Differential cytokine receptor expression, IL-15–mediated proliferation, and caspase activity of T_CM/Eand T_EM/E. (A) IL-15Rα, IL-2Rβ, or IL-2Rγ expression by T_CM/E and T_EM/E. Mean fluorescence intensity was normalized to that of isotype control staining in each case to determine ΔMFI. (B) Proliferation of T_CM/E and T_EM/E was determined after 48-hour incubation with different concentrations of rhIL-15 using a standard [³H]-thymidine incorporation assay. (C-D) CFSE-labeled T_CM/E and T_EM/E (10⁷) were injected intravenously into mice at day 0, and irradiated NS0-IL-15 cells (1.5 × 10⁷) were administered 3 times a week starting at day 0, until mice were killed at either day 9 or day 12. (C) CFSE profiles of the input and engrafted T_CM/E and T_EM/E in day 9 PBL was assessed by flow cytometry. Percentage of CFSE-diluted cells that fall within the first log are indicated. (D) Engraftment of the CD45⁺ human T cells in the PBL, bone marrow, and spleen was assessed on days 9 and 12 by flow cytometry. (E) FL-1 profiles of CD45⁺ human T cells in the PBL were assessed on day 9 as a readout for cleavage of the caspase substrate D₂R. The percentage of cells with cleaved D₂R is depicted.