Regulatory T cells converted from T helper 1 cells in tumors suppress cancer immunity via CD39

Although regulatory T (Treg) cells are widely believed to impede anti-tumor immunity, their regulation and functional mechanisms are not well understood. Here, through characterization of multiple cancer models, we identified substantial periphery-induced Treg cells in the tumor microenvironment (TME), depletion of which provoked anti-tumor responses and conferred potent therapeutic effects by increasing functional CD8+ T cells. Through fate-mapping and transfer experiments, we found IFN-γ-expressing T helper (Th) 1 cells developed into Treg cells in tumors, in response to TGF-β signaling. Pseudotime trajectory further revealed the terminal differentiation of Th1-like Treg cells from Th1 cells in the TME. Hence, tumor-resident Treg cells highly expressed T-bet, which was essential for their function in the TME. CD39, highly expressed by T-bet+ Treg cells in both mouse and human tumors, is required for Treg suppression of CD8+ T cell response. In summary, our study has elucidated a developmental pathway of intra-tumoral Treg cells and implicated new ways of targeting them in cancer patients.