Human fetal lung fibroblasts promote invasion of extracellular matrix by normal human tracheobronchial epithelial cells in vitro: a model of early airway gland development.

Epithelial invasion of extracellular matrix (ECM) is important during lung development and the pathogenesis of bronchogenic neoplasms. Airway submucosal gland development begins when clusters of surface epithelial cells invade the lamina propria between the tenth and thirtieth weeks of fetal life. The factors regulating this transient normal invasive behavior are unknown. We observed that normal human tracheobronchial epithelial (HTBE) cells from adult necropsy specimens penetrate collagen matrices when co-cultured with human fetal lung fibroblasts (HFLF). Invading clusters of epithelial cells resembled primordial glands, forming tubular structures and undergoing dichotomous branching. Using 48 different tracheobronchial specimens, we compared paired cultures of HTBE cells without (control) and with HFLF co-culture. Fixed, vertically sectioned culture substrates were examined, and invaginated epithelial cell clusters as well as total invading HTBE cells were counted. The co-cultured condition resulted in significantly more epithelial invaginations per two sections (4 +/- 1 versus 22 +/- 3, P < 0.0005) and more invading HTBE cells per two sections (31 +/- 9 versus 194 +/- 27, P < 0.0005) than controls. Epithelial invasion was noted by 36 h in culture and was greatest at HTBE cell/HFLF ratios near 1 and HFLF passages between 10 and 16. Epithelial cells co-cultured with a fibroblast cell line derived from the adult bronchiole showed no increase in ECM invasion compared with controls. These results demonstrate that fetal mesenchymal cells are capable of promoting invasive behavior in mature epithelial cells in vitro. Given the fibroblast type and passage specificity, this model should prove useful for investigating the cellular and molecular regulation of epithelial ECM invasion.