Surfactant protein A enhances the phagocytosis of C1q-coated particles by alveolar macrophages.
Surfactant protein-A (SP-A) plays multiple 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 alveolar macrophages in suspension that phagocytosed C1q-coated fluorescent beads. Furthermore, uptake of C1q-coated beads was enhanced when either beads or alveolar macrophages were preincubated with SP-A. In contrast, SP-A had no significant effect on the uptake of C1q-coated beads by alveolar macrophages adhered to plastic slides. We conclude that SP-A may serve a protective role in the lung by interacting with C1q to enhance the clearance of foreign particles.
Watford
Wendy T
WT
Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.
Smithers
Molly B
MB
Frank
Michael M
MM
Wright
Jo Rae
JR
D000818:Animals; D001665:Binding Sites; D002448:Cell Adhesion / Q000502:physiology; D015922:Complement C1q / Q000494:pharmacology*; D004912:Erythrocytes / Q000276:immunology / Q000502:physiology; D007074:Immunoglobulin G / Q000276:immunology; D037102:Lectins / Q000378:metabolism; D016676:Macrophages, Alveolar / Q000502:physiology*; D010587:Phagocytosis / Q000502:physiology*; D037662:Pulmonary Surfactant-Associated Protein A / Q000378:metabolism / Q000502:physiology*; D012756:Sheep
D007074:Immunoglobulin G; D037102:Lectins; D037662:Pulmonary Surfactant-Associated Protein A; D015922:Complement C1q