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Nonobese diabetic (NOD) mice develop type I diabetes spontaneously and have been utilized as a model for human autoimmune insulin-dependent diabetes. The disease is caused by the destruction of insulin-producing beta cells in the pancreatic islet of Langerhans by infiltrating inflammatory cells, which are primarily T lymphocytes. The incidence of diabetes in NOD mice is increased in females compared with males, suggesting that sex steroid hormones play an important role in the development of the disease. We therefore investigated the effect of a male steroid, 5-alpha-dihydrotestosterone (5DHT), on disease development, T-cell phenotype, T-cell proliferation, and cytokine profiles in this model. None of the mice that received 5DHT for 120 days (n = 7) developed insulitis, whereas all control mice (n = 8) developed the disease. The percentage of CD4+ T cells in peripheral blood mononuclear cells was markedly decreased in the 5DHT-treated females compared with those in controls (37.1 +/- 4.8 vs 51.3 +/- 9.3, P < 0.02), whereas no significant differences in the percentage of CD8+ T cells were observed between treated and control female mice. Results of a syngeneic mixed lymphocyte reaction (SMLR) also suggested that T cells are major target cells of 5DHT administration. An increased expression of IL-4 mRNA, representing T helper 2 (Th2) T cells, was observed in the SMLR. On the basis of these results, a systemic administration of 5DHT appears to have direct effects on the expansion of Th2 cell populations with subsequent restoration of normal immune responses.