Polystyrene Nanoplastics Increase Macrophage Bactericidal Activity Through a Mechanism Involving Reactive Oxygen Species and Itaconate

Nanoplastics are persistent environmental pollutants with potential risks to human health. Due to their small size nanoplastics are internalized by macrophages, potentially altering their function. In this study, we investigated the intracellular localization of polystyrene nanoplastics inside macrophages using confocal immunofluorescence microscopy. We observed their presence predominantly in endosomes, lysosomes, and in the endoplasmic reticulum. We next showed that internalization of polystyrene nanoplastics increases the bactericidal activity of macrophages, which was inhibited by the NADPH oxidase in-hibitor diphenyleneiodonium. Consistently, flow cytometry analysis using CellROX as well as MitoSOX revealed that polystyrene nanoplastics induce reactive oxygen species production in macrophages. In contrast, internalization of polystyrene nanoplastics re-duced the levels of nitric oxide released by macrophages in response to E. coli. We also investigated the impact of polystyrene nanoplastics on the expression of Acod1, a gene encoding for aconitate decarboxylase 1 which is responsible for the production of the mitochondrial metabolite itaconate. Internalization of polystyrene nanoplastics followed by the addition of E. coli induced high levels of Acod1 expression. In the absence of Acod1, the ability of macrophages exposed to polystyrene nanoplastics to kill E. coli was signif-icantly reduced with respect to control macrophages, indicating a role for itaconate in the increased bactericidal activity of macrophages exposed to polystyrene nanoplastics. Collectively, our results indicate that exposure of macrophages to polystyrene nanoplastics increases their bactericidal activity through the production of reactive oxygen species and of itaconate.