MyD88 signaling in CD8 T cells is critical for their survival after activation and proliferation in vivo. (A,B) A total of 10⁶ WT or MyD88^−/− clone 4 CD8 T cells (Thy1.1⁺) were transferred into B10.D2 mice, which were infected with 5 × 10⁶ pfu rVV-HA intraperitoneally or left uninfected (naive). (A) At 24 hours after infection, splenocytes were analyzed for T-cell activation phenotypically by staining with anti-CD44 and anti-CD62L (left panels) or anti-CD25 and anti-CD69 antibodies (right panels). FACS plots are gated on CD8⁺Thy1.1⁺ clonotypic cells, and percentages shown are of those cells possessing an activated phenotype of CD44^highCD62L^low or CD25^highCD69^high. (B) Three days after infection, proliferation of the clonotypic cells was analyzed by the CFSE dilution assay. Events were gated on CD8⁺Thy1.1⁺ cells. (C) A total of 10⁴ WT or MyD88^−/− clone 4 CD8 T cells were transferred into B10.D2 mice that were either left uninfected (naive) or infected with 5 × 10⁵ pfu rVV-HA (rVV-HA). Seven days after infection, splenocytes were harvested and the extent of clonotypic cells undergoing apoptosis was analyzed by annexin V staining. Events were gated on CD8⁺Thy1.1⁺ cells, and percentages shown are of those cells that are annexin V⁺. Representative data of 3 independent experiments with 3 mice per group for each experiment are shown.