Impact of ultra-small silver nanoparticles of 2 nm (AgNP) on neutrophil biology: AgNP alter the actin cytoskeleton and induce karyorrhexis by a mitogen-activated protein kinase-dependent mechanism in vitro and transitorily attract neutrophils in vivo.

Silver (Ag) is known as an antibacterial agent and there is a growing interest to use silver nanoparticles (AgNPs) in a variety of medical applications and other sectors. Some studies reported that one of the undesired effects of AgNPs is inflammation and that these NPs can alter the biology of neutrophils. Since it is commonly accepted that the more NPs are small, the more toxic they are the aim of this study was to determine the impact of ultra-small silver nanoparticles of 2 nm (AgNP) on the biology of neutrophils, key player cells in inflammation. We report that AgNP are potent neutrophil activators as they rapidly induce actin polymerization and dismantling the actin network. Although AgNP are not necrotic for neutrophils and do not induce ROS production, kinetic studies reveal that AgNP are rapid inducer of apoptosis. Pyknosis (mainly 1-2 large nuclear dots) was observed after only 1h of treatment followed by karyorrhexis (several small dots) and by a complete nuclear dissolution leading to anuclear neutrophils after 6h. These observations are not associated with the release of silver ions since treatment of neutrophils with 1-50 μg/ml AgNO (as a source of Ag) did not induce any apparent changes. AgNP induce p38 and Erk-1/2 mitogen-activated protein kinase (MAPK) and although karyorrhexis was markedly reversed by MAPK inhibitors, the cell nuclei remain with a pyknotic-like phenotype but do not return to the characteristic polylobed nucleus. Using the murine air pouch model of inflammation AgNP were found to induce a neutrophil influx. Our data indicate that AgNP are potent neutrophil activators targeting the actin cytoskeleton and the mechanism involved for inducing apoptosis is rapid, complex, and partially includes MAPK pathways. Therefore, the ultra-small AgNP are more potent than larger ones for inducing apoptosis and they can transitorily attract neutrophils in vivo.