SURFACTANT PROTEIN-A AND SURFACTANT PROTEIN-D IN HEALTH AND DISEASE

Surfactant protein (SP) A and SP-D are collagenous glycoproteins with multiple functions in the lung. Both of these proteins are calcium-dep endent lectins and are structurally similar to mannose-binding protein and bovine conglutinin. Both form polyvalent multimeric structures fo r interactions with pathogens, cells, or other molecules. SP-A is an i ntegral part of the surfactant system, binds phospholipids avidly, and is found in lamellar bodies and tubular myelin. Initially, most resea rch interest focused on its role in surfactant homeostasis. Recently, more attention has been placed on the role of SP-A as a host defense m olecule and its interactions with pathogens and phagocytic cells. SP-D is much less involved with the surfactant system. SP-D appears to be primarily a host defense molecule that binds surfactant phospholipids poorly and is not found in lamellar inclusion bodies or tubular myelin . Both SP-A and SP-D bind a wide spectrum of pathogens including virus es, bacteria, fungi, and pneumocystis. In addition, both molecules hav e been measured in the systemic circulation by immunologic methods and may be useful biomarkers of disease. The current challenges are chara cterization of the three-dimensional crystal structure of SP-A and SP- D, molecular cloning of their receptors, and determination of their pr ecise physiological functions in vivo.