Summary
Factor (F)VIIa/tissue factor (TF) rapidly activates FVIII activity by proteolysis
at Arg372 and Arg740, and subsequently inactivates FVIIIa activity by proteolysis at Arg336, although this activation is weaker than that by thrombin. The effects of anti-FVIII
inhibitor antibodies on these reactions remain unknown, however. In this study, 13
of anti-FVIII inhibitor antibodies recognising the A2 or C2 domain were prepared.
None of them, irrespective of epitope specificity, significantly affected FVIIa/TFcatalysed
FVIII activation in one-stage clotting assays. Anti-A2 and anti-C2 type 2 antibodies
had little effect on the inactivation phase. Anti-C2 type 1 antibodies, however, modulated
inactivation by 40–60% of that seen with control IgG, suggesting that the activity
of FVIIIa generated by FVIIa/TF persisted in the presence of this specific type of
inhibitor. SDS-PAGE analysis demonstrated that all antibodies had little effect on
FVIIa/TF-catalyzed proteolysis at Arg372 and Arg740. Anti-C2 type 1, however, significantly delayed cleavage at Arg336 in dose-dependent manners. Neither anti-A2 nor anti-C2 type 2 affected this reaction,
and the findings were consistent with the results of the functional assays. In addition,
anti-C2 monoclonal antibodies with type 1 and 2 demonstrated similar patterns of reaction
as the anti-C2 polyclonal antibodies in FVIIa/TF-mediated FVIII mechanisms. We demonstrated
that FVIIa/TF activated FVIII even in the presence of anti-FVIII antibodies, but inactivation
patterns appeared to depend on inhibitor type. It could be important to determine
the characteristic of these inhibitor antibodies for prediction of their effects on
FVIIa-related FVIII reactions, and the results could have significant therapeutic
implications.
Note: An account of this work was presented at the 51st annual meeting of the American
Society of Hematology, 2009, New Orleans, LA, USA. This work was supported by grants
for MEXT KAKENHI 21591370 in Japan and Bayer Hemophilia Award program.
Keywords
FVIII - FVIII inhibitor antibodies - FVIIa/TF - activation/inactivation - cleavage