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Free actin impairs macrophage bacterial defenses via scavenger receptor MARCO interaction with reversal by plasma
Christine M Ordija1, Terry Ting-Yu Chiou1,2, Zhiping Yang1
1Department of Environmental Health, Harvard T. H. Chan School of Public Health, Boston, Massachusetts.
Abstract:
Lung injury can release intracellular actin into the alveolar milieu and is also associated with increased susceptibility to secondary infections. We investigated the effect of free (extracellular) actin on lung macrophage host defense functions. Western blot analysis demonstrated free actin release into the lung lavage fluids of mouse models of ozone injury, influenza infection, and secondary pneumococcal pneumonia and in samples from patients following burn and inhalation injury. Using levels comparable with those observed in lung injury, we found that free actin markedly inhibited murine lung macrophage binding and uptake in vitro of S. pneumoniae, S. aureus, and E. coli, (e.g., S. pneumoniae, mean %inhibition, actin vs. vehicle: 85 ± 0.3 (SD); n = 22, P < .001). Similar effects were observed on the ability of primary human macrophages to bind and ingest fluorescent Saureus (~75% inhibition). Plasma gelsolin (pGSN), a protein that functions to bind and cleave actin, restored bacterial binding and uptake by both murine and human macrophages. Scavenger receptor inhibitors reduced binding of fluorescent actin by murine macrophages [fluorescence index (×10-3) after incubation with vehicle, actin, or actin + polyinosinic acid, respectively: 0.8 ± 0.7, 101.7 ± 50.7, or 52.7 ± 16.9; n = 5-6, P < 0.05]. In addition, actin binding was reduced in a MARCO/SR-AI/II-deficient cell line and by normal AMs obtained from MARCO-/- mice. After release from injured cells during lung injury, free actin likely contributes to impaired host defense by blocking scavenger receptor binding of bacteria. This mechanism for increased risk of secondary infections after lung injury or inflammation may represent another target for therapeutic intervention with pGSN.
Insights
Extracellular actin released during lung injury impairs macrophage bacterial clearance. Plasma gelsolin (pGSN) can restore this host defense function, offering a potential therapeutic target for secondary infections.
Area of Science:
- Pulmonary immunology
- Host-pathogen interactions
- Cellular microbiology
Background:
- Lung injury releases intracellular actin, potentially compromising host defense.
- Increased susceptibility to secondary infections is a known complication of lung injury.
- The role of extracellular actin in macrophage function during lung injury requires elucidation.
Purpose of the Study:
- To investigate the impact of extracellular actin on lung macrophage phagocytic capacity.
- To determine if plasma gelsolin (pGSN) can counteract the effects of extracellular actin.
- To identify the mechanisms by which extracellular actin interferes with macrophage bacterial uptake.
Main Methods:
- Western blot analysis of lung lavage fluid for free actin.
- In vitro assays measuring macrophage binding and uptake of bacteria (S. pneumoniae, S. aureus, E. coli).
- Experiments utilizing plasma gelsolin (pGSN), scavenger receptor inhibitors, and MARCO-deficient cell lines.
Main Results:
- Free actin significantly inhibited bacterial binding and uptake by murine and human macrophages.
- Plasma gelsolin (pGSN) restored macrophage phagocytic function in the presence of extracellular actin.
- Extracellular actin binding to macrophages was mediated by scavenger receptors, including MARCO.
Conclusions:
- Extracellular actin released during lung injury impairs macrophage-mediated bacterial clearance.
- This impairment is likely due to actin blocking scavenger receptor recognition of bacteria.
- Plasma gelsolin (pGSN) shows promise as a therapeutic agent to restore host defense against secondary infections following lung injury.