Anti-PD-1 amplifies costimulation in melanoma-infiltrating Th1-like Foxp3+ regulatory T cells to alleviate local immunosuppression.

Background: Immune checkpoint inhibitors targeting programmed cell death protein-1 (PD-1) are the first line of treatment for many solid tumors including melanoma. PD-1 blockade enhances the effector functions of melanoma-infiltrating CD8⁺ T cells, leading to durable tumor remissions. However, 55% of patients with melanoma do not respond to treatment. As Foxp3⁺ regulatory T (T_reg) cells play an important role in tumor-induced immunosuppression and express PD-1, we hypothesized that anti-PD-1 also increases the functions of melanoma-infiltrating T_reg cells, which could be detrimental to treatment efficacy.

Methods: The cellular and functional dynamics of T_reg cells were evaluated in C57Bl/6 Foxp3-reporter mice bearing highly immunogenic and PD-1 blockade-sensitive Yale University Mouse Melanoma Exposed to Radiation 1.7 (YUMMER1.7) tumors. T_reg cell responses in tumors and lymphoid compartments were examined throughout tumor growth or therapy and were assessed ex vivo by multiparametric flow cytometry analysis, with in vitro suppression assays using tumor-infiltrating lymphocytes isolated by fluorescence-activated cell sorting (FACS) and in situ through spatial proteomic and transcriptomic profiling.

Results: In this highly immunogenic melanoma model, anti-PD-1 monotherapy yielded high responders (HRs) and low responders (LRs). We show that the potent CD8⁺ T cell responses characteristic of HR tumors paradoxically coincide with the expansion of highly-activated, Helios-expressing T_reg cells. In both HRs and LRs, T_reg cells co-localize with CD8⁺ T cells in immunogenic regions of the tumor and display potent suppressive capacity in vitro. Further characterization revealed that melanoma-infiltrating T_reg cells progressively acquire T-bet and interferon gamma expression, exclusively in HRs, and induction of this T helper cell 1 (T_h1)-like phenotype in vitro led to CD8⁺ T cell evasion from T_reg cell-mediated suppression. Using spatial proteomic and transcriptomic profiling, we demonstrate that T_reg cells display an increased activity of PI3K/Akt signaling in regions of HR tumors with an elevated CD8:T_reg cell ratio.

Conclusions: PD-1 blockade promotes the expansion of a subset of highly-activated T_reg cells coexpressing PD-1 and Helios. While these cells are potently suppressive outside tumor environments, costimulatory and inflammatory signals present in the tumor microenvironment lead to their local acquisition of T_h1-like characteristics and loss of suppression of effector T cells.