The role of immunomodulation in the management of factor VIII inhibitors.

Approximately 25% of persons with hemophilia A will have their treatment complicated by the development of anti-FVIII inhibitory antibodies. This adverse event requires the use of alternative hemostatic agents to treat bleeding and the consideration of a protocol to generate immunological tolerance to FVIII. The pathogenetic factors contributing to FVIII inhibitor generation include both patient- and concentrate-related characteristics. The FVIII genotype contributes to this risk as do other, less well defined, immunogenetic factors. The role of the type of FVIII concentrate as a precipitant for inhibitor generation appears to be less influential. Immunomodulatory management of FVIII inhibitors requires sustained and repeated exposure to FVIII through a variety of intravenous immune tolerance induction (ITI) protocols. Certain pre-ITI characteristics predict for the likelihood of success, most especially the pre-ITI anti-FVIII inhibitor titer. Currently, two major areas of debate remain unresolved with relation to the optimal form of ITI schedule. The best FVIII dose to generate FVIII tolerance is under investigation in an international prospective trial, while the issue of whether von Willebrand factor-containing concentrates may provide more powerful tolerizing effects remains open for further discussion. With a variety of ITI protocols, success rates of approximately 80% have been achieved with good-risk patients. In those that fail initial attempts at ITI, additional treatments using agents such rituximab are now being explored with further evidence of success in 60-80% of these salvage patients. Finally, several pre-clinical studies of innovative approaches to achieving FVIII tolerance suggest that combinations of immunomodulatory therapy may be of benefit in the future.