The Mycobacterium tuberculosis Stress Response Factor SigH Is Required for Bacterial Burden as Well as Immunopathology in Primate Lungs

Background. Sigma H (sigH) is a major Mycobacterium tuberculosis (Mtb) stress response factor. It is induced in response to heat, oxidative stress, cell wall damage, and hypoxia. Infection of macrophages with the delta-sigH mutant generates more potent innate immune response than does infection with Mtb. The mutant is attenuated for pathology in mice. Methods. We used a nonhuman primate (NHP) model of acute tuberculosis, to better understand the phenotype of the delta-sigH mutant in vivo. NHPs were infected with high doses of Mtb or the mutant, and the progression of tuberculosis was analyzed in both groups using clinical, pathological, microbiological, and immunological parameters. Results. Animals exposed to Mtb rapidly progressed to acute pulmonary tuberculosis as indicated by worsening clinical correlates, high lung bacterial burden, and granulomatous immunopathology. All the animals rapidly succumbed to tuberculosis. On the other hand, the NHPs exposed to the Mtb:delta-sigH mutant did not exhibit acute tuberculosis, instead showing significantly blunted disease. These NHPs survived the entire duration of the study. Conclusions. The Mtb:delta-sigH mutant is completely attenuated for bacterial burden as well as immunopathology in NHPs. SigH and its regulon are required for complete virulence in primates. Further studies are needed to identify the molecular mechanism of this attenuation.