Longitudinal Analysis of Factor VIII Inhibitors in a Previously Untreated Mild Haemophilia A Patient with an Arg593→Cys Substitution

Abstract

Recent studies suggest that certain missense mutations associated with mild to moderate haemophilia A predispose to inhibitor development. In this study, we present a longitudinal analysis of the epitope specificity of an inhibitor that developed in a mild haemophiliac with an Arg593-->Cys mutation. Immunoprecipitation studies revealed the presence of antibodies directed towards the light chain and A2 domain of factor VIII. Limited reactivity was observed with metabolically labelled C2 domain. Almost complete inhibitor neutralization was achieved upon addition of A2 domain. Binding of the inhibitor was not affected by the presence of the Arg593-->Cys substitution in the recombinant A2 fragment. Evaluation of the epitope specificity of anti-factor VIII antibodies in plasma samples obtained at different time-points of inhibitor development revealed initial development of a low titre inhibitor directed towards the A2 domain and factor VIII light chain. A second period of factor VIII replacement therapy resulted in a dramatic rise in factor VIII inhibitor titre, which maintained their original epitope specificity. Based on the results of this and previous studies (Fijnvandraat et al., 1997; Thompson et al., 1997) it is argued that inhibitor development in patients with the Arg593-->Cys mutation may proceed via a similar mechanism.