[Vitamin D and the immune system]

There is now increasing evidence that the hormonal form of vitamin D, 1,25(OH)2D3, is involved in the regulation of the immune system. Local production of the hormone in various infectious diseases can benefit the immune environment. 1,25(OH)2D3 exerts most of its actions only after it has bound to its specific nuclear receptor. These receptors are present in monocytes and activated lymphocytes. The hormone inhibits lymphocyte proliferation and immunoglobulin production in a dose-dependent fashion. It also blocks the accumulation of the mRNAs for IL-2, IFN-gamma and GM-CSF. It interferes with T helper cell (Th) function, reducing Th-induction of immunoglobulin production by B-cells and inhibits the passive transfer of cellular immunity by Th in vivo. The steroid hormone promotes suppressor cell activity and inhibits the generation of cytotoxic and NK cells. The expression of Class II antigen by lymphocytes and monocytes is also affected. In vivo, 1,25(OH)2D3 is particularly effective in preventing auto-immune diseases such as experimental auto-immune encephalomyelitis, murine lupus, and diabetes in NOD mice. Synthetic analogues of vitamin D3 that bind to receptors but have no hypercalcemic effect in vivo have recently been developed for therapeutic use.