Curcumin induces the tolerogenic dendritic cell that promotes differentiation of intestine-protective regulatory T cells

The gut is home to a large number of Treg, with both CD4⁺ CD25⁺ Treg and bacterial antigen-specific Tr1 cells present in normal mouse intestinal lamina propria. It has been shown recently that intestinal mucosal DC are able to induce Foxp3⁺ Treg through production of TGF-β plus retinoic acid (RA). However, the factors instructing DC toward this mucosal phenotype are currently unknown. Curcumin has been shown to possess a number of biologic activities including the inhibition of NF-κB signaling. We asked whether curcumin could modulate DC to be tolerogenic whose function could mimic mucosal DC. We report here that curcumin modulated BM-derived DC to express ALDH1a and IL-10. These curcumin-treated DC induced differentiation of naïve CD4⁺ T cells into Treg resembling Treg in the intestine, including both CD4⁺CD25⁺ Foxp3 ⁺ Treg and IL-10-producing Tr1 cells. Such Treg induction required IL-10, TGF-β and retinoic acid produced by curcumin-modulated DC. Cell contact as well as IL-10 and TGF-β production were involved in the function of such induced Treg. More importantly, these Treg inhibited antigen-specific T-cell activation in vitro and inhibited colitis due to antigen-specific pathogenic T cells in vivo.