Tumor hypoxia drives suppressor function in exhausted T cells limiting antitumor immunity

Summary: While CD8+ cytotoxic T cells are clearly critical for identification and elimination of cancer cells, factors concentrated within the tumor microenvironment drive altered differentiation of these cells to a hypofunctional, short-lived state termed T cell exhaustion. Exhaustion is a progressive lineage, and it is now clear that terminally exhausted T (tTexh) cells are not the targets of checkpoint blockade immunotherapy but rather serve as factors that limit immunotherapeutic efficacy. Compared directly, tumor infiltrating CD8+ tTexh cells bear notable phenotypic similarity to CD4+Foxp3+ regulatory T (Treg) cells, upregulating immunosuppressive effector molecules in this terminally differentiated state. These data suggest that beyond loss of proinflammatory function, tTexh cells may be directly anti-functional and constrain tumor-specific immunity. Thus, we hypothesize that tTexh cells potentiate the suppressive microenvironment of solid tumor and that strategies to limit their generation or reprogram their immunosuppressive nature will improve control of tumor progression. Here we show, when isolated from the same tumor microenvironment, tTexh cells, but not their CD8+ progenitors, carried similar capacity to suppress T cell proliferation as CD4+Foxp3+ Treg cells. Unlike Treg cells, tTexh cells suppress not through numerous overlapping mechanisms but solely via the ectonucleotidase, CD39. As CD8+ cells differentiate to exhaustion, CD39 expression increases, restricting available extracellular ATP (eATP) and facilitating a suppressive adenosinergic microenvironment. Specific deletion of CD39 in CD8+ T cells slows tumor progression and improves response to checkpoint blockade therapy. CD39 expression in tTexh cells is driven by exposure to tumor hypoxia, such that therapeutic targeting of hypoxia limits the suppressive capacity for tTexh. Our data support a model that as CD8+ T cells progress to terminal exhaustion, hypoxia exposure enforces the upregulation of CD39, providing tTexh cells a mechanism to suppress proinflammatory processes and to aid tumor progression. These findings suggest tTexh cells are not solely dysfunctional but rather are deleterious to antitumor immunity and represent an unappreciated immunoregulatory population. Strategies should be designed to either limit their generation, reprogram their immunosuppressive nature, or deplete them from the tumor microenvironment to improve patient outcomes and response to current therapeutic options.