The Intermucosal Connection between the Mouth and Gut in Commensal Pathobiont-Driven Colitis.

The precise mechanism by which oral infection contributes to the pathogenesis of extra-oral diseases remains unclear. Here, we report that periodontal inflammation exacerbates gut inflammation in vivo. Periodontitis leads to expansion of oral pathobionts, including Klebsiella and Enterobacter species, in the oral cavity. Amassed oral pathobionts are ingested and translocate to the gut, where they activate the inflammasome in colonic mononuclear phagocytes, triggering inflammation. In parallel, periodontitis results in generation of oral pathobiont-reactive Th17 cells in the oral cavity. Oral pathobiont-reactive Th17 cells are imprinted with gut tropism and migrate to the inflamed gut. When in the gut, Th17 cells of oral origin can be activated by translocated oral pathobionts and cause development of colitis, but they are not activated by gut-resident microbes. Thus, oral inflammation, such as periodontitis, exacerbates gut inflammation by supplying the gut with both colitogenic pathobionts and pathogenic T cells.

A Dietary Fiber-Deprived Gut Microbiota Degrades the Colonic Mucus Barrier and Enhances Pathogen Susceptibility.

Despite the accepted health benefits of consuming dietary fiber, little is known about the mechanisms by which fiber deprivation impacts the gut microbiota and alters disease risk. Using a gnotobiotic mouse model, in which animals were colonized with a synthetic human gut microbiota composed of fully sequenced commensal bacteria, we elucidated the functional interactions between dietary fiber, the gut microbiota, and the colonic mucus barrier, which serves as a primary defense against enteric pathogens. We show that during chronic or intermittent dietary fiber deficiency, the gut microbiota resorts to host-secreted mucus glycoproteins as a nutrient source, leading to erosion of the colonic mucus barrier. Dietary fiber deprivation, together with a fiber-deprived, mucus-eroding microbiota, promotes greater epithelial access and lethal colitis by the mucosal pathogen, Citrobacter rodentium. Our work reveals intricate pathways linking diet, the gut microbiome, and intestinal barrier dysfunction, which could be exploited to improve health using dietary therapeutics.

Th17 Cell Induction by Adhesion of Microbes to Intestinal Epithelial Cells.

Intestinal Th17 cells are induced and accumulate in response to colonization with a subgroup of intestinal microbes such as segmented filamentous bacteria (SFB) and certain extracellular pathogens. Here, we show that adhesion of microbes to intestinal epithelial cells (ECs) is a critical cue for Th17 induction. Upon monocolonization of germ-free mice or rats with SFB indigenous to mice (M-SFB) or rats (R-SFB), M-SFB and R-SFB showed host-specific adhesion to small intestinal ECs, accompanied by host-specific induction of Th17 cells. Citrobacter rodentium and Escherichia coli O157 triggered similar Th17 responses, whereas adhesion-defective mutants of these microbes failed to do so. Moreover, a mixture of 20 bacterial strains, which were selected and isolated from fecal samples of a patient with ulcerative colitis on the basis of their ability to cause a robust induction of Th17 cells in the mouse colon, also exhibited EC-adhesive characteristics.

Control of pathogens and pathobionts by the gut microbiota.

A dense resident microbial community in the gut, referred as the commensal microbiota, coevolved with the host and is essential for many host physiological processes that include enhancement of the intestinal epithelial barrier, development of the immune system and acquisition of nutrients. A major function of the microbiota is protection against colonization by pathogens and overgrowth of indigenous pathobionts that can result from the disruption of the healthy microbial community. The mechanisms that regulate the ability of the microbiota to restrain pathogen growth are complex and include competitive metabolic interactions, localization to intestinal niches and induction of host immune responses. Pathogens, in turn, have evolved strategies to escape from commensal-mediated resistance to colonization. Thus, the interplay between commensals and pathogens or indigenous pathobionts is critical for controlling infection and disease. Understanding pathogen-commensal interactions may lead to new therapeutic approaches to treating infectious diseases.

NLRC4-driven production of IL-1ß discriminates between pathogenic and commensal bacteria and promotes host intestinal defense.

Intestinal phagocytes transport oral antigens and promote immune tolerance, but their role in innate immune responses remains unclear. Here we found that intestinal phagocytes were anergic to ligands for Toll-like receptors (TLRs) or commensals but constitutively expressed the precursor to interleukin 1ß (pro-IL-1ß). After infection with pathogenic Salmonella or Pseudomonas, intestinal phagocytes produced mature IL-1ß through the NLRC4 inflammasome but did not produce tumor necrosis factor (TNF) or IL-6. BALB/c mice deficient in NLRC4 or the IL-1 receptor were highly susceptible to orogastric but not intraperitoneal infection with Salmonella. That enhanced lethality was preceded by impaired expression of endothelial adhesion molecules, lower neutrophil recruitment and poor intestinal pathogen clearance. Thus, NLRC4-dependent production of IL-1ß by intestinal phagocytes represents a specific response that discriminates pathogenic bacteria from commensal bacteria and contributes to host defense in the intestine.

The role of the microbiota in myelopoiesis during homeostasis and inflammation.

The microbiota engages in the development and maintenance of the host immune system. The microbiota affects not only mucosal tissues where it localizes but also the distal organs. Myeloid cells are essential for host defense as first responders of the host immune system. Their generation, called myelopoiesis, is regulated by environmental signals, including commensal microbiota. Hematopoietic stem and progenitor cells in bone marrow can directly or indirectly sense microbiota-derived signals, thereby giving rise to myeloid cell lineages at steady-state and during inflammation. In this review, we discuss the role of commensal microorganisms in the homeostatic regulation of myelopoiesis in the bone marrow. We also outline the effects of microbial signals on myelopoiesis during inflammation and infection, with a particular focus on the development of innate immune memory. Studying the relationship between the microbiota and myelopoiesis will help us understand how the microbiota regulates immune responses at a systemic level beyond the local mucosa.

Distinct Commensals Induce Interleukin-1ß via NLRP3 Inflammasome in Inflammatory Monocytes to Promote Intestinal Inflammation in Response to Injury.

The microbiota stimulates inflammation, but the signaling pathways and the members of the microbiota involved remain poorly understood. We found that the microbiota induces interleukin-1ß (IL-1ß) release upon intestinal injury and that this is mediated via the NLRP3 inflammasome. Enterobacteriaceae and in particular the pathobiont Proteus mirabilis, induced robust IL-1ß release that was comparable to that induced by the pathogen Salmonella. Upon epithelial injury, production of IL-1ß in the intestine was largely mediated by intestinal Ly6C(high) monocytes, required chemokine receptor CCR2 and was abolished by deletion of IL-1ß in CCR2(+) blood monocytes. Furthermore, colonization with P. mirabilis promoted intestinal inflammation upon intestinal injury via the production of hemolysin, which required NLRP3 and IL-1 receptor signaling in vivo. Thus, upon intestinal injury, selective members of the microbiota stimulate newly recruited monocytes to induce NLRP3-dependent IL-1ß release, which promotes inflammation in the intestine.

Intestinal overgrowth of Candida albicans exacerbates bleomycin-induced pulmonary fibrosis in mice with dysbiosis.

Increasing evidence indicates an interaction between the intestinal microbiota and diseases in distal organs. However, the relationship between pulmonary fibrosis and the intestinal microbiota, especially intestinal fungal microbiota, is poorly understood. Thus, this study aimed to determine the effects of changes in the intestinal fungal microbiota on the pathogenesis of pulmonary fibrosis. Mice with intestinal overgrowth of Candida albicans, which was established by oral administration of antibiotics plus C. albicans, showed accelerated bleomycin-induced pulmonary fibrosis relative to the control mice (i.e. without C. albicans treatment). In addition, the mice with intestinal overgrowth of C. albicans showed enhanced Th17-type immunity, and treatment with IL-17A-neutralizing antibody alleviated pulmonary fibrosis in these mice but not in the control mice. This result indicates that IL-17A is involved in the pathogenesis of C. albicans-exacerbated pulmonary fibrosis. Even before bleomycin treatment, the expression of Rorc, the master regulator of Th17, was already upregulated in the pulmonary lymphocytes of the mice with intestinal overgrowth of C. albicans. Subsequent administration of bleomycin triggered these Th17-skewed lymphocytes to produce IL-17A, which enhanced endothelial-mesenchymal transition. These results suggest that intestinal overgrowth of C. albicans exacerbates pulmonary fibrosis via IL-17A-mediated endothelial-mesenchymal transition. Thus, it might be a potential therapeutic target in pulmonary fibrosis. This study may serve as a basis for using intestinal fungal microbiota as novel therapeutic targets in pulmonary fibrosis. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Inflammatory bowel disease and carcinogenesis.

Colorectal cancer (CRC) is the third most common cancer and the fourth most common cause of cancer mortality worldwide. Colitis-associated colorectal cancer (CAC) is a subtype of CRC associated with inflammatory bowel disease (IBD). It is well known that individuals with IBD have a 2-3 times higher risk of developing CRC than those who do not, rendering CAC a major cause of death in this group. Although the etiology and pathogenesis of CAC are incompletely understood, animal models of chronic inflammation and human cohort data indicate that changes in the intestinal environment, including host response dysregulation and gut microbiota perturbations, may contribute to the development of CAC. Genomic alterations are a hallmark of CAC, with patterns that are distinct from those in sporadic CRC. The discovery of the biological changes that underlie the development of CAC is ongoing; however, current data suggest that chronic inflammation in IBD increases the risk of developing CAC. Therefore, a deeper understanding of the precise mechanisms by which inflammation triggers genetic alterations and disrupts intestinal homeostasis may provide insight into novel therapeutic strategies for the prevention of CAC.

Untangling the oral-gut axis in the pathogenesis of intestinal inflammation.

An increasing body of literature reveals that host-microbe networks are well coordinated and impact human health and disease. Recently, it has become evident that an abnormal alteration in bacterial configuration in the oral cavity, namely oral dysbiosis, caused by periodontal inflammation, is associated with various distant inflammatory diseases, including inflammatory bowel disease. However, the extent to which the relationships between oral and distant disorders are merely an association or are causally triggered by oral microorganisms remains debated. In this mini-review, we highlight mechanisms in inter-related organ system diseases, particularly the one between oral and gut inflammation. Further, we discuss clinical perspectives and propose a novel concept of a multi-hit hypothesis in the pathogenesis of gut inflammation, on the basis of our updated knowledge of shared microbiological and immunological pathways between the oral and gut mucosae.

Interleukin-22 regulates the complement system to promote resistance against pathobionts after pathogen-induced intestinal damage.

Pathobionts play a critical role in disease development, but the immune mechanisms against pathobionts remain poorly understood. Here, we report a critical role for interleukin-22 (IL-22) in systemic protection against bacterial pathobionts that translocate into the circulation after infection with the pathogen Clostridium difficile. Infection with C. difficile induced IL-22, and infected Il22(-/-) mice harbored high numbers of pathobionts in extraintestinal organs despite comparable pathogen load and intestinal damage in mutant and wild-type mice. Pathobionts exhibited increased resistant against complement-mediated phagocytosis, and their intravenous administration resulted in high animal mortality. Selective removal of translocated commensals rescued Il22(-/-) mice, and IL-22 administration enhanced the elimination of pathobionts. Mechanistically, IL-22 augmented bacterial phagocytosis by increasing the expression and bacterial binding of complement C3. Our study demonstrates an unexpected role for IL-22 in controlling the elimination of pathobionts that enter the systemic circulation through the regulation of the complement system.

The Butyrate-Producing Bacterium Clostridium butyricum Suppresses Clostridioides difficile Infection via Neutrophil- and Antimicrobial Cytokine-Dependent but GPR43/109a-Independent Mechanisms.

Short-chain fatty acids, such as butyrate, are major gut microbial metabolites that are beneficial for gastrointestinal health. Clostridium butyricum MIYAIRI588 (CBM588) is a bacterium that produces a robust amount of butyrate and therefore has been used as a live biotherapeutic probiotic in clinical settings. Clostridioides difficile causes life-threatening diarrhea and colitis. The gut resident microbiota plays a critical role in the prevention of C. difficile infection (CDI), as the disruption of the healthy microbiota by antibiotics greatly increases the risk for CDI. We report that CBM588 treatment in mice significantly improved clinical symptoms associated with CDI and increased the number of neutrophils and Th1 and Th17 cells in the colonic lamina propria in the early phase of CDI. The protective effect of CBM588 was abolished when neutrophils, IFN-γ, or IL-17A were depleted, suggesting that induction of the immune reactants is required to elicit the protective effect of the probiotic. The administration of tributyrin, which elevates the concentration of butyrate in the colon, also increased the number of neutrophils in the colonic lamina propria, indicating that butyrate is a potent booster of neutrophil activity during infection. However, GPR43 and GPR109a, two G protein-coupled receptors activated by butyrate, were dispensable for the protective effect of CBM588. These results indicate that CBM588 and butyrate suppress CDI, in part by boosting antimicrobial innate and cytokine-mediated immunity.

Lipopolysaccharide O structure of adherent and invasive Escherichia coli regulates intestinal inflammation via complement C3.

Gut dysbiosis associated with intestinal inflammation is characterized by the blooming of particular bacteria such as adherent-invasive E. coli (AIEC). However, the precise mechanisms by which AIEC impact on colitis remain largely unknown. Here we show that antibiotic-induced dysbiosis worsened chemically-induced colitis in IL-22-deficient mice, but not in wild-type mice. The increase in intestinal inflammation was associated with the expansion of E. coli strains with genetic and functional features of AIEC. These E. coli isolates exhibited high ability to out compete related bacteria via colicins and resistance to the host complement system in vitro. Mutation of wzy, the lipopolysaccharide O polymerase gene, rendered AIEC more sensitive to the complement system and more susceptible to engulfment and killing by phagocytes while retaining its ability to outcompete related bacteria in vitro. The wzy AIEC mutant showed impaired fitness to colonize the intestine under colitic conditions, but protected mice from chemically-induced colitis. Importantly, the ability of the wzy mutant to protect from colitis was blocked by depletion of complement C3 which was associated with impaired intestinal eradication of AIEC in colitic mice. These studies link surface lipopolysaccharide O-antigen structure to the regulation of colitic activity in commensal AIEC via interactions with the complement system.

Periodontal connection with intestinal inflammation: Microbiological and immunological mechanisms.

Humans have coevolved with the trillions of resident microbes that populate every nook and cranny of the body. At each site, the resident microbiota creates a unique ecosystem specialized to its environment, benefiting the development and maintenance of human physiology through harmonious symbiotic relationships with the host. However, when the resident microbiota is perturbed, significant complications may arise with disastrous consequences that affect the local and distant ecosystems. In this context, periodontal disease results in inflammation beyond the oral cavity, such as in the gastrointestinal tract. Accumulating evidence indicates that potentially harmful oral resident bacteria (referred to as pathobionts) and pathogenic immune cells in the oral mucosa can migrate to the lower gastrointestinal tract and contribute to intestinal inflammation. We will review the most recent advances concerning the periodontal connection with intestinal inflammation from microbiological and immunological perspectives. Potential therapeutic approaches that target the connection between the mouth and the gut to treat gastrointestinal diseases, such as inflammatory bowel disease, will be examined. Deciphering the complex interplay between microbes and immunity along the mouth-gut axis will provide a better understanding of the pathogenesis of both oral and gut pathologies and present therapeutic opportunities.

Hyaluronic acid-bilirubin nanomedicine for targeted modulation of dysregulated intestinal barrier, microbiome and immune responses in colitis.

While conventional approaches for inflammatory bowel diseases mainly focus on suppressing hyperactive immune responses, it remains unclear how to address disrupted intestinal barriers, dysbiosis of the gut commensal microbiota and dysregulated mucosal immune responses in inflammatory bowel diseases. Moreover, immunosuppressive agents can cause off-target systemic side effects and complications. Here, we report the development of hyaluronic acid-bilirubin nanomedicine (HABN) that accumulates in inflamed colonic epithelium and restores the epithelium barriers in a murine model of acute colitis. Surprisingly, HABN also modulates the gut microbiota, increasing the overall richness and diversity and markedly augmenting the abundance of Akkermansia muciniphila and Clostridium XIVα, which are microorganisms with crucial roles in gut homeostasis. Importantly, HABN associated with pro-inflammatory macrophages, regulated innate immune responses and exerted potent therapeutic efficacy against colitis. Our work sheds light on the impact of nanotherapeutics on gut homeostasis, microbiome and innate immune responses for the treatment of inflammatory diseases.

The Nod2 sensor promotes intestinal pathogen eradication via the chemokine CCL2-dependent recruitment of inflammatory monocytes.

The intracellular sensor Nod2 is activated in response to bacteria, and the impairment of this response is linked to Crohn's disease. However, the function of Nod2 in host defense remains poorly understood. We found that Nod2-/- mice exhibited impaired intestinal clearance of Citrobacter rodentium, an enteric bacterium that models human infection by pathogenic Escherichia coli. The increased bacterial burden was preceded by reduced CCL2 chemokine production, inflammatory monocyte recruitment, and Th1 cell responses in the intestine. Colonic stromal cells, but not epithelial cells or resident CD11b+ phagocytic cells, produced CCL2 in response to C. rodentium in a Nod2-dependent manner. Unlike resident phagocytic cells, inflammatory monocytes produced IL-12, a cytokine that induces adaptive immunity required for pathogen clearance. Adoptive transfer of Ly6C(hi) monocytes restored the clearance of the pathogen in infected Ccr2-/- mice. Thus, Nod2 mediates CCL2-CCR2-dependent recruitment of inflammatory monocytes, which is important in promoting bacterial eradication in the intestine.

Quantitative proteomics identifies STEAP4 as a critical regulator of mitochondrial dysfunction linking inflammation and colon cancer.

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder and is a major risk factor for colorectal cancer (CRC). Hypoxia is a feature of IBD and modulates cellular and mitochondrial metabolism. However, the role of hypoxic metabolism in IBD is unclear. Because mitochondrial dysfunction is an early hallmark of hypoxia and inflammation, an unbiased proteomics approach was used to assess the mitochondria in a mouse model of colitis. Through this analysis, we identified a ferrireductase: six-transmembrane epithelial antigen of prostate 4 (STEAP4) was highly induced in mouse models of colitis and in IBD patients. STEAP4 was regulated in a hypoxia-dependent manner that led to a dysregulation in mitochondrial iron balance, enhanced reactive oxygen species production, and increased susceptibility to mouse models of colitis. Mitochondrial iron chelation therapy improved colitis and demonstrated an essential role of mitochondrial iron dysregulation in the pathogenesis of IBD. To address if mitochondrial iron dysregulation is a key mechanism by which inflammation impacts colon tumorigenesis, STEAP4 expression, function, and mitochondrial iron chelation were assessed in a colitis-associated colon cancer model (CAC). STEAP4 was increased in human CRC and predicted poor prognosis. STEAP4 and mitochondrial iron increased tumor number and burden in a CAC model. These studies demonstrate the importance of mitochondrial iron homeostasis in IBD and CRC.

The regenerating family member 3 ß 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 3ß 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.

Indoleamine 2,3-Dioxygenase 1, Increased in Human Gastric Pre-Neoplasia, Promotes Inflammation and Metaplasia in Mice and Is Associated With Type II Hypersensitivity/Autoimmunity.

BACKGROUND & AIMS: Chronic gastrointestinal inflammation increases the risk of cancer by mechanisms that are not well understood. Indoleamine-2,3-dioxygenase 1 (IDO1) is a heme-binding enzyme that regulates the immune response via catabolization and regulation of tryptophan availability for immune cell uptake. IDO1 expression is increased during the transition from chronic inflammation to gastric metaplasia. We investigated whether IDO1 contributes to the inflammatory response that mediates loss of parietal cells leading to metaplasia. METHODS: Chronic gastric inflammation was induced in Ido1-/- and CB57BL/6 (control) mice by gavage with Helicobacter felis or overexpression of interferon gamma in gastric parietal cells. We also performed studies in Jh-/- mice, which are devoid of B cells. Gastric tissues were collected and analyzed by flow cytometry, immunostaining, and real-time quantitative polymerase chain reaction. Plasma samples were analyzed by enzyme-linked immunosorbent assay. Gastric tissues were obtained from 20 patients with gastric metaplasia and 20 patients without gastric metaplasia (controls) and analyzed by real-time quantitative polymerase chain reaction; gastric tissue arrays were analyzed by immunohistochemistry. We collected genetic information on gastric cancers from The Cancer Genome Atlas database. RESULTS: H felis gavage induced significantly lower levels of pseudopyloric metaplasia in Ido1-/- mice, which had lower frequencies of gastric B cells, than in control mice. Blood plasma from H felis-infected control mice had increased levels of autoantibodies against parietal cells, compared to uninfected control mice, but this increase was lower in Ido1-/- mice. Chronically inflamed stomachs of Ido1-/- mice had significantly lower frequencies of natural killer cells in contact with parietal cells, compared with stomachs of control mice. Jh-/- mice had lower levels of pseudopyloric metaplasia than control mice in response to H felis infection. Human gastric pre-neoplasia and carcinoma specimens had increased levels of IDO1 messenger RNA compared with control gastric tissues, and IDO1 protein colocalized with B cells. Co-clustering of IDO1 messenger RNA with B-cell markers was corroborated by The Cancer Genome Atlas database. CONCLUSIONS: IDO1 mediates gastric metaplasia by regulating the B-cell compartment. This process appears to be associated with type II hypersensitivity/autoimmunity. The role of autoimmunity in the progression of pseudopyloric metaplasia warrants further investigation.

Fecal microbiota transplantation prevents Candida albicans from colonizing the gastrointestinal tract.

Gut microbes symbiotically colonize the gastrointestinal (GI) tract, interacting with each other and their host to maintain GI tract homeostasis. Recent reports have shown that gut microbes help protect the gut from colonization by pathogenic microbes. Here, we report that commensal microbes prevent colonization of the GI tract by the pathogenic fungus, Candida albicans. Wild-type specific pathogen-free (SPF) mice are resistant to C. albicans colonization of the GI tract. However, administering certain antibiotics to SPF mice enables C. albicans colonization. Quantitative kinetics of commensal bacteria are inversely correlated with the number of C. albicans in the gut. Here, we provide further evidence that transplantation of fecal microbiota is effective in preventing Candida colonization of the GI tract. These data demonstrate the importance of commensal bacteria as a barrier for the GI tract surface and highlight the potential clinical applications of commensal bacteria in preventing pathogenic fungal infections.