Epitope mapping of factor VIII inhibitor antibodies of Chinese origin

Epitopes recognized by factor VIII (FVIII) inhibitors of Chinese origin wer e analysed by immunoblotting with full-length recombinant FVIII (rFVIII), t hrombin-activated FVIII (FVIIIa) and 16 FVIII fusion proteins synthesized b y bacteria. Twenty-eight patients, 12 with haemophilia A and 16 with autoim mune diseases, were recruited, Antibodies from 22 patients showed reactivit y with rFVIII, 20 with FVIIIa, and one reacted only with FVIII fusion prote ins, Of these 22 cases, most were reactive with A2-a2 and A3-C1-C2 of FVIII (a). Of the nine cases that depicted binding to the fusion proteins, three were reactive with the A domains, three with only the B domain, and the oth er three with both the A and B (or C) domains. An epitope for a neutralizin g antibody of a haemophilia A patient, designated TWN-112, was localized to residues 323-390, specified by FVIII fusion proteins. The same epitope als o appeared on an FVIII-expression phage library screening, Immunoabsorption of antibodies from TWN-112 with the epitope reduced the neutralizing activ ity of the inhibitor by 33%. The incidence of nl of FVIII is higher, and th at of a3 is lower, than previously reported. Two novel epitopes, reported f or the first time in this paper, were localized on the 8B2 (amino acid resi dues 1022-1204) and 8A2(V) (residues 673-740) fusion proteins. These two ep itopes were able to reduce inhibitory antibody activity by 24% and 25% resp ectively. Changes of FVIII fragment specificity were also observed in one o f six patients for whom multiple samples, collected at different times, wer e available. Our initial finding showed that the FVIII inhibitors in these Chinese patients shared epitopes with those of patients from very different genetic backgrounds, suggesting a common mechanism for the development of FVIII inhibitors.