IRF4 expression by tetramer⁺CD8⁺T cells. PBMCs from HLA-A2⁺ healthy donors were prestained with Flu-M1–A2 tetramer and cultured with Flu-M1 GILGFVFTL peptide for 18 hours. (A) Frequency of tetramer⁺ cells in CD8⁺ T cells in unstimulated, DMSO-treated, and peptide-stimulated PBMCs. (B) Frequency of IRF4/CD69 (left panels), CD25/CD137 (middle panels), and IRF4/CD137 (right panels) of tetramer⁺ CD8⁺ T cells in unstimulated, DMSO-treated, and peptide-stimulated PBMCs. A representative result from 5 individual experiments is shown.