Some factor VIII (FVIII) inhibitors recognise a FVIII epitope(s) that is present only on FVIII-vWF complexes

A mild haemophilia A patient (LE) with an Ag2150His mutation in the C1 doma in of the factor VIII (FVIII) light chain was shown to have anti-FVIII anti bodies inhibiting wild type but not self FVIII. Polyclonal anti-FVIII antib odies of this patient were purified by affinity adsorption using recombinan t FVIII (rFVIII) and/or plasma-derived FVIII-von Willebrand factor (VWF) co mplexes. A distinct population of antibodies was obtained that bound to FVI II-vWF complexes but not to rFVIII, indicating that an epitope was created by the association of FVIII to VWF. Such antibodies belonged to the IgG, is otype, but the FVIII epitopes to which they bind could not be mapped with p recision due to VWF dependency. Depletion experiments showed that anti-FVII I antibodies recognising FVIII-vWF complex also distinguished wildtype from mutated self FVIII, indicating that the Arg2150His mutation alters the B c ell epitope formed by the association of FVIII to VWF. To determine whether the Arg2150His substitution also alters the formation of the FVIII-VWF com plex; the interaction between mutated or normal FVIII with VWF was evaluate d in plasma. The dissociation rate of mutated FVIII from VWF was found to b e significantly increased. The presence of an Arg2150His mutation therefore results in the disappearance of a FVIII B cell epitope generated by the as sociation of FVIII with vWF. Patients carrying such an Arg2150His mutation and receiving infusion of wild-type FVIII may therefore be at risk of devel oping inhibitors to allogeneic FVIII only.