Expression and immunogenicity of recombinant Mycobacterium bovis Bacillus Calmette-Guérin strains secreting the antigen ESAT-6 from Mycobacterium tuberculosis in mice.

Background: Tuberculosis remains the leading cause of human death. Currently, Bacillus Calmette-Guérin (BCG) is the only available vaccine against tuberculosis but its efficacy is highly variable. Thus, developing new tuberculosis vaccines becomes an urgent task. In this study, we evaluated in BALB/c mice the humoral and cellular immune responses of recombinant BCG expressing the antigen ESAT-6 from Mycobacterium tuberculosis.

Methods: Escherichia coli-BCG shuttle plasmid named pDE22-esat-6 was constructed by inserting the BamHI/EcoRI digested esat-6 gene PCR product into the similarly digested parental plasmid pDE22. BCG cells were transformed with pDE22-esat-6, which was named recombinant BCG (rBCG). BALB/c mice were immunized subcutaneously on the back with 100 microl normal saline containing 10(6) CFU of BCG or rBCG. They were sacrificed after 4 weeks to detect their humoral and cellular responses.

Results: There was no any significant differences in the growth characteristics between the conventional BCG and rBCG. In immunized mice, the IgG antibody titres of rBCG group were as high as 1:8000, which was significantly higher than that in BCG group (1:1400, P < 0.05). The elicited IFN-gamma level of rBCG group was (1993 +/- 106) pg/ml, which was also significantly higher than that in BCG group ((1463 +/- 105) pg/ml, P < 0.05). The splenocyte proliferation index of rBCG group reached 4.34 +/- 0.31, which was higher than that of BCG group (3.79 +/- 0.24, P < 0.05).

Conclusion: rBCG secreted expressing antigen ESAT-6 stimulated stronger humoral and cellular immune responses than BCG did, and, therefore may be the better vaccine against mycobacterium tuberculosis.