Neglected immunoregulation: M2 polarization of macrophages triggered by low‐dose irradiation plays an important role in bone regeneration

Abstract

Current studies have found that low-dose irradiation (IR) can promote bone regeneration. However, mechanism studies of IR-triggered bone regeneration mainly focus on the effects of osteoblasts, neglecting the role of the surrounding immune microenvironment. Here in this study, in vitro proliferation experiments showed that low-dose IR ≤2Gy could promote the proliferation of bone marrow mesenchymal stem cells (BMSCs), and qRT-PCR assay showed that low-dose IR ≤2Gy could exert the M2 polarization of Raw264.7 cells, while IR >2Gy inhibited BMSC proliferation and triggered M1 polarization in Raw264.7 cells. The ALP and mineralized nodules staining showed that low-dose IR ≤2Gy not only promoted osteoblast mineralization through IR-triggered osteoblast proliferation but also through M2 polarization of Raw264.7 cells, while high-dose IR >2Gy had the opposite effect. The co-incubation of BMSC with low-dose IR irradiated Raw264.7 cell supernatants increased the mRNA expression of BMP-2 and Osx. The rat cranial defects model revealed that low-dose IR ≤2Gy gradually promoted bone regeneration, while high-dose IR >2 Gy inhibited bone regeneration. Detection of macrophage polarity in peripheral blood samples showed that low-dose IR ≤2Gy increased the expression of CD206 and CD163, but decreased the expression of CD86 and CD80 in macrophages, which indicated M2 polarization of macrophages in vivo, while high-dose IR had the opposite effect. Our finding innovatively revealed that low-dose IR ≤2Gy promotes bone regeneration not only by directly promoting the proliferation of osteoblasts but also by triggering M2 polarization of macrophages, which provided a new perspective for immune mechanism study in the treatment of bone defects with low-dose IR.