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RANKL-induced M1 macrophages are involved in bone formation
Rong Huang1, Xin Wang1,2, Yinghong Zhou1
1The Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, Queensland, Australia.
Abstract:
The activation of M1 macrophages can be achieved by stimulating them with lipopolysaccharide (LPS) and interferon-γ (IFN-γ). However, M1 can be found under physiological conditions without any pathological stimuli. This study aimed to understand the involvement of RANKL-induced M1 macrophages in bone formation compared with pathologically induced macrophages. Fischer rats were used to investigate macrophage distribution in normal and injured femoral condyles in vivo. Bone marrow-derived macrophages (BMDMs) were activated with LPS+IFN-γ and RANKL to achieve M1 activation in vitro. Gene expression related to inflammation, osteoclastogenesis, angiogenesis, and migration was determined by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and fluorescence-activated cell sorting (FACS). Tissue macrophages showed distinct expression patterns at different bone regions. RANKL was found in close proximity to inducible nitric oxide synthase-positive (iNOS+) cells in vivo, suggesting an association between RANKL expression and iNOS+ cells, especially in trabecular bone. RANKL-induced macrophages showed a different cytokine secretion profile compared with pathologically induced macrophages. Both osteoclasts and M1 macrophages peaked on day 7 during bone healing. RANKL could trigger M1-like macrophages with properties that were different from those of LPS+IFN-γ-induced macrophages. These RANKL-activated M1 macrophages were actively involved in bone formation.
Insights
Receptor activator of nuclear factor-kappa B ligand (RANKL) triggers M1 macrophages involved in bone formation. These RANKL-induced M1 macrophages differ from pathologically induced ones, playing a key role in physiological bone remodeling.
Area of Science:
- Immunology
- Bone Biology
- Cellular Biology
Background:
- M1 macrophages, typically activated by lipopolysaccharide (LPS) and interferon-gamma (IFN-γ), are implicated in inflammation.
- M1 macrophages can also exist under physiological conditions, independent of pathological stimuli.
- The role of M1 macrophages in bone formation, particularly those induced by RANKL, requires further elucidation.
Purpose of the Study:
- To investigate the involvement of RANKL-induced M1 macrophages in bone formation.
- To compare RANKL-induced M1 macrophages with pathologically induced M1 macrophages.
- To understand macrophage distribution and function in normal and injured bone microenvironments.
Main Methods:
- In vivo analysis of macrophage distribution in Fischer rat femoral condyles.
- In vitro activation of bone marrow-derived macrophages (BMDMs) using LPS+IFN-γ and RANKL.
- Gene expression analysis (RT-qPCR) and fluorescence-activated cell sorting (FACS) for inflammation, osteoclastogenesis, angiogenesis, and migration markers.
Main Results:
- Distinct macrophage expression patterns were observed in different bone regions in vivo.
- RANKL expression was found near inducible nitric oxide synthase-positive (iNOS+) cells, particularly in trabecular bone.
- RANKL-induced M1 macrophages exhibited a different cytokine profile and actively participated in bone formation, peaking alongside osteoclasts during bone healing.
Conclusions:
- RANKL can induce M1-like macrophages with distinct properties compared to LPS+IFN-γ-induced macrophages.
- These RANKL-activated M1 macrophages play a significant role in physiological bone formation.
- Macrophage involvement in bone healing is dynamic, with M1 macrophages peaking during the repair process.
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