LOSS OF TOLERANCE TO EXOGENOUS AND ENDOGENOUS FACTOR-VIII IN A MILD HEMOPHILIA-A PATIENT WITH AN ARG(593) TO CYS MUTATION

A 42-year-old patient with mild hemophilia A developed spontaneous mus cle hematomas I month after intense therapy with factor VIII concentra tes. Factor VIII clotting activity was less than 1% and his factor VII I inhibitor was 10 Bethesda units (BU)/mL. The titer peaked al 128 BU despite daily infusions of factor VIII; 1 year later, the titer was 13 BU with no spontaneous bleeding for 4 months. The plasma inhibitor wa s 95% neutralized by factor VIII AZ domain but less than 15% neutraliz ed by light-chain or C2 domain, His inhibitor did not cross-react with porcine factor VIII and was at (east 10-fold less reactive to a serie s of hybrid factor VIII proteins in which human residues 484-508 are r eplaced by the homologous porcine sequence (Healey et al, J Biol Chem 270:14505, 1995). The inhibitor patient's DNA encoding his A2 domain a nd flanking sequences showed a C-T transition predicting Arg(593) to C ys. Thirteen patients from 5 unrelated families with Cys(593) have not developed inhibitors, Factor VIII clotting activity from one of them was inhibited similarly to diluted normal plasma by inhibitor patient plasma, In an homologous structure, ceruloplasmin (Zaitseva et al, J B iol Inorgan Chem 1:15, 1996), the residue equivalent to Arg(593), is i n a loop distinct from residues 484-508. On solution phase immunopreci pitation with labeled factor VIII fragments, A2, light chain, and C2 d omains bound. In contrast to typical immune responses to factor VIII i n patients with severe hemophilia A, this patient's inhibitor was almo st entirely reactive with common epitopes within the AZ domain whereas by more sensitive immunoprecipitation testing antibodies to light cha in epitopes were also present. Accordingly, immune responsiveness to e xogenous factor VIII (antigen burden) appears to be more critical than his endogenous, hemophilic factor VIII to his developing high-titer a nti-factor VIII antibodies and loss of tolerance to both native and he mophilic factor VIII proteins. (C) 1997 by The American Society of Hem atology.