Production of the acute-phase protein lipopolysaccharide-binding protein by respiratory type II epithelial cells: implications for local defense to bacterial endotoxins

Abstract

This study demonstrates for the first time that respiratory epithelial cells are able to produce the acute phase protein lipopolysaccharide (LPS)-binding protein (LBP), which is known to play a central role in the defense to bacterial endotoxins (or LPS). Indications for local presence of LBP in human lung was obtained via reverse transcriptase/polymerase chain reaction that showed LBP messenger RNA (mRNA) expression. Therefore, LBP production by the human lung epithelial cell line A549, a human adenocarcinoma with features of type II pneumocytes, was studied. These cells produced LBP in response to interleukin (IL)-1beta, IL-6, and tumor necrosis factor- alpha, a response that was strongly enhanced by dexamethasone. In addition, LBP mRNA was detected in A549 cells, in increasing amounts as a result of stimulation. The pattern of cytokine-induced LBP production in A549 cells was similar to the pattern in the human liver epithelial cell line HuH-7. Moreover, the molecular weight of A549-derived LBP was approximately 60 kD, which is similar to HuH-7-derived LBP. Biologic activity of LBP produced by A549 cells was evaluated on the basis of its ability to interact with LPS. Further indications that type II alveolar epithelial cells are able to produce LBP were obtained from the observations that the murine lung type II epithelial cell line C10 produced murine LBP, and that isolated human primary type II pneumocytes expressed LBP mRNA, which was enhanced after stimulation of cells. The local production of this endotoxin binding protein by lung epithelial cells might contribute to a highly specific response at the site of exposure to bacteria and bacterial endotoxins.