IL-7 induces proliferation of both naive and memory T cells uncoupled from T-cell activation, and memory-like phenotypic features in naive T cells. (A) Proliferation of naive and memory T cells treated with IL-7. Purified naive and memory CD4⁺ T cells were stained with CFSE and cultured in the presence or absence of low (0.2 ng/mL) or high (5 ng/mL) concentrations of IL-7. The lines represent the proportion of proliferating cells as evaluated by CFSE dilution over a period of 19 days in culture. The histograms in the insert show CSFE staining at day 19 in untreated (empty profile) and IL-7–treated (5 ng/mL; shaded histogram) CD4⁺ T lymphocytes. Data are representative of 3 independent experiments performed with similar results. (B) Expression of T-cell activation markers as evaluated by flow cytometry in purified naive and memory CD4⁺ T cells untreated or treated with the indicated cytokines over a period of 14 days. The values of MFI are shown for each marker at the indicated time points. Data are representative of 3 independent experiments performed with similar results. (C-F) Expression of memory-associated markers in naive and memory CD4⁺ T cells treated with IL-7. Purified naive and memory CD4⁺ T cells were left untreated or treated with the indicated doses of IL-7 and analyzed by flow cytometry after 3 days of culture for CD95 expression (C), cellular size (forward scatter, FSC; D), RNA content (pyronin Y; E), and TNF-α secretion (F). The ability to produce TNF-α was evaluated by intracellular staining followed by flow cytometry analysis on cells activated for an additional 6 hours with PMA and ionomycin. The shaded histograms denote the staining of IL-7–treated cells; the empty profiles denote the staining of control, untreated cells. Data are representative of 3 independent experiments performed with similar results.