The molecular mechanisms of immunomodulation and tolerance induction to factor VIII

Successful factor (F) VIII replacement therapy in hemophilia A patients is confounded by the generation of inhibitory anti-FVIII antibodies (Ab) in 25-30% of treated patients. These antibodies, termed 'inhibitors', significantly increase morbidity within the hemophilia population and lower the quality of life for these patients. For the past 30 years, immune tolerance induction (ITI) has been the standard therapy to elicit immunological tolerance to FVIII in the clinic. ITI works well in approximately 75% of patients, but it is expensive, can take years to show effect and is in many cases practically challenging. Therefore, new immunological tolerance induction strategies are now being designed and tested in hemophilia A animal models. This review attempts to provide a comprehensive description, at both the cellular and molecular levels, of these novel advances in tolerance induction and immunomodulation of FVIII. We begin by briefly reviewing why and how the immune system generates a protective response against exogenous FVIII. This leads to a discussion of the latest advances in FVIII tolerance/immunomodulation technology. These advances include interesting methodologies to induce B cell specific tolerance in FVIII primed humans and animals, as well as newer T cell-specific therapies that modify and/or block co-stimulation. We also discuss methods to manipulate FVIII loading of antigen-presenting cells.