Different in vitro exposure regimens of murine primary macrophages to silver nanoparticles induce different fates of nanoparticles and different toxicological and functional consequences.

Silver nanoparticles (Ag-NPs) are used in a variety of consumers' goods. Their toxicological impact is currently intensely studied, mostly upon acute exposure, but their intracellular dissolution and fate is rather poorly documented. In this study, murine primary macrophages were exposed to a single high but non-lethal dose of Ag-NPs or to repeated, low doses of Ag-NPs. Cells were either collected immediately after acute exposure or after 72 h of recovery in the NP-free exposure medium. Ag intracellular content and distribution were analyzed by particle-induced X-ray emission, transmission electron microscopy coupled to energy-dispersive spectroscopy analysis and inductively coupled plasma mass spectrometry. In parallel, macrophage functionality as well as inflammatory and thiol-responses were assessed after Ag-NP exposure. We show that Ag accumulation in macrophages is similar upon acute and repeated exposure to Ag-NPs, and that Ag is partly expelled from cells during the 72 h recovery stage. However, acute exposure leads to a strong response of macrophages, characterized by reduced mitochondrial membrane potential, phagocytic capacity and nitric oxide (NO) production upon lipopolysaccharide (LPS) stimulation. Under this condition, we also show an increased release of proinflammatory cytokines as well as a decreased release of anti-inflammatory cytokines. This response is reversible since these biomarkers reach their basal level after the recovery phase; and is much less intense in repeatedly exposed cells. These results suggest that repeated exposure of macrophages to Ag-NPs, which is a more realistic exposure scenario than acute exposure, leads to significant Ag intracellular accumulation but a much less intense toxicological response.