IL-15 administration increases CD8⁺ T, NK, and NK T-cells in young and old recipients of an allo TCD-BMT. Lethally irradiated (1300 cGy) CBA/J recipients received transplants of B10.BR TCD-BM (5 × 10⁶) and 2.5 μg/d IL-15 or PBS (control) on days 21 through 27 by intraperitoneal injection. BMT 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 with anti-CD3, -CD4, -CD8, and -NK1.1 antibodies. NK cells were defined as NK1.1⁺CD3^-, and NK T-cells were defined as NK1.1⁺CD3⁺. Data for young (3-month-old) recipients are shown in Figures 2A-B and for old (10-month-old) recipients in Figures 2C-D. (E) Lethally irradiated (1300 cGy) (B6xC3H)F1 recipients received transplants of B6 TCD-BM (5 × 10⁶). Mice were treated and harvested as shown in Figure 2A-2B. (F) Splenocytes from IL-15– or PBS-treated allo TCD-BMT recipients were harvested day +28 and cultured for 5 days with irradiated BALB/c splenocytes (third party) or CBA splenocytes (host). ³H-thymidine was added during the final 18 hours of culture, and proliferation was determined. Stimulation index was calculated as Proliferation in MLR (cpm)–spontaneous proliferation (cpm)/spontaneous proliferation (cpm). (G) Mice received transplants and were treated as described Figure 2A-D and NK 1.1⁺, CD3^- NK cells were sorted, and cytotoxicity against YAC-1 and A20 cell line was determined in a ⁵¹Cr release assay. 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 = 4-20).