Increased expression of Siglec-9 in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a common inflammatory lung disease. Sialic acid-binding immunoglobulin-type lectins 9 (Siglec-9) is predominantly expressed on innate immune cells and has been shown to exert regulatory effect on immune cells through glycan recognition. Soluble Siglec-9 (sSiglec-9), the extracellular region of Siglec-9, might fulfill its function partly by competitive inhibiting siglec-9 binding to its ligands; however, the role of Siglec-9 and sSiglec-9 in the pathogenesis COPD remain largely unknown. In this study, we showed that Siglec-9 expression in alveolar and peripheral blood neutrophil were increased in COPD patients by immunofluorescence and flow cytometry, respectively. Plasma levels of sSiglelc-9 were elevated in COPD patients by ELISA. In vitro, Siglec-9 expression and/or sSiglelc-9 levels were up-regulated by cigarette smoke extract (CSE), lipopolysaccharide (LPS), some cytokines, and dexamethasone (DEX). Recombinant sSiglce-9 increased oxidative burst in neutrophil and enhanced neutrophil chemotaxis toward IL-8 independent on CXCR1 and CXCR2 expression, but it did not affect neutrophil apoptosis or secretions of inflammatory cytokines. In conclusion, Siglec-9 was complementarily increased to induce a negative feedback loop to limit neutrophil activation in COPD, sSiglce-9 enhanced neutrophil ROS and chemotaxis toward IL-8 likely via competitively inhibiting ligands binding to Siglec-9.