Defective triggering of F5 T cells in the absence of IL-7R expression. F5 T cells from CD45.1⁺ control F5 and CD45.1⁻ F5 TetIL-7R mice off doxycycline for 7 days were CFSE-labeled, mixed at a 1:1 ratio, and transferred (3 × 10⁶ total T cells/mouse) to groups of Rag1^−/− hosts. Groups of recipient mice (n = 3) were further challenged with flu virus intravenously (24 U/mouse). At day 3 after transfer, mice were culled and donor populations among host splenocytes analyzed by FACS. (A) Dot plots are of CFSE versus CD45.1 expression by CD8⁺TCR^hi cells from naive or flu-challenged recipients. (B) Histograms are of CFSE profiles for CD45.1⁺ control F5 T cells (IL-7R⁺ F5) and CD45.1⁻ F5 T cells from F5 TetIL-7R donors (IL-7R⁻ F5). Numbers indicate the average percentage of F5 T cells triggered into division plus or minus SD in each case. *P < .008; n = 6. (C) Histograms show cell size as determined by Forward Scatter (FSc) signal for divided and undivided control (solid line) and IL-7R⁻ (gray fill) F5 T cells. Data are representative of 6 independent experiments.