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

Objective. Results of previous studies suggest that anti-beta(2)-glycoprotein I (anti-beta(2)GPI) antibodies in complex with beta(2)GPI 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-beta(2)GPI antibody-beta(2)GPI complex. Methods. In vitro methods were used in the present study. The interaction between beta(2)GPI and the GPIb alpha subunit of the GPIb-IX-V receptor was delineated using direct binding and competitive inhibition assays. The interaction between the anti-beta(2)GPI antibody-beta(2)GPI 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 B-2 production and the other involved assessment of the activation of the phosphatidylinositol 3-kinase/Akt/glycogen synthase kinase 3 beta intracellular signaling pathway. The contribution of the GPIb alpha receptor to platelet activation induced by the anti-beta(2)GPI antibody-beta(2)GPI complex was assessed by observing the influence of 2 anti-GPIb alpha antibodies (AK2 and SZ2) directed against distinct epitopes. Results. This study showed that beta(2)GPI could bind to the GPIba receptor. The anti-beta(2)GPI antibody-beta(2)GPI complex was able to activate platelets, and this effect was inhibited by anti-GPIb alpha antibody directed against epitope Leu-36-Gln-59, but not by anti-GPIb alpha antibody directed against residues Tyr-276-Glu-282. Conclusion. Our findings show that inappropriate platelet activation by the anti-beta(2)GPI antibody-beta(2)GPI complex via the GPIb alpha receptor may contribute to the prothrombotic tendency associated with APS.