IL-15 administration increases the proliferation of CD8⁺ T and NK cells and decreases the number of apoptotic CD8⁺ T-cells in recipients of an allo TCD-BMT. Mice received transplants as described Figure 1 and received 2.5 μg/d IL-15 or PBS (control) on days 21 through 27 by intraperitoneal injection. (A) Recipients were killed at day 28, and absolute numbers of donor-derived cell populations in the spleen were calculated from total cell counts and multicolor flow cytometric analyses of T-cells intracellularly stained with Ki67. (B) The percentage of apoptotic (Annexin-V(+) 7-AAD(-)) donor-derived CD8⁺ T-cells from young and old BMT recipients are shown. (C) The percentages of Bcl-2–expressing donor-derived CD8⁺ T-cells from young and old BMT recipients are shown. Values represent the median cell number ± 25% and 75% of the population (n = 5).