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1,25-Dihyroxyvitamin D3 Promotes FOXP3 Expression via Binding to Vitamin D Response Elements in Its Conserved Noncoding Sequence Region

FOXP3-positive regulatory T (Treg) cells are a unique subset of T cells with immune regulatory properties. Treg cells can be induced from non-Treg CD4+ T cells (induced Treg [iTreg] cells) by TCR triggering, IL-2, and TGF-β or retinoic acid. 1,25-Dihyroxyvitamin D3 [1,25(OH)2VD3] affects the functions of immune cells including T cells. 1,25(OH)2VD3 binds the nuclear VD receptor (VDR) that binds target DNA sequences known as the VD response element (VDRE). Although 1,25(OH)2VD3 can promote FOXP3 expression in CD4+ T cells with TCR triggering and IL-2, it is unknown whether this effect of 1,25(OH)2VD3 is mediated through direct binding of VDR to the FOXP3 gene without involving other molecules. Also, it is unclear whether FOXP3 expression in 1,25(OH)2VD3-induced Treg (VD-iTreg) cells is critical for the inhibitory function of these cells. In this study, we demonstrated the presence of VDREs in the intronic conserved noncoding sequence region +1714 to +2554 of the human FOXP3 gene and the enhancement of the FOXP3 promoter activity by such VDREs in response to 1,25(OH)2VD3. Additionally, VD-iTreg cells suppressed the proliferation of target CD4+ T cells and this activity was dependent on FOXP3 expression. These findings suggest that 1,25(OH)2VD3 can affect human immune responses by regulating FOXP3 expression in CD4+ T cells through direct VDR binding to the FOXP3 gene, which is essential for inhibitory function of VD-iTreg cells.