Abstract: Objective To investigate the effect of Escherichia coli heat-labile enterotoxin B subunit （LTB） as a genetic adjuvant in enhancing the immune response induced by Toxoplasma gondii tachyzoite compound gene vaccine. Methods The eukaryotic expression plasmids of pcDNA3.1-SAG1-ROP2 and pEASY-E1-LTB were constructed. Eighty-eight BALB/c mice were randomly divided into four groups: PBS （group A）, pcDNA3.1（B）, pcDNA3.1-SAG1-ROP2 （C） and pcDNA3.1-SAG1-ROP2+pEASY-E1-LTB （D）. Fifteen mice in each group were randomly selected, and intranasally immunized weekly with 20 μg plasmid or 20 μl PBS, respectively. Each mouse received four immunizations with the same dose of antigen. Two weeks after the final immunization, the antibodies and cytokines were detected, including the specific IgG and IgA antibodies in serum, sIgA in mucosa douche, IFN-γ and IL-4 in splenocyte culture supernatant. The remaining mice in each group were immunized three times weekly with 20 μg plasmid or 20 μl PBS, respectively, and challenged by T. gondii tachyzoites at four weeks after the final vaccination （1×10³ per mouse）. The survival time of mice was recorded. Results The recombinant plasmids pEASY-E1-LTB were constructed. The specific IgG (0.626±0.100) and IgA antibodies (1.086±0.138) in serum, sIgA (0.886±0.164) in mucosa douche, cytokines IFN-γ [(2 017±266)pg/ml] and IL-4 [(203±31)pg/ml] in splenocyte culture supernatant in group D were all higher than those in other groups （P<0.05）. After challenged with T. gondii tachyzoites, the median survival time of mice in groups A, B, C, and D were 3, 4, 6, and 10 d, respectively. The survival time of mice in group D was longest （P<0.05）. Conclusion E. coli heat-labile enterotoxin can enhance the immune response induced by the compound gene vaccine of T. gondii tachyzoites.

Key words: Toxoplasma gondii, Compound gene vaccine, Escherichia coli, Heat-labile enterotoxin, Mucosal immunity