Recombinant Mycobacterium smegmatis producing a functional M. tuberculosis ESX-1 system is protective in the murine model of bovine TB without sensitization to tuberculin.

Abstract

Bovine tuberculosis (bTB) is a chronic, productivity-limiting infection in livestock with significant zoonotic potential that is caused by members of the Mycobacterium tuberculosis-complex (MTBC) which includes Mycobacterium bovis. Although the live, attenuated M. bovis bacillus Calmette-Guérin (BCG) vaccine licensed for use in people has been shown to protect cattle against bTB in experimental settings, its capacity to sensitize vaccinated animals to tuberculin used in bTB diagnosis is a major impediment to its use in livestock. As such, a bTB vaccine that will allow for the differentiation of M. bovis-infected animals among vaccinated animals (DIVA) is preferred. In a previous study, we reported that a recombinant Mycobacterium smegmatis strain engineered to express a functional MTBC type-7 ESX-1 secretion system called MSX-1 protects C57BL/6 mice against the causative agent of human TB, M. tuberculosis, without sensitizing the vaccinated mice to tuberculin. In this study, we wanted to determine if MSX-1 will also protect mice against M. bovis. Accordingly, we found that C57BL/6 mice vaccinated with MSX-1 and challenged with M. bovis had reduced burdens of bTB bacilli in their lungs and spleens and presented with reduced lung pathology. Furthermore, MSX-1 vaccination reduced bTB-mediated weight-loss and lethality in challenged mice. Consistent with previous observations, we found that mice vaccinated with MSX-1 did not become sensitized to tuberculin nor to a peptide fragment of EsxA, a potent T-cell antigen and secreted protein of the M. tuberculosis ESX-1 system. While the lack of sensitization in mice by MSX-1 to tuberculin and EsxA underscores its promise as a DIVA bTB vaccine, it suggests the mechanism of protection of MSX-1 may be CD4and CD8T-cell and IFN-γ independent and will require further investigation. Nevertheless, our results indicate MSX-1 is an effective bTB vaccine that deserves further development for use in livestock.