Interleukin-1 receptor signaling protects mice from lethal intestinal damage caused by the attaching and effacing pathogen Citrobacter rodentium. Infect Immun. 2009;77:604-614. [PMID: 19075023 DOI: 10.1128/iai.00907-08]

Mitochondrial function and gastrointestinal diseases

Mitochondria are dynamic organelles that function in cellular energy metabolism, intracellular and extracellular signalling, cellular fate and stress responses. Mitochondria of the intestinal epithelium, the cellular interface between self and enteric microbiota, have emerged as crucial in intestinal health. Mitochondrial dysfunction occurs in gastrointestinal diseases, including inflammatory bowel diseases and colorectal cancer. In this Review, we provide an overview of the current understanding of intestinal epithelial cell mitochondrial metabolism, function and signalling to affect tissue homeostasis, including gut microbiota composition. We also discuss mitochondrial-targeted therapeutics for inflammatory bowel diseases and colorectal cancer and the evolving concept of mitochondrial impairment as a consequence versus initiator of the disease.

HOIL1 Regulates Group 3 Innate Lymphoid Cells in the Colon and Protects against Systemic Dissemination, Colonic Ulceration, and Lethality from Citrobacter rodentium Infection

Heme-oxidized IRP2 ubiquitin ligase-1 (HOIL1)-deficient patients experience chronic intestinal inflammation and diarrhea as well as increased susceptibility to bacterial infections. HOIL1 is a component of the linear ubiquitin chain assembly complex that regulates immune signaling pathways, including NF-κB-activating pathways. We have shown previously that HOIL1 is essential for survival following Citrobacter rodentium gastrointestinal infection of mice, but the mechanism of protection by HOIL1 was not examined. C. rodentium is an important murine model for human attaching and effacing pathogens, enteropathogenic and enterohemorrhagic Escherichia coli that cause diarrhea and foodborne illnesses and lead to severe disease in children and immunocompromised individuals. In this study, we found that C. rodentium infection resulted in severe colitis and dissemination of C. rodentium to systemic organs in HOIL1-deficient mice. HOIL1 was important in the innate immune response to limit early replication and dissemination of C. rodentium. Using bone marrow chimeras and cell type-specific knockout mice, we found that HOIL1 functioned in radiation-resistant cells and partly in radiation-sensitive cells and in myeloid cells to limit disease, but it was dispensable in intestinal epithelial cells. HOIL1 deficiency significantly impaired the expansion of group 3 innate lymphoid cells and their production of IL-22 during C. rodentium infection. Understanding the role HOIL1 plays in type 3 inflammation and in limiting the pathogenesis of attaching and effacing lesion-forming bacteria will provide further insight into the innate immune response to gastrointestinal pathogens and inflammatory disorders.

A secreted sirtuin from Campylobacter jejuni contributes to neutrophil activation and intestinal inflammation during infection

Histone modifications control numerous processes in eukaryotes, including inflammation. Some bacterial pathogens alter the activity or expression of host-derived factors, including sirtuins, to modify histones and induce responses that promote infection. In this study, we identified a deacetylase encoded by Campylobacter jejuni which has sirtuin activities and contributes to activation of human neutrophils by the pathogen. This sirtuin is secreted from the bacterium into neutrophils, where it associates with and deacetylates host histones to promote neutrophil activation and extracellular trap production. Using the murine model of campylobacteriosis, we found that a mutant of this bacterial sirtuin efficiently colonized the gastrointestinal tract but was unable to induce cytokine production, gastrointestinal inflammation, and tissue pathology. In conclusion, these results suggest that secreted bacterial sirtuins represent a previously unreported class of bacterial effector and that bacterial-mediated modification of host histones is responsible for the inflammation and pathology that occurs during campylobacteriosis.

IEC-intrinsic IL-1R signaling holds dual roles in regulating intestinal homeostasis and inflammation

Intestinal epithelial cells (IECs) constitute a critical first line of defense against microbes. While IECs are known to respond to various microbial signals, the precise upstream cues regulating diverse IEC responses are not clear. Here, we discover a dual role for IEC-intrinsic interleukin-1 receptor (IL-1R) signaling in regulating intestinal homeostasis and inflammation. Absence of IL-1R in epithelial cells abrogates a homeostatic antimicrobial program including production of antimicrobial peptides (AMPs). Mice deficient for IEC-intrinsic IL-1R are unable to clear Citrobacter rodentium (C. rodentium) but are protected from DSS-induced colitis. Mechanistically, IL-1R signaling enhances IL-22R-induced signal transducer and activator of transcription 3 (STAT3) phosphorylation in IECs leading to elevated production of AMPs. IL-1R signaling in IECs also directly induces expression of chemokines as well as genes involved in the production of reactive oxygen species. Our findings establish a protective role for IEC-intrinsic IL-1R signaling in combating infections but a detrimental role during colitis induced by epithelial damage.

Gasdermin D independent canonical inflammasome responses cooperate with caspase-8 to establish host defense against gastrointestinal Citrobacter rodentium infection

Citrobacter rodentium is an enteropathogen that causes intestinal inflammatory responses in mice reminiscent of the pathology provoked by enteropathogenic and enterohemorrhagic Escherichia coli infections in humans. C. rodentium expresses various virulence factors that target specific signaling proteins involved in executing apoptotic, necroptotic and pyroptotic cell death, suggesting that each of these distinct cell death modes performs essential host defense functions that the pathogen aims to disturb. However, the relative contributions of apoptosis, necroptosis and pyroptosis in protecting the host against C. rodentium have not been elucidated. Here we used mice with single or combined deficiencies in essential signaling proteins controlling apoptotic, necroptotic or pyroptotic cell death to reveal the roles of these cell death modes in host defense against C. rodentium. Gastrointestinal C. rodentium infections in mice lacking GSDMD and/or MLKL showed that both pyroptosis and necroptosis were dispensable for pathogen clearance. In contrast, while RIPK3-deficient mice showed normal C. rodentium clearance, mice with combined caspase-8 and RIPK3 deficiencies failed to clear intestinal pathogen loads. Although this demonstrated a crucial role for caspase-8 signaling in establishing intestinal host defense, Casp8^-/-Ripk3^-/- mice remained capable of preventing systemic pathogen persistence. This systemic host defense relied on inflammasome signaling, as Casp8^-/-Ripk3^-/- mice with combined caspase-1 and -11 deletion succumbed to C. rodentium infection. Interestingly, although it is known that C. rodentium can activate the non-canonical caspase-11 inflammasome, selectively disabling canonical inflammasome signaling by single caspase-1 deletion sufficed to render Casp8^-/-Ripk3^-/- mice vulnerable to C. rodentium-induced lethality. Moreover, Casp8^-/-Ripk3^-/- mice lacking GSDMD survived a C. rodentium infection, suggesting that pyroptosis was not crucial for the protective functions of canonical inflammasomes in these mice. Taken together, our mouse genetic experiments revealed an essential cooperation between caspase-8 signaling and GSDMD-independent canonical inflammasome signaling to establish intestinal and systemic host defense against gastrointestinal C. rodentium infection.

A Protective Role of NOD2 on Oxazolone-induced Intestinal Inflammation Through IL-1β-mediated Signalling Pathway

BACKGROUND AND AIMS

NOD2 has emerged as a critical player in the induction of both Th1 and Th2 responses for potentiation and polarisation of antigen-dependent immunity. Loss-of-function mutations in the NOD2-encoding gene and deregulation of its downstream signalling pathway have been linked to Crohn's disease. Although it is well documented that NOD2 is capable of sensing bacterial muramyl dipeptide, it remains counter-intuitive to link development of overt intestinal inflammation to a loss of bacterial-induced inflammatory response. We hypothesised that a T helper bias could also contribute to an autoimmune-like colitis different from inflammation that is fully fledged by Th1 type cells.

METHODS

An oedematous bowel wall with a mixed Th1/Th2 response was induced in mice by intrarectal instillation of the haptenating agent oxazolone. Survival and clinical scoring were evaluated. At several time points after instillation, colonic damage was assessed by macroscopic and microscopic observations. To evaluate the involvement of NOD2 in immunochemical phenomena, quantitative polymerase chain reaction [PCR] and flow cytometry analysis were performed. Bone marrow chimera experimentation allowed us to evaluate the role of haematopoietic/non-hematopoietic NOD2-expressing cells.

RESULTS

Herein, we identified a key regulatory circuit whereby NOD2-mediated sensing of a muramyl dipeptide [MDP] by radio-resistant cells improves colitis with a mixed Th1/Th2 response that is induced by oxazolone. Genetic ablation of either Nod2 or Ripk2 precipitated oxazolone colitis that is predominantly linked to a lack of interferon-gamma. Bone marrow chimera experiments revealed that inactivation of Nod2 signalling in non-haematopoietic cells is causing a biased M1-M2 polarisation of macrophages and a decreased frequency of splenic regulatory T cells that correlates with an impaired activation of CD4 + T cells within mesenteric lymph nodes. Mechanistically, mice were protected from oxazolone-induced colitis upon administration of MDP in an interleukin-1- and interleukin-23-dependent manner.

CONCLUSIONS

These findings indicate that Nod2 signalling may prevent pathological conversion of T helper cells for maintenance of tissue homeostasis.

NLRP3: A Promising Therapeutic Target for Inflammatory Bowel Disease

Inflammatory bowel disease (IBD), which includes Crohn's disease and ulcerative colitis, is an intestinal disease with complicated pathological mechanisms. The incidence of IBD has been increasing in recent years, which has a significant negative impact on the lives of patients. Therefore, it is particularly important to find new therapeutic targets and innovative drugs for the development of IBD. Recent studies have revealed that NLRP3 inflammatory vesicles can play an important role in maintaining intestinal homeostasis and sustaining the intestinal immune response in IBD. On the one hand, aberrant activation of NLRP3 inflammatory vesicles may cause excessive immune response by converting caspase-1, proIL-18, and proIL-1β to their active forms and releasing pro-inflammatory cytokines to stimulate the development and progression of IBD, and we can improve IBD by targeting blockade of NLRP3 activation. On the other hand, NLRP3 may also play an enter protective role by maintaining the homeostasis of the intestinal immune system. In this paper, we reviewed the activation mechanism of NLRP3 inflammasome, and the effects of NLRP3 inflammasome activation on IBD are discussed from two different perspectives: pathology and protection. At the same time, we listed the effects of direct inhibitors, indirect inhibitors, and natural inhibitors of NLRP3 inflammasome on IBD in combination with cutting-edge advances and clinical practice results, providing new targets and new ideas for the clinical treatment of IBD.

Interleukin-37 exacerbates experimental colitis in an intestinal microbiome-dependent fashion

Background: Inflammatory bowel disease (IBD) involves complicated crosstalk between host immunity and the gut microbiome, whereas the mechanics of how they govern intestinal inflammation remain poorly understood. In this study, we investigated the contribution of environmental factors to shaping gut microbiota composition in colitis mice that were transgenic for human IL-37, a natural anti-inflammatory cytokine possessing pathogenic and protective functions related to microbiota alterations. Methods: Mice transgenic expressing human IL-37 (IL-37tg) were housed under conventional and specific pathogen-free (SPF) conditions to develop a mouse model of dextran sulfate sodium (DSS)-induced colitis. 16S ribosomal RNA sequencing was used for analyzing fecal microbial communities. The efficacy of microbiota in the development of colitis in IL-37tg mice was investigated after antibiotic treatment and fecal microbiota transplantation (FMT). The mechanism by which IL-37 worsened colitis was studied by evaluating intestinal epithelial barrier function, immune cell infiltration, the expression of diverse cytokines and chemokines, as well as activated signaling pathways. Results: We found that IL-37 overexpression aggravated DSS-induced colitis in conventional mice but protected against colitis in SPF mice. These conflicting results from IL-37tg colitis mice are ascribed to a dysbiosis of the gut microbiota in which detrimental bacteria increased in IL-37tg conventional mice. We further identified that the outcome of IL-37-caused colon inflammation is strongly related to intestinal epithelial barrier impairment caused by pathogenic bacteria, neutrophils, and NK cells recruitment in colon lamina propria and mesenteric lymph node to enhance inflammatory responses in IL-37tg conventional mice. Conclusions: The immunoregulatory properties of IL-37 are detrimental in the face of dysbiosis of the intestinal microbiota, which contributes to exacerbated IBD occurrences that are uncontrollable by the immune system, suggesting that depleting gut pathogenic bacteria or maintaining intestinal microbial and immune homeostasis could be a promising therapeutic strategy for IBD.

Compound loss of GSDMD and GSDME function is necessary to achieve maximal therapeutic effect in colitis

Gasdermin D (GSDMD) and gasdermin E (GSDME) perpetuate inflammation by mediating the release of cytokines such as interleukin-1β (IL-1β) and IL-18. However, not only are the actions of GSDMD in colitis still controversial, but its interplay with GSDME in the pathogenesis of this disease has not been investigated. We sought to fill these knowledge gaps using the dextran sodium sulfate (DSS) experimental mouse colitis model. DSS ingestion by wild-type mice caused body weight loss as the result of severe gut inflammation, outcomes that were significantly attenuated in Gsdmd^−/− or Gsdme^−/− mice and nearly fully prevented in Gsdmd^−/−;Gsdme^−/− animals. To assess the translational implications of these findings, we tested the efficacy of the active metabolite of US Food and Drug Administration (FDA)-approved disulfiram, which inhibits GSDMD and GSDME function. The severe DSS-induced gut toxicity was significantly decreased in mice treated with the inhibitor. Collectively, our findings indicate that disruption of the function of both GSDMD and GSDME is necessary to achieve maximal therapeutic effect in colitis.

IL-1R1-dependent signaling coordinates epithelial regeneration in response to intestinal damage

Repair of the intestinal epithelium is tightly regulated to maintain homeostasis. The response after epithelial damage needs to be local and proportional to the insult. How different types of damage are coupled to repair remains incompletely understood. We report that after distinct types of intestinal epithelial damage, IL-1R1 signaling in GREM1+ mesenchymal cells increases production of R-spondin 3 (RSPO3), a Wnt agonist required for intestinal stem cell self-renewal. In parallel, IL-1R1 signaling regulates IL-22 production by innate lymphoid cells and promotes epithelial hyperplasia and regeneration. Although the regulation of both RSPO3 and IL-22 is critical for epithelial recovery from Citrobacter rodentium infection, IL-1R1-dependent RSPO3 production by GREM1+ mesenchymal cells alone is sufficient and required for recovery after dextran sulfate sodium-induced colitis. These data demonstrate how IL-1R1-dependent signaling orchestrates distinct repair programs tailored to the type of injury sustained that are required to restore intestinal epithelial barrier function.

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.

Act Locally, Act Globally-Microbiota, Barriers, and Cytokines in Atherosclerosis

Atherosclerosis is a lipid-driven chronic inflammatory disease that is characterized by the formation and progressive growth of atherosclerotic plaques in the wall of arteries. Atherosclerosis is a major predisposing factor for stroke and heart attack. Various immune-mediated mechanisms are implicated in the disease initiation and progression. Cytokines are key mediators of the crosstalk between innate and adaptive immune cells as well as non-hematopoietic cells in the aortic wall and are emerging players in the regulation of atherosclerosis. Progression of atherosclerosis is always associated with increased local and systemic levels of pro-inflammatory cytokines. The role of cytokines within atherosclerotic plaque has been extensively investigated; however, the cell-specific role of cytokine signaling, particularly the role of cytokines in the regulation of barrier tissues tightly associated with microbiota in the context of cardiovascular diseases has only recently come to light. Here, we summarize the knowledge about the function of cytokines at mucosal barriers and the interplay between cytokines, barriers, and microbiota and discuss their known and potential implications for atherosclerosis development.

Gut Microbiota and Liver Interaction through Immune System Cross-Talk: A Comprehensive Review at the Time of the SARS-CoV-2 Pandemic

BACKGROUND AND AIMS

The gut microbiota is a complex ecosystem containing bacteria, viruses, fungi, yeasts and other single-celled organisms. It is involved in the development and maintenance of both innate and systemic immunity of the body. Emerging evidence has shown its role in liver diseases through the immune system cross-talk. We review herein literature data regarding the triangular interaction between gut microbiota, immune system and liver in health and disease.

METHODS

We conducted a search on the main medical databases for original articles, reviews, meta-analyses, randomized clinical trials and case series using the following keywords and acronyms and their associations: gut microbiota, microbiome, gut virome, immunity, gastrointestinal-associated lymphoid tissue (GALT), non-alcoholic fatty liver disease (NAFLD), non-alcoholic steato-hepatitis (NASH), alcoholic liver disease, liver cirrhosis, hepatocellular carcinoma.

RESULTS

The gut microbiota consists of microorganisms that educate our systemic immunity through GALT and non-GALT interactions. The latter maintain health but are also involved in the pathophysiology and in the outcome of several liver diseases, particularly those with metabolic, toxic or immune-mediated etiology. In this context, gut virome has an emerging role in liver diseases and needs to be further investigated, especially due to the link reported between severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) infection and hepatic dysfunctions.

CONCLUSIONS

Changes in gut microbiota composition and alterations in the immune system response are involved in the pathogenesis of metabolic and immune-mediated liver diseases.

Induction of neutrophil extracellular traps by Campylobacter jejuni

Campylobacter jejuni is the leading cause of bacterial-derived gastroenteritis worldwide and can lead to several post-infectious inflammatory disorders. Despite the prevalence and health impacts of the bacterium, interactions between the host innate immune system and C. jejuni remain poorly understood. To expand on earlier work demonstrating that neutrophils traffic to the site of infection in an animal model of campylobacteriosis, we identified significant increases in several predominantly neutrophil-derived proteins in the faeces of C. jejuni-infected patients, including lipocalin-2, myeloperoxidase and neutrophil elastase. In addition to demonstrating that these proteins significantly inhibited C. jejuni growth, we determined they are released during formation of C. jejuni-induced neutrophil extracellular traps (NETs). Using quantitative and qualitative methods, we found that purified human neutrophils are activated by C. jejuni and exhibit signatures of NET generation, including presence of protein arginine deiminase-4, histone citrullination, myeloperoxidase, neutrophil elastase release and DNA extrusion. Production of NETs correlated with C. jejuni phagocytosis/endocytosis and invasion of neutrophils suggesting that host- and bacterial-mediated activities are responsible for NET induction. Further, NET-like structures were observed within intestinal tissue of C. jejuni-infected ferrets. Finally, induction of NETs significantly increased human colonocyte cytotoxicity, indicating that NET formation during C. jejuni infection may contribute to observed tissue pathology. These findings provide further understanding of C. jejuni-neutrophil interactions and inflammatory responses during campylobacteriosis.

Double-edged sword of inflammasome genetics in colorectal cancer prognosis

Inflammation is a colorectal cancer (CRC) hallmark. Inflammasome-dependent cytokines IL-1ß and IL-18 can play a beneficial or detrimental role in tumorigenesis depending on cancer type. Variants in inflammasome genes were associated with tumor development and/or outcome, and have been proposed as potential biomarkers for population screening. In this study, 215 CRC patients followed-up for 10 years were examined for 9 polymorphisms in selected inflammasome genes. Multivariate association analysis and survival analysis were performed to evaluate the association between the polymorphisms and CRC prognosis. Variants associated with lower levels of IL-18 (rs1834481, rs5744256), or with increased activation of inflammasome receptors NLRP1 (rs12150220) and NLRP3 (rs35829419) resulted detrimental to CRC prognosis and may be used as prognostic markers.

Inflammasomes in Colitis and Colorectal Cancer: Mechanism of Action and Therapies

Colorectal cancer is a multifactorial disease and a leading cause of cancer-related deaths worldwide. Inflammation is a driver across multiple stages in the development of colorectal cancer. The inflammasome is a cytosolic multiprotein complex of the innate immune system central to the regulation of inflammation, pyroptosis, and other cellular processes important for maintaining gut homeostasis. Studies using mouse models of colitis and colitis-associated colorectal cancer have highlighted diverse and sometimes contrasting roles of inflammasomes in maintaining a balance between intestinal barrier function and the gut microbiota. In addition, persistent and/or dysregulated stimulation of inflammasome sensors finetune inflammation and tumorigenesis in the intestine. This review highlights the emerging role of inflammasome signaling in colitis and colitis-associated colorectal cancer. We also review the key mechanisms by which inflammasome signaling modulate inflammation and tumor development. Finally, we speculate the importance of using more tightly regulated experimental approaches to examine the role of gut microbiota in colorectal cancer.

The regenerating family member 3 β instigates IL-17A-mediated neutrophil recruitment downstream of NOD1/2 signalling for controlling colonisation resistance independently of microbiota community structure

OBJECTIVE

Loss of the Crohn's disease predisposing NOD2 gene results in an intestinal microenvironment conducive for colonisation by attaching-and-effacing enteropathogens. However, it remains elusive whether it relies on the intracellular recruitment of the serine-threonine kinase RIPK2 by NOD2, a step that is required for its activation of the transcription factor NF-κB.

DESIGN

Colonisation resistance was evaluated in wild type and mutant mice, as well as in ex-germ-free (ex-GF) mice which were colonised either with faeces from Ripk2-deficient mice or with bacteria with similar preferences for carbohydrates to those acquired by the pathogen. The severity of the mucosal pathology was quantified at several time points postinfection by using a previously established scoring. The community resilience in response to infection was evaluated by 16S ribosomal RNA gene sequence analysis. The control of pathogen virulence was evaluated by monitoring the secretion of Citrobacter-specific antibody response in the faeces.

RESULTS

Primary infection was similarly outcompeted in ex-GF Ripk2-deficient and control mice, demonstrating that the susceptibility to infection resulting from RIPK2 deficiency cannot be solely attributed to specific microbiota community structures. In contrast, delayed clearance of Citrobacter rodentium and exacerbated histopathology were preceded by a weakened propensity of intestinal macrophages to afford innate lymphoid cell activation. This tissue protection unexpectedly required the regenerating family member 3β by instigating interleukin (IL) 17A-mediated neutrophil recruitment to the intestine and subsequent phosphorylation of signal transducer and activator of transcription 3.

CONCLUSIONS

These results unveil a previously unrecognised mechanism that efficiently protects from colonisation by diarrhoeagenic bacteria early in infection.

TLR9 limits enteric antimicrobial responses and promotes microbiota-based colonisation resistance during Citrobacter rodentium infection

Mammalian cells express an array of toll-like receptors to detect and respond to microbial pathogens, including enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC). These clinically important attaching and effacing (A/E) pathogens infect the apical surface of intestinal epithelial cells, causing inflammation as well as severe diarrheal disease. Because EPEC and EHEC are human-specific, the related murine pathogen Citrobacter rodentium has been widely used to define how hosts defend against A/E pathogens. This study explored the role of TLR9, a receptor that recognises unmethylated CpG dinucleotides present in bacterial DNA, in promoting host defence against C. rodentium. Infected Tlr9^-/- mice suffered exaggerated intestinal damage and carried significantly higher (10-100 fold) pathogen burdens in their intestinal tissues as compared with wild type (WT) mice. C. rodentium infection also induced increased antimicrobial responses, as well as hyperactivation of NF-κB signalling in the intestines of Tlr9^-/- mice. These changes were associated with accelerated depletion of the intestinal microbiota in Tlr9^-/- mice as compared with WT mice. Notably, antibiotic-based depletion of the gut microbiota in WT mice prior to infection increased their susceptibility to the levels seen in Tlr9^-/- mice. Our results therefore indicate that TLR9 signalling suppresses intestinal antimicrobial responses, thereby promoting microbiota-mediated colonisation resistance against C. rodentium infection.

IL-10 Family Cytokines IL-10 and IL-22: from Basic Science to Clinical Translation

Cytokines are among the most important effector and messenger molecules in the immune system. They profoundly participate in immune responses during infection and inflammation, protecting against or contributing to diseases such as allergy, autoimmunity, and cancer. Manipulating cytokine pathways, therefore, is one of the most effective strategies to treat various diseases. IL-10 family cytokines exert essential functions to maintain tissue homeostasis during infection and inflammation through restriction of excessive inflammatory responses, upregulation of innate immunity, and promotion of tissue repairing mechanisms. Their important functions in diseases are supported by data from many preclinical models, human genetic studies, and clinical interventions. Despite significant efforts, however, there is still no clinically approved therapy through manipulating IL-10 family cytokines. Here, we summarize the recent progress in understanding the biology of this family of cytokines, suggesting more specific strategies to maneuver these cytokines for the effective treatment of inflammatory diseases and cancers.

VSIG4 mediates transcriptional inhibition of Nlrp3 and Il-1β in macrophages

Hyperactivation of the NLRP3 inflammasome contributes to the pathogenesis of multiple diseases, but the mechanisms underlying transcriptional regulation of Nlrp3 remain elusive. We demonstrate here that macrophages lacking V-set and immunoglobulin domain-containing 4 (Vsig4) exhibit significant increases in Nlrp3 and Il-1β transcription, caspase-1 activation, pyroptosis, and interleukin-1β (IL-1β) secretion in response to NLRP3 inflammasome stimuli. VSIG4 interacts with MS4A6D in the formation of a surface signaling complex. VSIG4 occupancy triggers Ser²³² and Ser²³⁵ phosphorylation in MS4A6D, leading to activation of JAK2-STAT3-A20 cascades that further results in nuclear factor κB suppression and Nlrp3 and Il-1β repression. Exaggerated NLRP3 and IL-1β expression in Vsig4^-/- mice is accountable for deleterious disease severity in experimental autoimmune encephalomyelitis (EAE) and resistance to dextran sulfate sodium (DSS)-induced colitis. The agonistic VSIG4 antibodies (VG11), acting through NLRP3 and IL-1β suppression, show significant therapeutic efficacy in mouse EAE. These findings highlight VSIG4 as a prospective target for treating NLRP3-associated inflammatory disorders.

Loss-of-function CARD8 mutation causes NLRP3 inflammasome activation and Crohn's disease

In these studies, we evaluated the contribution of the NLRP3 inflammasome to Crohn's disease (CD) in a kindred containing individuals having a missense mutation in CARD8, a protein known to inhibit this inflammasome. Whole exome sequencing and PCR studies identified the affected individuals as having a V44I mutation in a single allele of the T60 isoform of CARD8. The serum levels of IL-1β in the affected individuals were increased compared with those in healthy controls, and their peripheral monocytes produced increased amounts of IL-1β when stimulated by NLRP3 activators. Immunoblot studies probing the basis of these findings showed that mutated T60 CARD8 failed to downregulate the NLRP3 inflammasome because it did not bind to NLRP3 and inhibit its oligomerization. In addition, these studies showed that mutated T60 CARD8 exerted a dominant-negative effect by its capacity to bind to and form oligomers with unmutated T60 or T48 CARD8 that impeded their binding to NLRP3. Finally, inflammasome activation studies revealed that intact but not mutated CARD8 prevented NLRP3 deubiquitination and serine dephosphorylation. CD due to a CARD8 mutation was not effectively treated by anti-TNF-α, but did respond to IL-1β inhibitors. Thus, patients with anti-TNF-α-resistant CD may respond to this treatment option.

Epithelial Cell Inflammasomes in Intestinal Immunity and Inflammation

Pattern recognition receptors (PRR), such as NOD-like receptors (NLRs), sense conserved microbial signatures, and host danger signals leading to the coordination of appropriate immune responses. Upon activation, a subset of NLR initiate the assembly of a multimeric protein complex known as the inflammasome, which processes pro-inflammatory cytokines and mediates a specialized form of cell death known as pyroptosis. The identification of inflammasome-associated genes as inflammatory bowel disease susceptibility genes implicates a role for the inflammasome in intestinal inflammation. Despite the fact that the functional importance of inflammasomes within immune cells has been well established, the contribution of inflammasome expression in non-hematopoietic cells remains comparatively understudied. Given that intestinal epithelial cells (IEC) act as a barrier between the host and the intestinal microbiota, inflammasome expression by these cells is likely important for intestinal immune homeostasis. Accumulating evidence suggests that the inflammasome plays a key role in shaping epithelial responses at the host-lumen interface with many inflammasome components highly expressed by IEC. Recent studies have exposed functional roles of IEC inflammasomes in mucosal immune defense, inflammation, and tumorigenesis. In this review, we present the main features of the predominant inflammasomes and their effector mechanisms contributing to intestinal homeostasis and inflammation. We also discuss existing controversies in the field and open questions related to their implications in disease. A comprehensive understanding of the molecular basis of intestinal inflammasome signaling could hold therapeutic potential for clinical translation.

Citrobacter rodentium: a model enteropathogen for understanding the interplay of innate and adaptive components of type 3 immunity

Citrobacter rodentium is a natural murine intestinal pathogen that shares a core set of virulence factors with the related human pathogens enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC). C. rodentium is now the most widely used small animal model for studying the molecular underpinnings of EPEC and EHEC infections in vivo, including: enterocyte attachment; virulence; colonization resistance; and mucosal immunity. In this review, we discuss type 3 immunity in the context of C. rodentium infection and discuss recent publications that use this model to understand how the innate and adaptive components of immunity intersect to mediate host protection against enteric pathogens and maintain homeostasis with the microbiota.

MyD88 signaling in dendritic cells and the intestinal epithelium controls immunity against intestinal infection with C. rodentium

MyD88-mediated signaling downstream of Toll-like receptors and the IL-1 receptor family is critically involved in the induction of protective host responses upon infections. Although it is known that MyD88-deficient mice are highly susceptible to a wide range of bacterial infections, the cell type-specific contribution of MyD88 in protecting the host against intestinal bacterial infection is only poorly understood. In order to investigate the importance of MyD88 in specific immune and nonimmune cell types during intestinal infection, we employed a novel murine knock-in model for MyD88 that enables the cell type-specific reactivation of functional MyD88 expression in otherwise MyD88-deficient mice. We report here that functional MyD88 signaling in CD11c⁺ cells was sufficient to activate intestinal dendritic cells (DC) and to induce the early group 3 innate lymphoid cell (ILC3) response as well as the development of colonic Th17/Th1 cells in response to infection with the intestinal pathogen C. rodentium. In contrast, restricting MyD88 signaling to several other cell types, including macrophages (MO), T cells or ILC3 did not induce efficient intestinal immune responses upon infection. However, we observed that the functional expression of MyD88 in intestinal epithelial cells (IEC) also partially protected the mice during intestinal infection, which was associated with enhanced epithelial barrier integrity and increased expression of the antimicrobial peptide RegIIIγ and the acute phase protein SAA1 by epithelial cells. Together, our data suggest that MyD88 signaling in DC and IEC is both essential and sufficient to induce a full spectrum of host responses upon intestinal infection with C. rodentium.

MyD88-dependent signaling pathways play a critical role in the protective immune response during intestinal infections. However, the significance of MyD88-mediated signaling in specific intestinal immune and nonimmune cell types for the activation of the early innate, adaptive and epithelial host responses upon infection remains poorly understood. Using a novel knock-in mouse model for MyD88, we report here that MyD88 signaling in CD11c⁺ dendritic cells (DC) is sufficient to activate RORγt⁺ group 3 innate lymphoid cells (ILC3) as well as Th17/Th1 cells in response to infection with C. rodentium. In contrast, restricting functional MyD88 signaling to several other immune cell types, including macrophages (MO), T cells and ILC3 did not result in intestinal immunity, while expression of MyD88 in intestinal epithelial cells (IEC) mainly enhanced epithelial barrier integrity. Together, our data suggest that MyD88 signaling in DC and IEC is both essential and sufficient to induce a full spectrum of host responses upon intestinal infection with C. rodentium.

Inflammasomes in the Gut Mucosal Homeostasis

Inflammasomes are critical checkpoints in inflammation. The activation of inflammasome can cause a series of inflammatory responses including maturation of interleukin (IL)-1β and IL-18 and a specialized form of cell death called pyroptosis. Since its identification in the early 2000s, inflammasomes have been implicated to play multifaceted roles in varied pathological and physiological conditions, especially in the mucosal compartments including the gut. Maintaining gut mucosal homeostasis has always been a remarkable challenge for the host due to both the vast mucosal surface that is exposed to the outside and the enormous amount of local microbiota. To accomplish this challenge, the host mounts a constant dynamic low-grade inflammatory response (physiological inflammation) in coping with insults of microbes in the intestine. This book chapter aims to summarize the current knowledge of how inflammasomes contribute to gut mucosal homeostasis.

Toll-like receptors in pathophysiology of liver diseases

Toll-like receptors (TLRs) are pattern recognition receptors that participate in host defense by recognizing pathogen-associated molecular patterns alongside inflammatory processes by recognizing damage associated molecular patterns. Given constant exposure to pathogens from gut, strict control of TLR-associated signaling pathways is essential in the liver, which otherwise may lead to inappropriate production of pro-inflammatory cytokines and interferons and may generate a predisposition to several autoimmune and chronic inflammatory diseases. The liver is considered to be a site of tolerance induction rather than immunity induction, with specificity in hepatic cell functions and distribution of TLR. Recent data emphasize significant contribution of TLR signaling in chronic liver diseases via complex immune responses mediating hepatocyte (i.e., hepatocellular injury and regeneration) or hepatic stellate cell (i.e., fibrosis and cirrhosis) inflammatory or immune pathologies. Herein, we review the available data on TLR signaling, hepatic expression of TLRs and associated ligands, as well as the contribution of TLRs to the pathophysiology of hepatic diseases.

Generation of a Novel T Cell Specific Interleukin-1 Receptor Type 1 Conditional Knock Out Mouse Reveals Intrinsic Defects in Survival, Expansion and Cytokine Production of CD4 T Cells

Interleukin-1 (IL-1) plays a crucial role in numerous inflammatory diseases via action on its only known signaling IL-1 receptor type 1 (IL-1R1). To investigate the role of IL-1 signaling in selected cell types, we generated a new mouse strain in which exon 5 of the Il1r1 gene is flanked by loxP sites. Crossing of these mice with CD4-Cre transgenic mice resulted in IL-1R1 loss of function specifically in T cells. These mice, termed IL-1R1ΔT, displayed normal development under steady state conditions. Importantly, isolated CD4 positive T cells retained their capacity to differentiate toward Th1 or Th17 cell lineages in vitro, and strongly proliferated in cultures supplemented with either anti-CD3/CD28 or Concanavalin A, but, as predicted, were completely unresponsive to IL-1β administration. Furthermore, IL-1R1ΔT mice were protected from gut inflammation in the anti-CD3 treatment model, due to dramatically reduced frequencies and absolute numbers of IL-17A and interferon (IFN)-γ producing cells. Taken together, our data shows the necessity of intact IL-1 signaling for survival and expansion of CD4 T cells that were developed in an otherwise IL-1 sufficient environment.

Mucosal immunity to pathogenic intestinal bacteria

The intestinal mucosa is a particularly dynamic environment in which the host constantly interacts with trillions of commensal microorganisms, known as the microbiota, and periodically interacts with pathogens of diverse nature. In this Review, we discuss how mucosal immunity is controlled in response to enteric bacterial pathogens, with a focus on the species that cause morbidity and mortality in humans. We explain how the microbiota can shape the immune response to pathogenic bacteria, and we detail innate and adaptive immune mechanisms that drive protective immunity against these pathogens. The vast diversity of the microbiota, pathogens and immune responses encountered in the intestines precludes discussion of all of the relevant players in this Review. Instead, we aim to provide a representative overview of how the intestinal immune system responds to pathogenic bacteria.

Microbiota, immunity and the liver

The gut harbors a complex community of over 100 trillion microbial cells known to exist in symbiotic harmony with the host influencing human physiology, metabolism, nutrition and immune function. It is now widely accepted that perturbations of this close partnership results in the pathogenesis of several major diseases with increasing evidence highlighting their role outside of the intestinal tract. The intimate proximity and circulatory loop of the liver and the gut has attracted significant attention regarding the role of the microbiota in the development and progression of liver disease. Here we give an overview of the interaction between the microbiota and the immune system and focus on their convincing role in both the propagation and treatment of liver disease.

IL-1 in Colon Inflammation, Colon Carcinogenesis and Invasiveness of Colon Cancer

Interleukin-1 (IL-1) is a major "alarm" upstream pro-inflammatory cytokine that mainly acts by inducing cascades of cytokine and inflammation-promoting mediators. In the tumor arena, IL-1 is produced by both malignant and microenvironmental cells. IL-1α and IL-1β are the major agonists of IL-1, while IL-1Ra is a physiological inhibitor of pre-formed IL-1. IL-1α and IL-1β differ in their compartmentalization and in the producing cells. IL-1β is only active in its inflammasome dependent processed and secreted form and has been considered as the major mediator of inflammation. On the other hand, IL-1α is mainly cell-associated in tissue resident cells, being also active in its precursor form. The role of the IL-1 molecules in the unique microenvironment in the colon is largely unknown. Here, we described the role of IL-1α and IL-1β in colon homeostasis, colon inflammation, colon carcinogenesis and invasiveness of colorectal cancer. Understanding of the integrative role of IL-1α and IL-1β in these processes will facilitate the application of novel IL-1 modulating approaches.

Functions of innate immune cells and commensal bacteria in gut homeostasis

The intestinal immune system remains unresponsive to beneficial microbes and dietary antigens while activating pro-inflammatory responses against pathogens for host defence. In intestinal mucosa, abnormal activation of innate immunity, which directs adaptive immune responses, causes the onset and/or progression of inflammatory bowel diseases. Thus, innate immunity is finely regulated in the gut. Multiple innate immune cell subsets have been identified in both murine and human intestinal lamina propria. Some innate immune cells play a key role in the maintenance of gut homeostasis by preventing inappropriate adaptive immune responses while others are associated with the pathogenesis of intestinal inflammation through development of Th1 and Th17 cells. In addition, intestinal microbiota and their metabolites contribute to the regulation of innate/adaptive immune responses. Accordingly, perturbation of microbiota composition can trigger intestinal inflammation by driving inappropriate immune responses.

IL-1RT1 signaling antagonizes IL-11 induced STAT3 dependent cardiac and antral stomach tumor development through myeloid cell enrichment

IL-1 is key driver of gastric tumorigenesis and is a downstream target of IL-11 signaling. Recently, IL-1 cytokines, particularly IL-1β, have been flagged as therapeutic targets for gastric cancer treatment. Here, we assess the requirement for IL-1 signaling in gastric tumorigenesis. gp130757FF xIL-1RT1-/- mice were generated to determine the pathological consequence of ablated IL-1 signaling in the IL-11 dependent gp130757FF mouse model of gastric tumorigenesis. Gastric lesions in gp130757FF xIL-1RT1-/- mice were increased in incidence and size compared to gp130757FF mice. Proximal gastric lesions originated from the cardiac region and were associated with elevated STAT3 activation, loss of specialized gastric cells and a modulated immune response including increased expression of TNF-α and MDSC associated genes. Administration of IL-11 to IL-1RT1-/- mice showed similar changes to gp130757FF xIL-1RT1-/- mice. Spleens from IL-11 treated wildtype mice showed an enrichment of MDSC and gp130757FF xIL-1RT1-/- mice had increased MDSCs in the stomach compared to gp130757FF mice. Furthermore, crossing TNF-α-/- to gp130757FF mice resulted in reduced lesion size. We conclude that IL-1 signaling antagonizes IL-11/STAT3 mediated pathology and the genetic deletion of IL-1RT1 results in increased tumor burden. We provide evidence that a likely mechanism is due to IL-11/STAT3 dependent enrichment of MDSCs.

Interleukin-1 (IL-1) signaling in intestinal stromal cells controls KC/ CXCL1 secretion, which correlates with recruitment of IL-22- secreting neutrophils at early stages of Citrobacter rodentium infection

Attaching and effacing pathogens, including enterohemorrhagic Escherichia coli in humans and Citrobacter rodentium in mice, raise serious public health concerns. Here we demonstrate that interleukin-1 receptor (IL-1R) signaling is indispensable for protection against C. rodentium infection in mice. Four days after infection with C. rodentium, there were significantly fewer neutrophils (CD11b+ Ly6C+ Ly6G+) in the colons of IL-1R−/− mice than in wild-type mice. Levels of mRNA and protein of KC/CXCL1 were also significantly reduced in colon homogenates of infected IL-1R−/− mice relative to wild-type mice. Of note, infiltrated CD11b+ Ly6C+ Ly6G+ neutrophils were the main source of IL-22 secretion after C. rodentium infection. Interestingly, intestinal stromal cells isolated from IL-1R−/− mice secreted lower levels of KC/CXCL1 than stromal cells from wild-type mice during C. rodentium infection. Similar effects were found when mouse intestinal stromal cells and human nasal polyp stromal cells were treated with IL-1R antagonists (i.e., anakinra) in vitro. These results suggest that IL-1 signaling plays a pivotal role in activating mucosal stromal cells to secrete KC/CXCL1, which is essential for infiltration of IL-22-secreting neutrophils upon bacterial infection.

Protective and detrimental roles of inflammasomes in disease

Over recent years, inflammasomes have emerged as key regulators of immune and inflammatory responses. They induce programmed cell death and direct the release of danger signals and the inflammatory cytokines interleukin (IL)-1β and IL-18. The concerted actions of inflammasomes are of utmost importance for responding adequately to harmful environmental agents and infections. However, deregulated inflammasome signaling is increasingly linked to a diversity of human pathologies, including rheumatoid arthritis, inflammatory bowel disease, and rare, hereditary periodic fever syndromes. In this review, we discuss recent insight in the protective and detrimental roles of inflammasomes in selected infectious, autoinflammatory and autoimmune diseases, and cover clinically approved therapies that interfere with inflammasome signaling. These findings highlight the importance of fine-balancing the Ying and Yang activities of inflammasomes for sustained homeostasis and suggest that further understanding of inflammasome mechanisms may offer new cures for human diseases.

The epithelial danger signal IL-1α is a potent activator of fibroblasts and reactivator of intestinal inflammation

Intestinal epithelial cell (IEC) death is typical of inflammatory bowel disease (IBD). We investigated: i) whether IEC-released necrotic cell products (proinflammatory mediators) amplify mucosal inflammation, ii) the capacity of necrotic cell lysates from HT29 cells or human IECs to induce human intestinal fibroblasts' (HIF) production of IL-6 and IL-8, and iii) whether IL-1α, released by injured colonocytes, exacerbated experimental IBD. Necrotic cell lysates potently induced HIF IL-6 and IL-8 production independent of Toll-like receptors 2 and 4, receptor for advanced glycation end-products, high-mobility group box 1, uric acid, IL-33, or inflammasome activation. IL-1α was the key IEC-derived necrotic cell product involved in HIF cytokine production. IL-1α-positive cells were identified in the epithelium in human IBD and dextran sulfate sodium (DSS)-induced colitis. IL-1α was detected in the stool of colitic mice before IL-1β. IL-1α enemas reactivated inflammation after DSS colitis recovery, induced IL-1 receptor expression in subepithelial fibroblasts, and activated de novo inflammation even in mice without overt colitis, after the administration of low-dose DSS. IL-1α amplifies gut inflammation by inducing cytokine production by mesenchymal cells. IL-1α-mediated IEC-fibroblast interaction may be involved in amplifying and perpetuating inflammation, even without obvious intestinal damage. IL-1α may be a target for treating early IBD or preventing the reactivation of IBD.

Citrobacter rodentium-induced colitis: A robust model to study mucosal immune responses in the gut

Citrobacter rodentium is a natural mouse pathogen which reproducibly infects mice and causes intestinal disease. The C. rodentium model of infection is very useful for investigating host-pathogen immune interactions in the gut, and can also be used to understand the pathogenesis of several important human intestinal disorders, including Crohn's disease, ulcerative colitis, dysbiosis and colon tumorigenesis. Both innate and adaptive immune responses play a critical role in protection against C. rodentium. Here, we summarize the role of immune components in protection against C. rodentium and describe techniques for the analysis of innate and adaptive mucosal immune responses, including setting up the infection, analysis of colonic hyperplasia and bacterial dissemination, evaluation of antibody responses, and purification and analysis of intestinal epithelial and lymphoid cells.

SIGNR3-dependent immune regulation by Lactobacillus acidophilus surface layer protein A in colitis

Intestinal immune regulatory signals govern gut homeostasis. Breakdown of such regulatory mechanisms may result in inflammatory bowel disease (IBD). Lactobacillus acidophilus contains unique surface layer proteins (Slps), including SlpA, SlpB, SlpX, and lipoteichoic acid (LTA), which interact with pattern recognition receptors to mobilize immune responses. Here, to elucidate the role of SlpA in protective immune regulation, the NCK2187 strain, which solely expresses SlpA, was generated. NCK2187 and its purified SlpA bind to the C-type lectin SIGNR3 to exert regulatory signals that result in mitigation of colitis, maintenance of healthy gastrointestinal microbiota, and protected gut mucosal barrier function. However, such protection was not observed in Signr3(-/-) mice, suggesting that the SlpA/SIGNR3 interaction plays a key regulatory role in colitis. Our work presents critical insights into SlpA/SIGNR3-induced responses that are integral to the potential development of novel biological therapies for autoinflammatory diseases, including IBD.

Role of inflammasomes in intestinal inflammation and Crohn's disease

: Inflammasomes are multiprotein complexes that process procytokines into mature forms of interleukin 1β and interleukin 18 and induce pyroptotic cell death. Evidence linking NLRP3, NLRC4, and NLRP6 inflammasomes to intestinal inflammation is reviewed to provide a basis to understand how the innate immune system discriminates pathogenic bacteria from commensal bacteria and shapes microbial ecology. Inflammasomes have a direct and important role limiting colitis by directing effective immune responses against pathogenic bacterial infections in the intestine. Chronic granulomatous disease is presented to reveal a contrasting proinflammatory effect of inflammasomes. This pathogenic effect is unmasked in a state of immunodeficiency where bacterial growth is poorly controlled increasing inflammasome activity. The role of inflammasomes in inflammation associated with Crohn's disease and ulcerative colitis is discussed. Finally, mechanistic studies linking genetic polymorphisms in ATG16L and NOD2 to inflammasome activation provide a basis for new hypotheses to explain how genetic polymorphism associated with Crohn's disease modulate intestinal inflammation. A deeper understanding of the role of inflammasomes in intestinal inflammation is expected to identify new ways of treating inflammatory bowel disease.

Fas regulates neutrophil lifespan during viral and bacterial infection

The regulation of neutrophil lifespan is critical for a circumscribed immune response. Neutrophils are sensitive to Fas/CD95 death receptor signaling in vitro, but it is unknown if Fas regulates neutrophil lifespan in vivo. We hypothesized that FasL-expressing CD8(+) T cells, which kill antigen-stimulated T cells during chronic viral infection, can also induce neutrophil death in tissues during infection. With the use of LysM-Cre Fas(fl/fl) mice, which lack Fas expression in macrophages and neutrophils, we show that Fas regulates neutrophil lifespan during lymphocytic choriomeningitis virus (LCMV) infection in the lung, peripheral blood, and spleen. Fas also contributed to the regulation of neutrophil numbers in the colon of Citrobacter rodentium-infected mice. To examine the effects of infection on Fas activation in neutrophils, we primed neutrophils with TLR ligands or IL-18, resulting in ablation of Fas death receptor signaling. These data provide the first in vivo genetic evidence that neutrophil lifespan is controlled by death receptor signaling and provide a mechanism to account for neutrophil resistance to Fas stimulation during infection.

Caspase-11 is expressed in the colonic mucosa and protects against dextran sodium sulfate-induced colitis

Ulcerative colitis and Crohn's disease are major inflammatory syndromes that affect millions of patients. Caspase-11 confers protection against Gram-negative enteropathogens, but its role during colitis is unknown. Here, we show that caspase-11 was constitutively expressed in the colon, and that caspase-11-deficient (caspase-11(-/-)) mice were hypersusceptible to dextran sodium sulfate (DSS)-induced colitis. Notably, pro-inflammatory Prevotella species were strongly reduced in the gut microbiota of caspase-11(-/-) mice. Co-housing with wild-type mice leveled Prevotella contents, but failed to protect caspase-11(-/-) mice from increased susceptibility to DSS-induced colitis. We therefore addressed the role of caspase-11 in immune signaling. DSS-induced tissue damage and inflammatory cell infiltration in the gut were markedly increased in caspase-11−/− mice, while release of the pyroptosis/necroptosis marker HMGB1 was abolished [Corrected]. Moreover, caspase-11(-/-) mice showed normal or increased production of mature interleukin (IL)-1β and IL-18, whereas IL-1β and IL-18 secretion was blunted in animals lacking both caspases 1 and 11. In conclusion, we showed that caspase-11 shapes the gut microbiota composition, and that caspase-11(-/-) mice are highly susceptible to DSS-induced colitis. Moreover, DSS-induced inflammasome activation relied on caspase-1, but not caspase-11. These results suggest a role for other caspase-11 effector mechanisms such as pyroptosis in protection against intestinal inflammation.

The Inflammatory Response during Enterohemorrhagic Escherichia coli Infection

The inflammatory response is an integral part of host defense against enterohemorrhagic Escherichia coli (EHEC) infection and also contributes to disease pathology. In this article we explore the factors leading to inflammation during EHEC infection and the mechanisms EHEC and other attaching and effacing (A/E) pathogens have evolved to suppress inflammatory signaling. EHEC stimulates an inflammatory response in the intestine through host recognition of bacterial components such as flagellin and lipopolysaccharide. In addition, the activity of Shiga toxin and some type III secretion system effectors leads to increased tissue inflammation. Various infection models of EHEC and other A/E pathogens have revealed many of the immune factors that mediate this response. In particular, the outcome of infection is greatly influenced by the ability of an infected epithelial cell to mount an effective host inflammatory response. The inflammatory response of infected enterocytes is counterbalanced by the activity of type III secretion system effectors such as NleE and NleC that modify and inhibit components of the signaling pathways that lead to proinflammatory cytokine production. Overall, A/E pathogens have taught us that innate mucosal immune responses in the gastrointestinal tract during infection with A/E pathogens are highly complex and ultimate clearance of the pathogen depends on multiple factors, including inflammatory mediators, bacterial burden, and the function and integrity of resident intestinal epithelial cells.

Nlrp3 activation in the intestinal epithelium protects against a mucosal pathogen

Polymorphisms in the intracellular pattern recognition receptor gene NLRP3 (NLR family, pyrin domain containing 3) have been associated with susceptibility to Crohn's disease, a type of inflammatory bowel disease. Following tissue damage or infection, NLRP3 triggers the formation of inflammasomes, containing NLRP3, ASC (apoptosis-associated speck-like protein containing a CARD domain), and caspase-1, that mediate secretion of interleukin (IL)-1β and IL-18. However, the precise role of NLRP3 inflammasomes in mucosal inflammation and barrier protection remains unclear. Here we show that upon infection with the attaching/effacing intestinal pathogen Citrobacter rodentium, Nlrp3(-/-) and Asc(-/-) mice displayed increased bacterial colonization and dispersion, more severe weight loss, and exacerbated intestinal inflammation. Analyses of irradiation bone marrow chimeras revealed that protection from disease was mediated through Nlrp3 activation in nonhematopoietic cells and was initiated very early after infection. Thus, early activation of Nlrp3 in intestinal epithelial cells limits pathogen colonization and prevents subsequent pathology, potentially providing a functional link between NLRP3 polymorphisms and susceptibility to inflammatory bowel disease.

NLRC4 expression in intestinal epithelial cells mediates protection against an enteric pathogen

The inflammasomes have an important role in connecting the detection of endogenous and microbial danger signals to caspase-1 activation and induction of protective immune responses. NLRC4 is a cytosolic NOD (nucleotide binding and oligomerization domain)-like receptor (NLR) that can trigger inflammasome formation in response to bacterial flagellin, an immunodominant antigen in the intestine. To characterize the role of NLRC4 in bacterially triggered intestinal inflammation, we used the murine pathogen Citrobacter rodentium, an extracellular, attaching/effacing bacterium similar to enterohemorrhagic Escherichia coli and enteropathogenic E. coli. Following infection with C. rodentium, we found that Nlrc4(-/-) mice developed more severe weight loss, increased bacterial colonization levels, and exacerbated intestinal inflammation compared with wild-type counterparts. Nlrc4(-/-) mice mounted robust adaptive immune responses but were unable to control early colonization by C. rodentium, suggesting that a defect in innate immunity was responsible. Experiments using bone marrow (BM) chimeras revealed that the protective effects of NLRC4 were dependent on its expression in non-hematopoietic cells, and quantitative PCR (Q-PCR) analyses revealed that NLRC4 was highly expressed in epithelial crypts but not in intestinal stroma. Thus, early NLRC4 sensing in intestinal epithelial cells regulates colonization by an extracellular bacterial pathogen and limits subsequent intestinal damage.

NOD-like receptors in intestinal homeostasis and epithelial tissue repair

The intestinal epithelium constitutes a dynamic physical barrier segregating the luminal content from the underlying mucosal tissue. Following injury, the epithelial integrity is restored by rapid migration of intestinal epithelial cells (IECs) across the denuded area in a process known as wound healing. Hence, through a sequence of events involving restitution, proliferation and differentiation of IECs the gap is resealed and homeostasis reestablished. Relapsing damage followed by healing of the inflamed mucosa is a hallmark of several intestinal disorders including inflammatory bowel diseases (IBD). While several regulatory peptides, growth factors and cytokines stimulate restitution of the epithelial layer after injury, recent evidence in the field underscores the contribution of innate immunity in controlling this process. In particular, nucleotide-binding and oligomerization domain-like receptors (NLRs) play critical roles in sensing the commensal microbiota, maintaining homeostasis, and regulating intestinal inflammation. Here, we review the process of intestinal epithelial tissue repair and we specifically focus on the impact of NLR-mediated signaling mechanisms involved in governing epithelial wound healing during disease.

Interactions between Nod-Like Receptors and Intestinal Bacteria

Nucleotide oligomerization domain (Nod)-like Receptors (NLRs) are cytosolic sensors that mediate the activation of Caspase-1 and the subsequent processing and secretion of the pro-inflammatory cytokines IL-1β and IL-18, as well as an inflammatory cell death termed pyroptosis. While a multitude of bacteria have been shown to activate one or more NLRs under in vitro conditions, the exact impact of NLR activation during the course of colonization, both of pathogenic and commensal nature, is less understood. In this review, we will focus on the role of intestinal NLRs during the various stages of infection with common gastrointestinal bacterial pathogens, as well as NLR function in controlling and shaping the microbiota.

A balanced IL-1β activity is required for host response to Citrobacter rodentium infection

Microbial sensing plays essential roles in the innate immune response to pathogens. In particular, NLRP3 forms a multiprotein inflammasome complex responsible for the maturation of interleukin (IL)-1β. Our aim was to delineate the role of the NLRP3 inflammasome in macrophages, and the contribution of IL-1β to the host defense against Citrobacter rodentium acute infection in mice. Nlrp3(-/-) and background C57BL/6 (WT) mice were infected by orogastric gavage, received IL-1β (0.5 µg/mouse; ip) on 0, 2, and 4 days post-infection (DPI), and assessed on 6 and 10 DPI. Infected Nlrp3(-/-) mice developed severe colitis; IL-1β treatments reduced colonization, abrogated dissemination of bacteria to mesenteric lymph nodes, and protected epithelial integrity of infected Nlrp3(-/-) mice. In contrast, IL-1β treatments of WT mice had an opposite effect with increased penetration of bacteria and barrier disruption. Microscopy showed reduced damage in Nlrp3(-/-) mice, and increased severity of disease in WT mice with IL-1β treatments, in particular on 10 DPI. Secretion of some pro-inflammatory plasma cytokines was dissipated in Nlrp3(-/-) compared to WT mice. IL-1β treatments elevated macrophage infiltration into infected crypts in Nlrp3(-/-) mice, suggesting that IL-1β may improve macrophage function, as exogenous administration of IL-1β increased phagocytosis of C. rodentium by peritoneal Nlrp3(-/-) macrophages in vitro. As well, the exogenous administration of IL-1β to WT peritoneal macrophages damaged the epithelial barrier of C. rodentium-infected polarized CMT-93 cells. Treatment of Nlrp3(-/-) mice with IL-1β seems to confer protection against C. rodentium infection by reducing colonization, protecting epithelial integrity, and improving macrophage activity, while extraneous IL-1β appeared to be detrimental to WT mice. Together, these findings highlight the importance of balanced cytokine responses as IL-1β improved bacterial clearance in Nlrp3(-/-) mice but increased tissue damage when given to WT mice.

SIGIRR, a negative regulator of TLR/IL-1R signalling promotes Microbiota dependent resistance to colonization by enteric bacterial pathogens

Enteric bacterial pathogens such as enterohemorrhagic E. coli (EHEC) and Salmonella Typhimurium target the intestinal epithelial cells (IEC) lining the mammalian gastrointestinal tract. Despite expressing innate Toll-like receptors (TLRs), IEC are innately hypo-responsive to most bacterial products. This is thought to prevent maladaptive inflammatory responses against commensal bacteria, but it also limits antimicrobial responses by IEC to invading bacterial pathogens, potentially increasing host susceptibility to infection. One reason for the innate hypo-responsiveness of IEC is their expression of Single Ig IL-1 Related Receptor (SIGIRR), a negative regulator of interleukin (IL)-1 and TLR signaling. To address whether SIGIRR expression and the innate hypo-responsiveness of IEC impacts on enteric host defense, Sigirr deficient (−/−) mice were infected with the EHEC related pathogen Citrobacter rodentium. Sigirr −/− mice responded with accelerated IEC proliferation and strong pro-inflammatory and antimicrobial responses but surprisingly, Sigirr −/− mice proved dramatically more susceptible to infection than wildtype mice. Through haematopoietic transplantation studies, it was determined that SIGIRR expression by non-haematopoietic cells (putative IEC) regulated these responses. Moreover, the exaggerated responses were found to be primarily dependent on IL-1R signaling. Whilst exploring the basis for their susceptibility, Sigirr −/− mice were found to be unusually susceptible to intestinal Salmonella Typhimurium colonization, developing enterocolitis without the typical requirement for antibiotic based removal of competing commensal microbes. Strikingly, the exaggerated antimicrobial responses seen in Sigirr −/− mice were found to cause a rapid and dramatic loss of commensal microbes from the infected intestine. This depletion appears to reduce the ability of the microbiota to compete for space and nutrients (colonization resistance) with the invading pathogens, leaving the intestine highly susceptible to pathogen colonization. Thus, SIGIRR expression by IEC reflects a strategy that sacrifices maximal innate responsiveness by IEC in order to promote commensal microbe based colonization resistance against bacterial pathogens.

Despite being in close contact with billions of commensal bacteria, the epithelial cells that line the intestine develop very weak innate inflammatory responses to bacterial products. The goal of this study was to explore why these cells respond so poorly, and how increasing their innate responsiveness would impact on host defense against invading bacterial pathogens. We show that a negative regulator of innate signaling called SIGIRR, limits the inflammatory responses of the intestine to bacteria. Following infection by the bacterial pathogen Citrobacter rodentium, the intestines of mice lacking SIGIRR showed exaggerated inflammatory, antimicrobial and proliferative responses. Through transplantation studies, we showed it was SIGIRR expression by intestinal epithelial cells that limits these responses, and that the exaggerated responses were driven by cytokine signaling through the interleukin-1 receptor. Despite their exaggerated responses, SIGIRR deficient mice proved extremely susceptible to infection by C. rodentium and other intestinal bacterial pathogens. We found the exaggerated inflammatory responses rapidly depleted intestinal commensal microbes, reducing their ability to outcompete invading pathogens for space and nutrients (colonization resistance). Our study thus clarifies that the hypo-responsiveness of epithelial cells plays an unexpected but critical role in host defense, by promoting commensal microbe based competition against enteric pathogens.

Inflammasomes in inflammatory bowel disease pathogenesis

PURPOSE OF REVIEW

Inflammasomes are molecular platforms assembled in response to infection or danger signals, and they regulate the activation of caspase-1 and the maturation of the inflammatory cytokines IL-1β and IL-18. In this review, we will summarize the centrality of Nod-like receptor proteins that assemble inflammasomes and regulate intestinal homeostasis by controlling host defense responses, microbiota composition, intestinal inflammation and tissue damage.

RECENT FINDINGS

In the intestine, the innate immune system evolved to tolerate commensal microorganisms while maintaining the capacity to trigger host defense responses to invading pathogens. Recent findings suggest that inflammasomes play a critical role in the intricate interplay between the local microbial community and the mucosal immune system by sensing commensal bacteria, regulating microbial ecology, establishing the host defense response that discriminates pathogenic from commensal microbes and preventing the emergence of pathobionts. A model to reconcile the conflicting results in the literature on the role of inflammasomes in experimental colitis will be discussed.

SUMMARY

A better understanding of the relationship between inflammasome signaling and the intestinal microbiota might provide insight into the complex interaction of the innate immune system with the intestinal environment, and identify new approaches for the treatment of inflammatory bowel disease and gastrointestinal cancer.

Inflammasome in intestinal inflammation and cancer

The activation of specific cytosolic pathogen recognition receptors, the nucleotide-binding-oligomerization-domain- (NOD-) like receptors (NLRs), leads to the assembly of the inflammasome, a multimeric complex platform that activates caspase-1. The caspase-1 pathway leads to the upregulation of important cytokines from the interleukin (IL)-1 family, IL-1β , and IL-18, with subsequent activation of the innate immune response. In this review, we discuss the molecular structure, the mechanisms behind the inflammasome activation, and its possible role in the pathogenesis of inflammatory bowel diseases and intestinal cancer. Here, we show that the available data points towards the importance of the inflammasome in the innate intestinal immune response, being the complex involved in the maintenance of intestinal homeostasis, correct intestinal barrier function and efficient elimination of invading pathogens.

HVEM: An unusual TNF receptor family member important for mucosal innate immune responses to microbes

The herpes virus entry mediator (HVEM or CD270) is a member of the tumor necrosis factor receptor superfamily (TNFRSF), and therefore it is also known as TNFRSF14. We have recently provided evidence showing a novel signaling pathway downstream of HVEM leading to signal transducer and activator of transcription 3 (STAT3) activation in epithelial cells. As STAT3 regulates the expression of genes important for host defense in epithelial cells, as well as the differentiation of retinoid-related orphan receptor (ROR)γt+ Th17 and innate lymphoid cells (ILC), our finding that HVEM activates STAT3 has revealed fresh insights into the potential regulatory function of HVEM in different cellular contexts. Therefore, although further investigations will be required, HVEM is emerging as a major player in mucosal host defense, capable of regulating several cellular responses.