Immunoregulation by 1,25-dihydroxyvitamin D3: basic concepts.

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), the biologically active metabolite of Vitamin D(3), not only regulates bone and calcium metabolism but also exerts other biological activities, including immunomodulation via the nuclear Vitamin D receptor expressed in antigen-presenting cells and activated T cells. This regulation is mediated through interference with nuclear transcription factors such as NF-AT and NF-kappaB or by direct interaction with Vitamin D responsive elements in the promoter regions of cytokine genes. Dendritic cells (DCs) are primary targets for the immunomodulatory activity of 1,25(OH)(2)D(3), as indicated by inhibited DC differentiation and maturation, leading to down-regulated expression of MHC-II, costimulatory molecules and IL-12. Moreover, 1,25(OH)(2)D(3) enhances IL-10 production and promotes DC apoptosis. Together, these effects of 1,25(OH)(2)D(3) inhibit DC-dependent T cell activation. Immunomodulation by 1,25(OH)(2)D(3) and its analogs in vivo has been demonstrated in different models of autoimmune diseases and transplantation. Moreover, combining analogs with other immunosuppressants leads to synergism in models of autoimmunity and transplantation. The availability of 1,25(OH)(2)D(3) analogs with immunomodulatory activity at non-hypercalcemic doses may allow exploitation of their immunomodulatory effects in a clinical setting of treatment of autoimmune diseases and prevention of allograft rejection.