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Targeted Injury of Type II Alveolar Epithelial Cells Induces Pulmonary Fibrosis

¹ Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, University of Michigan Hospital, Ann Arbor; ² Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Veterans Affairs Medical Center, Ann Arbor, Michigan; and ³ Department of Anatomy and Cell Biology, University of Iowa, Iowa City, Iowa

Correspondence and requests for reprints should be addressed to Thomas H. Sisson, M.D., Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, University of Michigan Medical Center, 1150 West Medical Center Drive, 6301 MSRB III, Ann Arbor, MI 48109-5642. E-mail: tsisson{at}umich.edu

Rationale: Ineffective repair of a damaged alveolar epithelium has been postulated to cause pulmonary fibrosis. In support of this theory, epithelial cell abnormalities, including hyperplasia, apoptosis, and persistent denudation of the alveolar basement membrane, are found in the lungs of humans with idiopathic pulmonary fibrosis and in animal models of fibrotic lung disease. Furthermore, mutations in genes that affect regenerative capacity or that cause injury/apoptosis of type II alveolar epithelial cells have been identified in familial forms of pulmonary fibrosis. Although these findings are compelling, there are no studies that demonstrate a direct role for the alveolar epithelium or, more specifically, type II cells in the scarring process.

Objectives: To determine if a targeted injury to type II cells would result in pulmonary fibrosis.

Methods: A transgenic mouse was generated to express the human diphtheria toxin receptor on type II alveolar epithelial cells. Diphtheria toxin was administered to these animals to specifically target the type II epithelium for injury. Lung fibrosis was assessed by histology and hydroxyproline measurement.

Measurements and Main Results: Transgenic mice treated with diphtheria toxin developed an approximately twofold increase in their lung hydroxyproline content on Days 21 and 28 after diphtheria toxin treatment. The fibrosis developed in conjunction with type II cell injury. Histological evaluation revealed diffuse collagen deposition with patchy areas of more confluent scarring and associated alveolar contraction.

Conclusions: The development of lung fibrosis in the setting of type II cell injury in our model provides evidence for a causal link between the epithelial defects seen in idiopathic pulmonary fibrosis and the corresponding areas of scarring.

Key Words: diphtheria toxin • lung • collagen • scarring

AT A GLANCE COMMENTARY Scientific Knowledge on the Subject Alveolar epithelial cell abnormalities are a common feature of pulmonary fibrosis. A mutation in a type II alveolar epithelial gene product that causes cell toxicity is associated with familial lung scarring. What This Study Adds to the Field We have established a causal relationship between type II alveolar epithelial cell injury and interstitial collagen accumulation.