A 360° view of the inflammasome: Mechanisms of activation, cell death, and diseases.

Inflammasomes are critical sentinels of the innate immune system that respond to threats to the host through recognition of distinct molecules, known as pathogen- or damage-associated molecular patterns (PAMPs/DAMPs), or disruptions of cellular homeostasis, referred to as homeostasis-altering molecular processes (HAMPs) or effector-triggered immunity (ETI). Several distinct proteins nucleate inflammasomes, including NLRP1, CARD8, NLRP3, NLRP6, NLRC4/NAIP, AIM2, pyrin, and caspases-4/-5/-11. This diverse array of sensors strengthens the inflammasome response through redundancy and plasticity. Here, we present an overview of these pathways, outlining the mechanisms of inflammasome formation, subcellular regulation, and pyroptosis, and discuss the wide-reaching effects of inflammasomes in human disease.

Multi-omics analyses reveal that HIV-1 alters CD4+ T cell immunometabolism to fuel virus replication.

Individuals infected with human immunodeficiency virus type-1 (HIV-1) show metabolic alterations of CD4+ T cells through unclear mechanisms with undefined consequences. We analyzed the transcriptome of CD4+ T cells from patients with HIV-1 and revealed that the elevated oxidative phosphorylation (OXPHOS) pathway is associated with poor outcomes. Inhibition of OXPHOS by the US Food and Drug Administration-approved drug metformin, which targets mitochondrial respiratory chain complex-I, suppresses HIV-1 replication in human CD4+ T cells and humanized mice. In patients, HIV-1 peak viremia positively correlates with the expression of NLRX1, a mitochondrial innate immune receptor. Quantitative proteomics and metabolic analyses reveal that NLRX1 enhances OXPHOS and glycolysis during HIV-1-infection of CD4+ T cells to promote viral replication. At the mechanistic level, HIV infection induces the association of NLRX1 with the mitochondrial protein FASTKD5 to promote expression of mitochondrial respiratory complex components. This study uncovers the OXPHOS pathway in CD4+ T cells as a target for HIV-1 therapy.

The inflammasome NLRs in immunity, inflammation, and associated diseases.

Inflammasome activation leads to caspase-1 activation, which causes the maturation and secretion of pro-IL-1ß and pro-IL-18 among other substrates. A subgroup of the NLR (nucleotide-binding domain, leucine-rich repeat containing) proteins are key mediators of the inflammasome. Studies of gene-deficient mice and cells have implicated NLR inflammasomes in a host of responses to a wide range of microbial pathogens, inflammatory diseases, cancer, and metabolic and autoimmune disorders. Determining exactly how the inflammasome is activated in these diseases and disease models remains a challenge. This review presents and integrates recent progress in the field.

Corrigendum: NLRP12 attenuates colon inflammation by maintaining colonic microbial diversity and promoting protective commensal bacterial growth.

This corrects the article DOI: 10.1038/ni.3690.

NLRP12 attenuates colon inflammation by maintaining colonic microbial diversity and promoting protective commensal bacterial growth.

Inflammatory bowel diseases involve the dynamic interaction of host genetics, the microbiome and inflammatory responses. Here we found lower expression of NLRP12 (which encodes a negative regulator of innate immunity) in human ulcerative colitis, by comparing monozygotic twins and other patient cohorts. In parallel, Nlrp12 deficiency in mice caused increased basal colonic inflammation, which led to a less-diverse microbiome and loss of protective gut commensal strains (of the family Lachnospiraceae) and a greater abundance of colitogenic strains (of the family Erysipelotrichaceae). Dysbiosis and susceptibility to colitis associated with Nlrp12 deficency were reversed equally by treatment with antibodies targeting inflammatory cytokines and by the administration of beneficial commensal Lachnospiraceae isolates. Fecal transplants from mice reared in specific-pathogen-free conditions into germ-free Nlrp12-deficient mice showed that NLRP12 and the microbiome each contributed to immunological signaling that culminated in colon inflammation. These findings reveal a feed-forward loop in which NLRP12 promotes specific commensals that can reverse gut inflammation, while cytokine blockade during NLRP12 deficiency can reverse dysbiosis.

NLRX1 promotes immediate IRF1-directed antiviral responses by limiting dsRNA-activated translational inhibition mediated by PKR.

NLRX1 is unique among the nucleotide-binding-domain and leucine-rich-repeat (NLR) proteins in its mitochondrial localization and ability to negatively regulate antiviral innate immunity dependent on the adaptors MAVS and STING. However, some studies have suggested a positive regulatory role for NLRX1 in inducing antiviral responses. We found that NLRX1 exerted opposing regulatory effects on viral activation of the transcription factors IRF1 and IRF3, which might potentially explain such contradictory results. Whereas NLRX1 suppressed MAVS-mediated activation of IRF3, it conversely facilitated virus-induced increases in IRF1 expression and thereby enhanced control of viral infection. NLRX1 had a minimal effect on the transcription of IRF1 mediated by the transcription factor NF-kB and regulated the abundance of IRF1 post-transcriptionally by preventing translational shutdown mediated by the double-stranded RNA (dsRNA)-activated kinase PKR and thereby allowed virus-induced increases in the abundance of IRF1 protein.

Mitochondrial GSDMD Pores DAMPen Pyroptosis.

Noncanoncial inflammasome activation by cytosolic lipopolysaccharide (LPS) causes pyroptotic cell death facilitated by gasdermin D (GSDMD) pore formation. In this issue of Immunity, Huang et al. describe how cytosolic LPS in endothelial cells does not cause cell death but restrains endothelial cell proliferation.

Viral DNA Binding to NLRC3, an Inhibitory Nucleic Acid Sensor, Unleashes STING, a Cyclic Dinucleotide Receptor that Activates Type I Interferon.

Immune suppression is a crucial component of immunoregulation and a subgroup of nucleotide-binding domain (NBD), leucine-rich repeat (LRR)-containing proteins (NLRs) attenuate innate immunity. How this inhibitory function is controlled is unknown. A key question is whether microbial ligands can regulate this inhibition. NLRC3 is a negative regulator that attenuates type I interferon (IFN-I) response by sequestering and attenuating stimulator of interferon genes (STING) activation. Here, we report that NLRC3 binds viral DNA and other nucleic acids through its LRR domain. DNA binding to NLRC3 increases its ATPase activity, and ATP-binding by NLRC3 diminishes its interaction with STING, thus licensing an IFN-I response. This work uncovers a mechanism wherein viral nucleic acid binding releases an inhibitory innate receptor from its target.

STING-induced regulatory B cells compromise NK function in cancer immunity.

An immunosuppressive tumour microenvironment is a major obstacle in the control of pancreatic and other solid cancers1-3. Agonists of the stimulator of interferon genes (STING) protein trigger inflammatory innate immune responses to potentially overcome tumour immunosuppression4. Although these agonists hold promise as potential cancer therapies5, tumour resistance to STING monotherapy has emerged in clinical trials and the mechanism(s) is unclear5-7. Here we show that the administration of five distinct STING agonists, including cGAMP, results in an expansion of human and mouse interleukin (IL)-35+ regulatory B cells in pancreatic cancer. Mechanistically, cGAMP drives expression of IL-35 by B cells in an IRF3-dependent but type I interferon-independent manner. In several preclinical cancer models, the loss of STING signalling in B cells increases tumour control. Furthermore, anti-IL-35 blockade or genetic ablation of IL-35 in B cells also reduces tumour growth. Unexpectedly, the STING-IL-35 axis in B cells reduces proliferation of natural killer (NK) cells and attenuates the NK-driven anti-tumour response. These findings reveal an intrinsic barrier to systemic STING agonist monotherapy and provide a combinatorial strategy to overcome immunosuppression in tumours.

The Innate Immune Sensor NLRC3 Acts as a Rheostat that Fine-Tunes T Cell Responses in Infection and Autoimmunity.

Appropriate immune responses require a fine balance between immune activation and attenuation. NLRC3, a non-inflammasome-forming member of the NLR innate immune receptor family, attenuates inflammation in myeloid cells and proliferation in epithelial cells. T lymphocytes express the highest amounts of Nlrc3 transcript where its physiologic relevance is unknown. We show that NLRC3 attenuated interferon-γ and TNF expression by CD4+ T cells and reduced T helper 1 (Th1) and Th17 cell proliferation. Nlrc3-/- mice exhibited increased and prolonged CD4+ T cell responses to lymphocytic choriomeningitis virus infection and worsened experimental autoimmune encephalomyelitis (EAE). These functions of NLRC3 were executed in a T-cell-intrinsic fashion: NLRC3 reduced K63-linked ubiquitination of TNF-receptor-associated factor 6 (TRAF6) to limit NF-κB activation, lowered phosphorylation of eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1), and diminished glycolysis and oxidative phosphorylation. This study reveals an unappreciated role for NLRC3 in attenuating CD4+ T cell signaling and metabolism.

AIM2 in regulatory T cells restrains autoimmune diseases.

The inflammasome initiates innate defence and inflammatory responses by activating caspase-1 and pyroptotic cell death in myeloid cells1,2. It consists of an innate immune receptor/sensor, pro-caspase-1, and a common adaptor molecule, ASC. Consistent with their pro-inflammatory function, caspase-1, ASC and the inflammasome component NLRP3 exacerbate autoimmunity during experimental autoimmune encephalomyelitis by enhancing the secretion of IL-1ß and IL-18 in myeloid cells3-6. Here we show that the DNA-binding inflammasome receptor AIM27-10 has a T cell-intrinsic and inflammasome-independent role in the function of T regulatory (Treg) cells. AIM2 is highly expressed by both human and mouse Treg cells, is induced by TGFß, and its promoter is occupied by transcription factors that are associated with Treg cells such as RUNX1, ETS1, BCL11B and CREB. RNA sequencing, biochemical and metabolic analyses demonstrated that AIM2 attenuates AKT phosphorylation, mTOR and MYC signalling, and glycolysis, but promotes oxidative phosphorylation of lipids in Treg cells. Mechanistically, AIM2 interacts with the RACK1-PP2A phosphatase complex to restrain AKT phosphorylation. Lineage-tracing analysis demonstrates that AIM2 promotes the stability of Treg cells during inflammation. Although AIM2 is generally accepted as an inflammasome effector in myeloid cells, our results demonstrate a T cell-intrinsic role of AIM2 in restraining autoimmunity by reducing AKT-mTOR signalling and altering immune metabolism to enhance the stability of Treg cells.

The TLR2/TLR6 ligand FSL-1 mitigates radiation-induced hematopoietic injury in mice and nonhuman primates.

Thrombocytopenia, hemorrhage, anemia, and infection are life-threatening issues following accidental or intentional radiation exposure. Since few therapeutics are available, safe and efficacious small molecules to mitigate radiation-induced injury need to be developed. Our previous study showed the synthetic TLR2/TLR6 ligand fibroblast stimulating lipopeptide (FSL-1) prolonged survival and provided MyD88-dependent mitigation of hematopoietic acute radiation syndrome (H-ARS) in mice. Although mice and humans differ in TLR number, expression, and function, nonhuman primate (NHP) TLRs are like those of humans; therefore, studying both animal models is critical for drug development. The objectives of this study were to determine the efficacy of FSL-1 on hematopoietic recovery in small and large animal models subjected to sublethal total body irradiation and investigate its mechanism of action. In mice, we demonstrate a lack of adverse effects, an easy route of delivery (subcutaneous) and efficacy in promoting hematopoietic progenitor cell proliferation by FSL-1. NHP given radiation, followed a day later with a single subcutaneous administration of FSL-1, displayed no adversity but showed elevated hematopoietic cells. Our analyses revealed that FSL-1 promoted red blood cell development and induced soluble effectors following radiation exposure. Cytologic analysis of bone marrow aspirates revealed a striking enhancement of mononuclear progenitor cells in FSL-1-treated NHP. Combining the efficacy of FSL-1 in promoting hematopoietic cell recovery with the lack of adverse effects induced by a single administration supports the application of FSL-1 as a viable countermeasure against H-ARS.

Human ACE2 expression, a major tropism determinant for SARS-CoV-2, is regulated by upstream and intragenic elements.

Angiotensin-converting enzyme 2 (ACE2), part of the renin-angiotensin system (RAS), serves as an entry point for SARS-CoV-2, leading to viral proliferation in permissive cell types. Using mouse lines in which the Ace2 locus has been humanized by syntenic replacement, we show that regulation of basal and interferon induced ACE2 expression, relative expression levels of different ACE2 transcripts, and sexual dimorphism in ACE2 expression are unique to each species, differ between tissues, and are determined by both intragenic and upstream promoter elements. Our results indicate that the higher levels of expression of ACE2 observed in the lungs of mice relative to humans may reflect the fact that the mouse promoter drives expression of ACE2 in populous airway club cells while the human promoter drives expression in alveolar type 2 (AT2) cells. In contrast to transgenic mice in which human ACE2 is expressed in ciliated cells under the control of the human FOXJ1 promoter, mice expressing ACE2 in club cells under the control of the endogenous Ace2 promoter show a robust immune response after infection with SARS-CoV-2, leading to rapid clearance of the virus. This supports a model in which differential expression of ACE2 determines which cell types in the lung are infected, and this in turn modulates the host response and outcome of COVID-19.

A role for the NLRP3 inflammasome in metabolic diseases--did Warburg miss inflammation?

The inflammasome is a protein complex that comprises an intracellular sensor (typically a Nod-like receptor), the precursor procaspase-1 and the adaptor ASC. Inflammasome activation leads to the maturation of caspase-1 and the processing of its substrates, interleukin 1ß (IL-1ß) and IL-18. Although initially the inflammasome was described as a complex that affects infection and inflammation, subsequent evidence has suggested that inflammasome activation influences many metabolic disorders, including atherosclerosis, type 2 diabetes, gout and obesity. Another feature of inflammation in general and the inflammasome specifically is that the activation process has a profound effect on aerobic glycolysis (the 'Warburg effect'). Here we explore how the Warburg effect might be linked to inflammation and inflammasome activation.

The innate immune sensor NLRC3 attenuates Toll-like receptor signaling via modification of the signaling adaptor TRAF6 and transcription factor NF-κB.

Several members of the NLR family of sensors activate innate immunity. In contrast, we found here that NLRC3 inhibited Toll-like receptor (TLR)-dependent activation of the transcription factor NF-κB by interacting with the TLR signaling adaptor TRAF6 to attenuate Lys63 (K63)-linked ubiquitination of TRAF6 and activation of NF-κB. We used bioinformatics to predict interactions between NLR and TRAF proteins, including interactions of TRAF with NLRC3. In vivo, macrophage expression of Nlrc3 mRNA was diminished by the administration of lipopolysaccharide (LPS) but was restored when cellular activation subsided. To assess biologic relevance, we generated Nlrc3(-/-) mice. LPS-treated Nlrc3(-/-) macrophages had more K63-ubiquitinated TRAF6, nuclear NF-κB and proinflammatory cytokines. Finally, LPS-treated Nlrc3(-/-) mice had more signs of inflammation. Thus, signaling via NLRC3 and TLR constitutes a negative feedback loop. Furthermore, prevalent NLR-TRAF interactions suggest the formation of a 'TRAFasome' complex.

Dynamic bidirectional regulation of NLRC3 and gammaherpesviruses during viral latency in B lymphocytes.

While most NOD-like receptors (NLRs) are predominately expressed by innate immune cells, NLRC3, an inhibitory NLR of immune signaling, exhibits the highest expression in lymphocytes. The role of NLRC3 or any NLRs in B lymphocytes is completely unknown. Gammaherpesviruses, including human Epstein-Barr virus (EBV) and murine gammaherpesvirus 68 (MHV-68), establish latent infection in B lymphocytes, which requires elevated NF-κB. This study shows that during latent EBV infection of human B cells, viral-encoded latent membrane protein 1 (LMP1) decreases NLRC3 transcript. LMP1-induced-NF-κB activation suppresses the promoter activity of NLRC3 via p65 binding to the promoter. Conversely, NLRC3 inhibits NF-κB activation by promoting the degradation of LMP1 in a proteasome-dependent manner. In vivo, MHV-68 infection reduces Nlrc3 transcripts in splenocytes, and Nlrc3-deficient mice show greater viral latency than controls. These results reveal a bidirectional regulatory circuit in B lymphocytes, where viral latent protein LMP1 reduces NLRC3 expression, while NLRC3 disrupts gammaherpesvirus latency, which is an important step for tumorigenesis.

Viral protease cleavage of MAVS in genetically modified mice with hepatitis A virus infection.

BACKGROUND & AIMS: Consistent with its relatively narrow host species range, hepatitis A virus (HAV) cannot infect C57BL/6 mice. However, in Mavs-/- mice with genetic deficiency of the innate immune signaling adaptor MAVS, HAV replicates robustly in the absence of disease. The HAV 3ABC protease cleaves MAVS in human cells, thereby disrupting virus-induced IFN responses, but it cannot cleave murine MAVS (mMAVS) due to sequence differences at the site of scission. Here, we sought to elucidate the role of 3ABC MAVS cleavage in determining HAV pathogenesis and host species range. METHODS: Using CRISPR/Cas9 gene editing, we established two independent lineages of C57BL/6 mice with knock-in mutations altering two amino acids in mMAVS ('mMAVS-VS'), rendering it susceptible to 3ABC cleavage without loss of signaling function. We challenged homozygous Mavsvs/vs mice with HAV, and compared infection outcomes with C57BL/6 and genetically deficient Mavs-/- mice. RESULTS: The humanized murine mMAVS-VS protein was cleaved as efficiently as human MAVS when co-expressed with 3ABC in Huh-7 cells. In embyronic fibroblasts from Mavsvs/vs mice, mMAVS-VS was cleaved by ectopically expressed 3ABC, significantly disrupting Sendai virus-induced IFN responses. However, in contrast to Mavs-/- mice with genetic MAVS deficiency, HAV failed to establish infection in Mavsvs/vs mice, even with additional genetic knockout of Trif or Irf1. Nonetheless, when crossed with permissive Ifnar1-/- mice lacking type I IFN receptors, Mavsvs/vsIfnar1-/- mice demonstrated enhanced viral replication coupled with significant reductions in serum alanine aminotransferase, hepatocellular apoptosis, and intrahepatic inflammatory cell infiltrates compared with Ifnar1-/- mice. CONCLUSIONS: MAVS cleavage by 3ABC boosts viral replication and disrupts disease pathogenesis, but it is not by itself sufficient to break the host-species barrier to HAV infection in mice. IMPACT AND IMPLICATIONS: The limited host range of human hepatitis viruses could be explained by species-specific viral strategies that disrupt innate immune responses. Both hepatitis A virus (HAV) and hepatitis C virus express viral proteases that cleave the innate immune adaptor protein MAVS, in human but not mouse cells. However, the impact of this immune evasion strategy has never been assessed in vivo. Here we show that HAV 3ABC protease cleavage of MAVS enhances viral replication and lessens liver inflammation in mice lacking interferon receptors, but that it is insufficient by itself to overcome the cross-species barrier to infection in mice. These results enhance our understanding of how hepatitis viruses interact with the host and their impact on innate immune responses.

Fatty acid-induced NLRP3-ASC inflammasome activation interferes with insulin signaling.

High-fat diet (HFD) and inflammation are key contributors to insulin resistance and type 2 diabetes (T2D). Interleukin (IL)-1ß plays a role in insulin resistance, yet how IL-1ß is induced by the fatty acids in an HFD, and how this alters insulin signaling, is unclear. We show that the saturated fatty acid palmitate, but not unsaturated oleate, induces the activation of the NLRP3-ASC inflammasome, causing caspase-1, IL-1ß and IL-18 production. This pathway involves mitochondrial reactive oxygen species and the AMP-activated protein kinase and unc-51-like kinase-1 (ULK1) autophagy signaling cascade. Inflammasome activation in hematopoietic cells impairs insulin signaling in several target tissues to reduce glucose tolerance and insulin sensitivity. Furthermore, IL-1ß affects insulin sensitivity through tumor necrosis factor-independent and dependent pathways. These findings provide insights into the association of inflammation, diet and T2D.