Current Knowledge of the AHF-Like Antigen
Factor VIII and von Willebrand Factor activities are associated with a high molecular weight protein which can be isolated from plasma and may be studied by immunological methods. Homologous antibodies to Factor VIII are directed towards the active site of the Factor VIII molecule; they do not neutralize Willebrand Factor activity and do not precipitate with normal plasma. The use of such antibodies has allowed the distinction between Hemophilia A+ and A-. Specific precipitation of Factor VIII antibodies using polyethylene glycol will be reported, allowing typing of heavy and light chains of purified antibodies. Heterologous antisera prepared in rabbits against purified human Factor VIII complex neutralize Factor VIII and von Willebrand Factor activities and precipitate with AHF-like antigen. Estimation of this antigen in plasma has allowed (1) the differenciation of the molecular abnormalities in Hemophilia A and classical von Willebrand’s disease; (2) the comparison between normal and Hemophilic AHF-like antigen; (3) the detection of carriers of Hemophilia A; (4) the study of variants of von Willebrand’s disease; (5) the demonstration of this antigen in platelets and in endothelial cells. Factor VIII activity and AHF-like antigen are probably separate entities, circulating as a complex in normal plasma, as suggested by the following experiments: transfusion studies in von Willebrand’s disease; immuno-adsorption studies; comparison of Factor VIII complex in cryoprecipitate and supernatant; and dissociation in high salt buffer, demonstrating that Factor VIII includes two biologically linked but distinct fragments, of high (HMW) and low (LMW) molecular weight. The non functional HMW subunit, controlled by an autosomal locus, is identified by the presence of AHF-like antigen and Willebrand Factor activity. The LMW subunit, product of an X-chromosome locus, does not contain AHF-like antigen, but it carries Factor VIII activity, as demonstrated by the following facts: inactivation by both human and rabbit antibodies to Factor VIII; transient activation by thrombin; obtention of antisera which specifically inactivate Factor VIII.