Adoptive transfer of immunomodulatory macrophages reduces the pro-inflammatory microenvironment and increases bone formation on titanium implants

Macrophages play a central role in orchestrating the inflammatory response to implanted biomaterials and are sensitive to changes in the chemical and physical characteristics of the implant. Macrophages respond to biological, chemical, and physical cues by polarizing into pro-inflammatory (M1) or anti-inflammatory (M2) states. We previously showed that rough-hydrophilic titanium (Ti) implants skew macrophage polarization towards an anti-inflammatory phenotype and increase mesenchymal stem cell (MSC) recruitment and bone formation around the implant. In the present study, we aimed to investigate whether the adoptive transfer of macrophages in different polarization states would alter the inflammatory microenvironment and improve biomaterial integration in macrophage-competent and macrophage-ablated mice. We found that ablating macrophages increased the presence of neutrophils, reduced T cells and MSCs, and compromised the healing and biomaterial integration process. These effects could not be rescued with adoptive transfer of naïve or polarized macrophages. Adoptive transfer of M1 macrophages into macrophage-competent mice increased inflammatory cells and inflammatory microenvironment, resulting in decreased bone-to-implant contact. Adoptive transfer of M2 macrophages into macrophage-competent mice reduced the pro-inflammatory environment in the peri‑implant tissue and increased bone-to-implant contact. Taken together, our results show the importance of macrophages in controlling and modulating the inflammatory process in response to implanted biomaterials and suggest they can be used to improve outcomes following biomaterial implantation. STATEMENT OF SIGNIFICANCE: Macrophages are central in orchestrating the inflammatory response to implanted biomaterials and are sensitive to biomaterial chemical and physical characteristics. Our study shows that a deficiency of macrophages results in prolonged inflammation and abolishes bone-biomaterial integration. Adoptive transfer of immunomodulatory macrophages into macrophage-competent mice reduced the inflammatory environment and increased bone-implant contact.