[Immunity characteristics induced by a genetic vaccine expressing Ag85B-ESAT6 fusion protein]

To evaluate the humoral and cell-mediated immune responses induced by a genetic vaccine expressing the Ag85B-ESAT6 fusion protein, and to investigate its protective effect against Mycobacterium tuberculosis (MTB) challenge.

Fifty BALB/c mice were randomized into 5 groups and subjected to the following treatments respectively: immunization with normal saline, BCG, pcDNA3, A(Z)-pcDNA3-E(F) and E(Z)-pcDNA3-A(F) for 3 times at 2-week intervals. The stimulation index (SI) of the splenic lymphocytes from the immunized mice was measured by the methyl thiazolyl tetrazolium (MTT) method, and the level of secreted IFN-gamma upon antigen-specific stimulation was detected by ELISA. The immunized mice were intravenously infected with 10(5) colony forming unit (CFU) of MTB H(37)Rv. The numbers of MTB CFU in spleens were determined 4 weeks later.

The specific antibody titers in the sera of mice immunized with plasmid A(Z)-pcDNA3-E(F) and E(Z)-pcDNA3-A(F) were 1:1,000 and 1:1,500 respectively, and the SI was 2.2 and 2.4 respectively, while the SI of the normal saline group and the plasmid pcDNA3 immunized group was only 0.9 and 1.1 respectively. The IFN-gamma concentrations in cultured supernatant of splenic lymphocytes from mice immunized with plasmid A(Z)-pcDNA3-E(F) [(5.48 +/- 0.38) ng/ml] and E(Z)-pcDNA3-A(F) [(5.76 +/- 0.51) ng/ml] were significantly higher than those of the normal saline group [(0.50 +/- 0.25) ng/ml] and the plasmid pcDNA3 immunized group [(1.20 +/- 0.33) ng/ml, P < 0.05], but were not significantly different with that of the BCG immunized group [(5.55 +/- 0.31) ng/ml]. Compared with plasmid pcDNA3 immunized group, the bacterial load (lg, CFU/g) in spleen was 6.08 +/- 0.25 which dramatically reduced in mice immunized with recombinant plasmids, but the protective efficacy of mice immunized with plasmid A(Z)-pcDNA3-E(F) (4.63 +/- 0.11) or E(Z)-pcDNA3-A(F) (4.50 +/- 0.32) was lower than that of the BCG vaccination group (4.09 +/- 0.27).

The cell-mediated immune response induced by genetic vaccine expressing the Ag85B-ESAT6 fusion protein was similar to that induced by BCG immunization.