Expression of the co-stimulatory molecule B7-1 (CD80) on pancreatic β cells can overcome peripheral T cell tolerance In transgenic models of autoimmune disease. This study aimed to determine if aberrant B7-1 or B7-2 (CD86) expression on pancreatic β cells is involved in the pathogenesis of autoimmune diabetes in non-obese diabetic (NOD) mice. Immunohistochemlcal analysis of NOD pancreas sections revealed no evidence of B7-1 or B7-2 expression on pancreatic β cells at any stage prior to the onset of either spontaneously arising or cyclophosphamldeaccelerated diabetes. Likewise, the NOD-derived NIT-1 β cell line did not express surface B7 orB7-1 mRNA either constitutively or following exposure to IFN-γ and TNF-α, two cytokines known to be present in the Insulitis lesion of NOD mice, or cAMP which can induce B7-1 expression on B cells. Both B7-1 and B7-2 were, however, highly expressed on the majority of islet-infiltrating inflammatory cells in NOD mice between days 7 and 12 after the administration of cyclophosphamide which results in accelerated β cell destruction. Likewise B7-1 and B7-2 were extensively expressed on islet-infiltrating cells present at the time of diabetes onset in NOD SCID mice with adoptively transferred diabetes. By immunohistochemistry and flow cytometry, it was determined that the phenotype of B7+ cells in the pancreas of NOD mice 9 days after cyclophosphamide included a mixture of macrophages and both CD4+ and CD8+ T cells. B7-2 was also expressed on islet-infiltrating cells in the spontaneously occurring diabetes of female NOD mice, but the levels of B7-1 expression were low in comparison with the accelerated models of diabetes. RIP-IL-2 transgenic mice, which have extensive islet infiltration but no autoimmune β cell destruction, also had virtually no B7-1 expression and a minority of B7-2-expressing inflammatory cells. Thus, the activation of β cell-specific T cells in NOD mice does not appear to be a result of aberrant expression of B7 on the β cells. Expression of B7-1 and B7-2 on islet-infiltrating cells is, however, associated with autoimmune β cell destruction, suggesting a role for the B7-CD28 interaction in this process.