We previously demonstrated that immunotherapy with dendritic cells (DC) prevented diabetes development in prediabetic NOD mice and that this effect was optimal when using a stimulatory DC population generated from bone marrow cells cultured with granulocyte-macrophage colony-stimulating factor (GM-CSF) and IL-4. In this study, we have investigated the mechanism by which GM-CSF- and IL-4-cultured DC prevent diabetes in prediabetic NOD mice. Histological analysis of pancreatic tissue from DC-treated mice revealed a reduction in the severity of insulitis compared to controls. Analysisof the T cell response in DC-treated mice suggested a general shift towards a Th2-dominated response, as determined by cytokine production following either concanavalin A or anti-TCR stimulation. Furthermore, sorted CD45RB(lo) CD25+ CD4+ T cells from the spleen of DC-treated mice produced high amounts of Th2 cytokines following anti-TCR stimulation, suggesting that these cells are responsible for the apparent Th2 shift. We conclude that DC therapy may have corrected the immunoregulatory defect in the NOD mouse, thus restoring a balance between pathogenic Th1 cells and protective Th2 cells.