Toxicological Aspects, Safety Assessment, and Green Toxicology of Silver Nanoparticles (AgNPs)—Critical Review: State of the Art

AgNPs are the most widely used nanoparticles in consumer products. However, their toxicity has raised concerns that limit their use. This section of our review presents the toxicity of AgNPs on immune cells. The physicochemical properties of nanoparticles, such as size, distribution, crystallinity, surface charge, surface coating, synthesis methods, and reactivity, are one of the key factors affecting the immune response [ 47 ]. Modification of the surface of the nanoparticles seems to be of greatest relevance to the immune system. The outer coating determines the toxicity of nanoparticles [ 48 ]. Silver nanoparticles have stimulating and inhibitory effects on cytokine production associated with the inflammatory response and are likely to depend on cell type and dose [ 3 ]. Silver nanoparticles can modulate cytokines involved in wound healing [ 49 ]. Interactions between nanoparticles and the innate immune system can affect the adaptive immune response through the production of cytokines and chemokines. IL-1β, a crucial cytokine involved in lymphocyte activation and proliferation, is produced by monocytes in response to exposure to AgNPs. The decreased amount of IL-1β may be related to the impairment of the innate immune response caused by AgNPs [ 50 ]. On the other hand, in human epidermal cells, the result was an increased number of IL-1β, IL-6, IL-8, and TNF-α [ 32 ]. The expression of the genes TLR2, Myd88, IL-8, NF-κB, and IL-1β was downregulated with increasing concentration of silver nanoparticles [ 50 ]. Silver nanoparticles have a toxic effect on the proliferation and expression of human lymphocyte cells and peripheral blood mononuclear cells (PBMCs) [ 51 ]. Primary human blood mononuclear cells were exposed to AgNPs with a particle size of 5 nm and 28 nm. Based on the measurement of IL-1β and the induction of inflammatory body formation, it turned out that smaller particles have a greater potential to activate innate immunity [ 52 ]. Furthermore, smaller particles also caused higher cytotoxicity in monocytes and macrophages than larger particles [ 52 53 ]. In addition, small AgNPs (10 nm) have toxic effects on human blood mononuclear cells, and their toxicity is both time- and dose-dependent [ 53 54 ]. AgNPs, in a concentration-dependent manner, induce cytotoxicity and inhibit cytokine proliferation and production, including IL-5, INF-γ, and TNF-α in peripheral blood mononuclear cells. Furthermore, silver nanoparticles can accumulate in the immune organs and affect the number of immune cells and the production of cytokines [ 55 ]. Exposure to silver nanoparticles in isolated monocytic THP-1 cells and peripheral blood mononuclear cells (PBMC) causes significant immunotoxicity. In both PBMC and THP-1 cells, the internalisation of AgNPs’ results in increased expression of Myd88, MEKK1, and early regulation of oxidative stress genes [ 56 ]. AgNPs internalised into the cytosol and nucleus of human THP-1 monocytes induce monocytic cell death by degradation of the stress sensor ATF-6 and activation of the NLRP-3 inflammasome [ 56 57 ]. AgNPs showed high dose-dependent immunomodulation of T cells and monocytes [ 58 ]. The immunomodulatory activity of AgNPs varies depending on the type of immune cell and the stage of differentiation. In the example of differentiation of human promyelocytic leukaemia cells (HL-60) into granulocytes or macrophage-like cells, and differentiation of human monocytic cells (U-937) into monocytes and macrophages, differentiated cells were found to exhibit greater resistance to AgNP-induced cell death than undifferentiated cells [ 58 59 ]. Alveolar macrophages responded with increased pro-inflammatory mediator production of pro-inflammatory mediators (TNF-α, MIP-2, and IL-1β) after exposure to AgNP [ 5 ]. The effect of human macrophages on concentrations of 5 nm or 100 nm silver nanoparticles showed that smaller nanoparticles induced stronger expression of pro-inflammatory cytokines (IL-8) and stress genes (hemeoxygenase-1 and heat shock protein-70) than exposure to 100 nm AgNP [ 60 ]. MAPK and NF-κB pathways, which lead to the transcription of many genes involved in inflammatory responses and induce deleterious inflammatory responses, can be activated by AgNP [ 60 61 ]. AgNPs exhibit independent immunomodulatory activity based on cytotoxicity by inducing NF-kB activation and the resulting triggering of pro-inflammatory genes, including IL-6 and IL-1β, by macrophages RAW264.7 after exposure to high concentrations (10 mg/mL) of silver nanoparticles [ 62 ]. In the case of AgNPs, exposure to these metallic nanoparticles can affect the immune system directly or indirectly. For example, exposure to AgNPs induces inflammation and releases chemokines by activating neutrophils [ 63 ]. Human neutrophils are modulated by silver nanoparticles. AgNPs interact with neutrophil cell membranes and infiltrate cells, and are located in vacuole-like structures distributed throughout the cytosol. Internalisation of AgNP increases the rate of neutrophil apoptosis and inhibits de novo protein synthesis [ 63 64 ]. Internalised AgNPs are known to increase oxidative stress, thus enhancing the production of reactive oxygen species in human neutrophils [ 64 65 ]. The effects of AgNPs on neutrophils include nanoparticles triggering the release of extracellular neutrophil traps and inhibiting the formation of nitric oxide, inhibiting the activity of protein phosphatase, and causing increased intracellular levels of reactive oxygen species [ 66 ]. Furthermore, human mesenchymal stem cells showed a decrease in pro-inflammatory factors (IL-6 and IL-8) after exposure to AgNPs. When cells were exposed to less than 5 µg/mL of AgNPs, an increase in the pro-inflammatory factor (IL-8) was observed [ 67 ].