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Extruded erythroblast nuclei are bound and phagocytosed by a novel macrophage receptor
L B Qiu1, H Dickson, N Hajibagheri
1Imperial Cancer Research Fund, Oxford University, UK.
Abstract:
Resident bone marrow macrophages (RBMM) play an important role in clearance of nuclei extruded from late-stage erythroblasts (Eb). To investigate the nature of macrophage receptors involved in this process, extruded erythroblast nuclei (EEN) were purified by cultivation of erythroblasts with erythropoietin, followed by density gradient centrifugation. By electron microscopy, the majority of free nuclei had an intact plasma membrane. EEN bound avidly to RBMM in a divalent cation-independent manner at both 4 degrees C and 37 degrees C. The use of specific monoclonal antibodies (MoAbs) and inhibitors showed that this adhesive interaction was not mediated by previously characterized macrophage receptors involved in recognition of either developing hematopoietic cells or apoptotic cells. The EEN receptor was expressed on resident macrophages isolated from murine bone marrow, spleen, lymph node, and peritoneal cavity, but was completely absent from alveolar macrophages. Despite high levels of EEN binding to RBMM, few were phagocytosed even after prolonged culture. Phorbol myristate acetate (PMA) was found to stimulate phagocytosis, suggesting that this is a regulated process. These findings indicate that EEN are recognized by a novel macrophage receptor and that recognition may be triggered during the membrane remodeling that accompanies enucleation.
Insights
Resident bone marrow macrophages (RBMM) recognize extruded erythroblast nuclei (EEN) via a novel receptor. This interaction is distinct from known pathways and suggests a regulated phagocytosis process.
Area of Science:
- Hematology
- Immunology
- Cell Biology
Background:
- Resident bone marrow macrophages (RBMM) are crucial for clearing extruded erythroblast nuclei (EEN).
- The specific macrophage receptors involved in this recognition process remain largely uncharacterized.
Purpose of the Study:
- To identify and characterize the macrophage receptors responsible for binding extruded erythroblast nuclei (EEN).
- To investigate the mechanism and regulation of EEN clearance by macrophages.
Main Methods:
- Purification of extruded erythroblast nuclei (EEN) using density gradient centrifugation.
- Assessment of EEN binding to RBMM using electron microscopy, monoclonal antibodies (MoAbs), and inhibitors.
- Analysis of EEN phagocytosis by RBMM, including the effect of phorbol myristate acetate (PMA).
Main Results:
- Extruded erythroblast nuclei (EEN) possess intact plasma membranes and bind avidly to RBMM independently of divalent cations.
- The EEN-RBMM interaction is mediated by a novel macrophage receptor, not previously identified receptors for hematopoietic or apoptotic cells.
- This novel EEN receptor is expressed on various resident macrophages but absent in alveolar macrophages.
- Phagocytosis of EEN by RBMM is limited but can be stimulated by PMA, indicating a regulated process.
Conclusions:
- Extruded erythroblast nuclei (EEN) are recognized by a novel macrophage receptor expressed on resident macrophages.
- The recognition and subsequent phagocytosis of EEN represent a regulated process potentially triggered during erythroblast enucleation.
- These findings shed light on the specific mechanisms of nuclear clearance in erythropoiesis.