Stimulation of eosinophil adherence to human vascular endothelial cells in vitro by platelet-activating factor

¹¹¹In-Labeled eosinophils from mildly eosinophilic subjects have been examined for their capacity to adhere to cultured human umbilical vein endothelial cells. In assay buffer alone, 32.0% ± 2.6 eosinophils adhered spontaneously to endothelial cells. Platelet-activating factor (PAF) (1-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine) at concentrations as low as 10^-9 M increased this adherence to a level of 46.7% ± 2.0. The effects of PAF were confirmed to be on eosinophils by parallel adherence assays done on serum-coated plastic plates where comparably enhanced adhesion of the eosinophils was seen. Lyso-PAF, the biologically inactive precursor/metabolite of PAF, had no stimulatory properties. FMLP caused an increase in eosinophil adherence, comparable to that of PAF, but only at high concentrations (10^-6 to 10^-7 M). Further examination of eosinophil subpopulations separated on metrizamide gradients indicated that 'hypodense' eosinophils had a significantly higher ability to adhere spontaneously to endothelial cells than 'normal' dense eosinophils, (35.5% ± 4.2 vs 23.8% ± 2.5, respectively) and could be stimulated with PAF to higher levels, although the magnitude of stimulation was similar for both populations. A mouse mAb TS1/18 to the common β-subunit of the Mac-1 cell surface glycoprotein complex (CDw18) reduced by up to 94.6% the PAF-induced increase in adherence, but had no effect on the spontaneous adhesion. Eosinophils were also shown by cytofluorography to be capable of binding the TS1/18 antibody on their cell surface, and in some experiments to exhibit an increased expression of the Mac-1 complex on stimulation with PAF. These studies indicate that eosinophils are capable of binding to endothelial cells in culture, that PAF is a potent stimulator of eosinophil adherence, and that the Mac-1 complex has a critical role in this adhesion process.