Summary During active intestinal inflammation granulocytes accumulate in the lumen of the gut where they damage the epithelium through the release of various products such as reactive oxygen species and proteolytic enzymes. Previously, using function blocking monoclonal antibodies, we showed that neutrophil migration across intestinal epithelial monolayers in response to various chemoattractants was partially β2 integrin Mac‐1 (CD11b/CD18)‐independent. Here, we show that treating neutrophils with intact monoclonal antibody (mAb) to CD18 activates the cells to express more CD11b. Thus our goal now was to determine whether neutrophil Mac‐1‐independent transepithelial migration proceeds independently of prior cell activation through Mac‐1. We took two approaches, one using blocking Fab′ fragments of mAb to CD18 and the second was to develop a neutrophil differentiated HL‐60 cell line which is Mac‐1 deficient to further study neutrophil/epithelial cell interaction. Anti‐CD18 Fab′ minimally activated neutrophils but inhibited approximately 75% of transepithelial migration to fMLP while having a minimal effect (≤25% inhibition) on the migration to C5a. Upon incubation with dimethylsulphoxide, HL‐60 cells differentiated and up‐regulated CD11b expression and migrated to C5a and n‐formyl methionyl leucyl phenylalanine in a similar manner to peripheral blood neutrophils. In contrast, CD11b expression was minimal on HL‐60 cells differentiated with dibutytyl cAMP to a neutrophil‐like phenotype. These cells, however, readily migrated across both intestinal and lung epithelial monolayers in response to C5a. We conclude that Mac‐1‐independent transepithelial migration does not require prior activation of cells via Mac‐1 ligation because HL‐60 cells lacking Mac‐1 (CD11b/CD18) expression migrate effectively. HL‐60 cells differentiated with dbcAMP should greatly assist in the search for the Mac‐1‐independent ligands for neutrophil migration across epithelium.

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.

In a previous study, the authors reported the development of an optimized model bilayer of endothelium with alveolar epithelium using A549 cells, and that neutrophil transendothelial migration across endotoxin (lipopolysaccharide [LPS])-activated endothelial cells was attenuated by the apposition of the epithelium. Here the authors investigated whether this modulation by the epithelium extended to other stimuli such as tumor necrosis factor (TNF)-alpha, which, like LPS, activates proinflammatory gene transcription via nuclear factor (NF)-kappa B-dependent mechanisms to induce neutrophil transendothelial migration. Unlike the response to LPS, neutrophil migration in response to TNF-alpha was not altered by the presence of lung epithelial cells, except at a low concentration of TNF-alpha upon alveolar directional exposure of the endothelium, i.e., from the epithelial side of the bilayer. Epithelial cells in the bilayer reduced expression of E-selectin on the endothelium in response to LPS, but not with TNF-alpha stimulation. The production of the chemokine CXCL8 was also differentially modulated by epithelium in response to these 2 mediators. The expression of Toll-like receptor 4 (TLR4), which is involved in LPS recognition by endothelium, was not altered by epithelial cells, suggesting that the anti-inflammatory effect on endothelium may be via downstream LPS-induced signaling events. Inhibition of some candidate anti-inflammatory mediators produced by epithelium, such as nitric oxide, or the activity of interleukin (IL)-10 or transforming growth factor (TGF)-beta had no effect on the inhibitory influence of the epithelium in the bilayers. The authors' findings demonstrate a selective role for alveolar epithelial cells, via either direct cell-cell contact or yet-to-be-identified but short-range or short-lived product(s) in attenuating endothelial responses to endotoxin.

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