PURIFICATION OF A CELL-SURFACE RECEPTOR FOR SURFACTANT PROTEIN-A

In the present report we have characterized the binding of surfactant protein A (SP-A) to bone marrow-derived macrophages, U937 cells, alveo lar macrophages, and type II epithelial cells. The binding of SP-A to all cell types is Ca2+-dependent and trypsin-sensitive, but type II ce lls express distinct Ca2+-independent binding sites. The binding of SP A to macrophages is independent of known cell surface carbohydrate-sp ecific receptors and of glycoconjugate binding sites on the surface of the cells and is distinct from binding to Clq receptors. Based on lig and blot analysis, both type II cells and macrophages express a 210-kD a SP-A-binding protein. The 210-kDa protein was purified to apparent h omogeneity from U937 macrophage membranes using affinity chromatograph y with noncovalently immobilized surfactant protein A, and was purifie d from rat lung by differential detergent and salt extraction of isola ted rat lung membranes. Polyclonal antibodies against the rat lung SP- A-binding protein inhibit binding of SP-A to both type II cells and ma crophages, indicating that the 210-kDa protein is expressed on the cel l surface. The polyclonal antibodies also block the SP-A-mediated inhi bition of phospholipid secretion by type II cells, indicating that the 210-kDa protein is a functional cell-surface receptor on type II cell s. In a separate report we have determined that antibodies to the SP-A receptor block the SP-A-mediated uptake of Mycobacterium bovis, indic ating that the macrophage SP-A receptor is involved in SP-A-mediated c learance of pathogens.