Tolerance to factor VIII in a transgenic mouse expressing human factor VIII cDNA carrying an Arg593 to Cys substitution

Inhibitory antibodies develop in approximately 25% of patients with severe hemophilia A following treatment with factorVIII. In E-16KO or E-17KO mice, in which the factor VIII gene has been inactivated by insertion of a neo cassette, inhibitors develop following administration of factor VIII. Here, we describe the generation of transgenic mice expressing human factor VIII-R593C (huFVIII-R593C). Human factor VIII-R593C cDNA under control of a mouse albumin enhancer/promoter was injected into fertilized oocytes. Analysis of transgenic mice revealed that human factor VIII-R593C was expressed in the liver.Transgenic mice were crossed with factor VIII-deficient mice (E-16KO mice). In plasma of E-16KO mice antibodies were detected after five serial intravenous injections of factor VIII, while plasma of huFVIII-R593C/E-16KO mice did not contain detectable levels of antibodies. No antibody secreting cells were observed in either spleen or bone marrow of huFVIII-R593C/E-16KO mice. Also, factor VIII-specific memory B cells were not observed in the spleen of huFVIII-R593C/E-16KO mice. Analysis of T cell responses revealed that splenocytes derived of E-16KO mice secreted IL-10 and IFN-? following restimulation with factorVIII in vitro . In contrast, no factor VIII-specific T cell responses were observed in huFVIII-R593C/E-16KO mice.These results indicate that huFVIII-R593C/E-16KO mice are tolerant to intravenously administered factor VIII. It is anticipated that this model may prove useful for studying immune responses in the context of factor VIII gene therapy.