Role of pertussis toxin-sensitive G proteins in stimulus-dependent human eosinophil degranulation.

Stimulation of human normodense eosinophils with immobilized secretory IgA (sIgA) or IgG, or with the soluble stimulus, FMLP, triggers the exocytotic release of the granule protein, eosinophil-derived neurotoxin (EDN). In this report, we demonstrate that these stimuli also provoke an increase in phospholipase C-mediated phosphoinositide breakdown in eosinophils. Pretreatment of eosinophils with pertussis toxin (PTX) for 2 h irreversibly abolished the increases in phospholipase C activity and EDN release induced by immobilized sIgA or FMLP. In contrast, PTX treatment only transiently inhibited eosinophil activation induced by immobilized IgG. Maximal inhibition of IgG-stimulated phosphoinositide hydrolysis and EDN release occurred after 2 h of PTX pretreatment with PTX, followed by a gradual recovery of cellular responsiveness to immobilized IgG as the duration of PTX pretreatment was extended to 16 h. Activated PTX catalyzed the in vitro ADP-ribosylation of 41- and 44-kDa proteins in eosinophil membranes. A 2-h pretreatment of intact cells with PTX markedly reduced the pools of unmodified 41- and 44-kDa substrates available for subsequent ADP-ribosylation in vitro, suggesting that both proteins were substrates for PTX in intact eosinophils. Continuous exposure of eosinophils to PTX for times ranging from 2 to 15 h resulted in the gradual reappearance of unmodified 44-kDa protein, whereas the levels of unmodified 41-kDa protein were persistently reduced in PTX-treated cells. The time course of the decline and reappearance of unmodified 44-kDa substrate in PTX-treated eosinophils closely paralleled the changes in the responsiveness of these cells to immobilized IgG. These results suggest that the receptors for sIgA, FMLP, or IgG transduce activating signals for eosinophil degranulation through differential coupling to at least two PTX-sensitive G proteins.     

IgA-induced eosinophil degranulation

Eosinophils play an important role as effector cells in allergic, parasitic, and other conditions. The mechanism(s) by which eosinophils mediate their effector functions was studied by incubation of human normodense eosinophils with Sepharose beads coupled to various Ig isotypes as targets. Controls included eosinophils incubated alone or incubated with uncoated beads, human serum albumin-, or OVA-coated beads. An eosinophil granule protein, the eosinophil-derived neurotoxin (EDN), was measured as an indicator of eosinophil degranulation. Eosinophils released eosinophil-derived neurotoxin when incubated with Sepharose beads coupled to Ig of the IgG or IgA isotypes, as well as IgA-Fc fragments. Mixtures of IgG and IgA on beads did not act synergistically. Secretory IgA (sIgA) provided the most potent signal for eosinophil degranulation and was two to three times more potent than IgG. Furthermore, 2 to 17% of the normodense eosinophils bound to IgG- or IgA-coated beads, whereas 24 to 27% of the eosinophils bound to sIgA-coated beads. Thus, sIgA may be the principal Ig mediating eosinophil effector function at mucosal surfaces in helminth infections and hypersensitivity diseases, especially bronchial asthma.     

Calcium ionophore A23187 calcium-dependent cytolytic degranulation in human eosinophils

The divalent cation ionophore A23187 is frequently used for studies of eosinophil degranulation. Nonetheless, the mechanism whereby A23187 induces degranulation in human eosinophils is still unclear. In the present experiments, A23187 caused human eosinophils to release a granule protein, eosinophil-derived neurotoxin (EDN) and a membrane-associated protein, Charcot-Leyden crystal (CLC) protein in a calcium and a concentration-dependent manner. However, A23187 at a concentration (1 microgram/ml) that caused 15% EDN release and 30% CLC protein release also produced release of the cytoplasmic enzyme lactic dehydrogenase (LDH) and loss of cell viability, both of which were calcium dependent. CLC protein release preceded EDN release and was detectable even at 15 min after the addition of 1 microgram/ml A23187, whereas EDN release occurred after a lag period of 30 min, and coincided with LDH release. At 1 microgram/ml A23187, neither the release of LDH nor the loss of viability occurred with purified neutrophils obtained in the same blood sample as a by-product of eosinophil purification. Electron microscopic examination demonstrated that exposure to A23187 for 15 min resulted in an increase and elongation of microridges on the cell surface, and exposure for 45 min caused cell disruption followed by extrusion of membrane-bound granules through breaks in the plasma membrane. Only once was granule exocytosis observed. These results indicate that A23187 treatment of eosinophils causes an initial release of membrane-associated CLC protein by a noncytolytic mechanism, and causes degranulation as a result of eosinophil lysis.     

Regulation of Ig-induced eosinophil degranulation by adenosine 3',5'-cyclic monophosphate.

We have investigated the effects of cAMP on Ig-induced human eosinophil activation. Stimulation of human normodense eosinophils with IgG- or secretory IgA (sIgA)-coated Sepharose beads induced cellular degranulation, as measured by the release of the granule protein, eosinophil-derived neurotoxin (EDN). Pretreatment with cAMP analogs (N6,O2,-dibutyryl adenosine-3,':5' cyclic monophosphate; 8-bromoadenosine 3':5' cyclic monophosphate; or N6-benzoyladenosine 3':5' cyclic monophosphate) or cAMP phosphodiesterase-inhibitors (theophylline or isobutylmethyl xanthine (IBMX] strongly inhibited Ig-induced human eosinophil degranulation. The beta-adrenoceptor agonists, isoproterenol and salbutamol, induced relatively low level increases in intracellular cAMP, and weakly suppressed EDN release induced by IgG-coated beads. However, cellular pretreatment with IBMX synergistically enhanced the inhibitory effects of isoproterenol or salbutamol on both IgG and sIgA-induced eosinophil degranulation. Similarly, PGE2 treatment increased intracellular cAMP concentrations in eosinophils and correspondingly inhibited the Ig-dependent cellular degranulation response: co-incubation with IBMX further enhanced both effects of PGE2. Finally, cholera toxin, which irreversibly activates the stimulatory guanine nucleotide-binding protein linked to adenylyl cyclase, strongly inhibited the release of EDN from IgG- or sIgA-stimulated eosinophils. The time-dependent accumulation of cAMP in cholera toxin-treated cells closely paralleled the time courses of inhibition of IgG- and sIgA-induced EDN release after toxin exposure. These data indicate that the cAMP-dependent signal transduction mechanism in eosinophils exerts a negative modulatory effect on the cellular degranulation responses induced by sIgA or IgG. The inhibitory effects of cAMP on eosinophil activation may provide an important physiologic and a clinically relevant therapeutic mechanism for limiting the release of eosinophil-derived cytotoxic proteins during certain allergic or inflammatory responses in vivo.     

Regulatory effect of cytokines on eosinophil degranulation

We tested the effects of different cytokines on IgA- and IgG-induced eosinophil degranulation in vitro to determine the potential interaction between eosinophils and mononuclear cells. Purified normodense eosinophils were incubated with cytokines (including rIL-1, rIL-2, rIL-3, rIL-4, rIL-5, rIL-6, IFN-gamma, granulocyte-macrophage CSF stimulating factor (GM-CSF), and TNF) for 1 to 3 h after which Ig-coupled Sepharose 4B beads were added as targets and the mixtures were incubated with the eosinophils at 37 degrees C for 4 h. The Ig used were secretory IgA (sIgA), serum IgA and IgG, and myeloma IgA and IgG. The release of eosinophil-derived neurotoxin (EDN) was measured by RIA as an index of degranulation. rIL-5 was the most potent enhancer of Ig-induced degranulation and increased EDN release by 48% for sIgA and 136% for IgG. The effect of rIL-5 appeared as quickly as 15 min after incubation of eosinophils, sIgA beads and IL-5. GM-CSF and rIL-3 also enhanced Ig-induced EDN release but less potently than rIL-5. GM-CSF and rIL-5 by themselves induced a small but significant release of EDN from eosinophils in the absence of Ig-coated beads; rIL-3 did not. However, IFN-gamma suppressed sIgA-induced EDN release by 23%. The other cytokines did not have any effect on eosinophil degranulation. These results suggest that cytokines which induce eosinophil differentiation and proliferation during hematopoiesis also enhance the effector function of mature eosinophils and that IFN-gamma partially down-regulates eosinophil degranulation.     

Release of granule proteins from eosinophils cultured with IL-5.

Eosinophils isolated from normal individuals were cultured in the presence of human rIL-5 (hrIL-5) for up to 14 days, and the effects of this exposure were determined. First, the hrIL-5-cultured eosinophils were activated and degranulated more readily than freshly isolated eosinophils. For example, eosinophils cultured for 7 days with hrIL-5 released 30 and 10% of granule eosinophil-derived neurotoxin (EDN) when exposed to Sepharose 4B beads coupled to secretory IgA and IgG, respectively, whereas freshly isolated eosinophils released only 19 and 4%, respectively, of their EDN in response to the same stimuli. Degranulation of hrIL-5-cultured eosinophils was not augmented by further exposure to hrIL-5, whereas degranulation of freshly isolated cells to secretory IgA and IgG beads was increased by exposure to hrIL-5. Second, eosinophils cultured with hrIL-5 had prolonged viability in vitro. For example, after four days of culture with 50 U/ml of hrIL-5, 86% of eosinophils were viable compared to 12% in medium alone. Third, hrIL-5-cultured eosinophils became hypodense, and electron microscopy showed that they contained granules with core and matrix lucency and with evidence of granule fusion. Fourth, hrIL-5-cultured eosinophils spontaneously lost 30 to 60% of their EDN, eosinophil cationic protein, and eosinophil peroxidase and about 50% of their eosinophil granule major basic protein content compared to freshly isolated eosinophils, and all four of the granule proteins were released into the culture medium. Fifth, detailed studies of eosinophils cultured in hrIL-5 showed that 89 +/- 10% of the starting quantity of EDN could be recovered at 7 days. Whereas 99 +/- 1% of the EDN at day 0 was cell associated, by 7 days 60 +/- 9% was in the cell supernatants. Thus, hrIL-5 activates eosinophils, increases their viability, decreases their density, and their content of granule proteins and causes release of the granule proteins into culture fluids. The striking loss of granule proteins during culture with hrIL-5 may be an important mechanism for deposition of these cationic toxins in various diseases where IL-5 plays a role.     

In vitro killing of microfilariae of Brugia pahangi and Brugia malayi by eosinophil granule proteins

Eosinophil infiltration and degranulation around the tissue-invasive stages of several species of helminths have been observed. Release of eosinophil granule contents upon the worms is supported by localization of two of the major granule proteins, major basic protein (MBP) and eosinophil peroxidase (EPO), on and around species of trematodes, nematodes, and cestodes. In the case of filarial worms, MBP is deposited on degenerating microfilariae (mf) of Onchocerca volvulus. Here, we performed in vitro assays of the toxicity of four purified eosinophil granule proteins, namely, MBP, EPO, eosinophil cationic protein (ECP), and eosinophil-derived neurotoxin (EDN), for the mf of Brugia pahangi and Brugia malayi. MBP, ECP, and EDN killed these worms in a dose-related manner although relatively high concentrations of EDN were necessary. EPO, in the presence of a H2O2-generating system and a halide, was the most potent toxin on a molar basis; here, the most potent halide was I- followed by Br- and Cl-. Surprisingly, EPO in the absence of H2O2 killed mf at concentrations comparable to those required for MBP and ECP. The toxicity of EPO + H2O2 + halide was inhibited by heparin, catalase, or 1% BSA, whereas the toxicity of EPO alone was inhibited only by heparin. Heparin also inhibited killing by both MBP and ECP. Despite the homology of ECP with certain RNases, placental RNasin, an RNase inhibitor, was unable to inhibit ECP-mediated toxicity. These results indicate that all of the eosinophil granule proteins are toxic to mf and they support the hypothesis that eosinophil degranulation causes death of mf in vivo.     

Atropine pretreatment enhances airway hyperreactivity in antigen-challenged guinea pigs through an eosinophil-dependent mechanism

Airway hyperreactivity in antigen-challenged animals is mediated by eosinophil major basic protein (MBP) that blocks inhibitory M(2) muscarinic receptors on parasympathetic nerves, increasing acetylcholine release onto M(3) muscarinic receptors on airway smooth muscle. Acutely, anticholinergics block hyperreactivity in antigen-challenged animals and reverse asthma exacerbations in the human, but are less effective in chronic asthma. We tested whether atropine, given before antigen challenge, affected hyperreactivity, M(2) receptor function, eosinophil accumulation, and activation. Sensitized guinea pigs received atropine (1 mg/kg ip) 1 h before challenge and 6 h later. Twenty-four hours after challenge, animals were anesthetized, vagotomized, paralyzed, and ventilated. Airway reactivity to electrical stimulation of the vagi and to intravenous acetylcholine was not altered by atropine pretreatment in nonsensitized animals, indicating that atropine was no longer blocking postjunctional muscarinic receptors. Antigen challenge induced airway hyperreactivity to vagal stimulation that was significantly potentiated by atropine pretreatment. Bronchoconstriction induced by acetylcholine was not changed by antigen challenge or by atropine pretreatment. M(2) receptor function was lost in challenged animals but protected by atropine pretreatment. Eosinophils in bronchoalveolar lavage and within airway tissues were significantly increased by challenge but significantly reduced by atropine pretreatment. However, extracellular MBP in challenged airways was significantly increased by atropine pretreatment, which may account for reduced eosinophils. Depleting eosinophils with antibody to IL-5 before challenge prevented hyperreactivity and significantly reduced MBP in airways of atropine-pretreated animals. Thus atropine pretreatment potentiated airway hyperreactivity by increasing eosinophil activation and degranulation. These data suggest that anticholinergics enhance eosinophil interactions with airway nerves.     

Degranulation of human eosinophils induced by Paragonimus westermani-secreted protease

Eosinophil degranulation is considered to be a key effector function for the killing of helminthic worms and tissue inflammation at worm-infected lesion sites. However, relatively little data are available with regard to eosinophil response after stimulation with worm-secreted products which contain a large quantity of cysteine proteases. In this study, we attempted to determine whether the degranulation of human eosinophils could be induced by the direct stimulation of the excretory-secretory products (ESP) of Paragonimus westermani, which causes pulmonary paragonimiasis in human beings. Incubation of eosinophils for 3 hr with Paragonimus-secreted products resulted in marked degranulation, as evidenced by the release of eosinophil-derived neurotoxin (EDN) in the culture supernatants. Moreover, superoxide anion was produced by eosinophils after stimulation of the ESP. The ESP-induced EDN release was found to be significantly inhibited when the ESP was pretreated with protease inhibitor cocktail or the cysteine protease inhibitor, E-64. These findings suggest that human eosinophils become degranulated in response to P. westermani-secreted proteases, which may contribute to in vivo tissue inflammation around the worms.     

Nonpathogenic, environmental fungi induce activation and degranulation of human eosinophils

Eosinophils and their products are probably important in the pathophysiology of allergic diseases, such as bronchial asthma, and in host immunity to certain organisms. An association between environmental fungal exposure and asthma has been long recognized clinically. Although products of microorganisms (e.g., lipopolysaccharides) directly activate certain inflammatory cells (e.g., macrophages), the mechanism(s) that triggers eosinophil degranulation is unknown. In this study we investigated whether human eosinophils have an innate immune response to certain fungal organisms. We incubated human eosinophils with extracts from seven environmental airborne fungi (Alternaria alternata, Aspergillus versicolor, Bipolaris sorokiniana, Candida albicans, Cladosporium herbarum, Curvularia spicifera, and Penicillium notatum). Alternaria and Penicillium induced calcium-dependent exocytosis (e.g., eosinophil-derived neurotoxin release) in eosinophils from normal individuals. Alternaria also strongly induced other activation events in eosinophils, including increases in intracellular calcium concentration, cell surface expression of CD63 and CD11b, and production of IL-8. Other fungi did not induce eosinophil degranulation, and Alternaria did not induce neutrophil activation, suggesting specificity for fungal species and cell type. The Alternaria-induced eosinophil degranulation was pertussis toxin sensitive and desensitized by preincubating cells with G protein-coupled receptor agonists, platelet-activating factor, or FMLP. The eosinophil-stimulating activity in Alternaria extract was highly heat labile and had an M(r) of approximately 60 kDa. Thus, eosinophils, but not neutrophils, possess G protein-dependent cellular activation machinery that directly responds to an Alternaria protein product(s). This innate response by eosinophils to certain environmental fungi may be important in host defense and in the exacerbation of inflammation in asthma and allergic diseases.     

IL-3 and TNFα increase Thymic Stromal Lymphopoietin Receptor (TSLPR) expression on eosinophils and enhance TSLP-stimulated degranulation

Background

Thymic stromal lymphopoietin (TSLP) and eosinophils are prominent components of allergic inflammation. Therefore, we sought to determine whether TSLP could activate eosinophils, focusing on measuring the regulation of TSLPR expression on eosinophils and degranulation in response to TSLP, as well as other eosinophil activation responses.

Methods

Eosinophil mRNA expression of TSLPR and IL-7R?? was examined by real-time quantitative PCR of human eosinophils treated with TNF?? and IL-5 family cytokines, and TSLPR surface expression on eosinophils was analyzed by flow cytometry. Eosinophils were stimulated with TSLP (with and without pre-activation with TNF?? and IL-3) and evaluated for release of eosinophil derived neurotoxin (EDN), phosphorylation of STAT5, and survival by trypan blue exclusion. A blocking antibody for TSLPR was used to confirm the specificity of TSLP mediated signaling on eosinophil degranulation.

Results

Eosinophil expression of cell surface TSLPR and TSLPR mRNA was upregulated by stimulation with TNF?? and IL-3. TSLP stimulation resulted in release of EDN, phosphorylation of STAT5 as well as promotion of viability and survival. TSLP-stimulated eosinophil degranulation was inhibited by a functional blocking antibody to TSLPR. Pre-activation of eosinophils with TNF?? and IL-3 promoted eosinophil degranulation at lower concentrations of TSLP stimulation.

Conclusions

This study demonstrates that eosinophils are activated by TSLP and that eosinophil degranulation in response to TSLP may be enhanced on exposure to cytokines present in allergic inflammation, indicating that the eosinophil has the capacity to participate in TSLP-driven allergic responses.     

In vitro Release of Eosinophil Proteins in Allergic and Atopic Dermatitis Patients

To investigate whether eosinophils are stimulated in vivo or have acquired an increased susceptibility to stimuli from the coagulation cascade, the release of eosinophil proteins was compared for three groups of donors with different levels of serum IgE. (1) with atopic dermatitis (s-IgE > 5000 IU/ml, n = 11); (2) with inhalant allergy (200 < s-IgE < 2 000 IU/ml, n = 10); and (3) non-allergic (s- IgE < 100 IU/ml, n = 10). The levels of eosinophil cationic protein and eosinophil protein X (ECP, EPX) were determined in serum (clotting time = 2.0 h) and plasma. Serum and plasma ECP in normal donors demonstrated large intra-personal variations (C.V. 50-80%), but serum-ECP (mean 8.1 ng/ml) was clearly distinguishable from plasma ECP (mean 1.0 ng/ml) by a factor of 8 (range: 5.6-11.6). The ECP released during clotting was markedly increased in the atopic dermatitis group (serum:plasma ratio 13.5, p < 0.003) compared with the other groups (6.7 and 5.6). EPX, having a higher plasma level, demonstrated a less pronounced release (serum: plasma ratios 2.0, 1.7 and 1.4), with no statistical difference between donor groups. Considering all donors together the levels of ECP and EPX in plasma and in serum were correlated to the number of eosinophils (coefficients of correlation 0.54-0.58, p < 0.002).     

Human eosinophils exert TNF-α and granzyme A-mediated tumoricidal activity toward colon carcinoma cells

Peripheral blood and tissue eosinophilia is a prominent feature in allergic diseases and helminth infections. In cancer patients, tumor-associated tissue eosinophilia is frequently observed. Tumor-associated tissue eosinophilia can be associated with a favorable prognosis, notably in colorectal carcinoma. However, underlying mechanisms of eosinophil contribution to antitumor responses are poorly understood. We have in this study investigated the direct interactions of human eosinophils with Colo-205, a colorectal carcinoma cell line, and show that eosinophils induce apoptosis and directly kill tumor cells. Using blocking Abs, we found that CD11a/CD18 complex is involved in the tumoricidal activity. Coculture of eosinophils with Colo-205 led to the release of eosinophil cationic protein and eosinophil-derived neurotoxin as well as TNF-α secretion. Moreover, eosinophils expressed granzyme A, which was released upon interaction with Colo-205, whereas cytotoxicity was partially inhibited by FUT-175, an inhibitor of trypsin-like enzymatic activity. Our data present the first demonstration, to our knowledge, that granzyme A is a cytotoxic mediator of the eosinophil protein arsenal, exerting eosinophil tumoricidal activity toward Colo-205, and provide mechanistic evidence for innate responses of eosinophil against tumor cells.     

Sensitization of isolated rat vagal pulmonary sensory neurons by eosinophil-derived cationic proteins

It has been shown that airway exposure to eosinophil-derived cationic proteins stimulated vagal pulmonary C fibers and markedly potentiated their responses to lung inflation in anesthetized rats (Lee LY, Gu Q, Gleich GJ, J Appl Physiol 91: 1318-1326, 2001). However, whether the effects resulted from a direct action of these proteins on the sensory nerves was not known. The present study was therefore carried out to determine the effects of these proteins on isolated rat vagal pulmonary sensory neurons. Our results obtained from perforated whole cell patch-clamp recordings showed that pretreatment with eosinophil major basic protein (MBP; 2 microM, 60 s) significantly increased the capsaicin-evoked inward current in these neurons; this effect peaked approximately 10 min after MBP and lasted for >60 min; in current-clamp mode, MBP substantially increased the number of action potentials evoked by both capsaicin and electrical stimulation. Pretreatment with MBP did not significantly alter the input resistance of these sensory neurons. In addition, the sensitizing effect of MBP was completely abolished when its cationic charge was neutralized by mixing with a polyanion, such as low-molecular-weight heparin or poly-L-glutamic or poly-L-aspartic acid, before its delivery to the neurons. Moreover, a similar sensitizing effect was also generated by other eosinophil granule-derived proteins (e.g., eosinophil peroxidase). These results demonstrate a direct, charge-dependent, and long-lasting sensitizing effect of cationic proteins on pulmonary sensory neurons, which may contribute to the airway hyperresponsiveness associated with airway infiltration of eosinophils under pathophysiological conditions.     

Distinctive cationic proteins of the human eosinophil granule: major basic protein, eosinophil cationic protein, and eosinophil-derived neurotoxin

The human eosinophil granule contains a number of cationic proteins that have been identified and purified to homogeneity, including the major basic protein (MBP), the eosinophil cationic protein (ECP), and the eosinophil-derived neurotoxin (EDN). Because of confusion in the literature regarding the distinctiveness of MBP and ECP, we investigated the immunochemical and physicochemical properties of these purified proteins by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels (SDS-PAGE), by specific double antibody radioimmunoassays (RIA) for MBP and ECP, and by fractionation of acid-solubilized eosinophil granules on Sephadex G-50 columns. Analysis of a mixture of the three purified proteins by SDS-PAGE showed that they migrated as three distinct bands with differing m.w. Comparison by specific RIA for MBP and ECP did not demonstrate any appreciable immunochemical cross-reactivities among the three proteins. Sephadex G-50 column fractions of acid-solubilized eosinophil granules were analyzed by RIA and by SDS-PAGE analysis of individual column fractions. MBP, ECP, and EDN eluted at different volumes from Sephadex G-50 columns as determined by RIA and SDS-PAGE. Soluble extracts of eosinophil granules from patients with the hypereosinophilic syndrome contained between six and 64 times more MBP than ECP on a weight basis. These observations demonstrate that MBP, ECP, and EDN are distinctive cationic proteins of the human eosinophil granule and that eosinophil granules from patients with eosinophilia contain considerably greater quantities of MBP than ECP.     

Stimulation of basophil and rat mast cell histamine release by eosinophil granule-derived cationic proteins

Major basic protein (MBP), an arginine-rich basic polypeptide that constitutes the crystalloid core of the large specific eosinophil granule, has previously been shown to stimulate noncytolytic histamine release from human basophils and rat mast cells by an IgE-independent mechanism. Two additional basic polypeptides present in eosinophil granules, eosinophil cationic protein (ECP) and eosinophil-derived neurotoxin (EDN), were examined for similar activity in the present study. Acid-solubilized eosinophil granules were fractionated by chromatography on a Sephadex G-50 column. Incubation of basophil-containing human mononuclear cells with the individual column fractions demonstrated that histamine release occurred only with the fractions that contained MBP. The selectivity of the basophil response for MBP was confirmed by using equimolar concentrations of purified MBP, ECP, and EDN. In contrast, both MBP and ECP, but not EDN, stimulated histamine release from purified rat peritoneal mast cells. Reduction and alkylation of the MBP molecule diminished the response of human basophils to MBP but enhanced the potency of the molecule with rat mast cells. The distinct potency of MBP as a stimulus for histamine secretion from human basophils suggests that eosinophil release of MBP may be a specific event in the augmentation of immediate hypersensitivity reactions and other disorders characterized by eosinophilia.     

In vitro leukotriene (LT) C4 synthesis by blood eosinophils from atopic asthmatics: Predominance of eosinophil subpopulations with high potency for LTC4 generation

We compared the leukotriene (LT) C4 generation by the eosinophil density subpopulations isolated from the blood of asthmatics and normal subjects. Using discontinuous Percoll gradients, eosinophil subpopulations with densities of 1.075, 1.078, 1.081, 1.084, 1.087 and 1.100 g/ml were isolated from the blood of six atopic asthmatics and seven normals. In normals, most eosinophils (94%) were recovered in the density fractions (1.084, 1.087, and 1.100 g/ml). In asthmatics, the eosinophil density profile was shifted towards lower cell density: the eosinophil subsets 1.078, 1.081 and 1.084 g/ml were increased 4.5, 30.3 and 8.9-fold respectively compared to normals (p less than 0.0001); the intermediate subsets, the hypodense 1.081 and normodense 1.084 g/ml fractions being the predominant subpopulations. The ability of asthmatic eosinophil subsets 1.078 to 1.087 g/ml to release LTC4 was measured by reversed-phase HPLC, LTC4 production was highest with the normodense 1.084 g/ml eosinophil subset (149 +/- 28 pmol/10(6) eosinophils, p less than 0.01 compared to the 1.081 fraction). The eosinophils from the hypodense 1.081 g/ml fraction also released more LTC4 (112 +/- 19 pmol/10(6) eosinophils) than the 1.078 and 1.087 g/ml fractions (70 +/- 16 and 67 +/- 12 respectively, p less than 0.01). These results show that, compared to normals, blood of mild atopic asthmatics contains an elevated number of eosinophils of intermediate density which have a high capacity for LTC4 production.     

Major basic protein homolog (MBP2): a specific human eosinophil marker

Human eosinophil granule major basic protein (MBP1) is an exceedingly basic (isoelectric point >11) 14-kDa protein, comprising the core of the secondary eosinophil granule. Recently, a less cationic homolog of MBP, termed MBPH or simply, MBP2, has been discovered. We prepared a panel of mAbs to MBP2 and used these Abs to localize and quantitate this molecule in leukocytes and biological fluids. Specific mAbs for MBP2 were selected using slot-blot analyses and used in a two-site immunoassay, Western blotting, and immunofluorescence microscopy. The sensitivity of the immunoassay was markedly improved by reduction and alkylation of MBP2. MBP1 is more abundant than MBP2 in lysates of eosinophils and their granules, as judged by immunoassay and Western blotting. By immunofluorescence, MBP1 is present in eosinophils, basophils, and a human mast cell line (HMC1), whereas MBP2 is only detected in eosinophils. Neither MBP1 nor MBP2 could be detected in any other peripheral blood leukocyte. MBP2 levels measured in plasma and serum were essentially identical. In contrast to past measurements for MBP1, MBP2 was not detected above normal levels in sera from pregnant donors. However, measurement of serum MBP2 discriminated patients with elevated eosinophils from normal subjects, and MBP2 was also detectable in other biological specimens, such as bronchoalveolar lavage, sputum, and stool. These results indicate that MBP2 is present only in eosinophils and that it may be a useful biomarker for eosinophil-associated diseases.