Thrombin contributes to cancer immune evasion via proteolysis of platelet-bound GARP for LTGF-β activation

Abstract

Abstract Cancer-associated thrombocytosis and high concentration of circulating TGFβ1 are hallmarks of progressive tumor disease in cancer patients. Using genetic and pharmacological approaches, we show a direct link between thrombin catalytic activity and maturation of LTGFβ bound to platelet GARP. We found that GARP protein has two conserved thrombin binding sites through which thrombin cleaves platelet-bound GARP to release active TGFβ from GARP/LTGFβ complex. Crystal structure models docked thrombin onto the concave surface of GARP supporting the notion that thrombin is able to bind and cleave GARP/LTGFβ complex. We observed that inhibition of thrombin obliterates TGFβ maturation in platelet releasate and in serum of WT mice, which translates in better tumor control alone and in combination with checkpoint blockade therapy, in two tumor preclinical models. Of note, the reduction of serum active TGFβ and the anti-tumor properties of thrombin were not observed in absence of platelet GARP. In the tumor microenvironment we found that blocking thrombin reduced active TGFβ signaling and increased the infiltrating CD8+ T cells as well as the innate NK cells and neutrophils. Also, we demonstrated that thrombin cleavage releases soluble GARP/LTGFβ complex and that high concentration of GARP in plasma, alone or in complex with LTGFβ, is a biomarker of poor clinical outcome in prostate cancer patients. We conclude that thrombin-GARP interaction and consequent GARP cleavage induce the activation of TGFβ derived from platelets. Considering that platelets are the major TGFβ reservoir, these observations demonstrated that blocking GARP cleavage opens a new therapeutic window to overcome TGFβ-mediated resistance to immune check point inhibitors.