AIM: To seek for an effective method to improve the immuneresponses induced by DNA vaccine expressing HBV surfaceantigen (pCR3.1-S) in Balb/c mice (H-2d).METHODS: The pCR3.1-S plasmid and the eukaryoticexpression vectors expressing murine IL-2 (pDOR-IL-2) orIL-12 (pWRG3169) were injected into mice subcutaneously.The immune responses to pCR3.1-S and the adjuvant effectof the cytokines plasmid were studied. Meanwhile the effectof pCR3.1-S on anti-translated subcutaneous tumor of P815mastocytoma cells stably expressing HBsAg (P815-HBV-S)was also studied. Anti-HBs in serum was detected by enzyme-linked immunoadsordent assay (ELISA) and HBsAg specificcytotoxic T lymphocytes (CTLs) activity was measured by 51Crrelease assay. After three weeks of DNA immunization, thecells of P815-HBV-S were inoculated into mice subcutaneouslyand the tumor growth was measured every five days. Thesurvival rate and living periods of mice were also calculated.RESULTS: After 8 wk DNA immunization, the ,4 450 nmvalues of sera in mice immunized with pCR3.1, pCR3.1-Sand pCR3.1-S codeliveried with IL-2 or IL-12 plasmids were0.03±0.01, 1.24±0.10, 1.98±0.17 and 1.67±0.12respectively. Data in mice codeliveried pCR3.1-S with IL-2or IL-12 plasmids were significantly higher than that of miceinjected pCR3.1 or pCR3.1-S only. The HBsAg specific CTLactivities in mice coinjected with pCR3.1-S and IL-2 or IL-12 eukaryotic expression vectors were (61.9±7.1) % and(73.3±8.8) %, which were significantly higher than that ofmice injected with pCR3.1 (10.1±2.1) % or pCR3.1-S (50.5±6.4) %. The HBsAg specific CTL activities in mice injectedwith pCR3.1, pCR3.1-S, pCR3.1-S combined with IL-2 or IL-12 eukaryotic expression vectors decreased significantly to(3.2±0.8) %, (10.6±1.4) %, (13.6±1.3) % and (16.9±2.3)% respectively after the spleen cells were treated by anti-CD8+ monoclonal antibody, but presented no significantchange to anti-CD4+ monoclonal antibody or unrelated tomonoclonal antibody. The HBV-S DNA vaccine (pCR3.1-S)could evidently inhibit the tumor growth, prolong the survivalperiod of mice and improve the survival rate of mice andthese effects could be improved by IL-12 gene codeliveried.CONCLUSION: HBV DNA vaccine has a strong antigenicityin humoral and cellular immunities, which can be promotedby plasmid expressing IL-2 or IL-12. CD8+ cells executedthe CTL activities. DNA vaccine may be useful for bothprophylaxis and treatment of HBV infection.