The immunological implications of the new Vitamin D metabolism

Vitamin D is actually a neurohormone whose pleiotropic activities encompass regulation of calcium-phosphate metabolism, cell proliferation and immunomodulation. Starting from a cutaneous compound, 2 hydroxylation steps are required to produce the active form of vitamin D3, named calcitriol [1, 25-(OH)2-cholecalciferol]. The second hydroxylation step may occur at different tis- sues and cell types, including kidney, lung, prostate, brain, immune cells and placenta. Based on the advancing knowledge of Cytochrome P450 functions, a new conception of Vitamin D metabolism emerged. It implies that, depending on the site where the second hydroxylation step occurs, the active hormone can act as a calcium-phosphorus-homeostasis regulator, or an immune system modulator, or a cell proliferation and differentiation regulator. A detailed description of new Vitamin D metabolism and Vitamin D regulation of immune response is provided in this review.