Enhanced protection against fatal mycobacterial infection in SCID beige mice by reshaping innate immunity with IFN-γ transgene

Humans with immune-compromised conditions such as SCID are unable to control infection caused by normally nonpathogenic intracellular pathogens such as Mycobacterium bovis bacillus Calmette-Guérin. We found that SCID beige mice lacking both lymphocytes and NK cells had functionally normal lung macrophages and yet a selectively impaired response of type 1 cytokines IFN-γ and IL-12, but not TNF-α, during M. bovis bacillus Calmette-Guérin infection. These mice succumbed to such infection. A repeated lung gene transfer strategy was designed to reconstitute IFN-γ in the lung, which allowed investigation of whether adequate activation of innate macrophages could enhance host defense in the complete absence of lymphocytes. IFN-γ transgene-based treatment was initiated 10 days after the establishment of mycobacterial infection and led to increased levels of both IFN-γ and IL-12, but not TNF-α, in the lung. Lung macrophages were activated to express increased MHC molecules, type 1 cytokines and NO, and increased phagocytic and mycobactericidal activities. Activation of innate immunity markedly inhibited otherwise uncontrollable growth of mycobacteria and prolonged the survival of infected SCID hosts. Thus, our study proposes a cytokine transgene-based therapeutic modality to enhance host defense in immune-compromised hosts against intracellular bacterial infection, and suggests a central effector activity played by IFN-γ-activated macrophages in antimycobacterial cell-mediated immunity.