Intravenous Immunization with Triple Auxotrophs of Mycobacterium tuberculosis: A novel vaccine strategy against tuberculosis

Tuberculosis, caused by Mycobacterium tuberculosis (Mtb), remains a leading infectious cause of mortality worldwide despite widespread use of the BCG vaccine and the availability of sterilizing pharmacopoeia. Recent research indicates that the intravenous administration of BCG confers sterilizing immunity against Mtb pulmonary challenge in non-human primates. However, while BCG is relatively safe, complications such as disseminated BCGosis have been observed in immunocompromised individuals. Double auxotrophic mutants of Mtb lacking the ability to synthesize leucine and pantothenate are safe and sterilized in immunocompromised mice and SIV-infected Rhesus macaques. We examined how immunization with a Mtb triple auxotrophic strain, mc27902, which cannot synthesize leucine, pantothenate, and arginine, protects immunocompetent mice from a virulent Mtb infection. The route of immunization was a crucial factor for protection with mc27902 with intravenous immunization being 100 times more effective in protecting immunocompetent mice from Mtb challenge when compared to conventional subcutaneous vaccination with BCG. To further increase the safety of the attenuated auxotroph for vaccine purposes, the type VII secretion system Esx1 responsible for BCG attenuation was deleted in mc27902. When tested by prime-boost immunization of immunocompetent mice followed by aerosol challenge with virulent Mtb, mc27902 {Delta}esx1 provided similar protection to mc27902. This robust protection against Mtb infection conferred by mc27902 and mc27902 {Delta}esx1 in a mouse model paves the way for new TB vaccine development using highly attenuated, auxotrophic Mtb strains.