SURFACTANT | Surfactant Protein A (SP-A)

Surfactant protein A (SP-A), the most abundant apoprotein of pulmonary surfactant, is a hydrophilic hybrid molecule that belongs to the Ca2+-dependent animal lectin superfamily termed collectins. Collectins are carbohydrate-binding proteins other than antibodies and enzymes that share the common structural characteristics of an N-terminal collagen-like domain connected to a Ca2+-dependent carbohydrate-recognition domain. SP-A has multiple functions including participating in tubular myelin formation, regulating the recycling of surfactant lipids, and acting synergistically with the two lipophilic surfactant apoproteins to lower surface tension. In addition, SP-A plays a major role in host defense of the lung: it is an essential component of innate immunity and helps to activate acquired immunity. SP-A can act either as a proinflammatory or anti-inflammatory agent, increasing or decreasing production of inflammatory cytokines by inflammatory cells to either enhance pathogen killing or suppress inflammation under basal conditions. Finally, it has been shown that SP-A levels are altered during a number of disease states including adult respiratory distress syndrome (ARDS) and cystic fibrosis. Concomitantly, exposure of SP-A to severe inflammation may cause oxidation or nitration of the protein and result in inactivation. Inactivation of SP-A during disease states may therefore cause increased susceptibility to secondary lung infections.