Basal cells (BCs) are p63-expressing multipotent progenitors of skin, tracheoesophageal and urinary tracts. p63 is abundant in developing airways; however, it remains largely unclear how embryonic p63+ cells contribute to the developing and postnatal respiratory tract epithelium, and ultimately how they relate to adult BCs. Using lineage-tracing and functional approaches in vivo, we show that p63+ cells arising from the lung primordium are initially multipotent progenitors of airway and alveolar lineages but later become restricted proximally to generate the tracheal adult stem cell pool. In intrapulmonary airways, these cells are maintained immature to adulthood in bronchi, establishing a rare p63+Krt5- progenitor cell population that responds to H1N1 virus-induced severe injury. Intriguingly, this pool includes a CC10 lineage-labeled p63+Krt5- cell subpopulation required for a full H1N1-response. These data elucidate key aspects in the establishment of regionally distinct adult stem cell pools in the respiratory system, potentially with relevance to other organs.
The differentiation of alveolar epithelial type I (AT1) and type II (AT2) cells is essential for the lung gas exchange function. Disruption of this process results in neonatal death or in severe lung diseases that last into adulthood. We developed live imaging techniques to characterize the mechanisms that control alveolar epithelial cell differentiation. We discovered that mechanical forces generated from the inhalation of amniotic fluid by fetal breathing movements are essential for AT1 cell differentiation. We found that a large subset of alveolar progenitor cells is able to protrude from the airway epithelium toward the mesenchyme in an FGF10/FGFR2 signaling-dependent manner. The cell protrusion process results in enrichment of myosin in the apical region of protruded cells; this myosin prevents these cells from being flattened by mechanical forces, thereby ensuring their AT2 cell fate. Our study demonstrates that mechanical forces and local growth factors synergistically control alveolar epithelial cell differentiation.
To investigate the role of immune cells in lung regeneration, we used a unilateral pneumonectomy model that promotes the formation of new alveoli in the remaining lobes. Immunofluorescence and single-cell RNA sequencing found CD115+ and CCR2+ monocytes and M2-like macrophages accumulating in the lung during the peak of type 2 alveolar epithelial stem cell (AEC2) proliferation. Genetic loss of function in mice and adoptive transfer studies revealed that bone marrow-derived macrophages (BMDMs) traffic to the lung through a CCL2-CCR2 chemokine axis and are required for optimal lung regeneration, along with Il4ra-expressing leukocytes. Our data suggest that these cells modulate AEC2 proliferation and differentiation. Finally, we provide evidence that group 2 innate lymphoid cells are a source of IL-13, which promotes lung regeneration. Together, our data highlight the potential for immunomodulatory therapies to stimulate alveologenesis in adults.