title: 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.
abstract: 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.
authors.affiliation: St George Hospital, University of New South Wales, Sydney, New South Wales, Australia.
authors.forename: Soheila
authors.initials: S
authors.lastname: Rahgozar
authors.forename: Qiuxia
authors.initials: Q
authors.lastname: Yang
authors.forename: Bill
authors.initials: B
authors.lastname: Giannakopoulos
authors.forename: Xiaokai
authors.initials: X
authors.lastname: Yan
authors.forename: Spiros
authors.initials: S
authors.lastname: Miyakis
authors.forename: Steven A
authors.initials: SA
authors.lastname: Krilis
mesh_terms: 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*
chemical_list: 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