Titanium Dioxide Particle Size Influences Macrophage Signal Switch Towards Osteogenesis In Vitro

Objective: The aim of this study was to investigate the impact of titanium dioxide microparticles (TiO2 MPs) and titanium dioxide nanoparticles (TiO2 NPs) on macrophage internalisation and their downstream influence on regulating osteogenesis in vitro. Design: The viability of THP-1-derived macrophages exposed to TiO2 MPs and TiO2 NPs was evaluated using XTT assays. Conditioned medium (CM) from macrophages treated with TiO2 MPs and NPs was collected and applied to human osteoblast (HOB) cells, with their functionality analysed at gene and protein expression levels, receptor activator of nuclear factor kappa-B ligand/osteoprotegerin (RANKL/OPG) ratio, and matrix mineralisation. Results: Both TiO2 MPs and NPs were nontoxic even at 100 µg/mL. Exposure of HOB to MP and NP CM induced significant proinflammatory responses, including increased IL-1β and IL-8 expression, alongside elevated TNF-α cytokine and MMP8 release. The TiO2 MP CM group elicited stronger inflammatory responses, whereas the TiO2 NP CM group demonstrated enhanced osteogenic activity, evidenced by upregulation of RUNX2, increased alkaline phosphatase activity, collagen type I secretion, and extracellular matrix mineralisation, generating a lower RANKL/OPG ratio in HOB cells. Western blotting revealed elevated osteocalcin and BMP-7 levels in NP CM-treated cells. Conclusion: Titanium dioxide particle sizes are able to "switch" macrophage signalling toward either bone formation (nano) or bone resorption (micro) and may play a crucial role in osteo-immunomodulation.