In vivo IFN-γ production by CD8⁺ T cells in various tissues from WT and prf^−/− mice after LCMV infection. (A) Seven days after LCMV infection, CD8⁺ T cells producing IFN-γ in vivo were assessed by flow cytometry (fixed immediately ex vivo without additional stimulation; see “Methods”). Live gated, CD8⁺/CD4⁻ cells are shown from each of the indicated tissues (lymph node represents mesenteric lymph nodes; peritoneum represents cells in the peritoneal cavity). (B) The percentage of CD8⁺ cells producing IFN-γ in vivo in the indicated tissue is displayed (minus any background staining seen in uninfected animals). (C) The absolute number of CD8⁺ cells producing IFN-γ in vivo in the indicated organ is displayed. The number plotted for lymph nodes is an extrapolation based on the estimate that the mesenteric lymph nodes represent 25% of all lymph nodes (ie, the number for all lymph nodes = mesenteric lymph nodes × 4). IFN-γ⁺ cells from lung, liver, and the peritoneal cavity are combined in the final bar graph in order to plot them on the same scale as lymphoid tissues. *P < .01, compared with the same tissue in WT mice. LN, lymph node; Per, peritoneal cavity; Lu, lung; Liv, liver.