Some Factor VI 11 Inhibitor Antibodies Recognize a Common Epitope Corresponding to C 2 Domain Amino Acids 2248 Through 2312 , Which Overlap a Phospholipid - Binding Site

The finding that human factor Vlll (NIII) inhibitor antibodies with C2 domain epitopes interfere with the binding of Nlll to phosphatidylserine (PS) suggested that this is the mechanism by which they inactivate NIII. We constructed a recombinant C2 domain polypeptide and demonstrated that it bound to all six human inhibitors with Nlll light chain specificity. Thus, some antibodies within the polyclonal anti-light chain population require only amino acids within C2 for binding. Recombinant C2 also partially or completely neutralized the inhibitor titer of these plasmas, demonstrating that antiC2 antibodies inhibit Nlll activity. lmmunoblotting of a series of C2 deletion polypeptides, expressed in Escherichia coli, with inhibitor plasmas showed that the epitopes for human inhibitors consist of a common core of amino acid residues 2248 through 2312 with differing extensions for individual inhibitors. The epitope of inhibitory monoclonal antibody (MoAb) ESH8 was localized to residues 2248 through