Evaluating the toxicity of polystyrene micro- and nanoplastics in human bronchial epithelial cells: differences and challenges using aerosol and suspension exposures

Studies investigating toxicity of airborne micro- and nanoplastics (MNPs) are mostly based on conventional submerged cell cultures while limited studies utilize air-liquid interface (ALI) systems. Inherent differences in these culture/exposure modalities, such as particle behavior and deposited dose, likely affect cellular responses. To investigate this, we exposed submerged human bronchial epithelial cells (BEAS-2B) to polystyrene (PS) MNPs vs. aerosol exposure of ALI cultures. First, submerged bronchial epithelial cells (BEAS-2B) were exposed to suspensions of PS particles (50 nm or 1 µm) in four different applied concentrations (0.79–50 µg/cm2; 24 h). Second, BEAS-2B cells were cultured at ALI and exposed in a cloud system to PS 1 µm particles (deposited dose: 55.4 µg/cm2; 24 h). Toxicity readouts focused on cytotoxicity (LDH release), inflammation (IL-8 release and transcriptional activation of inflammatory genes), and oxidative stress (DCFH-DA assay, antioxidant gene expression, and assessment of reduced/oxidized glutathione). In both models, PS exposure did not induce cell death, or an antioxidant response. However, NF-κB transcriptional activity was strongly upregulated in submerged cells in response to both sizes of PS particles in a dose-dependent manner. Gene expression of CXCL1, CXCL2, and CXCL8 increased up to 7-fold after PS microplastic exposure (50 µg/cm2) in the submerged model (which was less pronounced in response to PS nanoplastics) and 2-fold in the ALI model. In contrast, IL-8 secretion increased 1.6-fold for the ALI, but not the submerged model. Overall, both exposure modalities revealed an inflammatory response toward PS MNPs although with differences, likely due to significant differences in deposited dose. Very small plastic particles, called microplastics and nanoplastics (MNPs), are present in the air and can be inhaled into the lungs. To study possible health effects, scientists often expose lung cells to these particles in liquid (submerged) culture systems. However, this does not closely match how people are exposed through breathing. This study used a traditional liquid-based method and a more realistic system that exposes lung cells to plastic particles through the air. More specifically, human lung cells were exposed to polystyrene (PS) plastic particles of different sizes using either liquid exposure or an air–liquid interface system, where particles were delivered as an aerosol, similar to inhalation. The researchers looked for signs of cell damage, inflammation, and oxidative stress. The results showed that the plastic particles did not kill the cells and did not cause oxidative stress in either exposure method. However, both methods caused signs of inflammation, which is a normal response of the body to foreign substances. In the liquid-based system, inflammation-related genes were strongly activated. In the air-exposed system, gene activation was weaker, but cells released higher levels of an inflammatory signaling protein. Importantly, the air-based exposure better reflects how people actually breathe in particles and avoids some problems linked to liquid-based testing. This study shows that there are differences in cellular responses to MNPs depending on the dose, particle behavior and culture system, which are all connected to each other.