Objective: Beta(2)-glycoprotein I (beta(2)GPI) 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 beta(2)GPI with thrombin in beta(2)GPI-regulated factor XI activation by thrombin and to evaluate the effect of anti-beta(2)GPI antibodies on this system.
Methods: The beta(2)GPI interaction with thrombin was investigated in direct and competitive assays using beta(2)GPI domain mutants and thrombin-binding exosite oligonucleotides. Beta(2)-glycoprotein I inhibition of thrombin-mediated factor XI activation was assessed in the presence of 8 anti-beta(2)GPI monoclonal antibodies (mAb) directed against domain I.
Results: Domain V of beta(2)GPI was involved in direct binding to thrombin, and exosite I and exosite II on thrombin took part in this interaction. Anti-beta(2)GPI mAb produced a >70% inhibition of thrombin-mediated factor XI activation in the presence of beta(2)GPI.
Conclusion: We demonstrate that beta(2)GPI 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-beta(2)GPI antibodies potentiate the inhibitory effect of beta(2)GPI on thrombin-mediated factor XIa generation.