High Antigenicity for Treg Cells Confers Resistance to PD-1 Blockade Therapy via High PD-1 Expression in Treg Cells

Regulatory T (Treg) cells have an immunosuppressive function, and programmed death-1 (PD-1)-expressing Treg cells reportedly induce resistance to PD-1 blockade therapies through their reactivation. However, the effects of antigenicity on PD-1 expression in Treg cells and the resistance to PD-1 blockade therapy remain unclear. Here, we show that Treg cells gain high PD-1 expression through an antigen with high antigenicity. Additionally, tumors with high antigenicity for Treg cells were resistant to PD-1 blockade in vivo due to PD-1+ Treg-cell infiltration. Because such PD-1+ Treg cells have high cytotoxic T lymphocyte antigen (CTLA)-4 expression, resistance could be overcome by combination with an anti-CTLA-4 monoclonal antibody (mAb). Patients who responded to combination therapy with anti-PD-1 and anti-CTLA-4 mAbs sequentially after primary resistance to PD-1 blockade monotherapy showed high Treg cell infiltration. We propose that the high antigenicity of Treg cells confers resistance to PD-1 blockade therapy via high PD-1 expression in Treg cells, which can be overcome by combination therapy with an anti-CTLA-4 mAb.