Type 2 alveolar cells are stem cells in adult lung
Christina E. Barkauskas, … , Paul W. Noble, Brigid L.M. Hogan
Christina E. Barkauskas, … , Paul W. Noble, Brigid L.M. Hogan
Published June 10, 2013
Citation Information: J Clin Invest. 2013;123(7):3025-3036. https://doi.org/10.1172/JCI68782.
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Research Article Pulmonology

Type 2 alveolar cells are stem cells in adult lung

Abstract

Gas exchange in the lung occurs within alveoli, air-filled sacs composed of type 2 and type 1 epithelial cells (AEC2s and AEC1s), capillaries, and various resident mesenchymal cells. Here, we use a combination of in vivo clonal lineage analysis, different injury/repair systems, and in vitro culture of purified cell populations to obtain new information about the contribution of AEC2s to alveolar maintenance and repair. Genetic lineage-tracing experiments showed that surfactant protein C–positive (SFTPC-positive) AEC2s self renew and differentiate over about a year, consistent with the population containing long-term alveolar stem cells. Moreover, if many AEC2s were specifically ablated, high-resolution imaging of intact lungs showed that individual survivors undergo rapid clonal expansion and daughter cell dispersal. Individual lineage-labeled AEC2s placed into 3D culture gave rise to self-renewing “alveolospheres,” which contained both AEC2s and cells expressing multiple AEC1 markers, including HOPX, a new marker for AEC1s. Growth and differentiation of the alveolospheres occurred most readily when cocultured with primary PDGFRα+ lung stromal cells. This population included lipofibroblasts that normally reside close to AEC2s and may therefore contribute to a stem cell niche in the murine lung. Results suggest that a similar dynamic exists between AEC2s and mesenchymal cells in the human lung.

Authors

Christina E. Barkauskas, Michael J. Cronce, Craig R. Rackley, Emily J. Bowie, Douglas R. Keene, Barry R. Stripp, Scott H. Randell, Paul W. Noble, Brigid L.M. Hogan

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Figure 1

Long term self renewal of SFTPC lineage-labeled alveolar cells.

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Long term self renewal of SFTPC lineage-labeled alveolar cells. Adult Sf...
Adult Sftpc-CreER;Rosa-Tm mice were dosed ×4 with Tmx (0.2 mg/g). Tiled confocal images of lung sections 4 dpi show (A) that all Tm+ cells express SFTPC. The majority locate in alveoli and resemble AEC2s. No label is seen in bronchi/bronchioles (Br), indicating that the Sftpc-CreER allele is not active in proximal epithelial precursors in the absence of Tmx. (B) Tm+ epithelial cells in the BADJ coexpress SFTPC and SCGB1A1. Insets are high magnifications of cell marked with arrow: Left (Tm, red; SFTPC, green); right (SCGB1A1, purple; SFTPC, green). (C) A cohort of 8- to 12-week-old mice was dosed ×2 with 0.25 mg/g Tmx. Controls received vehicle alone. At different dpi, confocal z-stack images of lung sections (n ≥ 3 mice per point, 6 sections/mouse) were analyzed to give the proportion of SFTPC+ cells with lineage label. This does not significantly decline over the 48-week chase (red circles). Low recombination was seen without Tmx (black circles). (D) Section from a 24-week-old Tmx– control mouse from C showing small clusters of Tm+ AEC2s (arrows). (E) To confirm proliferation of lineage-labeled (Tm+) cells in C, sections of lungs (n = 3 from Tmx+ group) were stained for Ki67 and percentage of dual positive cells recorded. Error bars indicate mean ± SEM. Scale bars: 250 μm (A); 100 μm (B); 50 μm (D). See also Supplemental Figure 1.