A BCG-boosting subunit vaccine RS2/DMI induces Th1-polarized immunity by targeting RD6 and secretory antigens of Mycobacterium tuberculosis.

Bacillus Calmette-Guérin (BCG) vaccine has limited protection against tuberculosis (TB) in adults, and there is an urgent need to develop novel TB vaccines. Currently, booster vaccines based on initial immunization with BCG are important research hotspots. In this study, we constructed recombinant protein RS2 by innovatively fusing Mycobacterium tuberculosis (M. tb) region of difference (RD) antigen Rv1506c with secretory phase antigen Rv3204, and developed a novel composite adjuvant DMI (dimethyl dioctadecylammonium bromide [DDA] + colloidal manganese salt [Mn jelly, MnJ] + imiquimod) to prepare a novel TB subunit vaccine, RS2/DMI. Population experiments showed that RS2 efficiently activated Th1-type cytokine secretion (IFN-γ/TNF-α/IL-2) in infected (active/ latent) individuals; in a mouse model, RS2/DMI as a BCG booster vaccine induced the highest level of IgG antibody response and the strongest Th1-polarized immune response - including the highest levels of Th1-type cytokines (IFN-γ/TNF-α /IL-2) and IFN-γ⁺IL-2⁺ double-positive CD4⁺/CD8⁺ T-cell production, and significantly better immunogenicity than the other adjuvant combinations (RS2/DMV, RS2/DIV) and BCG-immunized groups (P < 0.01). This immunological strategy also demonstrated matched preventive efficacy in ex vivo challenge experiment, confirming its potential as an efficient BCG booster vaccine and providing a new strategy to overcome the bottleneck of TB protection in adults.