An immunogenic region within residues Val1670-Glu1684 of the factor VIII light chain induces antibodies which inhibit binding of factor VIII to von Willebrand factor.

We have identified a monoclonal anti-factor VIII (FVIII) antibody, C4, which inhibits the binding of purified human FVIII to purified human von Willebrand factor (vWF). Both whole immunoglobulin C4 and its Fab fragment demonstrated dose-dependent inhibition of FVIII binding to vWF immobilized on the surface of polystyrene beads. Synthetic peptides based on the amino acid sequence of FVIII were tested for the ability to block the binding of C4 to FVIII in an enzyme-linked immunosorbent assay system. A single synthetic FVIII pentadecapeptide, consisting of residues Val1670-Glu1684, was able to inhibit C4 binding to FVIII. Under the conditions used, the Val1670-Glu1684 peptide demonstrated total inhibition of C4 binding at a concentration of 1 microM. Synthetic FVIII peptides flanking and overlapping the Val1670-Glu1684 peptide had no significant inhibitory activity on C4 binding in concentrations up to 100 microM. A polyclonal antibody made to the Val1670-Glu1684 peptide also demonstrated inhibition of FVIII binding to vWF. Polyclonal antibodies made to synthetic FVIII peptides flanking and partially overlapping the Val1670-Glu1684 sequence did not demonstrate such inhibition. Localization of the binding region of the monoclonal anti-FVIII antibody C4 to residues Val1670-Glu1684 suggests that this site is at, or near, a major vWF binding domain of FVIII.