Heterologous Boosting With <i>Listeria</i>-Based Recombinant Strains in BCG-Primed Mice Improved Protection Against Pulmonary Mycobacterial Infection

While Baccillus Calmette-Guerin (BCG) is used worldwide, tuberculosis (TB) is still a global concern due to the poor efficacy of BCG. Novel vaccine candidates are therefore urgently required. In this study, two attenuated recombinant Listeria strains, LMΔ -msv and LIΔ -msv were constructed by deletion of actA and plcB and expression of a fusion protein consisting of T cell epitopes from four Mycobacterium tuberculosis ( Mtb ) antigens ( Rv2460c, Rv2660c, Rv3875 , and Rv3804c ). The safety and immunogenicity of the two recombinant strains were evaluated in C57BL/6J mice. After intravenous immunization individually, both recombinant strains entered liver and spleen but eventually were eliminated from these organs after several days. Simultaneously, they induced antigen-specific cell-mediated immunity, indicating that the recombinant Listeria strains were immunogenic and safe in vivo . LMΔ -msv immunization induced stronger cellular immune responses than LIΔ -msv immunization, and when boosted with LIΔ -msv , antigen-specific IFN-γ CD8 + T cell responses were notably magnified. Furthermore, we evaluated the protection conferred by the vaccine candidates against mycobacterial infection via challenging the mice with 1 × 10 7 CFU of BCG. Especially, we tested the feasibility of application of them as heterologous BCG supplement vaccine by immunization of mice with BCG firstly, and boosted with LMΔ -msv and LIΔ -msv sequentially before challenging. Combination immune strategy (LMΔ -msv prime-LIΔ -msv boost) conferred comparable protection efficacy as BCG alone. More importantly, BCG-vaccinated mice acquired stronger resistance to Mycobacterial challenge when boosted with LMΔ -msv and LIΔ -msv sequentially. Our results inferred that heterologous immunization with Listeria -based recombinant strains boosted BCG-primed protection against pulmonary mycobacterial infection.