Based on their immunodominant nature and ability to induce appropriate immune responses in the host, several
antigens of Mycobacterium tuberculosis have shown promise of protection. However, most of the candidate
vaccines developed by employing various strategies have afforded protection that is at best comparable with bacillus Calmette-Guérin (BCG) in animal models. Due to the inherent ability of BCG to prime cellular responses in the host, it has become an attractive vehicle for development of a
vaccine against intracellular
infections. In this study, we have cloned the genes of three
immunodominant antigens of M.
tuberculosis viz. the ESAT6 (Rv3875), the 19 kDa
lipoprotein (Rv3763) and the 38 kDa
antigen (Pst homolog) (Rv0934) in pSD5 under the transcriptional control of Trrn, a strong mycobacterial promoter, and expressed them in BCG. The19 kDa
antigen and the 38 kDa
antigen were expressed at levels that were approximately five and eightfolds higher in the cytosols of recombinant BCG strains rBCG19T and rBCG38T, respectively, as compared with their corresponding levels in M. bovis BCG. Both these
antigens were also secreted into the extracellular medium at enhanced levels (19 kDa
antigen fourfold and 38 kDa
antigen twofold) by rBCG strains in comparison with the wild type BCG. ESAT6
antigen, which is absent in M. bovis BCG, was also expressed at a very high level in the cytosol of the rBCG strain (rBCGE6T). Evaluation of immune responses induced by these three rBCG strains in mice shows a markedly different pattern. The rBCG strain overexpressing the 38 kDa
antigen exhibited a predominant T helper 1 (Th1) response with high levels of
interferon-gamma (IFN-gamma) production, whereas overexpression of the 19 kDa
antigen resulted in completely polarized Th2 responses against the BCG sonicate. The rBCG-expressing ESAT6
antigen induced a mixed Th1/Th2 response. Our observations suggest that the 38 kDa
antigen may hold excellent promise in the rBCG approach for the development of a
vaccine against
tuberculosis.