Anti–β2‐glycoprotein I antibodies in complex with β2‐glycoprotein I can activate platelets in a dysregulated manner via glycoprotein Ib‐IX‐V

Abstract

Objective Results of previous studies suggest that anti–β₂-glycoprotein I (anti-β₂GPI) antibodies in complex with β₂GPI activate platelets in a dysregulated manner, potentially contributing to the prothrombotic tendency associated with the antiphospholipid syndrome (APS). We undertook this study to investigate the possible contribution of the GPIb-IX-V receptor to platelet activation mediated by the anti-β₂GPI antibody–β₂GPI complex. Methods In vitro methods were used in the present study. The interaction between β₂GPI and the GPIbα subunit of the GPIb-IX-V receptor was delineated using direct binding and competitive inhibition assays. The interaction between the anti-β₂GPI antibody–β₂GPI complex and platelets was studied using a novel method in which the Fc portion of the antibody was immobilized using protein A coated onto a microtiter plate. Platelet activation was assessed by two methods; one involved measuring thromboxane B₂ production and the other involved assessment of the activation of the phosphatidylinositol 3-kinase/Akt/glycogen synthase kinase 3β intracellular signaling pathway. The contribution of the GPIbα receptor to platelet activation induced by the anti-β₂GPI antibody–β₂GPI complex was assessed by observing the influence of 2 anti-GPIbα antibodies (AK2 and SZ2) directed against distinct epitopes. Results This study showed that β₂GPI could bind to the GPIbα receptor. The anti-β₂GPI antibody–β₂GPI complex was able to activate platelets, and this effect was inhibited by anti-GPIbα antibody directed against epitope Leu-36–Gln-59, but not by anti-GPIbα antibody directed against residues Tyr-276–Glu-282. Conclusion Our findings show that inappropriate platelet activation by the anti-β₂GPI antibody–β₂GPI complex via the GPIbα receptor may contribute to the prothrombotic tendency associated with APS.