LECTIN-MEDIATED INTERACTIONS OF SURFACTANT PROTEIN-D WITH ALVEOLAR MACROPHAGES

Surfactant protein D (SP-D) is a calcium-dependent carbohydrate-bindin g protein that is secreted into the pulmonary airspaces by type II epi thelial and Clara cells. Previous studies have shown that SP-D can bin d to specific surfactant phospholipids and to glycoconjugates associat ed with the surface of various microorganisms, consistent with possibl e roles in surfactant metabolism and pulmonary host defense. We now de scribe specific saccharide-mediated interactions of SP-D with alveolar macrophages in lung tissue and in vitro. Biotinylated rat SP-D showed specific binding to alveolar macrophages in sections of rat lung; thi s labeling was inhibited by competing saccharides or EDTA. In addition , the binding of I-125-SP-D to isolated alveolar macrophages in the pr esence of calcium was time-dependent, saturable, and reversible and wa s preferentially inhibited by known monosaccharide and disaccharide li gands for SP-D. Scatchard analysis gave an apparent single class of bi nding sites with a K-d = 1.4 X 10(-6) M. We speculate that the multiva lent structure of SP-D mediates bridging interactions between microbia l glycoconjugates or surfactant phospholipids and specific glycosylate d ligands expressed on the surface of phagocytic cells.