Generation of CD45RA revertants in CD8⁺ T cells by IL-15. CD8⁺ CD45RO⁺ T cells were negatively depleted from freshly isolated PBMCs and placed in culture with fresh medium supplemented by a single dose of either fibroblast-conditioned medium (FCM), IL-2, IL-7, or exogenous IL-15. The extent of proliferation in CD8⁺ tetramer⁺ T cells (A) was examined by using antibodies to Ki67 on days 0, 7, and 13. CD45R expression was also determined on days 0, 7, and 13 in CD8⁺ tetramer⁺ T-cell populations (B) and in total CD8⁺ T cells (B) exposed to either FCM or IL-15. (C) Western blot analysis of cell-cycle proteins (cyclin D₂/D₃, p27, and actin) in freshly isolated, resting CD8⁺ CD45RO⁺ memory T cells (lane 1). These CD8⁺ CD45RO⁺ memory T cells were subsequently exposed to a single dose of exogenous IL-15. The relative expression of cyclin D₂/D₃, p27, and actin was determined by Western blot analysis 7 (lane 2) and 13 days (lane 3) after exposure to IL-15. Cell-cycle–associated protein expression was determined by Western blot analysis in freshly isolated CD45RA⁺ CD27^– revertant T cells (lane 4). These results are representative of 3 separate experiments.