Human bronchoalveolar fibronectin. Quantification of an opsonic protein and its synthesis by alveolar macrophages

Opsonins are critical for phagocytosis of particulate material and microorganisms by pulmonary alveolar macrophages (PAM) in the terminal airways. The opsonic protein fibronectin (FN) is a 440,000 mol wt glycoprotein mediating clearance of gelatin-coated particles from blood by reticuloendothelial cells. Increasing evidence suggests that specific cell-membrane receptors for FN mediate these interactions. Because of its potential importance as a nonimmune opsonin for PAM in the bronchoalveolar space, we studied the following: 1) the FN content in bronchoalveolar lavage (BAL) fluid; 2) whether PAM are a possible source of BAL-FN, and 3) if endogenous PAM-FN serves as an opsonin for particles with known FN-binding properties. (1) FN is present in BAL fluid of healthy humans; (2) mean FN levels in BAL fluid are significantly higher in smokers than in nonsmokers; (3) PAM are a likely source of FN in BAL fluid; (4) FN is not a membrane-bound receptor in PAM, but appears at the cell-membrane binding sites for gelatin-coated particles. Taken together, these findings suggest that PAM-FN may serve as a specific endogenous opsonin for uptake of denatured collagen, fibrin, or staphylococci in the lower respiratory tract of humans.