The effect of rIL-21 injection on thymi of old WT C57Bl/6 mice. (A) Schematic diagram of the experimental design. Three mice per group were tested. (B) Representative photograph of thymi derived from PBS- versus rIL-21–treated mice. (C-D) Thymic weight and cellularity. (E) Absolute numbers of DP, SP CD4, and CD8 T cells. (F) Analysis of IL-21Rα chain on the surface of preselected (CD69⁻) or postselected (CD69⁺) DP subsets. Data are representative of 3 separate experiments.