Pulmonary Surfactant

Pulmonary surfactant, synthesized by type II alveolar cells and nonciliated epithelial cells (Clara cells), is a surface-active material that reduces the surface tension in the lungs. The main components of surfactant are phospholipids, neutral lipids, serum proteins, and surfactant proteins, including SP-A, SP-B, SPC, and SP-D [159]. Surfactant proteins play integral roles in lowering surface tension and maintaining host defense [159]. SP-B and SP-C are hydrophobic proteins involved primarily in the adsorption and spreading of surfactant phospholipids at the air-liquid interface. SP-A (specific for mannose, glucose, galactose, and fucose) and SP-D (specific for maltose, mannose, and glucose) are hydrophilic proteins that act as the first line of host defense to inhaled pathogens. SP-A and SP-D are members of the collectin subgroup of mammalian C-type lectins that function in host immunity by binding carbohydrate ligands on the surface of invading particles, marking the complexes for destruction by alveolar macrophages [8,160,161]. SP-A stimulates the chemotaxis of alveolar macrophages and enhances their binding to viral and bacterial invaders [8,162]. SP-D binds to alveolar macrophages, polymorphonuclear cells, and various viral, fungal, and bacterial components, thereby playing a role in the regulation of inflammation [163]. As a result, surfactant proteins may reduce the efficiency of gene delivery when DNA vectors contain carbohydrate moieties [8]. For more information on the role of surfactants in the lung, see Refs. 159 and 164.

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