Transport of water, ions, and larger molecules across the pulmonary epithelium is becoming an important area of research in light of the increasing interest in using the lung as a route for the delivery of drugs to the body. The large surface area of the alveolar epithelium is composed primarily of thin, morphologically differentiated Type 1 cells. These Type 1 cells are derived from a relatively small number of Type 2 stem cells that also produce surfactant. Historically, most alveolar cell cultures have been Type 2 cell-like in nature. They have been derived from broncho-alveolar cell tumors and show such characteristics of Type 2 cells as low transepithelial cell resistance. The low electrical resistance corresponds to a lack of tight junctions between cells. In contrast, increased transepithelial resistance and the presence of tight juctions between cells characterize cultures of Type 1 cells. Unfortunately most primary Type 1 cell cultures are derived from rodents. Although human Type 1 cells cultures have been developed, they are used to study a variety of endpoints other than that of transepithelial resistance.

This study describes the establishment of a human Type 1 cell culture system and its electrical resistance characteristics. Evidence is also presented showing that these alveolar epithelial cells form tight junctions and desmosomes similar to those found in vivo. Desmosomes join cells together and link their intermediate filament cytoskeletons through the tissue. The Type I culture was derived from Type 2 cells isolated from human lungs. This leads to the conclusion that the Type 2 cells differentiated into Type 1 cells in culture. Morphological and histochemical changes indicating differentiation from Type 2 to Type 1 cells were observed. The final Type 1 culture was a tight confluent monolayer.

The techniques used to establish the Type I cell culture in this study were fairly standard. Human distal lung tissue was treated with enzymes, purified, plated on filter inserts and cultured in a low-serum media. Cell cultures were characterized by bioelectric measurements, immunohistochemical staining of cell junctions, cell-specific lectin binding, and measurement of transepithelial transport of various molecular weight dextrans. The final Type 1 monolayer had a peak transepithelial resistance of about 2,180 ohm X cm2 and potential difference of about 13.5 mV. Lectin binding indicated some Type 2 cells interspersed among the Type 1 cells in a manner similar to the alveolar region in vivo. Permeability of hydrophilic FITC-dextrans across the cell monolayer was inversely related to molecular size.

By: Susan G. Shami, ScD