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

Although Foxp3-expressing 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, depletion of which provoked anti-tumor responses and conferred potent therapeutic effects by increasing CD8+ T cell numbers and cytolytic function. 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. Hence, tumor-resident Treg cells highly expressed T-bet, which was essential for their development and function. 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. Treg cells contain several functional subsets analogous to CD4+ helper T (Th) cells. Thus, we analyzed if intra-tumoral pTreg cells were similar to any of the Th subsets in their gene expression. We performed transcriptomic analyses on intra-tumoral Treg cells derived from both Hepa1-6 and E.G7 murine tumor models by RNA sequencing. Given that intra-tumoral Treg cells were composed of a substantial proportion of pTreg cells whereas Treg cells derived from lymph nodes (LN) were mainly composed of tTreg cells, we included Treg cells purified from tumor-free LNs as a control. Intra-tumoral Treg cells were sorted from E.G7 and Hepa1-6 tumor-bearing Foxp3YFP-cre mice on day 21 and day 23, respectively. Treg cells sorted from LNs of tumor-free mice were included as a control.