Decreased expression of a phagocytic receptor Siglec-1 on alveolar macrophages in chronic obstructive pulmonary disease

Background: Alveolar macrophages are professional phagocytes that remove microbial pathogens inhaled into the lung. The phagocytic ability is compromised in chronic obstructive pulmonary disease (COPD). However, the molecular mechanisms underlying this defect in phagocytosis are not clearly defined. Materials and methods: Cell suspensions were collected from lung tissues of patients undergoing lung resection. Alveolar macrophages were detected as FSC^hi/ SSC^hi/CD45⁺/CD206⁺ cells in the isolated cell suspension by flow-cytometry. The cell surface expression of plasma membrane-bound phagocytic receptors (Fcγreceptor I (FcγRI), a complement receptor CD11b, macrophage scavenger receptor-1 (MSR-1), CD36 and Siglec-1) was determined on the alveolar macrophages. Correlations between the expression levels of the phagocytic receptors and disease severity were analysed. Phagocytosis of fluorescence-tagged bacteria by human alveolar macrophages was evaluated. Results: The flow-cytometry analyses revealed that FcγRI, CD11b, MSR-1 and Siglec-1, but not CD36, were expressed on human alveolar macrophages. Among these receptors, Siglec-1 expression was significantly decreased on alveolar macrophages in COPD ex-smokers (n = 11), compared to control never-smokers (n = 11) or control ex-smokers (n = 9). The Siglec-1 expression on alveolar macrophages was significantly correlated with lung function (forced expiratory volume in 1 s) and with the severity of emphysema. Treatment of human alveolar macrophages with an anti-Siglec1 blocking antibody decreased phagocytosis of non-typeable Haemophilus influenzae (NTHi). Conclusion: Our findings demonstrated reduced expression of Siglec-1 on alveolar macrophages in COPD, which is involved in engulfment of NTHi.