LAUSR

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 SCGB1A1 (secretoglobin family 1A member 1)+ club cells, a hypothesis verified by demonstrating in vitro that a pure population of human KRT5+ SCGB1A1− small airway epithelial basal cells differentiate into SCGB1A1+KRT5− club cells on air‐liquid interface culture. Using SCGB1A1 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 club cells.