IL-15 administration specifically enhances CD8⁺CD44⁺CD122⁺ memory 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 14 through 21 or 21 through 27 by intraperitoneal injection (panels A and B, respectively). Recipients were killed at day 21 or 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 with anti-CD8, -CD44, and -CD122 antibodies. Naive CD8 cells were defined as CD8⁺ CD44^-(low) and memory/activated (MEM/ACT) CD8 cells were defined as CD8⁺ CD122⁺CD44^+(high) and CD8⁺ CD122^-CD44^+(high). Donor or host origin was determined with anti-Ly 9.1, which is present on host leukocytes. Values represent the median cell number ± 25% and 75% of the population (n = 5).