Early pulmonary disease manifestations in cystic fibrosis mice

Abstract

Background: Altered pulmonary function is present early in the course of cystic fibrosis (CF), independent of documented infections or onset of pulmonary symptoms. New initiatives in clinical care are focusing on detection and characterization of preclinical disease. Thus, animal models are needed which recapitulate the pulmonary phenotype characteristic of early stage CF.

Methods: We investigated young CF mice to determine if they exhibit pulmonary pathophysiology consistent with the early CF lung phenotype. Lung histology and pulmonary mechanics were examined in 12- to 16-week-old congenic C57bl/6 F508del and R117H CF mice using a forced oscillation technique (flexiVent).

Results: There were no significant differences in the resistance of the large airways. However, in both CF mouse models, prominent differences in the mechanical properties of the peripheral lung compartment were identified including decreased static lung compliance, increased elastance and increased tissue damping. CF mice also had distal airspace enlargement with significantly increased mean linear intercept distances.

Conclusions: An impaired ability to stretch and expand the peripheral lung compartment, as well as increased distances between gas exchange surfaces, were present in young CF mice carrying two independent Cftr mutations. This altered pulmonary histopathophysiology in the peripheral lung compartment, which develops in the absence of infection, is similar to the early lung phenotype of CF patients.

Keywords: Cystic fibrosis; Lung mechanics; Mouse models.

Figure 1. Overall mechanics of the respiratory system are altered in CF mice

Single compartment analysis of respiratory system mechanics using flexiVent forced oscillation technique in wild type mice (black bars, n=7), F508del (grey bars, n=6) and R117H (white bars, n=6) cystic fibrosis mice. Compliance of the respiratory system was significantly decreased in both CF mice as compared to WT control mice (*p<0.05). Overall resistance of the respiratory system tended to be greater in CF mice (p=0.09). Inspiratory capacity normalized to body weight was not different between groups.

Figure 2. Significant differences in the mechanical properties of the CF lung are localized to the peripheral tissue compartment and not the central airways

Constant phase analysis was used to discriminate central and peripheral lung mechanics in wild type mice (black bars, n=7), F508del (grey bars, n=6) and R117H (white bars, n=6) cystic fibrosis mice. Newtonian airway resistance (central compartment) was similar between groups. Tissue damping (G) and tissue elastance (H) of the peripheral lung compartment were both significantly increased in both CF mice (*p<0.05).

Figure 3. Pressure-volume loops identify differences in lung mechanics

Pressure-volume loops (top panel) generated by stepped inflation and deflation maneuvers in wild type mice (solid black line, n=7), F508del (dashed grey line, n=6) and R117H (solid grey line, n=6) cystic fibrosis mice. Static lung compliance (Cst) was significantly reduced in CF (grey and white bars) as compared to WT mice (black bars). The area between the inflation and deflation limbs of the pressure-volume loop was similar among all groups. Significant differences were present in the shape of the deflation limb of the pressure-volume loop as quantified by the shape parameter (K). *p<0.05

Figure 4. Histologic analysis of the airways and lung parenchyma

Representative micrographs of WT, F508del, and R117H mice are displayed. Bronchioles were visualized at 10× (A–C), and all others were visualized at 20× (D–O). Slides were stained with H&E (A–L), and the terminal bronchioles were also stained with PAS to verify the absence or presence of any mucoid substance in the airway lumen (M–O).

Figure 5. Quantitative assessment of the airway luminal diameter and peripheral lung airspace structure

Measurements of the luminal diameter of the bronchioles, terminal bronchioles, and respiratory bronchioles showed no difference in CF (grey bars, n=6; white bars, n=5) and WT (black bars, n=6) lung. The mean linear intercept was significantly increased in CF peripheral lung compared to WT. *p<0.05