Complex Urban Atmospheres Alter Alveolar Mesenchymal Stem Cell Niche Properties and Drive Lung Fibrosis

There is growing evidence suggesting that urban pollution has adverse effects on lung health. However, how urban pollution affects alveolar mesenchymal and epithelial stem cell niches remains unknown. This study aimed to determine how complex representative urban atmospheres alter alveolar stem cell niche properties. Mice were placed in an innovative chamber realistically simulating the atmosphere of a megalopolis, or "clean air," for 7 days. Lungs were collected, and fibroblasts and epithelial cells (EpCAM(+)) were isolated. Proliferative capacities of fibroblasts were tested by population doubling levels (PDL), and microarray analyses were performed. Fibroblasts and EpCAM+ cells from exposed, nonexposed, or naive mice were cocultured in organoid assays to assess the stem cell properties. Collagen deposition (Sirius red), lipofibroblasts (ADRP, COL1A1), myofibroblasts (alpha SMA), alveolar type 2 cells (AT2, SFTPC+), and alveolar differentiation intermediate cell [ADI, keratin-8-positive (KRT8(+))/claudin-4-positive (CLDN4(+))] markers were quantified in the lungs. Fibroblasts obtained from mice exposed to urban atmosphere had lower PDL and survival and produced fewer and smaller organoids. Microarray analysis showed a decrease of adipogenesis and an increase of genes associated with fibrosis, suggesting a lipofibroblast to myofibroblast transition. Collagen deposition and myofibroblast number increased in the lungs of urban atmosphere-exposed mice. AT2 number was reduced and associated with an increase in ADI cells KRT8(+)/CLDN4(+). Furthermore, EpCAM(+) cells from exposed mice also produced fewer and smaller organoids. In conclusion, urban atmosphere alters alveolar mesenchymal stem cell niche properties by inducing a lipofibroblast to myofibroblast shift. It also results in alveolar epithelial dysfunction and a fibrotic-like phenotype.