ABSTRACT
Surfactant protein A (SP-A) is an innate immune molecule that binds foreign organisms that invade the lungs and targets them for phagocytic clearance by the resident pulmonary phagocyte, the alveolar macrophage (AM). We hypothesized that SP-A binds to and enhances macrophage uptake of other nonself particles, specifically apoptotic polymorphonuclear neutrophils (PMNs). PMNs are recruited into the lungs during inflammation, but as inflammation is resolved, PMNs undergo apoptosis and are phagocytosed by AMs. We determined that SP-A increases AM phagocytosis of apoptotic PMNs 280 +/- 62% above the no protein control value. The increase is dose dependent, and heat-treated SP-A still enhanced uptake, whereas deglycosylated SP-A had significantly diminished ability to enhance phagocytosis. Surfactant protein D also increased phagocytosis of apoptotic PMNs by approximately 125%. However, other proteins that are structurally homologous to SP-A, mannose-binding lectin and complement protein 1q, did not. SP-A enhances phagocytosis via an opsonization-dependent mechanism and binds apoptotic PMNs approximately 4-fold more than viable PMNs. Also, binding of SP-A to apoptotic PMNs does not appear to involve SP-A's lectin domain. These data suggest that the pulmonary collectins SP-A and SP-D facilitate the resolution of inflammation by accelerating apoptotic PMN clearance.
Subject(s)
Adjuvants, Immunologic/physiology , Apoptosis/immunology , Macrophages, Alveolar/immunology , Neutrophils/cytology , Neutrophils/immunology , Phagocytosis/immunology , Proteolipids/physiology , Pulmonary Surfactants/physiology , Animals , Carrier Proteins/physiology , Collectins , Complement C1q/physiology , Glycoproteins/physiology , Glycosylation , Hot Temperature , Humans , Jurkat Cells , Ligands , Macrophage Activation , Macrophages, Alveolar/enzymology , Macrophages, Alveolar/metabolism , Male , Neutrophils/enzymology , Neutrophils/metabolism , Opsonin Proteins/physiology , Peroxidase/metabolism , Protein Binding/immunology , Proteolipids/chemistry , Proteolipids/metabolism , Pulmonary Surfactant-Associated Protein A , Pulmonary Surfactant-Associated Protein D , Pulmonary Surfactant-Associated Proteins , Pulmonary Surfactants/chemistry , Pulmonary Surfactants/metabolism , Rats , Rats, Sprague-DawleyABSTRACT
Surfactant protein A (SP-A), a pulmonary member of the collectin family of proteins, facilitates the rapid clearance of pathogens by upregulating immune cell functions in the lungs. SP-A binds to bacteria and targets them for rapid phagocytosis by alveolar macrophages, but the mechanism by which this stimulation occurs is not clear. To characterize the intracellular events that may be involved, we examined the roles of protein phosphorylation and cytoskeletal polymerization in SP-A-stimulated phagocytosis. In rat alveolar macrophages, SP-A stimulated rapid tyrosine phosphorylation of specific proteins in a dose- and time-dependent manner. The pattern of proteins that were phosphorylated in response to SP-A, as resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was similar to that observed for immunoglobulin G (IgG)-stimulated macrophages. Both SP-A and IgG stimulated increases in phagocytosis of Streptococcus pneumoniae above levels in the absence of added protein by 394% +/- 81% and 200% +/- 25%, respectively. Phagocytosis in both cases was dependent on tyrosine kinases, protein kinase C, and actin polymerization but not on microtubule activity. These studies show that SP-A utilizes pathways similar to those used by IgG to increase macrophage phagocytosis of bacteria.
