Low plasma CC16 levels in smokers are associated with a higher risk for chronic bronchitis.

Club cell protein (CC16 in humans) plays a crucial role in maintaining homeostasis of the airway epithelium [1] and has anti-inflammatory activity in the lung [2]. Serum CC16 levels are low in cigarette smokers [3] and increase following smoking cessation [4], but patients with chronic obstructive pulmonary disease (COPD) show progressive decrease in CC16 with disease severity [5]. Thus, although the association between circulating levels of CC16 and airflow obstruction is accepted, the association between circulating CC16 and chronic bronchitis, and whether a link in association exists between circulating CC16 levels, lung function decline and chronic bronchitis have not been shown. Therefore, we tested the hypothesis that circulating levels of CC16 are associated with the presence of chronic bronchitis in ever smokers.