Animal vaccination was carried out by a gene gun delivery system and the preparation of DNA-coated gold particle cartridges was performed as previously described [2] . Briefly, cartridges were prepared by precipitating the plasmid DNA on 1 lm gold particles (Bio-Rad, Hercules, CA, USA) in 0.05 M spermidine (Sigma-Aldrich Corp) and 1 M CaCl 2 . The microcarrier/DNA suspension mixed with polyvinylpyrrolidone (5 mg/mL, Sigma-Aldrich Corp) was coated into the plastic tubing. The coated tubing was cut into 0.5 inch cartridges, and each cartridge contained 1 lg of DNA. DNA-coated particles were delivered to the shaved abdominal region of the HHD mice using a Helios gene gun (Bio-Rad, Hercules, CA, USA) with a discharge pressure of 400 psi. The mice received DNA (1 lg) for each vaccination. All animal groups (three mice per group) received a total of three vaccinations at 1-week intervals.