Structure of a factor VIIIC2 domain-immunoglobulin G4κ Fab complex:: identification of an inhibitory antibody epitope on the surface of factor VIII

The development of an immune response to infused factor VIII is a complication affecting many patients with hemophilia A. Inhibitor antibodies bind to antigenic determinants on the factor VIII molecule and block its procoagulant activity. A patient-derived inhibitory immunoglobulin G4 kappa antibody (BO2C11) produced by an immortalized memory B-lymphocyte cell line interferes with the binding of factor VIII to phospholipid surfaces and to von Willebrand factor, The structure of a Fab fragment derived from this antibody complexed with the factor VIII C2 domain was determined at 2.0 Angstrom resolution, The Fab interacts with solvent-exposed basic and hydrophobic side chains that form a membrane-association surface of factor VIII, This atomic resolution structure suggests a variety of amino acid substitutions in the C2 domain of factor VIII that might prevent the binding of anti-C2 inhibitor antibodies without significantly compromising the procoagulant functions of factor VIII.(C) 2001 by The American Society of Hematology.