Animal models of fibrotic lung disease

Abstract

Interstitial lung fibrosis can develop as a consequence of occupational or medical exposure, as a result of genetic defects, and after trauma or acute lung injury leading to fibroproliferative acute respiratory distress syndrome, or it can develop in an idiopathic manner. The pathogenesis of each form of lung fibrosis remains poorly understood. They each result in a progressive loss of lung function with increasing dyspnea, and most forms ultimately result in mortality. To better understand the pathogenesis of lung fibrotic disorders, multiple animal models have been developed. This review summarizes the common and emerging models of lung fibrosis to highlight their usefulness in understanding the cell-cell and soluble mediator interactions that drive fibrotic responses. Recent advances have allowed for the development of models to study targeted injuries of Type II alveolar epithelial cells, fibroblastic autonomous effects, and targeted genetic defects. Repetitive dosing in some models has more closely mimicked the pathology of human fibrotic lung disease. We also have a much better understanding of the fact that the aged lung has increased susceptibility to fibrosis. Each of the models reviewed in this report offers a powerful tool for studying some aspect of fibrotic lung disease.

Figure 1.

Asbestos induced pulmonary fibrosis. (A) Control lung shows normal terminal bronchi and alveolar parenchyma. (B) Intratracheal instillation of crocidolite asbestos induces robust peribronchial fibrosis with extension into the adjacent alveolar parenchyma (14 d after exposure). Scale bars = 50 μm.

Figure 2.

Intratracheal silica injury leads to multinodular fibrosis (A), predominantly in the alveolar parenchyma and terminal bronchioles (C). Activated macrophages are predominant in these lesions (D, arrows). (B) Control lung section.

Figure 3.

Bleomycin can be administered in different manners to induce lung fibrosis in mice. (A) Trichome blue–stained lung sections from normal wild-type C57BL/6 mice. (B) Lung section at 3 weeks after 0.08 of a unit of intratracheal bleomycin demonstrates the development of an area of fibrosis typically seen with this model. (C) Lung section from a mouse at 2 weeks after the eighth biweekly repetitive intratracheal 0.04-unit bleomycin dose. This repetitive intratracheal model not only induces prominent lung fibrosis, but also results in regions with prominent alveolar epithelial cell (AEC) hyperplasia lining areas of fibrosis. Arrow points to hyperplastic AECs. (D) Lung section from a mouse harvested on Day 33 in a twice-weekly intraperitoneal 0.035-U/g bleomycin study. With systemic delivery modalities such as intraperitoneal injections, fibrosis develops prominently in the subpleural regions. Arrowhead points to pleural edge. All sections, magnification, ×400.

Figure 4.

Fibrosis develops in areas of fluorescein isothiocyanate (FITC) deposition. Serial lung sections were prepared on Day 21 after FITC intratracheal instillation in C57Bl/6 mice. Hematoxylin-and-eosin staining shows patchy areas of fibrosis and consolidation (A), which line up well with areas of FITC deposition, as seen in the immunofluorescent image captured of a serial lung section (B).

Figure 5.

Targeted Type II AEC injury induced fibrosis. Trichrome-stained sections of lungs from wild-type (WT) or surfactant protein–C–diphtheria toxin receptor (SpC DTR) mice harvested on Day 28 after diphtheria toxin (DT; 100 μg/kg) treatment on Days 0–14. Magnification: ×400.

Figure 6.

Lung contusion induced fibrosis. Hematoxylin and eosin–stained lung sections from a rat on Day 7 after contusion injury show the development of fibrotic lesions, especially around bronchioles. Magnification: ×400.

Figure 7.

Humanized model of lung fibrosis. Pulmonary fibroblasts were grown from idiopathic pulmonary fibrosis (IPF) surgical lung biopsies and labeled with PKH26. After the PKH26 labeling, 1 ml of the fibroblast suspension and PKH26 dye solution containing 1 × 10⁶ fibroblasts were then injected intravenously into each mouse, using a 26-gauge needle and tuberculin syringe. The histological appearance of the lungs of these mice on Day 63 after injection is shown in A (trichrome staining; original magnification, ×40). The effects of a human IgG1-λ isotype control (anti–chi-lysozyme–MOR03207), anti-PDGFRα monoclonal antibody (mAb) (IMC-3G3; ImClone, Summerville, NJ), or anti-PDGFRβ antibody (IMC-2C5; ImClone) were examined using histological (trichrome staining) (B) IgG1, (C) anti-PDGFRα mAb, and (D) anti-PDGFRβ mAb (original magnification, ×40), biochemical analysis (hydroxyproline; E), and transforming growth factor–β and PDGF-BB (ELISA; F and G, respectively) analyses for the presence of pulmonary fibrosis, performed at various times after the intravenous injection of human fibroblasts. All antibodies and control IgG were administered at 1 mg per mouse every other day, beginning on Day 35. Groups of mice were killed on either Day 49 or Day 63. All data shown in E–G represent means ± SEMs for groups of n = 5 mice. *P ≤ 0.05. **P ≤ 0.01. PDGFR, platelet-derived growth factor receptor; hPDGFR, human platelet-derived growth factor receptor.