The effects of intramuscular immunization against influenza, with the adjuvant MF59, on FVIII immunogenicity. Mice were immunized intramuscularly with a scaled standard human dose (4.8 μg/kg) within 24 hours of the first of 7 rhFVIII (6 IU, ∼240 IU/kg) infusions. Blood was collected by cardiac puncture 4 weeks later and plasma was isolated by centrifugation. (A) Incidence of FVIII-specific IgG. Total cohort sizes are indicated above each bar. (B) Incidence of inhibitors. (C) Comparison of influenza- and FVIII-specific IgG titers among FVIII responders. (D) Comparison of inhibitory activity among FVIII responders. (E) Anti-FVIII IgG subclasses IgG1 and IgG2a among FVIII responders. The horizontal lines and error bars represent the mean and SEM.