Analysis of factor VIII inhibitors in a haemophilia A patient with an Arg593-->Cys mutation using phage display.

We characterized anti-factor VIII antibodies in a mild haemophilia A patient with an Arg593-->Cys mutation in the A2 domain, using V gene phage-display technology. All isolated single-chain variable-domain antibody fragments were directed against residues Arg484-Ile508, a binding site for factor VIII inhibitors in the A2 domain. After a further period of replacement therapy, a transient rise in inhibitor titre was observed. These antibodies were directed against the A2 domain. Activation of a pre-existing pool of B cells, which express antibodies against residues Arg484-Ile508, could explain the rapid anamnestic response.