Carbon load in airway macrophages as a biomarker of exposure to particulate air pollution; a longitudinal study of an international Panel

Abstract

Background: Carbon load in airway macrophages (AM) has been proposed as an internal marker to assess long-term exposure to combustion-derived pollutant particles. However, it is not known how this biomarker is affected by changes in exposure. We studied the clearance kinetics of black carbon (BC) in AM, obtained by sputum induction, in a one-year panel study.

Methods: AM BC was measured 8 times with 6 weeks intervals in healthy young subjects: 15 long-term residents in Leuven, Belgium (BE, mean annual PM₁₀ 20-30 μg/m³) and 30 newcomers having arrived recently (< 3 weeks) in Leuven from highly polluted cities (mean annual PM₁₀ > 50 μg/m³) in low and middle-income countries (LMIC, n = 15), or from low to moderately polluted cities in high-income countries (HIC, n = 15). The median and 90th percentile values of AM BC were quantified by image analysis of 25 macrophages per sputum sample; the carbonaceous nature of the black inclusions in AM was verified by Femtosecond Pulsed Laser Microscopy in 30 macrophages. We used a Bayesian hierarchical single-exponential decay model to describe the evolution of AM BC.

Results: In the LMIC group, the mean (95% credible interval) initial quantity (R₀) of median AM BC [1.122 (0.750-1.509) μm²] was higher than in the HIC group [0.387 (0.168-0.613) μm²] and BE group [0.275 (0.147-0.404) μm²]. Median AM BC content decreased in the LMIC group (decay constant 0.013 μm²/day), but remained stable over one year in the other two groups. In the LMIC group, clearance half-lives of 53 (30-99) and 116 (63-231) days, were calculated for median and 90th percentile AM BC, respectively.

Conclusions: In this real-life study of an international panel of healthy young subjects, we demonstrated that carbon load in airway macrophages obtained by induced sputum reflects past long-term exposure to particulate air pollution. Values of AM BC do not change over one year when exposure remains stable, but AM BC decreases upon moving from high to moderate exposure, with average half-lives of 53 and 116 days depending on the carbon load.

Keywords: Air pollution; Airway macrophages; Carbon load; Exposure assessment; Global health; Internal biomarker; Kinetic behavior; Lung clearance.

Fig. 1

Illustration of Femtosecond Pulsed Laser Microscopy detection of BC in a macrophage. a Illustration of an image captured for image analysis showing two airway macrophages (AMs) stained by Diff-Quik from a participant in the LMIC group. The black arrow points out carbon in AM. Image at × 100, bar = 10 μm. b Light microscope (× 100) image of a single AM stained by Diff-Quik. The surface of black inclusions is used to estimate the amount of engulfed BC. c Montage retrieved from the same AM as the one shown in Panel b, of successive images obtained by white light 3D z-stacks scanning with 0.2 μm interval (from left top to right bottom), showing engulfed BC in red. d Bland-Altman plot comparing the difference between the median areas of 30 AM BC obtained from 3D converted images and 2D images. The green line represents the mean of differences in individual values (expressed as natural logarithm) obtained by the two methods; solid lines represent 2 SDs

Fig. 2

AM BC profiles with posterior predictive check for groups. Observed loess-smoothed AM BC profiles (red lines) versus model predicted loess-smoothed AM BC profiles (blue lines). Each dot represents the median value of AM BC obtained from one participant at each one of 8 time points (15 participants in each group). LMIC: low and middle-income countries; BE: Belgium; HIC: high-income countries (except BE)