Integration of sub-organ quantitative imaging LA-ICP-MS and fractionation reveals differences in translocation and transformation of CeO₂ and Ce³⁺ in mice

Information on the risk of exposure to cerium oxide (CeO₂) nanoparticles (NPs) is limited. To assess risk, we must know where and how such NPs are distributed to the body after exposure, both short- and long-term. In this work, an integrated approach of quantitative LA-ICP-MS bioimaging and fractionation was employed to study the translocation and transformation of CeO₂ and Ce³⁺ in mouse spleen and liver. The complementary information retrieved by the two techniques above on the accumulation of Ce and dissolution/aggregation were found consistent. In brief, a detailed fine scanning of a region of interest in the organ was performed after fast-screening at low spatial resolution. In the spleen, after short-term high-dose exposure, CeO₂ NPs was found mainly in the marginal zone and caused an up-regulation of Zn in the white pulp. After long-term low-dose exposure, CeO₂ was found in the marginal zone and white pulp. In the liver, CeO₂ NPs were mainly distributed in the Kupffer cells and lobule periphery. The high spatial resolution LA maps of H&E-stained liver sections allowed imaging close to cell level; this enabled an estimation of Ce content in Kupffer cells. Furthermore, fractionation by ultrafiltration was also employed to differentiate the ionic and NP species in the organs. This fractionation showed aggregation of Ce ions in spleen, supporting the LA-ICP-MS results. Transmission electron microscopy revealed that long-term CeO₂ exposure triggered an immune response to infection in the spleen and confirmed the differential deposition of Ce in the marginal zone. The integrated analyses based on ICP-MS together with histology and TEM investigation suggests that long-term low doses of CeO₂ NPs may cause toxicity in the liver and impair functions of the immune system.