Early graft failure, graft rejection, and autoimmune recurrence remain unresolved issues in islet xenotransplantation in type 1 diabetes. The first aim of this study was to examine the existence of early graft failure in spontaneously diabetic autoimmune NOD mice after rat islet transplantation under technically controlled circumstances. The second aim was to examine the mediators of this early xenograft dysfunction. First, we demonstrated a higher percentage of early xenograft failure (48%) in spontaneously diabetic NOD mice as compared with chemically diabetic old NOD (13%, P < 0.05) and C57Bl/6 (7%, P < 0.01) mice. In addition, in spontaneously diabetic NOD mice, xenogeneic islets displayed early graft failure more frequently than allogeneic (23%, P < or = 0.05) or isogeneic islets (7%, P < 0.01). No early graft failure was observed in allotransplantation or isotransplantation in chemically diabetic mice. Reverse transcriptase-polymerase chain reaction analysis of cytokine mRNA in islet xenografts 8 h after transplantation showed higher levels of interleukin (IL)-1 mRNA in autoimmune diabetic mice compared with chemically diabetic old NOD mice (1.40 +/- 0.32 vs. 0.90 +/- 0.14 IL-1 copies/beta-actin copies, P < 0.05). In contrast, mRNA levels of transforming growth factor (TGF)-beta were lower in spontaneously diabetic NOD mice than in chemically diabetic old NOD mice (0.67 +/- 0.16 vs. 1.36 +/- 0.50 TGF-beta copies/beta-actin copies, P < 0.05). No differences in tumor necrosis factor-alpha, IL-6, and inducible nitric oxide synthase were seen between autoimmune and nonautoimmune diabetic mice. T-cell cytokines (IL-2, IL-4, IL-10, and gamma-interferon) were absent in all mice until 48 h after transplantation. These data suggest that early islet xenograft failure is more common in spontaneously diabetic NOD mice and could be due to a nonspecific inflammatory reaction locally in the grafts.