Uptake of silver from metallic silver surfaces induces cell death and a pro-inflammatory response in cultured J774 macrophages

In clinical medicine metallic silver is used as anti-bacterial coating on various catheters, bandages and prostheses. By means of dissolucytosis, i.e. extracellular macrophage-mediated bio-liberation of metal ions, silver ions are continuously liberated from silver surfaces starting within minutes of exposure. The present study investigates how bio-liberation and subsequent cellular uptake of silver ions affects cell viability and cell signalling within the first 3-24 hours of exposure when J774 macrophages are grown directly on a silver surface. Autometallography (AMG) was applied to demonstrate cytoplasmatic silver uptake and localisation after 1, 3, 12 and 24 hours of exposure to metallic silver. From 12 hours onwards the cells were completely filled with silver enhanced silver-sulphur nanocrystals (AMG-silver grains). At the ultrastructural level, the silver accumulations were located to lysosome-like structures. An immunoassay cell death kit found silver-induced apoptosis after 12 and 24 hours of exposure. Necrosis was seen at the same times. Judged by mRNA analysis silver exposure statistically significantly induces TNF-α and m-CSF gene expression, especially at 3 hours. Furthermore, anti-inflammatory IL-10 transcription is reduced by silver uptake and 24 hours of silver exposure induces massive iNOS-2 gene expression. At the same time silver exposure increases the gene expression of metallothionein (MT-I/MT-II), a cystein-rich protein known for its role in detoxifying heavy metals. Our data suggest that silver ions liberated from metallic silver surfaces accumulate in lysosomes, reduce macrophage viability by apoptosis and necrosis and induce a pro-inflammatory response.