TLR-induced local metabolism of vitamin D3 plays an important role in the diversification of adaptive immune responses

The addition of monophosphoryl lipid A, a minimally toxic derivative of LPS, to nonmucosally administered vaccines induced both systemic and mucosal immune responses to coadministered Ags. This was dependent on an up-regulated expression of 1α-hydroxylase (CYP27B1, 1αOHase), the enzyme that converts 25-hydroxycholecalciferol, a circulating inactive metabolite of vitamin D3, into 1,25(OH)2D3 (calcitriol). In response to locally produced calcitriol, myeloid dendritic cells (DCs) migrated from cutaneous vaccination sites into multiple secondary lymphoid organs, including classical inductive sites of mucosal immunity, where they effectively stimulated B and T cell immune responses. The endogenous production of calcitriol by monophosphoryl lipid A-stimulated DCs appeared to be Toll-IL-1R domain-containing adapter-inducing IFN-β-dependent, mediated through a type 1 IFN-induced expression of 1αOHase. Responsiveness to calcitriol was essential to promote the trafficking of mobilized DCs to nondraining lymphoid organs. Collectively, these studies help to expand our understanding of the physiologically important roles played by locally metabolized vitamin D3 in the initiation and diversification of adaptive immune responses. The influences of locally produced calcitriol on the migration of activated DCs from sites of vaccination/infection into both draining and nondraining lymphoid organs create a condition whereby Ag-responsive B and T cells residing in multiple lymphoid organs are able to simultaneously engage in the induction of adaptive immune responses to peripherally administered Ags as if they were responding to an infection of peripheral or mucosal tissues they were designed to protect.