Polystyrene nanoplastics induce mitochondrial dysfunction and stress responses in human PBMCs.

Plastics continuously fragment into micro- and nanoplastics (MPs/NPs), which are increasingly recognized as emerging environmental contaminants of global concern. Human exposure to nanoplastics through air, food, and water is becoming unavoidable; however, their direct effects on human immune cells remain poorly understood. Due to their small size, NPs can enter the circulation and directly interact with immune cells, yet their cellular effects in humans remain poorly understood. In this study, we investigated the impact of polystyrene NPs on human peripheral blood mononuclear cells (PBMCs) using an integrated approach that combined imaging, mitochondrial stress testing, basophil activation assays, and single-cell RNA sequencing. Confocal microscopy confirmed efficient cytoplasmic internalization of 25-nm NPs. Optical diffraction tomography revealed that even short-term (1 h) exposure induced pronounced biophysical remodeling, including reduced cell volume and dry mass alongside increased intracellular density and refractive index. Seahorse metabolic profiling demonstrated substantial suppression of mitochondrial respiration across major immune subsets, reflected in reduced basal and maximal respiration, ATP-linked oxygen consumption, and spare respiratory capacity. Basophil activation remained unaffected by NP exposure. Single-cell transcriptomics identified a distinct NP-induced "stress-cell" population, characterized by upregulation of heat-shock and proteostasis pathways and concomitant downregulation of mitochondrial-encoded transcripts. Together, these data show that NPs rapidly disrupt mitochondrial function and activate proteotoxic stress programs in human immune cells. By situating these mechanisms within the One Health framework (human, animal and the planet health), our findings highlight how environmental nanoplastic pollution may translate into immune dysregulation and inform integrated environmental-public health risk assessments.