The influence of subcutaneous and intravenous exposure to the MMR vaccine on FVIII immunogenicity. (A) HA mice were challenged with the MMR vaccine, at 10 times the standard scaled human dose, 24 hours before the first of 4 weekly infusions of 2 IU rhFVIII (∼80 IU/kg). Week 5 plasma was obtained at week 5 by retro-orbital sampling. Mice were subsequently rechallenged with the same dose of MMR vaccine followed by 4 biweekly infusions of 6 IU rhFVIII (∼240 IU/kg). Plasma samples at week 9 were obtained via cardiac puncture. (B) Incidence of FVIII-specific IgG in subcutaneous (SQ) and intravenous (IV) treatment arms at weeks 5 and 9, assessed by indirect ELISA. Total cohort sizes are indicated above the bars. (C) Incidence of FVIII inhibitors. (D) Comparison of FVIII-specific IgG titers among FVIII responders. (E) Comparison of inhibitory activity among FVIII responders. The horizontal lines and error bars represent the mean and standard error of the mean (SEM).