PLATELET-LEUKOCYTE INTERACTION IN ADHESION TO ENDOTHELIAL-CELLS INDUCED BY PLATELET-ACTIVATING-FACTOR INVITRO

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 than 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.