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Experimental Analysis of Apoptotic Thymocyte Engulfment by Macrophages
Published on: May 24, 2019
Monocytes recruited to the lungs of mice during immune inflammation ingest apoptotic cells poorly
Jeffrey H Jennings1, Derek J Linderman, Bin Hu
1Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, University of Michigan Health System, Ann Arbor, MI 48105, USA.
Abstract:
Apoptotic cells must be cleared to resolve inflammation, but few resident alveolar macrophages (AMo) from normal lungs ingest apoptotic cells. We examined how Mo ingestion of apoptotic cells is altered during immune inflammation induced by intratracheal challenge of primed C57BL/6 mice using sheep red blood cells. Resident AMo were labeled in situ before challenge using intravenous PKH26 to distinguish them from recruited monocytes. Using flow cytometry, we identified phagocytosis of fluorescently-labeled apoptotic thymocytes by alveolar mononuclear phagocytes in vitro and in vivo, and measured surface molecule expression. Intratracheal challenge induced rapid recruitment of monocytes, peaking at Day 3 and decreasing thereafter, whereas numbers of resident AMo did not change significantly. At all times, the percentage of phagocytes ingesting apoptotic thymocytes in vitro was greater among resident AMo (28-45%) than among recruited monocytes (9-19%), but was low in both cell types relative to ingestion of immunoglobulin-opsonized targets. There was also a nonsignificant trend toward lower ingestion by monocytes in vivo. MerTK, a receptor tyrosine kinase crucial for apoptotic cell phagocytosis, was expressed by resident AMo, but not by recruited monocytes. Relative to resident AMo, monocytes recruited to the alveolus ingest apoptotic cells meagerly, possibly due to absence of MerTK expression.
Insights
Resident alveolar macrophages (AMo) clear apoptotic cells better than recruited monocytes during lung inflammation. This difference may be due to monocytes lacking MerTK expression, impacting their phagocytic ability.
Area of Science:
- Immunology
- Cell Biology
- Respiratory Medicine
Background:
- Efficient clearance of apoptotic cells by macrophages is crucial for resolving inflammation.
- Resident alveolar macrophages (AMo) in normal lungs show limited phagocytosis of apoptotic cells.
- Immune inflammation models are needed to study macrophage function during lung injury.
Purpose of the Study:
- To investigate how monocyte and resident AMo phagocytosis of apoptotic cells changes during induced lung inflammation.
- To compare the phagocytic capacity of resident AMo versus recruited monocytes in vivo and in vitro.
- To identify molecular differences, such as receptor expression, that may explain variations in phagocytosis.
Main Methods:
- Induction of immune inflammation in C57BL/6 mice via intratracheal sheep red blood cell challenge.
- In vivo and in vitro labeling of resident AMo using intravenous PKH26.
- Flow cytometry analysis of apoptotic thymocyte phagocytosis by alveolar mononuclear phagocytes.
- Assessment of surface molecule expression, including MerTK, on different macrophage populations.
Main Results:
- Intratracheal challenge rapidly recruited monocytes to the lungs, while resident AMo numbers remained stable.
- Resident AMo demonstrated significantly higher in vitro phagocytosis of apoptotic cells compared to recruited monocytes.
- MerTK, a key receptor for apoptotic cell clearance, was expressed on resident AMo but absent on recruited monocytes.
- A trend towards lower in vivo phagocytosis by monocytes was observed, correlating with absent MerTK expression.
Conclusions:
- Recruited monocytes exhibit significantly impaired phagocytosis of apoptotic cells in the alveolus compared to resident AMo.
- The absence of MerTK expression on recruited monocytes is a likely reason for their reduced ability to clear apoptotic cells.
- Understanding these macrophage population differences is vital for developing strategies to resolve lung inflammation.
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