View Abstract

A2655 - Interleukin-13 Affects Type 2 Alveolar Cell Growth and Differentiation
Author Block: K. Glisinski1, S. Paramore2, C. Barkauskas1; 1Pulmonary, Allergy, and Critical Care Medicine, Duke University, Durham, NC, United States, 2Department of Molecular Biology, Princeton University, Princeton, NJ, United States.
Rationale: The pathogenesis of idiopathic pulmonary fibrosis (IPF) is poorly understood. It is generally accepted that IPF is driven by defects in alveolar epithelial stem cells (specifically type 2 alveolar epithelial cells (AEC2s)), yet specific mechanisms through which this occurs have not been fully elucidated. In searching for clues to IPF pathogenesis, it has been found that interleukin-13 (IL-13) is present in the bronchoalveolar lavage fluid (BALF) of some patients. While it has been hypothesized that IL-13 promotes a fibrotic phenotype in fibroblasts, the effect of this cytokine on the alveolar epithelium was unexplored. We, therefore, evaluated the effect of IL-13 on the stem cell properties of AEC2s to determine if this IPF-associated cytokine contributes to alveolar epithelial cell dysfunction. Methods: We used our ex vivo 3-dimensional organoid culture system to determine if IL-13 affects growth and differentiation of AEC2s. AEC2s and AEC2-supporting fibroblasts were isolated from mice as previously described. Cells were plated in standard organoid culture conditions and were grown in the presence or absence of recombinant mouse IL-13 (10 ng/ml). AEC2 stem cell activity was quantified with colony-forming efficiency (CFE) and sphere morphology/size. Differentiation was assessed through immunofluorescence staining and Western blot. We then assessed the effect of IL-13 on the alveolar epithelium in vivo through morphometric and histologic study of IL-13 overexpressing mice versus controls at steady state and after injury. Results: In normal organoid culture conditions, individual AEC2s form multilayer organoids containing AEC2s and type 1 alveolar epithelial cells (AEC1s). When exposed to IL-13, CFE is lower and some organoids form single-layer spheres with abnormal AEC2s and few AEC1s; this defect requires intact STAT6 signaling in the epithelium. Unexpectedly, some organoids express ectopic markers typically associated with tracheal basal cells. In vivo, overexpression of IL-13 by bronchiolar epithelial cells leads to emphysema. This is characterized by an increase in AEC2s and a paucity of AEC1s, mirroring our findings in the organoid culture system. After bleomycin injury, these abnormalities are exaggerated further, with damaged areas often displaying a honeycomb-like morphology as typically seen in IPF. Conclusions: IL-13 directly affects AEC2 growth and differentiation. By understanding how IL-13 alters AEC2 stem cells, we will better understand the pathogenesis of lung diseases that may be IL-13 mediated. Because some, but not all, patients with IPF have elevated IL-13 levels in BALF, these findings may guide future personalized treatment modalities.