Chronic bronchitis, also known as chronic mucus hypersecretion, is common in cigarette smokers and is a key component of chronic obstructive pulmonary disease (COPD). After the publication of the seminal study by Fletcher and Peto in the 1970s, 1 chronic bronchitis was recognized as being associated with cigarette smoking and conferring a predisposition to lower respiratory infections, but was reported as not influencing the progressive airway obstruction that is characteristic of COPD. 2 However, a number of later studies showed that chronic bronchitis was indeed not a benign feature of COPD and was clearly linked to disease progression, hospital admission, and mortality. 3 Recent data suggest that chronic bronchitis may particularly develop in smokers during middle age and runs a dynamic course, and the longer a patient with COPD has chronic bronchitis, the faster airway disease progresses. 4 Patients with chronic bronchitis are more prone to exacerbations of their COPD than those without chronic bronchitis, and thus sputum production is more common in the frequent-exacerbation phenotype. 5 This is not surprising, because chronic bronchitis is associated with airway bacterial infection and increased airway inflammation. Chronic bronchitis is classically diagnosed from the patient’s history and defined as 3 consecutive months of cough and sputum production over a period of 2 years. 6 However, symptoms of chronic bronchitis may be variable over time, decreasing with smoking reduction or cessation and increasing with COPD exacerbations. 4 Because chronic bronchitis is an important risk factor for the development and progression of COPD, early diagnosis in these patients is important. It would advance the art if there were a validated biomarker for chronic bronchitis, because diagnosis based on patient report may not be accurate.

In this issue of the Journal, Kesimer and colleagues describe the relationship between airway mucins and features of chronic bronchitis. 7 Mucus that is formed in the airways is a protective barrier and consists of globular proteins, macromolecular mucins (especially MUC5AC and MUC5B), and water. Airway mucins are thought to be important in airway mucus transport, but when mucin levels rise above a threshold, the “two-gel hypothesis,” detailed in the report by Kesimer et al., predicts that mucus accumulation occurs, leading to sputum production, increased airway inflammation, infection, and airflow obstruction, which are features consistent with progressive COPD. 8