In vitro response of macrophages to ceramic scaffolds used for bone regeneration

Macrophages, the primary cells of the inflammatory response, are major regulators of healing, and mediate both bone fracture healing and the inflammatory response to implanted biomaterials. However, their phenotypic contributions to biomaterial-mediated bone repair are incompletely understood. Therefore, we used gene expression and protein secretion analysis to investigate the interactions in vitro between primary human monocyte-derived macrophages and ceramic scaffolds that have been shown to have varying degrees of success in promoting bone regeneration in vivo. Specifically, baghdadite (Ca3ZrSi2O9) and strontium hardystonite gahnite (Sr Ca2ZnSi2O7-ZnAl2O4) scaffolds were chosen as two materials that enhanced bone regeneration in vivo in large defects under load compared with clinically used tricalcium phosphate hydroxyapatite (TCP HA). Principal component analysis revealed that the scaffolds differentially regulated macrophage phenotype. Temporal changes in gene expression included shifts in markers of pro-inflammatory M1, anti-inflammatory M2a and pro-remodelling M2c macrophage phenotypes. Of note, TCP HA scaffolds promoted upregulation of many M1-related genes and downregulation of many M2a- and M2c-related genes. Effects of the scaffolds on macrophages were attributed primarily to direct cell scaffold interactions because of only minor changes observed in transwell culture. Ultimately, elucidating macrophage biomaterial interactions will facilitate the design of immunomodulatory biomaterials for bone repair.