Interruption of the intratumor CD8+ T cell:Treg crosstalk improves the efficacy of PD-1 immunotherapy

PD -1 blockade unleashes potent antitumor activity in CD8 + T cells but can also promote immunosuppressive T regulatory (Treg) cells, which may worsen the response to immunotherapy. Tumor-Treg inhibition is a promising strategy to improve the efficacy of checkpoint blockade immunotherapy; however, our understanding of the mechanisms supporting tumor-Tregs during PD -1 immunotherapy is incomplete. Here, we show that PD -1 blockade increases tumor-Tregs in mouse models of melanoma and metastatic melanoma patients. Mechanistically, Treg accumulation is not caused by Treg-intrinsic inhibition of PD -1 signaling but depends on an indirect effect of activated CD8 + T cells. CD8 + T cells produce IL -2 and colocalize with Tregs in mouse and human melanomas. IL -2 upregulates the anti-apoptotic protein ICOS on tumor-Tregs, promoting their accumulation. Inhibition of ICOS signaling before PD -1 immunotherapy improves control over immunogenic melanoma. Thus, interrupting the intratumor CD8 + T cell:Treg crosstalk represents a strategy to enhance the therapeutic efficacy of PD -1 immunotherapy.