A novel antiplatelet antibody therapy that induces cAMP-dependent endocytosis of the GPVI/Fc receptor γ-chain complex

Platelet adhesion to vascular subendothelium, mediated in part by interactions between collagen and glycoprotein VI (GPVI) complexed with Fc receptor gamma-chain, is crucial for thrombus formation. Antiplatelet therapy benefits patients with various thrombotic and ischemic diseases, but the safety and efficacy of existing treatments are limited. Recent data suggest GPVI as a promising target for a novel antiplatelet therapy, for example, GPVI-specific Abs that deplete GPVI from the surface of platelets. Here, we characterized GPVI-specific auto-Abs (YA-Abs) from the first reported patient with ongoing platelet GPVI deficiency caused by the YA-Abs. To obtain experimentally useful human GPVI-specific mAbs with characteristics similar to YA-Abs, we generated human GPVI-specific mouse mAbs and selected 2 representative mAbs, mF1201 and mF1232, whose binding to GPVI was inhibited by YA-Abs. In vitro, mF1201, but not m171232, induced human platelet activation and GPVI shedding, and mF1232 inhibited collagen-induced human platelet aggregation. Administration of mF1201 and m171232 to monkeys caused GPVI immunodepletion with and without both significant thrombocytopenia and GPVI shedding, respectively. When a human/mouse chimeric form of mF1232 (cF1232) was labeled with a fluorescent endocytosis probe and administered to monkeys, fluorescence increased in circulating platelets and surface GPVI was lost. Loss of platelet surface GPVI mediated by cF1232 was successfully reproduced in vitro in the presence of a cAMP-elevating agent. Thus, we have characterized cAMP-dependent endocytosis of GPVI mediated by a human GPVI-specific mAb as what we believe to be a novel antiplatelet therapy.