CXCR4 and CX3CR1 expression on T cell subsets. Dot plots from a representative subject of CXCR4⁺ (red, A,C) and CX3CR1⁺ (blue, B,D) naive (N, CD45RA⁺CD62L⁺), central memory (CM, CD45RA⁻CD62L⁺), effector memory (EM, CD45RA⁻CD62L⁻), and effector (E, CD45RA⁺CD62L⁻) CD4⁺ (left) and CD8⁺ (right) T cells. Chemokine receptor-postitive cells are shown as colored dots, and mean percentages indicate respective subpopulation with the highest proportion of chemokine receptor positive cells; n = 6. Bar charts show mean plus or minus SEM of (E,F) CXCR4 and (G,H) CX3CR1 expression indicated as MFI; n = 6. *P < .05, **P < .01, ***P < .001, pairwise comparison between morning (09:00 h, red and blue bars) and evening (21:00 h, dark red and dark blue bars) levels and pairwise comparisons between naive vs central memory, central memory vs effector memory, and effector memory vs effector T cell counts (collapsed across morning and evening samples). (I) Correlation of CXCR4/CX3CR1 ratio (ln(CXCR4 MFI/CX3CR1 MFI), mean of n = 6) and percentage changes from night to day (Table 2) for naive (N), central memory (CM), effector memory (EM), and effector (E) CD4⁺ (○) and CD8⁺ (●) T cells. *P < .05. A high ratio predicts a strong daytime decrease, whereas a low ratio predicts a daytime increase. In an additional experiment, whole blood was sampled at 21:00 h and cultured in the presence or absence of cortisol at normal daytime concentrations (20 μg/dL). CXCR4 expression in naive (CD45RA⁺CD62L⁺) (J) CD4⁺ and (K) CD8⁺ T cells is presented as fluorescence intensity. Cells were stained with control immunoglobulin (filled histograms) or anti-CXCR4 antibody (empty histograms). FACS profiles are shown (from a representative subject) for gated T cells before (left empty histogram, thin line) and after 3 hours of culture without (middle empty histogram, gray line) and with cortisol (right empty histogram, red line).