1,25-DIHYDROXYVITAMIN D-3 RESTORES SENSITIVITY TO CYCLOPHOSPHAMIDE-INDUCED APOPTOSIS IN NONOBESE DIABETIC (NOD) MICE AND PROTECTS AGAINST DIABETES

The activated form of vitamin D, 1,25(OH)(2)D-3, and its analogues can prevent type I diabetes in NOD mice. Protection is achieved without s igns of systemic immunosuppression and is associated with a restoratio n of the defective immune regulator system of the NOD mice. The aim of the present study was to investigate whether this restoration of regu lator cell function is the only mechanism in the prevention of diabete s by 1,25(OH)(2)D-3. We tested therefore if 1,25(OH)(2)D-3 could preve nt cyclophosphamide induced diabetes, since diabetes occurring after c yclophosphamide injection is believed to be due to an elimination of s uppresser cells. NOD mice treated with 1,25(OH)(2)D-3 (5 mu g/kg every 2 days) from the time of weaning were clearly protected against diabe tes induced by cyclophosphamide (200 mg/kg body wt at 70 days old) (2/ 12 (17%) versus 36/53 (68%) in control mice, P < 0.005). By co-transfe r experiments it was demonstrated that cyclophosphamide had indeed eli minated the suppresser cells present in 1,25(OH)(2)D-3-treated mice. S ince cyclophosphamide injection did not break the protection offered b y 1,25(OH)(2)D-3, it was clear that diabetogenic effector cells were a ffected by 1,25(OH)(2)D-3 treatment as well. This was confirmed by the finding that splenocytes from 1,25(OH)(2)D-3-treated mice were less c apable of transferring diabetes in young, irradiated NOD mice, and by the demonstration of lower Th 1 cytokine levels in the pancreases of 1 ,25(OH)(2)D-3-treated, cyclophosphamide-injected mice. This better eli mination of effector cells in 1,25(OH)(2)D-3-treated mice could be exp lained by a restoration of the sensitivity to cyclophosphamide-induced apoptosis in both thymocytes and splenocytes, in normally apoptosis-r esistant NOD mice. Altogether, these data indicate that the protection against diabetes offered by 1,25(OH)(2)D-3 may be independent of the presence of suppresser cells, and may involve increased apoptosis of T h1 autoimmune effector cells.