Factor VIII inhibitor in a patient with mild haemophilia A and an Asn618-->Ser mutation responsive to immune tolerance induction and cyclophosphamide.
We describe a patient with mild haemophilia A (original value of factor VIII activity 0.30 U/ml) who developed an inhibitor (36.1 Bethesda U/ml) which cross-reacted with his endogenous factor VIII. This caused a decline in basal factor VIII level (< 0.01 U/ml) and severe haemorrhagic events. Treatment to induce immune tolerance was started with factor VIII/von Willebrand factor (VWF) concentrates, but inhibitor levels increased progressively and the patient suffered serious bleeding. Cyclophosphamide was administered and, after 8 months treatment, factor VIII levels increased to 0.20 U/ml and the inhibitor could no longer be detected. Screening of his factor VIII gene revealed a missense mutation in exon 13 that predicts substitution of Asn618-->Ser in the A2 domain of factor VIII. Immunoprecipitation analysis showed that the antibodies present in the patient's plasma reacted with metabolically labelled A2 domain and, to a lesser extent, with factor VIII light chain. Inhibitory antibodies were completely neutralized by recombinant A2 domain, whereas no neutralization was observed after the addition of factor VIII light chain (A3-C1-C2) and C2 domain. More detailed analysis showed that the majority of inhibitory antibodies were directed against residues Arg484-Ile508, a previously identified binding site for factor VIII inhibitors. Our findings suggest that immune tolerance therapy and cyclophosphamide were successful in eradicating inhibitory antibodies against a common epitope on factor VIII.
Vlot
André J
AJ
Department of Internal Medicine, Eemland Hospital, Amersfoort, the Netherlands. a.vlot@hemat.azn.nl
Wittebol
Shulamiet
S
Strengers
Paul F W
PF
Turenhout
Ellen A M
EA
Voorberg
Jan
J
van den Berg
H Marijke
HM
Mauser-Bunschoten
Eveline P
EP
D000368:Aged; D001323:Autoantibodies / Q000032:analysis*; D003520:Cyclophosphamide / Q000627:therapeutic use; D018604:Epitope Mapping; D005169:Factor VIII / Q000008:administration & dosage / Q000235:genetics / Q000276:immunology*; D006467:Hemophilia A / Q000097:blood* / Q000628:therapy; D006801:Humans; D007166:Immunosuppressive Agents / Q000627:therapeutic use; D008297:Male; D020125:Mutation, Missense; D011233:Precipitin Tests
D001323:Autoantibodies; D007166:Immunosuppressive Agents; D003520:Cyclophosphamide; D005169:Factor VIII