Monocyte Chemoattractant Protein-1 from a Conditioned Medium of Bone Marrow-Derived Mesenchymal Stem Cells Promotes Bone Regeneration by Enhancing Macrophage Phenotype Switching

Objectives: Cytokines and chemokines contained in conditioned media from human bone marrow-derived mesenchymal stem cells (MSC-CM) promote bone regeneration. We recently reported macrophage phenotype switching towards the anti-inflammatory M2 phenotype induced by MSC-CM and its potential to promote bone regeneration. However, the specific factors in the MSC-CM responsible for this process remain unclear. Monocyte chemoattractant protein (MCP) -1, present in MSC-CM, promotes cell migration and activation of the monocyte-macrophage lineage; therefore, we hypothesized that MCP-1 is a key factor in MSC-CM-induced macrophage phenotype switching. In this study, we aimed to elucidate the effect of MCP-1 on MSC-CM-induced macrophage phenotype switching and subsequent bone regeneration. Methods: MCP-1 was depleted from MSC-CM (depMSC-CM) and used in subsequent experiments. Rat bone marrow macrophages were incubated in MSC-CM or depMSC-CM and expression of macrophage markers was examined in vitro. In addition, the effect of MSC-CM and depMSC-CM on bone regeneration and macrophage phenotype switching were evaluated using rat calvaria defect model in vivo. Results: MSC-CM enhanced M2 macrophage marker expression in rat bone marrow macrophages compared to those treated with depMSC-CM in vitro. In addition, MSC-CM increased the number of M2 macrophage marker-positive cells in bone defects and enhanced subsequent bone regeneration in a rat calvarial defect model. Conclusions: MCP-1 plays an essential role in MSC-CM-induced macrophage phenotypic switching and subsequent bone regeneration.