4-1BBL is required for viral control early but has limited impact late in LCMV clone 13 infection. (a) Analysis of WT versus 4-1BBL^−/− mice 8 d after LCMV clone 13 infection. (top) Percentage of LCMV-tetramer⁺ CD8 T cells in blood. Each data point represents one mouse. Data are pooled from three independent experiments. (bottom) Viral titer as measured in lung and kidney. Data are representative of two independent experiments. (b) Kinetic analysis of the frequencies of NP396-specific T cells in the blood of WT and 4-1BBL^−/− mice after LCMV clone 13 infection. Note that the day 8 data from this experiment were included in the compiled day 8 data in panel a. (c) Analysis of WT versus 4-1BBL^−/− mice 60 d after LCMV clone 13 infection. (left) Percentage of GP33-tetramer⁺ CD8 T cells in spleen. Data are pooled from two independent experiments. (right) Viral titers in kidney. Data are representative of two independent experiments.