Subunit Vaccine Candidate AMM Down-Regulated the Regulatory T Cells and Enhanced the Protective Immunity of BCG on a Suitable Schedule

Mycobacterium bovis bacillus Calmette-Guérin (BCG) priming and subunit vaccine boosting strategies are urgently needed to improve the protective efficacy of BCG in adult population. However, the schedule of subunit vaccine boosting is not well investigated, especially the optimal immune responses and vaccine immunization schedules are still not clear. We have constructed a novel subunit vaccine candidate consisting of fusion protein Ag85B-Mpt64 (190-198)-Mtb8.4 (AMM) in a complex adjuvant composed of dimo-thylidioctyl ammonium bromide (DDA) and BCG polysaccharide nucleic acid (BCG-PSN). In this study, we compared the effect of different boosting schedules of the subunit vaccine in the prime-boost strategies. C57BL/6 mice were primed with BCG first and then boosted with the AMM vaccine once at 10th week, twice at 8th, 10th week, or thrice at 6th, 8th, 10th week, respectively. The immune responses were evaluated at the 14th and 20th weeks, respectively. Twelve weeks after the last immunization, the mice were challenged with virulent Mycobacterium tuberculosis strain H37Rv, and the protective effect was evaluated. The results showed that BCG priming and the AMM vaccine boosting twice induced the strongest antigen-specific IFN-γ and IL-2 production, down-regulated CD4 ⁺CD25 ⁺FoxP3 ⁺ regulatory T cells (Tregs) and had the best protective effect among all groups, while boosting thrice induced the strongest IL-4 production and did not improve BCG-primed protection significantly. Boosting BCG with the AMM vaccine twice instead of once or thrice induced strong Th1-type immunity and down-regulated Tregs significantly, which correlated with the best protection against M. tuberculosis infection in mice.