Humoral response in Kunming mice induced by DNA vaccination. (A). Determination of IgG antibodies in the sera of Kunming mice immunized with pVAX-CDPK1, pVAX-IL-21-IL-15, pVAX-CDPK1 + pVAX-IL-21-IL-15, pVAX I, PBS and blank controls on weeks 0, 2, 4, 6. (B). Determination of IgG subclass profiles (IgG1 and IgG2a) in sera of the immunized mice two weeks after the last immunization. Results are expressed as mean of the OD405 ± S.E. (n = 3) and statistically significant differences (P < 0.05) are indicated by (*).