Agents Actions 41:5-10 (1994) 0065-4299/94/020005-06 $1.50+ 0.20/0 9 1994 Birkh/iuser Verlag, Basel Potentiation of histamine release from human leucocytes by PAF R. Louis, T. Bury, J.-L. Corhay, and M. F. Radermecker Department of PneumologyC.H.U., University of Liege, Sart Tilman, B-4000 Lidge,Belgium Received 2 April 1993; accepted by W. Lorenz 16 November 1993 Abstract. Studies on the effects of PAF on histamine release from human leucocytes have yielded conflicting results. We therefore investigated the effects of PAF on leucocytic histamine release (HR) focusing on direct as well as on modulating effects. Peripheral blood leucocytes of normal and atopic subjects were incubated with PAF, anti-IgE and FMP for 30 min at 37 ~ and histamine was measured fluorometrically. Unlike anti-IgE (1/2000) and FMP (10 -s M) which caused histamine release (HR) of 34___7% and 31+8%, respectively, PAF by itself (10-11_10- 5 M) failed to induce any significant HR from human leucocytes (<3%) in normal (n--14) and atopic subjects (n = 6). Nevertheless, in normals as well as atopics, PAF, but not lyso-PAF, enhanced anti-IgE (1/2000) and FMP (10 .5 M)-induced HR in a concentration-related manner. Maximal potentiation of histamine release caused by FMP and anti-IgE was achieved with PAF (10 -7) (mean__+SEM: 26_+5%, n=5, p<0.01) and PAF (10 -5) (mean __SEM: 20___7%, n=7, p<0.05), respec- tively. This potentiation was suppressed by WEB2086 (10-5 M), a specific PAF antagonist. The time course of the enhancing effect produced by PAF was dependent on the type of secretagogue. The enhancement was nearly maximal when PAF and FMP were added simultaneously to the leucocytes, whereas a preincubation of 20 min with PAF was required to get maximal enhancement with anti-IgE. The enhancing activity of PAF on HR induced by both anti-IgE and FMP was reversed by washing the cells after preincubation. While PAF enhancement of FMP-induced HR persisted on mononuclear cell fraction containing basophils, that of anti-IgE-induced HR was considerably reduced under these conditions. Further, preincubation of mononuclear cells containing basophils with the supernatant of granulocytes stimulated by PAF resulted in a stronger potentiation of immunologica/his- tamine release than that afforded by the sole PAF on the mononuclear cell fraction. We conclude that PAF appears to be more an enhancing rather than a triggering agent for the basophil degranulation. The mechanism of this en- hancing effect is different according to the secretagogue Correspondence to." R. Louis and probably involves an intermediate mediator regard- ing immunologically induced histamine release. Key words: PAF - Histamine - Leucocytes Introduction Platelet-activating factor (PAF) is a phospholipid medi- ator produced by PMN, eosinophils, monocytes/macro- phages and endothelial cells upon chemical or immune stimulation [1]. Inhalation of PAF can cause an acute bronchospasm and an increase in bronchial responsive- ness in man [2]. Furthermore, exacerbations of asthma is associated with an increase in PAF production by the granulocytes [3]. This phospholipid can therefore be pro- posed as a mediator of central importance in asthma. PAF exerts a wide range of biological effects on inflammatory [4] and even endothelial cells [5]. PAF has mainly been reported to release histamine from human basophils [6]. To date, however, the studies devoted to this issue have yielded conflicting results. Indeed, Okuda et al. [6] firstly reported a substantial PAF-induced histamine release from mixed leucocytes of allergic asthmatics (but not of normals) only if the cells were shortly preincubated with cytochalasin B. In contrast, Columbo et al. [7] found that PAF without cytochalasin B caused a histamine release from human basophils both in normal and allergic sub- jects. On the other hand, Brunner et al. [8] concluded that PAF could induce histamine secretion from human basophils, provided these cells were preincubated with hematopoetic growth factors. PAF has been shown to act as a direct triggering agent for many cellular events. For instance, it induces platelet aggregation and exocytosis [9], endothelium activation [10], neutrophil and eosinophil chemotaxis [11], exocyto- sis [12] and superoxide production [13]. Nevertheless, concerning other cellular functions, such as neutrophil burst activity or cytokine production by monocytes, PAF acts as a potentiating agent rather than as a direct stimu- lating one [14, 15].