Targeting airway inflammation: PMX464 and the epithelial bulls eye.

Increasing evidence places the epithelial cell at the centre of inflammatory processes in human airways. Crucial to this function and the maintenance of inflammatory homoeostasis is a balanced oxidant-antioxidant status in the airway, in part controlled by thioredoxin and thioredoxin reductase, which together can alter the NF-kappaB pathway. PMX464, a thiol-reactive quinol and putative thioredoxin inhibitor, has been investigated in endothelial cells, fibroblasts and colorectal cancer cell lines but in the present issue of the BJP, these investigations were extended to A549 airway epithelial cells. Thioredoxin inhibition was confirmed as was NF-kappaB and IKK suppression but siRNA knockdown of thioredoxin did not alter inflammatory marker expression or activity, suggesting that PMX464 has targets other than thioredoxin. Future consolidation of this evidence will involve concomitant knockdown of thioredoxin reductase, the use of primary airway epithelial cells and, potentially, the employment of three-dimensional (3D) culture systems for both A549 and primary cells.

Control of eosinophil toxicity in the lung.

The inappropriate accumulation of eosinophils and the subsequent release of their potent pro-inflammatory mediator arsenal are thought to be important contributors to the pathogenesis of asthma and other allergic diseases. It is also becoming apparent that eosinophils may play a role in the orchestration of immune responses in the asthmatic lung. There is therefore much interest in the development of strategies to limit or prevent eosinophil-induced toxicity. The mechanisms by which eosinophils accumulate in the peribronchial tissues of the lung are complex and include enhanced differentiation and release from the bone marrow, selective adhesion and transendothelial migration, directed movement in response to specific chemotactic mediators and finally prolonged survival as a consequence of delayed apoptosis. Thus it can be appreciated that there are many points at which the toxicity of eosinophils can be limited or even prevented. Important areas for potential advances in glucocorticoid (GC) development include efforts to dissociate their anti-inflammatory effects from unwanted side effects. Other areas include the development of humanized monoclonal antibodies against IL-4, IL-13 and IL-5 together with the inhibition of adhesion pathways and/or chemokines responsible for eosinophil accumulation in the asthmatic lung. Several avenues of research are currently underway in an attempt to define mechanisms by which pro-inflammatory cells such as eosinophils can be safely removed from the asthmatic lung through apoptosis induction and their subsequent ingestion by phagocytes. This review will discuss both the potential and shortcomings of these diverse approaches to limit eosinophil toxicity in the asthmatic lung.

Phagocytosis of apoptotic eosinophils but not neutrophils by bronchial epithelial cells.

BACKGROUND: We have previously demonstrated that human bronchial epithelial cells engulf apoptotic eosinophils. OBJECTIVES: To compare and contrast the phagocytic capabilities of monocyte-derived macrophage and primary airway epithelial cells for apoptotic granulocytes. RESULTS: Here we compared phagocytosis of human apoptotic eosinophils and neutrophils by small and large airway epithelial cells (SAEC and LAEC) and monocyte-derived macrophages. Confocal microscopy of F-actin staining and scanning and transmission electron microscopy revealed phagocytic cup formation around apoptotic eosinophils by airway epithelial cells (AEC) membranes with evidence of their digestion. Resting and cytokine-stimulated AEC did not recognize and ingest apoptotic neutrophils. The latter were phagocytosed by macrophages that exhibited greater ingestion of and higher capacity for, apoptotic eosinophils over apoptotic neutrophils. Cytochalasin D completely abolished uptake of apoptotic eosinophils by SAEC, LAEC or macrophage monolayers. Ligation of epithelial cell CD44 receptors for 24 h increased phagocytosis of apoptotic eosinophils by SAEC and LAEC with a potency comparable with that of IL-1. Phagocytosis was a specific receptor-mediated process involving integrin- (alphavbeta3, alphavbeta5, CD36), phosphatidylserine receptor- and lectin-dependent mechanisms. No significant differences were observed in avarice for apoptotic eosinophils by SAEC or LAEC either resting, CD44 monoclonal antibodies- or cytokine- stimulated, or in their usage and expression of recognition receptors. CONCLUSION: These findings further suggest and define an important role for the bronchial epithelium in the selective removal of apoptotic eosinophils from the airways in asthma.

Corticosteroids, eosinophils and bronchial epithelial cells: new insights into the resolution of inflammation in asthma.

Anti-inflammatory therapy in asthma is reliant on corticosteroids, particularly in their inhaled form. However, steroids are rather non-specific in their actions and they also raise concerns regarding compliance and side-effect Issues. Furthermore, a small proportion of patients with asthma fail to respond to oral glucocorticoids even at high doses. This Article will review the role that steroids and membrane receptor ligation play in the induction of eosinophil apoptosis together with the mechanisms by which corticosteroids enhance the disposal of apoptotic eosinophils by both professional and non-professional phagocytes. Eosinophils are thought to be the major pro-inflammatory effector cell in asthma and their persistence in the airways is probably enhanced by the presence of several asthma-relevant cytokines that prolong eosinophil survival by inhibition of apoptosis (interleukin (IL)-3, IL-5, granulocyte-macrophage colony-stimulating factor, IL-9, IL-13, IL-15). In contrast, a number of signals have been described that accelerate apoptosis in human eosinophils including corticosteroids or ligation of membrane receptors (CD95, CD45, CD69). Thus, the load of lung eosinophils in asthmatic disease is likely to be related to a balance in the tIssue microenvironment between pro- and anti-apoptotic signals. Furthermore, removal of apoptotic eosinophils by phagocytosis by alveolar macrophages or bronchial epithelial cells in a specific receptor-mediated way is as important as the process of apoptosis induction. Corticosteroids enhance the recognition and engulfment of apoptotic eosinophils by macrophages or bronchial epithelial cells. Caspases are key intracellular molecules in the control of apoptosis and defects in caspase-induced apoptosis in eosinophils from steroid-resistant individuals may contribute to the molecular mechanisms underlying glucocorticoid insensitivity in these cells. These findings point the way to new and more targeted anti-inflammatory therapy for asthma and may provide important clues for the development of alternative therapies for glucocorticoid resistance.

Granule protein changes and membrane receptor phenotype in maturing human eosinophils cultured from CD34+ progenitors.

BACKGROUND: Eosinophils are now recognized as major effector cells in allergic and asthmatic disease with a potent armoury of mediators whose release makes a major contribution to the inflammation underlying these conditions. OBJECTIVES: The purpose of this study was to compare cultured eosinophils (CE) with normal-density peripheral blood eosinophils (PBE) in terms of their membrane receptor expression and to analyse the expression and storage of the eosinophil granule proteins major basic protein (MBP) and eosinophil cationic protein (ECP) during eosinophil maturation in vitro. METHODS: Purified human peripheral blood CD34+ cells were cultured in the presence of recombinant human IL-3, IL-5, rhGM-CSF, SCF, and FLT-3 ligand. PBE were isolated by density gradient centrifugation and negative immunomagnetic selection. Expression of CD11b, CD18, CD45, CD45RA, CD45RB, CD45RO, CD69, CD95, IL-5Ralpha, IL-9Ralpha, CCR1, CCR3, and CXCR4 by CE as they matured in culture were assessed by immunostaining and flow cytometry and expression of these receptors compared with freshly isolated PBE. Immunohistochemical staining and labophot-2TM light microscopy determined expression of MBP, ECP, and CD69 during eosinophil maturation. RESULTS: Positive immunostaining for MBP and ECP was detectable in a proportion (15-20%) of CE as early as 3 days of culture even though these cells were mononuclear in appearance. The numbers of CE positive for both granule proteins increased in rhIL-3 and rhIL-5 treated cells to a maximum of approximately 80% by day 28. Maturing eosinophils exhibited positive immunostaining for CD69 after 14, 21 and 28 days of culture. Compared with PBE, CE had lower expression of pan-CD45 and CD45 isoforms, CD95 and CD11b. In contrast, the specific mean fluorescence for CD69, CD18, IL-5Ralpha, and IL-9Ralpha was significantly elevated for CE compared with PBE. CCR3 expression by CE and PBE was similar with no expression of CXCR4 detected by either CE or PBE. No significant difference in expression of CCR1 was found between CE and PBE. CONCLUSION: These data suggest that CE and PBE share many phenotypic properties and both MBP and ECP appear early in eosinophil development in vitro. However, there are quantitative differences that may be a consequence of their immaturity and/or the influence of the cytokines used in their culture.

Cetirizine and levocetirizine inhibit eotaxin-induced eosinophil transendothelial migration through human dermal or lung microvascular endothelial cells.

BACKGROUND: Several second-generation antihistamines have documented anti-inflammatory effects which appear independent of H1-receptor blockade. We investigated the inhibitory effect of cetirizine and its active enantiomer levocetirizine on eosinophil transendothelial migration (TEM) through monolayers of normal human dermal microvascular endothelial cells (HMVEC-d) or human lung microvascular endothelial cells (HMVEC-l). METHODS: HMVEC-d or HMVEC-l were grown to confluence on micropore filters in transwells inserted into a 24-well tissue culture dish. Eosinophils were isolated by density gradient centrifugation and negative immunomagnetic selection. Untreated eosinophils or eosinophils pre-incubated (30 min at 37 degrees C) with a concentration range of cetirizine or levocetirizine (10-5 to 10-9 m) were added to the upper chamber of the transwell which was incubated for 60 min at 37 degrees C. Both spontaneous eosinophil TEM and TEM to 100 ng/mL of human eotaxin in the lower chamber were assessed. RESULTS: Between 8 and 10% of the eosinophils added to the upper chamber underwent spontaneous TEM through HMVEC-d or HMVEC-l. The addition of eotaxin to the lower chamber enhanced eosinophil TEM through HMVEC-d or HMVEC-l monolayers to over 20%, i.e. an enhanced TEM of approximately 100% in each case. Pre-incubation of eosinophils with cetirizine or levocetirizine dose-dependently inhibited eosinophil TEM to eotaxin through both HMVEC-d or HMVEC-l with total inhibition of eotaxin-induced TEM observed at 10-8 m for HMVEC-d and 10-7 m for HMVEC-l. Both drugs gave a reduced but significant inhibition of eosinophil TEM at lower concentrations. No concentration of cetirizine or levocetirizine had any significant effect on expression of CD11b, CD18 or CD49d by either resting or eotaxin-stimulated eosinophils. Furthermore, no effect on spontaneous eosinophil TEM, or eosinophil viability was seen with any concentration of cetirizine or levocetirizine. CONCLUSION: Levocetirizine inhibits eotaxin-induced eosinophil TEM through both dermal and lung microvascular endothelial cells suggesting that, like cetirizine, levocetirizine has potential anti-inflammatory effects.

Human alveolar epithelial cells engulf apoptotic eosinophils by means of integrin- and phosphatidylserine receptor-dependent mechanisms: a process upregulated by dexamethasone.

BACKGROUND: We previously demonstrated receptor-mediated apoptotic-eosinophil engulfment by human small airway epithelial cells, which may represent a potentially important mechanism in the resolution of allergic and asthmatic inflammation. OBJECTIVE: A549 cells were selected as being representative of alveolar epithelial cells, and their ability to ingest human apoptotic eosinophils was examined in terms of the effects of dexamethasone treatment and the receptor-mediated recognition mechanisms important in this process. METHODS: A549 epithelial-cell expression of alpha(v)beta3, alpha(v)beta5, CD36, and the phosphatidylserine receptor was established by using immunostaining and flow cytometry, and inhibition assays were examined by using the role of these receptors in apoptotic-eosinophil recognition by resting and dexamethasone-treated A549 epithelial cells. Electron microscopy confirmed engulfment of apoptotic eosinophils, and receptor clustering around apoptotic eosinophils was examined by using immunofluorescent labeling. RESULTS: A549 epithelial cells recognized and engulfed apoptotic eosinophils but not freshly isolated cells. Dexamethasone enhanced the number of A549 cells ingesting apoptotic eosinophils and dose dependently increased their capacity for ingestion. The tetrapeptide RGDS significantly inhibited apoptotic-eosinophil uptake by A549 cells, indicating a role for integrins in the recognition process. A549 cells expressed alpha(v)beta3, alpha(v)beta5, beta5, CD36, and the phosphatidylserine receptor, and expression of these receptors was not significantly increased after dexamethasone treatment. Engulfment of apoptotic eosinophils by A549 cells involved alpha(v)beta3-, CD36-, alpha(v)beta5-, and phosphatidylserine receptor-mediated recognition mechanisms and was associated with the formation of integrin focal adhesion complexes around apoptotic eosinophils. CONCLUSIONS: These data further suggest a nonpassive role for airway epithelium in the resolution of eosinophilic inflammation in asthma.

Ligation of CD45 and the isoforms CD45RA and CD45RB accelerates the rate of constitutive apoptosis in human eosinophils.

BACKGROUND: Eosinophils are important effector cells in asthma pathogenesis, and an understanding of the mechanisms involved in eosinophil apoptosis induction might thus be relevant to the resolution of asthmatic inflammation. OBJECTIVE: Our aim was to determine the role of the common leukocyte antigen CD45 and the isoforms CD45RA, CD45RB, and CD45RO in human eosinophil apoptosis induction. METHODS: Immmunostaining and flow cytometry were used to assess CD45 and CD45 isoform expression by eosinophils purified with use of density gradients and immunomagnetic negative selection. Apoptosis induction was measured by binding of fluorescein isothiocyanate-labeled annexin V to eosinophils cultured for 20 hours alone or with saturating quantities of mAb against CD45, CD45RA, CD45RB, CD45RO, CD9, CD11b, and isotype-matched controls in the presence or absence of GM-CSF. RESULTS: Freshly isolated eosinophils had high expression of CD45 and CD45RO, modest expression of CD45RB, and low expression of CD45RA. Eosinophils cultured alone for 20 hours were found to be approximately 20% to 25% apoptotic. Incubation with mAb against CD45, CD45RA, and CD45RB resulted in significant (P <.005) enhancement (>100%) of their constitutive rate of apoptosis. Incubation with CD45RO, CD11b, CD9 mAb, or isotype controls had no significant effect on the rate of eosinophil constitutive apoptosis. The addition of GM-CSF inhibited eosinophil apoptosis but did not prevent CD45, CD45RA, or CD45RB mAb-dependent apoptosis induction. CONCLUSION: These data indicate that ligation of CD45, CD45RA, or CD45RB represents a novel pathway for the induction of apoptosis in human eosinophils.

Resting and cytokine-stimulated human small airway epithelial cells recognize and engulf apoptotic eosinophils.

Eosinophils, which are prominent cells in asthmatic inflammation, undergo apoptosis and are recognized and engulfed by phagocytic macrophages in vitro. We have examined the ability of human small airway epithelial cells (SAEC) to recognize and ingest apoptotic human eosinophils. Cultured SAEC ingested apoptotic eosinophils but not freshly isolated eosinophils or opsonized erythrocytes. The ability of SAEC to ingest apoptotic eosinophils was enhanced by interleukin-1alpha (IL-1alpha) or tumor necrosis factor alpha (TNFalpha) in a time- and concentration-dependent fashion. IL-1alpha was found to be more potent than TNFalpha and each was optimal at 10(-10) mol/L, with a significant (P <.05) effect observed at 1 hour postcytokine incubation that was maximal at 5 hours. IL-1alpha stimulation not only increased the number of SAEC engulfing apoptotic eosinophils, but also enhanced their capacity for ingestion. The amino sugars glucosamine, n-acetyl glucosamine, and galactosamine significantly inhibited uptake of apoptotic eosinophils by both resting and IL-1alpha-stimulated SAEC, in contrast to the parent sugars glucose, galactose, mannose, and fucose. Incubation of apoptotic eosinophils with the tetrapeptide RGDS, but not RGES, significantly inhibited their uptake by both resting and IL-1alpha-stimulated SAEC, as did monoclonal antibody against alphavbeta3 and CD36. Thus, SAEC recognize apoptotic eosinophils via lectin- and integrin-dependent mechanisms. These data demonstrate a novel function for human bronchial epithelial cells that might represent an important mechanism in the resolution of eosinophil-induced asthmatic inflammation.