Functional mapping of anti-factor IX inhibitors developed in patients withsevere hemophilia B

Development of inhibitory antibodies is a serious complication of treatment with repeated factor IX infusions in a minority of patients with hemophili a B. Such antibodies detected in 8 patients have been characterized. Typing studies revealed that patients' immune response toward factor IX Is highly heterogeneous and involves immunoglobulin G (IgG) antibodies, preferential ly IgG1 and IgG4. The preservation of the sequence and the 3-dimensional or ientation of the amino acids constituting one epitope are highly important for the assembly of an antibody-antigen complex. To localize the epitopes o n the factor IX molecule, an original approach was designed using a set of factor X chimeras carrying regions of factor IX. Results showed that some p atients' antibodies were directed against both the domain containing the ga mma -carboxy glutamic acid residues (Gla domain) and the protease domain of factor IX. In contrast, no binding was observed to the epidermal growth fa ctor-like domains or to the activation peptide. Functional characterization showed that the purified IgG from patients' serum Inhibited the factor VII Ia-dependent activation of factor X. Moreover, patients' IgG directed again st the Gla domain inhibited the binding of factor IX to phospholipids as we ll as the binding of factor VIII light chain to factor IXa. These data demo nstrate that Inhibitors appearing In patients with severe hemophilia B disp lay specificity against restricted functional domains of factor IX. (C) 200 1 by The American Society of Hematology.