Urban dust nanoparticles drive inflammatory activation of human neutrophils and macrophages in vitro

Today, the majority of the world's population lives in urban environments, where they are constantly exposed to dust particles of varying sizes, including urban dust nanoparticles (NPs). The minute size of NPs ensures their quick and efficient penetration into inner body parts, where they can interact with many cell types, including epithelial, endothelial, and immune cells. Despite their pervasive distribution and considerable potential health hazards, little is known about the ability of NPs to activate innate immune cells of the first line of defense (neutrophils) and the second line of defense (macrophages). The majority of research focuses on coarse (< 10 μm) and fine (< 2.5 μm) particles or employs engineered NPs. The aim of the study was to evaluate the potential of urban dust NPs to activate human neutrophils and macrophages in vitro. NPs were separated from urban dust samples collected in central Moscow (Russia) using coiled tube field-flow fractionation followed by ultrafiltration. Pre-incubation of human neutrophils with NPs primed the cells for fMLP-induced oxidative burst. The NPs also induced the formation of neutrophil extracellular traps (NETs) when neutrophils were adsorbed onto poly-L-lysine, which mimics some extracellular matrix proteins. In THP-1 monocytes differentiated to macrophages, NPs upregulated mRNA expression of the pro-inflammatory cytokines TNF, IL-6, IL-8, and IL-1β as well as of monocyte chemoattractant protein-1 (MCP-1) and stimulated secretion of TNF and IL-6, indicating inflammatory activation of macrophages. Notably, heat treatment of the NPs to remove organic components decreased all observed immunostimulatory effects. Similarly, the antibiotic polymyxin B, which binds bacterial lipopolysaccharides (LPSs), partially prevented the effects of NPs. These data indicate that the organic compounds, including LPS, sorbed on urban NPs are predominantly responsible for the activation of human neutrophils and macrophages in vitro. It can be assumed that dust NPs are important components of urban air pollution that initiate the activation of the innate immune system, contributing to the pathogenesis of inflammatory respiratory diseases.