Virulence, immunogenicity, and protective efficacy of two recombinant Mycobacterium bovis bacillus Calmette-Guérin strains expressing the antigen ESAT-6 from Mycobacterium tuberculosis.

We constructed two recombinant Mycobacterium bovis BCG (rBCG) strains expressing ESAT-6 of Mycobacterium tuberculosis, named rBCG-1 and rBCG-2. rBCG-1 contained the ESAT-6 gene linked to BCG hsp60 and expressed a fusion protein, while rBCG-2, with a secretory sequence, could secret ESAT-6 into the culture medium. There was no evidence for increased virulence of the two rBCG strains when we made a comparison between them and BCG with regard to organ bacterial loads, lung histology, and survival time. rBCG-1 induced significantly higher specific antibody titers and stronger cellular immune response than BCG, whereas rBCG-2 had immunogenicity similar to that of the parental BCG strain. Both rBCG-1 and rBCG-2 conferred marked protection against M. tuberculosis infection, yet in terms of protective efficacy, they showed no significant improvements upon conventional BCG vaccine.