Investigations on the Toxicity of CeO2 Nanoparticles after Subchronic Inhalation of Low Doses

The present research focused on the toxicology of nanomaterial. It was based on a 90-day inhalation toxicity study with cerium oxide and barium sulfate nanoparticles, performed according to OECD TG 413. CeO2 is for example used as diesel fuel additive. BaSO4 is used in paintings and coatings. The project was an addition to a combined chronic inhalation toxicity and carcinogenicity study with CeO2 and BaSO4 (BASF, Ludwigshafen, Germany; NANoREG program, 81|0661/10|170). It aimed on investigating low dose nanoparticle exposure. Therefore, concentrations of 0.1, 0.3, 1.0, and 3.0 mg/m³ CeO2 were applied. BaSO4 was used as representative for a non-toxic nanomaterial and was applied in one high concentration of 50.0 mg/m³. Rats were exposed for 1, 28 and 90 days. Also post-exposure investigations were performed after 28 and 90 days. Respective investigations should provide data for better risk assessment of nanoparticles and better differentiation between materials. Lung burden analytics, hematology, histopathology and BAL fluid analysis were included as guideline required endpoints. Broad gene expression analysis and immunohistochemistry were performed as additional sensitive endpoints. The latter should help identify early biomarkers for potential long-term effects and get better insights in the mechanisms of action of poorly soluble nanomaterial after inhalation. Gene expression analysis was done in AEII cells to determine their contribution on cellular responses to nanoparticles. Exposure to 3.0 mg/m³ CeO2 revealed impaired particle clearance and overload. This was accompanied by inflammation, verified by histopathology, BAL fluid analysis, and gene expression. Gene expression analysis further showed upregulation of oxidative stress and fibrosis related enzymes (Lpo and Mmp12) in AEII cells. Immunohistochemical examinations indicated increased cell proliferation and DNA damage. Post-exposure examinations displayed persistence of effects, especially inflammation. 1.0 mg/m³ CeO2 exposure caused similar effects as the high dose group but less severe, with clearance half-times at the overload threshold. 0.3 mg/m³ CeO2 exposure induced slight upregulation of inflammatory mediators, including Il-1α and Il-1s, at absent overload. 0.1 mg/m³ CeO2 nanoparticle exposure did not cause any effects. The translocation of CeO2 to the liver and kidney was low. Although a significant increase was measured in liver tissue, the overall amount of particulate cerium was less than 1%. Gene regulation was restricted to slight overexpression of inflammatory mediators. In kidney tissue no significant amount of particulate cerium was measured. BaSO4 exposure caused slight histopathological findings, mainly in the nasal compartment. It was cleared rapidly from the respiratory tract. Although levels of Lpo and Mmp12 were increased, the overall contribution of AEII cells defense against BaSO4 nanoparticles was low. The results were all in all very consistent with clear time and concentration dependency. At high CeO2 concentrations the inflammation is related to overload. Effects at low dose levels suggest particle-specific reactivity e.g. due to CeO2 catalytic activity. AEII cells clearly contribute to the defense against inhaled nanomaterial. Histopathological findings and Mmp12 upregulation suggest a substantial risk of long-term fibrosis development. Based on the results a NOAEL of 0.41 mg/m³ could be determined for CeO2 nanoparticle exposure.