Neonatal RTEs, but not adult RTEs, proliferate in response to IL-7. (A) CD4⁺ RTEs, intermediate, and resident LN cells were sorted from adults and cultured with increasing concentrations of IL-7 or PBαCD3 and αCD28 for 48 hours. ³H-thymidine was added during the last 20 hours of culture. (B) Neonatal RTEs and intermediate cells were sorted and cultured in complete media, 10 ng/mL IL-7, or activated with PBαCD3 and αCD28 for 48 hours. ³H-thymidine was added for the last 20 hours of culture. Graphs represent pooled data from 3 or 4 independent experiments; n = 6 to 8 adults and 28 to 44 neonates per experiment. All data are shown as the mean ± SEM. (C) CD4⁺ RTEs and intermediate cells were sorted from neonatal LN and then cultured in complete media, 10 ng/mL IL-7, or activated with PBαCD3 and αCD28 for 72 hours. The percentage of proliferating cells was determined by propidium iodide (PI) staining, as described in “Measurement of proliferation and cell cycle.” Graphs represent pooled data from 3 independent experiments; n = 23 to 56 neonates per experiment. All data are shown as the mean ± SEM.