To investigate the potential role of endogenous IL-15 in mycobacterial infection, we examined protective immunity in IL-15-deficient (IL-15 -/-) mice after infection with Mycobacterium bovis bacillus Calmette-Guérin (BCG) or recombinant OVA-expressing BCG (rBCG-OVA). IL-15-/- mice exhibited an impaired protection in the lung on day 120 after BCG infection as assessed by bacterial growth. CD4+ Th1 response capable of producing IFN-γ was normally detected in spleen and lung of IL-15-/- mice on day 120 after infection. Although Ag-specific CD8 responses capable of producing IFN-γ and exhibiting cytotoxic activity were detected in the lung on day 21 after infection with rBCG-OVA, the responses were severely impaired on days 70 and 120 in IL-15 -/- mice. The degree of proliferation of Ag-specific CD8+ T cells in IL-15-/- mice was similar to that in wild-type mice during the course of infection with rBCG-OVA, whereas sensitivity to apoptosis of Ag-specific CD8+ T cells significantly increased in IL-15 -/- mice. These results suggest that IL-15 plays an important role in the development of long-lasting protective immunity to BCG infection via sustaining CD8 responses in the lung.
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