Macrophages play an important role in material-related immune responses and bone formation, but the functionality of macrophage-derived extracellular vesicles (EVs) in material-mediated bone regeneration is still unclear. Here, we evaluated intracellular communication through small extracellular vesicles (sEVs) and its effects on endogenous bone regeneration mediated by biomimetic intrafibrillarly mineralized collagen (IMC). After implantation in the bone defect area, IMC generated more neobone and recruited more mesenchymal stem cells (MSCs) than did extrafibrillarly mineralized collagen (EMC). More CD63+CD90+ and CD63+CD163+ cells were detected in the defect area in the IMC group than in the EMC group. To determine the functional roles of sEVs, extracellular vesicles from macrophages cultured on different mineralized collagen were isolated, and they showed no morphological differences. However, macrophage-derived sEVs in the IMC group showed an enhanced Young’s modulus and exerted beneficial effects on the osteogenic differentiation of bone marrow MSCs by increasing the expression of the osteoblastic differentiation markers BMP2, BGLAP, COL1, and OSX and calcium nodule formation. Mechanistically, sEVs from IMC-treated macrophages facilitated MSC osteogenesis through the BMP2/Smad5 pathway, and blocking sEV secretion with GW4869 significantly impaired MSC proliferative, immunomodulative and osteogenic potential. Taken together, these findings show that macrophage-derived sEVs may serve as an emerging functional tool in biomaterial-mediated endogenous bone regeneration.

The host immune response to bone biomaterials is vital in determining the fate of scaffolds and also the outcomes of bone regeneration. Mineralized collagen is an ideal tissue-engineering scaffold for bone repair; however, little is known about its immunomodulatory properties after implantation. In this study, extrafibrillarly-mineralized collagen (EMC) and intrafibrillarly-mineralized collagen (IMC) scaffolds with different nanostructures were fabricated and their immunomodulatory properties via macrophage polarization during bone regeneration were investigated. Micro-CT findings showed that the IMC scaffold yielded more new bone formation than the EMC scaffold. In the defect area, more CD68 + CD163 + M2like macrophages were observed in the IMC group, while M1-like macrophages positive for CD68 and inducible nitric oxide synthase (iNOS) increased dramatically in the EMC group. We further demonstrated, from the protein and RNA levels, that M2-associated anti-inflammatory cytokines interleukin (IL)-10 and arginase-1 were highly expressed in the macrophages seeded on the IMC scaffold, while those seeded on the EMC scaffold expressed more M1-related genes iNOS and IL-6. Moreover, the macrophage polarization in response to the nanostructure of mineralized collagen scaffolds influenced the osteogenesis of human bone marrow stromal cells. These findings suggest that the nanostructure of mineralized collagen scaffolds affects macrophage functional polarization during bone regeneration. The immunomodulatory properties of biomaterial scaffolds can be a dictator of bone regeneration outcomes.

The present study lays the foundation for the development of Col-Ap nanocomposite-based bone grafts for future clinical applications in bone regeneration of large periapical lesions after apical curettage or apicoectomy.