IFN-γ production by purified cell populations in response to IL-15 stimulation. CD4⁺ T cells, CD8⁺ T cells, and CD56⁺ NK cells were purified from PBMC by negative selection as described in Materials and Methods. The top panels represent the flow cytometry analyses of purified cell population (all 98% pure; black histograms) using anti-CD4 (left panel), anti-CD8 (middle), and anti-CD56 (right) antibodies. The isotype control antibody is represented by the white histograms. The bottom panels represent kinetics of IFN-γ production by corresponding cell population after IL-15 stimulation. Cells were seeded at 10⁶/mL in RPMI medium supplemented or not with IL-15. Cell-free supernatants were collected every 2 days starting on day 2 and were tested by ELISA for IFN-γ production. (○) Mock; (•) 50 ng/mL IL-15. Results are expressed as picograms per milliliter of IFN-γ (mean ± SD of triplicate cultures).