Tumor necrosis factor-alpha overexpression in lung disease: a single cause behind a complex phenotype

Abstract

Rationale: Tumor necrosis factor alpha (TNF-alpha) has been implicated as a key cytokine in many inflammatory lung diseases. These effects are currently unclear, because a transgenic mouse overexpressing TNF-alpha in the lung has been shown in separate studies to produce elements of both emphysema and pulmonary fibrosis.

Objectives: We sought to elucidate the phenotypic effects of TNF-alpha overexpression in a mouse model.

Measurements: We established the phenotype by measuring lung impedance and thoracic gas volume, and using micro-computed tomography and histology.

Main results: We found that airways resistance in this mouse was not different to control mice, but that lung tissue dampening, elastance, and hysteresivity were significantly elevated. Major heterogeneous abnormalities of the parenchyma were also apparent in histologic sections and in micro-computed tomography images of the lung. These changes included airspace enlargement, loss of small airspaces, increased collagen, and thickened pleural septa. We also found significant increases in lung and chest cavity volumes in the TNF-alpha-overexpressing mice.

Conclusions: We conclude that TNF-alpha overexpression causes pathologic changes consistent with both emphysema and pulmonary fibrosis combined with a general lung inflammation, and consequently does not model any single human disease. Our study thus confirms the pleiotropic effects of TNF-alpha, which has been implicated in multiple inflammatory disorders, and underscores the necessity of using a wide range of investigative techniques to link gene expression and phenotype in animal models of disease.

Figure 1.

Mean values of the parameters of the constant-phase model of respiratory input impedance (Zrs) in TG⁺ (n = 10; solid symbols) and control (n = 10; open symbols) mice versus time. R_N, H, G, and η reflect airway resistance, tissue stiffness, tissue damping, and hysteresivity, respectively. Error bars indicate SEM. Zrs was measured at a positive end-expiratory pressure (PEEP) of 0, 1, 3, and 6 cm H₂O in randomized order. After the first two Zrs at each PEEP, a pressure–volume (PV) loop and deep inhalation were administered to generate a standard volume history.

Figure 2.

(A) Quasi-static PV loops from TG⁺ (n = 10; solid symbols) and control (n = 10; open symbols) at four different PEEP levels (in cm H₂O). Loops proceed in a counterclockwise direction beginning with inspiration. Error bars indicate SEM. (B) Shape factor k (means ± SEM) calculated from PV loops in A using the Salazar-Knowles equation. Open bars are control animals (n = 10) and solid bars are TG⁺ mice (n = 10). Significant differences between groups are shown as follows: *p < 0.05, **p < 0.01, and ***p < 0.001.

Figure 3.

Representative micro–computed tomography (micro-CT) images of control (A and C) and TG⁺ mice (B and D). A and B are coronal sections and C and D are transverse sections. The mice were scanned at 80 kV(peak), 450 mA, 720 views for 80 minutes. The scans have a resolution of 47 μm/voxel side. Arrows point at interlobular fissures. Scale bar equals 1 mm.

Figure 4.

Surface renderings of the air–tissue interface from micro-CT scans of control (A) and TG⁺ (B) mice. Also shown are maximum intensity projection images of the same control (C) and TG⁺ (D) lungs from which the airway tree structure can be seen. Scale bar equals 1 mm.

Figure 5.

Representative images of lung tissue from control (A) and TG⁺ (B) mice stained with hematoxylin and eosin. The overview image was obtained at a magnification of 50×, whereas the breakout image was obtained at 200×. The dashed rectangular box indicates where the 200× image was obtained.

Figure 6.

Representative images of lung tissue from control (A) and TG⁺ (B) mice stained with Sirius red and fast green FCF. The overview image was obtained at a magnification of 50×, whereas the breakout image was obtained at 200×. The dashed rectangular box indicates where the 200× image was obtained.

Figure 7.

Frequency distributions of line segment lengths across airspaces from histologic images, weighted by segment length (arbitrary units). Mean distributions for control mice (thin line) and TG⁺ mice (thick line) are shown with vertical lines showing mean centroid of the histogram (L_C) values. The shift of L_C to the right indicates that TG⁺ mice have fewer small airspaces than do control mice.