RESOURCES

Abstract
Enhancing osteogenesis via modulating immune cells is emerging as a new approach to address the current challenges in repairing bone defects and fractures. However, much remains unknown about the crosstalk between immune cells and osteolineage cells during bone formation. Moreover, biomaterial scaffold-based approaches to effectively modulate this crosstalk to favor bone healing are also lacking. This study is the first to investigate the interactions between macrophages and mesenchymal stem cells (MSCs) in co-cultures with the sustained release of an anti-inflammatory and pro-osteogenesis drug (dexamethasone) from three-dimensional (3D)-printed scaffolds. We successfully achieved the sustained release of dexamethasone from polycaprolactone (PCL) by adding the excipient-sucrose acetate isobutyrate (SAIB). Dexamethasone was released over 35 days in the 17-163 nM range. The osteogenic differentiation of MSCs was enhanced by M1 macrophages at early time points. The late-stage mineralization was dominated by dexamethasone, with little contribution from the macrophages. Besides confirming BMP-2 whose secretion was promoted by both dexamethasone and M1 macrophages as a soluble mediator for enhanced osteogenesis, IL-6 was found to be a possible new soluble factor that mediated osteogenesis in macrophage-MSC co-cultures. The phenotype switching from M1 to M2 was drastically enhanced by the scaffold-released dexamethasone but only marginally by the co-cultured MSCs. Our results offer new insight into macrophage-MSC crosstalk and demonstrate the potential of using drug-release scaffolds to both modulate inflammation and enhance bone regeneration.