Increased expression of TRAIL receptor 3 on eosinophils in Churg-Strauss syndrome

OBJECTIVE: Prolonged survival of eosinophils plays an important role in the pathogenesis of Churg-Strauss syndrome (CSS); however, its detailed molecular mechanism is still unclear. TRAIL and its receptors are expressed on a variety of cells, including eosinophils. In this study, we examined the expression of TRAIL receptors on eosinophils from patients with CSS. METHODS: TRAIL receptor expression was assessed on eosinophils from healthy volunteers, patients with CSS, patients with asthma, and patients with hypereosinophilia due to parasitic infection. TRAIL-induced apoptosis of eosinophils was compared between the patients with CSS and patients with asthma. RNA interference was used to assess the effects of suppression of TRAIL receptor 3. RESULTS: Expression of TRAIL receptor 3, a decoy receptor that acts as an antiapoptotic receptor, on eosinophils from patients with CSS was significantly higher than that in the other subjects. Moreover, in CSS, serum TRAIL receptor 3 levels showed a significant positive correlation with peripheral eosinophil counts, tissue-infiltrating eosinophils stained positive for this receptor, and peripheral T cells expressed TRAIL on their surface. Compared with asthma patients, eosinophils from CSS patients showed a significantly lower percentage of recombinant TRAIL, less autologous T cell-induced apoptosis, and decreased level of active caspase 3. Suppression of TRAIL receptor 3 through RNA interference significantly increased the recombinant TRAIL-induced apoptosis of eosinophils from CSS patients. CONCLUSION: Increased expression of TRAIL receptor 3 on eosinophils from patients with CSS was observed. These alterations in TRAIL receptor 3 expression might be involved in the molecular pathogenesis of CSS eosinophilia.     

Increased expression of TRAIL receptor 3 on eosinophils in Churg‐Strauss syndrome

Objective

Prolonged survival of eosinophils plays an important role in the pathogenesis of Churg‐Strauss syndrome (CSS); however, its detailed molecular mechanism is still unclear. TRAIL and its receptors are expressed on a variety of cells, including eosinophils. In this study, we examined the expression of TRAIL receptors on eosinophils from patients with CSS.

Methods

TRAIL receptor expression was assessed on eosinophils from healthy volunteers, patients with CSS, patients with asthma, and patients with hypereosinophilia due to parasitic infection. TRAIL‐induced apoptosis of eosinophils was compared between the patients with CSS and patients with asthma. RNA interference was used to assess the effects of suppression of TRAIL receptor 3.

Results

Expression of TRAIL receptor 3, a decoy receptor that acts as an antiapoptotic receptor, on eosinophils from patients with CSS was significantly higher than that in the other subjects. Moreover, in CSS, serum TRAIL receptor 3 levels showed a significant positive correlation with peripheral eosinophil counts, tissue‐infiltrating eosinophils stained positive for this receptor, and peripheral T cells expressed TRAIL on their surface. Compared with asthma patients, eosinophils from CSS patients showed a significantly lower percentage of recombinant TRAIL, less autologous T cell–induced apoptosis, and decreased level of active caspase 3. Suppression of TRAIL receptor 3 through RNA interference significantly increased the recombinant TRAIL–induced apoptosis of eosinophils from CSS patients.

Conclusion

Increased expression of TRAIL receptor 3 on eosinophils from patients with CSS was observed. These alterations in TRAIL receptor 3 expression might be involved in the molecular pathogenesis of CSS eosinophilia.

     

Induction of apoptosis in human eosinophils by anti-Fas antibody treatment in vitro

Fas antigen (CD95) can induce apoptosis of cells such as lymphocytes and neutrophils. To determine whether Fas antigen is involved in eosinophil apoptosis, we examined its expression and function on eosinophils in vitro. Purified human eosinophils expressed low but consistently detectable levels of Fas antigen. Culture of eosinophils in up to 10 ng/mL interleukin-5 (IL-5) prolonged eosinophil survival; incorporation of 1 to 1,000 ng/mL Fas antibody led to significant reductions in IL-5-induced eosinophil viability after 48 to 72 hours of culture. Reductions in survival could not be overcome by IL-5 and also occurred in the absence of exogenous IL-5. Preactivation of eosinophils with platelet-activating factor (PAF) significantly reduced eosinophil viability without altering the survival-reducing effects of Fas antibody treatment. In contrast, RANTES did not affect eosinophil viability or Fas antibody-induced reductions in eosinophil survival. After treatment with Fas antibody, electron microscopy of eosinophils and gel electrophoresis of DNA extracted from eosinophils demonstrated changes consistent with apoptosis. These data demonstrate that Fas antigen can modify eosinophil survival by inducing apoptosis through a pathway that is, at least in part, independent of the survival- promoting effects of IL-5.     

Eosinophil active cytokines and surface analysis of eosinophils in Churg-Strauss syndrome.

There are few reports regarding the measurement of cytokines and surface analysis of eosinophils in Churg-Strauss syndrome (CSS). To examine the pathophysiology of CSS, concentrations of cytokines in serum and bronchoalveolar lavage fluid (BALF), and surface antigens on peripheral blood eosinophils were analyzed in five patients with CSS. Concentrations of cytokines (interleukin-1 beta (IL-1 beta), tumor necrosis factor-alpha (TNF-alpha), interleukin-3 (IL-3), interleukin-5 (IL-5) and granulocyte/macrophage colony stimulating factor (GM-CSF) were measured using ELISA. Surface antigens on eosinophils in peripheral blood were analyzed using flow cytometry. A concentration of interleukin-5 (IL-5) and TNF-alpha in serum was detected in five cases; however IL-1 beta, GM-CSF, and IL-3 were detected in 3 of 5, 2 of 5, and 1 of 5 patients, respectively. In BALF, TNF-alpha and IL-5 were detected in 2 of 3 and 1 of 3 patients, respectively; however, neither IL-1 beta, GM-CSF, nor IL-3 was detected in any. Newly expressed surface antigens such as CD25, CD4, and CD69 were observed on peripheral blood eosinophils in five cases. CD54 and HLA-DR were expressed in 4 of 5 and 3 of 5 patients, respectively. Eosinophils in peripheral blood are activated to various degrees, possibly depending on cytokine stimulation. This eosinophil activation may be related to the clinical stage of CSS.     

Adiponectin attenuates human eosinophil adhesion and chemotaxis: implications in allergic inflammation

Abstract

Objective: Growing evidence has shown an association between obesity and asthma. Adiponectin, an adipocyte-derived cytokine, is known to have anti-inflammatory effects with reduced concentrations in obese subjects. Recent findings raised the intriguing possibility that adiponectin might play a role in allergic inflammation, although the mechanistic basis for their relationship remains unclear. The purpose of this study was to examine whether adiponectin might affect functions of eosinophils, which play an important role in the pathogenesis of asthma. Methods: Human peripheral blood eosinophils were purified to study expression of adiponectin receptors AdipoR1 and AdipoR2 using RT-PCR and flow cytometry. The effect of adiponectin on eosinophil survival was investigated using annexin V and propidium iodide staining. Eotaxin-induced cell adhesion was investigated using ICAM-1-coated plates. A Boyden chamber and real-time horizontal migration system were used for eotaxin-directed chemotaxis assay. Expression of eotaxin receptor CCR3 and intracellular calcium influx were assessed by flow cytometry. Results: AdipoR1 and AdipoR2 were expressed in human eosinophils. Adiponectin did not affect eosinophil survival or CCR3 expression; however, eotaxin-enhanced adhesion was inhibited by pretreatment with adiponectin. Adiponectin also diminished eotaxin-directed chemotactic responses by disturbing both velocity and directionality. Calcium influx in response to eotaxin was attenuated by adiponectin. Conclusions: These results indicate that adiponectin attenuates the eosinophil functions induced by eotaxin without affecting cell viability. The inhibitory effect was associated with diminished calcium signaling rather than altering of surface receptor expression. Increasing circulating adiponectin might be a novel therapeutic modality for treatment of asthma, especially in obese asthmatics.     

Sputum eosinophilia in Churg-Strauss syndrome

Sputum induction (IS) can be used to study airway inflammation in asthmatics and other lung diseases. However, no data are available for patients with Churg-Strauss syndrome (CSS). A study was carried out to evaluate eosinophil counts and eosinophil cationic protein (ECP) levels in induced sputum during the follow-up of three patients with CSS. Induced sputum was carried out in 10 patients with corticosteroid-dependent asthma (used as a control group). Patients with CSS had significantly higher eosinophils percentages and ECP levels in sputum than those with stable corticosteroid-dependent asthma. During the follow-up, patients with CSS presented increased ECP levels sputum and eosinophils in sputum as well as increased blood eosinophils, despite their oral corticosteroid and immunosuppressive treatment. Eosinophil percentage in sputum and the total number of eosinophils in peripheral blood were more predictive of exacerbations of CSS than sputum ECP.     

Eosinophils from asthmatics release IL-5 in an autocrine fashion to prevent apoptosis through upregulation of Bcl-2 expression.

Interleukin (IL)-5 plays an important role in maintaining the survival of eosinophils via the specific alpha-subunit of its receptor. Apoptosis, a form of programmed cell death, is thought to represent a mechanism that promotes the resolution of eosinophilic inflammation in asthma. The aim of our present study is to investigate whether IL-5 acts in an autocrine fashion on eosinophil apoptosis in asthmatics. Immunoreactivities of intracellular IL-5 and IL-5 receptor alpha-subunit (Ralpha) were detected uniquely on the eosinophils. The magnitude of IL-5 and IL-5 Ralpha expression on eosinophils was significantly higher in asthmatics than that of normal subjects (p<0.05) determined by flow cytometry. Apoptosis of eosinophils was measured by the propidium iodide staining method and DNA ladder. The percent of apoptotic eosinophils from asthmatics was significantly increased by coincubation with anti-hIL-5 Ralpha Ab (0.1, 0.5, and 2.5 microg/mL) for 1, 2, or 16 hours than was those of corresponding controls (p<0.05, n=8). However, there was no significant effect of anti-hIL-5 Ralpha Ab on eosinophil apoptosis in normal subjects. Furthermore, the expression of B-cell lymphoma-2 (Bcl-2) proteins was significantly inhibited by the anti-hIL-5 Ralpha Ab or antisense IL-5 oligonucleotides in asthmatics (p<0.05, n=8), but there was no significant change in eosinophils from normal subjects. This study demonstrates that eosinophils from asthmatics release IL-5 in an autocrine fashion to act on their own IL-5 receptors in prevention of apoptosis through the upregulation of Bcl-2 expression.     

Hypodense eosinophils: characterization of surface molecule expression.

Distinct eosinophil populations have been characterized on the basis of discontinuous Percoll density gradients. In peripheral blood, normal individuals show a low number of normodense and hypodense eosinophils, contrasting with the high amount of hypodense cells in patients who have allergies. To characterize these two eosinophil populations, we analyzed membrane expression of several antigens and cytokine receptors in normodense and hypodense eosinophils from patients who have allergies and controls. Hypodense eosinophils expressed higher levels of CD122, CD69, and CD4 in both patients with allergies and control individuals when compared to normodense eosinophils. The expression of CD125, CD124, CD25, CD132, and CD23 were similar in both cell types.     

Eosinophils from Asthmatics Release IL-5 in an Autocrine Fashion to Prevent Apoptosis Through Upregulation of Bcl-2 Expression

Interleukin (IL)-5 plays an important role in maintaining the survival of eosinophils via the specific α-subunit of its receptor. Apoptosis, a form of programmed cell death, is thought to represent a mechanism that promotes the resolution of eosinophilic inflammation in asthma. The aim of our present study is to investigate whether IL-5 acts in an autocrine fashion on eosinophil apoptosis in asthmatics. Immunoreactivities of intracellular IL-5 and IL-5 receptor α-subunit (Rα) were detected uniquely on the eosinophils. The magnitude of IL-5 and IL-5 Rα expression on eosinophils was significantly higher in asthmatics than that of normal subjects (p < 0.05) determined by flow cytometry. Apoptosis of eosinophils was measured by the propidium iodide staining method and DNA ladder. The percent of apoptotic eosinophils from asthmatics was significantly increased by coincubation with anti-hIL-5 Rα Ab (0.1, 0.5, and 2.5 µg/mL) for 1, 2, or 16 hours than was those of corresponding controls (p < 0.05, n = 8). However, there was no significant effect of anti-hIL-5 Rα Ab on eosinophil apoptosis in normal subjects. Furthermore, the expression of B-cell lymphoma-2 (Bcl-2) proteins was significantly inhibited by the anti-hIL-5 Rα Ab or antisense IL-5 oligonucleotides in asthmatics (p < 0.05, n = 8), but there was no significant change in eosinophils from normal subjects. This study demonstrates that eosinophils from asthmatics release IL-5 in an autocrine fashion to act on their own IL-5 receptors in prevention of apoptosis through the upregulation of Bcl-2 expression.     

Non-specific activation of human eosinophil functional responses by vasoactive intestinal peptide

Eosinophils and neuropeptides are thought to play effector roles in allergic diseases, such as rhinitis; however, little is known about the biological effects of neuromediators, especially vasoactive intestinal peptide (VIP), on eosinophil functional responses. In the present study, it is shown that VIP induces eosinophil chemotaxis and eosinophil-derived neurotoxin (EDN) release in potency comparable with that induced by platelet activator factor, and in a novel synergistic manner with recombinant human interleukin-5. Contrary to chemotaxis, EDN release was sensitive to staurosporine, the protein kinase C inhibitor, as well as intracellular calcium chelation. However, eosinophil treatment with inhibitors of tyrosine kinases (herbimycin A) and phosphatases (pervanadate) resulted in a dose-dependent potentiation and blockage of VIP-induced eosinophil chemotaxis, respectively. Treatment of eosinophils with VIP receptor antagonist did not modify VIP-induced chemotaxis or EDN release. Furthermore, exploration of vasoactive intestinal peptide receptor I expression was lacking in human eosinophils, but not lymphocytes. These results demonstrate two different mechanisms in triggering eosinophil activation of functional responses by VIP, a calcium-dependent degranulation and a calcium-independent chemotaxis, and elaborate on a novel cytokine–neuropeptide interaction in eosinophilic inflammation.     

Stem cell factor induces eosinophil activation and degranulation: mediator release and gene array analysis

Eosinophils are effector cells that play an important role in the damage induced by the allergic process by releasing inflammatory mediators and proteolytic factors after activation. Stem cell factor (SCF) is a primary cytokine involved in hematopoiesis and mast cell differentiation, proliferation, and activation. Studies have also indicated that SCF is directly involved in pathogenesis of allergic airway inflammation. In the present study, we examined the ability of SCF to activate murine eosinophils for increased mediator release and up-regulation of chemokines. Initial data demonstrated that eosinophils have significant levels of surface c-kit protein, SCF receptor. SCF-activated eosinophils degranulate and release eosinophil peroxidase and leukotriene C(4) in a dose-dependent manner. In addition, SCF was further shown to induce the release of CC chemokines, RANTES, macrophage-derived chemokine (MDC), macrophage inflammatory protein-1beta (MIP-1beta), and C10 from eosinophils. To identify the extent of SCF-induced activation of eosinophils, we also performed gene array analysis using an array containing 1153 genes related to inflammation, including cytokines and their receptors, growth factors, structural and cytoskeletal genes, signal transduction genes as well as several other classes related to immune/inflammatory responses. The gene analysis indicated that more than 150 genes were significantly up-regulated in eosinophils after SCF stimulation. The gene array results were verified using a quantitative real-time polymerase chain reaction analysis to identify the expression of several chemokine and chemokine receptor genes. Altogether, these studies indicate that SCF is a potent eosinophil degranulator and activator that may play a number of roles during an inflammatory/immune response.     

Functional Fas expression in human thymic epithelial cells

Fas, a cell surface receptor, can induce apoptosis after cross-linking with its ligand. We report that Fas antigen is constitutively expressed in medullary epithelial cells of the human thymus. Expression is decreased in cultured thymic epithelial cells (TEC), similarly to HLA-DR antigen. TEC are resistant to anti-Fas-induced apoptosis after 4 days of primary culture, and this resistance is reversed by concomitant addition of cycloheximide. Cycloheximide also downregulated the expression of Fas-associated phosphatase-1, which has been found to inhibit Fas-induced apoptosis. This phosphatase could be involved in the resistance to Fas-induced apoptosis observed on day 4 of TEC culture. When TEC were subcultured after 10 to 13 days of primary culture, exposure to interleukin-1-beta, tumor necrosis factor-alpha, and interferon-gamma, alone or together, reinduced Fas mRNA and protein expression. In coculture with activated thymocytes, TEC also upregulated Fas protein expression. Cytokine-activated TEC became sensitive to apoptosis induced by an agonistic anti-Fas antibody. This apoptosis was inhibited by Z-VAD-fmk but not by Z-DEVD-fmk and DEVDase activity was slightly increased in Fas-stimulated TEC, suggesting that DEVDase activity is not sufficient to induce TEC apoptosis. Taken together, these data show that the Fas receptor is expressed in medullary epithelial cells of the human thymus and is able to induce apoptosis.     

Fas抗体介导的嗜酸细胞凋亡及其清除机制

目的探讨Fas单抗对嗜酸细胞(EOS)凋亡的诱导作用及凋亡EOS的清除机制。方法分离正常人外周血EOS,加入白细胞介素(IL)5与不同浓度(1～1000ng/ml)Fas单抗培养,台盼蓝拒染法和末端标记法分别检测细胞存活和凋亡变化,观察凋亡EOS能否被巨噬细胞(MΦ)吞噬。结果Fas单抗对IL-5介导的EOS存活呈浓度和时间依赖性抑制作用。高浓度的IL-5(106U/L)不能抑制Fas单抗的作用。Fas单抗(100ng/ml)处理24h后EOS即有明显的凋亡［（35±6）%］,72h后增至(96±3)%,前后比较差异有显著性（P<0.01）。Fas单抗(100ng/ml)处理24、48和72h的EOS与MΦ培养,吞噬凋亡EOS的阳性MΦ百分率分别为(20±5)%、(38±6)%和(45±6)%,MΦ吞噬新分离的EOS阳性百分率为（1±2）%,与前者比较差异均有显著性（P<0.01）。结论Fas抗体能有效诱导EOS凋亡,凋亡EOS能被MΦ所吞噬清除。     

Increased adhesive properties of eosinophils in sickle cell disease

OBJECTIVE: A role for leukocytes in vasoocclusion is becoming increasingly recognized. Here we investigate a possible role for the eosinophil in sickle cell anemia (SCA). PATIENTS AND METHODS: Eosinophils were isolated from whole blood samples of 59 steady-state SCA homozygous SS and control individuals using Percoll gradient separation, followed by immunomagnetic sorting. Adhesion of cells to FN was evaluated using static adhesion assays (60 min, 37 degrees C, 5% CO2) and eosinophil adhesion molecular expression was observed by flow cytometry. RESULTS: SCA patients presented significantly elevated absolute eosinophil numbers. Furthermore, eosinophils isolated from these individuals demonstrated a significantly greater adhesion ( approximately 70% increased) to fibronectin (FN) than normal eosinophils in static adhesion assays. Coincubation of eosinophils with integrin-blocking monoclonal antibodies in adhesion assays showed that an association of the VLA-4, LFA-1, and Mac-1 integrins mediate the adhesion of SCA eosinophils to FN. Flow cytometry demonstrated that the expression of these integrins, however, was unaltered on the surface of SCA eosinophils, suggesting that the increased SCA eosinophil adhesion is a consequence of increased integrin affinity or avidity. SCA eosinophil adhesion to FN was further increased by the cytokine, GM-CSF, indicating that inflammation processes may further stimulate eosinophil adhesion in these patients. CONCLUSION: We report that eosinophil numbers may be significantly increased in SCA individuals and that these cells appear to exist in an activated state. Such alterations may indicate a role for the eosinophil in the vasooclusive process.     

Surface and intracellular Fas expression associated with cytokine-induced apoptosis in rodent islet and insulinoma cells

During the process of insulitis in the pathogenesis of type I (insulin-dependent) diabetes mellitus, proinflammatory cytokines induce expression of the death receptor Fas on the surface of pancreatic beta-cells and thereby contribute to the enhanced susceptibility of beta-cells for apoptosis. The aim of this study was to compare cell-surface and intracellular Fas expression associated with cytokine-induced apoptosis in commonly used beta-cell models such as isolated islets and insulinoma lines derived from mouse and rat. The cell line NIT-1 responded to the interleukin (IL)-1beta+interferon (IFN)-gamma stimulus with translocation of Fas to the cell surface. Likewise, islet cells from non-obese diabetic (NOD) mice and BB/OK rats expressed increasing amounts of the Fas receptor on their surfaces after exposure to IL-1beta in combination with IFN-gamma and tumour necrosis factor-alpha. Moreover, islets obtained from BB/OK rats at an age near the onset of diabetes had an increased surface expression of Fas compared with young rats. In contrast, western blot analysis of cell lysates from cytokine-exposed islets and insulinoma cells revealed total Fas expression levels comparable to those of untreated controls. In conclusion, islets from BB/OK rats and NOD mice, in addition to NIT-1 insulinoma cells, responded to cytokine exposure with surface expression of the Fas receptor, whereas in cell lysates the levels of expression of Fas were found to be independent of cytokine exposure. Taken together, the findings indicate that cytokine-treated beta-cells might possess two pools of Fas protein, one of which is inducible by cytokines and accounts for surface Fas expression, whereas the other is constitutively expressed in cytoplasmic compartments. The underlying mechanisms, including possible interactions between these two sources of cellular Fas expression, need to be investigated in future studies.     

Monoclonal anti-interleukin-5 treatment suppresses eosinophil but not T-cell functions.

Influx of eosinophils in airway mucosa and airway lumen is a hallmark of bronchial asthma. In-vitro data and animal studies indicate that the T-helper (Th) type-2 cell cytokine, interleukin (IL)-5, plays an important role in eosinophil maturation, differentiation, recruitment, and survival. The objective of this study was to determine whether intravenous treatment with monoclonal anti-IL-5 would affect the number of peripheral blood eosinophils, their activation status, T-cell activation or the pattern of Th1 and Th2 cytokine production. Over a period of 6 months, 19 asthmatics were investigated in a double-blind, placebo-controlled, multicentre study with mepolizumab (SB 240563) anti-IL-5 antibody administered three times. Before each infusion and 12 weeks after the last infusion, peripheral blood leukocytes were examined, qualitative and quantitative distribution of eosinophils and lymphocyte subpopulations, frequencies of IL-2, -3, -4, -5, -10, -13, interferon-gamma-producing CD4 T-cells and serum eosinophil cationic protein (ECP) levels were determined. Treatment with mepolizumab resulted in a marked, rapid and sustained decrease of eosinophil numbers (median values from 300 to 45 per microL) paralleled by decreased levels of serum ECP (median values from 15 to 5 microg x L(-1)). Distribution of T-cell subsets and T-cell cytokine production were not altered during antibody treatment. In conclusion, administration of mepolizumab to asthmatic patients markedly reduces peripheral blood eosinophils without altering the distribution and activation status of lymphocytes.     

Churg-Strauss syndrome: update on recent developments.

Churg-Strauss syndrome (CSS) is a form of primary vasculitis characterized by allergy and angiitis. In the organ systems involved (lung, heart, peripheral nervous system, and so forth), eosinophilic infiltration can be found. Eosinophilia and normochromic normocytic anemia are leading laboratory findings together with elevated IgE. New seromarkers for the activation of endothelial cells, lymphocytes, and eosinophils (soluble thrombomodulin, soluble interleukin-2 receptor, eosinophil cationic protein) may be able to predict a relapse. Antineutrophil cytoplasmic antibodies are found in only approximately 50% of all patients with CSS, and their diagnostic value is questionable. Etiologically, hyperresponsiveness to an antigenic stimulus seems to underlie the syndrome. In asthmatics, cysteinyl leukotriene receptor type 1 antagonists are reported to trigger the disease. Cytokine profile findings on the cells involved in CSS remain contradictory. Some think CSS may be a Th2-mediated disease; its pathophysiology is not known fully. Interleukin-5 and tumor necrosis factor-alpha are elevated in serum and fluid of bronchoalveolar lavage, suggesting target cytokines for future treatment protocols. Treatment consists of glucocorticoid monotherapy. Data on outcome and effectiveness is lacking for other immunosuppressive regimens, such as cyclophosphamide or glucocorticoid plus cyclophosphamide. Treatment with interferon-alpha has been effective in patients refractory to glucocorticoid plus cyclophosphamide.     

Histamine reverses IL-5-afforded human eosinophil survival by inducing apoptosis: pharmacological evidence for a novel mechanism of action of histamine

Eosinophils are essential inflammatory cells in the pathogenesis of asthma and atopic conditions. Histamine, released from mast cells and basophils in response to allergen exposure, is a critical mediator in the allergic response. Histamine exerts its effects via four unequivocally characterized histamine receptors, H(1-4). Several functions of eosinophils have previously been shown to be stimulated by histamine. However, its effects on eosinophil apoptosis are unknown. The aim of the present study was to resolve the effects of histamine on constitutive apoptosis of human eosinophils and on the survival-enhancing action of interleukin (IL)-5. Additional experiments were conducted to elucidate the histamine receptor(s) involved in any response seen and the associated signal transduction cascade. Human isolated peripheral blood eosinophils were cultured in the absence or presence of histamine, IL-5 and receptor antagonists/agonists or mediator inhibitors/analogues. Apoptosis was assessed by measuring the relative DNA content of propidium iodide (PI)-stained cells and the effects were confirmed by morphological analysis with bright field microscopy. Caspase activities were assessed by using commercial Caspase-Glo 3/7, 8 and 9 luminescence assays. Histamine (10-100 microM) partially reversed IL-5-induced human eosinophil survival by enhancing apoptosis as assessed by measuring the relative DNA content of PI-stained cells. This effect was not mediated through any of the known histamine receptors or through non-specific activation of 5-hydroxytryptamine receptors or alpha-adrenoceptors. Moreover, the reversal of IL-5-inhibited eosinophil apoptosis by histamine seemed not to utilize the conventional intracellular second-messenger pathways including cyclic AMP, protein kinase A or phospholipase C. Inhibition of caspase 6 and caspases 1, 10 or 12 reversed the effects of histamine but also inhibited apoptosis in general. In conclusion, the data presented herein indicate that histamine induces human eosinophil apoptosis in the presence of a survival-prolonging cytokine by a mechanism that does not apparently involve the activation of any of the currently known histamine receptor subtypes. The possibility exists that another, as yet unidentified, histamine receptor may exist in human eosinophils that regulates survival, although the participation of histamine receptor-independent mechanisms cannot be excluded.