Abstract: Objective To study the protective effect of ROP2 nuclei acid vaccine in mice. Methods Forty-two BALB/c mice were divided into three groups. Each mouse in experiment group was injected with 50 μg recombinant plasmid pc-DNA3-ROP2 through musculus quadriceps fexoris. In control groups, each mouse was injected with 50 μg blank plasmid pc-DNA3 and with 50 μl PBS respectively. All mice were immunized for three times with an interval of three weeks. The volume was doubled for the final injection in the two plasmid groups. Blood, spleens and lymph nodes of 4 mice in each group were taken for the detection of CD4⁺, CD8⁺ T cells and cytokines 2 weeks after the final immunization. The rest mice in 3 groups were challenged with 500 tachyzoites of Toxoplasm gondii RH strain for further observation. Results The vaccine induced strong cellular and humoral immune response. The titer of antibody in serum was high after inoculation and recognized ROP2 protein antigen expressed in vitro. The lymphocyte phenotype was analyzed. CD4⁺ T cells proliferated sharply(69.5±3.4)%, and the ratio of CD4⁺/CD8⁺ increased considerably by （4.69±1.32）%（P<0.01）. The level of IL-2, IL4, IL-6, IL-12, IFN-γ and TNF in serum and cultured supernatant of spleen cells and lymph cells was higher in the experiment group than that in control groups, especially in serum. 88.9% mice in the experiment group were protected 180 hours after the challenge of T.gondii. The death time of mice in experiment group was delayed and the survival time was prolonged in comparison to that in control groups with a significant difference（P<0.01）. Conclusion The recombinant ROP2 nuclei acid vaccine shows fair immunogeni-city and obviously produces immuno-protection.

Key words: Toxoplasma gondii, Rhoptry protein (ROP2), Nuclei acid vaccine, Immuno-protection