Protective efficacy of recombinant BCG over-expressing protective, stage-specific antigens of Mycobacterium...

Tuberculosis (TB) remains a major cause of mortality and morbidity worldwide, yet current control strategies, including the existing BCG vaccine, have had little impact on disease control. CysVac2, a fusion protein comprising stage-specific Mycobacterium tuberculosis antigens, provided superior protective efficacy against chronic M. tuberculosis infection in mice, compared to BCG. To determine if the delivery of CysVac2 in the context of BCG could improve BCG-induced immunity and protection, we generated a recombinant strain of BCG overexpressing CysVac2 (rBCG:CysVac2). Expression of CysVac2 in BCG was facilitated by the M. tuberculosis hspX promoter, which is highly induced inside phagocytic cells and induces strong cellular immune responses to antigens expressed under its regulation. Intradermal vaccination with rBCG:CysVac2 resulted in increased monocyte/macrophage recruitment and enhanced antigen-specific CD4⁺ T cell priming compared to parental BCG, indicating CysVac2 overexpression had a marked effect on rBCG induced-immunity. Further, rBCG:CysVac2 was a more potent inducer of antigen-specific multifunctional CD4⁺ T cells (CD4⁺IFN-γ⁺TNF⁺IL-2⁺) than BCG after vaccination of mice. This improved immunogenicity however did not influence protective efficacy, with both BCG and rBCG:CysVac2 affording comparable level of protection aerosol infection with M. tuberculosis. Boosting either BCG or rBCG:CysVac2 with the CysVac2 fusion protein resulted in a similar improvement in protective efficacy. These results demonstrate that the expression of protective antigens in BCG can augment antigen-specific immunity after vaccination but does not alter protection against infection, further highlighting the challenge of developing effective vaccines to control TB.