The vitamin A metabolite retinoic acid (RA) has been reported to suppress Th1 responses and enhance Th2 responses. Here, we investigated whether differences in vitamin A metabolism could underlie the differences between C57BL/6 and BALB/c mice, which are reportedly seen as Th1 and Th2 responders, respectively. BALB/c mice were shown to have higher intestinal epithelial expression of RALDH1 (where RALDH is retinaldehyde dehydrogenase), and, consequently, higher RALDH activity in MLN-DCs, leading to an increased ability to induce IgA class switching in B cells. Furthermore, within BALB/c mice, induction of IgA secretion as well as increased accumulation of regulatory T cells (Treg) in the intestinal lamina propria was observed. Additionally, as BALB/c mice are more resistant to dextran sulphate sodium (DSS) induced colitis, mice that lacked vitamin A in their diet had a more severe form of DSS-induced colitis compared to control mice. Therefore, the level of RA production and consequently the degree of RA-mediated signaling is crucial for the efficiency of the mucosal immune system.
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