNA. The increased expression of the proinflammatory cytokine IL-1β has been found in the cutaneous lesion and PBMCs from lupus patients, suggesting a potential involvement of this cytokine in the pathogenesis of lupus. IL-1β is produced primarily by innate immune cells such as monocytes and can promote a Th17 cell response, which is increased in lupus. IL-1β production requires cleaving pro-IL-β into IL-1β by the caspase-1-associated multiprotein complex called inflammasomes. In this study we show that self dsDNA induces IL-1β production from human monocytes dependent on serum or purified IgG containing anti-dsDNA Abs by activating the nucleotide-binding oligomerization domain-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome. Reactive oxygen species (ROS) and K(+) efflux were involved in this activation. Knocking down the NLRP3 or inhibiting caspase-1, ROS, and K(+) efflux decreased IL-1β production. Supernatants from monocytes treated with a combination of self dsDNA and anti-dsDNA Ab(+) serum promoted IL-17 production from CD4(+) T cells in an IL-1β-dependent manner. These findings provide new insights in lupus pathogenesis by demonstrating that self dsDNA together with its autoantibodies induces IL-1β production from human monocytes by activating the NLRP3 inflammasome through inducing ROS synthesis and K(+) efflux, leading to the increased Th17 cell response.