Surfactant protein A (SP-A): the alveolus and beyond

Surfactant protein A (SP-A) is the major protein component of pulmonary sur factant, a material secreted by the alveolar type II cell that reduces surf ace tension at the alveolar air-liquid interface, The function of SP-A in t he alveolus is to facilitate the surface tension-lowering properties of sur factant phospholipids, regulate surfactant phospholipid synthesis, secretio n, and recycling, and counteract the inhibitory effects of plasma proteins released during lung injury on surfactant function. It has also been shown that SP-A modulates host response to microbes and particulates at the level of the alveolus. More recently, several investigators have reported that p ulmonary surfactant phospholipids and SP-A are present in nonalveolar pulmo nary sites as well as in other organs of the body. We describe the structur e and possible functions of alveolar SP-A as well as the sites of extra-alv eolar SP-A expression and the possible functions of SP-A in these sites.