Aerosol infection of mice with recombinant BCG secreting murine IFN-γ partially reconstitutes local protective immunity

Abstract

To better understand the contribution of interferon-gamma (IFN-γ) to the immune response during the first 60 days of mycobacterial infection in the lungs, IFN-γ gene disrupted (IFN-γ-/-) mice were infected via aerosol with recombinant Mycobacterium bovis Bacillus Calmette-Guerin (BCG) secreting murine IFN-γ (BCG-IFN-γ) and compared to mice infected with recombinant BCG containing the vector only (BCG-vector). When IFN-γ-/- mice were infected with BCG-vector, increasing bacillary loads and large undifferentiated granulomas that did not express inducible nitric oxide synthase (iNOS) were observed in the lungs. In contrast, infection with BCG-IFN-γ resulted in reduced bacillary load and better differentiated granulomas containing epithelioid macrophages expressing iNOS as well as reduced levels of interleukin 10 (IL-10) mRNA. However, local production of IFN-γ by the recombinant BCG did not protect IFN-γ-/- mice from subsequent challenge with M. tuberculosis. Infection of IFN-γ-/- peritoneal macrophages in vitro with BCG-IFN-γ led to induction of iNOS expression and lower IL-10 mRNA levels. Nevertheless, the growth of the intracellular BCG was unaffected. Since IFN-γ induced-iNOS protein and reduced IL-10 production were insufficient to control mycobacterial growth in vitro, the results suggest that additional mediator(s) present in vivo are required for control of mycobacterial growth.