Epitope mapping of human factor VIII inhibitor antibodies by site-directed mutagenesis of a factor VIII polypeptide.
Previous epitope mapping studies of human factor VIII (FVIII) inhibitor antibodies with heavy chain specificity localized epitopes to the amino-terminal half of the FVIII A2 domain. In this report we have used unidirectional deletion analysis and site-directed mutagenesis to identify a minimum length polypeptide and amino acid residues that contribute to the FVIII conformation recognized by these antibodies. Bacterial expression plasmids were exploited to demonstrate that a FVIII polypeptide of approximately 150 residues is required to generate a common heavy chain epitope(s). Another series of plasmids were constructed that synthesize: a FVIII polypeptide containing an internal deletion; four polypeptides with single residue substitutions; two polypeptides with triple residue changes; and a quadruple amino acid replacement within one polypeptide. The relative reactivities of the wild-type and mutant FVIII polypeptides were tested by immunoblotting, inhibitor neutralization assays and ELISA with a variety of human FVIII inhibitor auto- and alloantibodies. These techniques illustrate that the internal deletion mutant and one of the relatively conservative amino acid substitution triple mutants, mutant 389, resulted in significantly decreased immunoreactivity. The data identify FVIII Glu389,390,391 as three critical components of an epitope for human FVIII inhibitor antibodies and identify a major inhibitory epitope involved in the immune response to FVIII.