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Surfactant protein A stimulates phagocytosis of specific pulmonary pathogens by alveolar macrophages
1Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.
Abstract:
Surfactant protein A (SP-A) regulates alveolar macrophage function and has been implicated in the mediation of pulmonary host defense. Our goals were to characterize the interaction of SP-A with various pulmonary pathogens, to investigate the mechanism of SP-A-mediated phagocytosis using an assay that distinguishes bound from internalized bacteria by quenching the fluorescence of extracellular bacteria, and to examine further the interactions of SP-A and the structurally homologous protein complement component 1q (C1q) with alveolar macrophages and peripheral blood monocytes. We found that SP-A binds to and increases the phagocytosis of Haemophilus influenzae, Streptococcus pneumoniae, and Group A Streptococcus; SP-A aggregates only H. influenzae. SP-A neither binds to, aggregates, nor stimulates the phagocytosis of Pseudomonas aeruginosa. We have also found that bronchoalveolar lavage stimulates phagocytosis and that this stimulation is reduced by an anti-SP-A antibody. While the enhancement of phagocytosis by SP-A is inhibited in blood monocytes adhered to C1q-coated surfaces, which presumably clusters the C1q receptor on the basal surface of the cell, alveolar macrophages on C1q-coated slides show no significant change in their response to SP-A. In summary, SP-A stimulates the phagocytosis by alveolar macrophages of specific pulmonary pathogens to which it binds, but aggregation is not required for the effect. Additionally, the role of the C1q receptor in the response to SP-A may differ between monocytes and alveolar macrophages.
Insights
Surfactant protein A (SP-A) enhances the engulfment of specific bacteria by lung macrophages, aiding host defense. Its interaction with C1q receptors differs between lung and blood immune cells.
Area of Science:
- Pulmonary immunology
- Host defense mechanisms
- Innate immunity
Background:
- Surfactant protein A (SP-A) is crucial for alveolar macrophage function and pulmonary host defense.
- Understanding SP-A's interactions with pathogens and immune cells is key to elucidating its role in lung immunity.
Purpose of the Study:
- To characterize SP-A's interaction with diverse pulmonary pathogens.
- To investigate the mechanism of SP-A-mediated phagocytosis.
- To compare SP-A and C1q interactions with alveolar macrophages and peripheral blood monocytes.
Main Methods:
- Bacterial binding and aggregation assays.
- Phagocytosis assays using fluorescence quenching to differentiate bound and internalized bacteria.
- Co-culture experiments with C1q-coated surfaces to assess immune cell responses.
Main Results:
- SP-A binds and enhances phagocytosis of *Haemophilus influenzae*, *Streptococcus pneumoniae*, and Group A Streptococcus, but only aggregates *H. influenzae*.
- SP-A does not interact with *Pseudomonas aeruginosa*.
- Bronchoalveolar lavage fluid enhances phagocytosis, an effect reduced by anti-SP-A antibodies.
- SP-A-mediated phagocytosis enhancement is inhibited in monocytes on C1q surfaces, but not in alveolar macrophages.
Conclusions:
- SP-A stimulates phagocytosis of specific pulmonary pathogens by alveolar macrophages, independent of bacterial aggregation.
- The C1q receptor's role in SP-A-mediated responses may differ between alveolar macrophages and peripheral blood monocytes.