Phagocytosis of microparticles by alveolar macrophages during acute lung injury requires MerTK.

Abstract

Microparticles are a newly recognized class of mediators in the pathophysiology of lung inflammation and injury, but little is known about the factors that regulate their accumulation and clearance. The primary objective of our study was to determine whether alveolar macrophages engulf microparticles and to elucidate the mechanisms by which this occurs. Alveolar microparticles were quantified in bronchoalveolar fluid of mice with lung injury induced by LPS and hydrochloric acid. Microparticle numbers were greatest at the peak of inflammation and declined as inflammation resolved. Isolated, fluorescently labeled particles were placed in culture with macrophages to evaluate ingestion in the presence of endocytosis inhibitors. Ingestion was blocked with cytochalasin D and wortmannin, consistent with a phagocytic process. In separate experiments, mice were treated intratracheally with labeled microparticles, and their uptake was assessed though microscopy and flow cytometry. Resident alveolar macrophages, not recruited macrophages, were the primary cell-ingesting microparticles in the alveolus during lung injury. In vitro, microparticles promoted inflammatory signaling in LPS primed epithelial cells, signifying the importance of microparticle clearance in resolving lung injury. Microparticles were found to have phosphatidylserine exposed on their surfaces. Accordingly, we measured expression of phosphatidylserine receptors on macrophages and found high expression of MerTK and Axl in the resident macrophage population. Endocytosis of microparticles was markedly reduced in MerTK-deficient macrophages in vitro and in vivo. In conclusion, microparticles are released during acute lung injury and peak in number at the height of inflammation. Resident alveolar macrophages efficiently clear these microparticles through MerTK-mediated phagocytosis.

• Authors: Michael P Mohning, Stacey M Thomas, Lea Barthel, Kara J Mould, Alexandria L McCubbrey, S Courtney Frasch, Donna L Bratton, Peter M Henson, William J Janssen
• Journal: American journal of physiology. Lung cellular and molecular physiology (Am J Physiol Lung Cell Mol Physiol) Vol. 314 Issue 1 Pg. L69-L82 (01 01 2018) ISSN: 1522-1504 [Electronic] United States
• PMID: 28935638 (Publication Type: Journal Article, Research Support, N.I.H., Extramural)

Chemical References

• Proto-Oncogene Proteins
• Mertk protein, mouse
• Receptor Protein-Tyrosine Kinases
• c-Mer Tyrosine Kinase
• Axl Receptor Tyrosine Kinase
• AXL receptor tyrosine kinase, mouse

Topics

• Acute Lung Injury (metabolism, physiopathology)
• Animals
• Apoptosis
• Cell-Derived Microparticles (physiology)
• Inflammation (pathology)
• Macrophages, Alveolar (physiology)
• Male
• Mice
• Mice, Inbred C57BL
• Mice, Knockout
• Phagocytosis
• Proto-Oncogene Proteins (genetics, metabolism)
• Receptor Protein-Tyrosine Kinases (genetics, metabolism)
• Signal Transduction
• c-Mer Tyrosine Kinase (physiology)
• Axl Receptor Tyrosine Kinase