CD8 memory fails to form in the absence of both IL-7R and IL-15. F5 TreIL-7R^OFF and control F5 effector cells were generated in vitro, mixed in equal numbers, and 2 × 10⁷ total cells injected into either Rag1^–/– or Il15ra^–/–Rag1^–/– recipients. Recipients were bled at various times after transfer and frequencies of F5 T cells in PBLs determined by staining cells for CD8, TCR, and Ly5.1 expression and analyzed by FACS. (A) The graph shows the frequency of F5 TreIL-7R^OFF (open symbols) and control F5 T cells (filled symbols) in either Rag1^–/– (circles) or Il15ra^–/–Rag1^–/– (squares) hosts. F5 TreIL-7R^OFF T cells in Il15ra^–/–Rag1^–/– were undetectable more than 14 days after transfer. (B) The graph shows the chimerism between F5 TreIL-7R^OFF and control F5 T cells in either Rag1^–/– (•) or Il15ra^–/–Rag1^–/– hosts (▪). Data are from 2 independent experiments.