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CD14 is a component of multiple recognition systems used by macrophages to phagocytose apoptotic lymphocytes
R A Schlegel1, S Krahling, M K Callahan
1Department of Biochemistry and Molecular Biology, Penn State University, University Park, Pennsylvania 16802, USA. ur3@psu.edu
Abstract:
Expression of the aminophospholipid phosphatidylserine (PS) on the surface of apoptotic lymphocytes and lipid-symmetric erythrocytes triggers their phagocytosis by macrophages. Phagocytosis by both activated and unactivated macrophages, which utilize different recognition systems, can be blocked by certain monoclonal antibodies directed against the LPS receptor, CD14. Here we investigate the requirement for CD14 in the phagocytosis of both apoptotic thymocytes and lipid-symmetric erythrocytes by both activated and unactivated macrophages. We show that phagocytosis of lipid-symmetric erythrocytes by both activated and unactivated macrophages is completely abolished when CD14 is removed from macrophages by cleaving its glycosylphosphatidylinositol tether with phospholipase C. This treatment also substantially reduces phagocytosis of apoptotic lymphocytes by both types of macrophages. Unactivated LR-9 mouse macrophages which are deficient in CD14 expression are completely unable to phagocytose either apoptotic thymocytes or lipid-symmetric erythrocytes. These results argue that CD14 is an absolute requirement for the phagocytosis of lipid-symmetric erythrocytes by both activated and unactivated macrophages, despite their different recognition systems, that CD14 contributes at least substantially to the phagocytosis of apoptotic lymphocytes by both activated and unactivated macrophages, and that activated macrophages may also possess an alternate, CD14-independent mechanism for phagocytosis of apoptotic lymphocytes.
Insights
CD14 is essential for macrophages to engulf lipid-symmetric erythrocytes. This receptor also significantly aids in the uptake of apoptotic lymphocytes, though macrophages may have alternative pathways.
Area of Science:
- Immunology
- Cell Biology
Background:
- Macrophages engulf cells displaying phosphatidylserine (PS) on their surface.
- CD14, the LPS receptor, is implicated in macrophage recognition systems.
Purpose of the Study:
- To investigate the role of CD14 in phagocytosis of apoptotic thymocytes and lipid-symmetric erythrocytes.
- To compare the requirement of CD14 in both activated and unactivated macrophages.
Main Methods:
- Macrophages were treated with phospholipase C to cleave CD14.
- CD14-deficient LR-9 mouse macrophages were used.
- Phagocytosis assays were performed with apoptotic thymocytes and lipid-symmetric erythrocytes.
Main Results:
- CD14 removal completely abolished lipid-symmetric erythrocyte phagocytosis by both macrophage types.
- CD14 removal substantially reduced apoptotic lymphocyte phagocytosis.
- CD14-deficient macrophages were unable to phagocytose either cell type.
Conclusions:
- CD14 is indispensable for lipid-symmetric erythrocyte phagocytosis by macrophages.
- CD14 significantly contributes to apoptotic lymphocyte phagocytosis.
- Activated macrophages may utilize a CD14-independent mechanism for apoptotic lymphocyte uptake.