Slowed Release of Thrombin-cleaved Factor VIII from von Willebrand Factor by a Monoclonal and a Human Antibody Is a Novel Mechanism for Factor VIII Inhibition

These results suggest the existence of a second mechanism of inhibition of factor VIII coagulant activity by anti-C2 antibodies, which we have investigated in this study

Evgueni L. Saenko

2002

Scholarcy highlights

  • The plasma glycoprotein factor VIII1 functions as a cofactor for the factor Xase enzyme complex of the intrinsic ʈ Recipient of a Clinical Investigator Award HL02587. ** To whom correspondence should be addressed: American Red Cross, 15601 Crabbs Branch Way, Rockville, MD 20855
  • This possibility was tested by incubation of increasing concentrations of monoclonal antibody ESH8 IgG or FabЈ with factor VIII in the presence or absence of a 6-fold excess of von Willebrand factor, which was followed by activation of fVIII by thrombin and determination of fVIIIa activity in the factor Xase assay. mAb ESH8 inhibited factor Xase activity Ͼ90% only in the presence of vWf
  • Our examination of the effect of mAb ESH8 on the ability of activated fVIII to act as a cofactor for factor IXa in the activation of factor X demonstrated that ESH8 was inhibitory in this assay only in the presence of vWf; vWf was not required for high affinity ESH8 binding to fVIII
  • FVIIIa release from vWf after thrombin activation is an ultimate requirement for fVIIIa to exert its cofactor activity, since bound vWf prevents fVIII from binding to a phospholipid surface and to factor IXa 45, which are both required for assembly of the factor Xase complex
  • Our studies have demonstrated that ESH8 inhibits fVIIIa release from vWf upon thrombin cleavage of the fVIII1⁄7vWf complex
  • The concentration of mAb ESH8 required to reduce the release of fVIIIa by 50% compared to that in the absence of ESH8 was 6 nM
  • The simplest explanation for the mechanism of this effect is that ESH8 inhibits thrombin cleavage at Arg1689, which is critical for fVIII release from vWf
  • The similarity of calculated fVIIIa concentrations and those determined experimentally in the factor Xase assay is consistent with the hypothesis that inhibition of factor VIII coagulant activity by monoclonal antibody ESH8 is caused only by its effect on fVIIIa release from von Willebrand factor after thrombin cleavage

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