A BCG kill switch strain protects against Mycobacterium tuberculosis in mice and non-human primates with improved safety and immunogenicity.

Improved vaccination strategies for tuberculosis are needed. Intravenous (i.v.) delivery of live attenuated Mycobacterium bovis BCG provides protection against Mycobacterium tuberculosis (Mtb) in macaques but poses safety challenges. Here we genetically engineered two strains, BCG-TetON-DL and BCG-TetOFF-DL, to either induce or inhibit expression of two phage lysin operons, respectively, upon tetracycline exposure. We show that lysin expression kills BCG in vitro, in infected macrophages, and following infection of immunocompetent (C57BL/6) and immunocompromised (SCID) mice. Modified BCG elicited similar immune responses and provided similar protection against Mtb challenge as wild-type BCG in mice. In macaques, cessation of tetracycline treatment reduced i.v.-administered BCG-TetOFF-DL numbers. Intravenous BCG-TetOFF-DL increased pulmonary CD4 T-cell responses compared with wild-type BCG-induced responses and provided robust protection against Mtb challenge. Sterilizing immunity occurred in 6 of 8 macaques compared with 2 of 8 wild-type BCG-immunized macaques. Thus, a 'kill-switch' BCG strain provides additional safety and robust protection against Mtb infection.