In Vitro Models of the Alveolar Epithelial Barrier

In Vitro Models of the Alveolar Epithelial Barrier for Drug Absorption Studies 

While a number of immortalised cell lines emanating from different cell types of the airway (i.e., tracheo-bronchial) epithelium of lungs from various mammalian species are available (see Chap. 10), reliable and continuously growing cell lines that possess AEC morphology and phenotype are not reported to date. Most studies use cell lines of alveolar epithelial origin for drug absorption studies, while the observations are hard or meaningless to extend to in vivo human situation. 

L-2 cells (ATCC HTB-149) have been isolated by clonal culture techniques from the adult rat lung. These cells appear to retain differentiated functions that are present in ATII cells of intact rat lungs. L-2 cells are diploid, epithelial cells. They contain osmiophilic lamellar bodies in their cytoplasm and synthesise lecithin by the same de novo pathways as in a whole lung [78], It is not known if L-2 cells are capable to form confluent and electrically tight monolayers. L-2 cells have not been systematically investigated regarding their suitability as a model for absorption studies. 

The rat cell line R3/1 was established from cells obtained from broncho-alveolar tissues of foetal Wistar rats at 20 days of gestation. This cell line displays a phenotype with several characteristic features of ATI cells. R3/1 cells were analysed to show a positive expression for mRNA and protein content of markers related to the ATI cell type (e.g., Tla, ICAM-1, connexin-43 and caveolin-1 and -2) [79], Whether or not this cell line can form functional tight junctions is currently under investigation in our laboratories. 

Primary Alveolar Epithelial Cell Culture Models 

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We used in vitro models of lung epithelial cell lines or primary cells to determine E25 permeability. Two different cell types were used to mimic the airway and alveolar epithelium of the lung to study transport. Calu-3, a human cell line derived from an airway carcinoma, when grown at an air/liquid interface, differentiate to form a secretory airway epithelium (17). Rat primary epithelial cells isolated as described by Cheek et al. (18) form a tight barrier similar in structure and function to the alveolar surface. Both cell types when grown to confluence form tight junctions and differentiate and polarize so that the apical or air surface has different characteristics than the basolateral or blood side. The typical transepithelial resistance observed was 350 or >1000 ohms-cm for Calu-3 cells or primary rat alveolar cells, respectively. Once an acceptable resistance was achieved, E25 (2 mg/mL) was placed in either the apical or the basolateral chamber. Cell monolayers were incubated at 37°C for up to 3 hours and ELISA measured the amount of E25 that translocated the epithelial layer and appeared in the receiver well. The apparent permeability (Papp) of the epithelium for E25 was calculated as ... 

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