Gene therapy in adult hemophilia A mice is known to generate a robust immune response against the newly produced FVIII protein. This phenomenon has been extensively reported for both viral and non-viral vectors. T cell activation promotes the proliferation of B cells that produce antibodies against FVIII (inhibitors) leading to a loss of circulating FVIII protein and activity. Approaches to attenuate this immune response have included the use of cytotoxic chemotherapeutic agents (such as cyclophosphamide) or the use of antibodies or soluble fusion proteins that prevent T cell activation (such as the fusion protein between the cytotoxic T lymphocyte-associated antigen 4 and the immunoglobulin heavy chain, CTLA-4-Ig). Recently, the mechanism by which CTLA-4-Ig exerts its effect has been elucidated and is thought to involve tryptophan catabolism by upregulation of the enzyme indoleamine 2,3-dioxygenase (IDO). Catabolites of tryptophan degradation, such as kynurenine, have been shown to block T cell proliferation and promote T cell apoptosis. Here we report that hydrodynamic co-administration of Sleeping Beauty transposons encoding both FVIII and IDO are able to attenuate, but not fully block, inhibitor formation. With this approach, long term expression of FVIII at therapeutic levels (∼10% of normal) can be achieved in adult animals without the need for additional immune suppression. Only in animals receiving FVIII and IDO together can we detect FVIII protein by western blot at 24 weeks. Serum kynurenine levels are slightly elevated after gene delivery in animals receiving the IDO gene, but fall to within normal levels by 24 weeks. These results suggest that modulation of the levels of tryptophan catabolites in vivo can influence the formation of FVIII inhibitors.
Disclosure:Research Funding: National Hemophilia Foundation grant to Dr. Liu.