Macrophage M1/M2 polarization dynamically adapts to changes in microenvironment and modulates alveolar bone...

As one of the first arriving immune cells after dental implantation, Mϕs own the abilities to polarize into to a spectrum of diverse phenotypes, from "classically activated" M1 Mϕs to "alternatively activated" M2 Mϕs. Herein, it was hypothesized that Mϕ phenotypes dynamically adapt after dental implantation, and the changes ensue a cascade of coordinated interplay with the bone-forming osteoblast and the bone-resorbing osteoclast. Results showed that the remodelling process after dental implantation was similar with the standard response to tissue injury (exampled by tooth extraction models), only with the delay of bone regeneration phases. Additionally, Mϕ activation in both groups underwent a transition from M1 Mϕs dominated to M2-type dominated stage, but the persistence of M1 Mϕs occurred in rat model of dental implantation. Further research in vitro showed that M1 Mϕs are involved in osteoclast activities via secreting the highest levels of TNF-α and IL-1β, as well as being the potential precursor of osteoclasts. Besides, they also recruited BMSCs by secreting the highest levels of chemoattractants, CCL2 and VEGF. M2 Mϕs accelerated osteogenesis in the subsequent stage via their capability to secrete osteogenesis-related proteins, BMP-2 and TGF-β1. However, the osteogenic differentiation of BMSCs was inhibited when cultured in a high concentration of conditioned media from each Mϕ phenotype, meaning that the immune strategies should be controlled within the proper ranges. These results suggest that coordinated efforts by both M1 and M2 Mϕs for bone remodelling, which may highlight an optimization strategy for tissue engineering implants.