Ontogeny and Biology of Human Small Airway Epithelial Club Cells

Rationale: Little is known about human club cells, dome-shaped cells with dense cytoplasmic granules and microvilli that represent the major secretory cells of the human small airways (at least sixth-generation bronchi). Objectives: To define the ontogeny and biology of the human small airway epithelium club cell. Methods: The small airway epithelium was sampled from the normal human lung by bronchoscopy and brushing. Single-cell transcriptome analysis and air-liquid interface culture were used to assess club cell ontogeny and biology. Measurements and Main Results: We identified the club cell population by unbiased clustering using single-cell transcriptome sequencing. Principal component gradient analysis uncovered an ontologic link between KRT5 (keratin 5)(+) basal cells and SCGBIA1 (secretoglobin family 1 A member 1)(+) club cells, a hypothesis verified by demonstrating in vitro that a pure population of human KRT5(+) SCGBIA1(-) small airway epithelial basal cells differentiate into SCGBIA1(+) KRT5(-) club cells on air-liquid interface culture. Using SCGBIA1 as the marker of club cells, the single-cell analysis identified novel roles for these cells in host defense, xenobiotic metabolism, antiprotease, physical barrier function, monogenic lung disorders, and receptors for human viruses. Conclusions: These observations provide novel insights into the molecular phenotype and biologic functions of the human club cell population and identify basal cells as the human progenitor cells for dub cells.