Unraveling the impacts of photolysis-induced aging microplastics on enhanced immunotoxicity and nephrotoxicity

Photolytically aged microplastics i.e. photoaged MPs in aquatic environments could potentially enter human body through ingestion with associated health risks. However, most studies have focused on pristine MPs, single polymer types with single-cell models, leaving critical gaps in achieving a comprehensive toxicological assessment of pristine and photoaged MPs exposure. This study examines the cytotoxic effects of photoaged, environmentally relevant polymers-polyethylene (PE) and polyethylene terephthalate (PET)-on human embryonic kidney cells (HEK-293T) and murine macrophages (RAW264.7). The toxic properties of MPs, including the formation of environmentally persistent free radicals (EPFRs), reactive oxygen species (ROS), and leaching organics from pristine and photoaged MPs, are comprehensively characterized using electron paramagnetic resonance spectroscopy and orbitrap liquid chromatography-mass spectrometry. By integrating viability, oxidative stress, and apoptosis analysis, we provide mechanistic insight into how polymer type and photolytic aging influence MP-induced toxicity. Results demonstrate that photoaged MPs induced significantly higher cytotoxicity than their pristine counterparts, with differences of up to 40 %. Furthermore, PET exhibited greater cytotoxic effects than PE, with differences reaching 30 %. These effects are attributed to the presence of stable EPFRs, which intensify intracellular ROS generation and subsequently trigger apoptosis, whereas the leachate-only test showed no significant cytotoxicity. Photoaged PET, containing carbon- and oxygen-centered EPFRs (g-factor > 2.00400), induce the highest ROS levels and apoptosis in both cell types (P < 0.01). These findings underscore the polymer- and cell type-dependent toxicity of photoaged MPs and provide a foundation for further in vitro co-culture studies.