LUNG SURFACTANT PROTEIN-A (SP-A) ACTIVATES A PHOSPHOINOSITIDE CALCIUMSIGNALING PATHWAY IN ALVEOLAR MACROPHAGES/

Lung surfactant protein A (SP-A), the main protein component of lung s urfactant which lines the alveoli, strongly enhances serum-independent phagocytosis of bacteria by rat alveolar macrophages, We tested if th e effect of SP-A is due to interaction with the macrophages or to opso nization of the bacteria, In phagocytosis assays with fluorescein isot hiocyanate labeled bacteria, SP-A had no opsonic effect on Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus, but enhanced phagocytosis by acting only on the macrophages, We characterized this activation mechanism. With single cell measurements of fura-2 loaded c ells we demonstrate that SP-A raises the intracellular free calcium io n concentration 6 to 8 seconds after addition, This calcium mobilizati on is dose-dependent in that increased SP-A concentrations lead to a h igher percentage of responding cells, Additionally, SP-A leads to a do se-dependent and transient generation of inositol 1,4,5-trisphosphate. Release of intracellular stored calcium by SP-A is a prerequisite for its stimulatory effect on phagocytosis, since SP-A-induced enhancemen t of phagocytosis can be impaired by prior addition of thapsigargin, a Ca2+-ATPase inhibitor that leads to depletion of intracellular calciu m stores. We conclude that SP-A activates phosphoinositide/calcium sig naling pathway in alveolar macrophages leading to enhanced serum-indep endent phagocytosis of bacteria.