| Summary: | Background & Aims: Mesalamine is a first-line drug for treatment of inflammatory bowel diseases (IBD). However, its mechanisms are not fully understood. CD4+ Foxp3+ regulatory T cells (Tregs) play a potential role in suppressing IBD. This study determined whether the anti-inflammatory activity of mesalamine is related to Treg induction in the colon. Methods: We examined the frequencies of Tregs in the colons of wild-type mice, mice deficient for aryl hydrocarbon receptor (AhR-/- mice), and bone marrowâchimeric mice lacking AhR in hematopoietic cells (BM-AhR-/- mice), following oral treatment with mesalamine. We also examined the effects of mesalamine on transforming growth factor (TGF)-β expression in the colon. Results: Treatment of wild-type mice with mesalamine increased the accumulation of Tregs in the colon and up-regulated the AhR target gene Cyp1A1, but this effect was not observed in AhR-/- or BM-AhR-/- mice. In addition, mesalamine promoted in vitro differentiation of naive T cells to Tregs, concomitant with AhR activation. Mice treated with mesalamine exhibited increased levels of the active form of TGF-β in the colon in an AhR-dependent manner and blockade of TGF-β signaling suppressed induction of Tregs by mesalamine in the colon. Furthermore, mice pretreated with mesalamine acquired resistance to dextran sodium sulfateâinduced colitis. Conclusions: We propose a novel anti-inflammatory mechanism of mesalamine for colitis: induction of Tregs in the colon via the AhR pathway, followed by TGF-β activation. Keywords: Mesalamine, Aryl Hydrocarbon Receptor, TGF-β, Regulatory T Cells
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