Determination of specific IgG antibodies in sera of immunized mice. Sera from 13 mice per group were collected one day prior to each immunization and the subtype levels were determined using enzyme-linked immunosorbent assays. All samples were run in triplicate. The results are representative of 3 experiments and shown as the mean of the OD490 ± SD. *, statistically significant differences (P < 0.05) compared to the phosphate-buffered saline (PBS) or pBudCE4.1 controls.