Detection and IgG subclass analysis of antibodies to factor VIII in multitransfused haemophiliacs and healthy individuals

Summary. Using a binding assay to immobilized factor VIII (F VIII) (ELISA) we measured the amount of IgG with binding capacity to FVIII, in the plasma of patients with an inhibitor to F VIII, in multitransfused haemophiliacs without inhibitor and in a control group of blood donors. It was shown that the amount of IgG bound to VIII was elevated in patients with an inhibitor although a weak correlation could be established between the inhibitor titre (BU) and the amount of bound IgG. In all haemophiliacs without inhibitor, IgG bound to F VIII were present. Although the mean value of IgG bound to F VIII was significantly lower than the amount detected in patients with F VIII inhibitors, a group of patients developed an equal amount of IgG recognizing the F VIII molecules to the amount of IgG measured in inhibitor patients. These results indicate that the presence of an inhibitor is not related to the amount of specific IgG bound to F VIII but more likely to the position of epitopes recognized by specific IgG. The presence of IgG bound to F VIII was detected in 92% of control blood donors and an inhibitor to F VIII ranging from 0.5 to 1.3 BU mL^-1 in 17% of them. The isotypes of bound immunoglobins were identified in patients and controls: IgG4 subclass was predominant only in patients with an inhibitor and usually associated with antibodies of one or more of the other subclasses. In noninhibitor patients, very few had antibodies of IgG4 subclass with binding capacity to F VIII. These results raised the question of the clinical significance of these antibodies in multitransfused patients. The study indicates that binding assay is a complementary test to be used in multitransfused patients but cannot be used instead of the coagulation tests for detection of inhibitors.