Vitamin D has become an important player beyond bone and calcium metabolism. Receptors for 1,25(OH)2D3 or calcitriol as well as the machinery to produce 1,25(OH)2D3 locally are present all over the immune system. Regulation of the enzyme responsible for the final activation of 1,25-dihydroxyvitamin D3 (CYP27B1) is however completely different in immune cells than in kidney cells. Particular immunomodulating effects of 1,25(OH)2D3 are observed on dendritic cells, the central cell in the immune system, determining the balance between tolerance and autoimmunity.
Type 1 diabetes is an autoimmune disease where the insulin producing cell of the pancreas, the β-cell, is under attack. Vitamin D not only has effects on the immune system, but also affects the behaviour of the β-cell in a way that will alter its fate. Vitamin D deficiency dramatically affects disease presentation in an animal models of type 1 diabetes, whereas NOD mice lacking the vitamin D receptor have an unaltered diabetes presentation. Thus, vitamin D and its structural analogues may provide tools for the prevention or cure of type 1 diabetes.