Naive CD8+ T cells acquire cytotoxic properties upon IL-15 stimulation. The purified subsets were stimulated with IL-15 (10 ng/mL) for 7 days. (A) Naive CD8⁺ T cells were stained intracellular for perforin and granzyme B, followed by FACS analysis. Dotted line indicates control mAbs; thin line, resting; and bold line, IL-15-stimulated cells. (B) Purified CD8⁺ T-cell subsets were tested in a redirected cytotoxicity assay. Targets (P815 cells) were mixed with CD8⁺ T cells from UCB (•), peripheral naive CD8⁺ T cells (▴), cytotoxic CD8⁺ T cells (CD27^-CD45RA^bright) (♦), and noncytotoxic CD8⁺ T cells (CD27⁺CD45RA^dull/neg) (•) in the presence of anti-CD3 mAbs, resting (left panel) or IL-15-stimulated (right panel). Resting and IL-15-stimulated purified subsets did not lyse P815 cells in the absence of CD3 mAbs (data not shown). Data are one representative experiment of 3 performed. E/T ratio indicates effector-to-target ratio.