H65 fusion protein fails to improve the protection of a rationally attenuated live vaccine candidate against bovine tuberculosis in a mouse model of tuberculosis

Background The fusion protein H65, composed of Mycobacterium tuberculosis (TB) ESX-secreted antigens, has improved the bacillus Calmette-Guerin-induced immune protection in a mouse model of bovine TB when formulated in the liposomal adjuvant CAF01. In this study, we aimed to evaluate the protective efficacy of an attenuated Mycobacterium bovis strain - a mutant in mce2 and phoP genes - combined with H65+CAF01 immunization. We evaluated the protection of MbΔmce2-phoP alone or combined with H65+CAF01 against M. bovis challenge in mice. Methods Groups of BALBc mice were inoculated with the vaccine candidates or phosphate buffered saline (PBS), and 6 weeks after the last immunization, the animals were aerogenically challenged with virulent M. bovis. Bacterial load in organs was counted after 45 days of the challenge. One-way analysis of variance and Bonferroni's posttest were used for statistical analysis. Results All vaccinated mice showed reduced bacterial loads in lungs compared to unvaccinated animals. However, the protection level was similar between vaccinated groups. Conclusions The MbΔmce2-phoP strain combined with three doses of H65+CAF01 induced equivalent protection than the MbΔmce2-phoP strain alone. Thus, the use of combined vaccination strategies requires a careful analysis of the potential interactions of each of their components with the host's immune system.