Differential expression of CD45RA, CCR7, and CD27 cell surface molecules on total CD8⁺ T cells from healthy individuals. CD8⁺ gated cells were separated into 4 subsets (N, E, CM, EM) based on CD45RA and CCR7 labeling as previously described³  (A,C). Each of these subsets was analyzed for CD27 expression (B), and 6 subpopulations of CD8⁺ T cells could be distinguished (D). The percentage of CD27⁺ cells is indicated. Analysis performed on a representative healthy donor is depicted by the black histogram. The RA⁺CCR7^–27⁺ T-cell subset (gray peak) is referred to as “?” (unknown). The distribution of the 6 defined CD8⁺ T-cell subsets among 20 healthy individuals ranging in age from 20 to 70 years is shown in panel E as mean percentage (range). N indicates naive; E, effector; CM, central-memory; and EM, effector-memory.