Caspase-11 activation in response to bacterial secretion systems that access the host cytosol

Insights into the effect of guanylate-binding protein 1 on the survival of Brucella intracellularly

Brucellosis is a zoonotic disease that affects wild and domestic animals. It is caused by members of the bacterial genus Brucella. Guanylate-binding protein 1 (GBP1) is associated with microbial infections. However, the role of GBP1 during Brucella infection remains unclear. This investigation aimed to identify the association of GBP1 with brucellosis. Results showed that Brucella infection induced GBP1 upregulation in RAW 264.7 murine macrophages. Small interfering GBP1 targeting RNAs were utilized to explore how GBP1 regulates the survival of Brucella intracellularly. Results revealed that GBP1 knockdown promoted Brucella's survival ability, activated Nod-like receptor (NLR) containing a pyrin domain 3 (NLRP3) and absent in melanoma 2 (AIM2) inflammatory corpuscles, and induced pro-inflammatory cytokines IFN-γ and IL-1β. Furthermore, Brucella stimulated the expression of GBP1 in bone marrow-derived macrophages (BMDMs) and mice. During the inhibition of GBP1 in BMDMs, the intracellular growth of Brucella increased. In comparison, GBP1 downregulation enhanced the accumulation of Brucella-induced reactive oxygen species (ROS) in macrophages. Overall, the data indicate a significant role of GBP1 in regulating brucellosis and suggest the function underlying its suppressive effect on the survival and growth of Brucella intracellularly.

A bacterial toxin co-opts caspase-3 to disable active gasdermin D and limit macrophage pyroptosis

During infections, host cells are exposed to pathogen-associated molecular patterns (PAMPs) and virulence factors that stimulate multiple signaling pathways that interact additively, synergistically, or antagonistically. The net effect of such higher-order interactions is a vital determinant of the outcome of host-pathogen interactions. Here, we demonstrate one such complex interplay between bacterial exotoxin- and PAMP-induced innate immune pathways. We show that two caspases activated during enterohemorrhagic Escherichia coli (EHEC) infection by lipopolysaccharide (LPS) and Shiga toxin (Stx) interact in a functionally antagonistic manner; cytosolic LPS-activated caspase-11 cleaves full-length gasdermin D (GSDMD), generating an active pore-forming N-terminal fragment (NT-GSDMD); subsequently, caspase-3 activated by EHEC Stx cleaves the caspase-11-generated NT-GSDMD to render it nonfunctional, thereby inhibiting pyroptosis and interleukin-1β maturation. Bacteria typically subvert inflammasomes by targeting upstream components such as NLR sensors or full-length GSDMD but not active NT-GSDMD. Thus, our findings uncover a distinct immune evasion strategy where a bacterial toxin disables active NT-GSDMD by co-opting caspase-3.

Catalytic activity and autoprocessing of murine caspase-11 mediate noncanonical inflammasome assembly in response to cytosolic LPS

Inflammatory caspases are cysteine protease zymogens whose activation following infection or cellular damage occurs within supramolecular organizing centers (SMOCs) known as inflammasomes. Inflammasomes recruit caspases to undergo proximity-induced autoprocessing into an enzymatically active form that cleaves downstream targets. Binding of bacterial LPS to its cytosolic sensor, caspase-11 (Casp11), promotes Casp11 aggregation within a high-molecular-weight complex known as the noncanonical inflammasome, where it is activated to cleave gasdermin D and induce pyroptosis. However, the cellular correlates of Casp11 oligomerization and whether Casp11 forms an LPS-induced SMOC within cells remain unknown. Expression of fluorescently labeled Casp11 in macrophages revealed that cytosolic LPS induced Casp11 speck formation. Unexpectedly, catalytic activity and autoprocessing were required for Casp11 to form LPS-induced specks in macrophages. Furthermore, both catalytic activity and autoprocessing were required for Casp11 speck formation in an ectopic expression system, and processing of Casp11 via ectopically expressed TEV protease was sufficient to induce Casp11 speck formation. These data reveal a previously undescribed role for Casp11 catalytic activity and autoprocessing in noncanonical inflammasome assembly, and shed new light on the molecular requirements for noncanonical inflammasome assembly in response to cytosolic LPS.

Human GBP1 facilitates the rupture of the Legionella-containing vacuole and inflammasome activation

IMPORTANCE

Inflammasomes are essential for host defense against intracellular bacterial pathogens like Legionella, as they activate caspases, which promote cytokine release and cell death to control infection. In mice, interferon (IFN) signaling promotes inflammasome responses against bacteria by inducing a family of IFN-inducible GTPases known as guanylate-binding proteins (GBPs). Within murine macrophages, IFN promotes the rupture of the Legionella-containing vacuole (LCV), while GBPs are dispensable for this process. Instead, GBPs facilitate the lysis of cytosol-exposed Legionella. In contrast, the functions of IFN and GBPs in human inflammasome responses to Legionella are poorly understood. We show that IFN-γ enhances inflammasome responses to Legionella in human macrophages. Human GBP1 is required for these IFN-γ-driven inflammasome responses. Furthermore, GBP1 co-localizes with Legionella and/or LCVs in a type IV secretion system (T4SS)-dependent manner and promotes damage to the LCV, which leads to increased exposure of the bacteria to the host cell cytosol. Thus, our findings reveal species- and pathogen-specific differences in how GBPs function to promote inflammasome responses.

The resolution of phagosomes

Phagocytosis is a fundamental immunobiological process responsible for the removal of harmful particulates. While the number of phagocytic events achieved by a single phagocyte can be remarkable, exceeding hundreds per day, the same phagocytic cells are relatively long-lived. It should therefore be obvious that phagocytic meals must be resolved in order to maintain the responsiveness of the phagocyte and to avoid storage defects. In this article, we discuss the mechanisms involved in the resolution process, including solute transport pathways and membrane traffic. We describe how products liberated in phagolysosomes support phagocyte metabolism and the immune response. We also speculate on mechanisms involved in the redistribution of phagosomal metabolites back to circulation. Finally, we highlight the pathologies owed to impaired phagosome resolution, which range from storage disorders to neurodegenerative diseases.

TNF licenses macrophages to undergo rapid caspase-1, -11, and -8-mediated cell death that restricts Legionella pneumophila infection

The inflammatory cytokine tumor necrosis factor (TNF) is necessary for host defense against many intracellular pathogens, including Legionella pneumophila. Legionella causes the severe pneumonia Legionnaires' disease and predominantly affects individuals with a suppressed immune system, including those receiving therapeutic TNF blockade to treat autoinflammatory disorders. TNF induces pro-inflammatory gene expression, cellular proliferation, and survival signals in certain contexts, but can also trigger programmed cell death in others. It remains unclear, however, which of the pleiotropic functions of TNF mediate control of intracellular bacterial pathogens like Legionella. In this study, we demonstrate that TNF signaling licenses macrophages to die rapidly in response to Legionella infection. We find that TNF-licensed cells undergo rapid gasdermin-dependent, pyroptotic death downstream of inflammasome activation. We also find that TNF signaling upregulates components of the inflammasome response, and that the caspase-11-mediated non-canonical inflammasome is the first inflammasome to be activated, with caspase-1 and caspase-8 mediating delayed pyroptotic death. We find that all three caspases are collectively required for optimal TNF-mediated restriction of bacterial replication in macrophages. Furthermore, caspase-8 is required for control of pulmonary Legionella infection. These findings reveal a TNF-dependent mechanism in macrophages for activating rapid cell death that is collectively mediated by caspases-1, -8, and -11 and subsequent restriction of Legionella infection.

Brucella induces heme oxygenase-1 expression to promote its infection

Brucellosis is a zoonotic and contagious infectious disease caused by Brucella spp, which causes substantial economic losses to animal husbandry and leads to severe public health problems. Brucella have evolved multiple strategies to escape host immunity and survive within host cells. Elucidating the immune evasion strategies during Brucella infection will facilitate the control of brucellosis. The host enzyme, heme oxygenase-1 (HO-1), is a multifunctional protein that functions during inflammatory diseases and microbial infections. However, how HO-1 functions during Brucella infection is rarely studied. In this study, we evaluated the role of HO-1 during Brucella infection. We found that Brucella infection induced HO-1 expression in macrophages. We further showed that HO-1 was regulated by PI3K, AMPK kinase, and nuclear erythroid-related factor 2 (Nrf2) in macrophages. Interestingly, knocking out HO-1 or inhibiting the activity of HO-1 significantly decreased Brucella intracellular growth. Inducing the expression of HO-1 by treatment with CoPP promoted Brucella intracellular growth. Mechanistic analyses indicated that the effect of HO-1 was not meditated by HO-1 metabolites, but by decreasing the production of reactive oxygen species (ROS), TNF-α, and IL-1β. Moreover, Brucella induced HO-1 expression in bone marrow-derived macrophages (BMDMs) and mice. When the expression of HO-1 was knocked down in BMDMs, the intracellular survival of Brucella was reduced. Furthermore, the induction of HO-1 by CoPP significantly increased bacterial loads in vivo. Thus, we demonstrated that Brucella induced HO-1 expression to promote its survival and growth in vitro and in vivo. This study also identified HO-1 as a novel innate immune evasion factor during Brucella infection.

Innate Sensors Trigger Regulated Cell Death to Combat Intracellular Infection

Intracellular pathogens pose a significant threat to animals. In defense, innate immune sensors attempt to detect these pathogens using pattern recognition receptors that either directly detect microbial molecules or indirectly detect their pathogenic activity. These sensors trigger different forms of regulated cell death, including pyroptosis, apoptosis, and necroptosis, which eliminate the infected host cell niche while simultaneously promoting beneficial immune responses. These defenses force intracellular pathogens to evolve strategies to minimize or completely evade the sensors. In this review, we discuss recent advances in our understanding of the cytosolic pattern recognition receptors that drive cell death, including NLRP1, NLRP3, NLRP6, NLRP9, NLRC4, AIM2, IFI16, and ZBP1. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Role of Inflammasomes in the Pathogenesis of Neurodegenerative Diseases

Abstract—Protein misfolding and accumulation of protein aggregates is a distinctive feature of most neurodegenerative diseases. They lead to disruption of cellular homeostasis, loss of synaptic connections, and therefore cellular apoptosis. It has been demonstrated that some innate immune responses play an important role in the emergence and progression of neurodegenerative diseases. Inflammasomes are components of innate immunity that play a major role in the maintenance of chronic inflammation. Inflammasomes function as intracellular sensors, detecting both exogenous and endogenous stimuli. They also take part in caspase-1 activation and the synthesis of pro-inflammatory cytokines. In the central nervous system (CNS), inflammasomes are predominantly expressed by microglia, the key cells of innate immunity responsible for activation and maintenance of inflammation. In addition to microglia, inflammasomes can be expressed and activated by astrocytes and neurons, as well as infiltrating myeloid cells. Understanding the mechanisms of activation and functioning of inflammasomes is crucial for the development of novel drugs targeted at modulation of the immune response associated with their excessive activation. This review provides up-to-date information on the inflammasome structure and mechanisms of action, the role of protein misfolding, aggregation and the influence of these factors on inflammasome activation, as well as potential therapeutic targets in neurodegenerative diseases.

Role of the Yersinia pseudotuberculosis Virulence Plasmid in Pathogen-Phagocyte Interactions in Mesenteric Lymph Nodes

Yersinia pseudotuberculosis is an Enterobacteriaceae family member that is commonly transmitted by the fecal-oral route to cause infections. From the small intestine, Y. pseudotuberculosis can invade through Peyer's patches and lymph vessels to infect the mesenteric lymph nodes (MLNs). Infection of MLNs by Y. pseudotuberculosis results in the clinical presentation of mesenteric lymphadenitis. MLNs are important for immune responses to intestinal pathogens and microbiota in addition to their clinical relevance to Y. pseudotuberculosis infections. A characteristic of Y. pseudotuberculosis infection in MLNs is the formation of pyogranulomas. Pyogranulomas are composed of neutrophils, inflammatory monocytes, and lymphocytes surrounding extracellular microcolonies of Y. pseudotuberculosis. Key elements of the complex pathogen-host interaction in MLNs have been identified using mouse infection models. Y. pseudotuberculosis requires the virulence plasmid pYV to induce the formation of pyogranulomas in MLNs. The YadA adhesin and the Ysc-Yop type III secretion system (T3SS) are encoded on pYV. YadA mediates bacterial binding to host receptors, which engages the T3SS to preferentially translocate seven Yop effectors into phagocytes. The effectors promote pathogenesis by blocking innate immune defenses such as superoxide production, degranulation, and inflammasome activation, resulting in survival and growth of Y. pseudotuberculosis. On the other hand, certain effectors can trigger immune defenses in phagocytes. For example, YopJ triggers activation of caspase-8 and an apoptotic cell death response in monocytes within pyogranulomas that limits dissemination of Y. pseudotuberculosis from MLNs to the bloodstream. YopE can be processed as an antigen by phagocytes in MLNs, resulting in T and B cell responses to Y. pseudotuberculosis. Immune responses to Y. pseudotuberculosis in MLNs can also be detrimental to the host in the form of chronic lymphadenopathy. This review focuses on interactions between Y. pseudotuberculosis and phagocytes mediated by pYV that concurrently promote pathogenesis and host defense in MLNs. We propose that MLN pyogranulomas are immunological arenas in which opposing pYV-driven forces determine the outcome of infection in favor of the pathogen or host.

Mitochondrial uncoupling protein 1 antagonizes atherosclerosis by blocking NLRP3 inflammasome-dependent interleukin-1β production

Mitochondrial uncoupling protein 1 (UCP1) is the hallmark of brown adipocytes responsible for cold- and diet-induced thermogenesis. Here, we report a previously unidentified role of UCP1 in maintaining vascular health through its anti-inflammatory actions possibly in perivascular adipose tissue. UCP1 deficiency exacerbates dietary obesity-induced endothelial dysfunction, vascular inflammation, and atherogenesis in mice, which was not rectified by reconstitution of UCP1 in interscapular brown adipose tissue. Mechanistically, lack of UCP1 augments mitochondrial membrane potential and mitochondrial superoxide, leading to hyperactivation of the NLRP3-inflammasome and caspase-1–mediated maturation of interleukin-1β (IL-1β). UCP1 deficiency–evoked deterioration of vascular dysfunction and atherogenesis is reversed by IL-1β neutralization or a chemical mitochondrial uncoupler. Furthermore, UCP1 knockin pigs (which lack endogenous UCP1) are refractory to vascular inflammation and coronary atherosclerosis. Thus, UCP1 acts as a gatekeeper to prevent NLRP3 inflammasome activation and IL-1β production in the vasculature, thereby conferring a protective effect against cardiovascular diseases.

Microglia in the Pathophysiology of Hemorrhagic Stroke and the Relationship Between Microglia and Pain After Stroke: A Narrative Review

Stroke is a leading cause of death worldwide, and about a quarter of stroke patients are dead within 1 month. The prognosis is even worse for those with hemorrhagic stroke because the 1-month mortality approaches 50%. Besides, most patients who survive experience complications such as nausea, vomiting, and chronic pain. These adverse experiences, especially the existence of chronic pain, can lead to a decline in the patient's quality of life. In order to improve the treatment and prognosis of hemorrhagic stroke, there is an urgent need to understand its pathophysiological mechanism as well as the chronic pain it induces. This paper reviews studies of the molecular mechanisms of hemorrhagic stroke, especially the activation of microglia and the relationship between microglia and pain after stroke, which could shed new light on hemorrhagic stroke treatment.

Differential involvement of the canonical and noncanonical inflammasomes in the immune response against infection by the periodontal bacteria Porphyromonas gingivalis and Fusobacterium nucleatum

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The canonical P2 × 7-Caspase-1 pathway is necessary for secretion of IL-1β in oral tissues and macrophages infected with P. gingivalis.

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P2 × 7 receptor controls bacterial load of F. nucleatum and P. gingivalis in macrophages and in mice.

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Caspase-11 is essential for F. nucleatum-induced secretion of IL-1β in macrophages, limits F. nucleatum infection in macrophages and in mice, and is required for cell death induced by F. nucleatum infection.

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The canonical inflammasome is activated preferentially in response to P. gingivalis infection, while the noncanonical inflammasome plays a predominant role during F. nucleatum infection.

We examined the involvement of the P2 × 7 receptor and the canonical and noncanonical inflammasomes in the control of single-species or dual-species infection by the periodontal bacteria Porphyromonas gingivalis and Fusobacterium nucleatum in cells and mice. Stimulation of the P2 × 7 receptor leads to activation of the canonical NLRP3 inflammasome and activation of caspase-1, which leads to cleavage of pro-IL-1β to IL-1β, a key cytokine in the host inflammatory response in periodontal disease. The non-canonical inflammasome pathway involves caspase-11. Thus, wildtype (WT), P2 × 7^−/−, caspase-11^−/− and caspase-1/11^−/− mice were co-infected with both bacterial species. In parallel, bone marrow-derived macrophages (BMDMs) from WT mice and the different knockout mice were infected with P. gingivalis and/or F. nucleatum, and treated or not with extracellular ATP, which is recognized by P2 × 7. F. nucleatum infection alone promoted secretion of IL-1β in BMDMs. Conversely, the canonical pathway involving P2 × 7 and caspase-1 was necessary for secretion of IL-1β in BMDMs infected with P. gingivalis and in the mandible of mice coinfected with P. gingivalis and F. nucleatum. The P2 × 7 pathway can limit bacterial load in single-species and dual-species infection with P. gingivalis and F. nucleatum in BMDMs and in mice. The non-canonical pathway involving caspase-11 was required for secretion of IL-1β induced by F. nucleatum infection in BMDMs, without treatment with ATP. Caspase-11 was also required for induction of cell death during infection with F. nucleatum and contributed to limiting bacterial load during F. nucleatum infection in BMDMs and in the gingival tissue of mice coinfected with P. gingivalis and F. nucleatum. Together, these data suggest that the P2 × 7-caspase-1 and caspase-11 pathways are involved in the immune response against infection by P. gingivalis and F. nucleatum, respectively.

Cytosolic detection of phagosomal bacteria-Mechanisms underlying PAMP exodus from the phagosome into the cytosol

The metazoan innate immune system senses bacterial infections by detecting highly conserved bacterial molecules, termed pathogen-associated molecular patterns (PAMPs). PAMPs are detected by a variety of host pattern recognition receptors (PRRs), whose function is to coordinate downstream immune responses. PRR activities are, in part, regulated by their subcellular localizations. Accordingly, professional phagocytes can detect extracellular bacteria and their PAMPs via plasma membrane-oriented PRRs. Conversely, phagocytosed bacteria and their PAMPs are detected by transmembrane PRRs oriented toward the phagosomal lumen. Even though PAMPs are unable to passively diffuse across membranes, phagocytosed bacteria are also detected by PRRs localized within the host cell cytosol. This phenomenon is explained by phagocytosis of bacteria that specialize in phagosomal escape and cytosolic residence. Contrary to this cytosolic lifestyle, most bacteria studied to date spend their entire intracellular lifestyle contained within phagosomes, yet they also stimulate cytosolic PRRs. Herein, we will review our current understanding of how phagosomal PAMPs become accessible to cytosolic PRRs, as well as highlight knowledge gaps that should inspire future investigations.

Disruption of the endopeptidase ADAM10-Notch signaling axis leads to skin dysbiosis and innate lymphoid cell-mediated hair follicle destruction

Hair follicles (HFs) function as hubs for stem cells, immune cells, and commensal microbes, which must be tightly regulated during homeostasis and transient inflammation. Here we found that transmembrane endopeptidase ADAM10 expression in upper HFs was crucial for regulating the skin microbiota and protecting HFs and their stem cell niche from inflammatory destruction. Ablation of the ADAM10-Notch signaling axis impaired the innate epithelial barrier and enabled Corynebacterium species to predominate the microbiome. Dysbiosis triggered group 2 innate lymphoid cell-mediated inflammation in an interleukin-7 (IL-7) receptor-, S1P receptor 1-, and CCR6-dependent manner, leading to pyroptotic cell death of HFs and irreversible alopecia. Double-stranded RNA-induced ablation models indicated that the ADAM10-Notch signaling axis bolsters epithelial innate immunity by promoting β-defensin-6 expression downstream of type I interferon responses. Thus, ADAM10-Notch signaling axis-mediated regulation of host-microbial symbiosis crucially protects HFs from inflammatory destruction, which has implications for strategies to sustain tissue integrity during chronic inflammation.

Galectin-3 promotes noncanonical inflammasome activation through intracellular binding to lipopolysaccharide glycans

Cytosolic lipopolysaccharides (LPSs) bind directly to caspase-4/5/11 through their lipid A moiety, inducing inflammatory caspase oligomerization and activation, which is identified as the noncanonical inflammasome pathway. Galectins, β-galactoside-binding proteins, bind to various gram-negative bacterial LPS, which display β-galactoside-containing polysaccharide chains. Galectins are mainly present intracellularly, but their interactions with cytosolic microbial glycans have not been investigated. We report that in cell-free systems, galectin-3 augments the LPS-induced assembly of caspase-4/11 oligomers, leading to increased caspase-4/11 activation. Its carboxyl-terminal carbohydrate-recognition domain is essential for this effect, and its N-terminal domain, which contributes to the self-association property of the protein, is also critical, suggesting that this promoting effect is dependent on the functional multivalency of galectin-3. Moreover, galectin-3 enhances intracellular LPS-induced caspase-4/11 oligomerization and activation, as well as gasdermin D cleavage in human embryonic kidney (HEK) 293T cells, and it additionally promotes interleukin-1β production and pyroptotic death in macrophages. Galectin-3 also promotes caspase-11 activation and gasdermin D cleavage in macrophages treated with outer membrane vesicles, which are known to be taken up by cells and release LPSs into the cytosol. Coimmunoprecipitation confirmed that galectin-3 associates with caspase-11 after intracellular delivery of LPSs. Immunofluorescence staining revealed colocalization of LPSs, galectin-3, and caspase-11 independent of host N-glycans. Thus, we conclude that galectin-3 amplifies caspase-4/11 oligomerization and activation through LPS glycan binding, resulting in more intense pyroptosis-a critical mechanism of host resistance against bacterial infection that may provide opportunities for new therapeutic interventions.

Avoidance of the NLRP3 Inflammasome by the Stealth Pathogen, Coxiella burnetii

Coxiella burnetii, a highly adapted obligate intracellular bacterial pathogen and the cause of the zoonosis Q fever, is a reemerging public health threat. C. burnetii employs a Type IV secretion system (T4SS) to establish and maintain its intracellular niche and modulate host immune responses including the inhibition of apoptosis. Interactions between C. burnetii and caspase-1-mediated inflammasomes are not fully elucidated. This study confirms that C. burnetii does not activate caspase-1 during infection of mouse macrophages in vitro. C. burnetii-infected cells did not develop NLRP3 and ASC foci indicating its ability to avoid cytosolic detection. C. burnetii is unable to inhibit the pyroptosis and IL-1β secretion that is induced by potent inflammasome stimuli but rather enhances these caspase-1-mediated effects. We found that C. burnetii upregulates pro-IL-1β and robustly primes NLRP3 inflammasomes via TLR2 and MyD88 signaling. As for wildtype C. burnetii, T4SS-deficient mutants primed and potentiated NLRP3 inflammasomes. An in vivo model of pulmonary infection in C57BL/6 mice was developed. Mice deficient in NLRP3 or caspase-1 were like wildtype mice in the development and resolution of splenomegaly due to red pulp hyperplasia, and histologic lesions and macrophage kinetics, but had slightly higher pulmonary bacterial burdens at the greatest measured time point. Together these findings indicate that C. burnetii primes but avoids cytosolic detection by NLRP3 inflammasomes, which are not required for the clinical resistance of C57BL/6 mice. Determining mechanisms employed by C. burnetii to avoid cytosolic detection via NLRP3 inflammasomes will be beneficial to the development of preventative and interventional therapies for Q fever.

Alendronate Augments Lipid A-Induced IL-1α Release via Activation of ASC but Not Caspase-11

Nitrogen-containing bisphosphonates (NBPs), such as alendronate (ALN), are anti-bone-resorptive drugs that have inflammatory side effects. We previously reported that ALN augmented lipid A-induced interleukin (IL)-1β production and NOD-like receptor pyrin domain-containing-3 (NLRP3)/apoptosis-associated speck-like protein containing a CARD (ASC)-dependent cell death. The present study aimed to examine whether ALN augments lipid A-induced IL-1α release and necroptosis, which is induced by the activation of receptor-interacting protein kinase (RIPK) 3. Treatment of J774.1 cells with ALN augmented lipid A-induced IL-1α release, which was not inhibited by Ac-IETD-CHO, a caspase-8 inhibitor. ALN also activated mixed lineage kinase domain-like (MLKL), a key mediator of the necroptosis pathway, and upregulated the expression of caspase-11, a lipid A receptor. GSK'872, a RIPK3 inhibitor, suppressed the ALN-upregulated expression of caspase-11 and augmented lipid A-induced caspase-8 activation. Moreover, ALN induced the release of NLRP3 and ASC into culture supernatants. GSK'872, but not Ac-IETD-CHO, reduced the ALN-induced release of NLRP3, but not ASC, into culture supernatants, and reduced ALN-induced cell death, but not ALN-induced LDH release. Antibodies against NLRP3 and ASC upregulated caspase-11 expression in the cytosol by inhibiting ALN-induced cell death. However, pretreating cells with an antibody against ASC, but not NLRP3, before ALN addition also inhibited lipid A-induced IL-1α release. Pretreating cells with an antibody against caspase-11 before the addition of ALN or lipid A did not downregulate lipid A-induced production of IL-1α. Taken together, our findings suggest that ALN augments lipid A-induced IL-1α release via activation of ASC, but not caspase-11.

Interaction of nanoparticles with endotoxin Importance in nanosafety testing and exploitation for endotoxin binding

The interaction between engineered nanoparticles and the bacterial lipopolysaccharide, or endotoxin, is an event that warrants attention. Endotoxin is one of the most potent stimulators of inflammation and immune reactions in human beings, and is a very common contaminant in research labs. In nanotoxicology and nanomedicine, the presence of endotoxin on the nanoparticle surface affects their biological properties leading to misinterpretation of results. This review discusses the importance of detecting the endotoxin contamination on nanoparticles, focusing on the current method of endotoxin detection and their suitability for nanoparticulate materials. Conversely, the capacity of nanoparticles to bind endotoxin can be enhanced by functionalization with endotoxin-capturing molecules, opening the way to the development of novel endotoxin detection assays.

An overview of the non-canonical inflammasome

Inflammasomes are large cytosolic multiprotein complexes assembled in response to infection and cellular stress, and are crucial for the activation of inflammatory caspases and the subsequent processing and release of pro-inflammatory mediators. While caspase-1 is activated within the canonical inflammasome, the related caspase-4 (also known as caspase-11 in mice) and caspase-5 are activated within the non-canonical inflammasome upon sensing of cytosolic lipopolysaccharide (LPS) from Gram-negative bacteria. However, the consequences of canonical and non-canonical inflammasome activation are similar. Caspase-1 promotes the processing and release of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 and the release of danger signals, as well as a lytic form of cell death called pyroptosis, whereas caspase-4, caspase-5 and caspase-11 directly promote pyroptosis through cleavage of the pore-forming protein gasdermin D (GSDMD), and trigger a secondary activation of the canonical NLRP3 inflammasome for cytokine release. Since the presence of the non-canonical inflammasome activator LPS leads to endotoxemia and sepsis, non-canonical inflammasome activation and regulation has important clinical ramifications. Here we discuss the mechanism of non-canonical inflammasome activation, mechanisms regulating its activity and its contribution to health and disease.

Crystal structure of caspase-11 CARD provides insights into caspase-11 activation

Murine caspase-11 is the centerpiece of the non-canonical inflammasome pathway that can respond to intracellular LPS and induce pyroptosis. Caspase-11 contains two components, an N-terminal caspase recruitment domain (CARD) and a C-terminal catalytic domain. The aggregation of caspase-11 is thought to promote the auto-processing and activation of caspase-11. However, the activation mechanism of caspase-11 remains unclear. In this study, we purified the caspase-11 CARD fused to an MBP tag and found it tetramerizes in solution. Crystallographic analysis reveals an extensive hydrophobic interface formed by the H1–2 helix mediating homotypic CARD interactions. Importantly, mutations of the helix H1–2 hydrophobic residues abolished the tetramerization of MBP-tagged CARD in solution and failed to induce pyroptosis in cells. Our study provides the first evidence of the homotypic interaction mode for an inflammatory caspase by crystal model. This finding demonstrates that the tetramerization of the N-terminal CARD can promote releasing of the catalytic domain auto-inhibition, leading to the caspase-11 activation.

Caspase-8 mediates inflammation and disease in rodent malaria

Earlier studies indicate that either the canonical or non-canonical pathways of inflammasome activation have a limited role on malaria pathogenesis. Here, we report that caspase-8 is a central mediator of systemic inflammation, septic shock in the Plasmodium chabaudi-infected mice and the P. berghei-induced experimental cerebral malaria (ECM). Importantly, our results indicate that the combined deficiencies of caspases-8/1/11 or caspase-8/gasdermin-D (GSDM-D) renders mice impaired to produce both TNFα and IL-1β and highly resistant to lethality in these models, disclosing a complementary, but independent role of caspase-8 and caspases-1/11/GSDM-D in the pathogenesis of malaria. Further, we find that monocytes from malaria patients express active caspases-1, -4 and -8 suggesting that these inflammatory caspases may also play a role in the pathogenesis of human disease.

Inflammasome activation plays a role in malaria pathogenesis, but details aren’t well understood. Here, the authors show that caspase-8 is a central mediator of systemic inflammation in rodent malaria and that monocytes from malaria patients express active caspases-1, -4 and -8.

The Yersinia Type III Secretion System as a Tool for Studying Cytosolic Innate Immune Surveillance

Microbial pathogens have evolved complex mechanisms to interface with host cells in order to evade host defenses and replicate. However, mammalian innate immune receptors detect the presence of molecules unique to the microbial world or sense the activity of virulence factors, activating antimicrobial and inflammatory pathways. We focus on how studies of the major virulence factor of one group of microbial pathogens, the type III secretion system (T3SS) of human pathogenic Yersinia, have shed light on these important innate immune responses. Yersinia are largely extracellular pathogens, yet they insert T3SS cargo into target host cells that modulate the activity of cytosolic innate immune receptors. This review covers both the host pathways that detect the Yersinia T3SS and the effector proteins used by Yersinia to manipulate innate immune signaling.

Legionella-Infected Macrophages Engage the Alveolar Epithelium to Metabolically Reprogram Myeloid Cells and Promote Antibacterial Inflammation

Alveolar macrophages are among the first immune cells that respond to inhaled pathogens. However, numerous pathogens block macrophage-intrinsic immune responses, making it unclear how robust antimicrobial responses are generated. The intracellular bacterium Legionella pneumophila inhibits host translation, thereby impairing cytokine production by infected macrophages. Nevertheless, Legionella-infected macrophages induce an interleukin-1 (IL-1)-dependent inflammatory cytokine response by recruited monocytes and other cells that controls infection. How IL-1 directs these cells to produce inflammatory cytokines is unknown. Here, we show that collaboration with the alveolar epithelium is critical for controlling infection. IL-1 induces the alveolar epithelium to produce granulocyte-macrophage colony-stimulating factor (GM-CSF). Intriguingly, GM-CSF signaling amplifies inflammatory cytokine production in recruited monocytes by enhancing Toll-like receptor (TLR)-induced glycolysis. Our findings reveal that alveolar macrophages engage alveolar epithelial signals to metabolically reprogram monocytes for antibacterial inflammation.

Inflammasome and Mitophagy Connection in Health and Disease

The inflammasome is a large intracellular protein complex that activates inflammatory caspase-1 and induces the maturation of interleukin (IL)-1β and IL-18. Mitophagy plays an essential role in the maintenance of mitochondrial homeostasis during stress. Previous studies have indicated compelling evidence of the crosstalk between inflammasome and mitophagy. Mitophagy regulation of the inflammasome, or vice versa, is crucial for various biological functions, such as controlling inflammation and metabolism, immune and anti-tumor responses, and pyroptotic cell death. Uncontrolled regulation of the inflammasome often results in pathological inflammation and pyroptosis, and causes a variety of human diseases, including metabolic and inflammatory diseases, infection, and cancer. Here, we discuss how improved understanding of the interactions between inflammasome and mitophagy can lead to novel therapies against various disease pathologies, and how the inflammasome-mitophagy connection is currently being targeted pharmacologically by diverse agents and small molecules. A deeper understanding of the inflammasome-mitophagy connection will provide new insights into human health and disease through the balance between mitochondrial clearance and pathology.

Cellular Models and Assays to Study NLRP3 Inflammasome Biology

The NLRP3 inflammasome is a multi-protein complex that initiates innate immunity responses when exposed to a wide range of stimuli, including pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs). Inflammasome activation leads to the release of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 and to pyroptotic cell death. Over-activation of NLRP3 inflammasome has been associated with several chronic inflammatory diseases. A deep knowledge of NLRP3 inflammasome biology is required to better exploit its potential as therapeutic target and for the development of new selective drugs. To this purpose, in the past few years, several tools have been developed for the biological characterization of the multimeric inflammasome complex, the identification of the upstream signaling cascade leading to inflammasome activation, and the downstream effects triggered by NLRP3 activation. In this review, we will report cellular models and cellular, biochemical, and biophysical assays that are currently available for studying inflammasome biology. A special focus will be on those models/assays that have been used to identify NLRP3 inhibitors and their mechanism of action.

Carbon Monoxide Inhibits the Expression of Proteins Associated with Intestinal Mucosal Pyroptosis in a Rat Model of Sepsis Induced by Cecal Ligation and Puncture

BACKGROUND Carbon monoxide (CO) has anti-inflammatory effects and protects the intestinal mucosal barrier in sepsis. Pyroptosis, or cell death associated with sepsis, is mediated by caspase-1 activation. This study aimed to investigate the role of CO on the expression of proteins associated with intestinal mucosal pyroptosis in a rat model of sepsis induced by cecal ligation and puncture (CLP). MATERIAL AND METHODS The rat model of sepsis was developed using CLP. Male Sprague-Dawley rats (n=120) were divided into six study groups: the sham group (n=20); the CLP group (n=20); the hemin group (treated with ferric chloride and heme) (n=20); the zinc protoporphyrin IX (ZnPPIX) group (n=20); the CO-releasing molecule 2 (CORM-2) group (n=20); and the inactive CORM-2 (iCORM-2) group (n=20). Hemin and CORM-2 were CO donors, and ZnPPIX was a CO inhibitor. In the six groups, the seven-day survival curves, the fluorescein isothiocyanate (FITC)-labeled dextran 4000 Da (FD-4) permeability assay, levels of intestinal pyroptosis proteins caspase-1, caspase-11, and gasdermin D (GSDMD) were measured by confocal fluorescence microscopy. Proinflammatory cytokines interleukin (IL)-18, IL-1ß, and high mobility group box protein 1 (HMGB1) were measured by Western blot and enzyme-linked immunosorbent assay (ELISA). RESULTS CO reduced the mortality rate in rats with sepsis and reduced intestinal mucosal permeability and mucosal damage. CO also reduced the expression levels of IL-18, IL-1ß, and HMGB1, and reduced pyroptosis by preventing the cleavage of caspase-1 and caspase-11. CONCLUSIONS In a rat model of sepsis induced by CLP, CO had a protective role by inhibiting intestinal mucosal pyroptosis.

Vying for the control of inflammasomes: The cytosolic frontier of enteric bacterial pathogen-host interactions

Enteric pathogen-host interactions occur at multiple interfaces, including the intestinal epithelium and deeper organs of the immune system. Microbial ligands and activities are detected by host sensors that elicit a range of immune responses. Membrane-bound toll-like receptors and cytosolic inflammasome pathways are key signal transducers that trigger the production of pro-inflammatory molecules, such as cytokines and chemokines, and regulate cell death in response to infection. In recent years, the inflammasomes have emerged as a key frontier in the tussle between bacterial pathogens and the host. Inflammasomes are complexes that activate caspase-1 and are regulated by related caspases, such as caspase-11, -4, -5 and -8. Importantly, enteric bacterial pathogens can actively engage or evade inflammasome signalling systems. Extracellular, vacuolar and cytosolic bacteria have developed divergent strategies to subvert inflammasomes. While some pathogens take advantage of inflammasome activation (e.g. Listeria monocytogenes, Helicobacter pylori), others (e.g. E. coli, Salmonella, Shigella, Yersinia sp.) deploy a range of virulence factors, mainly type 3 secretion system effectors, that subvert or inhibit inflammasomes. In this review we focus on inflammasome pathways and their immune functions, and discuss how enteric bacterial pathogens interact with them. These studies have not only shed light on inflammasome-mediated immunity, but also the exciting area of mammalian cytosolic immune surveillance.

Pyroptosis: The missing puzzle among innate and adaptive immunity crosstalk

Pyroptosis is a newly discovered programmed cell death with inflammasome formation. Pattern recognition receptors that identify repetitive motifs of prospective pathogens such as LPS of gram-negative bacteria are crucial to pyroptosis. Upon stimulation by pathogen-associated molecular patterns or damage-associated molecular patterns, proinflammatory cytokines, mainly IL-1 family members IL-1β and IL-18, are released through pyroptosis specific pore-forming protein, gasdermin D. Even though IL-1 family members are mainly involved in innate immunity, they can be factors in adaptive immunity. Given the importance of IL-1 family members in health and diseases, deciphering the role of pyroptosis in the regulation of innate and adaptive immunity is of great importance, especially with the recent progress in identifying the exact mechanism of such a pathway. In this review, we will focus on how the innate inflammatory mediators can regulate the adaptive immune system and vice versa via pyroptosis.

Redundant and Cooperative Roles for Yersinia pestis Yop Effectors in the Inhibition of Human Neutrophil Exocytic Responses Revealed by Gain-of-Function Approach

Yersinia pestis causes a rapid, lethal disease referred to as plague. Y. pestis actively inhibits the innate immune system to generate a noninflammatory environment during early stages of infection to promote colonization. The ability of Y. pestis to create this early noninflammatory environment is in part due to the action of seven Yop effector proteins that are directly injected into host cells via a type 3 secretion system (T3SS). While each Yop effector interacts with specific host proteins to inhibit their function, several Yop effectors either target the same host protein or inhibit converging signaling pathways, leading to functional redundancy. Previous work established that Y. pestis uses the T3SS to inhibit neutrophil respiratory burst, phagocytosis, and release of inflammatory cytokines. Here, we show that Y. pestis also inhibits release of granules in a T3SS-dependent manner. Moreover, using a gain-of-function approach, we discovered previously hidden contributions of YpkA and YopJ to inhibition and that cooperative actions by multiple Yop effectors are required to effectively inhibit degranulation. Independent from degranulation, we also show that multiple Yop effectors can inhibit synthesis of leukotriene B₄ (LTB₄), a potent lipid mediator released by neutrophils early during infection to promote inflammation. Together, inhibition of these two arms of the neutrophil response likely contributes to the noninflammatory environment needed for Y. pestis colonization and proliferation.

Functional crosstalk between non-canonical caspase-11 and canonical NLRP3 inflammasomes during infection-mediated inflammation

Inflammation is a part of the body's immune response for protection against pathogenic infections and other cellular damages; however, chronic inflammation is a major cause of various diseases. One key step in the inflammatory response is the activation of inflammasomes, intracellular protein complexes comprising pattern recognition receptors and other inflammatory molecules. The role of the NLRP3 inflammasome in inflammatory responses has been extensively investigated; however, the caspase-11 inflammasome has been recently identified and has been classified as a 'non-canonical' inflammasome, and emerging studies have highlighted its role in inflammatory responses. Because the ligands and the mechanisms for the activation of these two inflammasomes are different, studies to date have separately described their roles, although recent studies have reported the functional cooperation between these two inflammasomes during an inflammatory response. This review discusses the studies investigating the functional crosstalk between non-canonical caspase-11 and canonical NLRP3 inflammasomes in the context of inflammatory responses; moreover, it provides insight for the development of novel anti-inflammatory therapeutics to prevent and treat infectious and inflammatory diseases.

NLRP3 Inflammasome in the Pathophysiology of Hemorrhagic Stroke: A Review

Hemorrhagic stroke is a devastating disease with high morbidity and mortality. There is still a lack of effective ther-apeutic approach. The recent studies have shown that the innate immune system plays a significant role in hemorrhagic stroke. Microglia, as major components in innate immune system, are activated and then can release cytokines and chemo-kines in response to hemorrhagic stroke, and ultimately led to neuroinflammation and brain injury. The NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome is predominantly released by microglia and is believed as the main contributor of neuroinflammation. Several studies have focused on the role of NLRP3 inflammasome in hemorrhagic stroke-induced brain injury, however, the specific mechanism of NLRP3 activation and regulation remains unclear. This re-view summarized the mechanism of NLRP3 activation and its role in hemorrhagic stroke and discussed the translational sig-nificance.

Caspase-11 counteracts mitochondrial ROS-mediated clearance of Staphylococcus aureus in macrophages

Methicillin-resistant Staphylococcus aureus (MRSA) is a growing health concern due to increasing resistance to antibiotics. As a facultative intracellular pathogen, MRSA is capable of persisting within professional phagocytes including macrophages. Here, we identify a role for CASP11 in facilitating MRSA survival within murine macrophages. We show that MRSA actively prevents the recruitment of mitochondria to the vicinity of the vacuoles they reside in to avoid intracellular demise. This process requires CASP11 since its deficiency allows increased association of MRSA-containing vacuoles with mitochondria. The induction of mitochondrial superoxide by antimycin A (Ant A) improves MRSA eradication in casp11-/- cells, where mitochondria remain in the vicinity of the bacterium. In WT macrophages, Ant A does not affect MRSA persistence. When mitochondrial dissociation is prevented by the actin depolymerizing agent cytochalasin D, Ant A effectively reduces MRSA numbers. Moreover, the absence of CASP11 leads to reduced cleavage of CASP1, IL-1β, and CASP7, as well as to reduced production of CXCL1/KC. Our study provides a new role for CASP11 in promoting the persistence of Gram-positive bacteria.

Viewing Legionella pneumophila Pathogenesis through an Immunological Lens

Legionella pneumophila is the causative agent of the severe pneumonia Legionnaires' disease. L. pneumophila is ubiquitously found in freshwater environments, where it replicates within free-living protozoa. Aerosolization of contaminated water supplies allows the bacteria to be inhaled into the human lung, where L. pneumophila can be phagocytosed by alveolar macrophages and replicate intracellularly. The Dot/Icm type IV secretion system (T4SS) is one of the key virulence factors required for intracellular bacterial replication and subsequent disease. The Dot/Icm apparatus translocates more than 300 effector proteins into the host cell cytosol. These effectors interfere with a variety of cellular processes, thus enabling the bacterium to evade phagosome-lysosome fusion and establish an endoplasmic reticulum-derived Legionella-containing vacuole, which facilitates bacterial replication. In turn, the immune system has evolved numerous strategies to recognize intracellular bacteria such as L. pneumophila, leading to potent inflammatory responses that aid in eliminating infection. This review aims to provide an overview of L. pneumophila pathogenesis in the context of the host immune response.

Caspase-11 Contributes to Oviduct Pathology during Genital Chlamydia Infection in Mice

It has been shown that caspase-1, but not its upstream activator, ASC, contributes to oviduct pathology during mouse genital Chlamydia muridarum infection. We hypothesized that this dichotomy is due to the inadvertent absence of caspase-11 in previously used caspase-1-deficient mice. To address this, we studied the independent contributions of caspase-1 and -11 during genital Chlamydia infection. Our results show that caspase-11 deficiency was sufficient to recapitulate the effect of the combined absence of both caspase-1 and caspase-11 on oviduct pathology. Further, mice that were deficient for both caspase-1 and -11 but that expressed caspase-11 as a transgene (essentially, caspase-1-deficient mice) had no significant difference in oviduct pathology from control mice. Caspase-11-deficient mice showed reduced dilation in both the oviducts and uterus. To determine the mechanism by which caspase-11-deficient mice developed reduced pathology, the chlamydial burden and immune cell infiltration were determined in the oviducts. In the caspase-11-deficient mice, we observed increased chlamydial burdens in the upper genital tract, which correlated with increased CD4 T cell recruitment, suggesting a contribution of caspase-11 in infection control. Additionally, there were significantly fewer neutrophils in the oviducts of caspase-11-deficient mice, supporting the observed decrease in the incidence of oviduct pathology. Therefore, caspase-11 activation contributes to pathogen control and oviduct disease independently of caspase-1 activation.

Gasdermin-D and Caspase-7 are the key Caspase-1/8 substrates downstream of the NAIP5/NLRC4 inflammasome required for restriction of Legionella pneumophila

Inflammasomes are cytosolic multi-protein complexes that detect infection or cellular damage and activate the Caspase-1 (CASP1) protease. The NAIP5/NLRC4 inflammasome detects bacterial flagellin and is essential for resistance to the flagellated intracellular bacterium Legionella pneumophila. The effectors required downstream of NAIP5/NLRC4 to restrict bacterial replication remain unclear. Upon NAIP5/NLRC4 activation, CASP1 cleaves and activates the pore-forming protein Gasdermin-D (GSDMD) and the effector caspase-7 (CASP7). However, Casp1^–/– (and Casp1/11^–/–) mice are only partially susceptible to L. pneumophila and do not phenocopy Nlrc4^–/–mice, because NAIP5/NLRC4 also activates CASP8 for restriction of L. pneumophila infection. Here we show that CASP8 promotes the activation of CASP7 and that Casp7/1/11^–/– and Casp8/1/11^–/– mice recapitulate the full susceptibility of Nlrc4^–/– mice. Gsdmd^–/– mice exhibit only mild susceptibility to L. pneumophila, but Gsdmd^–/–Casp7^–/– mice are as susceptible as the Nlrc4^–/– mice. These results demonstrate that GSDMD and CASP7 are the key substrates downstream of NAIP5/NLRC4/CASP1/8 required for resistance to L. pneumophila.

Inflammasomes are multi-protein complexes that detect infection and other stimuli and activate the Caspase-1 (CASP1) protease. The effectors required downstream of NAIP5/NLRC4 to restrict bacterial replication remain unclear. Active CASP1 cleaves and activates the pore-forming protein gasdermin D (GSDMD) to induce inflammation and cell death. We have previously shown that CASP8 is activated by the NAIP5/NLRC4 inflammasome independently of CASP1 and functions to restrict replication of the intracellular bacterium Legionella pneumophila. Here, we show that CASP7 is activated downstream of CASP8 and is required for CASP8-dependent restriction of L. pneumophila replication in macrophages and in vivo. In addition, CASP7 is also activated by CASP1. Taken together, these results imply that CASP7 and GSDMD are the two key caspase substrates downstream of NAIP5/NLRC4. In support of this hypothesis, we found that mice double deficient in CASP7 and GSDMD are more susceptible than the single knockouts and are as susceptible as the Nlrc4 deficient mice for restriction of L. pneumophila replication in vivo. Collectively, our data indicate that GSDMD and CASP7 are activated by CASP1 and induce cell death and restriction of bacterial infection. Therefore, GSDMD and multiple caspases (CASP1, CASP7 and CASP8) operate downstream of the NAIP5/NLRC4 inflammasome for restriction of infection by pathogenic bacteria.

Immunological Pathways Triggered by Porphyromonas gingivalis and Fusobacterium nucleatum: Therapeutic Possibilities?

Porphyromonas gingivalis (P. gingivalis) and Fusobacterium nucleatum (F. nucleatum) are Gram-negative anaerobic bacteria possessing several virulence factors that make them potential pathogens associated with periodontal disease. Periodontal diseases are chronic inflammatory diseases of the oral cavity, including gingivitis and periodontitis. Periodontitis can lead to tooth loss and is considered one of the most prevalent diseases worldwide. P. gingivalis and F. nucleatum possess virulence factors that allow them to survive in hostile environments by selectively modulating the host's immune-inflammatory response, thereby creating major challenges to host cell survival. Studies have demonstrated that bacterial infection and the host immune responses are involved in the induction of periodontitis. The NLRP3 inflammasome and its effector molecules (IL-1β and caspase-1) play roles in the development of periodontitis. We and others have reported that the purinergic P2X7 receptor plays a role in the modulation of periodontal disease and intracellular pathogen control. Caspase-4/5 (in humans) and caspase-11 (in mice) are important effectors for combating bacterial pathogens via mediation of cell death and IL-1β release. The exact molecular events of the host's response to these bacteria are not fully understood. Here, we review innate and adaptive immune responses induced by P. gingivalis and F. nucleatum infections and discuss the possibility of manipulations of the immune response as therapeutic strategies. Given the global burden of periodontitis, it is important to develop therapeutic targets for the prophylaxis of periodontopathogen infections.

Caspase-11, a specific sensor for intracellular lipopolysaccharide recognition, mediates the non-canonical inflammatory pathway of pyroptosis

Pyroptosis, a type of programmed cell death that along with inflammation, is mainly regulated by two main pathways, cysteinyl aspartate specific proteinase (caspase)-1-induced canonical inflammatory pathway and caspase-11-induced non-canonical inflammatory pathway. The non-canonical inflammatory pathway-induced pyroptosis is a unique immune response in response to gram-negative (G⁻) bacteria. It is induced by lipopolysaccharide (LPS) on the surface of G⁻ bacteria. This activates caspase-11 which, in turn, activates a series of downstream proteins eventually forming protein pores on the cell membrane and inducing cell sacrificial processes. Caspase-11 belongs to the caspase family and is an homologous protein of caspase-1. It has the ability to specifically hydrolyze proteins, but it is still unclear how it regulates cell death caused by non-canonical inflammatory pathways. The present study describes a pathway that enables LPS to directly enter the cell and activate caspase-11, and the key role caspase-11 plays in the activation of pyroptosis and inflammation.

AIM2 Inflammasome Activation Leads to IL-1α and TGF-β Release From Exacerbated Chronic Obstructive Pulmonary Disease-Derived Peripheral Blood Mononuclear Cells

Chronic obstructive pulmonary disease (COPD) is now the fourth-leading cause of death worldwide and its prevalence is increasing. The progressive decline of lung function and airway remodelling are a consequence of chronic inflammatory responses. It was recently postulated the involvement of the inflammasome in COPD, although the underlying mechanism/s still need to be elucidated. Therefore, we isolated peripheral blood mononuclear cells (PBMCs) from exacerbated/unstable COPD patients. The stimulation of PBMCs with an AIM2 inflammasome activator, Poly dA:dT, led to IL-1α, but not IL-1β, release. The release of this cytokine was caspase-1- and caspase-4-dependent and correlated to higher levels of 8-OH-dG in COPD compared to non-smoker and smoker-derived PBMCs. Interestingly, AIM2-depedent IL-1α release was responsible for higher TGF-β levels, crucial mediator during pro-fibrotic processes associated to COPD progression. In conclusion, our data highlight the involvement of AIM2/caspase-1/caspase-4 in IL-1α-induced TGF-β release in unstable COPD-derived PBMCs, opening new therapeutic perspectives for unstable COPD patients.

Detecting lipopolysaccharide in the cytosol of mammalian cells: Lessons from MD-2/TLR4

Proinflammatory immune responses to Gram-negative bacterial lipopolysaccharides (LPS) are crucial to innate host defenses but can also contribute to pathology. How host cells sensitively detect structural features of LPS was a mystery for years, especially given that a portion of the molecule essential for its potent proinflammatory properties-lipid A-is buried in the bacterial membrane. Studies of responses to extracellular and vacuolar LPS revealed a crucial role for accessory proteins that specifically bind LPS-rich membranes and extract LPS monomers to generate a complex of LPS, MD-2, and TLR4. These insights provided means to understand better both the remarkable host sensitivity to LPS and the means whereby specific LPS structural features are discerned. More recently, the noncanonical inflammasome, consisting of caspases-4/5 in humans and caspase-11 in mice, has been demonstrated to mediate responses to LPS that has reached the host cytosol. Precisely how LPS gains access to cytosolic caspases-and in what form-is not well characterized, and understanding this process will provide crucial insights into how the noncanonical inflammasome is regulated during infection. Herein, we briefly review what is known about LPS detection by cytosolic caspases-4/5/11, focusing on lessons derived from studies of the better-characterized TLR4 system that might direct future mechanistic questions.

Inflammasome Activation in Legionella-Infected Macrophages

Legionella pneumophila is a gram-negative bacterium that infects many species of unicellular protozoa in freshwater environments. The human infection is accidental, and the bacteria may not have evolved strategies to bypass innate immune signaling in mammalian macrophages. Thus, L. pneumophila triggers many innate immune pathways including inflammasome activation. The inflammasomes are multimolecular platforms assembled in the host cell cytoplasm and lead to activation of inflammatory caspases. Inflammasome activation leads to secretion of inflammatory cytokines, such as IL-1β and IL-18, and an inflammatory form of cell death called pyroptosis, which initiates with the induction of a pore in the macrophage membranes. In this chapter we provide detailed protocols to evaluate Legionella-induced inflammasome activation in macrophages, including real-time pore formation assay, western blotting to detect activation of inflammatory caspases (cleavage and pulldown), and the measurement of inflammatory cytokines.

Nucleic Acid Induced Interferon and Inflammasome Responses in Regulating Host Defense to Gastrointestinal Viruses

The gut bacterial and fungal communities residing in the gastrointestinal tract have undisputed far-reaching effects in regulating host health. In the meantime, however, metagenomic sequencing efforts are revealing enteric viruses as the most abundant dimension of the intestinal gut ecosystem, and the first gut virome-wide association studies showed that inflammatory bowel disease as well as type 1 diabetes could be linked to the presence or absence of particular viral inhabitants in the intestine. In line with the genetic component of these human diseases, mouse model studies demonstrated how beneficial functions of a resident virus can switch to detrimental inflammatory effects in a genetically predisposed host. Such viral-induced intestinal immune disturbances are also recapitulated by several gastrointestinal infectious viruses such as rotavirus and human norovirus. This wide range of viral effects on intestinal immunity emphasizes the need for understanding the innate immune responses to gastrointestinal viruses. Numerous nucleic acid sensors such as DexD/H helicases and AIM2 serve as cytosolic viral guardians to induce antiviral interferon and/or pro-inflammatory inflammasome responses. In both cases, pioneering examples are emerging in which RNA helicases cooperate with particular Nod-like receptors to trigger these cellular responses to enteric viruses. Here we summarize the reported beneficial versus detrimental effects of enteric viruses in the intestinal immune system, and we zoom in on the mechanisms through which sensing of nucleic acids from these enteric viruses trigger interferon and inflammasome responses.

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.

Overlapping Roles for Interleukin-36 Cytokines in Protective Host Defense against Murine Legionella pneumophila Pneumonia

Legionella pneumophila causes life-threatening pneumonia culminating in acute lung injury. Innate and adaptive cytokines play an important role in host defense against L. pneumophila infection. Interleukin-36 (IL-36) cytokines are recently described members of the larger IL-1 cytokine family known to exert potent inflammatory effects. In this study, we elucidated the role for IL-36 cytokines in experimental pneumonia caused by L. pneumophila Intratracheal (i.t.) administration of L. pneumophila induced the upregulation of both IL-36α and IL-36γ mRNA and protein production in the lung. Compared to the findings for L. pneumophila-infected wild-type (WT) mice, the i.t. administration of L. pneumophila to IL-36 receptor-deficient (IL-36R^-/-) mice resulted in increased mortality, a delay in lung bacterial clearance, increased L. pneumophila dissemination to extrapulmonary organs, and impaired glucose homeostasis. Impaired lung bacterial clearance in IL-36R^-/- mice was associated with a significantly reduced accumulation of inflammatory cells and the decreased production of proinflammatory cytokines and chemokines. Ex vivo, reduced expression of costimulatory molecules and impaired M1 polarization were observed in alveolar macrophages isolated from infected IL-36R^-/- mice compared to macrophages from WT mice. While L. pneumophila-induced mortality in IL-36α- or IL-36γ-deficient mice was not different from that in WT animals, antibody-mediated neutralization of IL-36γ in IL-36α^-/- mice resulted in mortality similar to that observed in IL-36R^-/- mice, indicating redundant and overlapping roles for these cytokines in experimental murine L. pneumophila pneumonia.

Betaine Inhibits Interleukin-1β Production and Release: Potential Mechanisms

Betaine is a critical nutrient for mammal health, and has been found to alleviate inflammation by lowering interleukin (IL)-1β secretion; however, the underlying mechanisms by which betaine inhibits IL-1β secretion remain to be uncovered. In this review, we summarize the current understanding about the mechanisms of betaine in IL-1β production and release. For IL-1β production, betaine affects canonical and non-canonical inflammasome-mediated processing of IL-1β through signaling pathways, such as NF-κB, NLRP3 and caspase-8/11. For IL-1β release, betaine inhibits IL-1β release through blocking the exocytosis of IL-1β-containing secretory lysosomes, reducing the shedding of IL-1β-containing plasma membrane microvesicles, suppressing the exocytosis of IL-1β-containing exosomes, and attenuating the passive efflux of IL-1β across hyperpermeable plasma membrane during pyroptotic cell death, which are associated with ERK1/2/PLA₂ and caspase-8/A-SMase signaling pathways. Collectively, this review highlights the anti-inflammatory property of betaine by inhibiting the production and release of IL-1β, and indicates the potential application of betaine supplementation as an adjuvant therapy in various inflammatory diseases associating with IL-1β secretion.

Legionella feeleii: pneumonia or Pontiac fever? Bacterial virulence traits and host immune response

Gram-negative bacterium Legionella is able to proliferate intracellularly in mammalian host cells and amoeba, which became known in 1976 since they caused a large outbreak of pneumonia. It had been reported that different strains of Legionella pneumophila, Legionella micdadei, Legionella longbeachae, and Legionella feeleii caused human respiratory diseases, which were known as Pontiac fever or Legionnaires' disease. However, the differences of the virulence traits among the strains of the single species and the pathogenesis of the two diseases that were due to the bacterial virulence factors had not been well elucidated. L. feeleii is an important pathogenic organism in Legionellae, which attracted attention due to cause an outbreak of Pontiac fever in 1981 in Canada. In published researches, it has been found that L. feeleii serogroup 2 (ATCC 35849, LfLD) possess mono-polar flagellum, and L. feeleii serogroup 1 (ATCC 35072, WRLf) could secrete some exopolysaccharide (EPS) materials to the surrounding. Although the virulence of the L. feeleii strain was evidenced that could be promoted, the EPS might be dispensable for the bacteria that caused Pontiac fever. Based on the current knowledge, we focused on bacterial infection in human and murine host cells, intracellular growth, cytopathogenicity, stimulatory capacity of cytokines secretion, and pathogenic effects of the EPS of L. feeleii in this review.