Immune responses during single and repeated murine endotracheal exposures of zinc oxide nanoparticles

Abstract The increasing use of zinc oxide nanoparticles (ZnO-NPs) has raised concerns about their human health and environmental risks. Towards understanding their potential hazard, we investigated the in vivo responses of two commercially available ZnO-NPs, which have been designated as representative of manufactured materials and are used in sunscreen formulations. One such commercial sunscreen product had a zinc concentration of 10.0 ± 2.6 wt%, and the average particle dimension measured by transmission electron microscopy was 112 ± 64 nm. In comparison, ZnO-NP pristine materials appeared similar to that observed in the sunscreen, with agglomerated elongated, needle-like or prismatic structures. Healthy male BALB/c mice were exposed to either coated or uncoated pristine ZnO-NPs by endotracheal instillation with a single dose (5 μg/mouse) or with repeated doses (5 μg/mouse/week for 4 weeks). Histological examination indicated that single exposures caused some pulmonary inflammation. This was confirmed by elevated levels of pulmonary granulocytes, as well as macrophage and natural killer (NK) cells. These changes were accompanied by leukopenia and lymphopenia in the blood. After a month of weekly repeated exposures, a drop in mean body mass was observed. Pulmonary T-helper cells, NK cells, epithelial cells, and especially macrophage were elevated. Both acute and repeated exposures resulted in induction of pulmonary interleukin (IL)-6, keratinocyte chemoattractant (KC), monocyte chemotactic protein (MCP)-1 and macrophage inflammatory protein (MIP)-1α. These results demonstrate that both coated and uncoated ZnO-NPs can induce pulmonary inflammation, but that the uncoated NPs generated a stronger immune response. The acute response was macrophage and neutrophil-mediated, but repeated exposures resulted in a macrophage-dominant and possibly adaptive immunological response.