Endothelial PD-L1 expression inhibits CD8 T cell–mediated killing. (A) CD31⁺CD144⁺ cells from lungs and livers of naive WT, infected WT, and infected PD-1 KO mice were analyzed for PD-L1 expression and LCMV infection (VL-4 staining) on day 6 after 10⁶ pfu LCMV docile infection. Histograms depict data from one representative of two analyzed mice per group. Data from one representative of three experiments are shown. (B) Lung CD31⁺CD144⁺ cells of naive and 10² pfu LCMV WE–infected WT mice were analyzed for PD-L1 expression and LCMV infection (VL-4 staining) on day 6 p.i. Histograms depict data from one mouse each. Data from one representative of two experiments are shown. (C) Untreated, IFN-γ–treated, and IFN-γ + αPD-L1–treated MS-I cells were analyzed for PD-L1 expression and pulsed with GP33+NP396 peptide to serve as target cells in a chromium release assay. (D) PD-1 expression levels of endogenous CD8 T cells and transferred P14 cells were determined on day 6 after systemic infection. C and D: histograms depict data from one representative of two experiments. (E) CD8 T cells were purified from four P14-transferred mice on day 6 p.i. and mixed with chromium-labeled, antigen-pulsed MS-I cells at the indicated effector to target ratios. Chromium release was analyzed in duplicates. Means ± SEM from one representative of three experiments are shown.