Assessment of micro and nanoplastic toxicity and their protein corona using in vitro and in silico new approach methodologies

The invention of plastic has revolutionized our world by providing a readily available, moldable material which is light, strong and durable. However, excessive use of plastics has led to universal pollution of our planet. Over time plastic waste and products can degrade, leading to release of microscopic particles called microplastics and nanoplastics (MNPs). MNPs are abundant in our food, the water we drink and the air we breathe and in turn human exposure is unavoidable. Oral ingestion is the biggest source of MNP exposure for humans and amidst all human organs the intestine faces the largest MNP exposure which potentially affects intestinal health. In animals, MNP exposure can cause inflammatory effects and barrier disruption in the intestine, however it is unknown if these effects occur in humans. Given the diversity of MNPs found in nature, exhaustive safety testing through animal studies is undesirable and instead In vitro and in silico methods (collectively called New Approach Methodologies (NAMs)) can aid MNP toxicity assessment. Current MNP research with NAMs has limited ability to predict toxicity in humans due to lack of assessment of true-to-life secondary MNPs, aberrant phenotype of in vitro cell lines and a lack of understanding of mechanisms like protein-corona formation and biokinetic processes. The aim of this thesis was to improve in vitro and in silico NAMs for MNP toxicity assessment and to improve understanding of the formation and effects of the protein corona on MNPs. This thesis demonstrated that commonly used micro and nanoplastic models are not predictive of true microplastic toxicity to the intestine or resident immune cells. We show that this discrepancy is in part due to the overreliance on polystyrene materials and due to effects of a bound layer of proteins, called the protein corona, on toxicity and bioavailability. Finally we build one of the first in silico models that predict tissue concentrations after oral ingestion of micro and nanoplastics. Overall our results show that specific proteins of the protein corona increase immunogenicity, generation of reactive oxygen species and barrier disruption as well as particle uptake through phagocytosis. Our in silico model predicts that at the moment microplastic toxicity to humans is of low risk, however the differences between model microplastics and true-microplastics need to be considered as micro- and nanoplastic risk assessment matures. Overall the studies described in this thesis indicate that the gut is one of the target organs of environmental microplastics which may be harmful to the human intestine, and that a realistic protein corona composition needs to be considered for both toxicokinetic and toxicodynamic assessment of microplastics.