Differential Regulation of the Platelet GPIb‐IX Complex by Anti‐gpibβ Antibodies

Background Platelets' initial recognition of endothelial damage proceeds through the interaction between collagen, plasma von Willebrand factor (VWF), and the platelet glycoprotein (GP)Ib-IX complex (CD42). The GPIb-IX complex consists of one GPIb alpha, one GPIX, and two GPIb beta subunits. Once platelets are immobilized to the subendothelial matrix, shear generated by blood flow unfolds a membrane-proximal mechanosensory domain (MSD) in GPIb alpha, exposing a conserved trigger sequence and activating the receptor. Currently, GPIb alpha appears to solely facilitate ligand-induced activation because it contains both the MSD and the binding sites for all known ligands to GPIb-IX. Despite being positioned directly adjacent to the MSD, the roles of GPIb beta and GPIX in signal transduction remain murky. Objectives To characterize a novel rat monoclonal antibody 3G6 that binds GPIb beta. Methods Effects of 3G6 on activation of GPIb-IX are characterized in platelets and Chinese hamster ovary cells expressing GPIb-IX (CHO-Ib-IX) and compared with those of an inhibitory anti-GPIb beta antibody, RAM.1. Results Both RAM.1 and 3G6 bind to purified GPIb beta and GPIb-IX with high affinity. 3G6 potentiates GPIb-IX-associated filopodia formation in platelets or CHO-Ib-IX when they adhere VWF or antibodies against the ligand-binding domain (LBD) of GPIb alpha. Pretreatment with 3G6 also increased anti-LBD antibody-induced GPIb-IX activation. Conversely, RAM.1 inhibits nearly all GPIb-IX-related signaling in platelets and CHO-Ib-IX cells. Conclusions These data represent the first report of a positive modulator of GPIb-IX activation. The divergent modulatory effects of 3G6 and RAM.1, both targeting GPIb beta, strongly suggest that changes in the conformation of GPIb beta underlie outside-in activation via GPIb-IX.