Bronchoalveolar fibronectin in smokers and nonsmokers

Fibronectin (FN) is an important nonimmune plasma opsonin for binding of fibrin, gelatin (denatured collagen), and staphylococci by phagocytic cells. Alveolar macrophages play a key role in the clearance of particles and microorganisms in the terminal airways, but they lack ready access to high molecular weight plasma proteins such as FN. Therefore, to determine if FN is present in the terminal respiratory tract, we used a sensitive radioimmunoassay to measure FN concentrations in bronchoalveolar lavage (BAL) fluid from 13 nonsmoking and 16 smoking volunteers. Fibronectin was present in the BAL fluid of all subjects, and was immunologically similar to human plasma FN. The mean FN content in BAL fluid from nonsmokers was 7.3 ± 2.9 (SD) μg/mg albumin or 2.7 ± 1.1 μg/mg total protein. In smokers, the mean FN content was 11.3 ± 3.9 μg/mg albumin or 3.9 ± 1.4 μg/ml total protein, values significantly higher than those of nonsmokers (p<0.01 and p<0.02, respectively, for FN/albumin and FN/total protein in smokers versus nonsmokers). The similar (nonsmokers) or elevated (smokers) ratio of FN/albumin in BAL fluid and plasma suggest local production of FN in the lower respiratory tract of humans. One likely source of this FN is the alveolar macrophage because FN is a major secreted protein in vitro. However, there was a poor correlation between the number of recovered macrophages and BAL FN concentrations. Our study demonstrates that: (1) FN is uniformly present in BAL fluid from normal persons, (2) mean FN concentrations in BAL fluid are significantly higher in smokers than in nonsmokers, (3) BAL fluid FN is probably in part produced in the lower respiratory tract. We concluded that FN may serve as an important nonimmune opsonin for macrophage function in the lower respiratory tract of humans.