Stimulation of human polymorphonuclear leukocytes by leukotriene B4 and platelet-activating factor: an ultrastructural and pharmacological study.

Abstract

Human polymorphonuclear leukocytes (PMNs) stimulated by N-formylmethionyl-leucylphenylalanine (fMLP), leukotriene B4 (LTB4) and platelet-activating factor (PAF) showed a similar sequence of ultrastructural changes. Exposure of unstirred PMNs to these soluble stimuli induced a rapid (within 10 s) plasma membrane ruffling followed by polarization of cells associated with formation of broad agranular lamellipodia (maximal by 30 s). Similar changes were observed in stirred PMN suspensions except that the polarized cells formed multicellular aggregates with the agranular lamellipodia extending outward from the clumps. In addition, these receptor-directed agonists induced concentration-dependent PMN aggregation that was rapid and reversible, and in cells pretreated with cytochalasin B the agonists induced a degranulatory response. The calcium ionophore ionomycin also activated PMNs but, in contrast to the other stimuli, ionomycin produced irreversible aggregation and evoked enzyme release in the absence of cytochalasin B. Ionomycin stimulated LTB4 release, whereas the response induced by PAF or fMLP was barely detectable. The possible role of LTB4 and PAF in mediating responses to other agonists was investigated pharmacologically using the 5-lipoxygenase inhibitor PF 5901 and a specific PAF antagonist, kadsurenone. PF 5901 blocked ionomycin-induced LTB4 release but did not inhibit aggregation or degranulation induced by ionomycin, PAF, fMLP, or LTB4. Kadsurenone blocked PAF-induced aggregation and degranulation but not responses to LTB4, ionomycin, or fMLP. We conclude that LTB4 and PAF are potent activators of human PMNs that act independently of one another and that these lipids appear not to be involved in the responses to the other agonists studied.