Acute requirement for IL-7 for optimal triggering of T cells. F5 T cells from CD45.1⁺ control F5 and CD45.1⁻ F5 TetIL-7R mice off doxycycline 7 days were CFSE-labeled, mixed at a 1:1 ratio, and transferred (3 × 10⁶ total T cells/mouse) to groups of Rag1^−/− or Il7^−/−Rag1^−/− hosts. Recipient mice were further challenged with flu virus (24 U/mouse). At day 3, mice were culled and F5 T cells populations in spleen analyzed by FACS. (A) Histograms show CFSE profiles of CD8⁺ TCR^hi F5 T cells from the donor mice and in the hosts indicated. (B) Box plot shows the ratio of control F5 to F5 TetIL-7R donor T cells triggered into division in the indicated hosts. Statistics: *P < .016, comparing ratios in Rag1^−/− and Il7^−/−Rag1^−/−. Comparison of frequency of triggered control and F5 TetIL-7R T cells in Rag1^−/− hosts (**P < .001) and in Il7^−/−Rag1^−/− hosts (***P = .66).