Expression of IL-15Rα in T cells from WT and KI mice. (A) IL-15Rα mRNA levels in T cells. mRNA were prepared from naive and anti-CD3ϵ plus anti-CD28 activated splenic T cells from WT and KI mice. IL-15Rα mRNA levels were quantitated by real-time PCR and normalized to the level of 18S ribosomal RNA. Means plus or minus SEM are shown (n = 3/group). (B) Binding of ¹²⁵I–IL-15 to WT and KI T cells purified from WT or KI mouse spleens (6-mouse pool). Both naive and anti-CD3ϵ plus anti-CD28 activated T cells were analyzed (see “Methods”). The data are presented as Scatchard plots (bound vs bound/free: radiolabeled IL-15–specific binding expressed in sites per cell on the abscissa, and the ratio of specifically bound fraction in sites per cell to the concentration of free iodinated IL-15 expressed in picomoles is on the ordinate). (C) Expression levels of total and cell-surface IL-15Rα in WT and KI T cells. T cells were isolated from WT and KI splenocytes and activated by anti-CD3ϵ plus anti-CD28 for 3 days. Cell-surface IL-15Rα expression were detected by staining both naive and activated T cells with biotinylated anti–mIL-15Rα followed by streptavidin-APC. In parallel, some of the cells were fixed and permeabilized, then stained with biotinylated anti–mIL-15Rα, thus staining both cell-surface and intracellular IL-15Rα. Biotinylated mouse IgG was used as an isotype control.