RANKL-induced M1 macrophages are involved in bone formation

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 Ml-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.