The TCRζ^dim cell subset is enriched for antigen experienced T cells. (A) Healthy donor PBLs were stained with CFSE prior to stimulation with OKT3. At the indicated times, cells were harvested, stained for TCRζ together with CD3, CD4, or CD8, and dilution of CFSE fluorescence determined by flow cytometry. Proliferating TCRζ^bright cells are indicated (arrows). (B) Fresh PBLs from HLA-B*0702⁺ CMV-reactive donors were stimulated in vitro with p65 CMV peptide, IL-2, and IL-7. Eight days later, cells were stained for the presence of peptide-reactive CD8⁺ T cells using fluorescent HLA-B7/p65 MHC-peptide tetramers and anti-CD8. (C) Cells were stained for TCRζ expression and the percent CD8⁺ tetramer-positive T cells within TCRζ^bright and TCRζ^dim subsets determined. The percent tetramer-positive cells is indicated. Data are representative of multiple experiments.