Administration of TNF-alpha to autoimmune diabetes-prone nonobese diabetic mice and biobreeding rats inhibits diabetes development; however, the mechanism(s) of diabetes prevention by TNF-alpha has not been established. We used the model of syngeneic islet transplantation into diabetic nonobese diabetic mice to study the effects of TNF-alpha administration on the types of mononuclear cells and cytokines expressed in the islet grafts and on autoimmune diabetes recurrence. Twice daily i.p. injections of TNF-alpha (20 microg/day) from day 1 to day 30 after islet transplantation significantly prolonged islet graft survival; thus, 70% (16 of 23) of mice treated with TNF-alpha were normoglycemic at 30 days after islet transplantation compared with none (0 of 14) of vehicle-treated control mice. Islet grafts and spleens from TNF-alpha-treated mice at 10 days after islet transplantation contained significantly fewer CD4+ and CD8+ T cells, and significantly decreased mRNA levels of type 1 cytokines (IFN-gamma, IL-2, and TNF-beta) than islet grafts and spleens from control mice. Regarding type 2 cytokines, IL-4 mRNA levels were not changed significantly in islet grafts or spleens of TNF-alpha-treated mice, whereas IL-10 mRNA levels were decreased significantly in islet grafts of TNF-alpha-treated mice and not significantly changed in spleens. TGF-beta mRNA levels in islet grafts and spleens were similar in TNF-alpha-treated and control mice. These results suggest that TNF-alpha partially protects beta cells in syngeneic islet grafts from recurrent autoimmune destruction by reducing CD4+ and CD8+ T cells and down-regulating type 1 cytokines, both systemically and locally in the islet graft.