1,25-Dihydroxyvitamin D3 and the immune system.

There is substantial evidence that lymphocytes and monocytes are targets for the actions of the hormonal form of vitamin D, 1,25-(OH)2D3 and that 1,25-(OH)2D3 acts to modulate the proliferation, differentiation, and immune functions of these cells. The effects of the hormone on lymphocytes are mediated directly as well as indirectly via the accessory monocytes. Depending upon the presence or absence of monocytes and the mode of lymphocyte activation, 1,25-(OH)2D3 can either stimulate or suppress lymphocytes. This evidence as well as clinical information and in vivo studies support a role of 1,25-(OH)2D3 in immunobiology. The physiologic, pathophysiologic, and pharmacologic implications of the immunomodulating properties of 1,25-(OH)2D3 however have not been well established.