Sequential analysis of surfactant, lung function and inflammation in cystic fibrosis patients

Abstract

Background: In a cross-sectional analysis of cystic fibrosis (CF) patients with mild lung disease, reduced surfactant activity was correlated to increased neutrophilic airway inflammation, but not to lung function. So far, longitudinal measurements of surfactant function in CF patients are lacking and it remains unclear how these alterations relate to the progression of airway inflammation as well as decline in pulmonary function over time.

Methods: As part of the BEAT trial, a longitudinal study to assess the course of airway inflammation in CF, we studied lung function, surfactant function and endobronchial inflammation using bronchoalveolar lavage fluid from 20 CF patients with normal pulmonary function (median FEV1 94% of predicted) at three times over a three year period.

Results: There was a progressive loss of surfactant function, assessed as minimal surface tension. The decline in surfactant function was negatively correlated to an increase in neutrophilic inflammation and a decrease in lung function, assessed by FEV1, MEF(75/25%VC), and MEF(25%VC). The concentrations of the surfactant specific proteins A, C and D did not change, whereas SP-B increased during this time period.

Conclusion: Our findings suggest a link between loss of surfactant function driven by progressive airway inflammation and loss of small airway function in CF patients with limited lung disease.

Figure 1

Quality control assurance by the assessment the function of a bovine surfactant (minimal surface tension, closed squares; surface tension after adsorption, open squares) and of water (minimal surface tension, closed circles; surface tension after adsorption, open squares) in the pulsating bubble surfactometer (upper panel) and the capillary surfactometer (lower panel) (bovine surfactant, closed squares, water, open squares) over a period of more than two years.

Figure 2

Surface activity of a surfactant fraction obtained from bronchoalveolar lavages in 20 patients with cystic fibrosis assessed in a pulsating bubble surfactometer (upper left panel), i.e. expressed as surface tension at minimum bubble radius (squares) and surface tension at adsorption (triangles), assessed in a capillary surfactometer (lower left panel), i.e. expressed as the percentage of time open of a small capillary of total time. The percentage of neutrophils (upper right panel) and the absolute number of neutrophils (lower right panel) as markers of inflammation in their BAL fluids are also indicated. Data are given as mean and standard error of the mean. Differences between the different time points were assessed by nonparametric Friedman ANOVA, followed by Dunn's post hoc multiple comparison test. The asterisks indicates a significantly different value, compared to the first data point (* P < 0.05, ** P < 0.01). The open symbols represent values in normal subjects assessed with the same methodolgy [7,29].

Figure 3

Correlation of the change in surfactant activity, assessed as minimal surface tension, to the change of the fraction of polymorphonuclear leukocytes in BAL (upper panel) and to the change of small airway function, assessed as MEF_75/25 (lower panel). Changes were calculated as the difference between the last visit at 3 years and the first visit at start of the study. Spearman rank correlations and linear regression line are indicated.

Figure 4

Lung function over time in 20 subjects with CF. FEV₁, MEF_72/25%VC and MEF_25%VC were significantly lower 3 years after start of the study (Friedman ANOVA, followed by Dunn' post hoc multiple comparison test, * p < 0.05, *** p < 0.001). FVC did not change.

Figure 5

Surfactant activity assessed in the pulsating bubble surfactometer (upper panels) and the capillary surfactometer (lower panels). The large aggregate (LA) surfactant fraction was recombined with the appropriate small aggregates (SA) of surfactant, demonstrating significant inhibitory activity (left side panels). This inhibitory activity of SA was not dependent on its amount, i.e. was not the sole determinate of surface activity (right side panels). Comparisons by Mann-Whitney test and correlations by Spearman rank test.