Recombinant BCG approach for development of vaccines: cloning and expression of immunodominant antigens of M. tuberculosis.

In spite of major advances in our understanding of the biology and immunology of tuberculosis, the incidence of the disease has not reduced in most parts of the world. In an attempt to improve the protective efficacy of Mycobacterium bovis bacille Calmette-Guérin (BCG), we have developed a generic vector system, pSD5, for expression of genes at varying levels in mycobacteria. In this study, we have cloned and overexpressed three immunodominant secretory antigens of M. tuberculosis, 85A, 85B and 85C, belonging to the antigen 85 complex. All the genes were cloned under the control of a battery of mycobacterial promoters of varying strength. The expression was analysed in the fast-growing strain M. smegmatis and the slow-growing vaccine strain M. bovis BCG. The recombinant BCG constructs were able to express the antigens at high levels and the majority of the expressed antigens was secreted into the medium. These results show that by using this strategy the recombinant BCG approach can be successfully used for the development of candidate vaccines against infections associated with mycobacteria as well as other pathogens.