CD25 Identifies a Subset of CD4+FoxP3− TIL That Are Exhausted Yet Prognostically Favorable in Human Ovarian Cancer

CD25, the alpha subunit of the interleukin-2 (IL-2) receptor, is a canonical marker of regulatory T cells (Tregs) and hence has been implicated in immune suppression in cancer. However, CD25 is also required for optimal expansion and activity of effector T cells in peripheral tissues. Thus, we hypothesized that CD25, in addition to demarcating Tregs, might identify effector T cells in cancer. To investigate this, we used multi-parameter flow cytometry and immunohistochemistry to analyze tumor-infiltrating lymphocytes (TIL) in primary high-grade serous carcinomas (HGSC), the most common and fatal subtype of ovarian cancer. CD25 was expressed primarily by CD4+ TIL, with negligible expression by CD8+ TIL. In addition to conventional CD25+FoxP3+ Tregs, we identified a subset of CD25+FoxP3- T cells that comprised up to 13% of CD4+ TIL. In tumors with CD8+ TIL, CD25+FoxP3- T cells showed a strong positive association with patient survival (HR 0.56, P=0.02), which exceeded the negative effect of Tregs (HR 1.55, P=0.09). Amongst CD4+ TIL subsets, CD25+FoxP3- cells expressed the highest levels of PD-1. Moreover, after in vitro stimulation they failed to produce common T helper cytokines (IFN-γ, TNF-α, IL-2, IL-4, IL-10 or IL-17A), suggesting they were functionally exhausted. In contrast, the more abundant CD25-FoxP3- subset of CD4+ TIL expressed low levels of PD-1 and produced T helper 1 cytokines, yet conferred no prognostic benefit. Thus, CD25 identifies a subset of CD4+FoxP3- TIL that, despite being exhausted at diagnosis, have a strong, positive association with patient survival and warrant consideration as effector T cells for immunotherapy.