Mucin Binding to Moraxella catarrhalis during Airway Inflammation Is Dependent on Sialic Acid

Chronic obstructive pulmonary disease (COPD) is associated with colonization by bacterial pathogens and repeated airway infections, leading to exacerbations and impaired lung function. The highly glycosylated mucins in the mucus lining the airways are an important part of the host defense against pathogens. However, mucus accumulation can contribute to COPD pathology. Here, we examined whether inflammation is associated with glycosylation changes that affect interactions between airway mucins and pathogens. We isolated mucins from lower airway samples (n = 4-9) from long-term smokers with and without COPD and from never-smokers. The most abundant terminal glycan moiety was N-acetylneuraminic acid (Neu5Ac) among smokers with and without COPD and N-acetyl-hexoseamine among never-smokers. Moraxella catarrhalis bound to MUC5 mucins from smokers with and without COPD. M. catarrhalis binding correlated with inflammatory parameters and Neu5Ac content. M. catarrhalis binding was abolished by enzymatic removal of Neu5Ac. Furthermore, M. catarrhalis bound to α2,6 sialyl-lactose, suggesting that α2,6 sialic acid contributes to M. catarrhalis binding to mucins. Furthermore, we detected more M. catarrhalis binding to mucins from patients with pneumonia than to those from control subjects (n = 8-13), and this binding correlated with C-reactive protein and Neu5Ac levels. These results suggest a key role of inflammation-induced Neu5Ac in the adhesion of M. catarrhalis to airway mucins. The inflammation-induced ability of MUC5 mucins to bind M. catarrhalis is likely a host defense mechanism in the healthy lung, although it cannot be excluded that impaired mucociliary clearance limits the effectiveness of this defense in patients with COPD.