Toxicity of polycaprolactone nanoplastics, pristine or weathered in environmental conditions, to human intestinal epithelial cells, in vitro

Plastic, used in almost all everyday products, is a major source of pollution, particularly in the form of micro- and nanoplastics (MNPs). MNP impact on health, particularly on the digestive tract, is still poorly understood, especially in vulnerable populations such as those suffering from inflammatory bowel disease. The aim of this study was to assess the in vitro toxicity of nanoplastics (NPLs) from a biodegradable polymer, polycaprolactone (PCL), both in its pristine state and after accelerated weathering in environmental conditions, the latter resulting in the release of potentially toxic PCL oligomers. To do so, we used in vitro models of genetic susceptibility to Crohn's disease (CD), consisting in co-cultures of HT29-MTX cells with Caco-2 cells engineered to express either wild-type (WT) or mutated nucleotide-binding oligomerization domain 2 (NOD2), representative of healthy individuals (Caco-2 NOD2^WT) or individuals with susceptibility to CD (Caco-2 NOD2^1007fs). Physicochemical transformation of PCL NPLs upon weathering were characterized. Cells were exposed to pristine and aged PCL NPLs and their cytotoxicity, genotoxicity, inflammatory potential, impact on the cells' redox balance and unfolded protein response, as well as on the epithelial barrier integrity were evaluated. Results show that accelerated weathering increases the crystallinity and leads to fusion of PCL NPLs. PCL NPLs accumulate inside cells where they degrade and release some PCL oligomers. However, in the tested conditions, PCL particles, both pristine and aged, do not show any overt toxicity in both cell systems, irrespective of particle size and weathering status. These data confirm that PCL NPLs, may they be intact or partially dissolved, show only mild toxicity to colon cells, upon acute, short-term exposure of Caco-2 / HT29-MTX cells.