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The interaction of a lung surfactant protein (SP-A) with macrophages is mannose dependent
E Wintergerst1, H Manz-Keinke, H Plattner
1Faculty of Biology, University of Konstanz, Federal Republic of Germany.
Abstract:
Lung surfactant protein A (SP-A) is the main protein component of pulmonary surfactant, which lines the alveolar space. We examined the interaction between recombinant human SP-A and human macrophages or monocytes. Binding and uptake of SP-A adsorbed onto colloidal gold particles was followed by electron microscopy and quantitated on micrographs. SP-A particles were internalized via coated pits/vesicles and transported to secondary lysosomes. Uptake was inhibited in the presence of alpha-D-mannosyl-bovine serum albumin (BSA) but not by beta-D-galactosyl-BSA. Two mannose-dependent recognition mechanisms might mediate SP-A uptake by macrophages. First, as SP-A is a glycoprotein with N-glycosylated glycans it could act as a ligand for the mannose-specific receptor on macrophages. Second, as SP-A is a mannose-specific lectin itself it could bind to mannose residues on the macrophage's cell surface. Activity of the Man-receptor on macrophages was demonstrated with alpha-D-mannosyl-BSA coated onto gold particles. Exposed alpha-D-mannosyl residues on macrophages were identified by Concanavalin A adsorbed onto gold particles. Hence, both mechanisms may be involved in principle. As monocytes have no mannose-specific receptor activity on their cell surface but internalize SP-A gold particles in a mannose-dependent manner, we conclude that at least the second mechanism participates in the recognition of SP-A by macrophages.
Insights
Lung surfactant protein A (SP-A) interacts with macrophages via mannose-dependent pathways. SP-A is recognized by macrophage mannose receptors or binds directly to cell surface mannose residues.
Area of Science:
- Pulmonary biology
- Immunology
- Cellular biology
Background:
- Lung surfactant protein A (SP-A) is a key component of pulmonary surfactant in the alveoli.
- SP-A plays a role in innate immunity and lung defense mechanisms.
Purpose of the Study:
- To investigate the interaction mechanisms between recombinant human SP-A and human macrophages/monocytes.
- To elucidate the role of mannose-dependent recognition in SP-A uptake by immune cells.
Main Methods:
- SP-A adsorbed onto colloidal gold particles was used to study binding and uptake in macrophages and monocytes.
- Electron microscopy was employed to visualize SP-A particle internalization and transport.
- Inhibition assays using mannosyl-BSA and galactosyl-BSA were performed to identify recognition pathways.
Main Results:
- SP-A particles were internalized by macrophages via coated pits and transported to lysosomes.
- SP-A uptake was inhibited by alpha-D-mannosyl-BSA, indicating mannose-dependent recognition.
- Monocytes internalized SP-A in a mannose-dependent manner, despite lacking surface mannose receptor activity.
Conclusions:
- Two mannose-dependent mechanisms potentially mediate SP-A uptake by macrophages: interaction with the mannose receptor or direct binding of SP-A's lectin activity to cell surface mannose.
- The findings suggest that SP-A's lectin activity is crucial for its recognition by both macrophages and monocytes.