IL-21 is unique among γ-chain receptor cytokines in its ability to induce expansion of antigen-specific CTLs. Unmanipulated or CD25-depleted PBMCs from HLA-A2⁺ patients with melanoma were stimulated in vitro with autologous mature dendritic cells pulsed with the MART-1 M26 peptide as described in “In vitro generation of human antigen-specific CD8⁺ T cells.” Optimal concentrations of IL-2 (12.5 U/mL), IL-7 (10 ng/mL), IL-15 (30 U/mL), and IL-21 (30 ng/mL) were added to individual cultures at time of stimulation on day 1. Cells (5 × 10⁵) from each experiment group were harvested on day 7 and stained with 20 μg/mL peptide/MHC tetramer PE- and PerCP-conjugated CD8. Data are expressed as percentage of tetramer-positive cells among gated lymphocytes.