TiO2 particles induce CHIT1 and YKL-40 in human macrophages: implications for implant-associated inflammation

Titanium is widely used for orthopedic and dental implants because of its biocompatibility and mechanical properties. However, corrosion and mechanical wear can result in the release of titanium-containing particulate debris, which may contribute to chronic peri-implant inflammation and implant failure. Macrophages are primary innate immune cells that recognize and respond to implant-derived particles, yet the effects of TiO 2 particles on the macrophage-derived chitinase family members chitotriosidase (CHIT1) and YKL-40 (CHI3L1) remain poorly understood. Here, we investigated the effects of TiO 2 particles derived from a 15-nm rutile nanoparticulate starting material that exhibited substantial and preparation-dependent agglomeration during laser diffraction analysis on primary human M0, M1, and M2 macrophages. Metabolic activity, assessed using the Alamar Blue assay, was maintained or increased following exposure to 25, 50, 100, and 200 ppm TiO 2 , with a statistically significant increase observed at 100 ppm in M0 macrophages, while no significant changes were detected in M1 or M2 macrophages. Transcriptomic analysis identified CHIT1 and CHI3L1 as the two most upregulated members of the Glyco_18 protein family. RT-qPCR confirmed significant induction of CHIT1 following TiO 2 exposure in M0, M1, and M2 macrophages, while CHI3L1 expression was significantly increased in M0 and M2 macrophages. Increased gene expression was accompanied by a concentration-dependent increase in YKL-40 secretion, with statistically significant increases observed from 100 ppm TiO 2 onwards. At 100 ppm, extracellular CHIT1 protein levels increased significantly in M0 and M1 macrophages, despite no corresponding significant increase in CHIT1 enzymatic activity. Immunofluorescence analysis revealed altered intracellular localization of CHIT1 following TiO 2 exposure, with accumulation in the trans-Golgi network (TGN) of M1 macrophages and disruption of TGN organization in M0 and M2 macrophages. Furthermore, CHIT1 and YKL-40 were detected in CD68-positive macrophages within peri-implant tissues obtained from patients undergoing removal of titanium fracture fixation implants, supporting the clinical relevance of the in vitro findings. Collectively, these findings identify CHIT1 and YKL-40 as macrophage-associated proteins responsive to TiO 2 particle exposure and support their further investigation in biomaterial-associated inflammation.