Intervention in autoimmunity: the potential of vitamin D receptor agonists.

Vitamin D receptor (VDR) agonists are well known for their capacity to control calcium metabolism and to regulate growth and differentiation of many cell types. More recently, it has become clear that VDR agonists possess immunoregulatory properties and, in particular, pronounced pro-tolerogenic activities. VDR agonists can act directly on T cells, but DCs appear to be their primary targets. The capacity of VDR agonists to modulate DC and T cell functions is mediated by VDR expression in both cell types and by the presence of common targets in their signal transduction pathways, such as the nuclear factor NF-kappaB that is downregulated by VDR agonists in APCs and in T cells. A potentially very important activity of VDR agonists is their capacity to induce in vitro and in vivo tolerogenic DCs able to enhance CD4+CD25+ suppressor T cells that, in turn, inhibit Th1 cell responses. These mechanisms of action can explain some of the immunoregulatory properties of VDR agonists in the treatment of Th1-mediated autoimmune diseases, but may also represent a physiologic element in the VDR-mediated regulation of innate and adaptive immune responses.