A transient breach in the epithelial barrier leads to regulatory T-cell generation and resistance to experimental colitis

Sodium orthovanadate protects against ulcerative colitis and associated liver damage in mice: insights into modulations of Nrf2/Keap1 and NF-κB pathways

Ulcerative colitis (UC) is a prominent category of disease that is associated with bowel inflammation, it can occur at any period of life and is prevalently rising on a global scale. Dextran sulfate sodium (DSS) has been extensively used to develop colitis due to its ability to mimic human UC, providing consistent and reproducible inflammation, ulceration, and disruption of the epithelial barrier in the colon. Chronic inflammation in the gut can lead to alterations in the gut-liver axis, potentially impacting liver function over time, while direct evidence linking diversion colitis to liver damage is limited. Thus, the present study aims to assess the gut and liver damage against DSS and the possible molecular mechanisms. Forty-seven animals were randomly assigned to six groups. Ulcerative colitis was induced using 2.5% w/v DSS in three alternate cycles, each lasting 7 days, with 1-week remission periods in between. SOV (5 and 10 mg/kg, orally) and the standard drug 5-aminosalicylic acid (100 mg/kg, orally) were administered from the start of the 2nd DSS cycle until the end of the experiment. Biochemical parameters, ELISA, histopathological, and immunohistochemical analyses have been conducted to assess damage in the colon and liver. SOV significantly reduced colitis severity by lowering the DAI score, oxidative stress markers (LPS, IL-1β, MPO, nitrite), and restoring liver biomarkers (SGPT, SGOT). Histopathological findings supported these protective benefits in the liver and gut. Moreover, immunohistochemical analysis showed SOV enhanced the expression of the cytoprotective mediator Nrf2/Keap-1 and reduced the expression of inflammatory mediators NF-κB and IL-6. Present findings concluded that SOV demonstrated a dose-dependent effect against UC through anti-inflammatory and antioxidant pathways, with the highest dose of SOV 10 mg/kg having more significant (p < 0.001) results than the low dose of 5 mg/kg.

Periodontitis increases the risk of gastrointestinal dysfunction: an update on the plausible pathogenic molecular mechanisms

Periodontitis is an immuno-inflammatory disease of the soft tissues surrounding the teeth. Periodontitis is linked to many communicable and non-communicable diseases such as diabetes, cardiovascular disease, rheumatoid arthritis, and cancers. The oral-systemic link between periodontal disease and systemic diseases is attributed to the spread of inflammation, microbial products and microbes to distant organ systems. Oral bacteria reach the gut via swallowed saliva, whereby they induce gut dysbiosis and gastrointestinal dysfunctions. Some periodontal pathogens like Porphyromonas. gingivalis, Klebsiella, Helicobacter. Pylori, Streptococcus, Veillonella, Parvimonas micra, Fusobacterium nucleatum, Peptostreptococcus, Haemophilus, Aggregatibacter actinomycetomcommitans and Streptococcus mutans can withstand the unfavorable acidic, survive in the gut and result in gut dysbiosis. Gut dysbiosis increases gut inflammation, and induce dysplastic changes that lead to gut dysfunction. Various studies have linked oral bacteria, and oral-gut axis to various GIT disorders like inflammatory bowel disease, liver diseases, hepatocellular and pancreatic ductal carcinoma, ulcerative colitis, and Crohn's disease. Although the correlation between periodontitis and GIT disorders is well established, the intricate molecular mechanisms by which oral microflora induce these changes have not been discussed extensively. This review comprehensively discusses the intricate and unique molecular and immunological mechanisms by which periodontal pathogens can induce gut dysbiosis and dysfunction.

Mechanisms of mucosal healing: treating inflammatory bowel disease without immunosuppression?

Almost all currently available treatments for inflammatory bowel disease (IBD) act by inhibiting inflammation, often blocking specific inflammatory molecules. However, given the infectious and neoplastic disease burden associated with chronic immunosuppressive therapy, the goal of attaining mucosal healing without immunosuppression is attractive. The absence of treatments that directly promote mucosal healing and regeneration in IBD could be linked to the lack of understanding of the underlying pathways. The range of potential strategies to achieve mucosal healing is diverse. However, the targeting of regenerative mechanisms has not yet been achieved for IBD. Stem cells provide hope as a regenerative treatment and are used in limited clinical situations. Growth factors are available for the treatment of short bowel syndrome but have not yet been applied in IBD. The therapeutic application of organoid culture and stem cell therapy to generate new intestinal tissue could provide a novel mechanism to restore barrier function in IBD. Furthermore, blocking key effectors of barrier dysfunction (such as MLCK or damage-associated molecular pattern molecules) has shown promise in experimental IBD. Here, we review the diversity of molecular targets available to directly promote mucosal healing, experimental models to identify new potential pathways and some of the anticipated potential therapies for IBD.

Mechanisms of mucosal healing: treating inflammatory bowel disease without immunosuppression?

Almost all currently available treatments for inflammatory bowel disease (IBD) act by inhibiting inflammation, often blocking specific inflammatory molecules. However, given the infectious and neoplastic disease burden associated with chronic immunosuppressive therapy, the goal of attaining mucosal healing without immunosuppression is attractive. The absence of treatments that directly promote mucosal healing and regeneration in IBD could be linked to the lack of understanding of the underlying pathways. The range of potential strategies to achieve mucosal healing is diverse. However, the targeting of regenerative mechanisms has not yet been achieved for IBD. Stem cells provide hope as a regenerative treatment and are used in limited clinical situations. Growth factors are available for the treatment of short bowel syndrome but have not yet been applied in IBD. The therapeutic application of organoid culture and stem cell therapy to generate new intestinal tissue could provide a novel mechanism to restore barrier function in IBD. Furthermore, blocking key effectors of barrier dysfunction (such as MLCK or damage-associated molecular pattern molecules) has shown promise in experimental IBD. Here, we review the diversity of molecular targets available to directly promote mucosal healing, experimental models to identify new potential pathways and some of the anticipated potential therapies for IBD.

Pattern Recognition Receptor Signaling and Cytokine Networks in Microbial Defenses and Regulation of Intestinal Barriers: Implications for Inflammatory Bowel Disease

Inflammatory bowel disease is characterized by defects in epithelial function and dysregulated inflammatory signaling by lamina propria mononuclear cells including macrophages and dendritic cells in response to microbiota. In this review, we focus on the role of pattern recognition receptors in the inflammatory response as well as epithelial barrier regulation. We explore cytokine networks that increase inflammation, regulate paracellular permeability, cause epithelial damage, up-regulate epithelial proliferation, and trigger restitutive processes. We focus on studies using patient samples as well as speculate on pathways that can be targeted to more holistically treat patients with inflammatory bowel disease.

Gut barrier disruption and chronic disease

The intestinal barrier protects the host against gut microbes, food antigens, and toxins present in the gastrointestinal tract. However, gut barrier integrity can be affected by intrinsic and extrinsic factors, including genetic predisposition, the Western diet, antibiotics, alcohol, circadian rhythm disruption, psychological stress, and aging. Chronic disruption of the gut barrier can lead to translocation of microbial components into the body, producing systemic, low-grade inflammation. While the association between gut barrier integrity and inflammation in intestinal diseases is well established, we review here recent studies indicating that the gut barrier and microbiota dysbiosis may contribute to the development of metabolic, autoimmune, and aging-related disorders. Emerging interventions to improve gut barrier integrity and microbiota composition are also described.

Gut Microbial Metabolite-Mediated Regulation of the Intestinal Barrier in the Pathogenesis of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a chronic inflammatory disease. The disease has a multifactorial aetiology, involving genetic, microbial as well as environmental factors. The disease pathogenesis operates at the host-microbe interface in the gut. The intestinal epithelium plays a central role in IBD disease pathogenesis. Apart from being a physical barrier, the epithelium acts as a node that integrates environmental, dietary, and microbial cues to calibrate host immune response and maintain homeostasis in the gut. IBD patients display microbial dysbiosis in the gut, combined with an increased barrier permeability that contributes to disease pathogenesis. Metabolites produced by microbes in the gut are dynamic indicators of diet, host, and microbial interplay in the gut. Microbial metabolites are actively absorbed or diffused across the intestinal lining to affect the host response in the intestine as well as at systemic sites via the engagement of cognate receptors. In this review, we summarize insights from metabolomics studies, uncovering the dynamic changes in gut metabolite profiles in IBD and their importance as potential diagnostic and prognostic biomarkers of disease. We focus on gut microbial metabolites as key regulators of the intestinal barrier and their role in the pathogenesis of IBD.

Overexpression of Vitamin D Receptor in Intestinal Epithelia Protects Against Colitis via Upregulating Tight Junction Protein Claudin 15

BACKGROUND AND AIMS

Dysfunction of the vitamin D receptor [VDR] contributes to the aetiology of IBD by regulating autophagy, immune response, and mucosal permeability. VDR directly controls the paracellular tight junction protein Claudin-2. Claudin-2 and Claudin-15 are unique in maintaining paracellular permeability. Interestingly, claudin-15 mRNA was downregulated in patients with ulcerative colitis. However, the exact mechanism of Claudin-15 regulation in colitis is still unknown. Here, we investigated the protective role of VDR against intestinal inflammation via upregulating Claudin-15.

METHODS

We analysed the correlation of Claudin-15 with the reduction of VDR in human colitis. We generated intestinal epithelial overexpression of VDR [O-VDR] mice to study the gain of function of VDR in colitis. Intestinal epithelial VDR knockout [VDR∆IEC] mice were used for the loss of function study. Colonoids and SKCO15 cells were used as in vitro models.

RESULTS

Reduced Claudin-15 was significantly correlated with decreased VDR along the colonic epithelium of human IBD. O-VDR mice showed decreased susceptibility to chemically and bacterially induced colitis and marked increased Claudin-15 expression [both mRNA and protein] in the colon. Correspondingly, colonic Claudin-15 was reduced in VDR∆IEC mice, which were susceptible to colitis. Overexpression of intestinal epithelial VDR and vitamin D treatment resulted in a significantly increased Claudin-15. ChIP assays identified the direct binding of VDR to the claudin-15 promoter, suggesting that claudin-15 is a target gene of VDR.

CONCLUSION

We demonstrated the mechanism of VDR upregulation of Claudin-15 to protect against colitis. This might enlighten the mechanism of barrier dysfunction in IBD and potential therapeutic strategies to inhibit inflammation.

Increased Intestinal Permeability Is Associated With Later Development of Crohn's Disease

BACKGROUND & AIMS

Increased intestinal permeability has been associated with Crohn's disease (CD), but it is not clear whether it is a cause or result of the disease. We performed a prospective study to determine whether increased intestinal permeability is associated with future development of CD.

METHODS

We assessed the intestinal permeability, measured by the urinary fractional excretion of lactulose-to-mannitol ratio (LMR) at recruitment in 1420 asymptomatic first-degree relatives (6-35 years old) of patients with CD (collected from 2008 through 2015). Participants were then followed up for a diagnosis of CD from 2008 to 2017, with a median follow-up time of 7.8 years. We analyzed data from 50 participants who developed CD after a median of 2.7 years during the study period, along with 1370 individuals who remained asymptomatic until October 2017. We used the Cox proportional hazards model to evaluate time-related risk of CD based on the baseline LMR.

RESULTS

An abnormal LMR (>0.03) was associated with a diagnosis of CD during the follow-up period (hazard ratio, 3.03; 95% CI, 1.64-5.63; P = 3.97 × 10^-4). This association remained significant even when the test was performed more than 3 years before the diagnosis of CD (hazard ratio, 1.62; 95% CI, 1.051-2.50; P = .029).

CONCLUSIONS

Increased intestinal permeability is associated with later development of CD; these findings support a model in which altered intestinal barrier function contributes to pathogenesis. Abnormal gut barrier function might serve as a biomarker for risk of CD onset.

Acute Colitis Drives Tolerance by Persistently Altering the Epithelial Barrier and Innate and Adaptive Immunity

BACKGROUND

Inflammatory bowel disease (IBD) has a remitting and relapsing disease course; however, relatively little is understood regarding how inflammatory damage in acute colitis influences the microbiota, epithelial barrier, and immune function in subsequent colitis.

METHODS

Mice were administered trinitrobenzene sulphonic acid (TNBS) via enema, and inflammation was assessed 2 days (d2) or 28 days (d28) later. Colitis was reactivated in some mice by re-treating at 28 days with TNBS and assessing 2 days later (d30). Epithelial responsiveness to secretagogues, microbiota composition, colonic infiltration, and immune activation was compared between all groups.

RESULTS

At day 28, the distal colon had healed, mucosa was restored, and innate immune response had subsided, but colonic transepithelial transport (P = 0.048), regulatory T-cell (TREG) infiltration (P = 0.014), adherent microbiota composition (P = 0.0081), and responsiveness of stimulated innate immune bone marrow cells (P < 0.0001 for IL-1β) differed relative to health. Two days after subsequent instillation of TNBS (d30 mice), the effects on inflammatory damage (P < 0.0001), paracellular permeability (P < 0.0001), and innate immune infiltration (P < 0.0001 for Ly6C+ Ly6G- macrophages) were reduced relative to d2 colitis. However, TREG infiltration was increased (P < 0.0001), and the responsiveness of stimulated T cells in the mesenteric lymph nodes shifted from pro-inflammatory at d2 to immune-suppressive at d30 (P < 0.0001 for IL-10). These effects were observed despite similar colonic microbiota composition and degradation of the mucosal layer between d2 and d30.

CONCLUSIONS

Collectively, these results indicate that acute colitis chronically alters epithelial barrier function and both innate and adaptive immune responses. These effects reduce the consequences of a subsequent colitis event, warranting longitudinal studies in human IBD subjects.

CD3+CD4+LAP+Foxp3-Regulatory Cells of the Colonic Lamina Propria Limit Disease Extension in Ulcerative Colitis

Background and Aims: In ulcerative colitis (UC), inflammation begins in the rectum and can extend proximally throughout the entire colon. The extension of inflammation is an important determinant of disease course, and may be limited by the action of regulatory T cells (Tregs). In this cross-sectional study, we evaluated the relationship between UC extension and the proportions of CD3+CD4+Foxp3+ and CD3+CD4+LAP+Foxp3-Tregs in the colonic lamina propria (LP) of 79 UC patients and 29 controls. The role of these cells in UC extension was also investigated in the murine oxazolone-induced colitis model.

Methods: Patients: Disease extension was classified according to the Montreal classification. Where possible, endoscopic biopsies of involved and uninvolved tissue were obtained from UC patients. Mouse model: Colitis was induced by intrarectal oxazolone administration. Lamina propria mononuclear cells were isolated from patient biopsies and mouse colon tissue using enzymatic method and the percentage of CD3+CD4+Foxp3+ and CD3+CD4+LAP+Foxp3-cells evaluated by immunofluorescence. Confocal microscopy was applied for the visualization and quantification of CD4+LAP+ cells on tissue histological sections.

Results: In UC patients with distal colitis the proportion of LP CD3+CD4+Foxp3+ Tregs was significantly higher in inflamed tissue than uninvolved tissue. As opposite, the proportion of LP CD3+CD4+LAP+ Tregs was significantly higher in uninvolved tissue than involved tissue. Both LP CD3+CD4+Foxp3+ and LP CD3+CD4+LAP+ Tregs proportion in involved tissue was significantly higher than in controls irrespective of the extension of inflammation. In mice with oxazolone-induced distal colitis, treatment with LAP-depleting antibody was associated with the development of extensive colitis.

Conclusions: Our findings suggest that CD3+CD4+LAP+Foxp3-Tregs limit the extension of inflammatory lesions in UC patients.

Nod2 Deficiency in mice is Associated with Microbiota Variation Favouring the Expansion of mucosal CD4+ LAP+ Regulatory Cells

Nucleotide-binding Oligomerization Domain-2 (NOD2) mutations are associated with an increased risk to develop Crohn's Disease. In previous studies, we have shown that Nod2-/- mice manifest increased proportion of Lamina Propria (LP) CD4+ LAP+ Foxp3- regulatory cells, when compared with Nod2+/+ mice, while CD4+ Foxp3 + regulatory cells were not affected. Here, we investigated the Nod2 gut microbiota, by 16S rRNA pyrosequencing, at steady state and after TNBS-colitis induction in mice reared separately or in cohousing, correlating the microbial profiles with LP regulatory T cells proportion and tissue cytokines content. We found that enrichment of Rikenella and Alistipes (Rikenellaceae) in Nod2-/- mice at 8 weeks of age reared separately was associated with increased proportion of CD4+ LAP+ Foxp3- cells and less severe TNBS-colitis. In co-housed mice the acquisition of Rickenellaceae by Nod2+/+ mice was associated with increased CD4+ LAP+ Foxp3- proportion and less severe colitis. Severe colitis was associated with enrichment of gram-negative pathobionts (Escherichia and Enterococcus), while less severe colitis with protective bacteria (Barnesiella, Odoribacter and Clostridium IV). Environmental factors acting on genetic background with different outcomes according to their impact on microbiota, predispose in different ways to inflammation. These results open a new scenario for therapeutic attempt to re-establish eubiosis in Inflammatory Bowel Disease patients with NOD2 polymorphisms.

The Guanylate Cyclase C-cGMP Signaling Axis Opposes Intestinal Epithelial Injury and Neoplasia

Guanylate cyclase C (GUCY2C) is a transmembrane receptor expressed on the luminal aspect of the intestinal epithelium. Its ligands include bacterial heat-stable enterotoxins responsible for traveler's diarrhea, the endogenous peptide hormones uroguanylin and guanylin, and the synthetic agents, linaclotide, plecanatide, and dolcanatide. Ligand-activated GUCY2C catalyzes the synthesis of intracellular cyclic GMP (cGMP), initiating signaling cascades underlying homeostasis of the intestinal epithelium. Mouse models of GUCY2C ablation, and recently, human populations harboring GUCY2C mutations, have revealed the diverse contributions of this signaling axis to epithelial health, including regulating fluid secretion, microbiome composition, intestinal barrier integrity, epithelial renewal, cell cycle progression, responses to DNA damage, epithelial-mesenchymal cross-talk, cell migration, and cellular metabolic status. Because of these wide-ranging roles, dysregulation of the GUCY2C-cGMP signaling axis has been implicated in the pathogenesis of bowel transit disorders, inflammatory bowel disease, and colorectal cancer. This review explores the current understanding of cGMP signaling in the intestinal epithelium and mechanisms by which it opposes intestinal injury. Particular focus will be applied to its emerging role in tumor suppression. In colorectal tumors, endogenous GUCY2C ligand expression is lost by a yet undefined mechanism conserved in mice and humans. Further, reconstitution of GUCY2C signaling through genetic or oral ligand replacement opposes tumorigenesis in mice. Taken together, these findings suggest an intriguing hypothesis that colorectal cancer arises in a microenvironment of functional GUCY2C inactivation, which can be repaired by oral ligand replacement. Hence, the GUCY2C signaling axis represents a novel therapeutic target for preventing colorectal cancer.

Role of Short Chain Fatty Acid Receptors in Intestinal Physiology and Pathophysiology

Nutrient sensing is a mechanism for organisms to sense their environment. In larger animals, including humans, the intestinal tract is a major site of nutrient sensing for the body, not surprisingly, as this is the central location where nutrients are absorbed. In the gut, bacterial fermentation results in generation of short chain fatty acids (SCFAs), a class of nutrients, which are sensed by specific membrane bound receptors, FFA2, FFA3, GPR109a, and Olfr78. These receptors are expressed uniquely throughout the gut and signal through distinct mechanisms. To date, the emerging data suggests a role of these receptors in normal and pathological conditions. The overall function of these receptors is to regulate aspects of intestinal motility, hormone secretion, maintenance of the epithelial barrier, and immune cell function. Besides in intestinal health, a prominent role of these receptors has emerged in modulation of inflammatory and immune responses during pathological conditions. Moreover, these receptors are being revealed to interact with the gut microbiota. This review article updates the current body of knowledge on SCFA sensing receptors in the gut and their roles in intestinal health and disease as well as in whole body energy homeostasis. © 2017 American Physiological Society. Compr Physiol 8:1091-1115, 2018.

IL-10 control of CD11c+ myeloid cells is essential to maintain immune homeostasis in the small and large intestine

Although IL-10 promotes a regulatory phenotype of CD11c⁺ dendritic cells and macrophages in vitro, the role of IL-10 signaling in CD11c⁺ cells to maintain intestinal tolerance in vivo remains elusive. To this aim, we generated mice with a CD11c-specific deletion of the IL-10 receptor alpha (Cd11c^creIl10ra^fl/fl). In contrast to the colon, the small intestine of Cd11c^creIl10ra^fl/fl mice exhibited spontaneous crypt hyperplasia, increased numbers of intraepithelial lymphocytes and lamina propria T cells, associated with elevated levels of T cell-derived IFNγ and IL-17A. Whereas naive mucosal T-cell priming was not affected and oral tolerance to ovalbumin was intact, augmented T-cell function in the lamina propria was associated with elevated numbers of locally dividing T cells, expression of T-cell attracting chemokines and reduced T-cell apoptosis. Upon stimulation, intestinal IL-10Rα deficient CD11c⁺ cells exhibited increased activation associated with enhanced IL-6 and TNFα production. Following colonization with Helicobacter hepaticus Cd11c^creIl10ra^fl/fl mice developed severe large intestinal inflammation characterized by infiltrating T cells and increased levels of Il17a, Ifng, and Il12p40. Altogether these findings demonstrate a critical role of IL-10 signaling in CD11c⁺ cells to control small intestinal immune homeostasis by limiting reactivation of local memory T cells and to protect against Helicobacter hepaticus-induced colitis.

The Microbiome Activates CD4 T-cell-mediated Immunity to Compensate for Increased Intestinal Permeability

BACKGROUND & AIMS

Despite a prominent association, chronic intestinal barrier loss is insufficient to induce disease in human subjects or experimental animals. We hypothesized that compensatory mucosal immune activation might protect individuals with increased intestinal permeability from disease. We used a model in which intestinal barrier loss is triggered by intestinal epithelial-specific expression of constitutively active myosin light chain kinase (CA-MLCK). Here we asked whether constitutive tight junction barrier loss impacts susceptibility to enteric pathogens.

METHODS

Acute or chronic Toxoplasma gondii or Salmonella typhimurium infection was assessed in CA-MLCK transgenic or wild-type mice. Germ-free mice or those lacking specific immune cell populations were used to investigate the effect of microbial-activated immunity on pathogen translocation in the context of increased intestinal permeability.

RESULTS

Acute T gondii and S typhimurium translocation across the epithelial barrier was reduced in CA-MLCK mice. This protection was due to enhanced mucosal immune activation that required CD4⁺ T cells and interleukin 17A but not immunoglobulin A. The protective mucosal immune activation in CA-MLCK mice depended on segmented filamentous bacteria (SFB), because protection against early S typhimurium invasion was lost in germ-free CA-MLCK mice but could be restored by conventionalization with SFB-containing, not SFB-deficient, microbiota. In contrast, chronic S typhimurium infection was more severe in CA-MLCK mice, suggesting that despite activation of protective mucosal immunity, barrier defects ultimately result in enhanced disease progression.

CONCLUSIONS

Increased epithelial tight junction permeability synergizes with commensal bacteria to promote intestinal CD4⁺ T-cell expansion and interleukin 17A production that limits enteric pathogen invasion.

Gut permeability and mucosal inflammation: bad, good or context dependent

Inflammatory bowel disease (IBD) is a multifactorial disease. A breach in the mucosal barrier, otherwise known as "leaky gut," is alleged to promote mucosal inflammation by intensifying immune activation. However, interaction between the luminal antigen and mucosal immune system is necessary to maintain mucosal homeostasis. Furthermore, manipulations leading to deregulated gut permeability have resulted in susceptibility in mice to colitis as well as to creating adaptive immunity. These findings implicate a complex but dynamic association between mucosal permeability and immune homeostasis; however, they also emphasize that compromised gut permeability alone may not be sufficient to induce colitis. Emerging evidence further supports the role(s) of proteins associated with the mucosal barrier in epithelial injury and repair: manipulations of associated proteins also modified epithelial differentiation, proliferation, and apoptosis. Taken together, the role of gut permeability and proteins associated in regulating mucosal inflammatory diseases appears to be more complex than previously thought. Herein, we review outcomes from recent mouse models where gut permeability was altered by direct and indirect effects of manipulating mucosal barrier-associated proteins, to highlight the significance of mucosal permeability and the non-barrier-related roles of these proteins in regulating chronic mucosal inflammatory conditions.

Germ-free and Antibiotic-treated Mice are Highly Susceptible to Epithelial Injury in DSS Colitis

BACKGROUND AND AIMS

Intestinal microbiota is required to maintain immune homeostasis and intestinal barrier function. At the same time, intraluminal bacteria are considered to be involved in inflammatory bowel disease and are required for colitis induction in animal models, with the possible exception of dextran sulphate sodium [DSS] colitis. This study was carried out to ascertain the mechanism underlying the induction of colitis by DSS in the absence of bacteria.

METHODS

Conventional and germ-free [GF] Naval Medical Research Institute [NMRI] mice were used, plus conventional mice treated with an antibiotic cocktail to deplete the intestinal microbiota ['pseudo-GF' or PGF mice]. The differential response to DSS was assessed.

RESULTS

Conventional mice developed DSS-induced colitis normally, whereas GF mice showed only minimal inflammation [no colonic thickening, lower myeloperoxidase activity, IL-6, IL-17, TNF-α, and IFN-γ secretion by splenocytes and mesenteric cell cultures, etc.]. However, these mice suffered enhanced haemorrhage, epithelial injury and mortality as a consequence of a weakened intestinal barrier, as shown by lower occludin, claudin 4, TFF3, MUC3, and IL-22. In contrast, PGF mice had a relatively normal, albeit attenuated, inflammatory response, but were less prone to haemorrhage and epithelial injury than GF mice. This was correlated with an increased expression of IL-10 and Foxp3 and preservation barrier-related markers.

CONCLUSIONS

We conclude that enteric bacteria are essential for the development of normal DSS-induced colitis. The absence of microbiota reduces DSS colonic inflammation dramatically but it also impairs barrier function, whereas subtotal microbiota depletion has intermediate effects at both levels.

Zonulin, a regulator of epithelial and endothelial barrier functions, and its involvement in chronic inflammatory diseases

Beside digesting nutrients and absorbing solutes and electrolytes, the intestinal epithelium with its barrier function is in charge of a tightly controlled antigen trafficking from the intestinal lumen to the submucosa. This trafficking dictates the delicate balance between tolerance and immune response causing inflammation. Loss of barrier function secondary to upregulation of zonulin, the only known physiological modulator of intercellular tight junctions, leads to uncontrolled influx of dietary and microbial antigens. Additional insights on zonulin mechanism of action and the recent appreciation of the role that altered intestinal permeability can play in the development and progression of chronic inflammatory disorders has increased interest of both basic scientists and clinicians on the potential role of zonulin in the pathogenesis of these diseases. This review focuses on the recent research implicating zonulin as a master regulator of intestinal permeability linked to the development of several chronic inflammatory disorders.

Leaky gut - concept or clinical entity?

PURPOSE OF REVIEW

This article evaluates the current status of the gut barrier in gastrointestinal disorders.

RECENT FINDINGS

The gut barrier is a complex, multicomponent, interactive, and bidirectional entity that includes, but is not restricted to, the epithelial cell layer. Intestinal permeability, the phenomenon most readily and commonly studied, reflects just one (albeit an important one) function of the barrier that is intimately related to and interacts with luminal contents, including the microbiota. The mucosal immune response also influences barrier integrity; effects of inflammation per se must be accounted for in the interpretation of permeability studies in disease states.

SUMMARY

Although several aspects of barrier function can be assessed in man, one must be aware of exactly what a given test measures, as well as of its limitations. The temptation to employ results from a test of paracellular flux to imply a role for barrier dysfunction in disorders thought to be based on bacterial or macromolecular translocation must be resisted. Although changes in barrier function have been described in several gastrointestinal disorders, their primacy remains to be defined. At present, few studies support efficacy for an intervention that improves barrier function in altering the natural history of a disease process.

Lamina Propria CD4+LAP+ Regulatory T Cells Are Increased in Active Ulcerative Colitis but Show Increased IL-17 Expression and Reduced Suppressor Activity

BACKGROUND

A CD4+CD25- regulatory T cell population expressing the surface TGF-β in its latent form LAP+ [latency associated peptide] cells was proved to be protective in experimental colitis and to be suppressive of human peripheral blood [PB] T proliferation. We investigated the frequency and function of lamina propria [LP] CD4+LAP+ T cells in inflammatory bowel disease [IBD] patients.

METHODS

Specimens from patients undergoing colonoscopy or bowel resection for IBD and colonic cancer were used as source of lamina propria mononuclear cells [LPMC]. The ulcerative colitis [UC] group was divided according to endoscopic activity evaluated with modified Baron Score. IL-17, IFN-γ, IL-10, LAP, and Foxp3 expression in CD3+CD8- [CD4] or CD3+/CD4+ gated cell population was assessed by immunofluorescence. The ability of FACS-sorted LP CD3+CD8-[CD4] LAP+CD25- to inhibit stimulated autologous PB CD3+CD8-[CD4] LAP- CD25- cells proliferation was assessed.

RESULTS

LP CD4LAP+ cells were significantly increased, when compared with controls, in active UC patients and not in Crohn's disease patients. The majority of LP CD4+LAP+ cells were Foxp3-. The percentage of IL-17+ cells in LP CD3+CD8-[CD4] LAP+ cells was significantly higher in active UC patients when compared with controls. LP CD3+CD8-[CD4]LAP+CD25- isolated from UC patients showed reduced or no ability to inhibit autologous PB CD3+CD8-[CD4]LAP-CD25- cell proliferation when compared with controls. Removal of IL-17+ cells from LP CD3+CD8-[CD4] LAP+ cells increases their suppressive ability.

CONCLUSIONS

The percentage of LP CD4LAP+ cells is increased in active UC, showing reduced suppressor activity due to their increased proportion of intracellular IL-17 expression.

Does oral exposure to cadmium and lead mediate susceptibility to colitis? The dark-and-bright sides of heavy metals in gut ecology

Although the heavy metals cadmium (Cd) and lead (Pb) are known environmental health concerns, their long-term impacts on gut ecology and susceptibility to gastrointestinal autoimmune diseases have not been extensively investigated. We sought to determine whether subchronic oral exposure to Cd or Pb is a risk factor for the development and progression of inflammatory bowel disease (IBD). Mice were exposed to various doses of CdCl2 or PbCl2 in drinking water for 1, 4 or 6 weeks prior to infection with Salmonella, the induction of colitis with dextran sodium sulfate (DSS) or trinitrobenzene sulfonic acid (TNBS). In human cell-based models, exposure to Cd and Pb is associated with reduced transepithelial electric resistance and changes in bacteria-induced cytokine responses. Although 1- and 6-week exposures did not have clear effects on the response to Salmonella infectious challenges, 1-week short-term treatments with CdCl2 tended to enhance intestinal inflammation in mice. Unexpectedly, subchronic exposure to Cd and (to a lesser extent) Pb significantly mitigated some of the symptoms of DSS-induced colitis and reduced the severity of TNBS colitis in a dose-dependent manner. The possible adaptive and immunosuppressive mechanisms by which heavy metals might reduce intestinal inflammation are explored and discussed.

Experimental Models of Inflammatory Bowel Diseases

The understanding of the intestinal inflammation occurring in the inflammatory bowel diseases (IBD) has been immeasurably advanced by the development of the now numerous murine models of intestinal inflammation. The usefulness of this research tool in IBD studies has been enabled by our improved knowledge of mucosal immunity and thus our improved ability to interpret the complex responses of mice with various causes of colitis; in addition, it has been powered by the availability of models in which the mice have specific genetic and/or immunologic defects that can be related to the origin of the inflammation. Finally, and more recently, it has been enhanced by our newly acquired ability to define the intestinal microbiome under various conditions and thus to understand how intestinal microorganisms impact on inflammation. In this brief review of murine models of intestinal inflammation we focus mainly on the most often used models that are, not incidentally, also the models that have yielded major insights into IBD pathogenesis.

Targeted colonic claudin-2 expression renders resistance to epithelial injury, induces immune suppression, and protects from colitis

Expression of claudin-2, a tight junction protein, is highly upregulated during inflammatory bowel disease (IBD) and, due to its association with epithelial permeability, has been postulated to promote inflammation. Notably, claudin-2 has also been implicated in the regulation of intestinal epithelial proliferation. However, precise role of claudin-2 in regulating colonic homeostasis remains unclear. Here, we demonstrate, using Villin-Claudin-2 transgenic mice, that increased colonic claudin-2 expression augments mucosal permeability as well as colon and crypt length. Most notably, despite leaky colon, Cl-2TG mice were significantly protected against experimental colitis. Importantly, claudin-2 expression increased colonocyte proliferation and provided protection against colitis-induced colonocyte death in a PI-3Kinase/Bcl-2-dependent manner. However, Cl-2TG mice also demonstrated marked suppression of colitis-induced increases in immune activation and associated signaling, suggesting immune tolerance. Accordingly, colons from naive Cl-2TG mice harbored significantly increased numbers of regulatory (CD4(+)Foxp3(+)) T cells than WT littermates. Furthermore, macrophages isolated from Cl-2TG mouse colon exhibited immune anergy. Importantly, these immunosuppressive changes were associated with increased synthesis of the immunoregulatory cytokine TGF-β by colonic epithelial cells in Cl-2TG mice compared with WT littermates. Taken together, our findings reveal a critical albeit complex role of claudin-2 in intestinal homeostasis by regulating epithelial permeability, inflammation and proliferation and suggest novel therapeutic opportunities.

Immunomodulation by gut microbiota: role of Toll-like receptor expressed by T cells

A close relationship exists between gut microbiota and immune responses. An imbalance of this relationship can determine local and systemic immune diseases. In fact the immune system plays an essential role in maintaining the homeostasis with the microbiota that normally resides in the gut, while, at the same time, the gut microbiota influences the immune system, modulating number and function of effector and regulatory T cells. To achieve this aim, mutual regulation between immune system and microbiota is achieved through several mechanisms, including the engagement of toll-like receptors (TLRs), pathogen-specific receptors expressed on numerous cell types. TLRs are able to recognize ligands from commensal or pathogen microbiota to maintain the tolerance or trigger the immune response. In this review, we summarize the latest evidences about the role of TLRs expressed in adaptive T cells, to understand how the immune system promotes intestinal homeostasis, fights invasion by pathogens, and is modulated by the intestinal microbiota.

Induction of murine TNBS colitis is strictly controlled by a modified method using continuous inhalation anesthesia with sevoflurane

BACKGROUND

Trinitrobenzenesulfonic acid (TNBS)-induced colitis is one of the most widely used experimental colitis models. However, there is no standard procedure for inducing colitis by TNBS because it is difficult to achieve a uniform distribution of colitis. We have developed a modified method of murine TNBS-induced colitis that involves inhalation anesthesia with sevoflurane combined with both single and repeated TNBS administrations.

AIMS

To compare the usefulness of our newly developed method for inducing murine TNBS-induced colitis with that of conventional intraperitoneal anesthesia.

METHODS

TNBS in ethanol was administered to C57BL/6J mice held in an inverted vertical position either under continuous inhalation anesthesia with sevoflurane, in accordance with our newly developed method, or by intraperitoneal injection with 2.5 % avertin, in accordance with the conventional procedure. Body weight change, cytokine profile, and histological findings were examined during the course of colitis.

RESULTS

The dispersion of anesthesia time, TNBS retention time, and nadir weight during the course of colitis was decreased using the newly developed method compared with the conventional procedure. Optimization of the modified TNBS-induced colitis, as evidenced by the predominant expression of Th1 and Th17 cytokines on day 7, was attained by the injection of 2.25 mg TNBS in 55 % ethanol. Regulation of the TNBS retention time using inhalation anesthesia with sevoflurane allowed strict control of the disease severity of TNBS-induced colitis. Using the modified method we were also able to develop a chronic TNBS-induced colitis model by repeated TNBS administration without excessive mortality of the mice.

CONCLUSIONS

Our modified method for murine TNBS-induced colitis using continuous inhalation anesthesia with sevoflurane provides a better experimental colitis model following both single and repeated TNBS administrations.

Immunopathological characterization of selected mouse models of inflammatory bowel disease: Comparison to human disease

Inflammatory bowel diseases (IBD) are chronic, relapsing conditions of multifactorial etiology. The two primary diseases of IBD are Crohn's disease (CD) and ulcerative colitis (UC). Both entities are hypothesized to occur in genetically susceptible individuals due to microbial alterations and environmental contributions. The exact etiopathogenesis, however, is not known for either disease. A variety of mouse models of CD and UC have been developed to investigate the pathogenesis of these diseases and evaluate treatment modalities. Broadly speaking, the mouse models can be divided into 4 categories: genetically engineered, immune manipulated, spontaneous and erosive/chemically induced. No one mouse model completely recapitulates the immunopathology of CD or UC, however each model possesses particular similarities to human IBD and offers advantageous for specific details of IBD pathogenesis. Here we discuss the more commonly used models in each category and critically evaluate how the immunopathology induced compares to CD or UC, as well as the advantages and disadvantages associated with each model.

IL-13 orchestrates resolution of chronic intestinal inflammation via phosphorylation of glycogen synthase kinase-3β

Spontaneous amelioration of inflammation (often accompanied by fibrosis) is a well-known, but poorly understood, outcome of many chronic inflammatory processes. We studied this phenomenon in a chronic trinitrobenzene sulfonic acid-induced colitis model, an experimental colitis in mice that we showed to ultimately undergo spontaneous resolution, despite continued trinitrobenzene sulfonic acid stimulation. Analysis of the mechanism of this resolution revealed that it was critically dependent on IL-13 activation of STAT6, followed by phosphorylation (inactivation) of glycogen synthase kinase-3β, at least in part via STAT6 induction of p38 MAPK. Such glycogen synthase kinase-3β inactivation causes changes in CREB and p65 DNA-binding activity that favors decreased proinflammatory IL-17 production and increased anti-inflammatory IL-10 production. Thus, in this case, IL-13 acts as a molecular switch that leads to resolution of inflammation.

Nod2 deficiency is associated with an increased mucosal immunoregulatory response to commensal microorganisms

On the basis of previous studies demonstrating that a breach of the colonic epithelial barrier is associated with a microbiota-dependent increase in lamina propria (LP) regulatory cells, we investigated if the lack of spontaneous intestinal inflammation observed in nucleotide-binding oligomerization domain 2 (Nod2)-/- mice was due to enhanced intestinal regulatory function. We found that the LP CD4+ T-cell population of Nod2-/- mice contains an increased percentage of CD4+ regulatory T cells bearing transforming growth factor -β/latency peptide (LP CD4+LAP (latency-associated peptide) + T cells) both under baseline conditions and following an intentional breach of the colonic barrier induced by ethanol administration. In addition, we found that Nod2-/- mice manifest decreased severity of 2,4,6-trinitrobenzene sulfonic acid (TNBS)-colitis and that TNBS-colitis in Nod2-/- or Nod2+/+ mice is ameliorated by adoptive transfer of LP cells from ethanol-treated mice before, but not after, depletion of LAP+ T cells. This increased regulatory T-cell response in Nod2-/- mice could explain why NOD2 polymorphisms in humans are not in themselves sufficient to establish inflammatory lesions.

Immunology of pediatric HIV infection

Most infants born to human immunodeficiency virus (HIV)-infected women escape HIV infection. Infants evade infection despite an immature immune system and, in the case of breastfeeding, prolonged repetitive exposure. If infants become infected, the course of their infection and response to treatment differs dramatically depending upon the timing (in utero, intrapartum, or during breastfeeding) and potentially the route of their infection. Perinatally acquired HIV infection occurs during a critical window of immune development. HIV's perturbation of this dynamic process may account for the striking age-dependent differences in HIV disease progression. HIV infection also profoundly disrupts the maternal immune system upon which infants rely for protection and immune instruction. Therefore, it is not surprising that infants who escape HIV infection still suffer adverse effects. In this review, we highlight the unique aspects of pediatric HIV transmission and pathogenesis with a focus on mechanisms by which HIV infection during immune ontogeny may allow discovery of key elements for protection and control from HIV.

Intestinal permeability defects: is it time to treat?

An essential role of the intestinal epithelium is to separate luminal contents from the interstitium, a function primarily determined by the integrity of the epithelium and the tight junction that seals the paracellular space. Intestinal tight junctions are selectively permeable, and intestinal permeability can be increased physiologically in response to luminal nutrients or pathologically by mucosal immune cells and cytokines, the enteric nervous system, and pathogens. Compromised intestinal barrier function is associated with an array of clinical conditions, both intestinal and systemic. Although most available data are correlative, some studies support a model where cycles of increased intestinal permeability, intestinal immune activation, and subsequent immune-mediated barrier loss contribute to disease progression. This model is applicable to intestinal and systemic diseases. However, it has not been proven, and both mechanistic and therapeutic studies are ongoing. Nevertheless, the correlation between increased intestinal permeability and disease has caught the attention of the public, leading to a rise in popularity of the diagnosis of "leaky gut syndrome," which encompasses a range of systemic disorders. Proponents claim that barrier restoration will cure underlying disease, but this has not been demonstrated in clinical trials. Moreover, human and mouse studies show that intestinal barrier loss alone is insufficient to initiate disease. It is therefore uncertain whether increased permeability in these patients is a cause or effect of the underlying disorder. Although drug targets that may mediate barrier restoration have been proposed, none have been proven effective. As such, current treatments for barrier dysfunction should target the underlying disease.

Impact of the gut microbiome on mucosal inflammation

In the past 10 years it has become increasingly apparent that the gut microbiome has profound effects on the immune system to which it is juxtaposed, the mucosal immune system. Here, I explore recent studies in which the effects of the microbiota expand or facilitate anti-inflammatory or regulatory immunological machinery or which favor development of proinflammatory immunological machinery in this system. I then focus on how these opposing processes play out in inflammatory bowel disease (IBD); a disease in which normal immune homeostasis is disturbed and inflammation takes hold.

Probiotic VSL#3-induced TGF-β ameliorates food allergy inflammation in a mouse model of peanut sensitization through the induction of regulatory T cells in the gut mucosa

SCOPE

Among food allergies, peanut allergy is frequently associated with severe anaphylactic reactions. In the need for safe and effective therapeutic strategies, probiotics may be considered on the basis of their immunomodulatory properties. The aim of the present study was to investigate the immunological mediators involved in the effects of probiotic VSL#3 oral supplementation on Th2 inflammation and anaphylaxis in a mouse model of peanut allergy.

METHODS AND RESULTS

VSL#3 supplementation to peanut-sensitized mice was effective in ameliorating anaphylaxis and Th2-mediated inflammation, by promoting regulatory responses in the jejunum mucosa and in the mesenteric lymph node, as evaluated by ELISA, real-time PCR, histologic, and immunohistochemical analysis. Probiotic-induced TGF-β mediates its protective effects through the induction of regulatory T cells expressing FOXP3 and/or latency-associated peptide, as proven by in vivo blockade of TGF-β in VSL#3-treated mice with a neutralizing monoclonal antibody one day before challenge.

CONCLUSION

TGF-β, induced in the gut by VSL#3 supplementation, is capable of reducing the Th2 inflammation associated with food anaphylaxis in a mouse model of peanut sensitization. TGF-β acts through the induction/maintenance of regulatory T cells expressing FOXP3 and/or latency-associated peptide. Probiotics supplementation may represent an effective and safe strategy for treating food allergies in adult population.

Short-chain fatty acids activate GPR41 and GPR43 on intestinal epithelial cells to promote inflammatory responses in mice

BACKGROUND & AIMS

Short-chain fatty acids (SCFAs), the most abundant microbial metabolites in the intestine, activate cells via G-protein-coupled receptors (GPRs), such as GPR41 and GPR43. We studied regulation of the immune response by SCFAs and their receptors in the intestines of mice.

METHODS

Inflammatory responses were induced in GPR41(-/-), GPR43(-/-), and C57BL6 (control) mice by administration of ethanol; 2, 4, 6-trinitrobenzene sulfonic-acid (TNBS); or infection with Citrobacter rodentium. We examined the effects of C rodentium infection on control mice fed SCFAs and/or given injections of antibodies that delay the immune response. We also studied the kinetics of cytokine and chemokine production, leukocyte recruitment, intestinal permeability, and T-cell responses. Primary colon epithelial cells were isolated from GPR41(-/-), GPR43(-/-), and control mice; signaling pathways regulated by SCFAs were identified using immunohistochemical, enzyme-linked immunosorbent assay, and flow cytometry analyses.

RESULTS

GPR41(-/-) and GPR43(-/-) mice had reduced inflammatory responses after administration of ethanol or TNBS compared with control mice, and had a slower immune response against C rodentium infection, clearing the bacteria more slowly. SCFAs activated intestinal epithelial cells to produce chemokines and cytokines in culture and mice after administration of ethanol, TNBS, or C rodentium. These processes required GPR41 and GPR43 and were required to recruit leukocytes and activate effector T cells in the intestine. GPR41 and GPR43 activated extracellular signal-regulated kinase 1/2 and p38 mitogen-activated protein kinase signaling pathways in epithelial cells to induce production of chemokines and cytokines during immune responses.

CONCLUSIONS

SCFAs activate GPR41 and GPR43 on intestinal epithelial cells, leading to mitogen-activated protein kinase signaling and rapid production of chemokines and cytokines. These pathways mediate protective immunity and tissue inflammation in mice.

Bone marrow-derived alternatively activated macrophages reduce colitis without promoting fibrosis: participation of IL-10

Alternatively activated macrophages (AAMs) (or M2a) can inhibit colitis but may also be associated with fibrosis. Thus, by using the dinitrobenzene sulfonic (DNBS) murine model of colitis, this study aimed to determine whether 1) bone marrow (BM)-derived AAMs could reduce colitis, 2) any anticolitic effect of BM-AAMs was IL-10 dependent, and 3) repeated AAM treatments remained effective and were associated with fibrosis in the gut or other tissues. Balb/c mice received AAMs (10(6) intraperitoneally) from wild-type (WT) or IL-10(-/-) mice 48 h prior to DNBS (3 mg intrarectally) with disease assessed 72 h later, or they received three doses of DNBS at 2-wk intervals ± AAMs 6 h post-DNBS to mimic a treatment regimen. DNBS-treated mice developed colitis; this was significantly less severe in mice receiving WT AAMs and less so in animals given IL-10(-/-) AAMs, indicating a role for IL-10 in the inhibition of DNBS-driven colitis. Similarly, after the third AAM treatment lesser colonic histopathology was observed compared with time-matched DNBS-only-treated animals, and notably there was no evidence of increased fibroses in the colon, terminal ileum, lung, or liver of AAM-treated mice as assessed by quantitative PCR for prolyl-4-hydrolase, α-smooth muscle actin, and collagen (type IIIα) and histochemical and biochemical assessment of collagen deposition. This study provides mechanistic insight to the anticolitic capacity of AAMs and indicates that repeated adoptive transfer of ex vivo programmed BM-AAMs is safe and efficacious in the treatment of DNBS-induced murine colitis, providing additional support for their consideration as an immunotherapy.

Endothelial and epithelial barriers in graft-versus-host disease

Endothelial and epithelial cells form selectively permeable barriers that separate tissue compartments. These cells coordinate movement between the lumen and tissue via the transcellular and paracellular pathways. The primary determinant of paracellular permeability is the tight junction, which forms an apical belt-like structure around endothelial and epithelial cells. This chapter discusses endothelial and epithelial barriers in graft-versus-host disease after allogeneic bone marrow transplantation, with a focus on the tight junction and its role in regulating paracellular permeability. Recent studies suggest that in graft-versus-host disease, pathological increases in paracellular permeability, or barrier dysfunction, are initiated by pretransplant conditioning and sustained by alloreactive cells and the proinflammatory milieu. The intestinal epithelium is a significant focus, as it is a target organ of graft-versus-host disease, and the mechanisms of barrier regulation in intestinal epithelium have been well characterized. Finally, we propose a model that incorporates endothelial and epithelial barrier dysfunction in graft-versus-host disease and discuss modulating barrier properties as a therapeutic approach.

Gate-keeper function of the intestinal epithelium

There is currently a major focus on the role of the gut barrier function in balancing mucosal immune responses. Increased epithelial permeability for exogenous antigens is a crucial primary or secondary event in the pathogenesis of several disorders affecting body surfaces and beyond. The epithelial gate-keeper function is determined by the individual's age (e.g. preterm vs. term infant), diet, genetics, mucus composition, interactions between mast cells, nerves and neuropeptides, concurrent infection, the commensal microbiota and the epithelium-shielding effect of secretory IgA (SIgA) antibodies provided by breast milk or produced in the individual's gut. The integrity of the epithelial barrier furthermore depends on homeostatic regulatory mechanisms, including mucosal induction of regulatory T cells, where commensal microbiota-host interactions apparently play decisive roles. Thus, both extrinsic and intrinsic factors have been identified that may have an impact on the dynamics of the epithelial cell-cell junctions in the gut and thereby increase or reduce paracellular permeability. Experiments have shown that SIgA normally cooperates with innate defence factors to protect the epithelium and reinforce its barrier function. In the absence of SIgA commensal gut bacteria overstimulate innate epithelial immunity at the expense of expression of genes that regulate fat and carbohydrate metabolism, resulting in an epithelial gene signature that correlates with the development of lipid malabsorption. This shows that the intestinal epithelial barrier is a cross-road between defence and nutrition, and that SIgA is essential to keep the balance between these two functions.

The short isoform of the CEACAM1 receptor in intestinal T cells regulates mucosal immunity and homeostasis via Tfh cell induction

Carcinoembryonic antigen cell adhesion molecule like I (CEACAM1) is expressed on activated T cells and signals through either a long (L) cytoplasmic tail containing immune receptor tyrosine based inhibitory motifs, which provide inhibitory function, or a short (S) cytoplasmic tail with an unknown role. Previous studies on peripheral T cells show that CEACAM1-L isoforms predominate with little to no detectable CEACAM1-S isoforms in mouse and human. We show here that this was not the case in tissue resident T cells of intestines and gut associated lymphoid tissues, which demonstrated predominant expression of CEACAM1-S isoforms relative to CEACAM1-L isoforms in human and mouse. This tissue resident predominance of CEACAM1-S expression was determined by the intestinal environment where it served a stimulatory function leading to the regulation of T cell subsets associated with the generation of secretory IgA immunity, the regulation of mucosal commensalism, and defense of the barrier against enteropathogens.

The impact of perinatal immune development on mucosal homeostasis and chronic inflammation

The mucosal surfaces of the gut and airways have important barrier functions and regulate the induction of immunological tolerance. The rapidly increasing incidence of chronic inflammatory disorders of these surfaces, such as inflammatory bowel disease and asthma, indicates that the immune functions of these mucosae are becoming disrupted in humans. Recent data indicate that events in prenatal and neonatal life orchestrate mucosal homeostasis. Several environmental factors promote the perinatal programming of the immune system, including colonization of the gut and airways by commensal microorganisms. These complex microbial-host interactions operate in a delicate temporal and spatial manner and have an important role in the induction of homeostatic mechanisms.

Intestinal epithelial barrier dysfunction in food hypersensitivity

Intestinal epithelial barrier plays a critical role in the maintenance of gut homeostasis by limiting the penetration of luminal bacteria and dietary allergens, yet allowing antigen sampling for the generation of tolerance. Undigested proteins normally do not gain access to the lamina propria due to physical exclusion by tight junctions at the cell-cell contact sites and intracellular degradation by lysosomal enzymes in enterocytes. An intriguing question then arises: how do macromolecular food antigens cross the epithelial barrier? This review discusses the epithelial barrier dysfunction in sensitized intestine with special emphasis on the molecular mechanism of the enhanced transcytotic rates of allergens. The sensitization phase of allergy is characterized by antigen-induced cross-linking of IgE bound to high affinity FcεRI on mast cell surface, leading to anaphylactic responses. Recent studies have demonstrated that prior to mast cell activation, food allergens are transported in large quantity across the epithelium and are protected from lysosomal degradation by binding to cell surface IgE and low-affinity receptor CD23/FcεRII. Improved immunotherapies are currently under study including anti-IgE and anti-CD23 antibodies for the management of atopic disorders.

NOD2, an intracellular innate immune sensor involved in host defense and Crohn's disease

Nucleotide-binding oligomerization domain 2 (NOD2) is an intracellular sensor for small peptides derived from the bacterial cell wall component, peptidoglycan. Recent studies have uncovered unexpected functions of NOD2 in innate immune responses such as induction of type I interferon and facilitation of autophagy; moreover, they have disclosed extensive cross-talk between NOD2 and Toll-like receptors, which has an indispensable role both in host defense against microbial infection and in the development of autoimmunity. Of particular interest, polymorphisms of CARD15 encoding NOD2 are associated with Crohn's disease and other autoimmune states such as graft vs. host disease. In this review, we summarize recent findings regarding normal functions of NOD2 and discuss the mechanisms by which NOD2 polymorphisms associated with Crohn's disease lead to intestinal inflammation.

Lower expression of tight junction protein 1 gene and increased FOXP3 expression in the small bowel mucosa in coeliac disease and associated type 1 diabetes mellitus

BACKGROUND

The role of regulatory T cells expressing FOXP3 in the pathogenesis of coeliac disease (CD) and type 1 diabetes (T1D) has been reported. Recent data have placed special focus on the interplay between the intestinal barrier and immunoregulatory processes. We aimed to determine whether the expression of tight junction protein 1 (TJP1), which reflects small bowel mucosa permeability, is changed in CD and T1D.

METHODS

Transcription levels of TJP1 and FOXP3 genes were evaluated in the small bowel biopsies of 14 children with CD, 12 with CD and coexisting T1D and 40 controls using real-time PCR. Serum IgA and IgG to deamidated gliadin, bovine β-lactoglobulin, bovine α-casein and human tissue transglutaminase (tTG) were determined by ELISA.

RESULTS

The highest expression of FOXP3 mRNA was seen in patients with CD and T1D compared to patients with CD alone and controls (p = 0.02). In contrast, the lowest level of TJP1 mRNA expression was found in patients with CD and T1D (p = 0.01). The levels of IgA to deamidated gliadin and tTG were highest in patients with CD and T1D (p = 0.0001 and 0.01, respectively). The expression of FOXP3 mRNA correlated highly with the level of anti-gliadin IgA (p = 0.02) and anti-tTG IgA antibodies (p = 0.004).

CONCLUSION

The significant decline in TJP1 expression in CD patients, particularly in those with coexisting T1D, was accompanied by an increase in FOXP3 expression. This might reflect an attempt to maintain immune tolerance to counterbalance the loss of mucosal integrity in the small intestine in CD associated with T1D.

The gut as communicator between environment and host: immunological consequences

During human evolution, the mucosal immune system developed two anti-inflammatory mechanisms: immune exclusion by secretory antibodies (SIgA and SIgM) to control epithelial colonization of microorganisms and inhibit penetration of harmful substances; and immunosuppression to counteract local and peripheral hypersensitivity against innocuous antigens such as food proteins. The latter function is referred to as oral tolerance when induced via the gut. Similar mechanisms also control immunity to commensal bacteria. The development of immune homeostasis depends on "windows of opportunity" where adaptive and innate immunities are coordinated by antigen-presenting cells; their function is not only influenced by microbial products but also by dietary constituents, including vitamin A and lipids like polyunsaturated omega-3 fatty acids. These factors can in several ways exert beneficial effects on the immunophenotype of the infant. Also breast milk provides immune-modulating factors and SIgA antibodies - reinforcing the gut barrier. Mucosal immunity is most abundantly expressed in the gut, and the intestinal mucosa of an adult contains at least 80% of the body's activated B cells - terminally differentiated to plasmablasts and plasma cells (PCs). Most mucosal PCs produce dimeric IgA which is exported by secretory epithelia expressing the polymeric Ig receptor (pIgR), also called membrane secretory component (SC). Immune exclusion is therefore performed mainly by SIgA. Notably, pIgR knockout mice which lack SIgs show increased uptake of food and microbial antigens and they have a hyper-reactive immune system with disposition for anaphylaxis; but this untoward development is counteracted by cognate oral tolerance induction as a homeostatic back-up mechanism.

Tight junction pore and leak pathways: a dynamic duo

Tissue barriers that restrict passage of liquids, ions, and larger solutes are essential for the development of multicellular organisms. In simple organisms this allows distinct cell types to interface with the external environment. In more complex species, the diversity of cell types capable of forming barriers increases dramatically. Although the plasma membranes of these barrier-forming cells prevent flux of most hydrophilic solutes, the paracellular, or shunt, pathway between cells must also be sealed. This function is accomplished in vertebrates by the zonula occludens, or tight junction. The tight junction barrier is not absolute but is selectively permeable and is able to discriminate between solutes on the basis of size and charge. Many tight junction components have been identified over the past 20 years, and recent progress has provided new insights into the proteins and interactions that regulate structure and function. This review presents these data in a historical context and proposes an integrated model in which dynamic regulation of tight junction protein interactions determines barrier function.

Tight junction pore and leak pathways: a dynamic duo

Tissue barriers that restrict passage of liquids, ions, and larger solutes are essential for the development of multicellular organisms. In simple organisms this allows distinct cell types to interface with the external environment. In more complex species, the diversity of cell types capable of forming barriers increases dramatically. Although the plasma membranes of these barrier-forming cells prevent flux of most hydrophilic solutes, the paracellular, or shunt, pathway between cells must also be sealed. This function is accomplished in vertebrates by the zonula occludens, or tight junction. The tight junction barrier is not absolute but is selectively permeable and is able to discriminate between solutes on the basis of size and charge. Many tight junction components have been identified over the past 20 years, and recent progress has provided new insights into the proteins and interactions that regulate structure and function. This review presents these data in a historical context and proposes an integrated model in which dynamic regulation of tight junction protein interactions determines barrier function.