Abstract
A robust in vitro model of the human respiratory epithelium, including the alveolar and the airway epithelium, is essential for understanding the biology and pathology of the human respiratory system. We previously described a protocol to derive human lung organoids from primary lung tissues. We now describe a protocol to induce bidirectional differentiation to generate mature alveolar or airway organoids. The lung organoids are consecutively expanded for over one year with high stability, while the differentiated alveolar and airway organoids morphologically and functionally simulate the human alveolar and airway epithelium to a near-physiological level. Thus, we establish a robust organoid culture system of the entire human respiratory epithelium, the first two-phase bipotential organoid culture system that enables long-term expansion and bidirectional differentiation of respiratory epithelial cells. The long-term expandable lung organoids and differentiated organoids generate a stable and renewable source of respiratory epithelial cells, enabling scientists to reconstruct and expand the human respiratory epithelium in culture dishes. The respiratory organoid system provides a unique and physiologically active in vitro model of the human respiratory epithelium for various applications, including studying respiratory viral infection, disease modeling, drug screening, and pre-clinical testing. Graphical abstract.