Interactions between LPS and lung surfactant proteins.

After penetration into the lower airways, bacterial lipopolysaccharide (LPS) interacts with alveolar cells in a fluid environment consisting of pulmonary surfactant, a lipid-protein complex which prevents alveolar collapsing and participates in lung defense. The two hydrophilic surfactant components SP-A and SP-D are proteins with collagen-like and lectin domains (collectins) able to interact with carbohydrate-containing ligands present on microbial membranes, and with defined regions of LPS. This explains their capacity to damage the bacterial envelope and induce an antimicrobial effect. In addition, they modulate LPS-induced production of pro-inflammatory mediators in leukocytes by interaction with LPS or with leukocyte receptors. A third surfactant component, SP-C, is a small, highly hydrophobic lipopeptide which interacts with lipid A and reduces LPS-induced effects in macrophages and splenocyte cultures. The interaction of the different SPs with CD14 might explain their ability to modulate some LPS responses. Although the alveolar fluid contains other antiLPS and antimicrobial agents, SPs are the most abundant proteins which might contribute to protect the lung epithelium and reduce the incidence of LPS-induced lung injury. The presence of the surfactant collectins SP-A and SP-D in non-pulmonary tissues, such as the female genital tract, extends their field of action to other mucosal surfaces.