Concentration-dependent effect of PF-4 and IL-8 on neutrophil adherence to EC and secondary granule marker exocytosis. PMN were incubated in the presence of a monolayer of cultured EC with increasing concentrations of PF-4 (A) or IL-8 (B) and allowed to attach for 20 minutes. Nonadherenrent cells were removed from EC layers by centrifugating the plates at an angle of 45° (150g for 1 minute). Secondary granule marker lactoferrin (○) was determined in the cell-free supernatant from the same PMN tested for adhesion (•). Assay backgrounds were determined in samples of unstimulated cells run in parallel (adhesion, 8.8% ± 1.8%; exocytosis, 3.5% ± 1.9%) and were subtracted. Data are given as the mean ± standard deviation (SD) of 3 independent experiments, each performed in duplicate.