IL-15Rα expression by host cells is crucial for memory CD8 T-cell proliferation. 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. Mice were killed 4 weeks later. (A) Lymphocytes from spleen were analyzed for CFSE intensity after gating on either N-tetramer-positive CD8⁺ T cells or CD44^hiCD8⁺ T cells. R indicates the percent of cells that responded. Panel B shows the number of tetramer-positive cells recovered from the spleens of each group of mice. Error bars indicate SD (P < .05).