Early decrease of serum Clara cell protein in silica-exposed workers.

Clara cell protein (CC16) is a 16 kDa protein secreted by nonciliated cells of the tracheobronchial tree; it has recently been proposed as a peripheral marker of respiratory epithelial injury. The concentration of CC16 was measured in the serum and, when available, in the sputum of 86 miners exposed to silica and of 86 control subjects matched for age, body mass index and smoking status (26 lifelong nonsmokers and 60 current smokers in both groups). Workers were exposed to silica-rich dust in a quarry for 15.2 months on average. No difference between exposed and control workers could be detected with regard to respiratory symptoms, chest radiographs or lung function tests. By contrast, the concentration of CC16 in serum was decreased in silica-exposed workers (geometric mean 12.3 micrograms.l-1) compared to controls (16.3 micrograms.l-1). The decrease was found both in lifelong nonsmokers (14.7 vs 21.9) and current smokers (11.3 vs 14.5). In the latter, tobacco smoking caused a decrease of serum CC16 that was additional to that associated with silica exposure. The determination of CC16 in sputum samples, judged to be reliable on the basis of the CC16/alpha-amylase concentration ratio (mostly from smokers), also revealed a reduction of CC16 following silica exposure (46.2 vs 106 mg.l-1). We conclude that alterations in the serum concentrations of CC16 probably reflect very early toxic effects of silica particles on the respiratory epithelium. This reinforces the view that serum CC16 is a sensitive marker, which might improve our ability to detect exposure to chemicals potentially harmful to the respiratory tract.