Platelet-leukocyte interaction in adhesion to endothelial cells induced by platelet-activating factor in vitro

1. Platelet-activating factor (PAF, 10 nM) did not induce platelet adhesion to endothelial cells cultured in monolayer but it induced their adhesion to protein-coated plastic. However, PAF induced a marked platelet adhesion to endothelial cells when polymorphonuclear leukocytes (PMNs) were present. Lyso-PAF had no effect. 2. Phase-contrast microscopic examination showed that single platelets rather than their aggregates adhered to the endothelial cell surface around aggregating and adhering PMNs. 3. Significant platelet adhesion was induced by PAF at concentrations higher that 0.01 nM with the maximal response at 10 nM. Platelet adhesion occurred within minutes after PAF addition, reaching a maximum approximately after 30 min. Platelet adhesion also occurred significantly at a PMN:platelet ratio of 1:800, and linearly up to 1:50. 4. The PAF-induced platelet adhesion was suppressed by three structurally unrelated PAF antagonists, WEB 2086, ONO 6240 and BN 52021, in a concentration-dependent manner. 5. PAF also increased PMN adhesion to endothelial cell monolayers, which was further augmented by the presence of platelets. 6. The present study demonstrates that PAF induces platelet adhesion to endothelial cells in vitro when PMNs are present and that there is a close interaction between platelets and PMNs in their adhesion to endothelial cells. The present study further suggests that PMNs could play a central role in platelet adhesion to vascular endothlium in certain pathological conditions.