Summary Chronic obstructive pulmonary disease is a highly prevalent, complex disease, usually caused by cigarette smoke. It causes serious morbidity and mortality and costs the global community billions of dollars per year. While chronic inflammation, extracellular matrix destruction and increased airway epithelial cell apoptosis are reported in chronic obstructive pulmonary disease, the understanding of the basic pathogenesis of the disease is limited and there are no effective treatments. We hypothesized that the accumulation of apoptotic airway epithelial cells chronic obstructive pulmonary disease in could be due to defective phagocytic clearance by alveolar macrophages. There have been no previous studies of the phagocytic capacity of alveolar macrophages in chronic obstructive pulmonary disease using physiologically relevant apoptotic airway epithelial cells as phagocytic targets. We developed a phagocytosis assay whereby cultured 16HBE airway epithelial cells were induced to apoptosis with ultraviolet radiation and stained with mitotracker green. Alveolar macrophages from bronchoalveolar lavage from eight control and six chronic obstructive pulmonary disease subjects were analysed following 1.5 h incubation with apoptotic airway epithelial cells, then staining with macrophage marker anti CD33. CD33+/mitotracker green + events (i.e., alveolar macrophages which had phagocytosed apoptotic airway epithelial cells) were analysed using flow cytometry. Phagocytosis of polystyrene microbeads was investigated in parallel. A significantly reduced proportion of alveolar macrophages from chronic obstructive pulmonary disease subjects ingested apoptotic airway epithelial cells compared with controls (11.6 ± 4.1% for chronic obstructive pulmonary disease versus 25.6 ± 9.2% for control group). Importantly, the deficiency was not observed using polystyrene beads, suggesting that the failure to resolve epithelial damage in chronic obstructive pulmonary disease may result, at least partially, from specific defects in phagocytic ability of alveolar macrophages to ingest apoptotic airway epithelial cells.

Chronic obstructive pulmonary disease (COPD) is associated with defective efferocytosis (apoptosis and alveolar macrophage [AM] phagocytic function) that may lead to secondary necrosis and tissue damage. We investigated ex vivo AM phagocytic ability and recognition molecules (CD36, integrin alphaVbeta3, CD31, CD91, CD44) using flow cytometry. The transferrin receptor (CD71) was measured as an indicator of monocyte-macrophage differentiation in bronchoalveolar lavage (BAL). Proliferation was assessed with Ki-67. Based on evidence of systemic involvement in COPD, blood from 17 current smokers and 25 ex-smokers with COPD, 22 healthy smokers, and 20 never-smoking control subjects was also investigated. BAL was collected from 10 to 16 subjects in each group. Levels of recognition molecules and cAMP were assessed after exposure of AM to cigarette smoke in vitro. The phagocytic ability of AM was significantly decreased in both COPD groups and in healthy smokers compared with control subjects. However, phagocytic capacity was better in subjects with COPD who had ceased smoking, compared with those who were still smoking. AM from current smokers with COPD and healthy smokers exhibited reduced CD31, CD91, CD44, and CD71, and enhanced Ki-67 compared with healthy never-smoker control subjects. There were no differences in these markers in AM from ex-smokers with COPD compared with control subjects, or in blood monocytes from any group. Suppressive effects of cigarette smoke on AM recognition molecules associated with an increase in cAMP were confirmed in vitro. Our data indicates that a smoking-related reduction in AM phagocytic ability and expression of several important recognition molecules may be at least partially normalized in those subjects with COPD who have ceased smoking.

Our findings indicate a novel approach to supplement existing therapies in COPD.