Immune Responses to Candidate Vaccine Antigens Delivered Through Naked Plasmid and Mycobacterial Vectors

Tuberculosis (TB) is a major global health problem and has been declared “a global emergency” by the World Health Organization. The failures of the currently available vaccine against TB, i.e. Mycobacterium bovis BCG, to impart consistent protection against TB, have led to the need for alternative vaccines. The low molecular weight major antigenic proteins, i.e. PE35, CFP10 and ESAT6, encoded by Mycobacterium tuberculosis-specific region of difference-1 (RD1) are among the antigens considered important to develop new TB vaccines. To deliver these antigens, two DNA vaccine vectors (pUMVC6 and pUMVC7) and three live mycobacterial species (M. bovis BCG, M. vaccae and M. smegmatis) were evaluated for the induction of antigenspecific cellular immune responses in animals. DNA corresponding to pe35, esat6 and cfp10 genes were amplified using polymerase chain reaction (PCR) from the genomic DNA of M. tuberculosis and cloned into plasmids pUMVC6 and pUMVC7 to prepare DNA vaccine constructs. Furthermore, the PCR-amplified DNA were cloned into a shuttle plasmid (pDE22), and the recombinant shuttle plasmids (pDE22-PE35, pDE22-CFP10 and pDE22-ESAT6) were electroporated into mycobacteria. The induction of antigen specific cellular immune responses was studied by immunizing mice and guinea-pigs with the recombinant constructs. The results with all the recombinant constructs and both animal models showed the consistent induction of antigen-specific cellular immune responses (spleen cell proliferation and secretion of protective T helper 1 cytokine interferon-γ in mice, and delayed-type hypersensitivity skin responses in guinea-pigs) only with the recombinant constructs expressing PE35 protein. These results suggest the superiority of PE35 antigen in inducing protective cellular immune responses in animals.