How Much Do We Know About the Mouse Respiratory Epithelial Stem Cells?

Abstract

Background-Purpose of this study: Tissue-specific stem cell lines are useful tools for cell biology studies. Information on respiratory tissue cell lines is limited. A doxycycline-regulated epithelial precursor cell line was established from the lung tissue of a tTAxSV40 Tag double transgenic mouse. In this study, we have characterized this cell line in vitro & in vivo, and found to mimic a rare subpopulation of club- and pneumocyte type II-dual cells. Methods: It was partially characterized using cell viability and death assays, H3-thymidine incorporation assay, chloride efflux assay, Western blotting of proteins secreted, RT-PCR assays for RNA isolated. In addition, immune-deficient SCID mice were used as hosts for implantation of this precursor cell line, and feed with/without doxycycline containing water. Immunofluorescent typing using different antibodies were used to characterize the implanted lung. Results: This cell line was found to mimic a rare subpopulation of club- and pneumocyte type II- dual cells with multiple phenotypes. Cell growth was doxycycline-regulated and observed only when doxycycline was omitted from the medium or present at concentrations up to 1 µg/ml, higher concentrations were inhibitory. ACT+ ciliated cells were found upon implantation into immune-deficient mice, in addition. Cell growth was doxycycline-regulated in vitro. When transplanted subcutaneously into immune-deficient mice, these cells migrated to the lung to form organized chimeric structures of donor and host origins, with club cells in the terminal bronchioles, ACT+ ciliated cells along the epithelial lining, and pneumocyte type II-cells in the alveolar interstices. No such homing of donor cells to the lung was observed when the implanted mice were fed doxycycline-containing water. Discussions-Conclusions: This lung stem cell line might be able to provide us with an insight into the differentiation pathway of lung epithelial cells as well as with some understanding of the nature of air trophic-pulmonary epithelial cells. The results of this study underline the possibility of a future application for somatic (stem / precursor) cells in tissue replacement and tissue engineering of the damaged lung. Its ability to secrete and deliver soluble protein, might be a potential novel way for drug delivery. In addition, stem cells are thought to proliferate and differentiate in response to a deficiency or as a result of injury. Successful migration to the target organ and subsequent maturation of these precursors could be attributed to a requirement of lung stem cells to search for an aerated environment. Our findings challenge some current concepts of stem cell biology.This lung stem cell line may become a rich source of cells for tissue engineering and cell-based therapy for lung injury. The route and protocol established for cell introduction into the lung may provide a novel alternative to delivery of soluble protein substances through the airways. This lung stem cell line might also be modified to provide models for screening drugs against respiratory infection.