Foxp3+ regulatory T (Treg) cells are crucial for restraining inflammation in a variety of autoimmune diseases, including type 1 diabetes (T1D). However, the transcriptional and functional phenotypes of Treg cells within the pancreatic lesion remain poorly understood. Here we characterized pancreas-infiltrating Treg cells in the NOD mouse model of T1D and uncovered a substantial enrichment of the Treg subpopulation expressing the chemokine receptor CXCR3. Accumulation of CXCR3 + Treg cells within pancreatic islets was dependent on the transcription factor T-BET, and genetic ablation of T-BET increased the onset and penetrance of disease, abrogating the sex bias normally seen in the NOD model. Both male and female mice lacking T-BET + Treg cells showed a more aggressive insulitic infiltrate, reflected most prominently by elevated production of type 1 cytokines. Our results suggest the possibility of fine therapeutic targeting of Treg cells, in a tissue-and cell-subset-specific fashion, as a more focused immunotherapy for T1D.immunoregulation | immunogenomics | Treg subsets