Inhibitory antibody formation is the most serious complication of factor (F)VIII replacement
therapy in
hemophilia A patients. FVIII-deficient mice were used to study new approaches for induction of immune tolerance. Neither antiFVIII inhibitory
antibodies nor antiFVIII IgGs were observed in 13 of 14 adult mice that received 0.05 U g(-1)
body weight of human FVIII intravenously within 24 h after birth and repeated
injections as adults. In contrast, high FVIII antibody titers (>50 Bethesda Units mL(-1)) developed in seven of 13 mice injected on day 3 postpartum and in all adult mice not treated neonatally. One of nine mice and three of 17 mice developed high-titer antiFVIII inhibitory antibody when they were treated initially with 2-fold (0.1 U g(-1)
body weight) and 10-fold higher doses (0.5 U g(-1)
body weight) FVIII on day 0, respectively. A human FVIII-specific T-cell proliferative response was absent in splenocytes from neonatally treated mice. The tolerance was FVIII specific because antitoxoid
antibodies developed after immunization with
tetanus toxoid. Splenocytes failed to proliferate or produce
interferon (IFN)-gamma in response to FVIII stimulation, yet still secreted
interleukin-2. A proliferative response was restored with exogenous IFN-gamma or
interleukin-12, suggesting that lack of inhibitor to FVIII was due to IFN-gamma-dependent anergy. Thus, exposure on day 0 to physiological levels of FVIII
antigen might be important for induction of immune tolerance. This immune tolerance model may provide a basis for new approaches to prevention of FVIII inhibitors during replacement
therapy.