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

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.

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.

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.

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.