Yamano, S.; Umeda, Y.

Background: We have previously reported that inhalation exposure to titanium dioxide nanoparticles (TiO2 NPs) for 13 weeks causes early pneumoconiosis lesions in the alveolar region of F344 rats. We defined these characteristic lesions as pulmonary dust foci (PDF). In this report, we re-evaluate and detail the histopathological data regarding particle-induced pneumoconiosis lesions, including progressive lesions of the early PDF lesions, that developed in F344 rats exposed TiO2 NPs by whole body inhalation over a period of two years. Methods: Male and female F344 rats were exposed to 0.5, 2, and 8 mg/m3 anatase type TiO2 NPs for 6 hours/day, 5 days/week for 104 weeks using a whole-body inhalation exposure system. After the final exposure, the rats were euthanized. In the present study, the collected lungs were re-evaluated macroscopically and histopathologically. Results: Rats exposed to TiO2 NPs developed macroscopic white lesions, primarily in the subpleural and hilar regions of the lung, which increased in size and number with exposure concentration. Histologically, two lesion types were identified: (1) Fibrotic Pulmonary Dust Foci (fPDF), characterized by collagen deposition, inflammatory infiltration, and disrupted alveolar epithelial differentiation, and (2) Dust Macules (DM), characterized by macrophage accumulation without significant fibrosis or inflammation. Immunohistochemical analysis revealed abnormal alveolar epithelial differentiation and reduced capillary density within fPDFs. Importantly, no histological connection was found between the pneumoconiosis lesions and the observed lung tumors, which resembled spontaneous, age-related neoplasms. Conclusions: Chronic inhalation of TiO2 NPs induces advanced pneumoconiosis characterized by fPDF and DM, with distinct pathological features. However, these lesions were not directly linked to lung tumor development. Therefore, in this study PDF lesions developed into fPDF lesions but did not lead to tumorigenesis. This study provides critical insights into the long-term pulmonary effects of TiO2 NP exposure and the progression of pneumoconiosis lesions in the rats.