Differential Nanoparticle Sequestration by Macrophages and Scavenger Endothelial Cells Visualized In Vivo in Real-Time and at Ultrastructural Resolution

Despite the common knowledge that the reticuloendothelial system is largely responsible for blood clearance of systemically administered nanoparticles, the sequestration mechanism remains a "black box". Using transgenic zebrafish embryos with cell type-specific fluorescent reporters and fluorescently-labelled model nanoparticles (70 nm SiO2), we here demonstrate simultaneous three-color in vivo imaging of intravenously injected nanoparticles, macrophages and scavenger endothelial cells (SECs). The trafficking processes were further revealed at ultrastructural resolution by transmission electron microscopy. We also find, using a correlative light-electron microscopy approach, that macrophages rapidly sequester nanoparticles via membrane adhesion and endocytosis (including macropinocytosis) within minutes after injection. In contrast, SECs trap single nanoparticles via scavenger receptor-mediated endocytosis resulting in gradual sequestration with a time scale of hours. Inhibition of the scavenger receptors...