PD-1–deficient virus-specific CD8 T cells are more functional and pathogenic. (A and B) WT mice were transferred with 10⁴ PD-1^+/+ or PD-1^−/− P14 cells, followed by 10⁶ pfu LCMV docile infection on the next day. On day 6 p.i., percentages and total numbers of splenic P14 cells were determined. The percentage of IFN-γ–producing cells in transferred P14 cells (A) and the mean fluorescence intensity of IFN-γ (B) were measured after restimulation with GP33 peptide. n = 3–4 mice per group. Means ± SEM from one representative of two experiments are shown. (C) Serum concentrations of IFN-γ were analyzed on day 6 after infection. n = 3–4 mice per group. Means ± SEM from one representative of two experiments are shown. (D) Splenic virus titers were analyzed on day 6 after infection. n = 8 mice per group. Means ± SEM from two pooled experiments are shown. (E) Serum levels of liver AST were determined and photos of PBS-perfused liver lobes were taken on day 6 p.i. The arrow indicates a focal necrotic liver lesion. n = 3–4 mice per group. Means ± SEM from one representative of two experiments and photos from representative mice from one of two experiments are shown. (F) WT mice were transferred with 2.5 × 10⁴ PD-1^+/+ or PD-1^−/− P14 cells, followed by 10⁶ pfu LCMV docile infection and core temperature was monitored. n = 4 mice per group. Means ± SEM from one representative of two experiments are shown. *, P < 0.05; **, P < 0.01 (unpaired two-tailed Student’s t test).