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Development of a biomarker for lung inflammation in COPD through analysis of labelled leukocyte transit through the lung circulation

CORRESPONDENCE: Emma H. Baker, St. George's, University of London, London, UK

Measurement of pulmonary leukocyte margination could be a useful biomarker of lung inflammation in COPD, but analysis is complicated by recirculation of labeled leukocytes.

15 minutes of planar nuclear images were obtained after injecting autologous ^99mTc-labeled leukocytes in 4 never-smoked controls, 6 stable mild/moderate and 2 exacerbating COPD patients. COPD patients were also imaged for 10 minutes after in vivo red blood cell (RBC) labeling with ^99mTc and were re-imaged 2 weeks later to determine reproducibility. Activity as a function of time was measured in regions of interest over lungs and heart.

A multi-compartment mathematical model was used to correct for recirculation but failed to provide a biomarker that clearly separated controls from COPD. A simpler model for activity in lungs and heart as a function of time (A(t)) was applied only to time points before recirculation: [A(t) = X₁ x (1-exp(-X₂ x t)) x exp(-X₃ x t), where X_i are adjusted to match the data]. The ratio (R) of X₃ (downslope of curve) in the lungs to X₃ in the heart was investigated as a biomarker of margination. Values of R were reasonable (1.06±0.08 (SE)) for RBC (i.e. no margination) and 0.76±0.10 for controls (25% margination). In stable COPD patients R was significantly smaller (0.19±0.09, p<0.01) than controls and was reproducible (0.25±0.10). R during exacerbation was surprisingly large (0.88±0.22), possibly due to steroid treatment, but R was similar to stable COPD patients 2 weeks later (0.09±0.04).

R requires only 2–3 minutes of imaging and may be a useful biomarker of margination. However it remains to be shown whether R truly reflects inflammation in COPD.