Modulation of STAT1 levels in vivo-reduced levels in antigen-specific proliferating CD8 T cells. For these experiments, populations were prepared from uninfected or day 5, 6, 7, and 8 LCMV-infected mice. BrdU was injected at 2 hours before harvest, and the isolated cells were labeled to evaluated CD4, CD8, BrdU, and STAT1 expression by FACS (A) CD4 T cells experience low levels of proliferation during LCMV infection. Histograms represent the percentage of CD4 T cells in the splenic leukocytes. The contour plots show the staining for STAT1 and BrdU in the electronically gated CD4 T-cell subset. (B) The proliferating and antigen-specific CD8 T-cell subsets are contained within the low STAT1 populations. Histograms represent the percentage of CD8 T cells in the splenic leukocytes. The contour plots show the staining for the electronically gated CD8 T-cell subset. In the first set, STAT1 versus BrdU incorporation, as a marker of in vivo proliferation, is shown. In the second set, STAT1 versus the tetramer Db GP33-41, as a marker of cells specific for the LCMV immunodominant epitope, is shown. In the third set, Db GP33-41 versus STAT1 is shown. Data presented are means ± SDs. (C) STAT1 levels are low in Db GP33-41-positive and -negative expanding CD8 T cells. Histograms represent STAT1 levels in the electronically gated peptide specific or unspecific populations. Numbers show mean fluorescence intensities. These experiments have been repeated more than 3 times.