The role of the Thrombin/PAR axis in modulating CD8+ T cell anti-tumor immunity

Abstract CD8+ tumor infiltrating lymphocytes (TIL) are critical for tumor clearance following immune checkpoint inhibitor (ICI) therapy. Thrombin, the central hemostatic protease, is activated in the tumor microenvironment and CD8+ T cells express the thrombin-cleavable protease activated receptors (PAR-1, -2, -3, -4). Recent studies showed that thrombin can enhance CD8+ T cell effector functions, but the role of thrombin in ICI is unknown. We hypothesized that thrombin enhances CD8+ TIL anti-tumor effector functions (αTEF) during ICI therapy. Consistent with this hypothesis, we found that reducing circulating prothrombin to ~10% of normal completely abrogated the efficacy of PD-1 blockade therapy to reduce tumor growth, reduced CD8+ TIL infiltration into tumor tissue, and limited αTEF. Mechanistic in vitro studies showed that thrombin stimulation of isolated murine or human CD8+ T cells resulted in a dose-dependent increase in survival, proliferation and cytokine production (activation profile). Surprisingly, when we stimulated CD8+ T cells lacking PAR-1 or PAR-2, we observed an increase in their activation profile relative to WT CD8+T cells in the absence of thrombin, which was further enhanced with thrombin. Moreover, stimulation of PAR-2 deficient CD8+ T cells with a specific PAR-1 activating peptide resulted in an increased activation profile compared to WT CD8+ T cells stimulated with the same peptide. Together these data suggest that 1) thrombin promotes adaptive tumor immunity in the context of ICI therapy, 2) thrombin promotes αTEF through PAR-1 dependent and independent mechanisms and 3) PAR-2 suppresses CD8+ T cell functions independently of thrombin and limits the capacity of PAR-1 activation to enhance effector functions. Supported by grants from NIH T32 (5T32AI118697-04)