Fibroblast-specific perturbation of transforming growth factor beta signaling provides insight into potential pathogenic mechanisms of scleroderma-associated lung fibrosis - Exaggerated response to alveolar epithelial injury in a novel mouse model

Objective. To explore increased susceptibility to fibrosis following experimental injury to alveolar epithelial cells (AECs) in a novel transgenic mouse model of scleroderma with fibroblast-specific perturbation of transforming growth factor beta (TGF beta) signaling (T beta RII Delta k-fib mice). Methods. Wild-type (WT) and transgenic mice were injured with intratracheally administered saline or bleomycin, and the lungs were harvested for biochemical, histologic, and electron microscopic analysis. Results. Electron microscopy revealed AEC abnormalities in the lungs of untreated transgenic mice and bleomycin-treated WT mice; the lungs of transgenic mice treated with bleomycin showed severe epithelial damage. Compared with lungs from bleomycin-treated WT mice, lungs from bleomycin-treated transgenic mice demonstrated increased fibroproliferation, myofibroblast persistence, and impaired hyperplasia and increased apoptosis of type II AECs. The lungs from saline-treated transgenic mice and those from bleomycin-treated WT mice had phenotypic similarities, suggesting enhanced susceptibility to minor epithelial injury in the transgenic strain. The level of collagen was increased in the lungs from transgenic mice compared with that in the lungs from WT mice after treatment with either bleomycin or saline. Persistent fibrosis in bleomycin-treated transgenic mice was independent of ongoing neutrophil inflammation but was associated with impaired alveolar epithelial repair. Conclusion. These results suggest that in the context of fibroblast-specific perturbation of TGF beta signaling, even minor epithelial injury induces significant fibrosis. The model supports a central role for TGF beta in determining fibrosis and demonstrates that lung fibroblasts may regulate the response of AECs to injury. Our findings provide insight into likely pathogenic mechanisms in scleroderma-associated pulmonary fibrosis.