Antigen priming with IL-21 imparts CD8⁺ T cells with a distinct gene expression program. Pmel-1 CD8⁺ T cells were antigen-primed with the cytokine indicated or without cytokine for 4 days, then restimulated for 4 hours without cytokine. Microarray gene expression analysis was then performed. Expression levels relative to the reference cells primed without cytokine are indicated. (A) Dendrogram showing the relatedness of gene expression patterns. (B) Microarray expression of selected genes associated with mature and immature effector CD8⁺ T cells. (C,D) Wild-type CD8⁺ T cells were stimulated with anti-CD3/anti-CD28 and 10 ng/mL of the indicated cytokine for 3 days and then restimulated in cytokine-neutral conditions for (C) 4 hours or (D) 3 days before real-time RT-PCR analysis. Expression of Tcf7 and Lef1 relative to β-actin is shown.