The Cultivation Modality and Barrier Maturity Modulate the Toxicity of Industrial Zinc Oxide and Titanium Dioxide Nanoparticles on Nasal, Buccal, Bronchial, and Alveolar Mucosa Cell-Derived Barrier Models

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Abstract
As common industrial by-products, airborne engineered nanomaterials are considered important environmental toxicants to monitor due to their potential health risks to humans and animals. The main uptake routes of airborne nanoparticles are nasal and/or oral inhalation, which are known to enable the transfer of nanomaterials into the blood stream resulting in rapid distribution in the body. Consequently, mucosal barriers present in nose, buccal and lung have been identified and intensively studied as the key tissue barrier to nanoparticle translocation. Despite decades of research, surprisingly little is known about the differences among various mucosa tissue types to tolerate nanoparticle exposures. One limitation in comparing nanotoxicological data sets can be linked to a lack of harmonization and standardization of cell-based assays, where a) different cultivation conditions such as air-liquid interface or submerged cultures, b) varying barrier maturity and c) diverse media substitutes have been used. The current comparative nanotoxicological study therefore aims at analyzing the toxic effects of nanomaterials on four human mucosa barrier models including nasal (RPMI2650), buccal (TR146), alveolar (A549), and bronchial (Calu-3) mucosal cell lines to better understand the modulating effects of tissue maturity, cultivation conditions and tissue type using standard Transwell cultivations at liquid-liquid and air-liquid interfaces. Overall, cell size, confluency, tight junction localization, and cell viability as well as barrier formation using 50% and 100% confluency was monitored using trans-epithelial-electrical resistance (TEER) measurements and Presto Blue assays of immature (e.g. 5 days) and mature (e.g. 22 days) cultures in the presence and absence of corticosteroids such as hydrocortisone. Results of our study show that cellular responses to increasing nanoparticle exposures are highly cell type specific, where bronchial mucosal cell barriers models cultivated under ALI conditions showed less tolerance to acute ZnO nanoparticle exposures. Additionally, stronger toxicities are found using early mucosa barriers compared to later barrier models being maturated under air-liquid cultivation conditions.

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