The unique Legionella longbeachae capsule favors intracellular replication and immune evasion.

Legionella longbeachae and Legionella pneumophila are the most common causative agents of Legionnaires' disease. While the clinical manifestations caused by both species are similar, species-specific differences exist in environmental niches, disease epidemiology, and genomic content. One such difference is the presence of a genomic locus predicted to encode a capsule. Here, we show that L. longbeachae indeed expresses a capsule in post-exponential growth phase as evidenced by electron microscopy analyses, and that capsule expression is abrogated when deleting a capsule transporter gene. Capsule purification and its analysis via HLPC revealed the presence of a highly anionic polysaccharide that is absent in the capsule mutant. The capsule is important for replication and virulence in vivo in a mouse model of infection and in the natural host Acanthamoeba castellanii. It has anti-phagocytic function when encountering innate immune cells such as human macrophages and it is involved in the low cytokine responses in mice and in human monocyte derived macrophages, thus dampening the innate immune response. Thus, the here characterized L. longbeachae capsule is a novel virulence factor, unique among the known Legionella species, which may aid L. longbeachae to survive in its specific niches and which partly confers L. longbeachae its unique infection characteristics.

Pulmonary inflammation and viral replication define distinct clinical outcomes in fatal cases of COVID-19.

COVID-19 has affected more than half a billion people worldwide, with more than 6.3 million deaths, but the pathophysiological mechanisms involved in lethal cases and the host determinants that determine the different clinical outcomes are still unclear. In this study, we assessed lung autopsies of 47 COVID-19 patients and examined the inflammatory profiles, viral loads, and inflammasome activation. Additionally, we correlated these factors with the patient's clinical and histopathological conditions. Robust inflammasome activation was detected in the lungs of lethal cases of SARS-CoV-2. Experiments conducted on transgenic mice expressing hACE2 and infected with SARS-CoV-2 showed that Nlrp3-/- mice were protected from disease development and lethality compared to Nlrp3+/+ littermate mice, supporting the involvement of this inflammasome in disease exacerbation. An analysis of gene expression allowed for the classification of COVID-19 patients into two different clusters. Cluster 1 died with higher viral loads and exhibited a reduced inflammatory profile than Cluster 2. Illness time, mechanical ventilation time, pulmonary fibrosis, respiratory functions, histopathological status, thrombosis, viral loads, and inflammasome activation significantly differed between the two clusters. Our data demonstrated two distinct profiles in lethal cases of COVID-19, thus indicating that the balance of viral replication and inflammasome-mediated pulmonary inflammation led to different clinical outcomes. We provide important information to understand clinical variations in severe COVID-19, a process that is critical for decisions between immune-mediated or antiviral-mediated therapies for the treatment of critical cases of COVID-19.

Coxiella co-opts the Glutathione Peroxidase 4 to protect the host cell from oxidative stress-induced cell death.

The causative agent of human Q fever, Coxiella burnetii, is highly adapted to infect alveolar macrophages by inhibiting a range of host responses to infection. Despite the clinical and biological importance of this pathogen, the challenges related to genetic manipulation of both C. burnetii and macrophages have limited our knowledge of the mechanisms by which C. burnetii subverts macrophages functions. Here, we used the related bacterium Legionella pneumophila to perform a comprehensive screen of C. burnetii effectors that interfere with innate immune responses and host death using the greater wax moth Galleria mellonella and mouse bone marrow-derived macrophages. We identified MceF (Mitochondrial Coxiella effector protein F), a C. burnetii effector protein that localizes to mitochondria and contributes to host cell survival. MceF was shown to enhance mitochondrial function, delay membrane damage, and decrease mitochondrial ROS production induced by rotenone. Mechanistically, MceF recruits the host antioxidant protein Glutathione Peroxidase 4 (GPX4) to the mitochondria. The protective functions of MceF were absent in primary macrophages lacking GPX4, while overexpression of MceF in human cells protected against oxidative stress-induced cell death. C. burnetii lacking MceF was replication competent in mammalian cells but induced higher mortality in G. mellonella, indicating that MceF modulates the host response to infection. This study reveals an important C. burnetii strategy to subvert macrophage cell death and host immunity and demonstrates that modulation of the host antioxidant system is a viable strategy to promote the success of intracellular bacteria.

Inflammasome activation and CCR2-mediated monocyte-derived dendritic cell recruitment restrict Legionella pneumophila infection.

Flagellin-induced NAIP/NLRC4 inflammasome activation and pyroptosis are critical events restricting Legionella pneumophila infection. However, the cellular and molecular dynamics of the in vivo responses against this bacterium are still unclear. We have found temporal coordination of two independent innate immunity pathways in controlling Legionella infection, the inflammasome activation and the CCR2-mediated Mo-DC recruitment. Inflammasome activation was an important player at the early stage of infection by lowering the numbers of bacteria for an efficient bacterial clearance conferred by the Mo-DC at the late stage of the infection. Mo-DC emergence highly depended on CCR2-signaling and dispensed inflammasome activation and pyroptosis. Also, Mo-DC compartment did not rely on the inflammasome machinery to deliver proper immune responses and was the most abundant cytokine-producing among the monocyte-derived cells in the infected lung. Importantly, when the CCR2- and NLRC4-dependent axes of response were simultaneously ablated, we observed an aggravated bacterial burden in the lung of infected mice. Taken together, we showed that inflammasome activation and CCR2-mediated immune response interplay in distinct pathways to restrict pulmonary bacterial infection. These findings extend our understanding of the in vivo integration and cooperation of different innate immunity arms in controlling infectious agents.

Cell death induced by NLRP3-palmitate axis impairs pulmonary damage tolerance and aggravates immunopathology during obesity-tuberculosis comorbidity.

A low-grade and persistent inflammation, which is the hallmark of obesity, requires the participation of NLRP3 and cell death. During Mycobacterium tuberculosis infection, NLRP3 signaling is important for bacterial killing by macrophages in vitro but was shown to be dispensable for host protection in vivo. We hypothesized that during obesity-tuberculosis (TB) comorbidity, NLRP3 signaling might play a detrimental role by inducing excessive inflammation. We employed a model of high-fat-diet-induced obesity, followed by M. tuberculosis infection in C57BL/6 mice. Obese mice presented increased susceptibility to infection and pulmonary immunopathology compared to lean mice. Using treatment with NLRP3 antagonist and Nlrp3-/- mice, we showed that NLRP3 signaling promoted cell death, with no effect in bacterial loads. The levels of palmitate were higher in the lungs of obese infected mice compared to lean counterparts, and we observed that this lipid increased M. tuberculosis-induced macrophage death in vitro, which was dependent on NLRP3 and caspase-1. At the chronic phase, although lungs of obese Nlrp3-/- mice showed an indication of granuloma formation compared to obese wild-type mice, there was no difference in the bacterial load. Our findings indicate that NLRP3 may be a potential target for host-directed therapy to reduce initial and severe inflammation-mediated disease and to treat comorbidity-associated TB. © 2022 The Pathological Society of Great Britain and Ireland.

CASP4/11 Contributes to NLRP3 Activation and COVID-19 Exacerbation.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection triggers activation of the NLRP3 inflammasome, which promotes inflammation and aggravates severe COVID-19. Here, we report that SARS-CoV-2 induces upregulation and activation of human caspase-4/CASP4 (mouse caspase-11/CASP11), and this process contributes to NLRP3 activation. In vivo infections performed in transgenic hACE2 humanized mice, deficient or sufficient for Casp11, indicate that hACE2 Casp11-/- mice were protected from disease development, with the increased pulmonary parenchymal area, reduced clinical score of the disease, and reduced mortality. Assessing human samples from fatal cases of COVID-19, we found that CASP4 was expressed in patient lungs and correlated with the expression of inflammasome components and inflammatory mediators, including CASP1, IL1B, IL18, and IL6. Collectively, our data establish that CASP4/11 promotes NLRP3 activation and disease pathology, revealing a possible target for therapeutic interventions for COVID-19.

Lipid droplet accumulation occurs early following Salmonella infection and contributes to intracellular bacterial survival and replication.

Salmonellosis is a public health problem caused by Salmonella sp., a highly adapted facultative intracellular pathogen. After internalization, Salmonella sp. Manipulates several host processes, mainly through the activation of the type III secretion system (T3SS), including modification of host lipid metabolism and lipid droplet (LD) accumulation. LDs are dynamic and complex lipid-rich organelles involved in several cellular processes. The present study investigated the mechanism involved in LD biogenesis in Salmonella-infected macrophages and its role in bacterial pathogenicity. Here, we reported that S. Typhimurium induced a rapid time-dependent increase of LD formation in macrophages. The LD biogenesis was demonstrated to depend on Salmonella's viability and SPI1-related T3SS activity, with the participation of Toll-Like Receptor (TLR) signaling. We also observed that LD accumulation occurs through TLR2-dependent signaling and is counter-regulated by TLR4. Last, the pharmacologic modulation of LD formation by inhibiting diacylglycerol O-acyltransferase 1 (DGAT1) and cytosolic phospholipase A2 (cPLA2) significantly reduced the intracellular bacterial proliferation and impaired the prostaglandin E2 (PGE2 ) synthesis. Collectively, our data suggest the role of LDs on S. typhimurium intracellular survival and replication in macrophages. This data set provides new perspectives for future investigations about LDs in host-pathogen interaction.

Gasdermin D inhibition prevents multiple organ dysfunction during sepsis by blocking NET formation.

Multiple organ dysfunction is the most severe outcome of sepsis progression and is highly correlated with a worse prognosis. Excessive neutrophil extracellular traps (NETs) are critical players in the development of organ failure during sepsis. Therefore, interventions targeting NET release would likely effectively prevent NET-based organ injury associated with this disease. Herein, we demonstrate that the pore-forming protein gasdermin D (GSDMD) is active in neutrophils from septic humans and mice and plays a crucial role in NET release. Inhibition of GSDMD with disulfiram or genic deletion abrogated NET formation, reducing multiple organ dysfunction and sepsis lethality. Mechanistically, we demonstrate that during sepsis, activation of the caspase-11/GSDMD pathway controls NET release by neutrophils during sepsis. In summary, our findings uncover a novel therapeutic use for disulfiram and suggest that GSDMD is a therapeutic target to improve sepsis treatment.

Dietary Fiber Drives IL-1ß-Dependent Peritonitis Induced by Bacteroides fragilis via Activation of the NLRP3 Inflammasome.

Intestinal barrier is essential for dietary products and microbiota compartmentalization and therefore gut homeostasis. When this barrier is broken, cecal content overflows into the peritoneal cavity, leading to local and systemic robust inflammatory response, characterizing peritonitis and sepsis. It has been shown that IL-1ß contributes with inflammatory storm during peritonitis and sepsis and its inhibition has beneficial effects to the host. Therefore, we investigated the mechanisms underlying IL-1ß secretion using a widely adopted murine model of experimental peritonitis. The combined injection of sterile cecal content (SCC) and the gut commensal bacteria Bacteroides fragilis leads to IL-1ß-dependent peritonitis, which was mitigated in mice deficient in NLRP3 (nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3) inflammasome components. Typically acting as a damage signal, SCC, but not B. fragilis, activates canonical pathway of NLRP3 promoting IL-1ß secretion in vitro and in vivo. Strikingly, absence of fiber in the SCC drastically reduces IL-1ß production, whereas high-fiber SCC conversely increases this response in an NLRP3-dependent manner. In addition, NLRP3 was also required for IL-1ß production induced by purified dietary fiber in primed macrophages. Extending to the in vivo context, IL-1ß-dependent peritonitis was worsened in mice injected with B. fragilis and high-fiber SCC, whereas zero-fiber SCC ameliorates the pathology. Corroborating with the proinflammatory role of dietary fiber, IL-1R-deficient mice were protected from peritonitis induced by B. fragilis and particulate bran. Overall, our study highlights a function, previously unknown, for dietary fibers in fueling peritonitis through NLRP3 activation and IL-1ß secretion outside the gut.

Sepsis-Induced Immunosuppression Is Marked by an Expansion of a Highly Suppressive Repertoire of FOXP3+ T-Regulatory Cells Expressing TIGIT.

BACKGROUND: Although the literature shows that an increase in both the number and suppressive function of CD4+forkhead box P3 (FOXP3)+ T-regulatory cells (Tregs) during sepsis contributes to an immunosuppressed state, little is known about the identity of these cells. METHODS: Using the sepsis mouse model of cecal ligation and puncture (CLP), we analyzed the frequency and molecular signature of the T-cell immunoglobulin and ITIM domain (TIGIT)+ and TIGIT- Treg subsets, using flow cytometry and quantitative polymerase chain reaction. In addition, ST2-/- and signal transducer and activator of transcription 6 (STAT6)-/- mice were submitted to CLP or recombinant interleukin 33 (IL-33) treatment to investigate the mechanism whereby TIGIT+ Tregs differentiate during sepsis. RESULTS: Sepsis was marked by the sustained expansion of the highly suppressive TIGIT+ Treg subset, which expresses Helios, neuropilin 1, and high levels of Tnfrsf18 and Pdcd1 at 15 days after CLP. The increase in TIGIT+ Tregs was accompanied by higher susceptibility to nosocomial bacteria challenge, suggesting their association with post sepsis immunosuppression. Mechanistically, we found that the ST2 deletion abrogated the expansion of the TIGIT+ Treg subset during sepsis. Furthermore, treatment with recombinant IL-33 resulted in the expansion of TIGIT+ Tregs depending on the STAT6 and M2 macrophages. CONCLUSIONS: These findings demonstrated that only the TIGIT+ Tregs remain stably expanded at the late phase of sepsis. Moreover, the expansion of TIGIT+ Tregs is dependent on the IL-33/ST2/STAT6/M2 macrophage axis.

Gasdermin-D activation by SARS-CoV-2 triggers NET and mediate COVID-19 immunopathology.

BACKGROUND: The release of neutrophil extracellular traps (NETs) is associated with inflammation, coagulopathy, and organ damage found in severe cases of COVID-19. However, the molecular mechanisms underlying the release of NETs in COVID-19 remain unclear. OBJECTIVES: We aim to investigate the role of the Gasdermin-D (GSDMD) pathway on NETs release and the development of organ damage during COVID-19. METHODS: We performed a single-cell transcriptome analysis in public data of bronchoalveolar lavage. Then, we enrolled 63 hospitalized patients with moderate and severe COVID-19. We analyze in blood and lung tissue samples the expression of GSDMD, presence of NETs, and signaling pathways upstreaming. Furthermore, we analyzed the treatment with disulfiram in a mouse model of SARS-CoV-2 infection. RESULTS: We found that the SARS-CoV-2 virus directly activates the pore-forming protein GSDMD that triggers NET production and organ damage in COVID-19. Single-cell transcriptome analysis revealed that the expression of GSDMD and inflammasome-related genes were increased in COVID-19 patients. High expression of active GSDMD associated with NETs structures was found in the lung tissue of COVID-19 patients. Furthermore, we showed that activation of GSDMD in neutrophils requires active caspase1/4 and live SARS-CoV-2, which infects neutrophils. In a mouse model of SARS-CoV-2 infection, the treatment with disulfiram inhibited NETs release and reduced organ damage. CONCLUSION: These results demonstrated that GSDMD-dependent NETosis plays a critical role in COVID-19 immunopathology and suggests GSDMD as a novel potential target for improving the COVID-19 therapeutic strategy.

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.

The NLRP3 inflammasome is involved with the pathogenesis of Mayaro virus.

Mayaro virus (MAYV) is an arbovirus that circulates in Latin America and is emerging as a potential threat to public health. Infected individuals develop Mayaro fever, a severe inflammatory disease characterized by high fever, rash, arthralgia, myalgia and headache. The disease is often associated with a prolonged arthralgia mediated by a chronic inflammation that can last months. Although the immune response against other arboviruses, such as chikungunya virus (CHIKV), dengue virus (DENV) and Zika virus (ZIKV), has been extensively studied, little is known about the pathogenesis of MAYV infection. In this study, we established models of MAYV infection in macrophages and in mice and found that MAYV can replicate in bone marrow-derived macrophages and robustly induce expression of inflammasome proteins, such as NLRP3, ASC, AIM2, and Caspase-1 (CASP1). Infection performed in macrophages derived from Nlrp3-/-, Aim2-/-, Asc-/-and Casp1/11-/-mice indicate that the NLRP3, but not AIM2 inflammasome is essential for production of inflammatory cytokines, such as IL-1ß. We also determined that MAYV triggers NLRP3 inflammasome activation by inducing reactive oxygen species (ROS) and potassium efflux. In vivo infections performed in inflammasome-deficient mice indicate that NLRP3 is involved with footpad swelling, inflammation and pain, establishing a role of the NLRP3 inflammasome in the MAYV pathogenesis. Accordingly, we detected higher levels of caspase1-p20, IL-1ß and IL-18 in the serum of MAYV-infected patients as compared to healthy individuals, supporting the participation of the NLRP3-inflammasome during MAYV infection in humans.

Systems analysis of subjects acutely infected with the Chikungunya virus.

The largest ever recorded epidemic of the Chikungunya virus (CHIKV) broke out in 2004 and affected four continents. Acute symptomatic infections are typically associated with the onset of fever and often debilitating polyarthralgia/polyarthritis. In this study, a systems biology approach was adopted to analyze the blood transcriptomes of adults acutely infected with the CHIKV. Gene signatures that were associated with viral RNA levels and the onset of symptoms were identified. Among these genes, the putative role of the Eukaryotic Initiation Factor (eIF) family genes and apolipoprotein B mRNA editing catalytic polypeptide-like (APOBEC3A) in the CHIKV replication process were displayed. We further compared these signatures with signatures induced by the Dengue virus infection and rheumatoid arthritis. Finally, we demonstrated that the CHIKV in vitro infection of murine bone marrow-derived macrophages induced IL-1 beta production in a mechanism that is significantly dependent on the inflammasome NLRP3 activation. The observations provided valuable insights into virus-host interactions during the acute phase and can be instrumental in the investigation of new and effective therapeutic interventions.

The role of annexin A1 in the modulation of the NLRP3 inflammasome.

Annexins are well-known Ca2+ phospholipid-binding proteins, which have a wide variety of cellular functions. The role of annexin A1 (AnxA1) in the innate immune system has focused mainly on the anti-inflammatory and proresolving properties through its binding to the formyl-peptide receptor 2 (FPR2)/ALX receptor. However, studies suggesting an intracellular role of AnxA1 are emerging. In this study, we aimed to understand the role of AnxA1 for interleukin (IL)-1ß release in response to activators of the nucleotide-binding domain leucine-rich repeat (NLR) and pyrin domain containing receptor 3 (NLRP3) inflammasome. Using AnxA1 knockout mice, we observed that AnxA1 is required for IL-1ß release in vivo and in vitro. These effects were due to reduction of transcriptional levels of IL-1ß, NLRP3 and caspase-1, a step called NLRP3 priming. Moreover, we demonstrate that AnxA1 co-localize and directly bind to NLRP3, suggesting the role of AnxA1 in inflammasome activation is independent of its anti-inflammatory role via FPR2. Therefore, AnxA1 regulates NLRP3 inflammasome priming and activation in a FPR2-independent manner.

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 Gbpchr3-/- 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-1ß 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.

Molecular basis of carrageenan-induced cytokines production in macrophages.

BACKGROUND: Low molecular weight carrageenan (Cg) is a seaweed-derived sulfated polysaccharide widely used as inflammatory stimulus in preclinical studies. However, the molecular mechanisms of Cg-induced inflammation are not fully elucidated. The present study aimed to investigate the molecular basis involved in Cg-induced macrophages activation and cytokines production. METHODS: Primary culture of mouse peritoneal macrophages were stimulated with Kappa Cg. The supernatant and cell lysate were used for ELISA, western blotting, immunofluorescence. Cg-induced mouse colitis was also developed. RESULTS: Here we show that Cg activates peritoneal macrophages to produce pro-inflammatory cytokines such as TNF and IL-1ß. While Cg-induced TNF production/secretion depends on TLR4/MyD88 signaling, the production of pro-IL-1ß relies on TLR4/TRIF/SYK/reactive oxygen species (ROS) signaling pathway. The maturation of pro-IL1ß into IL-1ß is dependent on canonical NLRP3 inflammasome activation via Pannexin-1/P2X7/K+ efflux signaling. In vivo, Cg-induced colitis was reduced in mice in the absence of NLRP3 inflammasome components. CONCLUSIONS: In conclusion, we unravel a critical role of the NLRP3 inflammasome in Cg-induced pro-inflammatory cytokines production and colitis, which is an important discovery on the pro-inflammatory properties of this sulfated polysaccharide for pre-clinical studies. Video abstract Carrageenan (Cg) is one the most used flogistic stimulus in preclinical studies. Nevertheless, the molecular basis of Cg-induced inflammation is not totally elucidated. Herein, Lopes et al. unraveled the molecular basis for Cg-induced macrophages production of biological active IL-1ß. The Cg-stimulated macrophages produces pro-IL-1ß depends on TLR4/TRIF/Syk/ROS, whereas its processing into mature IL-1ß is dependent on the canonical NLRP3 inflammasome.

Inhibition of caspase-1 or gasdermin-D enable caspase-8 activation in the Naip5/NLRC4/ASC inflammasome.

Legionella pneumophila is a Gram-negative, flagellated bacterium that survives in phagocytes and causes Legionnaires' disease. Upon infection of mammalian macrophages, cytosolic flagellin triggers the activation of Naip/NLRC4 inflammasome, which culminates in pyroptosis and restriction of bacterial replication. Although NLRC4 and caspase-1 participate in the same inflammasome, Nlrc4-/- mice and their macrophages are more permissive to L. pneumophila replication compared with Casp1/11-/-. This feature supports the existence of a pathway that is NLRC4-dependent and caspase-1/11-independent. Here, we demonstrate that caspase-8 is recruited to the Naip5/NLRC4/ASC inflammasome in response to flagellin-positive bacteria. Accordingly, caspase-8 is activated in Casp1/11-/- macrophages in a process dependent on flagellin, Naip5, NLRC4 and ASC. Silencing caspase-8 in Casp1/11-/- cells culminated in macrophages that were as susceptible as Nlrc4-/- for the restriction of L. pneumophila replication. Accordingly, macrophages and mice deficient in Asc/Casp1/11-/- were more susceptible than Casp1/11-/- and as susceptible as Nlrc4-/- for the restriction of infection. Mechanistically, we found that caspase-8 activation triggers gasdermin-D-independent pore formation and cell death. Interestingly, caspase-8 is recruited to the Naip5/NLRC4/ASC inflammasome in wild-type macrophages, but it is only activated when caspase-1 or gasdermin-D is inhibited. Our data suggest that caspase-8 activation in the Naip5/NLRC4/ASC inflammasome enable induction of cell death when caspase-1 or gasdermin-D is suppressed.

Dectin-1 Activation during Leishmania amazonensis Phagocytosis Prompts Syk-Dependent Reactive Oxygen Species Production To Trigger Inflammasome Assembly and Restriction of Parasite Replication.

Protozoan parasites of the genus Leishmania are the causative agents of Leishmaniasis, a disease that can be lethal and affects 12 million people worldwide. Leishmania replicates intracellularly in macrophages, a process that is essential for disease progression. Although the production of reactive oxygen species (ROS) accounts for restriction of parasite replication, Leishmania is known to induce ROS upon macrophage infection. We have recently demonstrated NLRP3 inflammasome activation in infected macrophages, a process that is important for the outcome of infection. However, the molecular mechanisms responsible for inflammasome activation are unknown. In this article, we demonstrate that ROS induced via NADPH oxidase during the early stages of L. amazonensis infection is critical for inflammasome activation in macrophages. We identified that ROS production during L. amazonensis infection occurs upon engagement of Dectin-1, a C-type lectin receptor that signals via spleen tyrosine kinase (Syk) to induce ROS. Accordingly, inflammasome activation in response to L. amazonensis is impaired by inhibitors of NADPH oxidase, Syk, focal adhesion kinase, and proline-rich tyrosine kinase 2, and in the absence of Dectin-1. Experiments performed with Clec7a-/- mice support the critical role of Dectin-1 for inflammasome activation, restriction of parasite replication in macrophages, and mouse resistance to L. amazonensis infection in vivo. Thus, we reported that activation of the Dectin-1/Syk/ROS/NLRP3 pathway during L. amazonensis phagocytosis is important for macrophage restriction of the parasite replication and effectively accounts for host resistance to Leishmania infection.

Inflammasomes in host response to protozoan parasites.

Inflammasomes are multimeric complexes of proteins that are assembled in the host cell cytoplasm in response to specific stress signals or contamination of the cytoplasm by microbial molecules. The canonical inflammasomes are composed of at least three main components: an inflammatory caspase (caspase-1, caspase-11), an adapter molecule (such as ASC), and a sensor protein (such as NLRP1, NLRP3, NLRP12, NAIP1, NAIP2, NAIP5, or AIM2). The sensor molecule determines the inflammasome specificity by detecting specific microbial products or cell stress signals. Upon activation, these molecular platforms facilitate restriction of microbial replication and trigger an inflammatory form of cell death called pyroptosis, thus accounting for the genesis of inflammatory processes. Inflammasome activation has been widely reported in response to pathogenic bacteria. However, recent reports have highlighted the important role of the inflammasomes in the host response to the pathogenesis of infections caused by intracellular protozoan parasites. Herein, we review the activation and specific roles of inflammasomes in recognition and host responses to intracellular protozoan parasites such as Trypanosoma cruzi, Toxoplasma gondii, Plasmodium spp., and Leishmania spp.