Figure 1. Pivotal roles of vitamins in the maintenance of immunologic homeostasis in the gut. Vitamin A-derived retinoic acid promotes the differentiation of naive T cells to Treg cells and simultaneously inhibits the induction of Th17 cells in the steady state. Like retinoic acid, Vitamin D (as an active form1α,25-dihydroxyvitamin D₃) inhibits the production of pro-inflammatory cytokines such as IFN-γ, IL-17 and IL-21 from T cells together with the promoted differentiation of Treg cells. It also prevents differentiation and maturation of DCs and increases the expression of tight junction protein such as claudins in the epithelial cells. Upon the differentiation of Treg cells, they express high levels of vitamin B9 receptor (folate receptor 4, FR4), which essential for their survival. α-tocopherol, an isoform of vitamin E, can inhibit T cell infiltration into intestine through the negative regulation of signal transduction from VCAM-1 and ICAM-1 by antagonizing protein kinase C.