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Multi-layered antiviral activity of TRIM5 TRIM5 is an E3 ubiquitin ligase with known antiretroviral restriction factor activity, although the mechanisms involved are poorly understood. Luban and colleagues now demonstrate that TRIM5 activates innate immune signalling pathways and acts as a pattern recognition receptor specific for the retrovirus capsid lattice. TRIM5 is a RING domain-E3 ubiquitin ligase that restricts infection by human immunodeficiency virus (HIV)-1 and other retroviruses immediately following virus invasion of the target cell cytoplasm 1 , 2 . Antiviral potency correlates with TRIM5 avidity for the retrovirion capsid lattice 3 , 4 and several reports indicate that TRIM5 has a role in signal transduction 5 , 6 , 7 , but the precise mechanism of restriction is unknown 8 . Here we demonstrate that TRIM5 promotes innate immune signalling and that this activity is amplified by retroviral infection and interaction with the capsid lattice. Acting with the heterodimeric, ubiquitin-conjugating enzyme UBC13–UEV1A (also known as UBE2N–UBE2V1), TRIM5 catalyses the synthesis of unattached K63-linked ubiquitin chains that activate the TAK1 (also known as MAP3K7) kinase complex and stimulate AP-1 and NFκB signalling. Interaction with the HIV-1 capsid lattice greatly enhances the UBC13–UEV1A-dependent E3 activity of TRIM5 and challenge with retroviruses induces the transcription of AP-1 and NF-κB-dependent factors with a magnitude that tracks with TRIM5 avidity for the invading capsid. Finally, TAK1 and UBC13–UEV1A contribute to capsid-specific restriction by TRIM5. Thus, the retroviral restriction factor TRIM5 has two additional activities that are linked to restriction: it constitutively promotes innate immune signalling and it acts as a pattern recognition receptor specific for the retrovirus capsid lattice.

Sepsis, severe sepsis and septic shock are the main cause of mortality in non-cardiac intensive care units. Immunometabolism has been linked to sepsis; however, the precise mechanism by which metabolic reprogramming regulates the inflammatory response is unclear. Here we show that aerobic glycolysis contributes to sepsis by modulating inflammasome activation in macrophages. PKM2-mediated glycolysis promotes inflammasome activation by modulating EIF2AK2 phosphorylation in macrophages. Pharmacological and genetic inhibition of PKM2 or EIF2AK2 attenuates NLRP3 and AIM2 inflammasomes activation, and consequently suppresses the release of IL-1β, IL-18 and HMGB1 by macrophages. Pharmacological inhibition of the PKM2–EIF2AK2 pathway protects mice from lethal endotoxemia and polymicrobial sepsis. Moreover, conditional knockout of PKM2 in myeloid cells protects mice from septic death induced by NLRP3 and AIM2 inflammasome activation. These findings define an important role of PKM2 in immunometabolism and guide future development of therapeutic strategies to treat sepsis. Inflammation involves a Warburg effect that switches cellular metabolism to glycolysis. Here the authors show this switch drives IL-1β, IL-18 and HMGB1 release from macrophages by activating the NLRP3 and AIM2 inflammasomes via protein kinase R phosphorylation, a pathway that can be inhibited to prevent sepsis in mice.

The adaptor ASC is required for caspase-1 activation via the NLRP3 and AIM2 inflammasomes. Mitsuyama and colleagues show that signaling dependent on the kinases Syk and Jnk controls ASC speck formation through ASC phosphorylation. The inflammasome adaptor ASC contributes to innate immunity through the activation of caspase-1. Here we found that signaling pathways dependent on the kinases Syk and Jnk were required for the activation of caspase-1 via the ASC-dependent inflammasomes NLRP3 and AIM2. Inhibition of Syk or Jnk abolished the formation of ASC specks without affecting the interaction of ASC with NLRP3. ASC was phosphorylated during inflammasome activation in a Syk- and Jnk-dependent manner, which suggested that Syk and Jnk are upstream of ASC phosphorylation. Moreover, phosphorylation of Tyr144 in mouse ASC was critical for speck formation and caspase-1 activation. Our results suggest that phosphorylation of ASC controls inflammasome activity through the formation of ASC specks.

The NLRP3 inflammasome is involved in IL-1 production and pyroptosis. Pelegrín et al . demonstrate that it is also released extracellularly as a functional proinflammatory particle. Assembly of the NLRP3 inflammasome activates caspase-1 and mediates the processing and release of the leaderless cytokine IL-1β and thereby serves a central role in the inflammatory response and in diverse human diseases. Here we found that upon activation of caspase-1, oligomeric NLRP3 inflammasome particles were released from macrophages. Recombinant oligomeric protein particles composed of the adaptor ASC or the p.D303N mutant form of NLRP3 associated with cryopyrin-associated periodic syndromes (CAPS) stimulated further activation of caspase-1 extracellularly, as well as intracellularly after phagocytosis by surrounding macrophages. We found oligomeric ASC particles in the serum of patients with active CAPS but not in that of patients with other inherited autoinflammatory diseases. Our findings support a model whereby the NLRP3 inflammasome, acting as an extracellular oligomeric complex, amplifies the inflammatory response.

NEK7 is a serine-threonine kinase linked to mitosis. Beutler and colleagues show that NEK7 is required for assembly of the NLRP3 inflammasome and restricts NLRP3 activation to interphase of the cell cycle. The NLRP3 inflammasome responds to microbes and danger signals by processing and activating proinflammatory cytokines, including interleukin 1β (IL-1β) and IL-18. We found here that activation of the NLRP3 inflammasome was restricted to interphase of the cell cycle by NEK7, a serine-threonine kinase previously linked to mitosis. Activation of the NLRP3 inflammasome required NEK7, which bound to the leucine-rich repeat domain of NLRP3 in a kinase-independent manner downstream of the induction of mitochondrial reactive oxygen species (ROS). This interaction was necessary for the formation of a complex containing NLRP3 and the adaptor ASC, oligomerization of ASC and activation of caspase-1. NEK7 promoted the NLRP3-dependent cellular inflammatory response to intraperitoneal challenge with monosodium urate and the development of experimental autoimmune encephalitis in mice. Our findings suggest that NEK7 serves as a cellular switch that enforces mutual exclusivity of the inflammasome response and cell division.

Key Points Neuroinflammation is characterized by microglial and astroglial cell activation, and is frequently associated with neurodegenerative disease. Aberrant or misfolded proteins can activate pattern recognition receptors that are expressed by innate immune cells of the brain, leading to neuroinflammatory responses. Pro-inflammatory mediators — such as cytokines, chemokines, complement components and free radicals — can lead to functional impairments and structural changes in the brain. Mutations in genes that encode innate immune proteins, such as triggering receptor expressed by myeloid cells 2 (TREM2) and CD33, may increase the risk of developing certain neurodegenerative diseases. Several exogenous factors — such as midlife obesity, poor oral health or systemic inflammation — may drive the pathogenesis of neurodegenerative disease by augmenting neuroinflammation. A better understanding of the innate immune mechanisms that promote neuroinflammation may enable the future development of anti-inflammatory and neuroprotective therapies for neurodegenerative diseases. These therapies have to be carefully considered, as innate immune pathways can have beneficial, as well as pathological, roles in the brain. In this Review, the authors discuss how innate immune mechanisms can contribute to the development of neurodegenerative disease. They suggest that both local and systemic inflammation can drive pathological microglial cell activity, thereby leading to neuronal cell dysfunction and disease. The triggering of innate immune mechanisms is emerging as a crucial component of major neurodegenerative diseases. Microglia and other cell types in the brain can be activated in response to misfolded proteins or aberrantly localized nucleic acids. This diverts microglia from their physiological and beneficial functions, and leads to their sustained release of pro-inflammatory mediators. In this Review, we discuss how the activation of innate immune signalling pathways — in particular, the NOD-, LRR- and pyrin domain-containing 3 (NLRP3) inflammasome — by aberrant host proteins may be a common step in the development of diverse neurodegenerative disorders. During chronic activation of microglia, the sustained exposure of neurons to pro-inflammatory mediators can cause neuronal dysfunction and contribute to cell death. As chronic neuroinflammation is observed at relatively early stages of neurodegenerative disease, targeting the mechanisms that drive this process may be useful for diagnostic and therapeutic purposes.

Key Points For the known inflammasomes, new cofactors such as caspase 11 and the NAIPs (NLR family, apoptosis inhibitory proteins) have been described. In addition, inflammasome-independent pathways for the processing of interleukin-1β (IL-1β), such as caspase 8 activation, have recently been described. Cell-extrinsic signalling can regulate inflammasome activation. Signalling by pattern recognition or cytokine receptors primes the cell and induces NLRP3 (NOD-, LRR- and pyrin domain-containing 3) and pro-IL-1β expression, whereas signalling by type I interferons and activated T cells reduces inflammasome activation. Energy levels, mitochondrial health and lysosomal compartmentalization are constantly under the surveillance of cellular health sensors such as the apoptosome and inflammasomes. These signalling platforms detect changes in cellular homeostasis and share many structural and functional similarities. NLRP3 is strongly regulated by fluxes of K + , Cl − and Ca 2+ . In addition, reactive oxygen species, autophagy and endoplasmic reticulum stress are important modulators of NLRP3 activity. The inflammasomes are regulated by pyrin domain- or CARD (caspase activation and recruitment domain)-only proteins, which sequester the signalling molecules. Other proteins that are known to regulate apoptosis also have a role in inflammasome signalling. Understanding the regulatory mechanisms of inflammasome activation will facilitate the development of new classes of drugs that target the inflammasomes. Inflammasomes are multiprotein signalling platforms that activate the highly pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18 and induce cell death in response to pathogens and sterile stressors. This Review provides a comprehensive overview of our rapidly evolving understanding of the regulatory mechanisms that control the activation of distinct inflammasome components, as well as the non-canonical processing of IL-1β. Inflammasomes are key signalling platforms that detect pathogenic microorganisms and sterile stressors, and that activate the highly pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18. In this Review, we discuss the complex regulatory mechanisms that facilitate a balanced but effective inflammasome-mediated immune response, and we highlight the similarities to another molecular signalling platform — the apoptosome — that monitors cellular health. Extracellular regulatory mechanisms are discussed, as well as the intracellular control of inflammasome assembly, for example, via ion fluxes, free radicals and autophagy.

Nephrocalcinosis, acute calcium oxalate (CaOx) nephropathy, and renal stone disease can lead to inflammation and subsequent renal failure, but the underlying pathological mechanisms remain elusive. Other crystallopathies, such as gout, atherosclerosis, and asbestosis, trigger inflammation and tissue remodeling by inducing IL-1β secretion, leading us to hypothesize that CaOx crystals may induce inflammation in a similar manner. In mice, intrarenal CaOx deposition induced tubular damage, cytokine expression, neutrophil recruitment, and renal failure. We found that CaOx crystals activated murine renal DCs to secrete IL-1β through a pathway that included NLRP3, ASC, and caspase-1. Despite a similar amount of crystal deposits, intrarenal inflammation, tubular damage, and renal dysfunction were abrogated in mice deficient in MyD88; NLRP3, ASC, and caspase-1; IL-1R; or IL-18. Nephropathy was attenuated by DC depletion, ATP depletion, or therapeutic IL-1 antagonism. These data demonstrated that CaOx crystals trigger IL-1β-dependent innate immunity via the NLRP3/ASC/caspase-1 axis in intrarenal mononuclear phagocytes and directly damage tubular cells, leading to the release of the NLRP3 agonist ATP. Furthermore, these results suggest that IL-1β blockade may prevent renal damage in nephrocalcinosis.

The receptor NLRP3 is central to the formation of inflammasomes in myeloid cells. Ghiringhelli and colleagues demonstrate that NLRP3 also serves an important inflammasome-independent role in CD4 + T cells, in which it helps coordinate T H 2 differentiation. The receptor NLRP3 is involved in the formation of the NLRP3 inflammasome that activates caspase-1 and mediates the release of interleukin 1β (IL-1β) and IL-18. Whether NLRP3 can shape immunological function independently of inflammasomes is unclear. We found that NLRP3 expression in CD4 + T cells specifically supported a T helper type 2 (T H 2) transcriptional program in a cell-intrinsic manner. NLRP3, but not the inflammasome adaptor ASC or caspase-1, positively regulated a T H 2 program. In T H 2 cells, NLRP3 bound the Il4 promoter and transactivated it in conjunction with the transcription factor IRF4. Nlrp3 -deficient T H 2 cells supported melanoma tumor growth in an IL-4-dependent manner and also promoted asthma-like symptoms. Our results demonstrate the ability of NLRP3 to act as a key transcription factor in T H 2 differentiation.

Foxp3 (forkhead box P3 transcription factor)-expressing regulatory T cells (Tregs) are essential for immunological tolerance, best illustrated by uncontrolled effector T-cell responses and autoimmunity upon loss of Foxp3 expression. Tregs can adopt specific effector phenotypes upon activation, reflecting the diversity of functional demands in the different tissues of the body. Here, we report that Foxp3+CD4+ T cells coexpressing retinoic acid-related orphan receptor-γt (RORγt), the master transcription factor for T helper type 17 (Th17) cells, represent a stable effector Treg lineage. Transcriptomic and epigenetic profiling revealed that Foxp3+RORγt+ T cells display signatures of both Tregs and Th17 cells, although the degree of similarity was higher to Foxp3+RORγt− Tregs than to Foxp3−RORγt+ T cells. Importantly, Foxp3+RORγt+ T cells were significantly demethylated at Treg-specific epigenetic signature genes such as Foxp3, Ctla-4, Gitr, Eos, and Helios, suggesting that these cells have a stable regulatory rather than inflammatory function. Indeed, adoptive transfer of Foxp3+RORγt+ T cells in the T-cell transfer colitis model confirmed their Treg function and lineage stability in vivo, and revealed an enhanced suppressive capacity as compared with Foxp3+RORγt− Tregs. Thus, our data suggest that RORγt expression in Tregs contributes to an optimal suppressive capacity during gut-specific immune responses, rendering Foxp3+RORγt+ T cells as an important effector Treg subset in the intestinal system.