Memory CD8 T-cell responses to IL-15 require IL-15Rα expression by opposing cells and IL-15Rβ expression by memory CD8 T cells. (A) Spleen cells from VSV-infected B6 mice were enriched for CD8 T cells, labeled with CFSE, and transferred to either normal or IL-15Rα^-/- hosts. Approximately 10 × 10⁶ CD8 T cells containing 0.5 × 10⁶ tetramer-positive cells were transferred into each mouse, followed by treatment with 2.5 μg of rIL-15 intravenously. Three days after IL-15 treatment, lymphocytes from spleen and lung were analyzed for CFSE intensity on N-tetramer-positive CD8⁺ T cells and CD44^hiCD8⁺ T cells. In the second column, the division peaks are labeled with a demarcation between the zero and first divisions. (B) Spleen and lymph node (LN) cells from IL-2/15Rβ^-/- mice or wild-type mice were enriched for CD8 T cells, labeled with CFSE, and transferred into normal Ly5.2 hosts, and treated with rIL-15 intravenously. Three days later, lymphocytes from spleen were analyzed for CFSE intensity on Ly5.1⁺CD44^hiCD8⁺T cells.