Antibodies against CD99 and CD99L2 block transendothelial migration of WT and PECAM-1–deficient neutrophils in vitro. Bone marrow–derived polymorphonuclear leukocytes from WT or PECAM-1–deficient mice were allowed to migrate for 30 minutes through a monolayer of bEnd.5 cells grown on transwell filters in the presence of 40 ng/mL KC in the lower chamber. Endothelial cells were stimulated with IL-1β (A) or TNF-α (B) 16 hours before the experiment. Thirty minutes before the start of the experiment, endothelial cells were incubated with 30 μg/mL of preimmune control IgG (co-IgG) or affinity-purified anti-CD99 IgG (anti-CD99) or affinity-purified anti-CD99L2 IgG (anti-CD99L2) or an mAb against ICAM-1 (anti–ICAM-1). ***P < .001. *P < .05. Statistical analysis was done by ANOVA evaluating all available data (n = 9) of 3 experiments.