Regulation of leukotriene and platelet-activating factor synthesis in human alveolar macrophages

It has been suggested that phospholipase A2 (PLA2) contributes to the regulation of leukotriene (LT) and platelet-activating factor (PAF) synthesis by controlling the release of their precursors, arachidonic acid (AA) and lysophosphatidylcholine (lysoPC), from membrane phospholipids. In rat alveolar macrophages (AMs), PLA2 appears to have a major role in LT synthesis but a more limited role in PAF synthesis. The present study was designed to define the role of PLA2 in LT and PAF synthesis in human AMs and determine whether differences exist between AMs obtained from normal subjects and those from patients with asthma. In the normal subjects, the calcium ionophore A23187 (Cal) increased AM PAF synthesis (percent incorporation of tritiated acetate) by 135% (p < 0.01) and LTB4 synthesis 88-fold (p < 0.001). Phorbol myristate acetate (PMA) had little effect alone, but it had a synergistic effect with Cal, increasing PAF synthesis by 466% and LTB4 synthesis to 229-fold above the control values (p < 0.001 for both). Ro 25-4331, a combined cytosolic (c) and secretory (s) PLA2 inhibitor, had little effect on the Cal-stimulated PAF synthesis, but it completely blocked the effect of PMA. It also blocked the Cal- and Cal+PMA-stimulated LTB4 synthesis. AACOCF3, a cPLA2 inhibitor, had no effect on either Cal or Cal+PMA-stimulated PAF synthesis. It reduced LTB4 synthesis, but it did so less effectively than Ro 25-4331. CoA-independent transacylase (CoAI-TA) activity in the AMs increased after stimulation and exposure to Ro 25-4331. SK&F 45905, a CoAI-TA inhibitor, reduced stimulated PAF synthesis by 30% to 40%. Patients with asthma had similar results except that cPLA2 had a greater role in stimulated LTB4 synthesis. These data indicate that PLA2 plays a direct role in human AM LT synthesis; both the cytosolic and secretory forms contribute to LT synthesis; PLA2 appears to have a more limited role in PAF synthesis, although it mediates the synergistic effect of PMA, probably via sPLA2; and CoAI-TA contributes to PAF synthesis during PLA2 inhibition. With the exception of the greater role for cPLA2 in stimulated LTB4 synthesis in the patients with asthma, the contributions of PLA2 and CoAI-TA to AM LT and PAF synthesis appear to be similar in normal subjects and patients with asthma.     

Activation of human monocyte functions by tumor necrosis factor: rapid priming for enhanced release of superoxide and erythrophagocytosis, but no direct triggering of superoxide release

Tumor necrosis factor (TNF), like granulocyte-macrophage colony-stimul ating factor (GM-CSF), rapidly primed human monocytes for enhanced release of superoxide (O-2) stimulated by receptor-mediated agonists, N-formyl-methionyl-leucyl-phenylalanine (FMLP) and concanavalin A (Con A), but not by phorbol myristate acetate (PMA), which bypasses the receptors to stimulate the cells. The optimal priming was obtained by pretreatment of suspended monocytes with 10 U/mL TNF for 10 minutes at 37 degrees C. The potency of the maximal priming effect was TNF> GM-CSF, and the combined effect of TNF and GM-CSF was greater than that of each cytokine alone. GM-CSF induced an increase in cytoplasmic pH but TNF did not. These findings suggest that TNF and GM-CSF activate monocytes through different mechanisms. TNF and GM-CSF by themselves never triggered O-2 release in suspended monocytes or monocytes adherent to endothelial cells, although both cytokines triggered massive release of O-2 in human neutrophils. In additions, TNF and GM-CSF induced tyrosine phosphorylation of a 42-kD protein in neutrophils but not in monocytes. These findings suggest that the TNF-receptor- or GM-CSF-receptor-mediated signaling pathways for triggering O-(2) release is active in neutrophils but inactive or defective in monocytes. TNF also enhanced phagocytosis of sialidase-treated autologous erythrocytes by monocytes, and this effect was further potentiated in the presence of autologous fresh serum. The significant enhancement of erythrophagocytosis was obtained at 1 U/mL TNF. At this concentration of TNF, the expression of C3bi-receptor (CD11b/CD18) was upregulated. These findings show that TNF rapidly primes human monocytes for enhanced release of O-(2) and erythrophagocytosis and suggest that TNF activates monocytes through autocrine or paracrine mechanisms at the inflammatory sites inasmuch as TNF is primarily produced by activated monocytes/macrophages.     

Lipopolysaccharide induces prostaglandin H synthase-2 protein and mRNA in human alveolar macrophages and blood monocytes

We and others have previously demonstrated that human alveolar macrophages produce more PGE2 in response to lipopolysaccharide (LPS) than do blood monocytes. We hypothesized that this observation was due to a greater increase in prostaglandin H synthase-2 (PGHS-2) enzyme mass in the macrophage compared to the monocyte. To evaluate this hypothesis, alveolar macrophages and blood monocytes were obtained from healthy nonsmoking volunteers. The cells were cultured in the presence of 0 to 10 micrograms/ml LPS. LPS induced the synthesis of large amounts of a new 75-kD protein in human alveolar macrophages, and a lesser amount in monocytes. Synthesis of this protein required more than 6 h and peaked in 24 to 48 h; the protein reacted with an anti-PGHS-2 antibody prepared against mouse PGHS-2. Associated with synthesis of the protein was a marked increase in LPS-stimulated and arachidonic acid-stimulated synthesis of PGE2 by alveolar macrophages compared to monocytes. Cells not exposed to LPS contained only PGHS-1 and synthesized very little PGE2 during culture or in response to exogenous arachidonic acid. An LPS-induced mRNA, which hybridized to a human cDNA probe for PGHS-2 mRNA, was produced in parallel with production of this new protein and was produced in much greater amounts by alveolar macrophages compared to blood monocytes. This mRNA was not detectable in cells not exposed to LPS. In contrast, both types of cells contain mRNA, which hybridizes to a cDNA probe for PGHS-1. This mRNA did not increase in response to LPS. LPS also had no effect on PGHS-1 protein. These data demonstrate that PGE2 synthesis in human alveolar macrophages and blood monocytes correlates to the mass of PGHS-2 in the cell. We conclude that the greater ability of the macrophage to synthesize PGE2 in response to LPS is due to greater synthesis of PGHS-2 by the macrophage.     

Mechanisms of the priming effect of lipopolysaccharides on the biosynthesis of leukotriene B4 in chemotactic peptide-stimulated human neutrophils

The goal of this study was to explain the priming effect of lipopolysaccharides (LPS) in human polymorphonuclear leukocytes on leukotriene B4 (LTB4) biosynthesis after stimulation with the receptor-mediated agonist formyl-methionyl-leucyl-phenylalanine (fMLP). This priming effect for LTB4 biosynthesis was maximal after a 30 min preincubation with LPS but was lost when incubations were extended to 90 min or longer. Priming with LPS resulted in an enhanced maximal activation of 5-lipoxygenase (5- to15-fold above unprimed cells) as well as a prolonged activation of the enzyme after stimulation with fMLP compared to that measured in unprimed cells. The activation of 5-lipoxygenase was associated with its translocation to the nuclear fraction of the cell after stimulation of LPS-primed cells but not of unprimed cells. Priming of cells with LPS also resulted in an enhanced capacity (fivefold increase) for arachidonic acid (AA) release after stimulation with fMLP compared to unprimed cells as measured by mass spectrometry. This release of AA was very efficiently blocked in a dose-dependent manner by the 85 kDa cytosolic phospholipase A2 (PLA2) inhibitor MAFP (IC50=10nM) but not by the 14 kDa secretory PLA2 inhibitor SB 203347 (up to 5 microM), indicating that the 85 kDa cPLA2 is the PLA2 responsible for AA release in response to receptor-mediated agonists. In accord with inhibitor studies, the LPS-mediated phosphorylation of cPLA2 followed the same kinetics as the priming for AA release, and a measurable fMLP-induced translocation of cPLA2 was observed only in primed cells. As with AA release and LTB4 biosynthesis, both the phosphorylation and capacity to translocate cPLA2 were reversed when the preincubation period with LPS was extended to 120 min. These results explain some of the cellular events responsible for the potentiation and subsequent decline of functional responses of human polymorphonuclear leukocytes recruited to inflammatory foci.     

Interaction of Chlamydia pneumoniae and human alveolar macrophages: infection and inflammatory response

Our laboratory developed an isolated perfused superior mesenteric arterial vascular bed preparation to study and correlate vascular smooth-muscle mechanics with associated biochemical events. This preparation provides consistent dose-dependent contractile responses, contains most of the superior mesenteric artery as well as first-, second-, and third-generation arterioles, and has been used for concurrent functional and biochemical analysis of vascular smooth muscle. Preparations isolated from Sprague-Dawley rats produced rapid, dose-related vasoconstrictor responses to norepinephrine (NE) and KCl, while appearing to be unresponsive to periarterial nerve stimulation. Endothelial relaxations to bolus doses of acetylcholine (ACh) in the presence of a constant infusion of NE (10 microM) were limited, producing reductions of perfusion pressures of <25%. Receptor-binding studies conducted to evaluate alpha1-adrenoceptor subtypes revealed high- and low-affinity binding sites composing 91 and 9% of the overall population, respectively. A 60-s time course for contractile response and inositol 1,4,5-triphosphate (IP3) production revealed a significant but transient increase of IP3 that paralleled the contractile response generated by using bolus injections of NE (30 microg). This preparation offers the capacity to conduct perfusion studies investigating vasoconstrictor responses, as well as biochemical studies including receptor-binding and second-messenger assays in the same tissue.     

Chronic smoke exposure alters the phenotype pattern and the metabolic response in human alveolar macrophages

PURPOSE: To investigate the regional differences in small intestinal (SI) metabolism and permeability for several compounds and to ascertain the potential significance of these differences on the reported reductions in regional bioavailability in humans. METHODS: The regional SI metabolism and permeability of captopril, didanosine (ddI), mannitol, ofloxacin and zidovudine (ZDV) were investigated in rats using a Single Pass Perfusion (SPIP) procedure or intestinal homogenates. RESULTS: ddI was metabolized to a greater extent in the upper SI whereas captopril was metabolized to a greater extent in the lower SI. Relatively low homogenate concentrations resulted in significant degradation of captopril in the upper and lower SI. All other compounds were stable and changes in the buffer system or the initial concentration did not affect the results. The SI permeabilities of all compounds, with the exception of mannitol, decreased linearly with respect to SI location and the slopes of the corresponding normalized regression lines were not significantly different. CONCLUSIONS: It has been reported that captopril and ddI demonstrate regional intestinal bioavailability in several species including humans. The current results suggest that the reported reduction in the lower SI bioavailability of captopril may be a result of a reduction in permeability and an increase in intestinal metabolism whereas for ddI, the reduction in the lower SI bioavailability appears to be attributable to a reduction in intestinal permeability. Other factors such as luminal metabolism may also significantly effect regional differences in the intestinal bioavailability of ddI or captopril. Based on these results, a strong possibility exists that ofloxacin and ZDV may also demonstrate regional differences in intestinal bioavailability.     

Lipid mediators in the rat retina: light exposure and trauma elicit leukotriene B4 release in vitro

Light exposure not only elicits a visual response but may also alter functional and structural characteristics of the retina. Furthermore, light exposure can lead to reversible or irreversible lesions of photoreceptors and pigment epithelium. Previous studies in our laboratory have shown that light liberates arachidonic acid from retinal membrane phospholipids mainly by activating the phospholipase A2. In this study we show that light and trauma elicit the synthesis of leukotriene B4 in the isolated rat retina in vitro. Male albino rats were dark adapted for 36 h, isolated retinae were taken, incubated and exposed a) either to darkness or to 5,000 lux of cool white fluorescent light for 5, 10 or 15 min at 37 degrees C, b) either to darkness or to 5,000 lux of cool white fluorescent light for 15 min at 0 degrees C or c) either to darkness or to 5,000 lux of cool white fluorescent light for 15 min at 37 degrees C with a 5-lipoxygenase inhibitor (zileuton). Eicosanoids were extracted and leukotriene B4 levels were determined by radioimmunoassay. Removal of retinae and incubation in darkness caused a significant rise in leukotriene B4 levels with increasing incubation time. This rise was further augmented significantly after light exposure. The leukotriene B4 levels obtained when incubating the retinae either at 0 degree C or with the lipoxygenase inhibitor zileuton as well as the high specificity of the radioimmunoassay indicate that the light- and trauma-elicited synthesis of leukotriene B4 is mediated by activating the 5-lipoxygenase. Leukotriene B4 may be involved, at least in part, in the pathogenesis of retinal diseases including light damage. Curr. Eye Res. 14: 1001-1008, 1995.     

Late phase response during nasal challenge: effect of astemizole on leukotriene B4 levels

Two conditioned media were prepared by culturing human umbilical artery smooth muscle cells (SMC) in 75 cm2 flasks with minimum essential medium (MEM) under magnesium (Mg) sufficient (900 microM) or deficient (100 microM) conditions for 72 h ([900]- and [100]-MEM), respectively. A third conditioned medium was obtained by adjusting the Mg concentration of half of the [100]-MEM to 900 microM ([100-900]-MEM). SMC in 12-well plates were incubated in one of the three conditioned media and the growth rates of SMC were determined by [3H]-thymidine incorporation and cell counting. The growth rate in [100-900]-MEM was significantly higher than in [900]- and [100]-MEM. When platelet derived growth factor (PDGF) was neutralized by the addition of a mixture of anti-PDGF-AA and -BB antibodies, [3H]-thymidine incorporation in [100-900]-MEM decreased by 23.3 per cent, but only by 7.0 per cent in [900]-MEM. The quantity of PDGF in the Mg-deficient media was greater than in the magnesium-sufficient media at all indicated times, as shown by radioimmunoassay for PDGF-BB or -AB. These results indicate that Mg deficiency increases the secretion of PDGF by SMC.     

Role of the mitogen-activated protein kinases and tyrosine kinases during leukotriene B4-induced eosinophil activation

Exposure of guinea-pig eosinophils to leukotriene B4 (LTB4; 1 microM) resulted in a rapid generation of H2O2 (index of NADPH oxidase activation), stimulated [3H]arachidonic acid (AA) release (index of phospholipase A2 activity), and promoted CD18-dependent homotypic aggregation. Under similar conditions, LTB4 (1 microM) induced a rapid activation of extracellular-regulated kinases-1 and 2 (ERK-1/2) but not c-jun N-terminal kinases 46 and 54 (JNK-46/54) or p38 mitogen-activated protein kinase (p38 MAP kinase). To examine the role of ERK-1/2 in the mechanism of eosinophil activation, a selective inhibitor of MAP kinase kinase-1/2 (MEK-1/2), PD098059, was employed. However, PD 098059 at concentrations that attenuated ERK-1/2 activation had no significant affect on eosinophil activation. In contrast, a role for tyrosine kinases in LTB4-induced eosinophil activation was suggested by studies with the tyrosine kinase inhibitors, herbimycin A and lavendustin A. However, the results of those experiments implied divergent pathways for the control of eosinophil responses because the inhibitors were more effective at attenuating H2O2 generation than [3H]AA release, and had little effect on homotypic aggregation.     

CD11b/CD18-dependent stimulation of leukotriene B4 synthesis by human neutrophils (PMN) is synergistically enhanced by tumour necrosis factor alpha and low dose diacylglycerol

Unopsonised zymosan particles bind to the CD11b/CD18 integrin on human neutrophils (PMN) and are phagocytosed. Binding stimulates the release of leukotriene (LT) B4. The present study examined the effect on this interaction of two agents that 'prime' PMN for augmented responses to a variety of agonists. The cell permeable diacyl glycerol, 1,2-dioctanoyl-glycerol (DiC8) and TNF alpha each increased CD11b/CD18 expression on PMN [maximal at 10-9 M TNF alpha or 10-8 M DiC8]. There was a decrease, however, in CD11b/CD18 expression above 10-8 M DiC8, which was not observed at high concentrations of TNF alpha. Pre-treatment with either DiC8 or TNF alpha dose-dependently augmented the zymosan-stimulated release of LTB4 from PMN. DiC8 and TNF alpha in combination, however, synergistically increased LTB4 release. In contrast, at concentrations above 10-8 M DiC8, whether in the presence or absence of TNF alpha, LTB4 release was inhibited and this was ameliorated by protein kinase C inhibitors. The response to neither TNF alpha nor DiC8 (below 10-8 M) was kinase inhibitor sensitive. Doses of DAG, which activate protein kinase C, inhibit CD11b/CD18-dependent responses by down-regulating receptor expression. In contrast, the mechanisms of TNF alpha and low dose DAG 'priming' are not clear but are independent of PKC activation. The synergy between these two priming agents, however, suggests independent, complementary signalling pathways that provide a novel, potentially important mechanism for the control of PMN CD11b/CD18 integrin-dependent activation.     

Distinct phospholipases A2 regulate the release of arachidonic acid for eicosanoid production and superoxide anion generation in neutrophils

Arachidonic acid (AA) released from membrane phospholipids by phospholipase A2 (PLA2) is important as a substrate for eicosanoid formation and as a second messenger for superoxide anion (O2-) generation in neutrophils. Different isoforms of PLA2 in neutrophils might mobilize AA for different functions. To test this possibility, we sought to characterize the PLA2s that are activated by the neutrophil stimuli, Aroclor 1242, a mixture of polychlorinated biphenyls, and A23187, a calcium ionophore. Both Aroclor 1242 and A23187 caused release of [3H]AA; however, O2- production was seen only in response to Aroclor 1242. Eicosanoids accounted for >85% of the radioactivity recovered in the supernatant of A23187-stimulated cells but <20% of the radioactivity recovered from cells exposed to Aroclor 1242. Omission or chelation of calcium abolished A23187-induced AA release, but did not alter AA release in Aroclor 1242-stimulated neutrophils. AA release and O2- production in response to Aroclor 1242 were inhibited by bromoenol lactone (BEL), an inhibitor of calcium-independent PLA2. BEL, however, did not alter Calcium-independent activity was inhibited >80% by BEL, whereas calcium-dependent activity was inhibited <5%. Furthermore, calcium-independent, but not calcium-dependent, PLA2 activity was significantly enhanced by Aroclor 1242. These data suggest that Aroclor 1242 and A23187 activate distinct isoforms of PLA2 that are linked to different functions: Aroclor 1242 activates a calcium-independent PLA2 that releases AA for the generation of O2-, and A23187 activates a calcium-dependent PLA2 that mobilizes AA for eicosanoid production.     

Studies on the characteristics of leukotriene B4 receptor with radio-ligand binding assay

Leukotriene B4 (LTB4), one of the metabolites of arachidonic acid via 5-lipoxygenase (5-LO), plays important role in some inflammatory diseases as one of the most potent chemotaxis factor. A radio-ligand binding assay was set up and the characteristics of LTB4 receptor on guinea-pig splenocytes membrane were studied. At 25 degrees C, the Kd was found to be 1.55 x 10(-9) mol.L-1 and the Bmax was 2.59 x 10(-13) mol.mg-1 protein. The assay established was evaluated by nordihydroguaiaretic acid (NDGA) as positive control.     

Differential sensitivities of human blood monocytes and alveolar macrophages to the inhibition of prostaglandin endoperoxide synthase-2 by interleukin-4

Interleukin-4 (IL-4) is a potent immunomodulatory cytokine synthesized and released by Th2 lymphocytes, mast cells and basophils. It has important effects on monocyte/macrophage cell lines, regulating the secretion of several cytokines, and the production of eicosanoids. In human monocytes and macrophages, IL-4 increases the expression of 15-lipoxygenase and 15-HETE production, but suppresses the inducible isoform of the prostaglandin H synthase (PGHS-2) enzyme and prostanoid synthesis. Prostanoids, in particular prostaglandin E2 (PGE2) have important functions in modulating inflammatory and fibrotic processes. We compared the effect of IL-4 on the expression of PGHS-2 in human alveolar macrophages (AM) and blood monocytes (BM) activated with physiologically distinct stimuli, lipopolysaccharide (LPS) or IL-1 in vitro. The induction of PGHS-2 mRNA and protein, and prostanoid synthesis by all stimuli was inhibited by exogenous IL-4 in both cell types. However, monocytes were more susceptible to this effect of IL-4 than alveolar macrophages.     

Suppression of leukotriene B4 and tumour necrosis factor alpha release in acute inflammatory responses by novel prenylated hydroquinone derivatives

A series of prenyl hydroquinone derivatives synthesized as structural analogs of marine products were tested for their effects on inflammatory responses in vitro and in vivo. 2-Prenyl-1,4-hydroquinone (H1), 2-diprenyl-1,4-hydroquinone (H2), 2-triprenyl-1,4-hydroquinone (H3) and 2-tetraprenyl-1,4-hydroquinone (H4) scavenged reactive oxygen species and inhibited 5-lipoxygenase (5-LO) activity in human neutrophils. The inhibition of 5-LO activity was demonstrated in vivo in the mouse air pouch injected with zymosan and arachidonic acid-induced ear inflammation. The four compounds suppressed the production of tumour necrosis factor alpha (TNFalpha) in J774 cells stimulated with lipopolysaccharide (LPS) and also in vivo in the mouse air pouch injected with zymosan. In addition, all prenyl-hydroquinones inhibited the release of nitrite and PGE2 in LPS-stimulated J774 cells, without direct effects on cyclo-oxygenase-1 (COX-1), cyclo-oxygenase-2 (COX-2) or inducible nitric oxide synthase (iNOS) activities in several cell-free systems. The reduction in the length of the lateral chain in prenyl-hydroquinones (1-4 isoprene units) with respect to their marine analogs (7-8 isoprene units) has improved the anti-inflammatory activity of this class of compounds. Marine natural products may be a model to design new anti-inflammatory agents.     

Release of gelatinase and superoxide from human mononuclear phagocytes in response to particulate Tamm Horsfall protein

This study describes the in vitro activation of human mononuclear phagocytes by particulate Tamm Horsfall protein (THP). Peripheral blood monocytes phagocytosed THP particles with the accompanying release of superoxide radicals, N-acetyl-beta-D-glucosaminidase, and neutral metalloproteinase. Immunoprecipitation and substrate gel analysis identified the neutral proteinase as a 95-kd gelatinase. A comparison with other particulate ligands highlighted the specificity of the response to THP and showed that the magnitude of the response was comparable with that obtained with lipopolysaccharide (100 micrograms/ml). Parallel studies using peritoneal macrophages resulted in a similar pattern of enzyme release and reactive oxygen species synthesis. THP has been implicated in the pathogenesis of tubulointerstitial nephritis associated with reflux nephropathy. The present study indicates that an inflammatory response initiated by a neutrophil-THP interaction may be extended into a chronic phase via the activation of mononuclear phagocytes. The subsequent release of reactive oxygen metabolites and proteinases may contribute to the tissue damage and fibrosis associated with chronic immune-mediated tubulointerstitial nephritis.     

Asbestos fibres and man made mineral fibres: induction and release of tumour necrosis factor-alpha from rat alveolar macrophages

OBJECTIVES: Mounting evidence suggests that asbestos fibres can stimulate alveolar macrophages to generate the potent inflammatory and fibrogenic mediator, tumour necrosis factor-alpha (TNF-alpha), and that this may play an important part in the onset and development of airway inflammation and lung fibrosis due to asbestos fibre inhalation. Little is known, however, about the ability of other mineral fibres to initiate formation and release of TNF-alpha by alveolar macrophages. Therefore the effects of different fibres (crocidolite, chrysotile A, chrysotile B, two man made mineral fibres (MMVF 21 and MMVF 22), a ceramic fibre (RCF 1), and a silicon carbide whisker fibre (SiCwh)) on formation and release of TNF-alpha by rat alveolar macrophages were examined. METHODS: Cells were isolated and incubated at 37 degrees C with the different fibres, or with culture medium alone (controls), and the amounts of TNF-alpha messenger RNA (mRNA) in the cells and TNF-alpha bioactivity released into the culture medium were measured at different time points. RESULTS: Significantly (P < 0.05 v control) increased amounts of TNF-alpha mRNA were found in cells exposed to crocidolite, chrysotile A, chrysotile B, MMVF 21, RCF 1, or SiCwh for 90 minutes, and significantly (P < 0.05 v control) increased activities of TNF-alpha were found in the medium of macrophages exposed to crocidolite, chrysotile A, chrysotile B, or MMVF 21 for four hours. CONCLUSION: These observations suggest that not only natural mineral fibres but also certain man made mineral fibres are able to induce the formation and release of TNF-alpha by alveolar macrophages in vitro.     

The interaction of IgE antibody with human alveolar macrophages and its participation in the inflammatory processes of lung allergy

After the initial observation that human and animal mononuclear phagocytes can be activated into specific killer cells against larvae of the parasite Schistosoma mansoni by seric IgE antibody from infected patients, a possible interaction of IgE with human alveolar macrophages in asthmatic patients was investigated. In vitro, alveolar macrophages from non-atopic individuals can bind monoclonal IgE molecules, as well as IgE antibody from the serum of patients with respiratory allergy. A subsequent contact with anti-IgE antibody or with the specific allergen induces the extracellular release of a variety of mediators, such as lysosomal enzymes, neutral proteases, or superoxide anion. Due to the presence of allergen-specific IgE antibody on the macrophage surface in situ, the same results were obtained in vitro with freshly purified alveolar macrophages from allergic patients. Disodium cromoglycate, corticosteroids, or beta-adrenergic stimulants are strong inhibitors of this specific exocytosis of physiological mediators. The atopic cells formed rosettes with allergen-coated erythrocytes at 4 degrees C, except after pretreatment with aggregated monoclonal IgE or with the allergen.