Determinants of Clara cell protein (CC16) concentration in serum: a reassessment with two different immunoassays

Clara cell protein (CC16) is a 16 kiloDalton protein secreted by Clara cells in the lining fluid of bronchiolar and bronchial epithelium. Recently, Nomori et al., using a nephelometric latex immunoassay, reported a strong correlation between serum CC16 (sCC16) and serum lipids as well as the body mass index (BMI) [Nomori H, Horio H, Takagi M Kobayashi Y, Hirabayashi Y. Clara cell protein correlation with hyperlipidemia. Chest 1996;110:680-4]. The same authors found higher values of sCC16 in males compared to females and did not detect any significant influence of tobacco smoking. Since these results are in disagreement with previous observations showing consistently a decrease of sCC16 in smokers and no influence of sex, we have reassessed in healthy subjects the determinants of sCC16 using two different assays: a particle counting-based latex immunoassay (LIA) using polyclonal antibodies and a fluorescence enzyme immunoassay (FEIA) using monoclonal antibodies. sCC16 was determined in a group of 52 female and 44 male healthy subjects (age 18 to 66 years), including 35 smokers and 61 nonsmokers. sCC16 measured by LIA and FEIA were well correlated (r = 0.92, n = 96, P < 0.0001) with values (geometric mean and range) of 13.3 (5.2-34.5) and 14.7 (4.1-53.1) microg/l, respectively. The determinants of sCC16 measured by both techniques were traced by stepwise regression analysis using as independent variables age, sex, smoking status, BMI or serum lipids (total cholesterol and triglycerides) and the glomerular filtration rate (GFR) estimated on the basis of serum creatinine or beta2-microglobulin. Only two significant determinants emerged: tobacco smoking which correlated negatively and the GFR which correlated positively with sCC16. No influence of serum lipids, BMI, age and sex on sCC16 was detected. We think that an analytical interference with serum lipids explains the results by Nomori et al. which are not confirmed here by two independent techniques and are inconsistent with the current understanding of the physiopathology of the Clara cell and its main secretory product, CC16.