Dendritic cells (DCs) not only induce but also modulate T cell activation. 1,25-Dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] induces DCs with a tolerogenic phenotype, characterized by decreased expression of CD40, CD80, and CD86 co-stimulatory molecules, low IL-12, and enhanced IL-10 secretion. We have found that a short treatment with 1,25-(OH)(2)D(3) induces tolerance to fully mismatched mouse islet allografts, and that this tolerance is stable to challenge with donor-type spleen cells and allows acceptance of donor-type vascularized heart grafts. This effect is enhanced by co-administration of mycophenolate mofetil (MMF), a selective inhibitor of T and B cell proliferation, that also has effects similar to 1,25-(OH)(2)D(3) on DCs. Graft acceptance is associated with impaired development of type 1 CD4(+) and CD8(+) cells and an increased percentage of CD4(+)CD25(+) regulatory cells expressing CD152 in the spleen and in the draining lymph node. Transfer of CD4(+)CD25(+) cells from tolerant mice protects 100% of the syngeneic recipients from islet allograft rejection. CD4(+)CD25(+) cells that are able to inhibit the T cell response to a pancreatic autoantigen and to significantly delay disease transfer by pathogenic CD4(+)CD25(-) cells are also induced by treatment of adult nonobese diabetic (NOD) mice with a selected vitamin D receptor (VDR) ligand. This treatment arrests progression of insulitis and Th1 cell infiltration, and inhibits diabetes development at non-hypercalcemic doses. The enhancement of CD4(+)CD25(+) regulatory T cells able to mediate transplantation tolerance and to arrest type 1 diabetes development by a short oral treatment with small organic compounds that induce tolerogenic DCs, like VDR ligands, suggests possible clinical applications of this approach.