Figure 1. Memory CD4⁺ T cells recirculate between naive skin and blood.

(a) Microscopy of naive mouse flank, abdomen, ear and footpad skin, detecting CD4 (orange) and CD8 (green) using fluorescently labelled antibodies. Cell nuclei (blue) are visualized by Hoechst 33342. Scale bar, 100 μm. (b) Number of T cells in sections of naive flank, abdomen, ear and footpad skin; n=7–14. (c) Phenotype of CD4 T cells in naive flank skin. Left plots gated on CD45.2⁺ live cells, middle and right plots gated on TCRβ⁺ CD4⁺ cells. (d,e) Naive CD45.1⁺ and CD45.2⁺ mice were surgically joined for 12–16 weeks then assessed by flow cytometry. Graphs display the ratio of CD45.1⁺: CD45.2⁺ CD4 T cells in the spleen and skin of the CD45.1⁺ (d) and CD45.2⁺ (e) partners. (f,g) CD69 and CD103 expression by host and partner CD4 T cells from spleen and skin of parabiotic mice in d,e; middle plots gated on CD4⁺ cells from the CD45.1⁺ partner. (h) Kaede red CD4 T cells in the brachial lymph node analysed immediately after and 3 days following photo-conversion. (i) CD69 and CD103 expression of kaede red and green CD4 T cells in the brachial lymph node 3 days following photo-conversion. Each symbol in d,e,g represents an individual mouse; n=26 mice from 13 pairs. NS, not significant; *P<0.05, **P<0.01, ***P<0.001, **** P<0.0001; two-tailed paired t-test. Data in a,c,f,h,i) are representative of at least two experiments. Data in b,d,e,g are pooled from two to three experiments. Mean (d,e,g), mean and s.e.m. (b,i). Numbers in plots of c,f represent frequency of events in the respective gates. Abd; abdominal skin; LN, lymph node.