CHARACTERIZATION OF STEM-CELLS IN HUMAN AIRWAY CAPABLE OF RECONSTITUTING A FULLY DIFFERENTIATED BRONCHIAL EPITHELIUM

The epithelia of the lung are complex structures that play an importan t role in normal lung physiology and are often involved in the pathoph ysiology of pulmonary diseases. The dynamics of cell turnover, lineage , and differentiation within these epithelia are complex and poorly un derstood. We have coupled the technique of retrovirus-mediated gene tr ansfer with a xenograft model of proximal human airway to evaluate pat hways of cellular proliferation and differentiation in human bronchial epithelia. Primary isolates of human bronchial epithelial cells (HBEC s) were infected with mixtures of recombinant retroviruses expressing different reporter genes and seeded into denuded rat trachea, which we re implanted subcutaneously into athymic mice. The HBECs were allowed to regenerate for four weeks in xenografts, which were then explanted. Clonal expansion of individual retrovirus-marked cells in the regener ated human bronchial epithelium was detected as clusters of transgene- expressing cells. Clone size varied with seeding density, resulting in the largest clones comprising 10(3)-10(4) cells. A substantial number of clones showed transgene expression in basal as well as differentia ted columnar cells, a finding that appeared independent of clone size. These studies demonstrate the existence of a cell type within the hum an bronchial epithelium that is capable of extensive self-renewal and pluripotent development. Further characterization of these potential s tem cells will be important in defining pathogenesis of pulmonary dise ases and in developing novel approaches to treatment such as gene ther apy.