Interactions of 1,25-dihydroxyvitamin D3 and the immune system.

A series of recent discoveries indicate that the hormonal form of vitamin D3, namely, 1,25(OH)2D3 plays a role in the regulation of the immune system. Cells of the monocyte/macrophage lineage possess receptors for 1,25(OH)2D3 regardless of their activation stage; cells of the lymphoid lineage also express these receptors but only at certain stages of their differentiation pathway and upon activation. Further, 1,25(OH)2D3 promotes the differentiation of monocyte precursors towards monocyte/macrophages and enhances monocyte function in antigen presentation. In addition 1,25(OH)2D3 is a potent inhibitor of interleukin-2 (IL-2) and suppresses effector functions of both T and B lymphocytes via IL-2-dependent as well as via IL-2-independent mechanisms. The theoretical and clinical implications of these discoveries are discussed.