Pulmonary endothelial cells are driven into apoptosis after systemic LCMV infection. (A) In situ analysis of lung sections from naive and infected WT and PD-1 KO mice on days 5–6 after 10⁶ pfu LCMV docile infection. Representative immunofluorescence stainings with the indicated antibodies are shown. The arrows indicate apoptotic endothelial cells as identified by TUNEL⁺CD31⁺ cells. n = 7 mice. Bars, 20 µm. (B) Quantification of CD31⁺TUNEL⁺ cells in immunofluorescence stainings of pulmonary vessels on day 5 p.i. Numbers of CD31⁺TUNEL⁺ cells were normalized to vessel areas and two to three sections per mouse were considered for quantification. WT, n = 4 mice; wt αPD-L1, n = 4 mice; PD-1 KO, n = 3 mice; PKOB, n = 3 mice; PKOB αPD-L1, n = 3 mice. Means ± SEM from two pooled experiments are shown. n.s.: not significant. (C) Total numbers of lung CD31⁺CD144⁺ cells were determined in naive and infected WT and αPD-L1–treated WT mice on day 6 p.i. n = 5–6 mice per group. Means ± SEM from two pooled experiments are shown. *, P < 0.05; **, P < 0.01 (unpaired two-tailed Student’s t test).