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Cell death is a fundamental physiological process in all living organisms. Its roles extend from embryonic development, organ maintenance, and aging to the coordination of immune responses and autoimmunity. In recent years, our understanding of the mechanisms orchestrating cellular death and its consequences on immunity and homeostasis has increased substantially. Different modalities of what has become known as ‘programmed cell death’ have been described, and some key players in these processes have been identified. We have learned more about the intricacies that fine tune the activity of common players and ultimately shape the different types of cell death. These studies have highlighted the complex mechanisms tipping the balance between different cell fates. Here, we summarize the latest discoveries in the three most well understood modalities of cell death, namely, apoptosis, necroptosis, and pyroptosis, highlighting common and unique pathways and their effect on the surrounding cells and the organism as a whole.

Eicke Latz recalls the discovery of the inflammasome in 2002 and how it revolutionized our understanding of inflammation and is now a target of new immunotherapeutics for inflammatory disease.

Our immune system is based on the close collaboration of the innate and adaptive immune systems for the rapid detection of any threats to the host. Recognition of pathogen-derived molecules is entrusted to specific germline- encoded signaling receptors. The same receptors have now also emerged as efficient detectors of misplaced or altered self-molecules that signal tissue damage and cell death following, for example, disruption of the blood supply and subsequent hypoxia. Many types of endogenous molecules have been shown to provoke such sterile inflammatory states when released from dying cells. However, a group of proteins referred to as alarmins have both intracellular and extracellular functions which have been the subject of intense research. Indeed, alarmins can either exert beneficial cell housekeeping functions, leading to tissue repair, or provoke deleterious uncontrolled inflammation. This group of proteins includes the high-mobility group box 1 protein （HMGB1）, interleukin （IL）-1α, IL-33 and the Ca^2＋-binding S100 proteins. These dual-function proteins share conserved regulatory mechanisms, such as secretory routes, post-translational modifications and enzymatic processing, that govern their extracellular functions in time and space. Release of alarmins from mesenchymal cells is a highly relevant mechanism by which immune cells can be alerted of tissue damage, and alarmins play a key role in the development of acute or chronic inflammatory diseases and in cancer development.

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.

Increasing evidence suggests that infection with Sars-CoV-2 causes neurological deficits in a substantial proportion of affected patients. While these symptoms arise acutely during the course of infection, less is known about the possible long-term consequences for the brain. Severely affected COVID-19 cases experience high levels of proinflammatory cytokines and acute respiratory dysfunction and often require assisted ventilation. All these factors have been suggested to cause cognitive decline. Pathogenetically, this may result from direct negative effects of the immune reaction, acceleration or aggravation of pre-existing cognitive deficits, or de novo induction of a neurodegenerative disease. This article summarizes the current understanding of neurological symptoms of COVID-19 and hypothesizes that affected patients may be at higher risk of developing cognitive decline after overcoming the primary COVID-19 infection. A structured prospective evaluation should analyze the likelihood, time course, and severity of cognitive impairment following the COVID-19 pandemic.

Danger signals are a hallmark of many common inflammatory diseases, and these stimuli can function to activate the cytosolic innate immune signalling receptor NLRP3 (NOD-, LRR- and pyrin domain-containing 3). Once activated, NLRP3 nucleates the assembly of an inflammasome, leading to caspase 1-mediated proteolytic activation of the interleukin-1β (IL-1β) family of cytokines, and induces an inflammatory, pyroptotic cell death. Pharmacological inhibition of NLRP3 activation results in potent therapeutic effects in a wide variety of rodent models of inflammatory diseases, effects that are mirrored by genetic ablation of NLRP3. Although these findings highlight the potential of NLRP3 as a drug target, an understanding of NLRP3 structure and activation mechanisms is incomplete, which has hampered the discovery and development of novel therapeutics against this target. Here, we review recent advances in our understanding of NLRP3 activation and regulation, highlight the evolving landscape of NLRP3 modulators and discuss opportunities for pharmacologically targeting NLRP3 with novel small molecules.

Detection of microbial components such as lipopolysaccharide (LPS) by Toll-like receptor 4 (TLR4) on macrophages induces a robust pro-inflammatory response that is dependent on metabolic reprogramming. These innate metabolic changes have been compared to aerobic glycolysis in tumour cells. However, the mechanisms by which TLR4 activation leads to mitochondrial and glycolytic reprogramming are unknown. Here we show that TLR4 activation induces a signalling cascade recruiting TRAF6 and TBK-1, while TBK-1 phosphorylates STAT3 on S727. Using a genetically engineered mouse model incapable of undergoing STAT3 Ser727 phosphorylation, we show ex vivo and in vivo that STAT3 Ser727 phosphorylation is critical for LPS-induced glycolytic reprogramming, production of the central immune response metabolite succinate and inflammatory cytokine production in a model of LPS-induced inflammation. Our study identifies non-canonical STAT3 activation as the crucial signalling intermediary for TLR4-induced glycolysis, macrophage metabolic reprogramming and inflammation. TLR4 signalling can reprogram the metabolism of macrophages to be more glycolytic and proinflammatory. Here the authors show that LPS and TLR4 signalling results in recruitment of TBK1, which in turn phosphorylates serine 727 on STAT3 to enable a proinflammatory switch via an effect on mitochondrial metabolism.

NLRP3 is an intracellular sensor protein that when activated by a broad spectrum of exogenous and endogenous stimuli leads to inflammasome formation and pyroptosis 1 , 2 . The conformational states of NLRP3 and the way antagonistic small molecules act at the molecular level remain poorly understood 2 , 3 . Here we report the cryo-electron microscopy structures of full-length human NLRP3 in its native form and complexed with the inhibitor CRID3 (also named MCC950) 4 . Inactive, ADP-bound NLRP3 is a decamer composed of homodimers of intertwined leucine-rich repeat (LRR) domains that assemble back-to-back as pentamers. The NACHT domain is located at the apical axis of this spherical structure. One pyrin domain dimer is in addition formed inside the LRR cage. Molecular contacts between the concave sites of two opposing LRR domains are mediated by an acidic loop that extends from an LRR transition segment. Binding of CRID3 considerably stabilizes the NACHT and LRR domains relative to each other. CRID3 binds into a cleft, connecting four subdomains of the NACHT with the transition LRR. Its central sulfonylurea group interacts with the Walker A motif of the NLRP3 nucleotide-binding domain and is sandwiched between two arginine residues, which explains the specificity of NLRP3 for this chemical entity. With the determination of the binding site of this key therapeutic agent, specific targeting of NLRP3 for the treatment of autoinflammatory and autoimmune diseases and rational drug optimization is within reach. Cryo-electron microscopy structures of human NLRP3 in its resting state and bound to the inhibitor CRID3 provide insight into the binding mechanism of CRID3 and its mode of antagonism.

The COVID-19 pandemic continues to have an unprecedented impact on societies and economies worldwide. There remains an ongoing need for high-performance SARS-CoV-2 tests which may be broadly deployed for infection monitoring. Here we report a highly sensitive single molecule array (Simoa) immunoassay in development for detection of SARS-CoV-2 nucleocapsid protein (N-protein) in venous and capillary blood and saliva. In all matrices in the studies conducted to date we observe >98% negative percent agreement and >90% positive percent agreement with molecular testing for days 1–7 in symptomatic, asymptomatic, and pre-symptomatic PCR+ individuals. N-protein load decreases as anti-SARS-CoV-2 spike-IgG increases, and N-protein levels correlate with RT-PCR Ct-values in saliva, and between matched saliva and capillary blood samples. This Simoa SARS-CoV-2 N-protein assay effectively detects SARS-CoV-2 infection via measurement of antigen levels in blood or saliva, using non-invasive, swab-independent collection methods, offering potential for at home and point of care sample collection. Here the authors develop a single molecule array (Simoa) immunoassay for detection of SARS-CoV-2 nucleocapsid protein in venous and dried capillary blood as well as saliva. The assay shows good performance in symptomatic, asymptomatic, and pre-symptomatic PCR+ individuals.

The cytoplasmic DNA receptor cGAS catalyses the synthesis of the second messenger cGAMP, which in turn activates type I interferon via STING; this study shows that cGAMP is transmitted to neighbouring cells via gap junction channels and activates STING, thus inducing an antiviral state in these bystander cells independent of paracrine interferon signalling. A novel pathway for innate immunity This paper reports a novel innate immune signalling pathway that depends on intercellular communication. The cytoplasmic DNA receptor cGAMP synthase (cGAS) catalyses the synthesis of the second messenger cGAMP, which in turn activates type I interferon via the transmembrane protein STING. Veit Hornung and colleagues show that cGAMP is transmitted to neighbouring cells via gap junction channels and activates STING, thereby inducing an antiviral state in these bystander cells independent of paracrine interferon signalling. Bystander activation and signal amplification might prove beneficial for the restriction of viral infection, but might also aggravate disease manifestations in STING-dependent autoimmune syndromes, making it a potential target for therapeutics. The innate immune defence of multicellular organisms against microbial pathogens requires cellular collaboration. Information exchange allowing immune cells to collaborate is generally attributed to soluble protein factors secreted by pathogen-sensing cells. Cytokines, such as type I interferons (IFNs), serve to alert non-infected cells to the possibility of pathogen challenge 1 . Moreover, in conjunction with chemokines they can instruct specialized immune cells to contain and eradicate microbial infection. Several receptors and signalling pathways exist that couple pathogen sensing to the induction of cytokines, whereas cytosolic recognition of nucleic acids seems to be exquisitely important for the activation of type I IFNs, master regulators of antiviral immunity 2 . Cytosolic DNA is sensed by the receptor cyclic GMP-AMP (cGAMP) synthase (cGAS), which catalyses the synthesis of the second messenger cGAMP(2′-5′) 3 , 4 , 5 , 6 , 7 , 8 . This molecule in turn activates the endoplasmic reticulum (ER)-resident receptor STING 9 , 10 , 11 , thereby inducing an antiviral state and the secretion of type I IFNs. Here we find in murine and human cells that cGAS-synthesized cGAMP(2′-5′) is transferred from producing cells to neighbouring cells through gap junctions, where it promotes STING activation and thus antiviral immunity independently of type I IFN signalling. In line with the limited cargo specificity of connexins, the proteins that assemble gap junction channels, most connexins tested were able to confer this bystander immunity, thus indicating a broad physiological relevance of this local immune collaboration. Collectively, these observations identify cGAS-triggered cGAMP(2′-5′) transfer as a novel host strategy that serves to rapidly convey antiviral immunity in a transcription-independent, horizontal manner.