Epitope mapping of antibodies present in the inhibitor plasma by immunoprecipitation. (A) Structure of the factor VIII light chain and the carboxyterminal truncated polypeptides synthesized in vitro and used for immunoprecipitation. The bar at the top of the figure represents the domain structure of the factor VIII light chain with the neighboring B domain. The amino acid residue numbers below the bar indicate the boundaries between the domains. The epitope for MoAb CLB-CAg 69, consisting of the amino acid residues Lys¹⁶⁷³-Arg¹⁶⁸⁹ within the amino-terminal region of the A3 domain (22), is indicated by a black bar. The three lines at the bottom of the figure represent three carboxyterminal truncated polypeptides. 1, Asp¹⁵⁶³-Asp¹⁸⁴⁰; 2, Asp¹⁵⁶³-Gln¹⁷⁷⁸; 3, Asp¹⁵⁶³-Met¹⁸²³. (B) Reactivity of MoAb CLB-CAg 69 (left) and patient's plasma (right) with truncated recombinant fragments of the A3 domain of factor VIII. Binding of anti-factor VIII antibodies to the metabolically labeled factor VIII fragments was assessed by immunoprecipitation and analyzed under reducing conditions on a 10% (wt/vol) SDS-polyacrylamide gel. Lane 1, Asp¹⁵⁶³-Asp¹⁸⁴⁰; lane 2, Asp¹⁵⁶³-Gln¹⁷⁷⁸; lane 3, Asp¹⁵⁶³-Met¹⁸²³.