SOCS1-deficient CD8⁺ SP thymocytes hyperproliferate to IL-15 stimulation. (A) SOCS1-deficient thymocytes show increased proliferative responses to γ_c cytokines. Total thymocytes from 6-week-old SOCS1^-/-IFNγ^-/- mice and their littermate controls were stimulated with indicated concentrations of IL-7, IL-2, and IL-15 for 48 hours. ³H-thymidine incorporation during the last 8 hours is shown. Error bars indicate standard error. (B) SOCS1-deficient DN and CD8⁺ SP thymocytes selectively respond to IL-15 and IL-2. Total thymocytes from SOCS1^-/-IFNγ^-/- and control mice were loaded with CFSE and stimulated with a 1:20 dilution of IL-7- or IL-2-containing CS or with 40 ng mL^-1 IL-15. After 72 hours, the cells were stained for CD4 and CD8 and were analyzed for diluted CFSE as a function of cell division. Representative data from at least 6 animals for each group from 3 experiments are shown. Numbers within histograms denote the frequency of cells that underwent 1 or 3 cell divisions.