Surfactant protein A enhances the phagocytosis of C1q-coated particles by alveolar macrophages

ajplung.00366.2001.—Surfactant protein-A (SP-A) plays mul-tiple roles in pulmonary host defense, including stimulating bacterial phagocytosis by innate immune cells. Previously, SP-A was shown to interact with complement protein C1q. Our goal was to further characterize this interaction and elucidate its functional consequences. Radiolabeled SP-A bound solid-phase C1q but not other complement proteins tested. The lectin activity of SP-A was not required for binding to C1q. Because C1q is involved in bacterial clearance but alone does not efficiently enhance the phagocytosis of most bacteria, we hypothesize that SP-A enhances phagocytosis of C1q-coated antigens. SP-A enhanced by sixfold the percentage of rat alve-olar macrophages in suspension that phagocytosed C1q-coated fluorescent beads. Furthermore, uptake of C1q-coated beads was enhanced when either beads or alveolar macrophages wer