Macrophages at Low-Inflammatory Status Improved Osteogenesis via Autophagy Regulation

Abstract

Accumulating evidence indicates that the interaction between immune and skeletal systems is vital in bone homeostasis. However, the detailed mechanisms between macrophage polarization and osteogenic differentiation of mesenchymal stromal cells (bone marrow-derived stromal cells [BMSCs]) remain largely unknown. We observed enhanced macrophage infiltration along with bone formation in vivo, which showed a transition from early-stage M1 phenotype to later stage M2 phenotype, cells at the transitional stage expressed both M1 and M2 markers that actively participated in osteogenesis, which was mimicked by stimulating macrophages with lower inflammatory stimulus (compared with typical M1). Using conditioned medium (CM) from M0, typical M1, low-inflammatory M1 (M1^semi), and M2 macrophages, it was found that BMSCs treated with M1^semi CM showed significantly induced migration, osteogenic differentiation, and mineralization, compared with others. Along with the induced osteogenesis, the autophagy level was the highest in M1^semi CM-treated BMSCs, which was responsible for BMSC migration and osteogenic differentiation, as autophagy interruption significantly abolished this effect. This study indicated that low-inflammatory macrophages could activate autophagy in BMSCs to improve osteogenesis.

Impact statement

The interaction between macrophage polarization and osteogenic differentiation of mesenchymal stromal cells (bone marrow-derived stromal cells [BMSCs]) plays a decisive role in bone homeostasis, whereas the detailed mechanisms remain largely unknown. In this study, we found that the macrophage at low-inflammatory status (M1^semi), which expressed both M1 and M2 markers, was actively involved in osteogenesis, which induced the migration, osteogenic differentiation, and mineralization of BMSCs, compared with M0, M1, and M2 macrophages, partially achieved by activating autophagy in BMSCs. This study identified that the low-inflammatory macrophage, as the transitional macrophage during the M1-to-M2 switch in bone formation, could activate autophagy in BMSCs to improve osteogenesis.