Beta2-glycoprotein I binds thrombin via exosite I and exosite II: anti-beta2-glycoprotein I antibodies potentiate the inhibitory effect of beta2-glycoprotein I on thrombin-mediated factor XIa generation.
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.


CONCLUSIONS
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.
Rahgozar
Soheila
S
St George Hospital, University of New South Wales, Sydney, New South Wales, Australia.
Yang
Qiuxia
Q
Giannakopoulos
Bill
B
Yan
Xiaokai
X
Miyakis
Spiros
S
Krilis
Steven A
SA
D000818:Animals; D000838:Anions / Q000378:metabolism; D000906:Antibodies / Q000276:immunology / Q000378:metabolism; D017152:Antibodies, Antiphospholipid / Q000276:immunology / Q000502:physiology; D000911:Antibodies, Monoclonal / Q000276:immunology / Q000494:pharmacology; D016736:Antiphospholipid Syndrome / Q000276:immunology / Q000503:physiopathology; D001665:Binding Sites / Q000502:physiology; D015945:Factor XIa / Q000378:metabolism*; D006801:Humans; D051379:Mice; D009154:Mutation / Q000235:genetics; D009841:Oligonucleotides / Q000235:genetics; D011485:Protein Binding / Q000502:physiology; D017434:Protein Structure, Tertiary; D011516:Prothrombin / Q000378:metabolism; D013917:Thrombin / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism*; D053482:beta 2-Glycoprotein I / Q000737:chemistry* / Q000235:genetics / Q000276:immunology / Q000378:metabolism*
D000838:Anions; D000906:Antibodies; D017152:Antibodies, Antiphospholipid; D000911:Antibodies, Monoclonal; D009841:Oligonucleotides; D053482:beta 2-Glycoprotein I; D011516:Prothrombin; C025337:Factor IIa; D015945:Factor XIa; D013917:Thrombin