Immunogenicity of Human Factor VIII in Rhesus and Cynomolgus Monkeys.

Up to 30% of hemophilia A patients form inhibitory antibodies to therapeutic doses of human factor VIII (fVIII), thus rendering therapy ineffective. Advances in identifying the nature of the fVIII mutations (inversion, stop codons, etc), HLA linkage and other factors have suggested these as important predictors of inhibitor responsiveness in patients. The ability to predict which patients may respond with inhibitors would be important in order to identify suitable patients for tolerance therapies prior to exposure to fVIII and the development of high inhibitor titers. Animal models to evaluate the immunogenicity of fVIII would also be useful in this regard. We and others have previously shown that mice with normal hemostasis produce antibodies to fVIII in response to therapeutic doses of human fVIII given intravenously (i.v.), a normally tolerogenic route (Lin, Soukhareva, and Scott, ASH 2004) or in an adenovirus vector (Brown et al. JTH 2004; Lozier and Zhang, ASH 2005). To establish whether hemostatically normal non-human primates would respond to human fVIII and provide a model for future tolerogenic therapy, we immunized rhesus and cynomolgus monkeys with fVIII using a variety of routes. In the case of rhesus monkeys, fVIII was administered via three routes: i.v. in PBS, intramuscularly (i.m.) in alum, or i.m. in a fVIII-expressing adenoviral vector. All three routes led to significant titers as measured by ELISA, and all six animals responded to i.v. boosting with fVIII in PBS. Cynomolgus macaques (3) that received multiple doses of fVIII i.v. also produced antibody to fVIII, albeit of lower titer. Importantly, T cell proliferation to human fVIII in vitro could be elicited from cynomolgus peripheral blood cells. These results set the stage for future tolerogenic therapies in both naïve and fVIII primed non-human primates.