Kinetics of the responsiveness of IL-7–specific cell lines to stromal cells. (A and B) The kinetics of the secretion of IL-7 by FACS-purified stromal cells was determined by measuring changes in the percentage of responding IL-7–specific cells in S/G2/M stages of the cell cycle. (C and D) Age-related differences in the kinetics of the responses to stromal cells are not due to the death of the responding cells. (□) 0 ng/mL IL-7; (▵) 0.001 ng/mL IL-7; (◊) 1.0 ng/mL IL-7; (▴) 1-month-old SC; (•) 24-month-old SC. (A) and (C) show representative results with the IL-7–specific cell line BC77; (B) and (D) show representative results with BC715. Each cell line was tested in two or three separate experiments. In all panels, the responses of the IL-7–specific cells to 0.001 and 1.0 ng/mL of rIL-7 are shown solely for comparison.