Anionic pulmonary surfactant lipid regulation of innate immunity

Pulmonary surfactant is a proteinand lipid-rich complex synthesized by alveolar type 2 cells and secreted onto the air/ tissue interface of the alveoli [1]. Lipids constitute approximately 90% of pulmonary surfactant and proteins are approximately 10% by weight. The phospholipid content of surfactant is remarkably high, with concentrations estimated to be 35–50 mg/ml [2]. Phosphatidylcholine is the most abundant lipid class present in surfactant; and an unusual lipid molecular species, dipalmitoyl-phosphatidylcholine, is essential for reducing the alveolar surface tension and preventing collapse during the respiratory cycle [3,4]. Approximately 10% of the phospho lipid pool of surfactant consists of the anionic, phosphatidylglycerol (PG) class, with the major molecular species being palmitoyl-oleoyl-PG (POPG) [5,6]. The PG content of surfactant (~3–5 mg/ml) is the highest concentration of this lipid found for any mammalian organ or tissue system [7,8].