The DNA Inflammasome in Human Myeloid Cells Is Initiated by a STING-Cell Death Program Upstream of NLRP3.

Detection of cytosolic DNA constitutes a central event in the context of numerous infectious and sterile inflammatory conditions. Recent studies have uncovered a bipartite mode of cytosolic DNA recognition, in which the cGAS-STING axis triggers antiviral immunity, whereas AIM2 triggers inflammasome activation. Here, we show that AIM2 is dispensable for DNA-mediated inflammasome activation in human myeloid cells. Instead, detection of cytosolic DNA by the cGAS-STING axis induces a cell death program initiating potassium efflux upstream of NLRP3. Forward genetics identified regulators of lysosomal trafficking to modulate this cell death program, and subsequent studies revealed that activated STING traffics to the lysosome, where it triggers membrane permeabilization and thus lysosomal cell death (LCD). Importantly, the cGAS-STING-NLRP3 pathway constitutes the default inflammasome response during viral and bacterial infections in human myeloid cells. We conclude that targeting the cGAS-STING-LCD-NLRP3 pathway will ameliorate pathology in inflammatory conditions that are associated with cytosolic DNA sensing.

IKKß primes inflammasome formation by recruiting NLRP3 to the trans-Golgi network.

The NLRP3 inflammasome plays a central role in antimicrobial defense as well as in the context of sterile inflammatory conditions. NLRP3 activity is governed by two independent signals: the first signal primes NLRP3, rendering it responsive to the second signal, which then triggers inflammasome formation. Our understanding of how NLRP3 priming contributes to inflammasome activation remains limited. Here, we show that IKKß, a kinase activated during priming, induces recruitment of NLRP3 to phosphatidylinositol-4-phosphate (PI4P), a phospholipid enriched on the trans-Golgi network. NEK7, a mitotic spindle kinase that had previously been thought to be indispensable for NLRP3 activation, was redundant for inflammasome formation when IKKß recruited NLRP3 to PI4P. Studying iPSC-derived human macrophages revealed that the IKKß-mediated NEK7-independent pathway constitutes the predominant NLRP3 priming mechanism in human myeloid cells. Our results suggest that PI4P binding represents a primed state into which NLRP3 is brought by IKKß activity.

Post-injury immunosuppression and secondary infections are caused by an AIM2 inflammasome-driven signaling cascade.

Loss of lymphocytes, particularly T cell apoptosis, is a central pathological event after severe tissue injury that is associated with increased susceptibility for life-threatening infections. The precise immunological mechanisms leading to T cell death after acute injury are largely unknown. Here, we identified a monocyte-T cell interaction driving bystander cell death of T cells in ischemic stroke and burn injury. Specifically, we found that stroke induced a FasL-expressing monocyte population, which led to extrinsic T cell apoptosis. This phenomenon was driven by AIM2 inflammasome-dependent interleukin-1ß (IL-1ß) secretion after sensing cell-free DNA. Pharmacological inhibition of this pathway improved T cell survival and reduced post-stroke bacterial infections. As such, this study describes inflammasome-dependent monocyte activation as a previously unstudied cause of T cell death after injury and challenges the current paradigms of post-injury lymphopenia.

Human Monocytes Engage an Alternative Inflammasome Pathway.

Interleukin-1ß (IL-1ß) is a cytokine whose bioactivity is controlled by activation of the inflammasome. However, in response to lipopolysaccharide, human monocytes secrete IL-1ß independently of classical inflammasome stimuli. Here, we report that this constituted a species-specific response that is not observed in the murine system. Indeed, in human monocytes, lipopolysaccharide triggered an "alternative inflammasome" that relied on NLRP3-ASC-caspase-1 signaling, yet was devoid of any classical inflammasome characteristics including pyroptosome formation, pyroptosis induction, and K(+) efflux dependency. Genetic dissection of the underlying signaling pathway in a monocyte transdifferentiation system revealed that alternative inflammasome activation was propagated by TLR4-TRIF-RIPK1-FADD-CASP8 signaling upstream of NLRP3. Importantly, involvement of this signaling cascade was limited to alternative inflammasome activation and did not extend to classical NLRP3 activation. Because alternative inflammasome activation embraces both sensitivity and promiscuity of TLR4, we propose a pivotal role for this signaling cascade in TLR4-driven, IL-1ß-mediated immune responses and immunopathology in humans.

GSDMD drives canonical inflammasome-induced neutrophil pyroptosis and is dispensable for NETosis.

Neutrophils are the most prevalent immune cells in circulation, but the repertoire of canonical inflammasomes in neutrophils and their respective involvement in neutrophil IL-1ß secretion and neutrophil cell death remain unclear. Here, we show that neutrophil-targeted expression of the disease-associated gain-of-function Nlrp3A350V mutant suffices for systemic autoinflammatory disease and tissue pathology in vivo. We confirm the activity of the canonical NLRP3 and NLRC4 inflammasomes in neutrophils, and further show that the NLRP1b, Pyrin and AIM2 inflammasomes also promote maturation and secretion of interleukin (IL)-1ß in cultured bone marrow neutrophils. Notably, all tested canonical inflammasomes promote GSDMD cleavage in neutrophils, and canonical inflammasome-induced pyroptosis and secretion of mature IL-1ß are blunted in GSDMD-knockout neutrophils. In contrast, GSDMD is dispensable for PMA-induced NETosis. We also show that Salmonella Typhimurium-induced pyroptosis is markedly increased in Nox2/Gp91Phox -deficient neutrophils that lack NADPH oxidase activity and are defective in PMA-induced NETosis. In conclusion, we establish the canonical inflammasome repertoire in neutrophils and identify differential roles for GSDMD and the NADPH complex in canonical inflammasome-induced neutrophil pyroptosis and mitogen-induced NETosis, respectively.

Therapeutic modulation of inflammasome pathways.

Inflammasomes are macromolecular complexes formed in response to pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) that drive maturation of the pro-inflammatory cytokines interleukin (IL)-1ß and IL-18, and cleave gasdermin D (GSDMD) for induction of pyroptosis. Inflammasomes are highly important in protecting the host from various microbial pathogens and sterile insults. Inflammasome pathways are strictly regulated at both transcriptional and post-translational checkpoints. When these checkpoints are not properly imposed, undue inflammasome activation may promote inflammatory, metabolic and oncogenic processes that give rise to autoinflammatory, autoimmune, metabolic and malignant diseases. In addition to clinically approved IL-1-targeted biologics, upstream targeting of inflammasome pathways recently gained interest as a novel pharmacological strategy for selectively modulating inflammasome activation in pathological conditions.

Cytosolic RNA:DNA hybrids activate the cGAS-STING axis.

Intracellular recognition of non-self and also self-nucleic acids can result in the initiation of potent pro-inflammatory and antiviral cytokine responses. Most recently, cGAS was shown to be critical for the recognition of cytoplasmic dsDNA. Binding of dsDNA to cGAS results in the synthesis of cGAMP(2'-5'), which then binds to the endoplasmic reticulum resident protein STING. This initiates a signaling cascade that triggers the induction of an antiviral immune response. While most studies on intracellular nucleic acids have focused on dsRNA or dsDNA, it has remained unexplored whether cytosolic RNA:DNA hybrids are also sensed by the innate immune system. Studying synthetic RNA:DNA hybrids, we indeed observed a strong type I interferon response upon cytosolic delivery of this class of molecule. Studies in THP-1 knockout cells revealed that the recognition of RNA:DNA hybrids is completely attributable to the cGAS-STING pathway. Moreover, in vitro studies showed that recombinant cGAS produced cGAMP upon RNA:DNA hybrid recognition. Altogether, our results introduce RNA:DNA hybrids as a novel class of intracellular PAMP molecules and describe an alternative cGAS ligand next to dsDNA.

A Genome-wide CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) Screen Identifies NEK7 as an Essential Component of NLRP3 Inflammasome Activation.

Inflammasomes are high molecular weight protein complexes that assemble in the cytosol upon pathogen encounter. This results in caspase-1-dependent pro-inflammatory cytokine maturation, as well as a special type of cell death, known as pyroptosis. The Nlrp3 inflammasome plays a pivotal role in pathogen defense, but at the same time, its activity has also been implicated in many common sterile inflammatory conditions. To this effect, several studies have identified Nlrp3 inflammasome engagement in a number of common human diseases such as atherosclerosis, type 2 diabetes, Alzheimer disease, or gout. Although it has been shown that known Nlrp3 stimuli converge on potassium ion efflux upstream of Nlrp3 activation, the exact molecular mechanism of Nlrp3 activation remains elusive. Here, we describe a genome-wide CRISPR/Cas9 screen in immortalized mouse macrophages aiming at the unbiased identification of gene products involved in Nlrp3 inflammasome activation. We employed a FACS-based screen for Nlrp3-dependent cell death, using the ionophoric compound nigericin as a potassium efflux-inducing stimulus. Using a genome-wide guide RNA (gRNA) library, we found that targeting Nek7 rescued macrophages from nigericin-induced lethality. Subsequent studies revealed that murine macrophages deficient in Nek7 displayed a largely blunted Nlrp3 inflammasome response, whereas Aim2-mediated inflammasome activation proved to be fully intact. Although the mechanism of Nek7 functioning upstream of Nlrp3 yet remains elusive, these studies provide a first genetic handle of a component that specifically functions upstream of Nlrp3.

Structural basis for the oligomerization-facilitated NLRP3 activation.

The NACHT-, leucine-rich-repeat-, and pyrin domain-containing protein 3 (NLRP3) is a critical intracellular inflammasome sensor and an important clinical target against inflammation-driven human diseases. Recent studies have elucidated its transition from a closed cage to an activated disk-like inflammasome, but the intermediate activation mechanism remains elusive. Here we report the cryo-electron microscopy structure of NLRP3, which forms an open octamer and undergoes a ~ 90° hinge rotation at the NACHT domain. Mutations on open octamer's interfaces reduce IL-1ß signaling, highlighting its essential role in NLRP3 activation/inflammasome assembly. The centrosomal NIMA-related kinase 7 (NEK7) disrupts large NLRP3 oligomers and forms NEK7/NLRP3 monomers/dimers which is a critical step preceding the assembly of the disk-like inflammasome. These data demonstrate an oligomeric cooperative activation of NLRP3 and provide insight into its inflammasome assembly mechanism.

In Silico Genome Analysis Reveals the Evolution and Potential Impact of SARS-CoV-2 Omicron Structural Changes on Host Immune Evasion and Antiviral Therapeutics.

New variants of SARS-CoV-2 continue to evolve. The novel SARS-CoV-2 variant of concern (VOC) B.1.1.529 (Omicron) was particularly menacing due to the presence of numerous consequential mutations. In this study, we reviewed about 12 million SARS-CoV-2 genomic and associated metadata using extensive bioinformatic approaches to understand how evolutionary and mutational changes affect Omicron variant properties. Subsampled global data based analysis of molecular clock in the phylogenetic tree showed 29.56 substitutions per year as the evolutionary rate of five VOCs. We observed extensive mutational changes in the spike structural protein of the Omicron variant. A total of 20% of 7230 amino acid and structural changes exclusive to Omicron's spike protein were detected in the receptor binding domain (RBD), suggesting differential selection pressures exerted during evolution. Analyzing key drug targets revealed mutation-derived differential binding affinities between Delta and Omicron variants. Nine single-RBD substitutions were detected within the binding site of approved therapeutic monoclonal antibodies. T-cell epitope prediction revealed eight immunologically important functional hotspots in three conserved non-structural proteins. A universal vaccine based on these regions may likely protect against all these SARS-CoV-2 variants. We observed key structural changes in the spike protein, which decreased binding affinities, indicating that these changes may help the virus escape host cellular immunity. These findings emphasize the need for continuous genomic surveillance of SARS-CoV-2 to better understand how novel mutations may impact viral spread and disease outcome.

Immune homeostasis and regulation of the interferon pathway require myeloid-derived Regnase-3.

The RNase Regnase-1 is a master RNA regulator in macrophages and T cells that degrades cellular and viral RNA upon NF-κB signaling. The roles of its family members, however, remain largely unknown. Here, we analyzed Regnase-3-deficient mice, which develop hypertrophic lymph nodes. We used various mice with immune cell-specific deletions of Regnase-3 to demonstrate that Regnase-3 acts specifically within myeloid cells. Regnase-3 deficiency systemically increased IFN signaling, which increased the proportion of immature B and innate immune cells, and suppressed follicle and germinal center formation. Expression analysis revealed that Regnase-3 and Regnase-1 share protein degradation pathways. Unlike Regnase-1, Regnase-3 expression is high specifically in macrophages and is transcriptionally controlled by IFN signaling. Although direct targets in macrophages remain unknown, Regnase-3 can bind, degrade, and regulate mRNAs, such as Zc3h12a (Regnase-1), in vitro. These data indicate that Regnase-3, like Regnase-1, is an RNase essential for immune homeostasis but has diverged as key regulator in the IFN pathway in macrophages.

BAX/BAK-Induced Apoptosis Results in Caspase-8-Dependent IL-1ß Maturation in Macrophages.

IL-1ß is a cytokine of pivotal importance to the orchestration of inflammatory responses. Synthesized as an inactive pro-cytokine, IL-1ß requires proteolytic maturation to gain biological activity. Here, we identify intrinsic apoptosis as a non-canonical trigger of IL-1ß maturation. Guided by the discovery of the immunomodulatory activity of vioprolides, cyclic peptides isolated from myxobacteria, we observe IL-1ß maturation independent of canonical inflammasome pathways, yet dependent on intrinsic apoptosis. Mechanistically, vioprolides inhibit MCL-1 and BCL2, which in turn triggers BAX/BAK-dependent mitochondrial outer membrane permeabilization (MOMP). Induction of MOMP results in the release of pro-apoptotic factors initiating intrinsic apoptosis, as well as the depletion of IAPs (inhibitors of apoptosis proteins). IAP depletion, in turn, operates upstream of ripoptosome complex formation, subsequently resulting in caspase-8-dependent IL-1ß maturation. These results establish the ripoptosome/caspase-8 complex as a pro-inflammatory checkpoint that senses the perturbation of mitochondrial integrity.

Activation of the NLRP3 Inflammasome by Hyaboron, a New Asymmetric Boron-Containing Macrodiolide from the Myxobacterium Hyalangium minutum.

A Natural Compound Library containing myxobacterial secondary metabolites was screened in murine macrophages for novel activators of IL-1ß maturation and secretion. The most potent of three hits in total was a so far undescribed metabolite, which was identified from the myxobacterium Hyalangium minutum strain Hym3. While the planar structure of 1 was elucidated by high resolution mass spectrometry and NMR data yielding an asymmetric boron containing a macrodiolide core structure, its relative stereochemistry of all 20 stereocenters of the 42-membered ring was assigned by rotating frame Overhause effect spectroscopy correlations, 1H,1H, and 1H,13C coupling constants, and by comparison of 13C chemical shifts to those of the structurally related metabolites tartrolon B-D. The absolute stereochemistry was subsequently assigned by Mosher's and Marfey's methods. Further functional studies revealed that hyaboron and other boronated natural compounds resulted in NLRP3 inflammasome dependent IL-1ß maturation, which is most likely due to their ability to act as potassium ionophores. Moreover, besides its inflammasome-stimulatory activity in human and mouse cells, hyaboron (1) showed additional diverse biological activities, including antibacterial and antiparasitic effects.