Retinoic acid attenuates ileitis by restoring the balance between T-helper 17 and T regulatory cells.

BACKGROUND & AIMS: Retinoic acid (RA), produced by intestinal epithelial cells (IECs) and dendritic cells (DCs) promotes the induction of regulatory T cells (Tregs) and decreases the induction of T-helper (Th)17 cells. METHODS: We studied the roles of RA in mice that overproduce tumor necrosis factor (TNF) and develop chronic ileitis (TNF_ARE mice). We assessed the frequency and function of CD103+ DCs, Th17 cells, and Tregs by flow cytometry, and we measured expression of cytokines and retinaldehyde dehydrogenase (RALDH) enzymes in ileum samples, DCs, and IECs by real-time polymerase chain reaction. We quantified RA by electrochemical analysis and examined the effect of RA supplementation on TNF-induced ileitis using histologic, coculture, and suppression assays and flow cytometry. RESULTS: Numbers of CD103+ DCs decreased in the inflamed ilea of mice with chronic disease; RA synthetic machinery (RALDH1,2) was down-regulated. Nevertheless, the proportion of CD4+, CD25+, FoxP3+ Tregs increased, indicating an alternate source for RA. IECs responded to reduced levels of RA by up-regulating RALDH3 in vivo and in vitro. Net tissue levels of RA remained lower in TNF+ARE than wild-type mice, indicating that epithelial up-regulation of RALDH3 could not maintain adequate concentrations of RA, probably because of loss of IEC mass. RA supplementation significantly attenuated disease by increasing the number and function of CD103+ DCs and Tregs and reducing Th17 cells. CONCLUSIONS: Reduced levels of RA appear to induce IECs to up-regulate synthesis of RA. RA supplementation attenuates ileitis through its effects on CD103+ DCs, Tregs, and Th17 cells. RA supplementation might offer therapeutic benefit in Crohn's disease.

An endogenously anti-inflammatory role for methylation in mucosal inflammation identified through metabolite profiling.

Tissues of the mucosa are lined by an epithelium that provides barrier and transport functions. It is now appreciated that inflammatory responses in inflammatory bowel diseases are accompanied by striking shifts in tissue metabolism. In this paper, we examined global metabolic consequences of mucosal inflammation using both in vitro and in vivo models of disease. Initial analysis of the metabolic signature elicited by inflammation in epithelial models and in colonic tissue isolated from murine colitis demonstrated that levels of specific metabolites associated with cellular methylation reactions are significantly altered by model inflammatory systems. Furthermore, expression of enzymes central to all cellular methylation, S-adenosylmethionine synthetase and S-adenosylhomocysteine hydrolase, are increased in response to inflammation. Subsequent studies showed that DNA methylation is substantially increased during inflammation and that epithelial NF-κB activity is significantly inhibited following treatment with a reversible S-adenosylhomocysteine hydrolase inhibitor, DZ2002. Finally, these studies demonstrated that inhibition of cellular methylation in a murine model of colitis results in disease exacerbation while folate supplementation to promote methylation partially ameliorates the severity of murine colitis. Taken together, these results identify a global change in methylation, which during inflammation, translates to an overall protective role in mucosal epithelia.