Platelet-dependent induction and amplification of polymorphonuclear leucocyte lysosomal enzyme release

Degranulation of human polymorphonuclear leucocytes induced by opsonized zymosan (OpZ) was studied in the presence or absence of platelets, either resting or stimulated by thrombin. Lysozyme secretion from polymorphonuclear leucocytes stimulated with OpZ increased in the presence of platelets. A further increase was observed when platelets had been stimulated with thrombin. The effect was dependent on platelet concentration (10-80 platelets/polymorphonuclear leucocytes). Resting polymorphonuclear leucocytes could be activated by platelets that had been stimulated with thrombin with an optimal effect observed at 0.1 U/ml of thrombin. Substitution of platelets with platelet-released products only resulted in significant stimulation of polymorphonuclear leucocytes at concentrations above those used in the coincubations experiments. Zymosan coated with various opsonins showed a gradient in the relative intensity of polymorphonuclear leucocytes degranulation. Platelet-dependent enhancement of polymorphonuclear leucocytes degranulation was higher with opsonins that were poorer at inducing lysosomal secretion. The role of platelets as helper cells in the activation of polymorphonuclear leucocytes appears to be dominant when polymorphonuclear leucocytes are challenged in suboptimal conditions. Positive interactions between platelets and polymorphonuclear leucocytes may be relevant in vivo at the site of inflammation where platelets could enhance effector functions of polymorphonuclear leucocytes.