Pulmonary Surfactant Protein A Modulates the Cellular Response to Smooth and Rough Lipopolysaccharides by Interaction with CD14

Pulmonary surfactant protein A (SP-A) plays an important part in Ab-independent host defense mechanisms of the lung. In this study we investigated how SP-A interacts with distinct serotypes of bacterial LPS and modulates LPS-elicited cellular responses. SP-A bound to rough forms but not to smooth forms of LPS. In the macrophage-like cell line U937, SP-A inhibited mRNA expression and secretion of TNF-alpha induced by smooth LPS, but rough LPS-induced TNF-alpha expression was unaffected by SP-A. When U937 cells and rat alveolar macrophages were preincubated with SP-A, smooth LPS failed to induce TNF-alpha secretion, whereas rough LPS-induced TNF-alpha secretion was modestly increased. To clarify the mechanism by which SP-A modulates LPS-elicited cellular responses, we further examined the interaction of SP-A with CD14, which is known as a major LPS receptor. Western blot analysis revealed that CD14 was one of the SP-A binding proteins isolated from solubilized U937 cells. In addition, SP-A directly bound to recombinant soluble CD14 (rsCD14). When rsCD14 was preincubated with SP-A, the binding of rsCD14 to smooth LPS was significantly reduced but the association of rsCD14 with rough LPS was augmented. These results demonstrate the different actions of SP-A upon distinct serotypes of LPS and indicate that the direct interaction of SP-A with CD14 constitutes a likely mechanism by which SP-A modulates LPS-elicited cellular responses.