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ABSTRACTGamma interferon (IFN-γ)-secreting CD4+T cells have long been established as an essential component of the protective immune response against Mycobacterium tuberculosis. It is now becoming evident from studies with the murine model of tuberculosis that an important role also exists for major histocompatibility complex (MHC) class I-restricted CD8+T cells. These cells are capable of acting as both IFN-γ secretors and cytotoxic T lymphocyte (CTL) effectors; however, their exact role in immunity against tuberculosis remains unclear. This study demonstrates the presence ofMycobacterium bovisBCG-reactive CD8+T cells in healthy BCG-vaccinated donors and that these CD8+T cells are potent cytokine producers as well as cytotoxic effector cells. Using FACScan analysis, we have shown that restimulation with live M. bovisBCG induced more CD8+-T-cell activation than the soluble antigen purified protein derivative and that these cells are actively producing the type 1 cytokines IFN-γ and tumor necrosis factor alpha (TNF-α). These CD8+T cells also contain the cytolytic granule perforin and are capable of acting as potent CTLs against M. bovisBCG-infected macrophages. The mycobacterial antigens 85A and B (Ag85A and Ag85B, respectively), and to a lesser extent the 19- and 38-kDa proteins, are major antigenic targets for these mycobacterium-specific CD8+T cells, while whole-M. bovisBCG activated effector cells from these BCG-vaccinated donors, as expected, failed to recognize the 6-kDa ESAT-6 protein. The use of metabolic inhibitors and blocking antibodies revealed that the CD8+T cells recognize antigen processed and presented via the classical MHC class I pathway. These data suggest that CD8+T cells may play a critical role in the human immune response to tuberculosis infection.