Vibi B

Factor VIII (FVIII), an essential blood coagulation protein, is a key component of the fluid phase blood coagulation system. Human factor VIII is a single chain of about 300 kDa consisting of domains described as A1-A2-B-A3-C1-C2. The protein undergoes processing prior to secretion into blood resulting in a heavy chain of 200 kDa (A1-A2-B) and a light chain of 80 kDa (A3-C1-C2) linked by metal ions. The role of factor VIII is to increase the catalytic efficiency of factor IXa in the activation of factor X. Variants of these factors lead frequently also to severe bleeding disorders.

The development of persistent neutralizing antibodies against factor VIII (FVIII) is the most severe complication in the treatment of congenital hemophilia A patients with FVIII replacement therapies. Recently, we presented data which indicate that neutralizing antibodies are high-affinity antibodies which are mostly of the IgG1 and IgG4 subclasses. However, there are also FVIII-specific antibodies of low to moderate affinity which are found in some patients without neutralizing antibodies and in some healthy individuals. The underlying immune mechanisms which regulate the development of these different populations of FVIII-specific antibodies are poorly understood. In this review, we discuss potential mechanisms for the development of low-affinity and high-affinity antibodies. In particular, we discuss the role of memory B cells which are essential in maintaining long-lasting antibody responses and in driving the rapid anamnestic antibody response after re-exposure to the same antigen. We also present available data on the potential role of FVIII-specific memory B cells in the maintenance of neutralizing antibodies against FVIII.

The haemophilias are inherited disorders in which one of the coagulation factors is deficient. Although deficiencies of factor VIII (haemophilia A) and factor IX (haemophilia B) are well recognised, von Willebrand's disease is much more common. Rare defects can occur in any of the coagulation factors. In the past, men with haemophilia were likely to die in their youth. With advances in diagnosis, and especially with development of safe and effective treatment, affected individuals can now look forward to a normal life expectancy. Complications of the disorder, particularly the development of antibodies that make treatment ineffective, and of treatment, such as transfusion-transmitted infections, have taken a severe toll on these patients. The future holds the realistic possibility of gene therapy. However, we must not forget that haemophilia is a worldwide disorder that requires significant economic resources not available for the majority.