A human antibody directed to the factor VIIIC1 domain inhibits factor VIIIcofactor activity and binding to von Willebrand factor

The occurrence Of factor VIII (fVIII) inhibitory antibodies is a rare compl ication of fVIII substitution therapy in mild/moderate hemophilia A patient s, fVIII mutations in certain regions such as the C1 domain are, however, m ore frequently associated with inhibitor, for reasons which remain unclear, To determine whether inhibitors could map to the mutation site, we analyze d at the clonal level the immune response of such a patient with an inhibit or to wild-type but not self-fVIII and an Arg2150His substitution in the C1 domain. Immortalization of the patient B lymphocytes provided a cell line producing an anti-fVIII IgG4 kappa antibody, LE2E9, that inhibited fVIII co factor activity, following type 2 kinetics and prevented fVIII binding to v on Willebrand factor. Epitope mapping with recombinant fVIII fragments indi cated that LE2E9 recognized the fVIII C1 domain, but not the Arg2150His-sub stituted C1 domain. Accordingly, LE2E9 did not inhibit Arg2105His fVIII act ivity. These observations identify C1 as a novel target for fVIII inhibitor s and demonstrate that Arg2150His substitution alters a B-cell epitope in t he C1 domain, which may contribute to the higher inhibitor incidence in pat ients carrying such substitution. (C) 2000 by The American Society of Hemat ology.