Multiple stromal populations contribute to pulmonary fibrosis without evidence for epithelial to mesenchymal transition

Proc Natl Acad Sci U S A. 2011 Dec 27;108(52):E1475-83. doi: 10.1073/pnas.1117988108. Epub 2011 Nov 28.


There are currently few treatment options for pulmonary fibrosis. Innovations may come from a better understanding of the cellular origin of the characteristic fibrotic lesions. We have analyzed normal and fibrotic mouse and human lungs by confocal microscopy to define stromal cell populations with respect to several commonly used markers. In both species, we observed unexpected heterogeneity of stromal cells. These include numerous cells with molecular and morphological characteristics of pericytes, implicated as a source of myofibroblasts in other fibrotic tissues. We used mouse genetic tools to follow the fates of specific cell types in the bleomcyin-induced model of pulmonary fibrosis. Using inducible transgenic alleles to lineage trace pericyte-like cells in the alveolar interstitium, we show that this population proliferates in fibrotic regions. However, neither these cells nor their descendants express high levels of the myofibroblast marker alpha smooth muscle actin (Acta2, aSMA). We then used a Surfactant protein C-CreER(T2) knock-in allele to follow the fate of Type II alveolar cells (AEC2) in vivo. We find no evidence at the cellular or molecular level for epithelial to mesenchymal transition of labeled cells into myofibroblasts. Rather, bleomycin accelerates the previously reported conversion of AEC2 into AEC1 cells. Similarly, epithelial cells labeled with our Scgb1a1-CreER allele do not give rise to fibroblasts but generate both AEC2 and AEC1 cells in response to bleomycin-induced lung injury. Taken together, our results show a previously unappreciated heterogeneity of cell types proliferating in fibrotic lesions and exclude pericytes and two epithelial cell populations as the origin of myofibroblasts.

Publication types

  • Comparative Study
  • Research Support, N.I.H., Extramural

MeSH terms

  • Actins / metabolism
  • Animals
  • Biomarkers / metabolism
  • Bleomycin / toxicity
  • Bromodeoxyuridine
  • Cell Differentiation / physiology*
  • Cell Proliferation
  • Epithelial-Mesenchymal Transition / physiology
  • Extracellular Matrix Proteins / metabolism
  • Flow Cytometry
  • Humans
  • Immunohistochemistry
  • Mice
  • Myofibroblasts / cytology
  • Pericytes / metabolism
  • Pulmonary Alveoli / cytology*
  • Pulmonary Alveoli / pathology
  • Pulmonary Fibrosis / chemically induced
  • Pulmonary Fibrosis / pathology*
  • Real-Time Polymerase Chain Reaction
  • Stromal Cells / cytology*
  • Stromal Cells / metabolism


  • ACTA2 protein, human
  • Actins
  • Biomarkers
  • Extracellular Matrix Proteins
  • Bleomycin
  • Bromodeoxyuridine