Localization of the Binding Regions of a Murine Monoclonal Anti - Factor VilI Antibody and a Human Anti - Factor Vil Alloantibody

We have localized the binding region of a previously described monoclonal anti-factor VIII (FVIII) inhibitory antibody (C5) to amino acid residues Thr351-Ser '5 ofthe thrombin-generated 54-kD fragment of the heavy chain of FVIII. Synthetic FVIII peptides were examined for the ability to competitively inhibit the binding of C5 to FVIII in an ELISA system. The synthetic FV1lI peptide Thr35'-Ser36blocked C5 binding to FVIII in a dose-dependent manner in this system. Two other synthetic FVIII peptides, Asn340-Glu354 and Glu342-Asp3s' which partially overlapped Thr351-Ser*-5, also blocked C5 binding to FVIJI. Blocking of C5 binding with these peptides, however, required much greater concentrations (> 100 times stronger) than that required for Thr351-Ser365. The Thr3SI1Serm5 peptide also neutralized the FVIII inhibitory activity of C5 in plasma. A human FVIII inhibitor (anti-FVIII heavy chain alloantibody) was also partially neutralized by Thr351-Ser365. Thr351Ser15 lies between a thrombin cleavage site (Arg372) and an activated protein C cleavage site (ArgA36) and may be at or near a region of functional importance in the expression of FVIII procoagulant activity.