Epithelium-autonomous NAIP/NLRC4 prevents TNF-driven inflammatory destruction of the gut epithelial barrier in Salmonella-infected mice

Paralichthys olivaceus GSDME-mediated pyroptosis is regulated by multiple caspases in different manners

Pyroptosis is a type of programmed cell death mediated by gasdermin (GSDM). GSDM is activated by caspase (CASP), which cleaves GSDM to release the N-terminal (NT) fragment that forms channels in the plasma membrane and leads to cell death. To date, research on pyroptosis in teleost is limited. In this study, we examined the activation and regulation mechanism of pyroptosis in flounder Paralichthys olivaceus. P. olivaceus gasdermin E (PoGSDME) was found to be cleaved by six P. olivaceus caspases (PoCASP1/3a/3b/7/8a/8b). PoCASP1/3a/3b/7 cleaved primarily at 245FEAD248, which generated an NT fragment (NT248) that induced robust pyroptosis. PoCASP8a/8b cleaved both the full length PoGSDME and NT248 at 202IEKD205, thus destroying the biological activity of PoGSDME and NT248. Nine residues crucial for PoGSDME function were identified, of which, F2, L19, and G85 were essential to plasma membrane translocation. During bacterial infection, PoGSDME and PoCASP1 expressions were significantly upregulated in flounder tissues, and PoGSDME, as well as PoCASP1, activation occurred in flounder cells accompanied with the processing cleavage of IL-1β and IL-18. Together these results revealed both the activation and the inhibition mechanisms of GSDME-mediated pyroptosis in flounder, and added new insights into the regulation of pyroptosis in fish.

Pyroptosis regulation by Salmonella effectors

The genus Salmonella contains the most common foodborne pathogens frequently isolated from food-producing animals and is responsible for zoonotic infections in humans and animals. Salmonella infection in humans and animals can cause intestinal damage, resulting in intestinal inflammation and disruption of intestinal homeostasis more severe cases can lead to bacteremia. Pyroptosis, a proinflammatory form of programmed cell death, is involved in many disease processes. Inflammasomes, pyroptosis, along with their respective signaling cascades, are instrumental in the preservation of intestinal homeostasis. In recent years, with the in-depth study of pyroptosis, our comprehension of the virulence factors and effector proteins in Salmonella has reached an extensive level, a deficit persists in our knowledge regarding the intrinsic pathogenic mechanisms about pyroptosis, necessitating a continued pursuit of understanding and investigation. In this review, we discuss the occurrence of pyroptosis induced by Salmonella effectors to provide new ideas for elucidating the regulatory mechanisms through which Salmonella virulence factors and effector proteins trigger pyroptosis could pave the way for novel concepts and strategies in the clinical prevention of Salmonella infections and the treatment of associated diseases.

NLRC4, inflammation and colorectal cancer (Review)

Chronic inflammation is recognized as a major risk factor for cancer and is involved in every phase of the disease. Inflammasomes are central to the inflammatory response and play a crucial role in cancer development. The present review summarizes the role of Nod‑like receptor C4 (NLRC4) in inflammation and colorectal cancer (CRC). Reviews of the literature were conducted using Web of Science, PubMed and CNKI, with search terms including 'NLRC4', 'colorectal cancer', 'auto‑inflammatory diseases' and 'prognosis'. Variants of NLRC4 can cause recessive immune dysregulation and autoinflammation or lead to ulcerative colitis as a heterozygous risk factor. Additionally, genetic mutations in inflammasome components may increase susceptibility to cancer. NLRC4 is considered a tumor suppressor in CRC. The role of NLRC4 in CRC signaling pathways is currently understood to involve five key aspects (caspase 1, NLRP3/IL‑8, IL‑1β/IL‑1, NAIP and p53). The mechanisms by which NLRC4 is involved in CRC are considered to be threefold (through pyroptosis, apoptosis, necroptosis and PANoptosis; regulating the immune response; and protecting intestinal epithelial cells to prevent CRC). However, the impact of NLRC4 mutations on CRC remains unclear. In conclusion, NLRC4 is a significant inflammasome that protects against CRC through various signaling pathways and mechanisms. The association between NLRC4 mutations and CRC warrants further investigation.

The immune interactions of gut glycans and microbiota in health and disease

The human digestive system harbors a vast diversity of commensal bacteria and maintains a symbiotic relationship with them. However, imbalances in the gut microbiota accompany various diseases, such as inflammatory bowel diseases (IBDs) and colorectal cancers (CRCs), which significantly impact the well-being of populations globally. Glycosylation of the mucus layer is a crucial factor that plays a critical role in maintaining the homeostatic environment in the gut. This review delves into how the gut microbiota, immune cells, and gut mucus layer work together to establish a balanced gut environment. Specifically, the role of glycosylation in regulating immune cell responses and mucus metabolism in this process is examined.

Salmonella-driven intestinal edema in mice is characterized by tensed fibronectin fibers

Intestinal edema is a common manifestation of numerous gastrointestinal diseases and is characterized by the accumulation of fluid in the interstitial space of the intestinal wall. Technical advances in laser capture microdissection and low-biomass proteomics now allow us to specifically characterize the intestinal edema proteome. Using advanced proteomics, we identify peptides derived from antimicrobial factors with high signal intensity, but also highlight major contributions from the blood clotting system, extracellular matrix (ECM) and protease-protease inhibitor networks. The ECM is a complex fibrillar network of macromolecules that provides structural and mechanical support to the intestinal tissue. One abundant component of the ECM observed in Salmonella-driven intestinal edema is the glycoprotein fibronectin, recognized for its structure-function interplay regulated by mechanical forces. Using mechanosensitive staining of fibronectin fibers reveals that they are tensed in the edema, despite the high abundance of proteases able to cleave fibronectin. In contrast, fibronectin fibers increasingly relax in other cecal tissue areas as the infection progresses. Co-staining for fibrin(ogen) indicates the formation of a provisional matrix in the edema, similar to what is observed in response to skin injury, while collagen staining reveals a sparse and disrupted collagen fiber network. These observations plus the absence of low tensional fibronectin fibers and the additional finding of a high number of protease inhibitors in the edema proteome could indicate a critical role of stretched fibronectin fibers in maintaining tissue integrity in the severely inflamed cecum. Understanding these processes may also provide valuable functional diagnostic markers of intestinal disease progression in the future.

Destruction of the brush border by Salmonella enterica sv. Typhimurium subverts resorption by polarized epithelial cells

Salmonella enterica serovar Typhimurium is an invasive, facultative intracellular gastrointestinal pathogen that destroys the brush border of polarized epithelial cells (PEC). The brush border is critical for the functions of PEC because it resorbs nutrients from the intestinal lumen and builds a physical barrier to infecting pathogens. The manipuation of PEC during infection by Salmonella was investigated by live-cell imaging and ultrastructural analysed of the brush border. We demonstrate that the destruction of the brush border by Salmonella significantly reduces the resorption surface of PEC along with the abrogation of endocytosis at the apical side of PEC. Both these changes in the physiology of PEC were associated with the translocation of type III secretion system effector protein SopE. Additionally, the F-actin polymerization rate at the apical side of PEC was highly altered by SopE, indicating that reduced endocytosis observed in infected PEC is related to the manipulation of F-actin polymerization mediated by SopE and, to a lesser extent, by effectors SopE2 or SipA. We further observed that in the absence of SopE, Salmonella effaced microvilli and induced reticular F-actin by bacterial accumulation during prolonged infection periods. In contrast to strains translocating SopE, strains lacking SopE did not alter resorption by PEC. Finally, we observed that after engulfment of Salmonella, ezrin was lost from the apical side of PEC and found later in early endosomes containing Salmonella. Our observations suggest that the destruction of the brush border by Salmonella may contribute to the pathogenesis of diarrhea.

Exploring the Immunomodulatory Potential of Human Milk: Aryl Hydrocarbon Receptor Activation and Its Impact on Neonatal Gut Health

Several metabolites of the essential amino acid tryptophan have emerged as key players in gut homeostasis through different cellular pathways, particularly through metabolites which can activate the aryl hydrocarbon receptor (AHR). This study aimed to map the metabolism of tryptophan in early life and investigate the effects of specific metabolites on epithelial cells and barrier integrity. Twenty-one tryptophan metabolites were measured in the feces of full-term and preterm neonates as well as in human milk and formula. The ability of specific AHR metabolites to regulate cytokine-induced IL8 expression and maintain barrier integrity was assessed in Caco2 cells and human fetal organoids (HFOs). Overall, higher concentrations of tryptophan metabolites were measured in the feces of full-term neonates compared to those of preterm ones. Within AHR metabolites, indole-3-lactic acid (ILA) was significantly higher in the feces of full-term neonates. Human milk contained different levels of several tryptophan metabolites compared to formula. Particularly, within the AHR metabolites, indole-3-sulfate (I3S) and indole-3-acetic acid (IAA) were significantly higher compared to formula. Fecal-derived ILA and milk-derived IAA were capable of reducing TNFα-induced IL8 expression in Caco2 cells and HFOs in an AHR-dependent manner. Furthermore, fecal-derived ILA and milk-derived IAA significantly reduced TNFα-induced barrier disruption in HFOs.

Direct Salmonella injection into enteroid cells allows the study of host-pathogen interactions in the cytosol with high spatiotemporal resolution

Intestinal epithelial cells (IECs) play pivotal roles in nutrient uptake and in the protection against gut microorganisms. However, certain enteric pathogens, such as Salmonella enterica serovar Typhimurium (S. Tm), can invade IECs by employing flagella and type III secretion systems (T3SSs) with cognate effector proteins and exploit IECs as a replicative niche. Detection of flagella or T3SS proteins by IECs results in rapid host cell responses, i.e., the activation of inflammasomes. Here, we introduce a single-cell manipulation technology based on fluidic force microscopy (FluidFM) that enables direct bacteria delivery into the cytosol of single IECs within a murine enteroid monolayer. This approach allows to specifically study pathogen-host cell interactions in the cytosol uncoupled from preceding events such as docking, initiation of uptake, or vacuole escape. Consistent with current understanding, we show using a live-cell inflammasome reporter that exposure of the IEC cytosol to S. Tm induces NAIP/NLRC4 inflammasomes via its known ligands flagellin and T3SS rod and needle. Injected S. Tm mutants devoid of these invasion-relevant ligands were able to grow in the cytosol of IECs despite the absence of T3SS functions, suggesting that, in the absence of NAIP/NLRC4 inflammasome activation and the ensuing cell death, no effector-mediated host cell manipulation is required to render the epithelial cytosol growth-permissive for S. Tm. Overall, the experimental system to introduce S. Tm into single enteroid cells enables investigations into the molecular basis governing host-pathogen interactions in the cytosol with high spatiotemporal resolution.

Salmonella T3SS-2 virulence enhances gut-luminal colonization by enabling chemotaxis-dependent exploitation of intestinal inflammation

Salmonella Typhimurium (S.Tm) utilizes the chemotaxis receptor Tsr to exploit gut inflammation. However, the characteristics of this exploitation and the mechanism(s) employed by the pathogen to circumvent antimicrobial effects of inflammation are poorly defined. Here, using different naturally occurring S.Tm strains (SL1344 and 14028) and competitive infection experiments, we demonstrate that type-three secretion system (T3SS)-2 virulence is indispensable for the beneficial effects of Tsr-directed chemotaxis. The removal of the 14028-specific prophage Gifsy3, encoding virulence effectors, results in the loss of the Tsr-mediated fitness advantage in that strain. Surprisingly, without T3SS-2 effector secretion, chemotaxis toward the gut epithelium using Tsr becomes disadvantageous for either strain. Our findings reveal that luminal neutrophils recruited as a result of NLRC4 inflammasome activation locally counteract S.Tm cells exploiting the byproducts of the host immune response. This work highlights a mechanism by which S.Tm exploitation of gut inflammation for colonization relies on the coordinated effects of chemotaxis and T3SS activities.

Non-professional efferocytosis of Salmonella-infected intestinal epithelial cells in the neonatal host

The intestinal epithelium is the first line of defense against enteric pathogens. Removal of infected cells by exfoliation prevents mucosal translocation and systemic infection in the adult host, but is less commonly observed in the neonatal intestine. Instead, here, we describe non-professional efferocytosis of Salmonella-infected enterocytes by neighboring epithelial cells in the neonatal intestine. Intestinal epithelial stem cell organoid cocultures of neonatal and adult cell monolayers with damaged enterocytes replicated this observation, confirmed the age-dependent ability of intestinal epithelial cells for efferocytosis, and identified the involvement of the "eat-me" signals and adaptors phosphatidylserine and C1q as well as the "eat-me" receptors integrin-αv (CD51) and CD36 in cellular uptake. Consistent with this, massive epithelial cell membrane protrusions and CD36 accumulation at the contact site with apoptotic cells were observed in the infected neonatal host in vivo. Efferocytosis of infected small intestinal enterocytes by neighboring epithelial cells may represent a previously unrecognized mechanism of neonatal antimicrobial host defense to maintain barrier integrity.

Immune cell-derived signals governing epithelial phenotypes in homeostasis and inflammation

The intestinal epithelium fulfills important physiological functions and forms a physical barrier to the intestinal lumen. Barrier function is regulated by several pathways, and its impairment contributes to the pathogenesis of inflammatory bowel disease (IBD), a chronic inflammatory condition affecting more than seven million people worldwide. Current treatment options specifically target inflammatory mediators and have led to improvement of clinical outcomes; however, a significant proportion of patients experience treatment failure. Pro-repair effects of inflammatory mediators on the epithelium are emerging. In this review we summarize current knowledge on involved epithelial pathways, identify open questions, and put recent findings into clinical perspective, and pro-repair effects. A detailed understanding of epithelial pathways integrating mucosal stimuli in homeostasis and inflammation is crucial for the development of novel, more targeted therapies.

A two-step activation mechanism enables mast cells to differentiate their response between extracellular and invasive enterobacterial infection

Mast cells localize to mucosal tissues and contribute to innate immune defense against infection. How mast cells sense, differentiate between, and respond to bacterial pathogens remains a topic of ongoing debate. Using the prototype enteropathogen Salmonella Typhimurium (S.Tm) and other related enterobacteria, here we show that mast cells can regulate their cytokine secretion response to distinguish between extracellular and invasive bacterial infection. Tissue-invasive S.Tm and mast cells colocalize in the mouse gut during acute Salmonella infection. Toll-like Receptor 4 (TLR4) sensing of extracellular S.Tm, or pure lipopolysaccharide, causes a modest induction of cytokine transcripts and proteins, including IL-6, IL-13, and TNF. By contrast, type-III-secretion-system-1 (TTSS-1)-dependent S.Tm invasion of both mouse and human mast cells triggers rapid and potent inflammatory gene expression and >100-fold elevated cytokine secretion. The S.Tm TTSS-1 effectors SopB, SopE, and SopE2 here elicit a second activation signal, including Akt phosphorylation downstream of effector translocation, which combines with TLR activation to drive the full-blown mast cell response. Supernatants from S.Tm-infected mast cells boost macrophage survival and maturation from bone-marrow progenitors. Taken together, this study shows that mast cells can differentiate between extracellular and host-cell invasive enterobacteria via a two-step activation mechanism and tune their inflammatory output accordingly.

PANoptosis: Mechanisms, biology, and role in disease

Cell death can be executed through distinct subroutines. PANoptosis is a unique inflammatory cell death modality involving the interactions between pyroptosis, apoptosis, and necroptosis, which can be mediated by multifaceted PANoptosome complexes assembled via integrating components from other cell death modalities. There is growing interest in the process and function of PANoptosis. Accumulating evidence suggests that PANoptosis occurs under diverse stimuli, for example, viral or bacterial infection, cytokine storm, and cancer. Given the impact of PANoptosis across the disease spectrum, this review briefly describes the relationships between pyroptosis, apoptosis, and necroptosis, highlights the key molecules in PANoptosome formation and PANoptosis activation, and outlines the multifaceted roles of PANoptosis in diseases together with a potential for therapeutic targeting. We also discuss important concepts and pressing issues for future PANoptosis research. Improved understanding of PANoptosis and its mechanisms is crucial for identifying novel therapeutic targets and strategies.

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.

IL-23/IL-23R Promote Macrophage Pyroptosis and T Helper 1/T Helper 17 Cell Differentiation in Mycobacterial Infection

Pathogen-induced epigenetic modifications can reshape anti-infection immune processes and control the magnitude of host responses. DNA methylation profiling has identified crucial aberrant methylation changes associated with diseases, thus providing biological insights into the roles of epigenetic factors in mycobacterial infection. In this study, we performed a genome-wide methylation analysis of skin biopsies from patients with leprosy and healthy controls. T helper 17 differentiation pathway was found to be significantly associated with leprosy through functional enrichment analysis. As a key gene in this pathway, IL-23R was found to be critical to mycobacterial immunity in leprosy, according to integrated analysis with DNA methylation, RNA sequencing, and GWASs. Functional analysis revealed that IL-23/IL-23R-enhanced bacterial clearance by activating caspase-1/GSDMD-mediated pyroptosis in a manner dependent on NLRP3 through signal transducer and activator of transcription 3 signaling in macrophages. Moreover, IL23/IL-23R promoted T helper 1 and T helper 17 cell differentiation and proinflammatory cytokine secretion, thereby increasing host bactericidal activity. IL-23R knockout attenuated the effects and increased susceptibility to mycobacterial infection mentioned earlier. These findings illustrate the biological functions of IL-23/IL-23R in modulating intracellular bacterial clearance in macrophages and further support their regulatory effects in T helper cell differentiation. Our study highlights that IL-23/IL-23R might serve as potential targets for the prevention and treatment of leprosy and other mycobacterial infections.

Epithelial inflammasomes, gasdermins, and mucosal inflammation - Lessons from Salmonella and Shigella infected mice

Besides its crucial function in nutrient absorbance and as barrier against the microbiota, the gut epithelium is essential for sensing pathogenic insults and mounting of an appropriate early immune response. In mice, the activation of the canonical NAIP/NLRC4 inflammasome is critical for the defense against enterobacterial infections. Activation of the NAIP/NLRC4 inflammasome triggers the extrusion of infected intestinal epithelial cells (IEC) into the gut lumen, concomitant with inflammasome-mediated lytic cell death. The membrane permeabilization, a hallmark of pyroptosis, is caused by the pore-forming proteins called gasdermins (GSDMs). Recent work has revealed that NAIP/NLRC4-dependent extrusion of infected IECs can, however, also be executed in the absence of GSDMD. In fact, several reports highlighted that various cell death pathways (e.g., pyroptosis or apoptosis) and unique mechanisms specific to particular infection models and stages of gut infection are in action during epithelial inflammasome defense against intestinal pathogens. Here, we summarize the current knowledge regarding the underlying mechanisms and speculate on the putative functions of the epithelial inflammasome activation and cell death, with a particular emphasis on mouse infection models for two prominent enterobacterial pathogens, Salmonella Typhimurium and Shigella flexneri.

Redox Regulation of Nucleotide-Binding and Oligomerization Domain-Like Receptors Inflammasome

Significance: Inflammasomes are multimeric complexes that, as part of the innate immune response, sense a wide range of pathogenic and sterile stimuli. They consist of three components, namely a sensor protein, an adaptor, and procaspase-1, which once activated result in secretion of proinflammatory interleukin (IL)-1β and IL-18 and, eventually, in a gasdermin D-dependent lytic cell death called pyroptosis. Recent Advances: Since their discovery 20 years ago, the molecular mechanisms underlying the regulation of inflammasomes have been extensively studied. Oxidative stress appears as a major contributor to modulate inflammasomes, especially NLRP3 as well as NLRP1, NLRP6, and NLRC4. Growing evidence supports the idea that the positive feedback between redox imbalance and inflammasome-driven inflammation fuels an OxInflammatory state in a variety of human pathologies. Critical Issues: The current knowledge about the redox signaling pathways involved in inflammasomes activation and functions are here highlighted. In addition, we discuss the role of this complex molecular network interaction in the onset and progression of pathological conditions including neurological and metabolic diseases as well as skin disorders, also with an insight on COVID-19-related pathology. Finally, the therapeutic strategies able to mitigate the redox-mediated inflammasome activation with synthetic and natural compounds as well as by acting on inflammasome-related post-translational modifications and microRNAs are also addressed. Future Directions: Further investigations leading to a deeper understanding of the reciprocal interaction between inflammasomes and reactive oxygen species will help identify other molecular targets for modulating their hyperactivated state, and to design novel therapeutics for chronic OxInflammatory conditions. Antioxid. Redox Signal. 39, 744-770.

FlgM is required to evade NLRC4-mediated host protection against flagellated Salmonella

Salmonella enterica serovar Typhimurium is a leading cause of gastroenteritis worldwide and a deadly pathogen in children, immunocompromised patients, and the elderly. Salmonella induces innate immune responses through the NLRC4 inflammasome, which has been demonstrated to have distinct roles during systemic and mucosal detections of flagellin and non-flagellin molecules. We hypothesized that NLRC4 recognition of Salmonella flagellin is the dominant protective pathway during infection. To test this hypothesis, we used wild-type, flagellin-deficient, and flagellin-overproducing Salmonella to establish the role of flagellin in mediating NLRC4-dependent host resistance during systemic and mucosal infections in mice. We observed that during the systemic phase of infection, Salmonella efficiently evades NLRC4-mediated innate immunity. During mucosal Salmonella infection, flagellin recognition by the NLRC4 inflammasome pathway is the dominant mediator of protective innate immunity. Deletion of flgM results in constitutive expression of flagellin and severely limits systemic and mucosal Salmonella infections in an NLRC4 inflammasome-dependent manner. These data establish that recognition of Salmonella's flagellin by the NLRC4 inflammasome during mucosal infection is the dominant innate protective pathway for host resistance against the enteric pathogen and that FlgM-mediated evasion of the NLRC4 inflammasome enhances virulence and intestinal tissue destruction.

Unraveling the role of type 1 fimbriae in Salmonella pathogenesis: insights from a comparative analysis of Salmonella Enteritidis and Salmonella Gallinarum

Significant differences in pathogenicity between Salmonella Enteritidis and Salmonella Gallinarum exist despite the fact that S. Gallinarum is a direct descendant of S. Enteritidis. It was hypothesized that such various properties may be in part the result of differences in structure and functions of type 1 fimbriae (T1Fs). In S. Enteritidis, T1Fs bind to oligomannosidic structures carried by host cell glycoproteins and are called mannose-sensitive T1Fs (MST1F). In S. Gallinarum, T1Fs lost ability to bind such carbohydrate chains, and were named mannose-resistant MRT1Fs (MRT1F). Therefore, the present study was undertaken to evaluate the role of MST1Fs and MRT1Fs in the adhesion, invasion, intracellular survival and cytotoxicity of S. Enteritidis and S. Gallinarum toward chicken intestinal CHIC8-E11cells and macrophage-like HD11 cells. Using mutant strains: S. Enteritidis fimH::kan and S. Gallinarum fimH::kan devoid of T1Fs and in vitro assays the following observations were made. MST1Fs have a significant impact on the chicken cell invasion by S. Enteritidis as MST1F-mediated adhesion facilitates direct and stable contact of bacteria with host cells, in contrast to MRT1Fs expressed by S. Gallinarum. MST1Fs as well as MRT1Fs did not affected intracellular viability of S. Enteritidis and S. Gallinarum. However, absolute numbers of intracellular viable wild-type S. Enteritidis were significantly higher than S. Enteritidis fimH::kan mutant and wild-type S. Gallinarum and S. Gallinarum fimH::kan mutant. These differences, reflecting the numbers of adherent and invading bacteria, underline the importance of MST1Fs in the pathogenicity of S. Enteritidis infections. The cytotoxicity of wild-type S. Enteritidis and its mutant devoid of MST1Fs to HD11 cells was essentially the same, despite the fact that the number of viable intracellular bacteria was significantly lower in the mutated strain. Using HD11 cells with similar number of intracellular wild-type S. Enteritidis and S. Enteritidis fimH::kan mutant, it was found that the lack of MST1Fs did not affect directly the cytotoxicity, suggesting that the increase in cytotoxicity of S. Enteritidis devoid of MST1Fs may be associated with crosstalk between T1Fs and other virulence factors.

The microbiota conditions a gut milieu that selects for wild-type Salmonella Typhimurium virulence

Salmonella Typhimurium elicits gut inflammation by the costly expression of HilD-controlled virulence factors. This inflammation alleviates colonization resistance (CR) mediated by the microbiota and thereby promotes pathogen blooms. However, the inflamed gut-milieu can also select for hilD mutants, which cannot elicit or maintain inflammation, therefore causing a loss of the pathogen's virulence. This raises the question of which conditions support the maintenance of virulence in S. Typhimurium. Indeed, it remains unclear why the wild-type hilD allele is dominant among natural isolates. Here, we show that microbiota transfer from uninfected or recovered hosts leads to rapid clearance of hilD mutants that feature attenuated virulence, and thereby contributes to the preservation of the virulent S. Typhimurium genotype. Using mouse models featuring a range of microbiota compositions and antibiotic- or inflammation-inflicted microbiota disruptions, we found that irreversible disruption of the microbiota leads to the accumulation of hilD mutants. In contrast, in models with a transient microbiota disruption, selection for hilD mutants was prevented by the regrowing microbiota community dominated by Lachnospirales and Oscillospirales. Strikingly, even after an irreversible microbiota disruption, microbiota transfer from uninfected donors prevented the rise of hilD mutants. Our results establish that robust S. Typhimurium gut colonization hinges on optimizing its manipulation of the host: A transient and tempered microbiota perturbation is favorable for the pathogen to both flourish in the inflamed gut and also minimize loss of virulence. Moreover, besides conferring CR, the microbiota may have the additional consequence of maintaining costly enteropathogen virulence mechanisms.

Intraluminal neutrophils limit epithelium damage by reducing pathogen assault on intestinal epithelial cells during Salmonella gut infection

Recruitment of neutrophils into and across the gut mucosa is a cardinal feature of intestinal inflammation in response to enteric infections. Previous work using the model pathogen Salmonella enterica serovar Typhimurium (S.Tm) established that invasion of intestinal epithelial cells by S.Tm leads to recruitment of neutrophils into the gut lumen, where they can reduce pathogen loads transiently. Notably, a fraction of the pathogen population can survive this defense, re-grow to high density, and continue triggering enteropathy. However, the functions of intraluminal neutrophils in the defense against enteric pathogens and their effects on preventing or aggravating epithelial damage are still not fully understood. Here, we address this question via neutrophil depletion in different mouse models of Salmonella colitis, which differ in their degree of enteropathy. In an antibiotic pretreated mouse model, neutrophil depletion by an anti-Ly6G antibody exacerbated epithelial damage. This could be linked to compromised neutrophil-mediated elimination and reduced physical blocking of the gut-luminal S.Tm population, such that the pathogen density remained high near the epithelial surface throughout the infection. Control infections with a ssaV mutant and gentamicin-mediated elimination of gut-luminal pathogens further supported that neutrophils are protecting the luminal surface of the gut epithelium. Neutrophil depletion in germ-free and gnotobiotic mice hinted that the microbiota can modulate the infection kinetics and ameliorate epithelium-disruptive enteropathy even in the absence of neutrophil-protection. Together, our data indicate that the well-known protective effect of the microbiota is augmented by intraluminal neutrophils. After antibiotic-mediated microbiota disruption, neutrophils are central for maintaining epithelial barrier integrity during acute Salmonella-induced gut inflammation, by limiting the sustained pathogen assault on the epithelium in a critical window of the infection.

The gasdermin protein family: emerging roles in gastrointestinal health and disease

Since the identification and characterization of gasdermin (GSDM) D as the main effector of inflammatory regulated cell death (or pyroptosis), literature on the GSDM family of pore-forming proteins is rapidly expanding, revealing novel mechanisms regulating their expression and functions that go beyond pyroptosis. Indeed, a growing body of evidence corroborates the importance of GSDMs within the gastrointestinal system, underscoring their critical contributions to the pathophysiology of gastrointestinal cancers, enteric infections and gut mucosal inflammation, such as inflammatory bowel disease. However, with this increase in knowledge, several important and controversial issues have arisen regarding basic GSDM biology and its role(s) during health and disease states. These include critical questions centred around GSDM-dependent lytic versus non-lytic functions, the biological activities of cleaved versus full-length proteins, the differential roles of GSDM-expressing mucosal immune versus epithelial cells, and whether GSDMs promote pathogenic or protective effects during specific disease settings. This Review provides a comprehensive summary and interpretation of the current literature on GSDM biology, specifically focusing on the gastrointestinal tract, highlighting the main controversial issues and their clinical implications, and addressing future areas of research to unravel the specific role(s) of this intriguing, yet enigmatic, family of proteins.

The regulation of self-tolerance and the role of inflammasome molecules

Inflammasome molecules make up a family of receptors that typically function to initiate a proinflammatory response upon infection by microbial pathogens. Dysregulation of inflammasome activity has been linked to unwanted chronic inflammation, which has also been implicated in certain autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, type 1 diabetes, systemic lupus erythematosus, and related animal models. Classical inflammasome activation-dependent events have intrinsic and extrinsic effects on both innate and adaptive immune effectors, as well as resident cells in the target tissue, which all can contribute to an autoimmune response. Recently, inflammasome molecules have also been found to regulate the differentiation and function of immune effector cells independent of classical inflammasome-activated inflammation. These alternative functions for inflammasome molecules shape the nature of the adaptive immune response, that in turn can either promote or suppress the progression of autoimmunity. In this review we will summarize the roles of inflammasome molecules in regulating self-tolerance and the development of autoimmunity.

A hierarchy of cell death pathways confers layered resistance to shigellosis in mice

Bacteria of the genus Shigella cause shigellosis, a severe gastrointestinal disease driven by bacterial colonization of colonic intestinal epithelial cells. Vertebrates have evolved programmed cell death pathways that sense invasive enteric pathogens and eliminate their intracellular niche. Previously we reported that genetic removal of one such pathway, the NAIP-NLRC4 inflammasome, is sufficient to convert mice from resistant to susceptible to oral Shigella flexneri challenge (Mitchell et al., 2020). Here, we investigate the protective role of additional cell death pathways during oral mouse Shigella infection. We find that the Caspase-11 inflammasome, which senses Shigella LPS, restricts Shigella colonization of the intestinal epithelium in the absence of NAIP-NLRC4. However, this protection is limited when Shigella expresses OspC3, an effector that antagonizes Caspase-11 activity. TNFα, a cytokine that activates Caspase-8-dependent apoptosis, also provides potent protection from Shigella colonization of the intestinal epithelium when mice lack both NAIP-NLRC4 and Caspase-11. The combined genetic removal of Caspases-1, -11, and -8 renders mice hyper-susceptible to oral Shigella infection. Our findings uncover a layered hierarchy of cell death pathways that limit the ability of an invasive gastrointestinal pathogen to cause disease.

Salmonella effector SopF regulates PANoptosis of intestinal epithelial cells to aggravate systemic infection

SopF, a newly discovered effector secreted by Salmonella pathogenicity island-1 type III secretion system (T3SS1), was reported to target phosphoinositide on host cell membrane and aggravate systemic infection, while its functional relevance and underlying mechanisms have yet to be elucidated. PANoptosis (pyroptosis, apoptosis, and necroptosis) of intestinal epithelial cells (IECs) has been characterized as a pivotal host defense to limit the dissemination of foodborne pathogens, whereas the effect of SopF on IECs PANoptosis induced by Salmonella is rather limited. Here, we show that SopF can attenuate intestinal inflammation and suppress IECs expulsion to promote bacterial dissemination in mice infected with Salmonella enterica serovar Typhimurium (S. Typhimurium). We revealed that SopF could activate phosphoinositide-dependent protein kinase-1 (PDK1) to phosphorylate p90 ribosomal S6 kinase (RSK) which down-regulated Caspase-8 activation. Caspase-8 inactivated by SopF resulted in inhibition of pyroptosis and apoptosis, but promotion of necroptosis. The administration of both AR-12 (PDK1 inhibitor) and BI-D1870 (RSK inhibitor) potentially overcame Caspase-8 blockade and subverted PANoptosis challenged by SopF. Collectively, these findings demonstrate that this virulence strategy elicited by SopF aggregates systemic infection via modulating IEC PANoptosis through PDK1-RSK signaling, which throws light on novel functions of bacterial effectors, as well as a mechanism employed by pathogens to counteract host immune defense.

Mouse models for bacterial enteropathogen infections: insights into the role of colonization resistance

Globally, enteropathogenic bacteria are a major cause of morbidity and mortality.1-3 Campylobacter, Salmonella, Shiga-toxin-producing Escherichia coli, and Listeria are among the top five most commonly reported zoonotic pathogens in the European Union.4 However, not all individuals naturally exposed to enteropathogens go on to develop disease. This protection is attributable to colonization resistance (CR) conferred by the gut microbiota, as well as an array of physical, chemical, and immunological barriers that limit infection. Despite their importance for human health, a detailed understanding of gastrointestinal barriers to infection is lacking, and further research is required to investigate the mechanisms that underpin inter-individual differences in resistance to gastrointestinal infection. Here, we discuss the current mouse models available to study infections by non-typhoidal Salmonella strains, Citrobacter rodentium (as a model for enteropathogenic and enterohemorrhagic E. coli), Listeria monocytogenes, and Campylobacter jejuni. Clostridioides difficile is included as another important cause of enteric disease in which resistance is dependent upon CR. We outline which parameters of human infection are recapitulated in these mouse models, including the impact of CR, disease pathology, disease progression, and mucosal immune response. This will showcase common virulence strategies, highlight mechanistic differences, and help researchers from microbiology, infectiology, microbiome research, and mucosal immunology to select the optimal mouse model.

Time-Lapse Imaging of Inflammasome-Dependent Cell Death and Extrusion in Enteroid-Derived Intestinal Epithelial Monolayers

Inflammasome-induced cell death is an epithelium-intrinsic innate immune response to pathogenic onslaught on epithelial barriers, caused by invasive microbes such as Salmonella Typhimurium (S.Tm). Pattern recognition receptors detect pathogen- or damage-associated ligands and elicit inflammasome formation. This ultimately restricts bacterial loads within the epithelium, limits breaching of the barrier, and prevents detrimental inflammatory tissue damage. Pathogen restriction is mediated via the specific extrusion of dying intestinal epithelial cells (IECs) from the epithelial tissue, accompanied by membrane permeabilization at some stage of the process. These inflammasome-dependent mechanisms can be studied in real time in intestinal epithelial organoids (enteroids), which allow imaging at high temporal and spatial resolution in a stable focal plane when seeded as 2D monolayers. The protocols described here involve the establishment of murine and human enteroid-derived monolayers, as well as time-lapse imaging of IEC extrusion and membrane permeabilization following inflammasome activation by S.Tm infection. The protocols can be adapted to also study other pathogenic insults or combined with genetic and pharmacological manipulation of the involved pathways.

Barcoded Consortium Infections: A Scalable, Internally Controlled Method to Study Host Cell Binding and Invasion by Pathogenic Bacteria

Bacterial host cell invasion has routinely been investigated by gentamicin protection assays, which are laborsome and suffer from pronounced experimental noise. This chapter describes an internally controlled, medium- to high-throughput method that resolves the capacity of multiple Salmonella virulence factor mutant strains to bind and invade host cells. The method, widely applicable to also other pathogens, is based on the combination of consortia of genetically tagged isogenic bacterial strains and a modified gentamicin protection assay. These protocols provide a flexible tool box to stringently quantify host cell binding and invasive properties of different mutants. Moreover, the method can be applied to both infections of cultured host cells and in vivo animal models, providing a comparable genetic readout, which greatly facilitates comparisons across experimental models.

New insights into bacterial mechanisms and potential intestinal epithelial cell therapeutic targets of inflammatory bowel disease

The global incidence of inflammatory bowel disease (IBD) has increased rapidly in recent years, but its exact etiology remains unclear. In the past decade, IBD has been reported to be associated with dysbiosis of gut microbiota. Although not yet proven to be a cause or consequence of IBD, the common hypothesis is that at least some alterations in the microbiome are protective or pathogenic. Furthermore, intestinal epithelial cells (IECs) serve as a protective physical barrier for gut microbiota, essential for maintaining intestinal homeostasis and actively contributes to the mucosal immune system. Thus, dysregulation within the intestinal epithelium increases intestinal permeability, promotes the entry of bacteria, toxins, and macromolecules, and disrupts intestinal immune homeostasis, all of which are associated with the clinical course of IBD. This article presents a selective overview of recent studies on bacterial mechanisms that may be protective or promotive of IBD in biological models. Moreover, we summarize and discuss the recent discovery of key modulators and signaling pathways in the IECs that could serve as potential IBD therapeutic targets. Understanding the role of the IECs in the pathogenesis of IBD may help improve the understanding of the inflammatory process and the identification of potential therapeutic targets to help ameliorate this increasingly common disease.

[NLRC4 plays a regulatory role in F. nucleatum-induced pyroptosis in macrophages]

OBJECTIVE

To explore the mechanism of F.nucleatum-induced pyroptosis in macrophages and the regulatory role of inflammasomes.

METHODS

Lactate dehydrogenase (LDH) cytotoxicity assay and Hoechst 33342/PI double fluorescence staining were used to analyze cytolysis in F.nucleatum-infected macrophage RAW264.7 cells.The expressions of pyroptosis-related proteins caspase-1, GSDMD and IL-1β were determined using Western blotting.Inflammasome activation in the cells was analyzed by detecting the mRNA expressions of NLRP3, NLRC4, AIM2, and NLRP1 with qRT-PCR.RNA interference technique was used to knock down the key molecules involved in pyroptosis regulation in the macrophages, and the pyroptosis and necrosis rates of the cells following F.nucleatum infection were examined.

RESULTS

The results of LDH cytotoxicity assay and double-fluorescence staining showed that F.nucleatum infection caused swelling and lytic cell death in RAW264.7 cells.F.nucleatum infection resulted in the activation of caspase-1 and GSDMD and upregulated IL-1β expression in a multiplicity of infection (MOI)-and time-dependent manner (P < 0.05).qRT-PCR revealed significantly increased expression of NLRC4 mRNA in the macrophages after F.nucleatum infection (P < 0.05).NLRC4 silencing by siRNA strongly inhibited the activation of caspase-1/GSDMD pathway and reduced cell death (P < 0.05) and IL-1β expression in F.nucleatum-infected cells.

CONCLUSION

NLRC4 inflammasome drives caspase-1/GSDMD-mediated pyroptosis and inflammatory signaling in F.nucleatum-infected macrophages, suggesting the potential of NLRC4 inflammasome as a therapeutic target for F.nucleatum infections.

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.

Acinetobacter baumannii Outer Membrane Protein A Induces Pulmonary Epithelial Barrier Dysfunction and Bacterial Translocation Through The TLR2/IQGAP1 Axis

Pulmonary epithelial barrier dysfunction is a critical pathophysiological process in pneumonia and associated invasive infections, such as those caused by Acinetobacter baumannii. However, the mechanisms underlying A. baumannii-induced pulmonary epithelial barrier dysfunction and bacterial translocation remain unclear. In this study, lungs of mice and A549 human epithelial cell monolayers were challenged with the A. baumannii wild-type strain and an outer membrane protein A (ompA) deletion strain. In addition, epithelial cells in culture were treated with purified OmpA protein or transfected with a eukaryotic expression vector encoding ompA (pCMV-ompA). Bacterial translocation across cell monolayers and intrapulmonary burden were measured, barrier function was evaluated in vivo and in vitro; cell migration ability was determined. The specific inhibitors C29 and JSH-23 were used to suppress the activity of Toll-like receptor 2 (TLR2) and of NF-κB, respectively. IQ-GTPase-activating protein 1 (IQGAP1) small interfering RNA was used to knock down endogenous IQGAP1 expression. In this work, we show that OmpA from A. baumannii increased the production of pro-inflammatory cytokines, remodeled the cytoskeleton, and internalized intercellular adherens junctions (AJs); these changes eventually induced pulmonary epithelial barrier dysfunction to promote bacterial translocation. IQGAP1-targeting small interfering RNA and chemical inhibition of TLR2 or NF-κB prevented high permeability of the pulmonary epithelial barrier. TLR2/NF-κB signaling was involved in OmpA-induced inflammation, IQGAP1-mediated OmpA-induced opening of the pulmonary epithelial barrier via cytoskeleton dynamic remodeling, and cellular redistribution of the major AJ protein, E-cadherin. These observations indicate that A. baumannii uses OmpA to overcome epithelial defences and cross the pulmonary epithelial barrier.

Cross-Talk Between the Intestinal Epithelium and Salmonella Typhimurium

Salmonella enterica serovars are invasive gram-negative bacteria, causing a wide range of diseases from gastroenteritis to typhoid fever, representing a public health threat around the world. Salmonella gains access to the intestinal lumen after oral ingestion of contaminated food or water. The crucial initial step to establish infection is the interaction with the intestinal epithelium. Human-adapted serovars such as S. Typhi or S. Paratyphi disseminate to systemic organs and induce life-threatening disease known as typhoid fever, whereas broad-host serovars such as S. Typhimurium usually are limited to the intestine and responsible for gastroenteritis in humans. To overcome intestinal epithelial barrier, Salmonella developed mechanisms to induce cellular invasion, intracellular replication and to face host defence mechanisms. Depending on the serovar and the respective host organism, disease symptoms differ and are linked to the ability of the bacteria to manipulate the epithelial barrier for its own profit and cross the intestinal epithelium.

This review will focus on S. Typhimurium (STm). To better understand STm pathogenesis, it is crucial to characterize the crosstalk between STm and the intestinal epithelium and decipher the mechanisms and epithelial cell types involved. Thus, the purpose of this review is to summarize our current knowledge on the molecular dialogue between STm and the various cell types constituting the intestinal epithelium with a focus on the mechanisms developed by STm to cross the intestinal epithelium and access to subepithelial or systemic sites and survive host defense mechanisms.

Limosilactobacillus reuteri SLZX19-12 Protects the Colon from Infection by Enhancing Stability of the Gut Microbiota and Barrier Integrity and Reducing Inflammation

Limosilactobacillus reuteri plays an important role in regulating intestinal functions and maintaining barrier integrity in animals. In this study, Limosilactobacillus reuteri strain SLZX19-12 was isolated from the fecal microbiota of Tibetan pigs, and it was found that this strain is sensitive to common antibiotics and has strong resistance to stress. Upon being administered by gavage at different doses, including low, medium, and high doses, for 14 days, Limosilactobacillus reuteri SLZX19-12 may enhance the intestinal barrier. After administration of a high dose of SLZX19-12, mice were challenged with Salmonella enterica serovar Typhimurium SL1344. Infection with Salmonella Typhimurium SL1344 led to disordered colonic microbiotas, colonic inflammation through the S100A8/S100A9-NF-κB pathway and potential apoptosis, and translocation of pathogens to parenteral visceral organs in mice. However, the mice pretreated with Limosilactobacillus reuteri SLZX19-12 showed lower loads of Salmonella in visceral organs, less colonic inflammation, and higher barrier integrity. More importantly, the administration of strain SLZX19-12 resulted in a more stable microbiota structure of the colon, in which the abundance of Alloprevotella was greatly enhanced. Therefore, this study suggests that Limosilactobacillus reuteri SLZX19-12 can protect the colon from infection by enhancing the stability of gut microbiota and barrier integrity and reducing inflammation. IMPORTANCE The use of antibiotics to treat bacterial infections leads to a series of side effects. As an alternative method, the biocontrol strategy, which uses probiotics to suppress pathogens, is considered a potential way to deal with bacterial infections in gut. However, there are few probiotics that are currently safe and can protect against infection. In this study, Limosilactobacillus reuteri strain SLZX19-12 was obtained from Tibetan pigs, which have higher resistance to infection. This strain is sensitive to conventional antibiotics, secretes a wide spectrum of enzymes, and also promotes the intestinal barrier function in mice. In addition, Limosilactobacillus reuteri SLZX19-12 can promote the stability of the gut microbiota to avoid or alleviate the occurrence or development of foodborne infections.

Epithelial Pyroptosis in Host Defense

Pyroptosis is a lytic form of cell death that is executed by a family of pore-forming proteins called gasdermins (GSDMs). GSDMs are activated upon proteolysis by host proteases including the proinflammatory caspases downstream of inflammasome activation. In myeloid cells, GSDM pore formation serves two primary functions in host defense: the selective release of processed cytokines to initiate inflammatory responses, and cell death, which eliminates a replicative niche of the pathogen. Barrier epithelia also undergo pyroptosis. However, unique mechanisms are required for the removal of pyroptotic epithelial cells to maintain epithelial barrier integrity. In the following review, we discuss the role of epithelial inflammasomes and pyroptosis in host defense against pathogens. We use the well-established role of inflammasomes in intestinal epithelia to highlight principles of epithelial pyroptosis in host defense of barrier tissues, and discuss how these principles might be shared or distinctive across other epithelial sites.

Bacterial and Viral Co-Infection in the Intestine: Competition Scenario and Their Effect on Host Immunity

Bacteria and viruses are both important pathogens causing intestinal infections, and studies on their pathogenic mechanisms tend to focus on one pathogen alone. However, bacterial and viral co-infections occur frequently in clinical settings, and infection by one pathogen can affect the severity of infection by another pathogen, either directly or indirectly. The presence of synergistic or antagonistic effects of two pathogens in co-infection can affect disease progression to varying degrees. The triad of bacterial–viral–gut interactions involves multiple aspects of inflammatory and immune signaling, neuroimmunity, nutritional immunity, and the gut microbiome. In this review, we discussed the different scenarios triggered by different orders of bacterial and viral infections in the gut and summarized the possible mechanisms of synergy or antagonism involved in their co-infection. We also explored the regulatory mechanisms of bacterial–viral co-infection at the host intestinal immune interface from multiple perspectives.

STAT1 coordinates intestinal epithelial cell death during gastrointestinal infection upstream of Caspase-8

Intestinal homeostasis and the maintenance of the intestinal epithelial barrier are essential components of host defense during gastrointestinal Salmonella Typhimurium infection. Both require a strict regulation of cell death. However, the molecular pathways regulating epithelial cell death have not been completely understood. Here, we elucidated the contribution of central mechanisms of regulated cell death and upstream regulatory components during gastrointestinal infection. Mice lacking Caspase-8 in the intestinal epithelium are highly sensitive towards bacterial induced enteritis and intestinal inflammation, resulting in an enhanced lethality of these mice. This phenotype was associated with an increased STAT1 activation during Salmonella infection. Cell death, barrier breakdown and systemic infection were abrogated by an additional deletion of STAT1 in Casp8^ΔIEC mice. In the absence of epithelial STAT1, loss of epithelial cells was abolished which was accompanied by a reduced Caspase-8 activation. Mechanistically, we demonstrate that epithelial STAT1 acts upstream of Caspase-8-dependent as well as -independent cell death and thus might play a major role at the crossroad of several central cell death pathways in the intestinal epithelium. In summary, we uncovered that transcriptional control of STAT1 is an essential host response mechanism that is required for the maintenance of intestinal barrier function and host survival.

Inflammasome activation leads to cDC1-independent cross-priming of CD8 T cells by epithelial cell-derived antigen

The innate immune system detects pathogens and initiates adaptive immune responses. Inflammasomes are central components of the innate immune system, but whether inflammasomes provide sufficient signals to activate adaptive immunity is unclear. In intestinal epithelial cells (IECs), inflammasomes activate a lytic form of cell death called pyroptosis, leading to epithelial cell expulsion and the release of cytokines. Here, we employed a genetic system to show that simultaneous antigen expression and inflammasome activation specifically in IECs is sufficient to activate CD8+ T cells. By genetic elimination of direct T cell priming by IECs, we found that IEC-derived antigens were cross-presented to CD8+ T cells. However, cross-presentation of IEC-derived antigen to CD8+ T cells only partially depended on IEC pyroptosis. In the absence of inflammasome activation, cross-priming of CD8+ T cells required Batf3+ dendritic cells (conventional type one dendritic cells [cDC1]), whereas cross-priming in the presence of inflammasome activation required a Zbtb46+ but Batf3-independent cDC population. These data suggest the existence of parallel inflammasome-dependent and inflammasome-independent pathways for cross-presentation of IEC-derived antigens.

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.

Inflammasome-independent functions of NAIPs and NLRs in the intestinal epithelium

The gut relies on the complex interaction between epithelial, stromal and immune cells to maintain gut health in the face of food particles and pathogens. Innate sensing by the intestinal epithelium is critical for maintaining epithelial barrier function and also orchestrating mucosal immune responses. Numerous innate pattern recognition receptors (PRRs) are involved in such sensing. In recent years, several Nucleotide-binding-domain and Leucine-rich repeat-containing receptors (NLRs) have been found to partake in pathogen or damage sensing while also being implicated in gut pathologies, such as colitis and colorectal cancer (CRC). Here, we discuss the current literature focusing on NLR family apoptosis inhibitory proteins (NAIPs) and other NLRs that have non-inflammasome roles in the gut. The mechanisms behind NLR-mediated protection often converges on similar signalling pathways, such as STAT3, MAPK and NFκB. Further understanding of how these NLRs contribute to the maintenance of gut homeostasis will be important for understanding gut pathologies and developing new therapies.

Intercrypt sentinel macrophages tune antibacterial NF-κB responses in gut epithelial cells via TNF

Intestinal epithelial cell (IEC) NF-κB signaling regulates the balance between mucosal homeostasis and inflammation. It is not fully understood which signals tune this balance and how bacterial exposure elicits the process. Pure LPS induces epithelial NF-κB activation in vivo. However, we found that in mice, IECs do not respond directly to LPS. Instead, tissue-resident lamina propria intercrypt macrophages sense LPS via TLR4 and rapidly secrete TNF to elicit epithelial NF-κB signaling in their immediate neighborhood. This response pattern is relevant also during oral enteropathogen infection. The macrophage-TNF-IEC axis avoids responses to luminal microbiota LPS but enables crypt- or tissue-scale epithelial NF-κB responses in proportion to the microbial threat. Thereby, intercrypt macrophages fulfill important sentinel functions as first responders to Gram-negative microbes breaching the epithelial barrier. The tunability of this crypt response allows the induction of defense mechanisms at an appropriate scale according to the localization and intensity of microbial triggers.

Bacterial cancer therapy in autochthonous colorectal cancer affects tumor growth and metabolic landscape

Bacterial cancer therapy (BCT) shows great promise for treatment of solid tumors, yet basic mechanisms of bacterial-induced tumor suppression remain undefined. Attenuated strains of Salmonella enterica serovar Typhimurium (STm) have commonly been used in mouse models of BCT in xenograft and orthotopic transplant cancer models. We aimed to better understand the tumor epithelium-targeted mechanisms of BCT by using autochthonous mouse models of intestinal cancer and tumor organoid cultures to assess the effectiveness and consequences of oral treatment with aromatase A-deficient STm (STmΔaroA). STmΔaroA delivered by oral gavage significantly reduced tumor burden and tumor load in both a colitis-associated colorectal cancer (CAC) model and in a spontaneous Apcmin/+ intestinal cancer model. STmΔaroA colonization of tumors caused alterations in transcription of mRNAs associated with tumor stemness, epithelial-mesenchymal transition, and cell cycle. Metabolomic analysis of tumors demonstrated alteration in the metabolic environment of STmΔaroA-treated tumors, suggesting that STmΔaroA imposes metabolic competition on the tumor. Use of tumor organoid cultures in vitro recapitulated effects seen on tumor stemness, mesenchymal markers, and altered metabolome. Furthermore, live STmΔaroA was required, demonstrating active mechanisms including metabolite usage. We have demonstrated that oral BCT is efficacious in autochthonous intestinal cancer models, that BCT imposes metabolic competition, and that BCT has direct effects on the tumor epithelium affecting tumor stem cells.

Salmonella effector driven invasion of the gut epithelium: breaking in and setting the house on fire

Salmonella Typhimurium (S.Tm) is a major cause of diarrheal disease. The invasion into intestinal epithelial cells (IECs) is a central step in the infection cycle. It is associated with gut inflammation and thought to benefit S.Tm proliferation also in the intestinal lumen. Importantly, it is still not entirely clear how inflammation is elicited and to which extent it links to IEC invasion efficiency in vivo. In this review, we summarize recent findings explaining IEC invasion by type-three-secretion-system-1 (TTSS-1) effector proteins and discuss their effects on invasion and gut inflammation. In non-polarized tissue culture cells, the TTSS-1 effectors (mainly SopB/E/E2) elicit large membrane ruffles fueling cooperative invasion, and can directly trigger pro-inflammatory signaling. By contrast, in the murine gut, we observe discreet-invasion (mainly via the TTSS-1 effector SipA) and a prominent pro-inflammatory role of the host?"s epithelial inflammasome(s), which sense pathogen associated molecular patterns (PAMPs). We discuss why it has remained a major challenge to tease apart direct and indirect inflammatory effects of TTSS-1 effectors and explain why further research will be needed to fully determine their inflammation-modulating role(s).

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.

High-Definition DIC Imaging Uncovers Transient Stages of Pathogen Infection Cycles on the Surface of Human Adult Stem Cell-Derived Intestinal Epithelium

Interactions between individual pathogenic microbes and host tissues involve fast and dynamic processes that ultimately impact the outcome of infection. Using live-cell microscopy, these dynamics can be visualized to study, e.g., microbe motility, binding and invasion of host cells, and intrahost-cell survival. Such methodology typically employs confocal imaging of fluorescent tags in tumor-derived cell line infections on glass. This allows high-definition imaging but poorly reflects the host tissue's physiological architecture and may result in artifacts. We developed a method for live-cell imaging of microbial infection dynamics on human adult stem cell-derived intestinal epithelial cell (IEC) layers. These IEC layers are grown in apical imaging chambers, optimized for physiological cell arrangement and fast, but gentle, differential interference contrast (DIC) imaging. This allows subsecond visualization of both microbial and epithelial surface ultrastructure at high resolution without using fluorescent reporters. We employed this technology to probe the behavior of two model pathogens, Salmonella enterica serovar Typhimurium and Giardia intestinalis, at the intestinal epithelial surface. Our results reveal pathogen-specific swimming patterns on the epithelium and show that Salmonella lingers on the IEC surface for prolonged periods before host cell invasion, while Giardia uses circular swimming with intermittent attachments to scout for stable adhesion sites. The method even permits tracking of individual Giardia flagella, demonstrating that active flagellar beating and attachment to the IEC surface are not mutually exclusive. This work describes a generalizable and relatively inexpensive approach to resolving dynamic pathogen-IEC layer interactions, applicable even to genetically nontractable microorganisms. IMPORTANCE Knowledge of dynamic niche-specific interactions between single microbes and host cells is essential to understand infectious disease progression. However, advances in this field have been hampered by the inherent conflict between the technical requirements for high-resolution live-cell imaging on the one hand and conditions that best mimic physiological infection niche parameters on the other. Toward bridging this divide, we present a methodology for differential interference contrast (DIC) imaging of pathogen interactions at the apical surface of enteroid-derived intestinal epithelia, providing both high spatial and temporal resolution. This alleviates the need for fluorescent reporters in live-cell imaging and provides dynamic information about microbe interactions with a nontransformed, confluent, polarized, and microvilliated human gut epithelium. Using this methodology, we uncover previously unrecognized stages of Salmonella and Giardia infection cycles at the epithelial surface.