The IRF4-CD137 assay detects Ag-specific CD8⁺T cells with low background. PBMCs were cultured with CEF peptides for 18 hours and analyzed for the expression of the indicated molecules. Frequency of IRF4⁺CD69⁺ (top panels), IRF4⁺CD137⁺ (middle panels), and CD25⁺CD137⁺ cells (bottom panels) in CD8⁺ T cells. Two representative results are shown.