PD-1 KO mice develop fatal CD8 T cell–dependent pathology during early systemic LCMV infection. (A) PD-1 KO mice were infected with 10⁶ pfu LCMV clone 13, 10⁶ pfu LCMV docile, or 10² pfu LCMV docile and monitored for morbidity (hunched back, ruffled fur, and reduced motoric activity). (B and C) Core temperatures were measured in WT and PD-1 KO mice (B), untreated and αPD-L1–treated WT mice (C). (D) Morbidity was monitored in undepleted and CD8 T cell– or CD4 T cell–depleted PD-1 KO mice at the indicated times after infection with 10⁶ pfu LCMV docile. (E) Cytokine concentrations in the sera of WT and PD-1 KO mice were analyzed on day 6 after 10⁶ pfu LCMV docile infection. A–E: n = 3–4 mice per group. Means ± SEM from one representative of two experiments are shown. (F) Liver AST concentrations in the sera of WT and PD-1 KO mice were determined on day 6 after infection. WT, n = 8 mice; KO, n = 7 mice. Means ± SEM from two pooled experiments are shown. *, P < 0.05; **, P < 0.01 (unpaired two-tailed Student’s t test).