Silver nanoparticles (nanosilver) are widely available in a range of consumer products for their antibacterial properties. The increasing volume of nanosilver being manufactured has led to a need for toxicological investigations to understand possible hazards associated with exposure of workers and consumers. A major route of exposure to nanosilver for workers and consumers is via inhalation, and nanosilver can translocate from the lungs into the systemic circulation and from there into other organs and tissues. This makes the immune system an important secondary target for nanosilver exposure and possible toxicity.
Fundamental to understanding the interactions of nanosilver with the immune system is the characterisation of the uptake of these particles by immune cells in the circulation, e.g. monocytes, and in tissues, e.g. macrophages. THP-1 human monocytes, and phorbol ester-stimulated macrophages derived from these cells, were exposed to four types of nanosilver: laboratory-synthesised material (citrate capped, 9 ± 4 nm), commercially-available research material (98 ± 29 nm), and two test materials from OECD’s Sponsorship Programme for the Testing of Manufactured Nanomaterials (citrate capped, 9 ± 3 nm; and polyvinylpyrrolidone coated, 13 ± 5 nm).
The uptake and intracellular localisation of nanosilver in these cells over 24 hours was investigated by: changes in flow cytometric side-scatter intensity; ultra-thin section TEM; synchrotron X-ray fluorescence microscopy; and surface enhanced Raman scattering intensity mapping. THP-1 monocytes and macrophages were both observed to take up nanosilver particles and agglomerates, with most uptake occurring within 4 hours of exposure. Nanosilver was seen within cellular vesicles and the cytosol, but not the nucleus. This research has shown that human immune cells will actively take up nanosilver when exposed in vitro, in agglomerates ranging from just a few particles to micron-sized. These findings help to explain our previous research assessing the potential immunotoxicity of nanosilver in vitro.