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Immunological responses and protective immunity against tuberculosis conferred by vaccination of Balb/C mice with the attenuated Mycobacterium tuberculosis (phoP) SO2 strain

The Mycobacterium tuberculosis phoP mutant strain SO2 has been shown previously to be more attenuated than Mycobacterium bovis bacillus Calmette–Guérin (BCG) and confers protective immunity against tuberculosis in mice and guinea pig models. In this study we have investigated the survival and immunological responses of Balb/c mice infected with the M. tuberculosis SO2 strain. All Balb/C mice survived intratracheal infection with M. tuberculosis SO2 strain under conditions where all the mice infected with the parental M. tuberculosis MT103 had died after 9 weeks. Infection of Balb/c mice with M. tuberculosis SO2 was associated with comparatively lower levels of interferon (IFN)‐γ, interleukin (IL)‐4 and tumour necrosis factor (TNF)‐α and higher levels of inducible nitric oxide synthase (iNOS) during the late stage of infection, when compared with M. tuberculosis MT103 infection. The delayed‐type hypersensitivity (DTH) response against M. tuberculosis culture filtrates was similar in mice infected with either the M. tuberculosis phoP SO2 strain or M. tuberculosis MT103. The protective efficacy of M. tuberculosis SO2 was compared with M. bovis BCG when delivered subcutaneously to groups of Balb/C mice. Following intratracheal challenge with M. tuberculosis H37Rv, protection was generated by 60 days post‐challenge in mice vaccinated with either vaccine. At day 120 post‐challenge the levels of protection were still significantly greater when compared with the non‐vaccinated control group. The levels of protection conferred by vaccination with M. tuberculosis SO2 or with M. bovis BCG were similar, as measured by granuloma coalescence and pneumonia in addition to growth reduction of M. tuberculosis H37Rv.