Pore size-mediated macrophage M1 to M2 transition affects osseointegration of 3D-printed PEEK scaffolds

Increasing evidence indicates that macrophages play an important role in angiogenesis and bone regeneration. Because the phenotypic polarization of macrophage is extremely sensitive to the pore size of materials, poly(ether-ether-ketone) (PEEK) scaffolds with pore sizes of 0, 200, and 400 μm were prepared, and the influence of pore size-mediated macrophage polarization on subsequent angiogenesis and osteogenesis was examined. The interaction results of macrophages and scaffolds indicated that macrophages were responsive to the pore size of three-dimensional (3D)-printed PEEK scaffolds, and large pore size scaffolds showed greater potential in inducing M1 to M2 transition of macrophage and enhanced macrophage secretion of high concentrations of osteogenesis-related and angiogenesis-related cytokines. When human umbilical vein endothelial cells (HUVECs) and bone marrow mesenchymal stem cells (BMSCs) were cultured in the conditioned medium derived from co-culture of macrophages and scaffolds, HUVECs showed good angiogenic responses in terms of cell migration and angiogenic gene expression, while BMSCs showed good osteogenic differentiation effect in in vitro mineralization and osteogenesis-related gene expression. The results of bone defect repair showed that the bone volume/total volume ratio and trabecular thickness of the large pore size PEEK scaffold were significantly higher, and it had better biomechanical properties and achieved a better osseointegration effect. Our data demonstrate that large-pore PEEK scaffolds promote angiogenesis and osteogenic differentiation in vitro and osseointegration in vivo, most likely because scaffolds with larger pore size are able to mediate a higher degree of M1 to M2 transition in macrophages.