CD4 CD25 regulatory cells control the development of autoimmunity, including type 1 diabetes, and the transcription factor Foxp3 is crucial for the development and function of these cells. The decreased effectiveness of regulatory T cell function in diabetes-prone NOD mice is due to the failure of NOD APC to activate regulatory T cells properly. In the present study, we examined the parameters that modulate Foxp3 expression in CD4 CD25 regulatory cells. We found that CD4 CD25 cells from congenic diabetes-resistant NOD mice exhibited intermediate levels of Foxp3 compared with regulatory cells from B6 mice and Sick NOD mice. Using an in vitro Foxp3 induction system, we found that APC from diabetes-resistant congenic NOD mice also exhibited an intermediate ability to maintain expression of Foxp3 in CD4 CD25 cells. Finally, NOD APC pre-treated with a microbial component, lipoteichoic acid, regained their ability to maintain Foxp3 expression in CD4 CD25 cells in vitro. APC from CFA-treated NOD mice that were transferred into recipient NOD mice could induce Foxp3 expression in CD4 CD25 cells in vivo. Altogether, these data suggest that Foxp3 expression in CD4 CD25 cells from NOD mice can be modulated by optimal stimulation by APC.