Beta2-glycoprotein I binds thrombin via exosite I and exosite II: Anti-β2-glycoprotein I antibodies potentiate the inhibitory effect of β2-glycoprotein I on thrombin-mediated factor XIa generation

Abstract

Objective. Beta2-glycoprotein I (β2GPI) is a dominant antigenic target in antiphospholipid syndrome (APS). Beta 2-glycoprotein I may bind to factor XI and serve a physiologic function as a regulator of factor XI activation by thrombin. We undertook this study to investigate the possible interactions of β2GPI with thrombin in β2GPI-regulated factor XI activation by thrombin and to evaluate the effect of anti-β2GPI antibodies on this system. Methods. The β2GPI interaction with thrombin was investigated in direct and competitive assays using β2GPI domain mutants and thrombin-binding exosite oligonucleotides. Beta2-glycoprotein I inhibition of thrombin-mediated factor XI activation was assessed in the presence of 8 anti-β2GPI monoclonal antibodies (mAb) directed against domain I. Results. Domain V of β2GPI was involved in direct binding to thrombin, and exosite I and exosite II on thrombin took part in this interaction. Anti-β2GPI mAb produced a >70% inhibition of thrombin-mediated factor XI activation in the presence of β2GPI. Conclusion. We demonstrate that β2GPI interacts with thrombin exosites I and II. This novel finding necessitates a reinterpretation of previous studies from which the detection of anti-human thrombin antibodies in APS has been reported. We also show that anti-β2GPI antibodies potentiate the inhibitory effect of β2GPI on thrombin-mediated factor XIa generation. © 2007, American College of Rheumatology.