ANTI–ß2-GLYCOPROTEIN I ANTIBODIES IN COMPLEX WITH ß2-GLYCOPROTEIN I CAN ACTIVATE PLATELETS IN A DYSREGULATED MANNER VIA GLYCOPROTEIN IB-IX-V

Tong Shi, Bill Giannakopoulos, Xiaokai Yan, Pei Yu, Michael C. Berndt, Robert K. Andrews, Juan Rivera, G. Michael Iverson, Keith A. Cockerill, Matthew D. Linnik, and Steven A. Krilis

Abstract

Objective. Results of previous studies suggest that anti–ß2-glycoprotein I (anti-ß2GPI) antibodies in complex with ß2GPI 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-ß2GPI antibody–ß2GPI complex.

Methods. In vitro methods were used in the present study. The interaction between ß2GPI and the GPIba subunit of the GPIb-IX-V receptor was delineated using direct binding and competitive inhibition assays. The interaction between the anti-ß2GPI antibody–ß2GPI 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 3ß intracellular signaling pathway. The contribution of the GPIba receptor to platelet activation induced by the anti-ß2GPI antibody–ß2GPI complex was assessed by observing the influence of 2 anti-GPIba antibodies (AK2 and SZ2) directed against distinct epitopes.

Results. This study showed that ß2GPI could bind to the GPIba receptor. The anti-ß2GPI antibody–ß2GPI complex was able to activate platelets, and this effect was inhibited by anti-GPIba antibody directed against epitope Leu-36–Gln-59, but not by anti-GPIba antibody directed against residues Tyr-276–Glu-282.

Conclusion. Our findings show that inappropriate platelet activation by the anti-ß2GPI antibody–ß2GPI complex via the GPIba receptor may contribute to the prothrombotic tendency associated with APS.

Published in
Arthritis & Rheumatism
August 2006
Volume 54, No. 8, pp 2558-2567