Memory T cells exhibit greater effector function than naïve T cells. (a) Fluorescence-activated cell sorting analysis of CD4+ subpopulations. Resting CD4+ lymphocytes in normal peripheral blood contains CD45RA+CCR7+ (naïve) lymphocytes (left panel) together with CD45RO+CCR7+ central memory (TCM; right panel) and CD45RO+CCR7- effector memory populations (TEM; right panel). (b) Effector function resides mainly within the CD4+CD45RO+ population. Lymphocytes were sorted into naïve (CD4+CD45RA+) and memory (CD4+CD45RO+) populations at time zero, before stimulation for 8 days with IL-2, IL-6 and tumour necrosis factor (TNF)-α. Lymphocytes were then co-cultured with monocytes at a ratio of 0.5:1 and 1:1 for 18 hours and the supernatants assayed in triplicate for TNF-α production by ELISA. ***P < 0.001 comparing CD4+CD45RA+ versus CD4+CD45RO+ populations using one-way analysis of variance (ANOVA) with Bonferroni's multiple comparison. (c) Effector function resides predominates within CD4+ effector memory population. CD45RO+ cells were further sorted by flow cytometry into CCR7+ and CCR7- populations at day 0 (and stimulated for 8 days with cytokines) or at day 8 (after cytokine expansion) and then co-cultured with monocytes at a ratio of 1:1 for 18 hours, and the supernatants assayed in triplicate for TNF-α production by ELISA. ***P < 0.001 comparing CD4+CD45RO+CCR7- versus CD4+CD45RO+CCR7+ after sorting at day 0 sort or at day 8 using one-way ANOVA with Bonferroni's multiple comparison. In panels b and c monocytes and T cells cultured alone as negative controls produced under 20 pg/ml TNF-α. Results are shown from one donor representative of five different experiments (panel b) and three different experiments (panel c). CCR, CC chemokine receptor.