MECHANISM AND KINETICS OF FACTOR-VIII INACTIVATION - STUDY WITH AN IGG4 MONOCLONAL-ANTIBODY DERIVED FROM A HEMOPHILIA-A PATIENT WITH INHIBITOR

The development of an immune response towards factor VIII (fVIII) rema ins a major complication for hemophilia A patients receiving fVIII inf usions. The design of a specific therapy to restore unresponsiveness t o fVIII has been hampered by the diversity of the anti-fVIII antibody. Molecular analysis of the specific immune response is therefore requi red. To this end, we have characterized an fVIII-specific human IgG4 k appa monoclonal antibody (BO2C11) produced by a cell line derived from the memory B-cell repertoire of a hemophilia A patient with inhibitor . BO2C11 recognizes the C2 domain of fVIII and inhibits its binding to both von Willebrand factor (VWF) and phospholipids. It completely inh ibits the procoagulant activity of native and activated fVIII, with a specific activity of approximately 7,000 Bethesda units/mg. vWF reduce s the rate of fVIII inactivation by BO2C11. The antibody fVIII associa tion rate constant (k(ass) similar to 7.4 x 10(5) M-1 s(-1)) is eightf old lower than that for vWF-vWF association, whereas its dissociation rate constant (k(diss) less than or equal to 1 X 10(-5) s(-1)) is 100- fold lower than that for the vWF-fVIII complex, which suggests that BO 2C11 almost irreversibly neutralizes fVIII after its dissociation from vWF. BO2C11 is the first human monoclonal anti fVIII IgG antibody tha t has been isolated and allows the study of fVIII inactivation at the molecular level. (C) 1998 by The American Society of Hematology.