A combined DNA vaccine-prime, BCG-boost strategy results in better protection against Mycobacterium bovis challenge

In this study, we demonstrated that calves vaccinated with a combined DNA vaccine encoding Ag85B, MPT- 64, and MPT-83 antigens from the Mycobacterium tuberculosis for the priming and subsequently boosting with BCG prior to experimental challenge with virulent Mycobacterium bovis (M. bovis) resulted in improved immune responses over immunizing. Vaccination with the combined DNA/BCG induced higher levels of antigen- specific gamma interferon (IFN-gamma) in whole-blood cultures 4 weeks after final vaccination and the level of antigen-specific IFN-gamma in response to Ag85, MPT-64, and MPT-83 were still higher 4 weeks after challenge when compared to the combined DNA group. There was a significant bias toward induction of CD4+ T cells rather than CD8+ T cells responses, and the mean percentage of CD4+ T cells was increased about 2.6-fold in peripheral blood mononuclear cells (PBMC) cultures in DNA prime-BCG boost vaccination when compared to the nonvaccinated group. In addition, DNA prime-BCG boost vaccination resulted in stronger humoral immune responses, and the levels of the specific antibodies to three antigens were increased two- to 32- fold when compared to the combined DNA group. Vaccination with the combined DNA/BCG induced a high level of protection against an intratracheal challenge with virulent M. bovis, based on a significant enhancement of six pathological and microbiological parameters of protection compared to the nonvaccinated group. Finally, the combined DNA/BCG increased the protective efficacy by more than 10-100-fold as measured by reduced CFU counts in the lungs from calves challenged with M. bovis compared to the combined DNA and BCG groups. These results suggest that use of the prime-boost strategy offers better protection against bovine tuberculosis than does the combined DNA vaccines and BCG.