The antitumor efficacy of CD8⁺ T cells for adoptive transfer is impaired by IL-2 and IL-15, but enhanced by IL-21. Pmel-1 CD8⁺ T cells were primed with cognate antigen and 10 ng/mL of the specified cytokine for 4 days then adoptively transferred into tumor-bearing hosts. Vaccination and exogenous IL-2 were administered to mice in all but the “No treatment” group. (A) Tumor response to adoptive transfer of 2.5 × 10⁵ cells. The cytokine present during priming is indicated. Error bars reflect the standard error of the mean. (B) Pmel-1 Thy1.1 CD8⁺ T cells were antigen-primed with 10 ng/mL of the indicated cytokine. At 4 days after priming, 5 × 10⁵ cells per mouse were infused, and vaccine and IL-2 were administered. The number of Thy1.1 splenocytes was determined 12 days after transfer. The scatter plots indicate individual mice. The horizontal lines represent the means. *P < .05 compared with “No cytokine.”