Assessment of the percentage of peptide-specific CD8+T cells expressing CD5. Responder cells were labeled with CFSE, stimulated with the desired peptides for 7 days, surface stained for CD3, CD8 and CD5 and then analyzed by flow cytometry. CD3+CD8+ lymphocytes were positively gated from the live lymphocyte population (a) and the fraction of proliferating cells expressing CD5 was calculated by visualizing CFSE fluorescence intensity versus CD5 in plots b-d. Proliferating, peptide responsive cells have lower CFSE fluorescence intensity than the parental non-responding, non proliferating population (b). Numbers in the quadrants of plots (b-d) represent CD5+ (upper quadrants) and CD5-negative (lower quadrants) percentages of the non-responding (right hand quadrants) and peptide responsive CD8+ T cell populations (left hand quadrants). Representative examples where the percentage of peptide-responsive, proliferating cells not expressing CD5 spans a range of 1% (b) to 16% (d) are shown for CD8+ T cell responses to the CMV (b) PR (c) and Gag (d) peptides described in Table 1.