Adoptively transferred CMV-specific CD8⁺T_CM/Eexhibit a better response to antigen challenge in vivo than T_EM/E. (A) Schematic of in vivo antigenic stimulation of engrafted CMV-specific T_CM/E and T_EM/E. (B) Engraftment of CMV-specific T_CM/E (squares) or T_EM/E (circles) was carried out with (black) or without (white) administration of irradiated CMV pp65-expressing LCLs at days 3, 10, and17; and mean percentage (± SE) of human T cells (CD45⁺ CD8⁺) in mouse PBL was determined by flow cytometry (n = 6). *P < .05, engraftment of T_CM/E alone versus pp65-driven T_CM/E engraftment (unpaired Student t test). (C) On euthanasia at day 28, PBLs were harvested and analyzed by flow cytometry for percentage of Ki-67⁺ cells in the human T-cell population (left) and for the ability of CD45⁺ human T cells to cleave the caspase substrate D₂R (right).