Coupling between Macrophage Phenotype, Angiogenesis and Bone Formation by Calcium Phosphates

The ability of calcium phosphate (CaP) materials to induce bone formation varies with their physicochemical properties, with surface topography as one of the most crucial triggers. In view of the natural wound healing processes initiated after surgical implantation, involving inflammation, angiogenesis, tissue formation and remodeling, we here comparatively investigated the biological cascades occurring upon ectopic implantation of a tricalcium phosphate with submicron surface topography (TCP-S, osteoinductive) and a tricalcium phosphate with micron-scale topography (TCP-B, non-osteoinductive). In vitro, TCP-S facilitated M2 polarization of macrophages derived from a human leukemic cell line (THP-1) as shown by the enhanced secretion of TGF-β and CCL18. Interestingly, the conditioned media of polarized M2 macrophages on TCP-S enhanced tube formation by human umbilical vein endothelial cells (HUVECs), while had no influence on the osteogenic differentiation of human bone marrow stromal cells (HBMSCs). Following implantation in a canine intramuscular defect, TCP-S locally increased typical M2 macrophage markers (e.g. IL-10) at week 1 and enhanced blood vessel formation after week 3 compared to TCP-B. Bone formation was observed histologically for TCP-S 6 weeks after implantation, and bone formation was reduced when an angiogenesis inhibitor (KRN633) was added to TCP-S. No bone formation was observed for TCP-B. The data here presented demonstrates strong links between surface topography, macrophage polarization, angiogenesis and bone formation in CaP materials implanted in non-osseous sites.