Caspase-4 mediates non-canonical activation of the NLRP3 inflammasome in human myeloid cells

The role of interleukin-1 in general pathology

Interleukin-1, an inflammatory cytokine, is considered to have diverse physiological functions and pathological significances and play an important role in health and disease. In this decade, interleukin-1 family members have been expanding and evidence is accumulating that highlights the importance of interleukin-1 in linking innate immunity with a broad spectrum of diseases beyond inflammatory diseases. In this review, we look back on the definition of "inflammation" in traditional general pathology and discuss new insights into interleukin-1 in view of its history and the molecular bases of diseases, as well as current progress in therapeutics.

Role of Pyroptosis in Cardiovascular Diseases and its Therapeutic Implications

Pyroptotic cell death or pyroptosis is characterized by caspase-1-dependent formation of plasma membrane pores, leading to the release of pro-inflammatory cytokines and cell lysis. Pyroptosis tightly controls the inflammatory responses and coordinates antimicrobial host defenses by releasing pro-inflammatory cellular contents, such as interleukin (IL)-1β and IL-18, and consequently expands or sustains inflammation. It is recognized as an important innate immune effector mechanism against intracellular pathogens. The induction of pyroptosis is closely associated with the activation of the NOD-like receptor 3 (NLRP3) inflammasome which has been linked to key cardiovascular risk factors including hyperlipidemia, diabetes, hypertension, obesity, and hyperhomocysteinemia. Emerging evidence has indicated pyroptosis as an important trigger and endogenous regulator of cardiovascular inflammation. Thus, pyroptosis may play an important role in the pathogenesis of cardiovascular diseases. Design of therapeutic strategies targeting the activation of NLRP3 inflammasome and pyroptosis holds promise for the treatment of cardiovascular diseases.

Recognition of Intracellular Bacteria by Inflammasomes

Inflammasomes are multiprotein signaling complexes that are assembled by cytosolic sensors upon the detection of infectious or noxious stimuli. These complexes activate inflammatory caspases to induce host cell death and cytokine secretion and are an essential part of antimicrobial host defense. In this review, I discuss how intracellular bacteria are detected by inflammasomes, how the specific sensing mechanism of each inflammasome receptor restricts the ability of bacteria to evade immune recognition, and how host cell death is used to control bacterial replication in vivo.

Early endosome autoantigen 1 regulates IL-1β release upon caspase-1 activation independently of gasdermin D membrane permeabilization

Unconventional protein secretion represents an important process of the inflammatory response. The release of the pro-inflammatory cytokine interleukin (IL)-1β which burst during pyroptosis as a consequence of gasdermin D plasma membrane pore formation, can also occur through other unconventional secretion pathways dependent on caspase-1 activation. However, how caspase-1 mediates cytokine release independently of gasdermin D remains poorly understood. Here we show that following caspase-1 activation by different inflammasomes, caspase-1 cleaves early endosome autoantigen 1 (EEA1) protein at Asp^127/132. Caspase-1 activation also results in the release of the endosomal EEA1 protein in a gasdermin D-independent manner. EEA1 knock-down results in adecreased release of caspase-1 and IL-1β, but the pyroptotic release of other inflammasome components and lactate dehydrogenase was not affected. This study shows how caspase-1 control the release of EEA1 and IL-1β in a pyroptotic-independent manner.

Bacterial Outer Membrane Vesicles Provide Broad-Spectrum Protection against Influenza Virus Infection via Recruitment and Activation of Macrophages

Influenza A virus (IAV) poses a constant worldwide threat to human health. Although conventional vaccines are available, their protective efficacy is type or strain specific, and their production is time-consuming. For the control of an influenza pandemic in particular, agents that are immediately effective against a wide range of virus variants should be developed. Although pretreatment of various Toll-like receptor (TLR) ligands have already been reported to be effective in the defense against subsequent IAV infection, the efficacy was limited to specific subtypes, and safety concerns were also raised. In this study, we investigated the protective effect of an attenuated bacterial outer membrane vesicle -harboring modified lipid A moiety of lipopolysaccharide (fmOMV) against IAV infection and the underlying mechanisms. Administration of fmOMV conferred significant protection against a lethal dose of pandemic H1N1, PR8, H5N2, and highly pathogenic H5N1 viruses; this broad antiviral activity was dependent on macrophages but independent of neutrophils. fmOMV induced recruitment and activation of macrophages and elicited type I IFNs. Intriguingly, fmOMV showed a more significant protective effect than other TLR ligands tested in previous reports, without exhibiting any adverse effect. These results show the potential of fmOMV as a prophylactic agent for the defense against influenza virus infection.

Entamoeba histolytica-induced IL-1β secretion is dependent on caspase-4 and gasdermin D

During invasion, Entamoeba histolytica (Eh) encounter macrophages and activate them to elicit tissue damaging pro-inflammatory responses. When Eh binds macrophages via the Gal-lectin, surface EhCP-A5 RGD sequence ligates α₅β₁ integrin to activate caspase-1 in a complex known as the NLRP3 inflammasome. In this study, we investigated Eh requirements underlying macrophage caspase-4 and -1 activation and the role caspase-4 and gasdermin D (GSDMD) play in augmenting pro-inflammatory cytokine responses. Caspase-4 activation was similar to caspase-1 requiring live Eh attachment via the Gal-lectin and EhCP-A5. However, unlike caspase-1, caspase-4 activation was independent of ASC and NLRP3. Using CRISPR/Cas9 gene editing of caspase-4 and -1 and GSDMD, we determined that caspase-1 and bioactive IL-1β release was highly dependent on caspase-4 activation and cleavage of GSDMD in response to Eh. Formaldehyde cross-linking to stabilize protein-protein interactions in transfected COS-7 cells stimulated with Eh revealed that caspase-4 specifically interacted with caspase-1 in a protein complex that enhanced the cleavage of caspase-1 CARD domains to augment IL-1β release. Activated caspase-4 and -1 cleaved GSDMD liberating the N-terminal p30 pore-forming fragment that caused the secretion of IL-1β. These findings reveal a novel role for caspase-4 as a sensor molecule to amplify pro-inflammatory responses when macrophage encounters Eh.

The classical NLRP3 inflammasome controls FADD unconventional secretion through microvesicle shedding

Fas-associated death domain (FADD) is a key adaptor molecule involved in numerous physiological processes including cell death, proliferation, innate immunity and inflammation. Therefore, changes in FADD expression have dramatic cellular consequences. In mice and humans, FADD regulation can occur through protein secretion. However, the molecular mechanisms accounting for human FADD secretion were still unknown. Here we report that canonical, non-canonical, but not alternative, NLRP3 inflammasome activation in human monocytes/macrophages induced FADD secretion. NLRP3 inflammasome activation by the bacterial toxin nigericin led to the proinflammatory interleukin-1β (IL-1β) release and to the induction of cell death by pyroptosis. However, we showed that FADD secretion could occur in absence of increased IL-1β release and pyroptosis and, reciprocally, that IL-1β release and pyroptosis could occur in absence of FADD secretion. Especially, FADD, but not IL-1β, secretion following NLRP3 inflammasome activation required extracellular glucose. Thus, FADD secretion was an active process distinct from unspecific release of proteins during pyroptosis. This FADD secretion process required K⁺ efflux, NLRP3 sensor, ASC adaptor and CASPASE-1 molecule. Moreover, we identified FADD as a leaderless protein unconventionally secreted through microvesicle shedding, but not exosome release. Finally, we established human soluble FADD as a new marker of joint inflammation in gout and rheumatoid arthritis, two rheumatic diseases involving the NLRP3 inflammasome. Whether soluble FADD could be an actor in these diseases remains to be determined. Nevertheless, our results advance our understanding of the mechanisms contributing to the regulation of the FADD protein expression in human cells.

NOD-like receptors and inflammasomes: A review of their canonical and non-canonical signaling pathways

The NOD-like receptor (NLR) family of proteins is a group of pattern recognition receptors (PRRs) known to mediate the initial innate immune response to cellular injury and stress. The NLRP proteins represent a fourteen-member subset of the NLR family that contains an N-terminal pyrin domain. Some NLRs are known to form multi-protein complexes known as inflammasomes. Inflammasomes consist of an NLR, the adaptor protein ASC, and the effector molecule pro-caspase-1. Once activated, these inflammasomes facilitate the cleavage and activation of caspase-1, which in turn mediates the cleavage of the pro-inflammatory cytokines IL-1β and IL-18 into their active and secreted forms. Activated caspase-1 also drives the cleavage of gasdermin D, which triggers an inflammatory form of cell death known as pyroptosis. Several NLRs are also known to possess non-canonical, inflammasome-independent functions, regulating a variety of signaling pathways. In this review, a thorough overview of both inflammasome-dependent and -independent NLR signaling will be presented, with highlights from the field as well as promising future directions and postulates based on the known science.

Human Cytomegalovirus Immediate Early 86-kDa Protein Blocks Transcription and Induces Degradation of the Immature Interleukin-1β Protein during Virion-Mediated Activation of the AIM2 Inflammasome

Secretion of interleukin-1β (IL-1β) represents a fundamental innate immune response to microbial infection that, at the molecular level, occurs following activation of proteolytic caspases that cleave the immature protein into a secretable form. Human cytomegalovirus (HCMV) is the archetypal betaherpesvirus that is invariably capable of lifelong infection through the activity of numerous virally encoded immune evasion phenotypes. Innate immune pathways responsive to cytoplasmic double-stranded DNA (dsDNA) are known to be activated in response to contact between HCMV and host cells. Here, we used clustered regularly interspaced short palindromic repeat (CRISPR)-CRISPR-associated protein 9 (Cas9) genome editing to demonstrate that the dsDNA receptor absent in melanoma 2 (AIM2) is required for secretion of IL-1β following HCMV infection. Furthermore, dsDNA-responsive innate signaling induced by HCMV infection that leads to activation of the type I interferon response is also shown, unexpectedly, to play a contributory role in IL-1β secretion. Importantly, we also show that rendering virus particles inactive by UV exposure leads to substantially increased IL-1β processing and secretion and that live HCMV can inhibit this, suggesting the virus encodes factors that confer an inhibitory effect on this response. Further examination revealed that ectopic expression of the immediate early (IE) 86-kDa protein (IE86) is actually associated with a block in transcription of the pro-IL-1β gene and, independently, diminishment of the immature protein. Overall, these results reveal two new and distinct phenotypes conferred by the HCMV IE86 protein, as well as an unusual circumstance in which a single herpesviral protein exhibits inhibitory effects on multiple molecular processes within the same innate immune response.IMPORTANCE Persistent infection with HCMV is associated with the operation of diverse evasion phenotypes directed at antiviral immunity. Obstruction of intrinsic and innate immune responses is typically conferred by viral proteins either associated with the viral particle or expressed immediately after entry. In line with this, numerous phenotypes are attributed to the HCMV IE86 protein that involve interference with innate immune processes via transcriptional and protein-directed mechanisms. We describe novel IE86-mediated phenotypes aimed at virus-induced secretion of IL-1β. Intriguingly, while many viruses target the function of the molecular scaffold required for IL-1β maturation to prevent this response, we find that HCMV and IE86 target the IL-1β protein specifically. Moreover, we show that IE86 impairs both the synthesis of the IL-1β transcript and the stability of the immature protein. This indicates an unusual phenomenon in which a single viral protein exhibits two molecularly separate evasion phenotypes directed at a single innate cytokine.

Recent advances in the mechanisms of NLRP3 inflammasome activation and its inhibitors

The NLRP3 inflammasome is a multimeric protein complex that initiates an inflammatory form of cell death and triggers the release of proinflammatory cytokines IL-1β and IL-18. The NLRP3 inflammasome has been implicated in a wide range of diseases, including Alzheimer’s disease, Prion diseases, type 2 diabetes, and some infectious diseases. It has been found that a variety of stimuli including danger-associated molecular patterns (DAMPs, such as silica and uric acid crystals) and pathogen-associated molecular patterns (PAMPs) can activate NLRP3 inflammasome, but the specific regulatory mechanisms of NLRP3 inflammasome activation remain unclear. Understanding the mechanisms of NLRP3 activation will enable the development of its specific inhibitors to treat NLRP3-related diseases. In this review, we summarize current understanding of the regulatory mechanisms of NLRP3 inflammasome activation as well as inhibitors that specifically and directly target NLRP3.

Fli-1 transcription factor regulates the expression of caspase-1 in lung pericytes

Our previous data demonstrated that Friend leukemia virus integration 1 (Fli-1), an ETS transcription factor, governs pericyte loss and vascular dysfunction in cecal ligation and puncture-induced murine sepsis by regulating essential pyroptosis markers including caspase-1. However, whether Fli-1 regulates caspase-1 expression levels in vitro and how Fli-1 regulates caspase-1 remain unknown. Our present work further demonstrated that overexpressed Fli-1 significantly increased caspase-1 and IL-18 expression levels in cultured mouse lung pericytes. Bacterial outer membrane vesicles (OMVs) have been found to induce cell pyroptosis through transferring LPS intracellularly. Using OMVs to induce an in vitro model of pyroptosis, we observed that OMVs significantly increased protein levels of Fli-1 in mouse lung pericytes. Furthermore, knockdown of Fli-1 by siRNA blocked OMVs-induced caspase-1, caspase-11 and IL-18 expression levels. As caspase-1 was predicted as a potential target of Fli-1, we cloned murine caspase-1 promoter into a luciferase construct. Our data demonstrate for the first time that Fli-1 regulates caspase-1 expression by directly binding to its promoter regions measured by chromatin immunoprecipitation (ChIP) assay and luciferase reporter system. In summary, our findings demonstrated a novel role and mechanism of Fli-1 in regulating caspase-1 expression in lung pericytes.

Inflammasome activation: Neutrophils go their own way

Discussion on a new RIPK3-dependent pathway to cell death and cytokine secretion independent of the canonical necroptosis and inflammasome machinery in neutrophils.

Inflammasomes, Autophagy, and Cell Death: The Trinity of Innate Host Defense against Intracellular Bacteria

Inflammasome activation is an innate host defense mechanism initiated upon sensing pathogens or danger in the cytosol. Both autophagy and cell death are cell autonomous processes important in development, as well as in host defense against intracellular bacteria. Inflammasome, autophagy, and cell death pathways can be activated by pathogens, pathogen-associated molecular patterns (PAMPs), cell stress, and host-derived damage-associated molecular patterns (DAMPs). Phagocytosis and toll-like receptor (TLR) signaling induce reactive oxygen species (ROS), type I IFN, NFκB activation of proinflammatory cytokines, and the mitogen-activated protein kinase cascade. ROS and IFNγ are also prominent inducers of autophagy. Pathogens, PAMPs, and DAMPs activate TLRs and intracellular inflammasomes, inducing apoptotic and inflammatory caspases in a context-dependent manner to promote various forms of cell death to eliminate pathogens. Common downstream signaling molecules of inflammasomes, autophagy, and cell death pathways interact to initiate appropriate measures against pathogens and determine host survival as well as pathological consequences of infection. The integration of inflammasome activation, autophagy, and cell death is central to pathogen clearance. Various pathogens produce virulence factors to control inflammasomes, subvert autophagy, and modulate host cell death in order to evade host defense. This review highlights the interaction of inflammasomes, autophagy, and host cell death pathways in counteracting Burkholderia pseudomallei, the causative agent of melioidosis. Contrasting evasion strategies used by B. pseudomallei, Mycobacterium tuberculosis, and Legionella pneumophila to avoid and dampen these innate immune responses will be discussed.

Guanylate-binding protein 5 licenses caspase-11 for Gasdermin-D mediated host resistance to Brucella abortus infection

Innate immune response against Brucella abortus involves activation of Toll-like receptors (TLRs) and NOD-like receptors (NLRs). Among the NLRs involved in the recognition of B. abortus are NLRP3 and AIM2. Here, we demonstrate that B. abortus triggers non-canonical inflammasome activation dependent on caspase-11 and gasdermin-D (GSDMD). Additionally, we identify that Brucella-LPS is the ligand for caspase-11 activation. Interestingly, we determine that B. abortus is able to trigger pyroptosis leading to pore formation and cell death, and this process is dependent on caspase-11 and GSDMD but independently of caspase-1 protease activity and NLRP3. Mice lacking either caspase-11 or GSDMD were significantly more susceptible to infection with B. abortus than caspase-1 knockout or wild-type animals. Additionally, guanylate-binding proteins (GBPs) present in mouse chromosome 3 participate in the recognition of LPS by caspase-11 contributing to non-canonical inflammasome activation as observed by the response of Gbp^chr3-/- BMDMs to bacterial stimulation. We further determined by siRNA knockdown that among the GBPs contained in mouse chromosome 3, GBP5 is the most important for Brucella LPS to be recognized by caspase-11 triggering IL-1β secretion and LDH release. Additionally, we observed a reduction in neutrophil, dendritic cell and macrophage influx in spleens of Casp11^-/- and Gsdmd^-/- compared to wild-type mice, indicating that caspase-11 and GSDMD are implicated in the recruitment and activation of immune cells during Brucella infection. Finally, depletion of neutrophils renders wild-type mice more susceptible to Brucella infection. Taken together, these data suggest that caspase-11/GSDMD-dependent pyroptosis triggered by B. abortus is important to infection restriction in vivo and contributes to immune cell recruitment and activation.

Brucella abortus is the causative agent of brucellosis, a zoonotic disease that affects both humans and cattle. In humans, it is characterized by undulant fever and chronic symptoms as arthritis, endocarditis, and meningitis, while in cattle it causes abortion and infertility. Due to its difficult diagnosis and treatment, it leads to severe economic losses and human suffering. Recently, a novel non-canonical inflammasome pathway was described that involves sensing of bacterial LPS by an intracellular receptor termed caspase-11 and leads to pyroptosis and non-canonical NLRP3 inflammasome activation. Here, we show that B. abortus or its purified LPS is able to activate the non-canonical inflammasome. In this process, activated caspase-11 leads to GSDMD-dependent pyroptosis. Moreover, this pathway was dependent of IFN-induced GBP proteins, mainly GBP5. To analyze the role of caspase-1, caspase-11 and GSDMD in controlling B. abortus infection, we infected knockout (KO) mice for these molecules and we observed that caspase-11 and GSDMD KO animals were more susceptible to infection compared to wild-type animals. Casp11^-/- and Gsdmd^-/- animals also recruited less immune cells in mouse spleens compared to wild-type animals in response to B. abortus. Thus, caspase-11 and GSDMD are major components of the innate immune system to restrict B. abortus in vivo. This pathway of bacterial sensing by the host immune system is important to future development of drugs and vaccines that may contribute to the control of brucellosis worldwide.

Role of the NLRP3 inflammasome in cancer

Inflammasomes are large intracellular multi-protein signalling complexes that are formed in the cytosolic compartment as an inflammatory immune response to endogenous danger signals. The formation of the inflammasome enables activation of an inflammatory protease caspase-1, pyroptosis initiation with the subsequent cleaving of the pro-inflammatory cytokines interleukin (IL)-1β and proIL-18 to produce active forms. The inflammasome complex consists of a Nod-like receptor (NLR), the adapter apoptosis-associated speck-like (ASC) protein, and Caspase-1. Dysregulation of NLRP3 inflammasome activation is involved tumor pathogenesis, although its role in cancer development and progression remains controversial due to the inconsistent findings described. In this review, we summarize the current knowledge on the contribution of the NLRP3 inflammasome on potential cancer promotion and therapy.

Principles of inflammasome priming and inhibition: Implications for psychiatric disorders

The production of inflammatory proteins by the innate immune system is a tightly orchestrated procedure that allows the body to efficiently respond to exogenous and endogenous threats. Recently, accumulating evidence has indicated that disturbances in the inflammatory response system not only provoke autoimmune disorders, but also can have deleterious effects on neuronal function and mental health. As inflammation in the brain is primarily mediated by microglia, there has been an expanding focus on the mechanisms through which these cells initiate and propagate neuroinflammation. Microglia can enter persistently active states upon their initial recognition of an environmental stressor and are thereafter prone to elicit amplified and persistent inflammatory responses following subsequent exposures to stressors. A recent focus on why primed microglia cells are susceptible to environmental insults has been the NLRP3 inflammasome. Its function within the innate immune system is regulated in such a manner that supports a role for the complex in gating neuroinflammatory responses. The activation of NLRP3 inflammasome in microglia results in the cleavage of zymogen inflammatory interleukins into functional forms that elicit a number of consequential effects in the local neuronal environment. There is evidence to support the principle that within primed neuroimmune systems a lowered threshold for NLRP3 activation can cause persistent neuroinflammation or the amplified production of inflammatory cytokines, such as IL-1β and IL-18. Over the course of an individual's lifetime, persistent neuroinflammation can subsequently lead to the pathophysiological signatures that define psychological disorders. Therefore, targeting the NLRP3 inflammasome complex may represent an innovative and consequential approach to limit neuroinflammatory states in psychiatric disorders, such as major depressive disorder.

From Inflammasome to Exosome-Does Extracellular Vesicle Secretion Constitute an Inflammasome-Dependent Immune Response?

Inflammasomes are intracellular protein complexes of pattern recognition receptors and caspase-1, with essential functions in regulating inflammatory responses of macrophages and dendritic cells. The primary role of inflammasomes is to catalyze processing and secretion of pro-inflammatory cytokines IL-1β and IL-18. Recently, intracellular non-canonical inflammasome activation by caspases-4/5, which are also regulators of pyroptosis via processing gasdermin D, has been elucidated. Caspase-1, the effector protease of inflammasome complex, is also known to modulate secretion of large number of other proteins. Thereby, besides its known role in processing pro-inflammatory cytokines, the inflammasome turns into a universal regulator of protein secretion, which allows the danger-exposed cells to release various proteins in order to alert and guide neighboring cells. Majority of these proteins are not secreted through the conventional ER-Golgi secretory pathway. Instead, they are segregated in membrane-enclosed compartment and secreted in nanosized extracellular vesicles, which protect their cargo and guide it for delivery. Growing evidence indicates that inflammasome activity correlates with enhanced secretion of extracellular vesicles and modulation of their protein cargo. This inflammasome-driven unconventional, vesicle-mediated secretion of multitude of immunoregulatory proteins may constitute a novel paradigm in inflammatory responses. In this mini review we discuss the current knowledge and highlight unsolved questions about metabolic processes, signals, and mechanisms linking inflammasome activity with regulated extracellular vesicle secretion of proteins. Further investigations on this relationship may in the future help understanding the significance of extracellular vesicle secretion in inflammatory diseases such as atherosclerosis, gouty arthritis, asthma, Alzheimer's and many others.

Cytosolic Recognition of Microbes and Pathogens: Inflammasomes in Action

Infection is a dynamic biological process underpinned by a complex interplay between the pathogen and the host. Microbes from all domains of life, including bacteria, viruses, fungi, and protozoan parasites, have the capacity to cause infection. Infection is sensed by the host, which often leads to activation of the inflammasome, a cytosolic macromolecular signaling platform that mediates the release of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18 and cleavage of the pore-forming protein gasdermin D, leading to pyroptosis. Host-mediated sensing of the infection occurs when pathogens inject or carry pathogen-associated molecular patterns (PAMPs) into the cytoplasm or induce damage that causes cytosolic liberation of danger-associated molecular patterns (DAMPs) in the host cell. Recognition of PAMPs and DAMPs by inflammasome sensors, including NLRP1, NLRP3, NLRC4, NAIP, AIM2, and Pyrin, initiates a cascade of events that culminate in inflammation and cell death. However, pathogens can deploy virulence factors capable of minimizing or evading host detection. This review presents a comprehensive overview of the mechanisms of microbe-induced activation of the inflammasome and the functional consequences of inflammasome activation in infectious diseases. We also explore the microbial strategies used in the evasion of inflammasome sensing at the host-microbe interaction interface.

Gasdermin D Restrains Type I Interferon Response to Cytosolic DNA by Disrupting Ionic Homeostasis

Inflammasome-activated caspase-1 cleaves gasdermin D to unmask its pore-forming activity, the predominant consequence of which is pyroptosis. Here, we report an additional biological role for gasdermin D in limiting cytosolic DNA surveillance. Cytosolic DNA is sensed by Aim2 and cyclic GMP-AMP synthase (cGAS) leading to inflammasome and type I interferon responses, respectively. We found that gasdermin D activated by the Aim2 inflammasome suppressed cGAS-driven type I interferon response to cytosolic DNA and Francisella novicida in macrophages. Similarly, interferon-β (IFN-β) response to F. novicida infection was elevated in gasdermin D-deficient mice. Gasdermin D-mediated negative regulation of IFN-β occurred in a pyroptosis-, interleukin-1 (IL-1)-, and IL-18-independent manner. Mechanistically, gasdermin D depleted intracellular potassium (K⁺) via membrane pores, and this K⁺ efflux was necessary and sufficient to inhibit cGAS-dependent IFN-β response. Thus, our findings have uncovered an additional interferon regulatory module involving gasdermin D and K⁺ efflux.

Membrane vesicles from Pseudomonas aeruginosa activate the noncanonical inflammasome through caspase-5 in human monocytes

Outer membrane vesicles (OMVs) are constitutively produced by Gram-negative bacteria both in vivo and in vitro. These lipid-bound structures carry a range of immunogenic components derived from the parent cell, which are transported into host target cells and activate the innate immune system. Recent advances in the field have shed light on some of the multifaceted roles of OMVs in host-pathogen interactions. In this study, we investigated the ability of OMVs from two clinically important pathogens, Pseudomonas aeruginosa and Helicobacter pylori, to activate canonical and noncanonical inflammasomes. P. aeruginosa OMVs induced inflammasome activation in mouse macrophages, as evidenced by "speck" formation, as well as the cleavage and secretion of interleukin-1β and caspase-1. These responses were independent of AIM2 and NLRC4 canonical inflammasomes, but dependent on the noncanonical caspase-11 pathway. Moreover, P. aeruginosa OMVs alone were able to activate the inflammasome in a TLR-dependent manner, without requiring an exogenous priming signal. In contrast, H. pylori OMVs were not able to induce inflammasome activation in macrophages. Using CRISPR/Cas9 knockout THP-1 cells lacking the human caspase-11 homologs, caspase-4 and -5,we demonstrated that caspase-5 but not caspase-4 is required for inflammasome activation by P. aeruginosa OMVs in human monocytes. In contrast, free P. aeruginosa lipopolysaccharide (LPS) transfected into cells induced inflammasome responses via caspase-4. This suggests that caspase-4 and caspase-5 differentially recognize LPS depending on its physical form or route of delivery into the cell. These findings have relevance to Gram-negative infections in humans and the use of OMVs as novel vaccines.

Inflammatory cell death in intestinal pathologies

The intestinal tract is a site of intense immune cell activity that is poised to mount an effective response against a pathogen and yet maintain tolerance toward commensal bacteria and innocuous dietary antigens. The role of cell death in gut pathologies is particularly important as the intestinal epithelium undergoes self-renewal every 4-7 days through a continuous process of cell death and cell division. Cell death is also required for removal of infected, damaged, and cancerous cells. Certain forms of cell death trigger inflammation through release of damage-associated molecular patterns. Further, molecules involved in cell death decisions also moonlight as critical nodes in immune signaling. The manner of cell death is, therefore, highly instructive of the immunological consequences that ensue. Perturbations in cell death pathways can impact the regulation of the immune system with deleterious consequences. In this review, we discuss the various forms of cell death with a special emphasis on lytic cell death pathways of pyroptosis and necroptosis and their implications in inflammation and cancer in the gut. Understanding the implications of distinct cell death pathways will help in the development of therapeutic interventions in intestinal pathologies.

AIM2 Engages Active but Unprocessed Caspase-1 to Induce Noncanonical Activation of the NLRP3 Inflammasome

Inflammasomes are multimeric protein complexes that initiate inflammatory cascades. Their activation is a hallmark of many infectious or inflammatory diseases. Their composition and activity are specified by proinflammatory stimuli. For example, the NLRP3 inflammasome is activated in response to cell damage and K⁺ efflux, whereas the AIM2 inflammasome is activated in response to cytosolic DNA. We used Legionella pneumophila, an intracellular bacterial pathogen that activates multiple inflammasomes, to elucidate the molecular mechanisms regulating inflammasome activation during infection. Upon infection, the AIM2 inflammasome engaged caspase-1 to induce pore formation in the cell membrane, which then caused K⁺-efflux-mediated activation of NLRP3. Thus, the AIM2 inflammasome amplifies signals of infection, triggering noncanonical activation of NLRP3. During infection, AIM2 and caspase-11 induced membrane damage, which was sufficient and essential for activating the NLRP3 inflammasome. Our data reveal that different inflammasomes regulate one another's activity to ensure an effective immune response to infection.

The 'known-knowns', and 'known-unknowns' of extracellular Nm23-H1/NDPK proteins

Nucleoside diphosphate kinases (NDPKs/NDK/NME) are a multifunctional class of proteins conserved throughout evolution. Whilst many of the functions of NDPKs have been identified as intracellular, extracellular eukaryotic and prokaryotic NDPK proteins are also detected in multiple systems and have been implicated in both normal physiology and disease. This review provides an overview of where the field stands on our developing understanding of how NDPK proteins get out of cells, the physiological role of extracellular NDPKs, and how extracellular NDPKs may signal to cells. We will also discuss some of the unanswered questions, the 'known-unknowns' that particularly warrant further investigation.

Diverse pro-inflammatory endotoxin recognition systems of mammalian innate immunity

In humans and other mammals, recognition of endotoxins—abundant surface lipopolysaccharides (LPS) of Gram-negative bacteria—provides a potent stimulus for induction of inflammation and mobilization of host defenses. The structurally unique lipid A region of LPS is the principal determinant of this pro-inflammatory activity. This region of LPS is normally buried within the bacterial outer membrane and aggregates of purified LPS, making even more remarkable its picomolar potency and the ability of discrete variations in lipid A structure to markedly alter the pro-inflammatory activity of LPS. Two recognition systems—MD-2/TLR4 and “LPS-sensing” cytosolic caspases—together confer LPS responsiveness at the host cell surface, within endosomes, and at sites physically accessible to the cytosol. Understanding how the lipid A of LPS is delivered and recognized at these diverse sites is crucial to understanding how the magnitude and character of the inflammatory responses are regulated.

The E3 ubiquitin ligase Pellino2 mediates priming of the NLRP3 inflammasome

The NLRP3 inflammasome has an important function in inflammation by promoting the processing of pro-IL-1β and pro-IL-18 to their mature bioactive forms, and by inducing cell death via pyroptosis. Here we show a critical function of the E3 ubiquitin ligase Pellino2 in facilitating activation of the NLRP3 inflammasome. Pellino2-deficient mice and myeloid cells have impaired activation of NLRP3 in response to toll-like receptor priming, NLRP3 stimuli and bacterial challenge. These functions of Pellino2 in the NLRP3 pathway are dependent on Pellino2 FHA and RING-like domains, with Pellino2 promoting the ubiquitination of NLRP3 during the priming phase of activation. We also identify a negative function of IRAK1 in the NLRP3 inflammasome, and describe a counter-regulatory relationship between IRAK1 and Pellino2. Our findings reveal a Pellino2-mediated regulatory signaling system that controls activation of the NLRP3 inflammasome.

The NLRP3 inflammasome is important for inducing IL-1β and IL-18 inflammatory responses. Here the authors show, by generating and characterizing Peli2 deficient mice and immune cells, that an E3 ubiquitin ligase Pellino2 promotes inflammasome priming by inducing NLRP3 ubiquitination and by targeting IRAK1.

Expression and regulation of alarmin cytokine IL-1α in human retinal pigment epithelial cells

Human retinal pigment epithelial (hRPE) cells play important immune-regulatory roles in a variety of retinal pathologic processes, including the production of inflammatory cytokines that are essential mediators of the innate immune response within the ocular microenvironment. The pro-inflammatory "alarmin" cytokine IL-1α has been implicated in both infectious and non-infectious retinal diseases, but its regulation in the retina is poorly understood. The purpose of this study was to elucidate the expression and regulation of IL-1α within hRPE cells. To do this, IL-1α mRNA and protein in hRPE cells was assessed by RT-PCR, qPCR, ELISA, Western blot, and immunofluorescence following treatment with a variety of stimuli and inhibitors. ER stress, LPS, IL-1β, and TLR2 activation all significantly increased intracellular IL-1α protein. Increasing intracellular calcium synergized both LPS- and Pam3CSK4-induced IL-1α protein production. Accordingly, blocking calcium signaling and calpain activity strongly suppressed IL-1α protein expression. Significant but more moderate inhibition occurred following blockage of TLR4, caspase-4, or caspase-1. Neutralizing antibodies to IL-1β and TLR2 partially eliminated LPS- and TLR2 ligand Pam3CSK4-stimulated IL-1α protein production. IFN-β induced caspase-4 expression and activation, and also potentiated LPS-induced IL-1α expression, but IFN-β alone had no effect on IL-1α protein production. Interestingly, all inhibitors targeting the PI3K/Akt pathway, with the exception of Ly294002, strongly increased IL-1α protein expression. This study improves understanding of the complex mechanisms regulating IL-1α protein expression in hRPE cells by demonstrating that TLR4 and TLR2 stimulation and exposure to IL-1β, ER stress and intracellular calcium all induce hRPE cells to produce intracellular IL-1α, which is negatively regulated by the PI3K/Akt pathway. Additionally, the non-canonical inflammasome pathway was shown to be involved in LPS-induced hRPE IL-1α expression through caspase-4 signaling.

The oxidized phospholipid oxPAPC protects from septic shock by targeting the non-canonical inflammasome in macrophages

Lipopolysaccharide (LPS) of Gram-negative bacteria can elicit a strong immune response. Although extracellular LPS is sensed by TLR4 at the cell surface and triggers a transcriptional response, cytosolic LPS binds and activates non-canonical inflammasome caspases, resulting in pyroptotic cell death, as well as canonical NLRP3 inflammasome-dependent cytokine release. Contrary to the highly regulated multiprotein platform required for caspase-1 activation in the canonical inflammasomes, the non-canonical mouse caspase-11 and the orthologous human caspase-4 function simultaneously as innate sensors and effectors, and their regulation is unclear. Here we show that the oxidized phospholipid 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (oxPAPC) inhibits the non-canonical inflammasome in macrophages, but not in dendritic cells. Aside from a TLR4 antagonistic role, oxPAPC binds directly to caspase-4 and caspase-11, competes with LPS binding, and consequently inhibits LPS-induced pyroptosis, IL-1β release and septic shock. Therefore, oxPAPC and its derivatives might provide a basis for therapies that target non-canonical inflammasomes during Gram-negative bacterial sepsis.

Comparative 'omics analyses differentiate Mycobacterium tuberculosis and Mycobacterium bovis and reveal distinct macrophage responses to infection with the human and bovine tubercle bacilli

Members of the Mycobacterium tuberculosis complex (MTBC) are the causative agents of tuberculosis in a range of mammals, including humans. A key feature of MTBC pathogens is their high degree of genetic identity yet distinct host tropism. Notably, while Mycobacterium bovis is highly virulent and pathogenic for cattle, the human pathogen M. tuberculosis is attenuated in cattle. Previous research also suggests that host preference amongst MTBC members has a basis in host innate immune responses. To explore MTBC host tropism, we present in-depth profiling of the MTBC reference strains M. bovis AF2122/97 and M. tuberculosis H37Rv at both the global transcriptional and the translational level via RNA-sequencing and SWATH MS. Furthermore, a bovine alveolar macrophage infection time course model was used to investigate the shared and divergent host transcriptomic response to infection with M. tuberculosis H37Rv or M. bovis AF2122/97. Significant differential expression of virulence-associated pathways between the two bacilli was revealed, including the ESX-1 secretion system. A divergent transcriptional response was observed between M. tuberculosis H37Rv and M. bovis AF2122/97 infection of bovine alveolar macrophages, in particular cytosolic DNA-sensing pathways at 48 h post-infection, and highlights a distinct engagement of M. bovis with the bovine innate immune system. The work presented here therefore provides a basis for the identification of host innate immune mechanisms subverted by virulent host-adapted mycobacteria to promote their survival during the early stages of infection.

Inflammasome biology, molecular pathology and therapeutic implications

Inflammasomes are intracellular multiprotein signaling complexes, mainly present in myeloid cells. They commonly assemble around a cytoplasmic receptor of the nucleotide-binding leucine-rich repeat containing receptor (NLR) family, although other cytoplasmic receptors like pyrin have been shown to form inflammasomes. The nucleation of the multiprotein scaffolding platform occurs upon detection of a microbial, a danger or a homeostasis pattern by the receptor that will, most commonly, associate with the adaptor protein ASC (apoptosis-associated speck-like protein containing a CARD) through homotypic domain interactions resulting in recruitment of procaspase-1. This will lead to the autoproteolytic activation of caspase-1, which regulates the secretion of proinflammatory IL1β and IL18 cytokines and pyroptosis, a caspase-1-mediated form of cell death. Pyroptosis occurs through cleavage of Gasdermin D, a membrane pore forming protein. Recently, non-canonical inflammasomes have been described, which directly sense intracellular pathogens through caspase-4 and -5 in humans, leading to pyroptosis. Inflammasomes are important in host defense; however, a deregulated activity is associated with a number of inflammatory, immune and metabolic disorders. Furthermore, mutations in inflammasome receptor coding genes are causal for an increasing number of rare autoinflammatory diseases. Biotherapies targeting the products of inflammasome activation as well as molecules that directly or indirectly inhibit inflammasome nucleation and activation are promising therapeutic areas. This review discusses recent advances in inflammasome biology, the molecular pathology of several inflammasomes, and current therapeutic approaches in autoinflammatory diseases and in selected common multifactorial inflammasome-mediated disorders.

Extensive peptide and natural protein substrate screens reveal that mouse caspase-11 has much narrower substrate specificity than caspase-1

Inflammatory cell death, or pyroptosis, is triggered by pathogenic infections or events. It is executed by caspase-1 (in the canonical pyroptosis pathway) or caspase-11 (noncanonical pathway), each via production of a cell-lytic domain from the pyroptosis effector protein gasdermin D through specific and limited proteolysis. Pyroptosis is accompanied by the release of inflammatory mediators, including the proteolytically processed forms of interleukin-1β (IL-1β) and IL-18. Given the similar inflammatory outcomes of the canonical and noncanonical pyroptosis pathways, we hypothesized that caspase-1 and -11 should have very similar activities and substrate specificities. To test this hypothesis, we purified recombinant murine caspases and analyzed their primary specificities by massive hybrid combinatorial substrate library (HyCoSuL) screens. We correlated the substrate preferences of each caspase with their activities on the recombinant natural substrates IL-1β, IL-18, and gasdermin D. Although we identified highly selective and robust peptidyl substrates for caspase-1, we were unable to do so for caspase-11, because caspase-1 cleaved even the best caspase-11 substrates equally well. Caspase-1 rapidly processed pro-IL-1β and -18, but caspase-11 processed these two pro-ILs extremely poorly. However, both caspase-1 and -11 efficiently produced the cell-lytic domain from the gasdermin D precursor. We hypothesize that caspase-11 may have evolved a specific exosite to selectively engage pyroptosis without directly activating pro-IL-1β or -18. In summary, comparing the activities of caspase-1 and -11 in HyCoSuL screens and with three endogenous protein substrates, we conclude that caspase-11 has highly restricted substrate specificity, preferring gasdermin D over all other substrates examined.

Human caspase-4 detects tetra-acylated LPS and cytosolic Francisella and functions differently from murine caspase-11

Caspase-4/5 in humans and caspase-11 in mice bind hexa-acylated lipid A, the lipid moeity of lipopolysaccharide (LPS), to induce the activation of non-canonical inflammasome. Pathogens such as Francisella novicida express an under-acylated lipid A and escape caspase-11 recognition in mice. Here, we show that caspase-4 drives inflammasome responses to F. novicida infection in human macrophages. Caspase-4 triggers F. novicida-mediated, gasdermin D-dependent pyroptosis and activates the NLRP3 inflammasome. Inflammasome activation could be recapitulated by transfection of under-acylated LPS from different bacterial species or synthetic tetra-acylated lipid A into cytosol of human macrophage. Our results indicate functional differences between human caspase-4 and murine caspase-11. We further establish that human Guanylate-binding proteins promote inflammasome responses to under-acylated LPS. Altogether, our data demonstrate a broader reactivity of caspase-4 to under-acylated LPS than caspase-11, which may have important clinical implications for management of sepsis.

Lipid A from some bacteria is sensed differently by humans and mice for the activation of the inflammasomes and inflammatory responses, but the mechanisms are not clear. Here, the authors show that murine caspase-11 and human caspase-4/5 contribute to this differential response via their distinct recognition of under-acylated lipid A.

β-Sheet Breaker Peptide-HPYD for the Treatment of Alzheimer's Disease: Primary Studies on Behavioral Test and Transcriptional Profiling

Background: Alzheimer's disease (AD), is a progressive neurodegenerative disease that is characterized by cognitive loss. Most researchers believe that aggregation and accumulation of β-amyloid peptides (Aβ) in brain cells are the central pathological hallmark of this disease.

Methods: Based on the amyloid hypothesis, a 10 amino acids β-sheet breaker peptide HPYD (His-Lys-Gln-Leu-Pro-Phe-Tyr-Glu-Glu-Asp) was designed according to the structure and sequence of the previous designed peptide H102. Accelerated stability test, thioflavine T (ThT) fluorescence spectral analysis and transmission electron microscopy (TEM) imaging were performed to detect the stability and inhibitory effects on the aggregation of Aβ₁₋₄₂ by H102 and HPYD. FITC-labeled HPYD was first tested to determine whether it could be transferred along the olfactory pathway to the brain after nasal administration to mice. Subsequently, the Morris Water Maze (MWM) test for behavioral analysis was used to investigate the learning and memory ability of APP/PS1 transgenic mice by HPYD. Immunohistochemistry and western blot analysis was performed to determine the role of HPYD on Aβ and APP protein levels. In addition, microarray analysis was used to evaluate the effect of HPYD on gene expression in AD mouse models.

Results: Our in vitro results demonstrated that HPYD had enhanced stability and inhibitory effects on Aβ₁₋₄₂ aggregation compared to H102. HPYD could be delivered into the brain through nasal administration and improved the learning and memory ability in APP/PS1 transgenic mouse models by reducing Aβ and APP protein levels. In addition, microarray analyses suggested that several genes related to the inflammatory pathway, AD and gluco-lipid metabolism were dysregulated and could be restored to almost normal levels after HPYD administration to mice.

Conclusions: Our results demonstrated that HPYD could be a potential therapeutic drug candidate for the treatment of AD.

Emerging Concepts in Innate Immunity

This review introduces recent concepts in innate immunity highlighting some of the latest exciting findings. These include: the discovery of the initiator of pyroptosis, Gasdermin D, and mechanisms of inflammatory caspases in innate immune signaling; the formation of oligomeric signalosomes downstream of innate immune receptors; mechanisms that shape innate immune responses, such as cellular homeostasis, cell metabolism, and pathogen viability; rapid methods of cell-to-cell communication; the interplay between the host and its microbiome and the concept of innate immunological memory. Furthermore, we discuss open questions and illustrate how technological advances, such as CRISPR/Cas9, may provide important answers for outstanding questions in the field of innate immunity.

The NLRP3 Inflammasome Renders Cell Death Pro-inflammatory

NLRP3 is the most studied inflammasome sensor due to its crucial involvement in sterile and infection-triggered inflammation. Although its molecular mode of activation remains to be defined, it is well established that low intracellular potassium concentrations result in its activation. This functionality allows the classical NLRP3 pathway to serve as a highly sensitive, but non-specific surveillance mechanism responding to any type of perturbation that breaches plasma membrane integrity and the associated potassium gradient across the membrane. Here, we review our current knowledge on potassium efflux-dependent NLRP3 activation, with a special focus on how major cell death programs are rendered pro-inflammatory by secondary NLRP3 activation. Apart from the "alternative inflammasome" as the major exception to the rule, this connection explains the fundamental importance of NLRP3 in cell death-associated inflammation and firmly establishes NLRP3 as a principal surveillance mechanism of cellular integrity.

Inflammasome activation and assembly at a glance

Inflammasomes are multimeric protein complexes that typically comprise a sensor, an adaptor and the zymogen procaspase-1. An inflammasome assembles in response to a diverse range of pathogen-associated or danger-associated molecular patterns (PAMPs or DAMPs). The inflammasome platform leads to activation of caspase-1 through proximity-induced self-cleavage, which further induces maturation of interleukins 1β and 18 (IL-1β and IL-18) through proteolytic cleavage of pro-IL-1β and pro-IL-18. Activated caspase-1 also cleaves gasdermin D, which leads to a particular form of cell death called pyroptosis. Mutations in genes that encode inflammasome components are associated with many inflammatory disorders, and studies in the past decade have highlighted the importance of appropriate activation of the inflammasome in homeostasis and disease pathogenesis. Therefore, much attention is being paid to uncover the modulators and regulators of inflammasome assembly and pyroptosis. This Cell Science at a Glance article and accompanying poster outlines the concepts in the activation of inflammasome sensors and assembly of the inflammasome platform. We also discuss recent insights into the mechanisms of regulation of inflammasome activity and the induction of cell death by pyroptosis.

Bruton's Tyrosine Kinase: An Emerging Key Player in Innate Immunity

Bruton’s tyrosine kinase (BTK) was initially discovered as a critical mediator of B cell receptor signaling in the development and functioning of adaptive immunity. Growing evidence also suggests multiple roles for BTK in mononuclear cells of the innate immune system, especially in dendritic cells and macrophages. For example, BTK has been shown to function in Toll-like receptor-mediated recognition of infectious agents, cellular maturation and recruitment processes, and Fc receptor signaling. Most recently, BTK was additionally identified as a direct regulator of a key innate inflammatory machinery, the NLRP3 inflammasome. BTK has thus attracted interest not only for gaining a more thorough basic understanding of the human innate immune system but also as a target to therapeutically modulate innate immunity. We here review the latest developments on the role of BTK in mononuclear innate immune cells in mouse versus man, with specific emphasis on the sensing of infectious agents and the induction of inflammation. Therapeutic implications for modulating innate immunity and critical open questions are also discussed.

Innate sensing and cell-autonomous resistance pathways in Legionella pneumophila infection

Legionella pneumophila is a facultative intracellular bacterium which can cause a severe pneumonia called Legionnaires' disease after inhalation of contaminated water droplets and replication in alveolar macrophages. The innate immune system is generally able to sense and -in most cases- control L. pneumophila infection. Comorbidities and genetic risk factors, however, can compromise the immune system and high infection doses might overwhelm its capacity, thereby enabling L. pneumophila to grow and disseminate inside the lung. The innate immune system mediates sensing of L. pneumophila by employing e.g. NOD-like receptors (NLRs), Toll-like receptors (TLRs), as well as the cGAS/STING pathway to stimulate death of infected macrophages as well as production of proinflammatory cytokines and interferons (IFNs). Control of pulmonary L. pneumophila infection is largely mediated by inflammasome-, TNFα- and IFN-dependent macrophage-intrinsic resistance mechanisms. This article summarizes the current knowledge of innate immune responses to L. pneumophila infection in general, and of macrophage-intrinsic defense mechanisms in particular.

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.

Emerging Insights into Noncanonical Inflammasome Recognition of Microbes

Inflammasomes are cytosolic multi-molecular complexes that sense intracellular microbial danger signals and metabolic perturbations. Inflammasome activation leads to the activation of caspase-1 and the release of pro-inflammatory cytokines IL-1β and IL-18 accompanied by cell death. An inflammasome-based surveillance machinery for Gram-negative bacterial infections has been recently discovered. This noncanonical inflammasome relies on sensing the cytosolic presence of lipopolysaccharide of Gram-negative bacteria via inflammatory caspases such as caspase-4, -5, and -11. This review discusses the recent findings related to the mechanism of activation of the noncanonical inflammasome and its biological functions.

Guanylate Binding Proteins Regulate Inflammasome Activation in Response to Hyperinjected Yersinia Translocon Components

Gram-negative bacterial pathogens utilize virulence-associated secretion systems to inject, or translocate, effector proteins into host cells to manipulate cellular processes and promote bacterial replication. However, translocated bacterial products are sensed by nucleotide binding domain and leucine-rich repeat-containing proteins (NLRs), which trigger the formation of a multiprotein complex called the inflammasome, leading to secretion of interleukin-1 (IL-1) family cytokines, pyroptosis, and control of pathogen replication. Pathogenic Yersinia bacteria inject effector proteins termed Yops, as well as pore-forming proteins that comprise the translocon itself, into target cells. The Yersinia translocation regulatory protein YopK promotes bacterial virulence by limiting hyperinjection of the translocon proteins YopD and YopB into cells, thereby limiting cellular detection of Yersinia virulence activity. How hyperinjection of translocon proteins leads to inflammasome activation is currently unknown. We found that translocated YopB and YopD colocalized with the late endosomal/lysosomal protein LAMP1 and that the frequency of YopD and LAMP1 association correlated with the level of caspase-1 activation in individual cells. We also observed colocalization between YopD and Galectin-3, an indicator of endosomal membrane damage. Intriguingly, YopK limited the colocalization of Galectin-3 with YopD, suggesting that YopK limits the induction or sensing of endosomal membrane damage by components of the type III secretion system (T3SS) translocon. Furthermore, guanylate binding proteins (GBPs) encoded on chromosome 3 (Gbp^Chr3 ), which respond to pathogen-induced damage or alteration of host membranes, were necessary for inflammasome activation in response to hyperinjected YopB/-D. Our findings indicate that lysosomal damage by Yersinia translocon proteins promotes inflammasome activation and implicate GBPs as key regulators of this process.

Salmonella typhimurium-induced IL-1 release from primary human monocytes requires NLRP3 and can occur in the absence of pyroptosis

Large molecular complexes known as inflammasomes regulate the release of IL-1β from immune cells in response to infection and injury. Salmonella typhimurium infection is reported to activate NLRP3 and NLRC4 inflammasomes which are subsequently involved in pyroptosis of the cell and pathogen clearance. However, the response to S. typhimurium in primary human monocytes has not been studied in detail. The aim of this study was to investigate the effect of S. typhimurium on inflammasomes in primary human monocytes. Much of the previous research in the field has been conducted in murine models and human THP-1 cells, which may not reflect the responses of primary human monocytes. Here, we report that inhibiting NLRP3 with the selective inhibitor MCC950, blocked release of IL-1β and the related cytokine IL-1α from primary human monocytes in response to S. typhimurium. Additionally, under these conditions S. typhimurium-induced IL-1 release occurred independently of pyroptosis. We propose that IL-1β release without pyroptosis may occur in early-recruited monocytes to regulate a maximal innate immune response to Salmonella infection, allowing a sustained inflammatory signal. This insight into the mechanisms involved in IL-1 release from primary human monocytes highlights major differences between immune cell types, and the defences they employ during bacterial infection.

The Role of Interferons in Inflammation and Inflammasome Activation

Inflammation is an essential physiological process, which enables survival during infection and maintains tissue homeostasis. Interferons (IFNs) and pro- and anti-inflammatory cytokines are crucial for appropriate response to pathogens, damaged cells, or irritants in inflammatory response. The inflammasom is multiprotein complex, which initiates cleavage of pro-inflammatory cytokines IL-1β and IL-18 into active forms. In addition, inflammasomes initiate pyroptotic cell death. In the present review, I summarize and analyze recent findings regarding the cross talk of IFNs and inflammasomes.

Live-cell visualization of gasdermin D-driven pyroptotic cell death

Pyroptosis is a form of cell death important in defenses against pathogens that can also result in a potent and sometimes pathological inflammatory response. During pyroptosis, GSDMD (gasdermin D), the pore-forming effector protein, is cleaved, forms oligomers, and inserts into the membranes of the cell, resulting in rapid cell death. However, the potent cell death induction caused by GSDMD has complicated our ability to understand the biology of this protein. Studies aimed at visualizing GSDMD have relied on expression of GSDMD fragments in epithelial cell lines that naturally lack GSDMD expression and also lack the proteases necessary to cleave GSDMD. In this work, we performed mutagenesis and molecular modeling to strategically place tags and fluorescent proteins within GSDMD that support native pyroptosis and facilitate live-cell imaging of pyroptotic cell death. Here, we demonstrate that these fusion proteins are cleaved by caspases-1 and -11 at Asp-276. Mutations that disrupted the predicted p30-p20 autoinhibitory interface resulted in GSDMD aggregation, supporting the oligomerizing activity of these mutations. Furthermore, we show that these novel GSDMD fusions execute inflammasome-dependent pyroptotic cell death in response to multiple stimuli and allow for visualization of the morphological changes associated with pyroptotic cell death in real time. This work therefore provides new tools that not only expand the molecular understanding of pyroptosis but also enable its direct visualization.

Inflammasomes and intestinal inflammation

The inflammasome is a cytosolic multi-protein innate immune rheostat, sensing a variety of endogenous and environmental stimuli, and regulating homeostasis or damage control. In the gastrointestinal tract, inflammasomes orchestrate immune tolerance to microbial and potentially food-related signals or drive the initiation of inflammatory responses to invading pathogens. When inadequately regulated, intestinal inflammasome activation leads to a perpetuated inflammatory response leading to immune pathology and tissue damage. In this review, we present the main features of the predominant types of inflammasomes participating in intestinal homeostasis and inflammation. We then discuss current controversies and open questions related to their functions and implications in disease, highlighting how pathological inflammasome over-activation or impaired function impact gut homeostasis, the microbiome ecosystem, and the propensity to develop gut-associated diseases. Collectively, understanding of the molecular basis of intestinal inflammasome signaling may be translated into clinical manipulation of this fundamental pathway as a potential immune modulatory therapeutic intervention.