Salmonella Flagellin Activates NAIP/NLRC4 and Canonical NLRP3 Inflammasomes in Human Macrophages

NLRP3 inflammasome activity and periodontal disease pathogenesis-A bidirectional relationship

OBJECTIVE

Periodontitis is an inflammatory oral disease that occurs as a result of the damaging effects of the immune response against the subgingival microflora. Among the mechanisms involved, the nucleotide-binding oligomerization domain, leucine-rich repeat-containing proteins family member NLRP3 (NLR family pyrin domain-containing 3), proposed as the key regulator of macrophage-induced inflammation, is strongly associated with periodontal disease due to the bacterial activators. This paper aimed to present key general concepts of NLRP3 inflammasome activation and regulation in periodontal disease.

METHOD

A narrative review was conducted in order to depict the current knowledge on the relationship between NLRP3 inflammasome activity and periodontal disease. In vitro and in situ studies were retrieved and commented based on their relevance in the field.

RESULTS

The NLRP3 inflammasome activity stimulated by periodontal microbiota drive periodontal disease pathogenesis and progression. This occurs through the release of proinflammatory cytokines IL-1β, IL-18, and DAMPs (damage-associated molecular pattern molecules) following inflammasome activation. Moreover, the tissue expression of NLRP3 is dysregulated by oral microbiota, further exacerbating periodontal inflammation.

CONCLUSION

The review provides new insights into the relationship between the NLRP3 inflammasome activity and periodontal disease pathogenesis, highlighting the roles and regulatory mechanism of inflammatory molecules involved in the disease process.

Flagellin-modulated inflammasome pathways characterize the human alveolar macrophage response to Burkholderia pseudomallei, a lung-tropic pathogen

Melioidosis is an emerging tropical infection caused by inhalation, inoculation, or ingestion of the flagellated, facultatively intracellular pathogen Burkholderia pseudomallei. The melioidosis case fatality rate is often high, and pneumonia, the most common presentation, doubles the risk of death. The alveolar macrophage is a sentinel pulmonary host defense cell, but the human alveolar macrophage in B. pseudomallei infection has never been studied. The objective of this study was to investigate the host-pathogen interaction of B. pseudomallei infection with the human alveolar macrophage and to determine the role of flagellin in modulating inflammasome-mediated pathways. We found that B. pseudomallei infects primary human alveolar macrophages but is gradually restricted in the setting of concurrent cell death. Electron microscopy revealed cytosolic bacteria undergoing division, indicating that B. pseudomallei likely escapes the alveolar macrophage phagosome and may replicate in the cytosol, where it triggers immune responses. In paired human blood monocytes, uptake and intracellular restriction of B. pseudomallei are similar to those observed in alveolar macrophages, but cell death is reduced. The alveolar macrophage cytokine response to B. pseudomallei is characterized by marked interleukin (IL)-18 secretion compared to monocytes. Both cytotoxicity and IL-18 secretion in alveolar macrophages are partially flagellin dependent. However, the proportion of IL-18 release that is driven by flagellin is greater in alveolar macrophages than in monocytes. These findings suggest differential flagellin-mediated inflammasome pathway activation in the human alveolar macrophage response to B. pseudomallei infection and expand our understanding of intracellular pathogen recognition by this unique innate immune lung cell.

Salmonella Enteritidis antitoxin DinJ inhibits NLRP3-dependent canonical inflammasome activation in macrophages

The inflammasome is a pivotal component of the innate immune system, acting as a multiprotein complex that plays an essential role in detecting and responding to microbial infections. Salmonella Enteritidis have evolved multiple mechanisms to regulate inflammasome activation and evade host immune system clearance. Through screening S. Enteritidis C50336ΔfliC transposon mutant library, we found that the insertion mutant of dinJ increased inflammasome activation. In this study, we demonstrated the genetic connection between the antitoxin DinJ and the toxin YafQ in S. Enteritidis, confirming their co-transcription. The deletion mutant ΔfliCΔdinJ increased cell death and IL-1β secretion in J774A.1 cells. Western blotting analysis further showed elevated cleaved Caspase-1 product (p10 subunits) and IL-1β secretion in cells infected with ΔfliCΔdinJ compared to cells infected with ΔfliC. DinJ was found to inhibit canonical inflammasome activation using primary bone marrow-derived macrophages (BMDMs) from Casp-/- C57BL/6 mice. Furthermore, DinJ specifically inhibited NLRP3 inflammasome activation, as demonstrated in BMDMs from Nlrp3-/- and Nlrc4-/- mice. Fluorescence resonance energy transfer (FRET) experiments confirmed the translocation of DinJ into host cells during infection. Finally, we revealed that DinJ could inhibit the secretion of IL-1β and IL-18 in vivo, contributing to S. Enteritidis evading host immune clearance. In summary, our findings provide insights into the role of DinJ in modulating the inflammasome response during S. Enteritidis infection, highlighting its impact on inhibiting inflammasome activation and immune evasion.

The NLRP3 molecule is responsible for mediating the pyroptosis of intestinal mucosa cells and plays a crucial role in salmonellosis enteritis in chicks

Salmonella enteritis in poultry can result in reduced immune function, decreased growth rate, and increased mortality. Many farm salmonella strains have developed severe drug resistance and are less susceptible to multiple antibiotics. In the post-antibiotic era, it is of great significance to identify the mechanism of salmonella-induced enteritis in chicks to protect their health and ensure food safety. This article will elucidate the activation mechanism of NOD-like receptor protein 3 (NLRP3) inflammasomes in Salmonella enteritis and review the research on interventions targeting NLRP3 inflammasomes.

A host-directed oxadiazole compound potentiates antituberculosis treatment via zinc poisoning in human macrophages and in a mouse model of infection

Antituberculosis drugs, mostly developed over 60 years ago, combined with a poorly effective vaccine, have failed to eradicate tuberculosis. More worryingly, multiresistant strains of Mycobacterium tuberculosis (MTB) are constantly emerging. Innovative strategies are thus urgently needed to improve tuberculosis treatment. Recently, host-directed therapy has emerged as a promising strategy to be used in adjunct with existing or future antibiotics, by improving innate immunity or limiting immunopathology. Here, using high-content imaging, we identified novel 1,2,4-oxadiazole-based compounds, which allow human macrophages to control MTB replication. Genome-wide gene expression analysis revealed that these molecules induced zinc remobilization inside cells, resulting in bacterial zinc intoxication. More importantly, we also demonstrated that, upon treatment with these novel compounds, MTB became even more sensitive to antituberculosis drugs, in vitro and in vivo, in a mouse model of tuberculosis. Manipulation of heavy metal homeostasis holds thus great promise to be exploited to develop host-directed therapeutic interventions.

Salmonella T3SS-elicited inflammatory innate immune response inhibits type I IFN response in macrophages

As a gram-negative intracellular bacterial pathogen, Salmonella enterica serovar Typhimurium (S. Typhimurium) invades different cell types including macrophages. Its infection in macrophages induces robust innate immune responses that are featured by proinflammatory and type I interferon (IFN) responses. The type III secretion systems (T3SSs) of S. Typhimurium play a crucial role in activating host inflammasome pathways. It has been recognized that the inflammasome pathways inhibit the type I IFN cascade. However, the potential role of T3SS in regulating the type I IFN response and the underlying mechanisms are largely unknown. In this study, we showed that S. Typhimurium infection activated strong proinflammatory, type I IFN and IFN-stimulated genes (ISGs) expression in macrophages. Furthermore, we showed that T3SS-defective S. Typhimurium mutant ΔinvC elicited attenuated inflammatory response but enhanced type I IFN and ISGs expression. Additionally, the inhibition of caspase-1 by a specific inhibitor VX-765 resulted in increased type I IFN response. Moreover, cell-permeable pan-caspase inhibitor Z-VAD-FMK also enhanced the type I IFN response upon S. Typhimurium infection. Intriguingly, compared with exponential phase S. Typhimurium infection, stationary phase bacteria triggered higher levels of type I IFN responses. Finally, the inhibition of caspase-1 by VX-765 substantially increased the intracellular S. Typhimurium burden. In conclusion, we demonstrated that the proinflammatory response induced by S. Typhimurium T3SS can inhibit the type I IFN response, which provides insight into the role of T3SS in orchestrating innate immunity during S. Typhimurium infection.

The role of inflammasomes in human diseases and their potential as therapeutic targets

Inflammasomes are large protein complexes that play a major role in sensing inflammatory signals and triggering the innate immune response. Each inflammasome complex has three major components: an upstream sensor molecule that is connected to a downstream effector protein such as caspase-1 through the adapter protein ASC. Inflammasome formation typically occurs in response to infectious agents or cellular damage. The active inflammasome then triggers caspase-1 activation, followed by the secretion of pro-inflammatory cytokines and pyroptotic cell death. Aberrant inflammasome activation and activity contribute to the development of diabetes, cancer, and several cardiovascular and neurodegenerative disorders. As a result, recent research has increasingly focused on investigating the mechanisms that regulate inflammasome assembly and activation, as well as the potential of targeting inflammasomes to treat various diseases. Multiple clinical trials are currently underway to evaluate the therapeutic potential of several distinct inflammasome-targeting therapies. Therefore, understanding how different inflammasomes contribute to disease pathology may have significant implications for developing novel therapeutic strategies. In this article, we provide a summary of the biological and pathological roles of inflammasomes in health and disease. We also highlight key evidence that suggests targeting inflammasomes could be a novel strategy for developing new disease-modifying therapies that may be effective in several conditions.

Structural basis of the human NAIP/NLRC4 inflammasome assembly and pathogen sensing

The NLR family caspase activation and recruitment domain-containing 4 (NLRC4) inflammasome is a critical cytosolic innate immune machine formed upon the direct sensing of bacterial infection and in response to cell stress during sterile chronic inflammation. Despite its major role in instigating the subsequent host immune response, a more complete understanding of the molecular events in the formation of the NLRC4 inflammasome in humans is lacking. Here we identify Bacillus thailandensis type III secretion system needle protein (Needle) as a potent trigger of the human NLR family apoptosis inhibitory protein (NAIP)/NLRC4 inflammasome complex formation and determine its structural features by cryogenic electron microscopy. We also provide a detailed understanding of how type III secretion system pathogen components are sensed by human NAIP to form a cascade of NLRC4 protomer through a critical lasso-like motif, a 'lock-key' activation model and large structural rearrangement, ultimately forming the full human NLRC4 inflammasome. These results shed light on key regulatory mechanisms specific to the NLRC4 inflammasome assembly, and the innate immune modalities of pathogen sensing in humans.

Pyroptosis in microbial infectious diseases

Pyroptosis is a gasdermins-mediated programmed cell death that plays an essential role in immune regulation, and its role in autoimmune disease and cancer has been studied extensively. Increasing evidence shows that various microbial infections can lead to pyroptosis, associated with the occurrence and development of microbial infectious diseases. This study reviews the recent advances in pyroptosis in microbial infection, including bacterial, viral, and fungal infections. We also explore potential therapeutic strategies for treating microbial infection-related diseases by targeting pyroptosis.

The tetrapeptide sequence of IL-18 and IL-1β regulates their recruitment and activation by inflammatory caspases

Inflammasomes are multiprotein signaling complexes that activate the innate immune system. Canonical inflammasomes recruit and activate caspase-1, which then cleaves and activates IL-1β and IL-18, as well as gasdermin D (GSDMD) to induce pyroptosis. In contrast, non-canonical inflammasomes, caspases-4/-5 (CASP4/5) in humans and caspase-11 (CASP11) in mice, are known to cleave GSDMD, but their role in direct processing of other substrates besides GSDMD has remained unknown. Here, we show that CASP4/5 but not CASP11 can directly cleave and activate IL-18. However, CASP4/5/11 can all cleave IL-1β to generate a 27-kDa fragment that deactivates IL-1β signaling. Mechanistically, we demonstrate that the sequence identity of the tetrapeptide sequence adjacent to the caspase cleavage site regulates IL-18 and IL-1β recruitment and activation. Altogether, we have identified new substrates of the non-canonical inflammasomes and reveal key mechanistic details regulating inflammation that may aid in developing new therapeutics for immune-related disorders.

Characterization of Inflammasomes and Their Regulation in the Red Fox

BACKGROUND

Inflammasomes recognize endogenous and exogenous danger signals, and subsequently induce the secretion of IL-1β. Studying inflammasomes in the red fox (Vulpes vulpes) is crucial for wildlife veterinary medicine, as it can help control inflammatory diseases in foxes.

METHODS

We investigated the activation and intracellular mechanisms of three inflammasomes (NLRP3, AIM2, and NLRC4) in fox peripheral blood mononuclear cells (PBMCs), using established triggers and inhibitors derived from humans and mice.

RESULTS

Fox PBMCs exhibited normal activation and induction of IL-1β secretion in response to representative inflammasome triggers (ATP and nigericin for NLRP3, dsDNA for AIM2, flagellin for NLRC4). Additionally, PBMCs showed normal IL-1β secretion when inoculated with inflammasome-activating bacteria. In inhibitors of the inflammasome signaling pathway, fox inflammasome activation was compared with mouse inflammasomes. MCC950, a selective NLRP3 inhibitor, suppressed the secretion of dsDNA- and flagellin-mediated IL-1β in foxes, unlike mice.

CONCLUSIONS

These findings suggest that NLRP3 may have a common role in dsDNA- and flagellin-mediated inflammasome activation in the red fox. It implies that this fox inflammasome biology can be applied to the treatment of inflammasome-mediated diseases in the red fox.

Gasdermin D is the only Gasdermin that provides protection against acute Salmonella gut infection in mice

Gasdermins (GSDMs) share a common functional domain structure and are best known for their capacity to form membrane pores. These pores are hallmarks of a specific form of cell death called pyroptosis and mediate the secretion of pro-inflammatory cytokines such as interleukin 1β (IL1β) and interleukin 18 (IL18). Thereby, Gasdermins have been implicated in various immune responses against cancer and infectious diseases such as acute Salmonella Typhimurium (S.Tm) gut infection. However, to date, we lack a comprehensive functional assessment of the different Gasdermins (GSDMA-E) during S.Tm infection in vivo. Here, we used epithelium-specific ablation, bone marrow chimeras, and mouse lines lacking individual Gasdermins, combinations of Gasdermins or even all Gasdermins (GSDMA1-3C1-4DE) at once and performed littermate-controlled oral S.Tm infections in streptomycin-pretreated mice to investigate the impact of all murine Gasdermins. While GSDMA, C, and E appear dispensable, we show that GSDMD i) restricts S.Tm loads in the gut tissue and systemic organs, ii) controls gut inflammation kinetics, and iii) prevents epithelium disruption by 72 h of the infection. Full protection requires GSDMD expression by both bone-marrow-derived lamina propria cells and intestinal epithelial cells (IECs). In vivo experiments as well as 3D-, 2D-, and chimeric enteroid infections further show that infected IEC extrusion proceeds also without GSDMD, but that GSDMD controls the permeabilization and morphology of the extruding IECs, affects extrusion kinetics, and promotes overall mucosal barrier capacity. As such, this work identifies a unique multipronged role of GSDMD among the Gasdermins for mucosal tissue defense against a common enteric pathogen.

Yersinia deploys type III-secreted effectors to evade caspase-4 inflammasome activation in human cells

IMPORTANCE

Yersinia are responsible for significant disease burden in humans, ranging from recurrent disease outbreaks (yersiniosis) to pandemics (Yersinia pestis plague). Together with rising antibiotic resistance rates, there is a critical need to better understand Yersinia pathogenesis and host immune mechanisms, as this information will aid in developing improved immunomodulatory therapeutics. Inflammasome responses in human cells are less studied relative to murine models of infection, though recent studies have uncovered key differences in inflammasome responses between mice and humans. Here, we dissect human intestinal epithelial cell and macrophage inflammasome responses to Yersinia pseudotuberculosis. Our findings provide insight into species- and cell type-specific differences in inflammasome responses to Yersinia.

Targeting Desulfovibrio vulgaris flagellin-induced NAIP/NLRC4 inflammasome activation in macrophages attenuates ulcerative colitis

INTRODUCTION

The perturbations of gut microbiota could interact with excessively activated immune responses and play key roles in the etiopathogenesis of ulcerative colitis (UC). Desulfovibrio, the most predominant sulfate reducing bacteria (SRB) resided in the human gut, was observed to overgrow in patients with UC. The interactions between specific gut microbiota and drugs and their impacts on UC treatment have not been demonstrated well.

OBJECTIVES

This study aimed to elucidate whether Desulfovibrio vulgaris (D. vulgaris, DSV) and its flagellin could activate nucleotide-binding oligomerization domain-like receptors (NLR) family of apoptosis inhibitory proteins (NAIP) / NLR family caspase activation and recruitment domain-containing protein 4 (NLRC4) inflammasome and promote colitis, and further evaluate the efficacy of eugeniin targeting the interaction interface of D. vulgaris flagellin (DVF) and NAIP to attenuate UC.

METHODS

The abundance of DSV and the occurrence of macrophage pyroptosis in human UC tissues were investigated. Colitis in mice was established by dextran sulfate sodium (DSS) and gavaged with DSV or its purified flagellin. NAIP/NLRC4 inflammasome activation and macrophage pyroptosis were evaluated in vivo and in vitro. The effects of eugeniin on blocking the interaction of DVF and NAIP/NLRC4 and relieving colitis were also assessed.

RESULTS

The abundance of DSV increased in the feces of patients with UC and was found to be associated with disease activity. DSV and its flagellin facilitated DSS-induced colitis in mice. Mechanistically, RNA sequencing showed that gene expression associated with inflammasome complex and pyroptosis was upregulated after DVF treatment in macrophages. DVF was further demonstrated to induce significant macrophage pyroptosis in vitro, depending on NAIP/NLRC4 inflammasome activation. Furthermore, eugeniin was screened as an inhibitor of the interface between DVF and NAIP and successfully alleviated the proinflammatory effect of DVF in colitis.

CONCLUSION

Targeting DVF-induced NAIP/NLRC4 inflammasome activation and macrophage pyroptosis ameliorates UC. This finding is of great significance for exploring the gut microbiota-host interactions in UC development and providing new insights for precise treatment.

Inflammasome activation occurs in CD4+ and CD8+ T cells during graft-versus-host disease

A severe complication of hematopoietic stem cell transplantation is graft-versus-host disease (GvHD), a reaction that occurs following the transfer of donor immune cells (the graft) into an allogeneic host. Transplanted cells recognize host alloantigens as foreign, resulting in the activation of donor T cells and migration of these pathological cells into host tissues. In this study, we found that caspase-1 is activated in alloreactive murine and human CD4+ and CD8+ T cells early during acute GvHD (aGvHD). The presence of inflammasome-bound active caspase-1 (p33) and ASC-speck formation confirmed inflammasome activation in these cells. We further measured gasdermin D (GSDMD) cleavage and IL-18 secretion from alloreactive T cells ex vivo. Isolated T cells with high levels of active caspase-1 had a strong inflammatory transcriptional signature and a metabolic phenotype similar to inflammatory myeloid cells, including the upregulation of proinflammatory cytokines and metabolic switch from oxidative phosphorylation to aerobic glycolysis. We also observed oxidative stress, mitochondrial dysfunction, and cell death phenotypes consistent with inflammatory cell death in alloreactive T cells. For the first time, this study characterizes caspase-1 activation in transplanted T cells during aGvHD, using mouse and human models, adding to a body of literature supporting inflammasome function in cells of the adaptive immune system.

Salmonella enterica serovar Typhi uses two type 3 secretion systems to replicate in human macrophages and colonize humanized mice

Salmonella enterica serovar Typhi (S. Typhi) is a human-restricted pathogen that replicates in macrophages. In this study, we investigated the roles of the S. Typhi type 3 secretion systems (T3SSs) encoded on Salmonella pathogenicity islands (SPI)-1 (T3SS-1) and SPI-2 (T3SS-2) during human macrophage infection. We found that mutants of S. Typhi deficient for both T3SSs were defective for intramacrophage replication as measured by flow cytometry, viable bacterial counts, and live time-lapse microscopy. T3SS-secreted proteins PipB2 and SifA contributed to S. Typhi replication and were translocated into the cytosol of human macrophages through both T3SS-1 and T3SS-2, demonstrating functional redundancy for these secretion systems. Importantly, an S. Typhi mutant strain that is deficient for both T3SS-1 and T3SS-2 was severely attenuated in the ability to colonize systemic tissues in a humanized mouse model of typhoid fever. Overall, this study establishes a critical role for S. Typhi T3SSs during its replication within human macrophages and during systemic infection of humanized mice. IMPORTANCE Salmonella enterica serovar Typhi is a human-restricted pathogen that causes typhoid fever. Understanding the key virulence mechanisms that facilitate S. Typhi replication in human phagocytes will enable rational vaccine and antibiotic development to limit the spread of this pathogen. While S. Typhimurium replication in murine models has been studied extensively, there is limited information available about S. Typhi replication in human macrophages, some of which directly conflict with findings from S. Typhimurium murine models. This study establishes that both of S. Typhi's two type 3 secretion systems (T3SS-1 and T3SS-2) contribute to intramacrophage replication and virulence.

The Achromobacter type 3 secretion system drives pyroptosis and immunopathology via independent activation of NLRC4 and NLRP3 inflammasomes

How the opportunistic Gram-negative pathogens of the genus Achromobacter interact with the innate immune system is poorly understood. Using three Achromobacter clinical isolates from two species, we show that the type 3 secretion system (T3SS) is required to induce cell death in human macrophages by inflammasome-dependent pyroptosis. Macrophages deficient in the inflammasome sensors NLRC4 or NLRP3 undergo pyroptosis upon bacterial internalization, but those deficient in both NLRC4 and NLRP3 do not, suggesting either sensor mediates pyroptosis in a T3SS-dependent manner. Detailed analysis of the intracellular trafficking of one isolate indicates that the intracellular bacteria reside in a late phagolysosome. Using an intranasal mouse infection model, we observe that Achromobacter damages lung structure and causes severe illness, contingent on a functional T3SS. Together, we demonstrate that Achromobacter species can survive phagocytosis by promoting macrophage cell death and inflammation by redundant mechanisms of pyroptosis induction in a T3SS-dependent manner.

Divergent functional outcomes of NLRP3 blockade downstream of multi-inflammasome activation: therapeutic implications for ALS

NOD-Like Receptor Family Pyrin Domain Containing 3 (NLRP3) inflammasome modulation has emerged as a potential therapeutic approach targeting inflammation amplified by pyroptotic innate immune cell death. In diseases characterized by non-cell autonomous neurodegeneration including amyotrophic lateral sclerosis (ALS), the activation of several inflammasomes has been reported. Since functional redundancy can exist among inflammasome pathways, here we investigate the effects of NLRP3 inhibition on NLRP3, NLR family CARD Domain Containing 4 (NLRC4) and non-canonical pathways to understand whether NLRP3 blockade alone can mitigate pro-inflammatory cytokine release and pyroptotic cell death in contexts where single or multiple inflammasome pathways independent of NLRP3 are activated. In this study we do not limit our insights into inflammasome biology by solely relying on the THP-1 monocytic line under the LPS/nigericin-mediated NLRP3 pathway activation paradigm. We assess therapeutic potential and limitations of NLRP3 inhibition in multi-inflammasome activation contexts utilizing various human cellular systems including cell lines expressing gain of function (GoF) mutations for several inflammasomes, primary human monocytes, macrophages, healthy and Amyotrophic Lateral Sclerosis (ALS) patient induced pluripotent stem cells (iPSC)-derived microglia (iMGL) stimulated for canonical and non-canonical inflammasome pathways. We demonstrate that NLRP3 inhibition can modulate the NLRC4 and non-canonical inflammasome pathways; however, these effects differ between immortalized, human primary innate immune cells, and iMGL. We extend our investigation in more complex systems characterized by activation of multiple inflammasomes such as the SOD1G93A mouse model. Through deep immune phenotyping by single-cell mass cytometry we demonstrate that acute NLRP3 inhibition does not ameliorate spinal cord inflammation in this model. Taken together, our data suggests that NLRP3 inhibition alone may not be sufficient to address dynamic and complex neuroinflammatory pathobiological mechanisms including dysregulation of multiple inflammasome pathways in neurodegenerative disease such as ALS.

Inflammasomes primarily restrict cytosolic Salmonella replication within human macrophages

Salmonella enterica serovar Typhimurium is a facultative intracellular pathogen that utilizes its type III secretion systems (T3SSs) to inject virulence factors into the host cell and colonize the host. In turn, a subset of cytosolic immune receptors respond to T3SS ligands by forming multimeric signaling complexes called inflammasomes, which activate caspases that induce interleukin-1 (IL-1) family cytokine release and an inflammatory form of cell death called pyroptosis. Human macrophages mount a multifaceted inflammasome response to Salmonella infection that ultimately restricts intracellular bacterial replication. However, how inflammasomes restrict Salmonella replication remains unknown. We find that caspase-1 is essential for mediating inflammasome responses to Salmonella and subsequent restriction of bacterial replication within human macrophages, with caspase-4 contributing as well. We also demonstrate that the downstream pore-forming protein gasdermin D (GSDMD) and ninjurin-1 (NINJ1), a mediator of terminal cell lysis, play a role in controlling Salmonella replication in human macrophages. Notably, in the absence of inflammasome responses, we observed hyperreplication of Salmonella within the cytosol of infected cells, and we also observed increased bacterial replication within vacuoles, suggesting that inflammasomes control Salmonella replication primarily within the cytosol and also within vacuoles. These findings reveal that inflammatory caspases and pyroptotic factors mediate inflammasome responses that restrict the subcellular localization of intracellular Salmonella replication within human macrophages.

Yersinia Type III-Secreted Effectors Evade the Caspase-4 Inflammasome in Human Cells

Yersinia are gram-negative zoonotic bacteria that use a type III secretion system (T3SS) to inject Yersinia outer proteins (Yops) into the host cytosol to subvert essential components of innate immune signaling. However, Yersinia virulence activities can elicit activation of inflammasomes, which lead to inflammatory cell death and cytokine release to contain infection. Yersinia activation and evasion of inflammasomes have been characterized in murine macrophages but remain poorly defined in human cells, particularly intestinal epithelial cells (IECs), a primary site of intestinal Yersinia infection. In contrast to murine macrophages, we find that in both human IECs and macrophages, Yersinia pseudotuberculosis T3SS effectors enable evasion of the caspase-4 inflammasome, which senses cytosolic lipopolysaccharide (LPS). The antiphagocytic YopE and YopH, as well as the translocation regulator YopK, were collectively responsible for evading inflammasome activation, in part by inhibiting Yersinia internalization mediated by YadA and β1-integrin signaling. These data provide insight into the mechanisms of Yersinia-mediated inflammasome activation and evasion in human cells, and reveal species-specific differences underlying regulation of inflammasome responses to Yersinia .

Salmonella enterica serovar Typhi uses two type 3 secretion systems to replicate in human macrophages and to colonize humanized mice

Salmonella enterica serovar Typhi ( S. Typhi) is a human-restricted pathogen that replicates in macrophages. In this study, we investigated the roles of the S. Typhi Type 3 secretion systems (T3SSs) encoded on Salmonella Pathogenicity Islands (SPI) -1 (T3SS-1) and -2 (T3SS-2) during human macrophage infection. We found that mutants of S . Typhi deficient for both T3SSs were defective for intramacrophage replication as measured by flow cytometry, viable bacterial counts, and live time-lapse microscopy. T3SS-secreted proteins PipB2 and SifA contributed to S. Typhi replication and were translocated into the cytosol of human macrophages through both T3SS-1 and -2, demonstrating functional redundancy for these secretion systems. Importantly, an S . Typhi mutant strain that is deficient for both T3SS-1 and -2 was severely attenuated in the ability to colonize systemic tissues in a humanized mouse model of typhoid fever. Overall, this study establishes a critical role for S. Typhi T3SSs during its replication within human macrophages and during systemic infection of humanized mice.

Importance

Salmonella enterica serovar Typhi is a human-restricted pathogen that causes typhoid fever. Understanding the key virulence mechanisms that facilitate S. Typhi replication in human phagocytes will enable rational vaccine and antibiotic development to limit spread of this pathogen. While S. Typhimurium replication in murine models has been studied extensively, there is limited information available about S. Typhi replication in human macrophages, some of which directly conflicts with findings from S. Typhimurium murine models. This study establishes that both of S. Typhi's two Type 3 Secretion Systems (T3SS-1 and -2) contribute to intramacrophage replication and virulence.

Human and mouse NAIP/NLRC4 inflammasome responses to bacterial infection

Intracellular immune complexes known as inflammasomes sense breaches of cytosolic sanctity. Inflammasomes promote downstream proinflammatory events, including interleukin-1 (IL-1) family cytokine release and pyroptotic cell death. The nucleotide-binding leucine-rich repeat family, apoptosis inhibitory protein/nucleotide-binding leucine-rich repeat family, caspase recruitment domain (CARD) domain-containing protein 4 (NAIP/NLRC4) inflammasome is involved in a range of pathogenic and protective inflammatory processes in mammalian hosts. In particular, the NAIP/NLRC4 inflammasome responds to flagellin and components of the virulence-associated type III secretion (T3SS) apparatus in the host cytosol, thereby allowing it to be a critical mediator of host defense during bacterial infection. Notable species- and cell type-specific differences exist in NAIP/NLRC4 inflammasome responses to bacterial pathogens. With a focus on Salmonella enterica serovar Typhimurium as a model pathogen, we review differences between murine and human NAIP/NLRC4 inflammasome responses. Differences in NAIP/NLRC4 inflammasome responses across species and cell types may have arisen in part due to evolutionary pressures.

LNCGM1082-mediated NLRC4 activation drives resistance to bacterial infection

The activation of NLRC4 is a major host response against intracellular bacteria infection. However, NLRC4 activation after a host senses diverse stimuli is difficult to understand. Here, we found that the lncRNA LNCGM1082 plays a critical role in the activation of NLRC4. LNCGM1082 in macrophages affects the maturation of interleukin (IL)-1β and pyroptotic cell death only after exposure to an NLRC4 ligand. Similar to NLRC4-/- mice, LNCGM1082-/- mice were highly sensitive to Salmonella Typhimurium (S. T) infection. LNCGM1082 deficiency in mouse or human macrophages inhibited IL-1β maturation and pyroptosis. Mechanistically, LNCGM1082 induced the binding of PKCδ with NLRC4 in both mice and humans. In contrast, NLRC4 did not bind PKCδ in LNCGM1082-/- macrophages. The activity of the lncRNA LNCGM1082 induced by S. T may be mediated through TLR5 in the macrophages of both mice and humans. In summary, our data indicate that TLR5-mediated LNCGM1082 activity can promote the binding of PKCδ with NLRC4 to activate NLRC4 and induce resistance to bacterial infection.

Transcriptome Sequencing Reveals Salmonella Flagellin Activation of Interferon-β-Related Immune Responses in Macrophages

The flagellin (FliC) of Salmonella typhimurium is a potential vaccine adjuvant as it can activate innate immunity and promote acquired immune responses. Macrophages are an important component of the innate immune system. The mechanism of flagellin’s adjuvant activity has been shown to be related to its ability to activate macrophages. However, few studies have comprehensively investigated the effects of Salmonella flagellin in macrophages using transcriptome sequencing. In this study, RNA-Seq was used to analyze the expression patterns of RAW264.7 macrophages induced by FliC to identify novel transcriptomic signatures in macrophages. A total of 2204 differentially expressed genes were found in the FliC-treated group compared with the control. Gene ontology and KEGG pathway analyses identified the top significantly regulated functional classification and canonical pathways, which were mainly related to immune responses and regulation. Inflammatory cytokines (IL-6, IL-1β, TNF-α, etc.) and chemokines (CXCL2, CXCL10, CCL2, etc.) were highly expressed in RAW264.7 cells following stimulation. Notably, flagellin significantly increased the expression of interferon (IFN)-β. In addition, previously unidentified IFN regulatory factors (IRFs) and IFN-stimulated genes (ISGs) were also significantly upregulated. The results of RNA-Seq were verified, and furthermore, we demonstrated that flagellin increased the expression of IFN-β and IFN-related genes (IRFs and ISGs) in bone marrow-derived dendritic cells and macrophages. These results suggested that Salmonella flagellin can activate IFN-β-related immune responses in macrophages, which provides new insight into the immune mechanisms of flagellin adjuvant.

Bi-Directional Relationship Between Autophagy and Inflammasomes in Neurodegenerative Disorders

The innate immune system, as the first line of cellular defense, triggers a protective response called inflammation when encountered with invading pathogens. Inflammasome is a multi-protein cytosolic signaling complex that induces inflammation and is critical for inflammation-induced pyroptotic cell death. Inflammasome activation has been found associated with neurodegenerative disorders (NDs), inflammatory diseases, and cancer. Autophagy is a crucial intracellular quality control and homeostasis process which removes the dysfunctional organelles, damaged proteins, and pathogens by sequestering the cytosolic components in a double-membrane vesicle, which eventually fuses with lysosome resulting in cargo degradation. Autophagy disruption has been observed in many NDs presented with persistent neuroinflammation and excessive inflammasome activation. An interplay between inflammation activation and the autophagy process has been realized over the last decade. In the case of NDs, autophagy regulates neuroinflammation load and cellular damage either by engulfing the misfolded protein deposits, dysfunctional mitochondria, or the inflammasome complex itself. A healthy two-way regulation between both cellular processes has been realized for cell survival and cell defense during inflammatory conditions. Therefore, clinical interest in the modulation of inflammasome activation by autophagy inducers is rapidly growing. In this review, we discuss the structural basis of inflammasome activation and the mechanistic ideas of the autophagy process in NDs. Along with comments on multiple ways of neuroinflammation regulation by microglial autophagy, we also present a perspective on pharmacological opportunities in this molecular interplay pertaining to NDs.

A flagellin-conjugate protein induces dual NLRC4- and NLRP3-inflammasome activation which modulates inflammatory cytokine secretion from macrophages

Background

A recombinant fusion protein combining the adjuvant and TLR5-ligand flagellin with the major birch pollen allergen Bet v 1 (rFlaA:Betv1) has been suggested to prevent the manifestation of birch allergy. Noteworthy, rFlaA:Betv1 induced both pro- and anti-inflammatory responses which were differentially regulated. However, the mechanism by which flagellin fusion proteins modulate allergen-specific immune responses, especially the mechanisms underlying IL-1β secretion and their contribution to the overall immune responses remains elusive.

Objective

To investigate the mechanisms underlying the production of IL-1β from rFlaA:Betv1 stimulated macrophages.

Methods

Macrophages were derived from mouse peritoneal-, human buffy-coat-, and PMA-differentiated THP-1 (wild type or lacking either ASC, NLRP3, or NLRC4) cells. Macrophages were stimulated with non-modified rFlaA:Betv1, mutant variants lacking either the flagellin DC0 domain or a sequence motif formerly described to mediate TLR5-activation, and respective controls in the presence or absence of inhibitors interfering with MAPK- and NFκB-signaling. Cytokine secretion was analyzed by ELISA and intracellular signaling by Western Blot. To study the contribution of IL-1β to the overall immune responses, IL1R-deficient mouse peritoneal macrophages were used.

Results

rFlaA:Betv1 consistently activated all types of investigated macrophages, inducing higher IL-1β secretion compared with the equimolar mixture of both proteins. rFlaA:Betv1-induced activation of THP-1 macrophages was shown to be independent of either the TLR5-activating sequence motif or the flagellin DC0 domain but depended on both NLRP3- and NLRC4-inflammasomes. In addition, NFκB and SAP/JNK MAP kinases regulated rFlaA:Betv1-induced inflammasome activation and cytokine secretion by modulating pro-Caspase-1- and pro-IL-1β-expression in THP-1 macrophages. Finally, lack of IL-1β positive feedback via the IL1R strongly diminished the rFlaA:Betv1-induced secretion of IL-1β, IL-6, and TNF-α from peritoneal macrophages.

Conclusion

The mechanisms contributing to rFlaA:Betv1-induced IL-1β secretion from macrophages were shown to be complex, involving both NLRC4- and NLRP3-inflammsomes, as well as NFκB- and SAP/JNK MAP kinase-signaling. Better understanding the mechanisms regulating the activation of immune cells by novel therapeutic candidates like the rFlaA:Betv1 fusion protein will allow us to further improve and develop new treatment strategies when using flagellin as an adjuvant.

Lysosomal cathepsins act in concert with Gasdermin-D during NAIP/NLRC4-dependent IL-1β secretion

The NAIP/NLRC4 inflammasome is classically associated with the detection of bacterial invasion to the cytosol. However, recent studies have demonstrated that NAIP/NLRC4 is also activated in non-bacterial infections, and in sterile inflammation. Moreover, in addition to the well-established model for the detection of bacterial proteins by NAIP proteins, the participation of other cytosolic pathways in the regulation of NAIP/NLRC4-mediated responses has been reported in distinct contexts. Using pharmacological inhibition and genetic deletion, we demonstrate here that cathepsins, well known for their involvement in NLRP3 activation, also regulate NAIP/NLRC4 responses to cytosolic flagellin in murine and human macrophages. In contrast to that observed for NLRP3 agonists, cathepsins inhibition did not reduce ASC speck formation or caspase-1 maturation in response to flagellin, ruling out their participation in the effector phase of NAIP/NLRC4 activation. Moreover, cathepsins had no impact on NF-κB-mediated priming of pro-IL-1β, thus suggesting these proteases act downstream of the NAIP/NLRC4 inflammasome activation. IL-1β levels secreted in response to flagellin were reduced in the absence of either cathepsins or Gasdermin-D (GSDMD), a molecule involved in the induction of pyroptosis and cytokines release. Notably, IL-1β secretion was abrogated in the absence of both GSDMD and cathepsins, demonstrating their non-redundant roles for the optimal IL-1β release in response to cytosolic flagellin. Given the central role of NAIP/NLRC4 inflammasomes in controlling infection and, also, induction of inflammatory pathologies, many efforts have been made to uncover novel molecules involved in their regulation. Thus, our findings bring together a relevant contribution by describing the role of cathepsins as players in the NAIP/NLRC4-mediated responses.

Assessing the NLRP3 Inflammasome Activating Potential of a Large Panel of Micro- and Nanoplastics in THP-1 Cells

Due to the ubiquity of environmental micro- and nanoplastics (MNPs), inhalation and ingestion by humans is very likely, but human health effects remain largely unknown. The NLRP3 inflammasome is a key player of the innate immune system and is involved in responses towards foreign particulate matter and the development of chronic intestinal and respiratory inflammatory diseases. We established NLRP3-proficient and -deficient THP-1 cells as an alternative in vitro screening tool to assess the potential of MNPs to activate the NLRP3 inflammasome. By investigating cytokine release (IL-1β and IL-8) and cytotoxicity after treatment with engineered nanomaterials, this in vitro approach was compared to earlier published ex vivo murine bone marrow-derived macrophages and in vivo data. This approach showed a strong correlation with previously published data, verifying that THP-1 cells are a suitable model to investigate NLRP3 inflammasome activation. We then investigated the proinflammatory potential of eight MNPs of different size, shape, and chemical composition. Only amine-modified polystyrene (PS-NH₂) acted as a direct NLRP3 activator. However, polyethylene terephthalate (PET), polyacrylonitrile (PAN), and nylon (PA6) induced a significant increase in IL-8 release in NLRP3^−/− cells. Our results suggest that most MNPs are not direct activators of the NLRP3 inflammasome, but specific MNP types might still possess pro-inflammatory potential via other pathways.

Bacterial subversion of NLR-mediated immune responses

Members of the mammalian Nod-like receptor (NLR) protein family are important intracellular sensors for bacteria. Bacteria have evolved under the pressure of detection by host immune sensing systems, leading to adaptive subversion strategies to dampen immune responses for their benefits. These include modification of microbe-associated molecular patterns (MAMPs), interception of innate immune pathways by secreted effector proteins and sophisticated instruction of anti-inflammatory adaptive immune responses. Here, we summarise our current understanding of subversion strategies used by bacterial pathogens to manipulate NLR-mediated responses, focusing on the well-studied members NOD1/2, and the inflammasome forming NLRs NLRC4, and NLRP3. We discuss how bacterial pathogens and their products activate these NLRs to promote inflammation and disease and the range of mechanisms used by bacterial pathogens to evade detection by NLRs and to block or dampen NLR activation to ultimately interfere with the generation of host immunity. Moreover, we discuss how bacteria utilise NLRs to facilitate immunotolerance and persistence in the host and outline how various mechanisms used to attenuate innate immune responses towards bacterial pathogens can also aid the host by reducing immunopathologies. Finally, we describe the therapeutic potential of harnessing immune subversion strategies used by bacteria to treat chronic inflammatory conditions.

Salmonella enterica Serovar Typhimurium Induces NAIP/NLRC4- and NLRP3/ASC-Independent, Caspase-4-Dependent Inflammasome Activation in Human Intestinal Epithelial Cells

Salmonella enterica serovar Typhimurium is a Gram-negative pathogen that causes diseases ranging from gastroenteritis to systemic infection and sepsis. Salmonella uses type III secretion systems (T3SS) to inject effectors into host cells. While these effectors are necessary for bacterial invasion and intracellular survival, intracellular delivery of T3SS products also enables detection of translocated Salmonella ligands by cytosolic immune sensors. Some of these sensors form multimeric complexes called inflammasomes, which activate caspases that lead to interleukin-1 (IL-1) family cytokine release and pyroptosis. In particular, the Salmonella T3SS needle, inner rod, and flagellin proteins activate the NAIP/NLRC4 inflammasome in murine intestinal epithelial cells (IECs), which leads to restriction of bacterial replication and extrusion of infected IECs into the intestinal lumen, thereby preventing systemic dissemination of Salmonella. While these processes are quite well studied in mice, the role of the NAIP/NLRC4 inflammasome in human IECs remains unknown. Unexpectedly, we found the NAIP/NLRC4 inflammasome is dispensable for early inflammasome responses to Salmonella in both human IEC lines and enteroids. Additionally, NLRP3 and the adaptor protein ASC are not required for inflammasome activation in Caco-2 cells. Instead, we observed a necessity for caspase-4 and gasdermin D pore-forming activity in mediating inflammasome responses to Salmonella in Caco-2 cells. These findings suggest that unlike murine IECs, human IECs do not rely on NAIP/NLRC4 or NLRP3/ASC inflammasomes and instead primarily use caspase-4 to mediate inflammasome responses to Salmonella pathogenicity island 1 (SPI-1)-expressing Salmonella.

Gasdermins: New Therapeutic Targets in Host Defense, Inflammatory Diseases, and Cancer

Gasdermins (GSDMs) are a class of pore-forming proteins related to pyroptosis, a programmed cell death pathway that is induced by a range of inflammatory stimuli. Small-scale GSDM activation and pore formation allow the passive release of cytokines, such as IL-1β and IL-18, and alarmins, but, whenever numerous GSDM pores are assembled, osmotic lysis and cell death occur. Such GSDM-mediated pyroptosis promotes pathogen clearance and can help restore homeostasis, but recent studies have revealed that dysregulated pyroptosis is at the root of many inflammation-mediated disease conditions. Moreover, new homeostatic functions for gasdermins are beginning to be revealed. Here, we review the newly discovered mechanisms of GSDM activation and their prominent roles in host defense and human diseases associated with chronic inflammation. We also highlight the potential of targeting GSDMs as a new therapeutic approach to combat chronic inflammatory diseases and cancer and how we might overcome the current obstacles to realize this potential.

Pyroptosis in host defence against bacterial infection

Pyroptosis, a regulated form of pro-inflammatory cell death, is characterised by cell lysis and by the release of cytokines, damage- and pathogen-associated molecular patterns. It plays an important role during bacterial infection, where it can promote an inflammatory response and eliminate the replicative niche of intracellular pathogens. Recent work, using a variety of bacterial pathogens, has illuminated the versatility of pyroptosis, revealing unexpected and important concepts underlying host defence. In this Review, we overview the molecular mechanisms underlying pyroptosis and discuss their role in host defence, from the single cell to the whole organism. We focus on recent studies using three cellular microbiology paradigms - Mycobacterium tuberculosis, Salmonella Typhimurium and Shigella flexneri - that have transformed the field of pyroptosis. We compare insights discovered in tissue culture, zebrafish and mouse models, highlighting the advantages and disadvantages of using these complementary infection models to investigate pyroptosis and for modelling human infection. Moving forward, we propose that in-depth knowledge of pyroptosis obtained from complementary infection models can better inform future studies using higher vertebrates, including humans, and help develop innovative host-directed therapies to combat bacterial infection.

Cardiolipin Biosynthesis Genes Are Not Required for Salmonella enterica Serovar Typhimurium Pathogenesis in C57BL/6J Mice

Salmonella enterica serovar Typhimurium is an intracellular pathogen that parasitizes macrophages from within a vacuole. The vacuolar environment prompts the bacterium to regulate the lipid composition of the outer membrane (OM), and this influences host inflammation. S. Typhimurium regulates the levels of acidic glycerophospholipids known as cardiolipins (CL) within the OM, and mitochondrial CL molecules can prime and activate host inflammasomes. However, the contribution of S. Typhimurium’s CL biosynthesis genes to intracellular survival, inflammasome activation, and pathogenesis had not been examined. S. Typhimurium genes encode three CL synthases. Single, double, and triple mutants were constructed. Similar to other Enterobacteriaceae, ClsA is the primary CL synthase for S. Typhimurium during logarithmic growth, while ClsB and ClsC contribute CL production in stationary phase. It was necessary to delete all three genes to diminish the CL content of the envelope. Despite being devoid of CL molecules, ΔclsABC mutants were highly virulent during oral and systemic infection for C57BL/6J mice. In macrophages, ΔclsA, ΔclsB, ΔclsC, and ΔclsAC mutants behaved like the wild type, whereas ΔclsAB, ΔclsBC, and ΔclsABC mutants were attenuated and elicited reduced amounts of secreted interleukin-1 beta (IL-1β), IL-18, and lactate dehydrogenase. Hence, when clsA and clsC are deleted, clsB is necessary and sufficient to promote intracellular survival and inflammasome activation. Similarly, when clsB is deleted, clsA and clsC are necessary and sufficient. Therefore, the three CL synthase genes cooperatively and redundantly influence S. Typhimurium inflammasome activation and intracellular survival in C57BL/6J mouse macrophages but are dispensable for virulence in mice.

IMPORTANCESalmonella enterica serovar Typhimurium is a pathogenic Gram-negative bacterium that regulates the cardiolipin (CL) and lipopolysaccharide (LPS) composition of the outer membrane (OM) during infection. Mitochondrial CL molecules activate the inflammasome and its effector caspase-1, which initiates an inflammatory process called pyroptosis. Purified bacterial CL molecules also influence LPS activation of Toll-like receptor 4 (Tlr4). S. Typhimurium resides within macrophage vacuoles and activates Tlr4 and the inflammasome during infection. However, the contribution of the three bacterial CL synthase genes (cls) to microbial pathogenesis and inflammation had not been tested. This study supports that the genes encoding the CL synthases work coordinately to promote intracellular survival in macrophages and to activate the inflammasome but do not influence inflammatory cytokine production downstream of Tlr4 or virulence in C57BL/6J mice. The macrophage phenotypes are not directly attributable to CL production but are caused by deleting specific combinations of cls gene products.

Intratumoral injection of schwannoma with attenuated Salmonella typhimurium induces antitumor immunity and controls tumor growth

Schwannomas are slow-growing benign neoplasms that develop throughout the body causing pain, sensory/motor dysfunction, and death. Because bacterial immunotherapy has been used in the treatment of some malignant neoplasms, we evaluated attenuated Salmonella typhimurium strains as immunotherapies for benign murine schwannomas. Several bacterial strains were tested, including VNP20009, a highly attenuated strain that was previously shown to be safe in human subjects with advanced malignant neoplasms, and a VNP20009 mutant that was altered in motility and other properties that included adherence and invasion of cultured mammalian cells. VNP20009 controlled tumor growth in two murine schwannoma models and induced changes in cytokine and immune effector cell profiles that were consistent with induction of enhanced innate and adaptive host immune responses compared with controls. Intratumoral (i.t.) injection of S. typhimurium led to tumor cell apoptosis, decreased tumor angiogenesis, and lower growth of the injected schwannoma tumors. Invasive VNP20009 was significantly more efficacious than was a noninvasive derivative in controlling the growth of injected tumors. Bacterial treatment apparently induced systemic antitumor immunity in that the growth of rechallenge schwannomas implanted following primary bacterial treatment was also reduced. Checkpoint programmed death-1 (PD-1) blockade induced by systemic administration of anti-PD-1 antibodies controlled tumor growth to the same degree as i.t. injection of S. typhimurium, and together, these two therapies had an additive effect on suppressing schwannoma growth. These experiments represent validation of a bacterial therapy for a benign neoplasm and support development of S. typhimurium VNP20009, potentially in combination with PD-1 inhibition, as a schwannoma immunotherapy.

Mechanisms for the Invasion and Dissemination of Salmonella

Salmonella enterica is a gastroenteric Gram-negative bacterium that can infect both humans and animals and causes millions of illnesses per year around the world. Salmonella infections usually occur after the consumption of contaminated food or water. Infections with Salmonella species can cause diseases ranging from enterocolitis to typhoid fever. Salmonella has developed multiple strategies to invade and establish a systemic infection in the host. Different cell types, including epithelial cells, macrophages, dendritic cells, and M cells, are important in the infection process of Salmonella. Dissemination throughout the body and colonization of remote organs are hallmarks of Salmonella infection. There are several routes for the dissemination of Salmonella typhimurium. This review summarizes the current understanding of the infection mechanisms of Salmonella. Additionally, different routes of Salmonella infection will be discussed. In this review, the strategies used by Salmonella enterica to establish persistent infection will be discussed. Understanding both the bacterial and host factors leading to the successful colonization of Salmonella enterica may enable the rational design of effective therapeutic strategies.

Salmonella Induces the cGAS-STING-Dependent Type I Interferon Response in Murine Macrophages by Triggering mtDNA Release

Salmonella enterica serovar Typhimurium (S. Typhimurium) elicited strong innate immune responses in macrophages. To activate innate immunity, pattern recognition receptors (PRRs) in host cells can recognize highly conserved pathogen-associated molecular patterns (PAMPs). Here, we showed that S. Typhimurium induced a robust type I interferon (IFN) response in murine macrophages. Exposure of macrophages to S. Typhimurium activated a Toll-like receptor 4 (TLR4)-dependent type I IFN response. Next, we showed that type I IFN and IFN-stimulated genes (ISGs) were elicited in a TBK1-IFN-dependent manner. Furthermore, cytosolic DNA sensor cyclic GMP-AMP synthase (cGAS) and immune adaptor protein stimulator of interferon genes (STING) were also required for the induction of type I IFN response during infection. Intriguingly, S. Typhimurium infection triggered mitochondrial DNA (mtDNA) release into the cytosol to activate the type I IFN response. In addition, we also showed that bacterial DNA was enriched in cGAS during infection, which may contribute to cGAS activation. Finally, we showed that cGAS and STING deficient mice and cells were more susceptible to S. Typhimurium infection, signifying the critical role of the cGAS-STING pathway in host defense against S. Typhimurium infection. In conclusion, in addition to TLR4-dependent innate immune response, we demonstrated that S. Typhimurium induced the type I IFN response in a cGAS-STING-dependent manner and the S. Typhimurium-induced mtDNA release was important for the induction of type I IFN. This study elucidated a new mechanism by which bacterial pathogen activated the cGAS-STING pathway and also characterized the important role of cGAS-STING during S. Typhimurium infection.

Focus on the Mechanisms and Functions of Pyroptosis, Inflammasomes, and Inflammatory Caspases in Infectious Diseases

Eukaryotic cells can initiate several distinct self-destruction mechanisms to display essential roles for the homeostasis maintenance, development, and survival of an organism. Pyroptosis, a key response mode in innate immunity, also referred to as caspase-1-dependent proinflammatory programmed necrotic cell death activated by human caspase-1/4/5, or mouse caspase-1/11, plays indispensable roles in response to cytoplasmic insults and immune defense against infectious diseases. These inflammatory caspases are employed by the host to eliminate pathogen infections such as bacteria, viruses, protozoans, and fungi. Gasdermin D requires to be cleaved and activated by these inflammatory caspases to trigger the pyroptosis process. Physiological rupture of cells results in the release of proinflammatory cytokines, the alarmins IL-1β and IL-18, symbolizing the inflammatory potential of pyroptosis. Moreover, long noncoding RNAs play direct or indirect roles in the upstream of the pyroptosis trigger pathway. Here, we review in detail recently acquired insights into the central roles of inflammatory caspases, inflammasomes, and pyroptosis, as well as the crosstalk between pyroptosis and long noncoding RNAs in mediating infection immunity and pathogen clearance.

NLRC4 Inflammasome-Mediated Regulation of Eosinophilic Functions

Eosinophils play critical roles in the maintenance of homeostasis in innate and adaptive immunity. Although primarily known for their roles in parasitic infections and the development of Th2 cell responses, eosinophils also play complex roles in other immune responses ranging from anti-inflammation to defense against viral and bacterial infections. However, the contributions of pattern recognition receptors in general, and NOD-like receptors (NLRs) in particular, to eosinophil involvement in these immune responses remain relatively underappreciated. Our in vivo studies demonstrated that NLRC4 deficient mice had a decreased number of eosinophils and impaired Th2 responses after induction of an allergic airway disease model. Our in vitro data, utilizing human eosinophilic EoL-1 cells, suggested that TLR2 induction markedly induced pro-inflammatory responses and inflammasome forming NLRC4 and NLRP3. Moreover, activation by their specific ligands resulted in caspase-1 cleavage and mature IL-1β secretion. Interestingly, Th2 responses such as secretion of IL-5 and IL-13 decreased after transfection of EoL-1 cells with short interfering RNAs targeting human NLRC4. Specific induction of NLRC4 with PAM3CSK4 and flagellin upregulated the expression of IL-5 receptor and expression of Fc epsilon receptors (FcεR1α, FcεR2). Strikingly, activation of the NLRC4 inflammasome also promoted expression of the costimulatory receptor CD80 as well as expression of immunoregulatory receptors PD-L1 and Siglec-8. Concomitant with NLRC4 upregulation, we found an increase in expression and activation of matrix metalloproteinase (MMP)-9, but not MMP-2. Collectively, our results present new potential roles of NLRC4 in mediating a variety of eosinopilic functions.

The flagellin of Vibrio parahaemolyticus induces the inflammatory response of Tetraodon nigroviridis through TLR5M

Vibrio parahaemolyticus is an important marine pathogen that cause inflammation even death in teleost. It has brought huge economic losses to aquaculture and serious threats to the sustainable development of marine fisheries. Here, we isolated the DNA, RNA, and total flagellin from V. parahaemolyticus, and obtained the primary spleen and head kidney cells (including leukocytes) from Tetraodon nigroviridis. V. parahaemolyticus DNA, RNA, and total flagellin were used to treat the T. nigroviridis primary cells described above. The results show that the nitric oxide (NO) production and respiratory burst response were significantly induced after stimulation with V. parahaemolyticus total flagellin in T. nigroviridis head kidney and spleen cells. And total flagellin could promote the gene expression and protein production of IL-1β in T. nigroviridis leukocytes. T. nigroviridis TLR5M (TnTLR5M) and TLR5S (TnTLR5S) ORF sequences were obtained as the main recognition receptor for flagellin. Real-time fluorescent quantitative PCR (qRT-PCR) was performed to detect the expression of pattern recognition receptor TnTLR5M and TnTLR5S, the important signal molecule of inflammatory system TnMyD88 and TnTRAF6, and inflammatory cytokines TnIL-1β and TnIFN-γ2. The results show that there were a significant upregulation after challenge with V. parahaemolyticus total flagellin. We further demonstrated that the total flagellin of V. parahaemolyticus could activate the luciferase activity of the NF-κB reporter gene mediated by TnTLR5M. Overall, our results suggest that V. parahaemolyticus total flagellin activated the NO production, respiratory burst response, and inflammatory cytokines expressions, such as TnIL-1β and TnIFN-γ2, through the TnTLR5M-NF-κB signaling pathway in T. nigroviridis.

Human NAIP/NLRC4 and NLRP3 inflammasomes detect Salmonella type III secretion system activities to restrict intracellular bacterial replication

Salmonella enterica serovar Typhimurium is a Gram-negative pathogen that uses two distinct type III secretion systems (T3SSs), termed Salmonella pathogenicity island (SPI)-1 and SPI-2, to deliver virulence factors into the host cell. The SPI-1 T3SS enables Salmonella to invade host cells, while the SPI-2 T3SS facilitates Salmonella's intracellular survival. In mice, a family of cytosolic immune sensors, including NAIP1, NAIP2, and NAIP5/6, recognizes the SPI-1 T3SS needle, inner rod, and flagellin proteins, respectively. Ligand recognition triggers assembly of the NAIP/NLRC4 inflammasome, which mediates caspase-1 activation, IL-1 family cytokine secretion, and pyroptosis of infected cells. In contrast to mice, humans encode a single NAIP that broadly recognizes all three ligands. The role of NAIP/NLRC4 or other inflammasomes during Salmonella infection of human macrophages is unclear. We find that although the NAIP/NLRC4 inflammasome is essential for detecting T3SS ligands in human macrophages, it is partially required for responses to infection, as Salmonella also activated the NLRP3 and CASP4/5 inflammasomes. Importantly, we demonstrate that combinatorial NAIP/NLRC4 and NLRP3 inflammasome activation restricts Salmonella replication in human macrophages. In contrast to SPI-1, the SPI-2 T3SS inner rod is not sensed by human or murine NAIPs, which is thought to allow Salmonella to evade host recognition and replicate intracellularly. Intriguingly, we find that human NAIP detects the SPI-2 T3SS needle protein. Critically, in the absence of both flagellin and the SPI-1 T3SS, the NAIP/NLRC4 inflammasome still controlled intracellular Salmonella burden. These findings reveal that recognition of Salmonella SPI-1 and SPI-2 T3SSs and engagement of both the NAIP/NLRC4 and NLRP3 inflammasomes control Salmonella infection in human macrophages.

Gasdermin D and Beyond - Gasdermin-mediated Pyroptosis in Bacterial Infections

The discovery of pyroptosis and its subsequent implications in infection and immunity has uncovered a new angle of host-defence against pathogen assault. At its most simple, gasdermin-mediated pyroptosis in bacterial infection would be expected to remove pathogens from the relative safety of the cytosol or pathogen containing vacuole/phagosome whilst inducing a rapid and effective immune response. Differences in gasdermin-mediated pyroptosis between cell types, stimulation conditions, pathogen and even animal species, however, make things more complex. The excessive inflammation associated with the pathogen-induced gasdermin-mediated pyroptosis contributes to a downward spiral in sepsis. With no currently approved effective treatment options for sepsis understanding how gasdermin-mediated pyroptotic pathways are regulated provides an opportunity to identify novel therapeutic candidates against this complex disease. In this review we cover recent advances in the field of gasdermin-mediated pyroptosis with a focus on bacterial infection and sepsis models in the context of humans and other animal species. Importantly we also consider why there is considerable redundancy set into these ancient immune pathways.

DDX17 is an essential mediator of sterile NLRC4 inflammasome activation by retrotransposon RNAs

Detection of microbial products by multiprotein complexes known as inflammasomes is pivotal to host defense against pathogens. Nucleotide-binding domain leucine-rich repeat (NLR) CARD domain containing 4 (NLRC4) forms an inflammasome in response to bacterial products; this requires their detection by NLR family apoptosis inhibitory proteins (NAIPs), with which NLRC4 physically associates. However, the mechanisms underlying sterile NLRC4 inflammasome activation, which is implicated in chronic noninfectious diseases, remain unknown. Here, we report that endogenous short interspersed nuclear element (SINE) RNAs, which promote atrophic macular degeneration (AMD) and systemic lupus erythematosus (SLE), induce NLRC4 inflammasome activation independent of NAIPs. We identify DDX17, a DExD/H box RNA helicase, as the sensor of SINE RNAs that licenses assembly of an inflammasome comprising NLRC4, NLR pyrin domain–containing protein 3, and apoptosis-associated speck-like protein–containing CARD and induces caspase-1 activation and cytokine release. Inhibiting DDX17-mediated NLRC4 inflammasome activation decreased interleukin-18 release in peripheral blood mononuclear cells of patients with SLE and prevented retinal degeneration in an animal model of AMD. Our findings uncover a previously unrecognized noncanonical NLRC4 inflammasome activated by endogenous retrotransposons and provide potential therapeutic targets for SINE RNA–driven diseases.

Isolates of Salmonella typhimurium circumvent NLRP3 inflammasome recognition in macrophages during the chronic phase of infection

Inflammasome signaling results in cell death and release of cytokines from the IL-1 family, which facilitates control over an infection. However, some pathogens such as Salmonella typhimurium (ST) activate various innate immune signaling pathways, including inflammasomes, yet evade these cell death mechanisms, resulting in a chronic infection. Here we investigated inflammasome signaling induced by acute and chronic isolates of ST obtained from different organs. We show that ST isolated from infected mice during the acute phase displays an increased potential to activate inflammasome signaling, which then undergoes a protracted decline during the chronic phase of infection. This decline in inflammasome signaling was associated with reduced expression of virulence factors, including flagella and the Salmonella pathogenicity island I genes. This reduction in cell death of macrophages induced by chronic isolates had the greatest impact on the NLRP3 inflammasome, which correlated with a reduction in caspase-1 activation. Furthermore, rapid cell death induced by Casp-1/11 by ST in macrophages limited the subsequent activation of cell death cascade proteins Casp-8, RipK1, RipK3, and MLKL to prevent the activation of alternative forms of cell death. We observed that the lack of the ability to induce cell death conferred a competitive fitness advantage to ST only during the acute phase of infection. Finally, we show that the chronic isolates displayed a significant attenuation in their ability to infect mice through the oral route. These results reveal that ST adapts during chronic infection by circumventing inflammasome recognition to promote the survival of both the host and the pathogen.

Recombinant HA1-ΔfliC enhances adherence to respiratory epithelial cells and promotes the superiorly protective immune responses against H9N2 influenza virus in chickens

H9N2 subtype avian influenza virus (AIV) is an ongoing threat causing substantial loss to the poultry industry and thus necessitating the development of safe and effective vaccines against AIV. Given that inactivated vaccines are less effective in activating the mucosal immune system, we aimed to generate a vaccine that can actively engage the mucosal immunity which is the front line of the immune system. We generated a group of flagellin-based hemagglutinin globular head (HA1) fusion proteins and characterized their immunogenicity and efficacy. We found that Salmonella typhimurium flagellin (fliC) lacking the hypervariable domain (called herein as HA1-ΔfliC) was recognized by TLR5 and induced a moderate innate immune response compared to N-terminus of fliC (HA1-fliC) and C-terminus of fliC (fliC-HA1). The HA1-ΔfliC protein had increased adherence to the nasal cavity and trachea than HA1-fliC and fliC-HA1 and significantly increased the HA-specific sIgA titers. Our in vivo results revealed that chickens treated with HA1-ΔfliC had a significantly reduced level of viral loads in the cloaca and throat compared with chickens treated with inactivated vaccine. Overall, these results revealed that HA1-ΔfliC can protect chickens against H9N2 AIV by eliciting the efficient mucosal immune responses.

Inflammasome activation by Salmonella

Host recognition of bacteria such as Salmonella enterica serovar Typhimurium requires multiple host detection systems to generate complex inflammatory responses which can be cell type specific and has the potential for bacterial subversion of the host. Host detection of Salmonella requires Pattern Recognition Receptors (PRRs) sensing Pathogen Associated Molecular Patterns (PAMPs). These bacteria possess a diverse array of PAMPs including lipopolysaccharide, flagellin, proteins, lipoproteins, DNA, RNA and metabolites which can potentially activate multiple PRRs concurrently in different cell types. Salmonella is sensed by the inflammasome forming cytosolic nucleotide oligomerisation domain leucine rich repeat-like receptor (NLR) PRRs NLRC4 and NLRP3 as well as by the non canonical inflammasome formed by caspase 11 in mice, caspase 4 and 5 in humans. This review will discuss the different inflammasomes and how their activity regulates the host response to Salmonella infection.

Evolutionary loss of inflammasomes in the Carnivora and implications for the carriage of zoonotic infections

Zoonotic pathogens, such as COVID-19, reside in animal hosts before jumping species to infect humans. The Carnivora, like mink, carry many zoonoses, yet how diversity in host immune genes across species affect pathogen carriage is poorly understood. Here, we describe a progressive evolutionary downregulation of pathogen-sensing inflammasome pathways in Carnivora. This includes the loss of nucleotide-oligomerization domain leucine-rich repeat receptors (NLRs), acquisition of a unique caspase-1/-4 effector fusion protein that processes gasdermin D pore formation without inducing rapid lytic cell death, and the formation of a caspase-8 containing inflammasome that inefficiently processes interleukin-1β. Inflammasomes regulate gut immunity, but the carnivorous diet has antimicrobial properties that could compensate for the loss of these immune pathways. We speculate that the consequences of systemic inflammasome downregulation, however, can impair host sensing of specific pathogens such that they can reside undetected in the Carnivora.

Salmonella spvC Gene Inhibits Autophagy of Host Cells and Suppresses NLRP3 as Well as NLRC4

Salmonella spvC gene, encoding a phosphothreonine lyase on host mitogen-activated protein kinases, facilitates systemic infection of Salmonella while the precise mechanisms remain elusive. Autophagy and pyroptosis dependent on the activation of inflammasomes, as parts of innate immune response, contribute to host defense against Salmonella infection. Recently, we reported that spvC could inhibit pyroptosis. To explore the effect of spvC on autophagy and the relationship between its function in pyroptosis and autophagy, infection models of macrophages J774A.1 and epithelial HeLa cells co-cultured with Salmonella Typhimurium wild type, spvC deletion, site-directed mutant which lacks phosphothreonine lyase activity, or complemented strain were established. The levels of LC3 turnover and Beclin 1 of J774A.1 cells were determined by western blot. Confocal laser scanning microscopy was used to visualize the autophagic flux after being transfected with mRFP-GFP-LC3 plasmid in HeLa cells. Results showed that SpvC inhibited autophagosome formation through its phosphothreonine lyase activity. Additionally, analysis of nucleotide-binding oligomerization domain, leucine-rich repeat and pyrin domain-containing 3 (NLRP3) and NLR with CARD domain-containing 4 (NLRC4) in J774A.1 cells indicated that spvC decreased the protein levels of NLRP3 and NLRC4, which were significantly changed by autophagy inhibitor Bafilomycin A1. Together, our observations reveal a novel mechanism of spvC in Salmonella pathogenesis and host inflammatory response via inhibiting autophagy and NLRP3 as well as NLRC4. These pathways and their subversion by diverse pathogen virulence determinants are expected to throw light on the design of anti-infective agents.

Therapeutic implications of inflammasome in inflammatory bowel disease

Inflammatory bowel disease (IBD) remains a persistent health problem with a global burden surging over 6.8 million cases currently. Clinical pathology of IBD is complicated; however, hyperactive inflammatory and immune responses in the gut is shown to be one of the persistent causes of the disease. Human gut inflammasome, the activator of innate immune system is believed to be a primary underlying cause for the pathology and is largely associated with the progression of IBD. To manage IBD, there is a need to fully understand the role of inflammasome activation in IBD. Since inflammasome potentially play a significant role in IBD, systemic modulation of inflammasome may provide an effective therapeutic and clinical approach to control IBD symptoms. In this review, we have focused on this association between IBD and gut inflammasome, and recent advances in the research and therapeutic strategies for IBD. We have discussed inflammasomes and their components, outcomes from the experimental animals and human studies, inflammasome inhibitors, and developments in the inflammasome-targeted therapies for IBD.