Respective roles of Glycoprotein VI and FcγRIIA in the regulation of αIIbβ3-mediated platelet activation to fibrinogen, thrombus buildup, and stability

Background: The interplay between platelets and fibrinogen is the cornerstone of thrombus formation. Integrin αIIbβ3 is the main platelet adhesion receptor for fibrinogen and mediates an outside-in signal upon ligand binding that reinforces platelet activation. In addition, FcγRIIA and glycoprotein VI (GPVI) contribute to platelet activation on fibrinogen, thereby participating in thrombus growth and stability. To date, the relative importance of these two immunoreceptor tyrosine-based activation motif-bearing receptors in these processes remains unknown. Objective: The aim of this study was to evaluate the relative contributions of FcγRIIA and GPVI to platelet activation on fibrinogen and subsequent thrombus growth and stability. Methods: We evaluated human and mouse platelet adhesion to fibrinogen in static assays and a flow-based approach to evaluate the contribution of FcγRIIA and GPVI to thrombus growth and stability. Results: We first confirmed that integrin αIIbβ3 is the key receptor supporting platelet adhesion and spreading on fibrinogen. Using human platelets treated with pharmacological blocking agents and transgenic mouse platelets expressing human receptors, data indicate that GPVI, but not FcγRIIA, plays a prominent role in platelet activation on fibrinogen. Moreover, using a flow-based assay, we observed that blockade of GPVI with 1G5, but not FcγRIIA with IV.3, prevents thrombus growth. Finally, we observed that 1G5, but not IV.3, promotes the disaggregation of thrombi formed on collagen in vitro. Conclusion: This study provides evidence that GPVI, but not FcγRIIA, induces platelet activation and spreading on fibrinogen, and promotes thrombus buildup and stability.