Neutrophil degranulation: Evidence pertaining to its mediation by the combined effects of leukotriene B4, platelet‐activating factor, and 5‐HETE

Abstract

Stimulus-activated polymorphonuclear neutrophils (PMN) produce leukotriene B₄ (LTB₄), 5-hydroxyeicosatetraenoate (5-HETE), and platelet-activating factor (PAF). Each of these lipids promotes PMN degranulation; in combination they have additive and potentiating effects that result in prominent degranulation responses at relatively low concentrations. Thus, the combined interactions of LTB₄, 5-HETE, and PAF may mediate responses in PMN activated by other stimuli. This possibility was examined by measuring the responses of PMN made insensitive to one or more of these lipids. Cells were pretreated with LTB₄, 5-HETE, and/or PAF for 8 min; exposed for 2 min to cytochalasin B (which is required for lipid-induced degranulation); and then challenged. PMN challenged with only buffer released minimal amounts of granulebound enzymes. Furthermore, the lipid-pretreated cells were hyporesponsive to challenge with (1) various combinations of these same lipids or (2) ionophore A23187. The relative potencies of the lipids in producing hyporesponsiveness to themselves or A23187 were: 5-HETE < PAF ≤ LTB₄ < PAF + LTB₄ < PAF + LTB₄ + 5-HETE. For both types of challenge, reduced responsiveness occurred in cells pretreated with > 0.1 nM LTB₄ and/or > 0.2 nM PAF, persisted in cells washed after lipid pretreatment, and did not develop in cells pretreated with various combinations of bioinactive structural analogues of the lipids. Thus, PAF, LTB₄, and 5-HETE interacted to desensitize PMN, and the degranulating actions of A23187 required cells that were fully responsive to each of the three lipids. This supports the concept that the lipids act together in mediating certain of the ionophore's effects. However, lipid-desensitized PMN degranulated fully when challenged with C5a, a formylated oligopeptide, or phorbol myristate acetate. Degranulation responses, therefore, may proceed through various pathways, only some of which involve the lipid products studied here.