Overexpression of IL-15 in vivo enhances protection against Mycobacterium bovis bacillus Calmette-Guérin infection via augmentation of NK and T cytotoxic 1 responses

To investigate the immunomodulating effects of IL-15 in vivo on mycobacterial infection, we used IL-15-transgenic (Tg) mice, which were recently constructed with cDNA.encoding secretable isoform of IL-15 precursor protein under the control of a MHC class I promoter. The IL-15-Tg mice exhibited resistance against infection with Mycobacterium bovis bacillus Calmette. Guérin (BCG), as assessed by bacteria growth. IFN-γ level in serum was significantly higher in IL-15-Tg mice than in non-Tg mice after BCG infection. NK cells were remarkably increased, and Ag-specific T cytotoxic 1 response mediated by CD8⁺ T cells producing IFN-γ was significantly augmented in the IL-15-Tg mice following BCG infection. Neutralization of endogenous IFN-γ by in vivo administration of anti-IFN-γ mAb deteriorated the clearance of the bacteria. Depletion of NK cells or CD8⁺ T cells by in vivo administration of anti-asialo-GM₁ Ab or anti-CD8 mAb hampered the exclusion of bacteria. Thus, overexpression of IL-15 in vivo enhanced protection against BCG infection via augmentation of NK and T cytotoxic 1 responses.