Loss of CD99 from endothelial cells impairs neutrophil transmigration in vitro. (A) Bone marrow–derived neutrophils from WT mice were allowed to transmigrate through 16-hour TNF-α–stimulated, WT (red bars) or CD99^−/− (blue bars) primary lung endothelial cells in the presence (30 minutes) or absence (90 minutes) of CXCL1 (n = 24 per group from 4 independent experiments). Results are displayed as mean ± standard error of the mean (SEM). *P < .05 and ***P < .001, as per Mann-Whitney U test. (B) Bone marrow–derived neutrophils from WT mice (red bars) or CD99^−/− mice (blue bars), were allowed to transmigrate for 30 minutes through 16-hour TNF-α–stimulated WT primary lung endothelial cells in the presence of CXCL1 (n = 19 per group from 3 independent experiments). Results are displayed as mean ± SEM. n.s., *P < .05, and ***P < .001, as per unpaired Student t test. PMN, polymorphonuclear neutrophil.