Abstract: Objective To investigate the effect of immunostimulatory sequence on SjC23 DNA vaccine against Schistosoma japonicum infection. Methods SjC23 gene fragment was inserted into pcDNA3.1-CpG to construct pcDNA3.1-SjC23/CpG. BALB/c mice in 4 groups were immunized intramuscularly 3 times at 2 week intervals, with 100 μg plasmid DNA per injection. Four weeks after the 3rd immunization, all mice were challenged with 45±1 cercariae of S.japonicum by abdominal skin penetration. After 45 days post-challenge, mice were perfused and the number of recovered worms and of eggs in liver was counted. Blood samples were collected from the tail vein of all mice 2 days before the 1st immunization and before challenge respectively. IgG, IgG1 and IgG2a in sera were detected. Three weeks after the 3rd inoculation, the spleen cells of 2 mice from each group were cultured and stimulated with ConA and recombinant peptide. The supernatant was collected to detect IL-2, IL-4 and IFN-γ. Simultaneously, the cytotoxic activity was detected with 51 Cr release assay in vitro. Results The worm reduction rate in SjC23 group and SjC23/CpG group was 28.1% and 35.1%, the hepatic egg reduction rate was 21.6% and 26.5%, respectively, compared with the control group. The level of protection in SjC23/CpG group was higher than that in SjC23 group (P<0.05). ELISA results indicated that mice immunized with pcDNA3.1-SjC23 and SjC23/CpG produced specific IgG to rSjC23, while mice immunized with pcDNA3.1 and pcDNA3.1-CpG did not. Mice in SjC23 group and SjC23/CpG group also produced IgG1 and IgG2a antibody isotypes, with the ratio of IgG2a/IgG1 10.1 and 12.2, respectively. In comparison with the control, the level of IL-2 and IFN-γ in mice immunized with pcDNA3.1-SjC23 and pcDNA3.1-SjC23/CpG was augmented. The cytotoxic activity of spleen cells from mice in SjC23/CpG group was augmented from 9.7% to 40.0% compared with that in SjC23 group. Conclusion The study indicates that immunostimulatory sequence appears to increase the level of protection induced by immunization with pcDNA3.1-SjC23 vaccine.

Key words: Schistosoma japonicum, Immunostimulatory sequence (CpG), Mr 23 000 membrane protein, Recombinant antigen, DNA vaccine