A subset of epithelial cells with CCSP promoter activity participates in alveolar development

Alveolar formation is hallmarked by the transition of distal lung saccules into gas exchange units through the emergence of secondary crests and an exponential increase in surface area. Several cell types are involved in this complex process, including families of epithelial cells that differentiate into alveolar type I and II cells. Subsets of cells expressing Clara cell secretory protein (CCSP) have been identified in both lung and bone marrow compartments, and are described as a progenitor/stem cell pool involved in airway regeneration and alveolar homeostasis. Whether these cells also participate in alveolar formation during postnatal development remains unknown. Based on their regenerative capacity, we asked whether these cells participate in alveogenesis. We used a previously described transgenic mouse model (CCSP-tk) in which Ganciclovir exposure selectively depletes all cells with CCSP promoter activity through intracellular generation of a toxic metabolite of thymidine kinase. Our results showed that Ganciclovir treatment in newborn CCtk mice depleted this cell population in lung airways and bone marrow, and was associated with alveolar hypoplasia and respiratory failure. Hypoplastic lungs had fewer alveolar type I and II cells, with impaired secondary crest formation and decreased vascular endothelial growth factor expression in distal airways. These findings are consistent with a model in which a unique population of cells with CCSP promoter activity that expresses vascular endothelial growth factor participates in alveolar development.