A murine monoclonal anti-factor VIII inhibitory antibody and two human factor VIII inhibitors bind to different areas within a twenty amino acid segment of the acidic region of factor VIII heavy chain.

The factor VIII (FVIII) binding regions of the monoclonal anti-FVIII inhibitory antibody C5 and a human FVIII inhibitor antibody have previously been reported to be contained within amino acid residues 351-365 of FVIII. Localization of the binding regions of these two antibodies was based on their reactivity with four synthetic FVIII peptides. Nineteen synthetic FVIII peptides spanning the entire acidic region of the FVIII heavy chain have now been evaluated for the ability to inhibit the binding of C5 to FVIII in an ELISA assay. The smallest peptide tested that inhibited C5 binding to FVIII consisted of residues 351-361. Those peptides that were able to inhibit C5 binding in the ELISA assay were also able to neutralize the FVIII inhibitory activity of C5 in plasma. The FVIII inhibitory activity of two human FVIII inhibitor antibodies was also partially neutralized by peptides from this region. Evaluation of the pattern of peptides reactive with the three antibodies indicates that the binding regions of these antibodies are in very close proximity to each other, but are not identical. Their respective binding regions are contained within residues 351-361 (C5), 354-362 (inhibitor 1), and 342-354 (inhibitor 2). These results suggest that this 20 amino acid segment of the acidic region of the heavy chain of FVIII may be functionally important in the expression of FVIII procoagulant activity.