Haemophilia A: effects of inhibitory antibodies on factor VIII functional interactions and approaches to prevent their action.

Factor VIII (FVIII) is an essential component of the intrinsic pathway of blood coagulation. Normal functioning of FVIII requires its interactions with other components of the coagulation cascade. In the circulation, it exists as a complex with von Willebrand factor (vWF). Upon activation by thrombin or activated factor X (FXa), activated FVIII (FVIIIa) functions as a cofactor for the serine protease factor IXa. Their complex assembled on the phospholipid surface activates FX to FXa, which consequently participates in formation of thrombin, the key protease of the coagulation cascade. Genetic deficiency in FVIII results in a coagulation disorder haemophilia A, which is treated by infusions of FVIII products. Approximately 25-30% of patients develop antibodies inhibiting FVIII activity (FVIII inhibitors). The major epitopes of inhibitors are located within the A2, C2 and A3 domains of the FVIII molecule. The inhibitory effects of antibodies are manifested at various stages of the FVIII functional pathway, including FVIII binding to vWF, activation of FVIII by thrombin, and FVIIIa incorporation into the Xase complex. We summarize the current knowledge of the FVIII sites involved in interaction with its physiological ligands and different classes of inhibitory antibodies and describe their inhibitory mechanisms. We outline the strategies aimed to overcome the effects of inhibitory antibodies such as development of human/porcine FVIII molecules, resistant to inhibitors. We also discuss approaches to modulate the antibody response, as well as efforts to develop a long-term immunotolerance to FVIII protein.