Neutrophil transepithelial migration is dependent upon epithelial characteristics

An in vitro transepithelial migration assay to evaluate the role of neutrophils in Respiratory Syncytial Virus (RSV) induced epithelial damage

Large numbers of neutrophils migrate into the lungs of children with severe Respiratory Syncytial Virus (RSV) disease. It is unclear how these cells contribute to viral clearance and recovery from infection or whether they contribute to disease pathology. We have developed a novel in vitro model to study neutrophil migration through airway epithelial cells (AECs), the main cellular target of RSV infection. Our model reproduces a physiologically relevant cell polarity and directionality of neutrophil migration. Using this model, we found that RSV infected AECs induced rapid neutrophil transepithelial migration. We also detected increased AEC damage associated with RSV infection, with a further increase in epithelial cells shedding from the Transwell membrane following neutrophil migration. This was not observed in the mock infected controls. Neutrophils that migrated through the RSV infected AECs showed increased cell surface expression of CD11B and MPO compared to neutrophils that had not migrated. In conclusion, our in vitro co-culture assay can be used to identify critical mechanisms that mediate epithelial cell damage and promote inflammation in children with severe RSV disease.

Neutrophil-Epithelial Interactions: A Double-Edged Sword

In recent years, it has become clear that innate immune cells termed neutrophils act as double-edged swords by playing essential roles in clearing infection but also causing tissue damage, yet being critical for wound healing. Neutrophil recruitment to sites of injured tissue or infection has been well studied, and many of the molecular events that regulate passage of leukocytes out of the microcirculation are now understood. However, after exiting the circulation, the molecular details that regulate neutrophil passage to end targets, such mucosal surfaces, are just beginning to be appreciated. Given that migration of neutrophils across mucosal epithelia is associated with disease symptoms and disruption of critical barrier function in disorders such as inflammatory bowel disease, there has been long-standing interest in understanding the molecular basis and functional consequences of neutrophil-epithelial interactions. It is a great honor that my work was recognized by the Rous-Whipple Award this past year, giving me the opportunity to summarize what we have learned during the past few decades about leukocyte interactions with epithelial cells.

Assessment of cell line models of primary human cells by Raman spectral phenotyping

Researchers have previously questioned the suitability of cell lines as models for primary cells. In this study, we used Raman microspectroscopy to characterize live A549 cells from a unique molecular biochemical perspective to shed light on their suitability as a model for primary human pulmonary alveolar type II (ATII) cells. We also investigated a recently developed transduced type I (TT1) cell line as a model for alveolar type I (ATI) cells. Single-cell Raman spectra provide unique biomolecular fingerprints that can be used to characterize cellular phenotypes. A multivariate statistical analysis of Raman spectra indicated that the spectra of A549 and TT1 cells are characterized by significantly lower phospholipid content compared to ATII and ATI spectra because their cytoplasm contains fewer surfactant lamellar bodies. Furthermore, we found that A549 spectra are statistically more similar to ATI spectra than to ATII spectra. The spectral variation permitted phenotypic classification of cells based on Raman spectral signatures with >99% accuracy. These results suggest that A549 cells are not a good model for ATII cells, but TT1 cells do provide a reasonable model for ATI cells. The findings have far-reaching implications for the assessment of cell lines as suitable primary cellular models in live cultures.

Modulation of endotoxin-induced neutrophil transendothelial migration by alveolar epithelium in a defined bilayer model

Within the alveolus, epithelial cells, due to their close association with endothelial cells, can potentially influence endothelial cell responsiveness during inflammation and their interaction with leukocytes. To investigate this, three lung epithelial cell lines (A549, Calu-3, or NCI-H441) were grown with endothelium on opposing surfaces of Transwell filters and the formation and stability of bilayers was rigorously evaluated. All epithelial lines disrupted endothelial monolayer formation on filters with 3- or 5-microm pores by breaching the filter, and this occurred regardless of seeding density, matrix composition, or duration of culture. Endothelial disruption was not detectable by electrical resistance or permeability measurements but required cell-specific staining with immunofluorescence and microscopy. Distinct bilayers formed only on filters with 0.4-microm pores and only with A549 cells and human umbilical vein endothelial cells. Endotoxin (lipopolysaccharide [LPS]) stimulation of bilayers (4 hours) enhanced neutrophil transendothelial migration, but this was significantly decreased compared with the response of endothelium grown alone, irrespective of whether LPS exposure was via the epithelial or endothelial side of the bilayer. Down-modulation required epithelial-endothelial approximation and was not seen when these cells were separated by 0.5 to 1 mm. This study defines optimal conditions required for generation of intact bilayers of lung epithelial cells with endothelium for the study of leukocyte-transendothelial migration. Furthermore, it was demonstrated that lung epithelial cells can modulate endothelial cell responsiveness to an environmental inflammatory stimulus such as LPS and thus may have an important role in minimizing excessive and deleterious neutrophilic inflammation in the lung alveolus.

Neutrophil elastase induces IL-8 synthesis by lung epithelial cells via the mitogen-activated protein kinase pathway

The sequestration of neutrophils in the lung and the release of proinflammatory mediators, including neutrophil elastase, are responsible for sepsis-induced microvascular permeability and alveolar epithelial cell damage. To assess the underlying mechanism, human neutrophil elastase (0.01-0.5 microg/ml) was added to cultured A549 epithelial cells in the presence or absence of inhibitors. IL-8 was analyzed by ELISA or by RT-PCR to measure the IL-8 synthesis capacity. Mitogen-activated protein kinase (MAPK) activity was detected by Western blot analysis. Neutrophil elastase dose-dependently increased IL-8 release from cultured A549 epithelial cells. Pretreatment with a specific elastase inhibitor, elastase inhibitor II (at 0.5, 5, and 50 microg/ml), dose-dependently inhibited neutrophil elastase-induced IL-8 release. The activities of MAPK, p38, and extracellular signal-regulated kinase (ERK) were upregulated by neutrophil elastase. Nuclear transcriptional factor-kappa B (NF-kappaB) and activator protein 1 (AP-1) were also activated. These responses were significantly inhibited by elastase inhibitor II. A specific inhibitor of p38 MAPK (SB203580) and an NF-kappaB inhibitor (pyrrolidine dithiocarbamate), but not an ERK inhibitor (PD 98059), significantly inhibited neutrophil elastase-induced IL-8 release and mRNA expression. The specific tyrosine kinase inhibitor, genistein, and the protein kinase C (PKC) inhibitor, Ro 31-8220, also inhibited IL-8 release and mRNA expression as well as p38 and NF-kappaB activation. There was no significant effect by the protein kinase A inhibitor, H-89, on neutrophil elastase-induced IL-8 synthesis or p38 MAPK activation. Our results indicate that neutrophil elastase activates p38 MAPK which upregulates NF-kappaB and AP-1 activities, thus inducing IL-8 mRNA expression and protein synthesis. Tyrosine kinase and PKC are implicated in neutrophil elastase activation of the MAPK pathway.

Rapid and efficient clearance of airway tissue granulocytes through transepithelial migration

BACKGROUND

Clearance of tissue granulocytes is central to the resolution of airway inflammation. To date the focus has been on apoptotic mechanisms of cell removal and little attention has been given to alternative processes. The present study explores transepithelial migration as a mechanism of cell clearance.

METHOD

Guinea pig tracheobronchial airways where eosinophils are constitutively present in the mucosal tissue were studied. A complex topical stimulus (allergen challenge) was applied and the fate of the eosinophils was determined by selective tracheobronchial lavage and histological examination of the tissue.

RESULTS

Within 10 minutes of the allergen challenge, massive migration of eosinophils into the airway lumen occurred together with a reduction in tissue eosinophil numbers. Cell clearance into the lumen continued at high speed and by 30 and 60 minutes the tissue eosinophilia had been reduced by 63% and 73%, respectively. The marked transepithelial migration (estimated maximal speed 35,000 cells/min x cm2 mucosal surface) took place ubiquitously between epithelial cells without affecting epithelial integrity as assessed by transmission and scanning electron microscopy. Eosinophil apoptosis was not detected but occasional cytolytic eosinophils occurred.

CONCLUSION

This study shows that luminal entry has a remarkably high capacity as a granulocyte elimination process. The data also suggest that an appropriate stimulus of transepithelial migration may be used therapeutically to increase the resolution of inflammatory conditions of airway tissues.

Effects of rewarming on nuclear factor-kappaB and interleukin 8 expression in cold-preserved alveolar epithelial cells

BACKGROUND

Nuclear factor-kappaB (NF-kappaB) and interleukin (IL)-8 play important roles in the pathophysiology of acute lung injury after lung transplantation. Because alveolar epithelium is one of the most important sites at which IL-8 production takes place after reperfusion of donor lungs, we examined the effects of cold/rewarming on NF-kappaB and IL-8 expression in alveolar epithelial cells.

METHODS

A549 cells were preserved at 4 degrees C for 5 hr and then rewarmed for up to 20 hr. NF-kappaB was analyzed by electrophoretic mobility shift assay. IL-8 mRNA expression was examined by reverse transcription-polymerase chain reaction. IL-8 concentration in the cell culture medium after rewarming was measured by enzyme-linked immunosorbent assay.

RESULTS

NF-kappaB was increased in the nuclear extracts as early as 30 min after rewarming. There was a marked increase in the IL-8 mRNA expression at 1 and 3 hr after rewarming. IL-8 concentration in the cell culture medium was progressively increased during 20 hr following rewarming. The cell culture medium inhibited apoptosis of neutrophils significantly. The cold/rewarming-induced IL-8 production was reduced to approximately 50% by introducing an antisense oligonucleotide for the p65 subunit of NF-kappaB and by treatment with N-acetyl-leucinyl-leucinyl-norleucinal and pyrrolidine dithiocarbamate. The effect of dexamethasone treatment was dose dependent (reduced to approximately 30% at 10-5 M dexamethasone).

CONCLUSIONS

Our results indicate that rewarming of cold-preserved alveolar epithelial cells itself may be an important initiator of the inflammatory cascades, including NF-kappaB activation and IL-8 release. Inhibition of NF-kappaB would be worth trying to control unnecessary IL-8 production and the inflammatory response in the donor lungs.

Cadmium transport through type II alveolar cell monolayers: contribution of transcellular and paracellular pathways in the rat ATII and the human A549 cells

Cadmium (CD) transport in alveolar type II (ATII) cells has been studied using two in vitro models widely used to investigate lung function: primary cultures of rat ATII cells and the human cell line A549. Nonlinear regression analyses of the uptake time-course of (109)Cd revealed: a zero-time accumulation, a fast process of accumulation which proceeds within minutes, and a much slower process which takes hours. This three-step mechanism was characterized by different parameter values under dishes-or filter-growth conditions. A higher initial uptake rate (v(i)) and equilibrium accumulation (A(max)) of (109)Cd were found in the rat ATII cells; these differences were not related to a higher level of adsorption onto the external surface of the cell membrane. Specific transport systems of similar capacity but different affinity (threefold higher in rat cells) were characterized. A significant transepithelial transport of (109)Cd, with similar P(coeff) in both cell models, could not be exclusively related to cellular metal release. Results on 3H-mannitol permeability together with (109)Cd efflux data strongly suggest a greater contribution of the paracellular pathways in Cd transport through A549 cell monolayers. These differences in transport properties between the two lung cell models may modify the dose-response curve for Cd toxicity.

Neutrophil transmigration across human airway epithelial monolayers: mechanisms and dependence on electrical resistance

To examine neutrophil transepithelial migration in the basolateral-to-luminal direction, bronchial epithelial cells (16HBE) were grown at an air-medium interface on the lower face of permeable supports, and resistance across each membrane was recorded before measuring neutrophil transmigration over 2 h. Subconfluent monolayers (resistance < 250 Omega) permitted high spontaneous migration of neutrophils (7.4+/-1%), which was further enhanced (29.7+/-3%) in response to interleukin (IL)-8 (100 ng/ml). Confluent monolayers (250 to 700 Omega) showed low spontaneous migration (2+/- 0.5%) but responded markedly to IL-8 (12.4+/-1.3%). Left in culture, 16HBE resistances continued to increase and were associated with minimal spontaneous migration (< 0.5%) or responses to IL-8. Using cells in the 250 to 700 Omega range, neutrophil migration to IL-8 was dose-dependent and was enhanced when epithelial cells were incubated with a combination of tumor necrosis factor-alpha and interferon-gamma. Neutrophil migration was stimulus-specific and was reduced by preincubation of epithelial cells with a F(ab')(2) anti-intercellular adhesion molecule (ICAM)-1, or by preincubation of neutrophils with anti-CD18, anti-CD11a, anti-CD11b, or anti-CD11c, but not by anti-CD11d, indicating a role for beta(2)-integrin-ICAM-1 interaction in the migration process.

Cystic fibrosis transmembrane conductance regulator does not affect neutrophil migration across cystic fibrosis airway epithelial monolayers

Recent studies have shown that airway inflammation dominated by neutrophils, ie, polymorphonuclear cells (PMN) was observed in infants and children with cystic fibrosis (CF) even in the absence of detectable infection. To assess whether there is a CF-related anomaly of PMN migration across airway epithelial cells, we developed an in vitro model of chemotactic migration across tight and polarized CF(15) cells, a CF human nasal epithelial cell line, seeded on porous filters. To compare PMN migration across a pair of CF and control monolayers in the physiological direction, inverted CF(15) cells were infected with increasing concentrations of recombinant adenoviruses containing either the normal cystic fibrosis transmembrane conductance regulator (CFTR) cDNA, the DeltaF508 CFTR cDNA, or the beta-galactosidase gene. The number of PMN migrating in response to N-formyl-Met-Leu-Phe across inverted CF(15) monolayers expressing beta-galactosidase was similar to that seen across CF(15) monolayers rescued with CFTR, whatever the proportion of cells expressing the transgene. Moreover, PMN migration across monolayers expressing various amounts of mutated CFTR was not different from that observed across matched counterparts expressing normal CFTR. Finally, PMN migration in response to adherent or Pseudomonas aeruginosa was equivalent across CF and corrected monolayers. The possibility that mutated CFTR may exert indirect effects on PMN recruitment, via an abnormal production of the chemotactic cytokine interleukin-8, was also explored. Apical and basolateral production of interleukin-8 by polarized CF cells expressing mutated CFTR was not different from that observed with rescued cells, either in baseline or stimulated conditions. CF(15) cells displayed a CF phenotype that could be corrected by CFTR-containing adenoviruses, because two known CF defects, Cl(-) secretion and increased P. aeruginosa adherence, were normalized after infection with those viruses. Thus, we conclude that the presence of a mutated CFTR does not per se lead to an exaggerated inflammatory response of CF surface epithelial cells in the absence or presence of a bacterial infection.

The sequential migration of neutrophils through endothelium and epithelium: a new model system

To better understand the mechanisms by which neutrophils migrate into the airways, we constructed a novel in vitro model system with human umbilical vein endothelial cell (HUVE) monolayers grown on top of permeable filters and human lung Type II-like alveolar epithelial cell (A549) monolayers grown on the undersurface of the filters. The sequential migration of human neutrophils through the endothelium (apical to basal movement) and subsequently through the epithelium (basal to apical movement) in response to a stimulus located basally to the epithelium was measured. We found that the neutrophil chemoattractants, formylmethionylleucylphenylalanine (FMLP), leukotriene B4 (LTB4), and interleukin-8 (IL-8), induced dose-responsive migration through the double monolayer-filter complex. The pattern of migration was similar to that observed through either a naked filter or single monolayer-filter complex. Maximal chemotaxis through the double monolayer-filter complex was observed by 3 hours. Thus, we have established an in vitro model system to examine the sequential migration of neutrophils through endothelium and the respiratory epithelium in a manner analogous to that occurring with an in vivo airway stimulus causing neutrophil-rich airway inflammatory responses.

Characterization of the A549 cell line as a type II pulmonary epithelial cell model for drug metabolism

Multiple cell types contribute to the pulmonary barrier including Type I and Type II alveolar epithelium. The objective of this research was to establish and characterize an in vitro model of Type II alveolar epithelium using the A549 human lung adenocarcinoma cell line. A549 cells form confluent monolayers with Type II characteristic morphology and tannic acid staining for typical lamellar bodies. A549 cells possess P450 IA1 and P450 IIB6 as determined by Western blots. Both CYPIA1 and CYPIIB6 P450 isozymes were determined to be functional with the fluorescent resorufin assay. Only the IA1 isozyme was observed to be inducible with selected polycyclic hydrocarbons. Uptake and transport experiments were carried out in cluster plates and in Snapwells. Cationized ferritin, a nonspecific absorbtive marker, was found to be taken up by the cells in a concentration-, time-, and temperature-dependent fashion. Lucifer yellow, a fluid-phase marker, was not internalized by the A549 cells. Transferrin, a representative receptor-mediated endocytic marker, was found to be taken up by the cells in a concentration-dependent and competitive fashion. Transport experiments involving fluorescein-transferrin also showed that A549 monolayers were polarized, with a greater amount of intracellular transferrin being transported out of the basolateral side of the cells. The experimental data agree favorably with literature for primary cultures of Type II pulmonary epithelial cells. These results indicated that the A549 cell line may be useful for the studying the metabolic and macromolecule processing contributions of alveolar Type II cells to mechanisms of drug delivery at the pulmonary epithelium.

Endotoxin-stimulated alveolar macrophage recruitment of neutrophils and modulation with exogenous surfactant

OBJECTIVE

To determine whether endotoxin-stimulated alveolar macrophages would attract neutrophils and whether exogenous surfactant treatment would modulate this chemoattraction.

DESIGN

Alveolar macrophages were harvested from bronchoalveolar lavage fluid and neutrophils from the blood of anesthetized guinea pigs.

SUBJECTS

Hartley guinea pigs.

INTERVENTIONS

Alveolar macrophages were suspended in RPMI 1640 and stimulated with 1 microg/mL of lipopolysaccharide (LPS), the supernatant removed and the alveolar macrophages were incubated in either RPMI or RPMI with surfactant at two different doses (292 microg/mL or 875 microg/mL) for 16 hrs.

MEASUREMENTS AND MAIN RESULTS

The supernatant was extracted from the alveolar macrophages and placed in a chemotaxis plate and the migration of neutrophils was measured. Chemotaxis of all cell types to be tested was measured by a change of absorbance on a microplate reader set at 492 nm. Results were compared with alveolar macrophages not stimulated with LPS, RPMI alone, and N formyl-methionyl-leucyl-phenylalanine (FMLP). The supernatant of the stimulated alveolar macrophages increased neutrophil chemotaxis as compared with unstimulated alveolar macrophages, and RPMI (p < .05). Surfactant treatment with 292 microg/mL significantly decreased LPS-stimulated alveolar macrophages induced neutrophil chemotaxis. Treatment with 875 microg/mL of surfactant did not alter neutrophil chemotaxis.

CONCLUSIONS

Alveolar macrophages stimulation with LPS increased the chemotaxis of neutrophils. Treatment with surfactant at a concentration of 875 microg/mL did not alter neutrophil migration; however, treatment with 292 microg/mL significantly decreased neutrophil chemotaxis suggesting that at low concentrations, surfactant inhibits chemokine release and may reduce pulmonary neutrophil sequestration in vivo.

Cytokine-induced neutrophil transepithelial migration is dependent upon epithelial orientation

The mechanisms by which mediators and cytokines stimulate neutrophils to migrate across the lung epithelium are still unclear. We hypothesized that neutrophil transepithelial migration depends upon polarity of the epithelium. We therefore compared neutrophil migration through human lung Type II-like alveolar epithelial cell line (A549) monolayers grown on the upper versus lower surface of permeable filters to simulate apical-to-basal and basal-to-apical movement of neutrophils, respectively. The classic chemoattractants formyl-methionylleucylphenylalanine (FMLP), leukotriene B4 (LTB4), and interleukin-8 (IL-8) induced equivalent neutrophil transepithelial migration in the apical-to-basal and basal-to-apical directions. However, the degree of neutrophil transepithelial migration was significantly greater in the basal-to-apical direction in response to either IL-1beta or tumor necrosis factor-alpha (TNF-alpha). Enhanced TNF-alpha-induced neutrophil migration through A549 monolayers in the basal-to-apical direction occurred regardless of whether the TNF-alpha was above or below the filter/monolayer complex. Actinomycin D pretreatment of A549 monolayers had no effect on FMLP-induced neutrophil transepithelial migration, but markedly (about 75%) inhibited both TNF-alpha- and IL-1beta-induced neutrophil transepithelial migration, regardless of monolayer orientation. Thus, in contrast to classic chemoattractants, IL-1beta and TNF-alpha induced greater neutrophil transepithelial migration in a basal-to-apical direction, and this occurred independently of the cytokine location, but depended upon intact metabolic capacity of the A549 cells. These data suggest that the mechanisms important for neutrophil transepithelial migration in response to classic chemoattractants differ from those important for migration in response to inflammatory cytokines.

Asthma: a dynamic disease of inflammation and repair

It is now widely accepted that asthma in its varied forms is an inflammatory disorder of the airways in which mediator release from activated mast cells and eosinophils plays a major role. T lymphocytes take a primary role in orchestrating these processes through their capacity to generate a range of cytokines of the interleukin 4 gene cluster encoded on the long arm of chromosome 5. Additional cytokines derived from mast cells and eosinophils also play a key role, especially tumour necrosis factor alpha, which is responsible for initiating the up-regulation of vascular adhesion molecules involved in the recruitment of eosinophils and other inflammatory cells from the circulation. The importance of C-X-C and C-C chemokines as local chemoattractants and activating stimuli is also recognized. In addition to releasing an array of pharmacologically active autacoids, the inflammatory response in asthma results in the generation of proteolytic activities from mast cells (tryptase, chymase), eosinophils (MMP-9) and the epithelium itself (MMP-2, MMP-9), which exert tissue-destructive and cell-signalling effects. The epithelium is also highly activated, as evidenced by the up-regulation of cytokine production, inducible enzymes and soluble mediators. Increased surface expression of the epithelial isoform of CD44 (9v) and subepithelial proliferation of myofibroblasts are indicative of a simultaneous active repair process and the laying down of new interstitial collagens. Together, inflammatory and repair processes create the complex phenotype that characterizes asthma and its progression.