Come on mtDNA, light my fire.

Oxidized mitochondrial DNA (ox-mtDNA) activates NLRP3 inflammasome signaling through an ill-defined mechanism. In this issue of Immunity, Xian et al. reveal FEN1 endonuclease cleaves ox-mtDNA into fragments that escape mitochondria, igniting NLRP3 and cGAS-STING signaling and inflammation.

NLRC3 Restrains Responses to a T.

The NOD-like receptor protein NLRC3 attenuates myeloid cell inflammatory responses. In this issue of Immunity, Uchimura et al. (2018) reveal additional T-cell-intrinsic functions for NLRC3 in restricting T cell metabolism, T helper 1 and T helper 17 cell responses, and antiviral and autoimmune responses.

Parkinson's disease: connecting mitochondria to inflammasomes.

Activated microglia foster a neurotoxic, inflammatory environment in the mammalian central nervous system (CNS) that drives the pathology of neurodegenerative diseases including Parkinson's disease (PD). Moreover, mitochondrial fission promotes microglial inflammatory responses in vitro. Given that the NLRP3 inflammasome and mitochondria are central regulators of both inflammation and PD, we explore potential functions for the NLRP3 inflammasome and mitochondrial dynamics in PD. Specifically, we propose that inducible microglial mitochondrial fission can promote NLRP3-dependent neuroinflammation in hereditary and idiopathic PD. Further in-depth exploration of this topic can prompt valuable discoveries of the underlying molecular mechanisms of PD neuroinflammation, identify novel candidate anti-inflammatory therapeutics for PD, and ideally provide better outcomes for PD patients.

The septin modifier, forchlorfenuron, activates NLRP3 via a potassium-independent mitochondrial axis.

The Nod-like receptor protein 3 (NLRP3) inflammasome is activated by stimuli that induce perturbations in cell homeostasis, which commonly converge on cellular potassium efflux. NLRP3 has thus emerged as a sensor for ionic flux. Here, we identify forchlorfenuron (FCF) as an inflammasome activator that triggers NLRP3 signaling independently of potassium efflux. FCF triggers the rearrangement of septins, key cytoskeletal proteins that regulate mitochondrial function. We report that FCF triggered the rearrangement of SEPT2 into tubular aggregates and stimulated SEPT2-independent NLRP3 inflammasome signaling. Similar to imiquimod, FCF induced the collapse of the mitochondrial membrane potential and mitochondrial respiration. FCF thereby joins the imidazoquinolines as a structurally distinct class of molecules that triggers NLRP3 inflammasome signaling independent of potassium efflux, likely by inducing mitochondrial damage.

NLRP12 interacts with NLRP3 to block the activation of the human NLRP3 inflammasome.

Inflammasomes are multiprotein complexes that drive inflammation and contribute to protective immunity against pathogens and immune pathology in autoinflammatory diseases. Inflammasomes assemble when an inflammasome scaffold protein senses an activating signal and forms a signaling platform with the inflammasome adaptor protein ASC. The NLRP subfamily of NOD-like receptors (NLRs) includes inflammasome nucleators (such as NLRP3) and also NLRP12, which is genetically linked to familial autoinflammatory disorders that resemble diseases caused by gain-of-function NLRP3 mutants that generate a hyperactive NLRP3 inflammasome. We performed a screen to identify ASC inflammasome-nucleating proteins among NLRs that have the canonical pyrin-NACHT-LRR domain structure. Only NLRP3 and NLRP6 could initiate ASC polymerization to form "specks," and NLRP12 failed to nucleate ASC polymerization. However, wild-type NLRP12 inhibited ASC inflammasome assembly induced by wild-type and gain-of-function mutant NLRP3, an effect not seen with disease-associated NLRP12 mutants. The capacity of NLRP12 to suppress NLRP3 inflammasome assembly was limited to human NLRP3 and was not observed for wild-type murine NLRP3. Furthermore, peripheral blood mononuclear cells from patients with an NLRP12 mutant-associated inflammatory disorder produced increased amounts of the inflammatory cytokine IL-1ß in response to NLRP3 stimulation. Thus, our findings provide insights into NLRP12 biology and suggest that NLRP3 inhibitors in clinical trials for NLRP3-driven diseases may also be effective in treating NLRP12-associated autoinflammatory diseases.

Live Imaging of Pyroptosis in Primary Murine Macrophages.

Inflammasomes are the ultimate weapon of the macrophage immune arsenal. Inflammasome signalling in macrophages triggers pyroptosis, a lytic cell death pathway that facilitates inflammation-driven pathogen clearance. Imaging-based approaches to investigating cell death have proven useful, revealing cellular remodelling events such as the generation of extracellular vesicles, and continuing to uncover important structural changes in cells involved in inflammatory signalling. Pyroptosis has proved extremely challenging to image, because its lytic nature is incompatible with many well-established imaging approaches employed for other, non-lytic pathways. The complexities of ectopically expressing fluorescent constructs in primary macrophages and the sensitivity of such proteins to drug-based probes compound this difficulty. We and others have demonstrated key differences in pyroptosis induced by canonical versus noncanonical inflammasomes that delineate functional differences between these signalling pathways. Here, we describe a live imaging approach to study and compare canonical versus noncanonical inflammasome signalling and pyroptotic architecture in primary murine macrophages.

Macrophage ACE2 is necessary for SARS-CoV-2 replication and subsequent cytokine responses that restrict continued virion release.

Macrophages are key cellular contributors to the pathogenesis of COVID-19, the disease caused by the virus SARS-CoV-2. The SARS-CoV-2 entry receptor ACE2 is present only on a subset of macrophages at sites of SARS-CoV-2 infection in humans. Here, we investigated whether SARS-CoV-2 can enter macrophages, replicate, and release new viral progeny; whether macrophages need to sense a replicating virus to drive cytokine release; and, if so, whether ACE2 is involved in these mechanisms. We found that SARS-CoV-2 could enter, but did not replicate within, ACE2-deficient human primary macrophages and did not induce proinflammatory cytokine expression. By contrast, ACE2 overexpression in human THP-1-derived macrophages permitted SARS-CoV-2 entry, processing and replication, and virion release. ACE2-overexpressing THP-1 macrophages sensed active viral replication and triggered proinflammatory, antiviral programs mediated by the kinase TBK-1 that limited prolonged viral replication and release. These findings help elucidate the role of ACE2 and its absence in macrophage responses to SARS-CoV-2 infection.

Isolation and culture of pure adult mouse microglia and astrocytes for in vitro characterization and analyses.

Microglia and astrocytes are implicated in aging and age-related diseases. Here, we present a protocol to isolate and culture these glia cells from the murine brain. The protocol consists of two parts: magnetic sorting of adult microglia and mechanical/magnetic sorting of adult microglia and astrocytes. We then describe the characterization of these glial cells by flow cytometry and immunohistochemistry. Microglia isolated from aged mice maintain age-related phenotype during culture. These purified glia cells can be applied in ex vivo studies.

The rOX-stars of inflammation: links between the inflammasome and mitochondrial meltdown.

The nod-like receptor protein 3 (NLRP3) inflammasome drives inflammation in response to mitochondrial dysfunction. As metabolic powerhouses with prokaryotic ancestry, mitochondria are a cache for danger-associated molecular patterns and pathogen-associated molecular pattern-like molecules that elicit potent innate immune responses. Persistent mitochondrial damage caused by infection, or genetic or environmental factors, can lead to inappropriate or sustained inflammasome signalling. Here, we review the features of mitochondria that drive inflammatory signalling, with a particular focus on mitochondrial activation of the NLRP3 inflammasome. Given that mitochondrial network dynamics, metabolic activity and redox state are all intricately linked to each other and to NLRP3 inflammasome activity, we highlight the importance of a holistic approach to investigations of NLRP3 activation by dysfunctional mitochondria.

Author Correction: Mitochondrial DNA synthesis fuels NLRP3 inflammasome.

In the initial published version of this article, there was a mistake in the title. The correct title should be "Mitochondrial DNA synthesis fuels NLRP3 activation". This correction does not affect the description of the results or the conclusions of this work.

Interleukin-1ß Maturation Triggers Its Relocation to the Plasma Membrane for Gasdermin-D-Dependent and -Independent Secretion.

IL-1ß requires processing by caspase-1 to generate the active, pro-inflammatory cytokine. Acute IL-1ß secretion from inflammasome-activated macrophages requires caspase-1-dependent GSDMD cleavage, which also induces pyroptosis. Mechanisms of IL-1ß secretion by pyroptotic and non-pyroptotic cells, and the precise functions of caspase-1 and GSDMD therein, are unresolved. Here, we show that, while efficient early secretion of endogenous IL-1ß from primary non-pyroptotic myeloid cells in vitro requires GSDMD, later IL-1ß release in vitro and in vivo proceeds independently of GSDMD. IL-1ß maturation is sufficient for slow, caspase-1/GSDMD-independent secretion of ectopic IL-1ß from resting, non-pyroptotic macrophages, but the speed of IL-1ß release is boosted by inflammasome activation, via caspase-1 and GSDMD. IL-1ß cleavage induces IL-1ß enrichment at PIP2-enriched plasma membrane ruffles, and this is a prerequisite for IL-1ß secretion and is mediated by a polybasic motif within the cytokine. We thus reveal a mechanism in which maturation-induced IL-1ß trafficking facilitates its unconventional secretion.

Caspase-1 self-cleavage is an intrinsic mechanism to terminate inflammasome activity.

Host-protective caspase-1 activity must be tightly regulated to prevent pathology, but mechanisms controlling the duration of cellular caspase-1 activity are unknown. Caspase-1 is activated on inflammasomes, signaling platforms that facilitate caspase-1 dimerization and autoprocessing. Previous studies with recombinant protein identified a caspase-1 tetramer composed of two p20 and two p10 subunits (p20/p10) as an active species. In this study, we report that in the cell, the dominant species of active caspase-1 dimers elicited by inflammasomes are in fact full-length p46 and a transient species, p33/p10. Further p33/p10 autoprocessing occurs with kinetics specified by inflammasome size and cell type, and this releases p20/p10 from the inflammasome, whereupon the tetramer becomes unstable in cells and protease activity is terminated. The inflammasome-caspase-1 complex thus functions as a holoenzyme that directs the location of caspase-1 activity but also incorporates an intrinsic self-limiting mechanism that ensures timely caspase-1 deactivation. This intrinsic mechanism of inflammasome signal shutdown offers a molecular basis for the transient nature, and coordinated timing, of inflammasome-dependent inflammatory responses.

Sulfonylureas as Concomitant Insulin Secretagogues and NLRP3 Inflammasome Inhibitors.

Insulin-secretory sulfonylureas are widely used, cost-effective treatments for type 2 diabetes (T2D). However, pancreatic ß-cells are continually depleted as T2D progresses, thereby rendering the sulfonylurea drug class ineffective in controlling glycaemia. Dysregulation of the innate immune system via activation of the NLRP3 inflammasome, and the consequent production of interleukin-1ß, has been linked to pancreatic ß-cell death and multiple inflammatory complications of T2D disease. One proposed strategy for treating T2D is the use of sulfonylurea insulin secretagogues that are also NLRP3 inhibitors. We report the synthesis and biological evaluation of nine sulfonylureas that inhibit NLRP3 activation in murine bone-marrow- derived macrophages in a potent, dose-dependent manner. Six of these compounds inhibited NLRP3 at nanomolar concentrations and can also stimulate insulin secretion from a murine pancreatic cell line (MIN6). These novel compounds possess unprecedented dual modes of action, paving the way for a new generation of sulfonylureas that may be useful as therapeutic candidates and/or tool compounds in T2D and its associated inflammatory complications.

Familial autoinflammation with neutrophilic dermatosis reveals a regulatory mechanism of pyrin activation.

Pyrin responds to pathogen signals and loss of cellular homeostasis by forming an inflammasome complex that drives the cleavage and secretion of interleukin-1ß (IL-1ß). Mutations in the B30.2/SPRY domain cause pathogen-independent activation of pyrin and are responsible for the autoinflammatory disease familial Mediterranean fever (FMF). We studied a family with a dominantly inherited autoinflammatory disease, distinct from FMF, characterized by childhood-onset recurrent episodes of neutrophilic dermatosis, fever, elevated acute-phase reactants, arthralgia, and myalgia/myositis. The disease was caused by a mutation in MEFV, the gene encoding pyrin (S242R). The mutation results in the loss of a 14-3-3 binding motif at phosphorylated S242, which was not perturbed by FMF mutations in the B30.2/SPRY domain. However, loss of both S242 phosphorylation and 14-3-3 binding was observed for bacterial effectors that activate the pyrin inflammasome, such as Clostridium difficile toxin B (TcdB). The S242R mutation thus recapitulated the effect of pathogen sensing, triggering inflammasome activation and IL-1ß production. Successful therapy targeting IL-1ß has been initiated in one patient, resolving pyrin-associated autoinflammation with neutrophilic dermatosis. This disease provides evidence that a guard-like mechanism of pyrin regulation, originally identified for Nod-like receptors in plant innate immunity, also exists in humans.