Antibody formation and specificity in Bethesda-negative brother pairs with haemophilia A

Antibodies directed towards non-neutralizing epitopes on the factor VIII protein (FVIII) may be detected in patients with haemophilia A. We evaluated the prevalence of non-neutralizing antibodies, in 201 inhibitor-negative brother pairs with severe haemophilia A, enrolled in the Malmo International Brother Study and the Haemophilia Inhibitor Genetics Study. To evaluate binding specificity of the antibodies, ELISA plates were coated with two recombinant full-length (FL) FVIII-products and one recombinant B-domain-deleted (BDD) product. Seventy-nine patients (39.3%) had a history of positive inhibitor titre measured by Bethesda assay, and FVIII antibodies were detected in 20 of them (25.3%). Additional 23 samples from subjects without a history of FVIII inhibitors were ELISA-positive corresponding to a frequency of non-neutralizing antibodies of 18.9%. The antibody response towards the different FVIII products was heterogenous, and was raised not only towards the non-functional B-domain but also towards both FL-rFVIII and BDD-rFVIII. In patients considered successfully treated with immune tolerance induction, 25.4% had remaining FVIII antibodies. The number of families with an antibody response in all siblings was increased when the total antibody response was taken into account, further supporting the concept of a genetic predisposition of the immune response. Further studies and careful monitoring over time are required to appreciate the immune response on the risk of inhibitor development or recurrence in the future.