Enhancement of the enzymatic activity of activated coagulation factor IX by anti-factor IX antibodies

Abstract

Factor VIIIa (FVIIIa) binds to activated FIX and enhances the activation of FX by several orders of magnitude. Deficiency of FVIII causes the bleeding disorder hemophilia A and is treated by i.v. infusion of FVIII concentrates. To explore whether or not FVIII activity can be supplied by alternative molecules, e.g. molecules with FIXa-binding activity. Conventional hybdridoma technology was used to discover antibodies exhibiting FVIII-like activity. We identified a series of antibodies specific for human FIX that mimicked the stimulatory effect of FVIIIa on FIXa. Upon binding to human FIXa, these antibodies enhanced the protease activity of FIXa towards its natural substrate FX about tenfold. A similar enhancement was also achieved with 5 pm FVIIIa (i.e. 16 mU mL(-1) or 1.6% activated FVIII). Procoagulant activity of these anti-FIXa antibodies was observed in model systems containing purified proteins as well as in plasma. Our findings show that FVIII can, at least partially, be replaced by an unrelated molecule. Procoagulant antibodies might potentially aid the development of an FVIII substitute for hemophilia A treatment.