1,25-Dihydroxyvitamin D3 restores sensitivity to cyclophosphamide-induced apoptosis in non-obese diabetic (NOD) mice and protects against diabetes

The activated form of vitamin D, 1,25(OH)₂D₃, and its analogues can prevent type I diabetes in NOD mice. Protection is achieved without signs of systemic immunosuppression and is associated with a restoration of the defective immune regulator system of the NOD mice. The aim of the present study was to investigate whether this restoration of regulator cell function is the only mechanism in the prevention of diabetes by 1,25(OH)₂D₃. We tested therefore if 1,25(OH)₂D₃ could prevent cyclophosphamide-induced diabetes, since diabetes occurring after cyclophosphamide injection is believed to be due to an elimination of suppresser cells. NOD mice treated with 1,25(OH)₂D₃ (5 μg/kg every 2 days) from the time of weaning were clearly protected against diabetes 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-transfer experiments it was demonstrated that cyclophosphamide had indeed eliminated the suppresser cells present in 1,25(OH)₂D₃-treated mice. Since cyclophosphamide injection did not break the protection offered by 1,25(OH)₂D₃, it was clear that diabetogenic effector cells were affected by 1,25(OH)₂D₃ treatment as well. This was confirmed by the finding that splenocytes from 1,25(OH)₂D₃-treated mice were less capable of transferring diabetes in young, irradiated NOD mice, and by the demonstration of lower Th1 cytokine levels in the pancreases of 1,25(OH)₂D₃-treated, cyclophosphamide-injected mice. This better elimination of effector cells in 1,25(OH)₂D₃-treated mice could be explained by a restoration of the sensitivity to cyclophosphamide-induced apoptosis in both thymocytes and splenocytes, in normally apoptosis-resistant NOD mice. Altogether, these data indicate that the protection against diabetes offered by 1,25(OH)₂D₃ may be independent of the presence of suppresser cells, and may involve increased apoptosis of Th1 autoimmune effector cells.