Plastics are present in many different aspects of our life including medicine treatments, domestic appliances, and technology. Most of the plastic-based products are single use, which leads to their environmental accumulation and suffer continuous degradation thus generating micro- and nano-plastics residues. Despite their ubiquity, little is known about human exposure to MNPLs and their potential health risks. In this regard, the effect of nanoplastics on the immune system, specifically on the induction of inflammation is not well characterized. Inflammation is triggered by the activation of pattern recognition receptors including the NLRP3 inflammasome, which activation is a validated pathway to induce inflammation in sterile conditions, such as the internalization of particles by myeloid cells. The NLRP3 inflammasome consists of three proteins (NLRP3 protein, ASC protein and pro-caspase 1) which assembly forms a structure named the ASC speck as hallmark of inflammasome activation. This NLRP3 activation leads to caspase-1 activation which cleaves pro-inflammatory cytokines such as interleukin-1 beta (IL-1β) and interleukin-18 (IL-18) as well as gasdermin D, which N-terminal domain produces pores in the cell membrane by which IL-1β and IL-18 can be released. If these pores are not solved, more intracellular content can be release and extracellular content as water can go into the cell thus inducing a specific type of cell death called pyroptosis. In this study we have assessed if two of the most common nanoplastics found in the environment, polystyrene (PS) and polyethylene terephthalate (PET) nanoparticles, can induce NLRP3 inflammasome activation.

Mouse bone marrow derived macrophages (BMDMs) were primed with lipopolysaccharide (LPS), and then treated with different concentrations of PS and PET particles. To evaluate inflammasome activation, the release of IL-1β and IL-18 was measured in cell supernatants by ELISA. To assess the specific NLRP3 inflammasome activation, different NLRP3 inflammasome blockers and macrophages from Pycard, Nlrp3 and Caspase-1 knock out mice were used. Inflammasome activation was also tested in Pam3CSK4 primed human peripheral blood mononuclear cells (PBMCs) treated with PS or PET particles.

Our results show that PS particles were able to induce lactate dehydrogenase (LDH), IL-1β and IL-18 release in a dose-dependent manner in mouse macrophages. IL-1β production was blocked with the specific NLRP3 inhibitor MCC950 and with high K+ cell media. IL-1β was not detected in the supernatants of knock out cells after treatment with PS. These results indicate that PS particles activate specifically the NLRP3 inflammasome. The presence of IL-1β was also detected in the supernatants of human PBMCs treated with PS particles, and its production was blocked with MCC950 and high K+ cell media, thus suggesting that PS particles also activate the NLRP3 inflammasome in human PBMCs. PET particles didn´t induce any effects in mouse macrophages and human PBMCs.

• PS particles are able to activate the NLRP3 inflammasome in mouse BMDM and human PBMCs. • PET particles didn’t produce any inflammatory effects in mouse macrophages and human PBMCs.