TGFβ1-Mediated SMAD3 Enhances PD-1 Expression on Antigen-Specific T Cells in Cancer

Programmed death-1 (PD-1) is a coinhibitory receptor that downregulates the activity of tumor-infiltrating lymphocytes (TIL) in cancer and of virus-specific T cells in chronic infection. The molecular mechanisms driving high PD-1 expression on TILs have not been fully investigated. We demonstrate that TGF beta 1 enhances antigen-induced PD-1 expression through SMAD3-dependent, SMAD2-independent transcriptional activation in T cells in vitro and in TILs in vivo. The PD-1 hi subset seen in CD8(+) TILs is absent in Smad3-deficient tumor-specific CD8(+) TILs, resulting in enhanced cytokine production by TILs and in draining lymph nodes and antitumor activity. In addition to TGF beta 1's previously known effects on T-cell function, our findings suggest that TGF beta 1 mediates T-cell suppression via PD-1 upregulation in the tumor microenvironment (TME). They highlight bidirectional cross-talk between effector TILs and TGF beta-producing cells that upregulates multiple components of the PD-1 signaling pathway to inhibit antitumor immunity. SIGNIFICANCE: Engagement of the coinhibitory receptor PD-1 or its ligand, PD-L1, dramatically inhibits the antitumor function of TILs within the TME. Our findings represent a novel immunosuppressive function of TGF beta and demonstrate that TGF beta 1 allows tumors to evade host immune responses in part through enhanced SMAD3-mediated PD-1 expression on TILs.