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

Surfactant protein A (SP-A) is the major protein component of pulmonary surfactant, a material secreted by the alveolar type II cell that reduces surface tension at the alveolar air-liquid interface. The function of SP-A in the alveolus is to facilitate the surface tension-lowering properties of surfactant phospholipids, regulate surfactant phospholipid synthesis, secretion, 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 pulmonary surfactant phospholipids and SP-A are present in nonalveolar pulmonary sites as well as in other organs of the body. We describe the structure and possible functions of alveolar SP-A as well as the sites of extra-alveolar SP-A expression and the possible functions of SP-A in these sites.