Lectin-mediated interactions of surfactant protein D with alveolar macrophages

Surfactant protein D (SP-D) is a calcium-dependent carbohydrate-binding protein that is secreted into the pulmonary airspaces by type II epithelial and Clara cells. Previous studies have shown that SP-D can bind to specific surfactant phospholipids and to glycoconjugates associated with the surface of various microorganisms, consistent with possible roles in surfactant metabolism and pulmonary host defense. We now describe 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; this labeling was inhibited by competing saccharides or EDTA. In addition, the binding of 125I-SP-D to isolated alveolar macrophages in the presence of calcium was time-dependent, saturable, and reversible and was preferentially inhibited by known monosaccharide and disaccharide ligands for SP-D. Scatchard analysis gave an apparent single class of binding sites with a Kd = 1.4 x 10(-6) M. We speculate that the multivalent structure of SP-D mediates bridging interactions between microbial glycoconjugates or surfactant phospholipids and specific glycosylated ligands expressed on the surface of phagocytic cells.