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.

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.

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.

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.