Zebrafish let you see how blood clearance of 'non-self' nanoparticles triggers inflammation

Today, no "magic bullets" exist that can target tumours with 100% efficiency. In fact, off-targeting due to massive clearance by the liver is a common problem for the intravenous route of nanoparticle administration. Only little is known, however, about the rapidity of this process and how it could backfire on the proper functioning of such biological systems. The use of transgenic zebrafish embryos now offers a unique opportunity for real-time intravital imaging that can be combined with electron microscopy to enable (correlative) ultrastructural observation of nanoparticle sequestration in vivo.We first reveal that macrophages and scavenger endothelial cells (functional analogue of liver sinusoidal endothelial cells in mammals) are central to blood clearance of injected SiO2 nanoparticles (70 nm in size). Of particular interest are the two different uptake mechanisms, endocytosis and macropinocytosis, clearly visualized by electron microscopy approaches. We then demonstrate an application of this zebrafish model, inspired by our old findings – "species differences at nanoparticles". The basic idea behind this concept is that a corona of proteins can form around a nanoparticle representing a biological identity that critically contributes to the way the nanoparticle interacts with biological systems. Here, we introduced a non-self biological identity by mismatched combinations of zebrafish and a protein corona pre-formed of bovine serum. The upshot was strikingly rapid sequestration of the nanoparticles by scavenger endothelial cells. This triggered strong pro-inflammatory responses in macrophages which was not observed by simple injection of non-self proteins alone. We thus provide an experimental in vivo evidence that cells "see" the biological identity. This highlights the critical importance of species compatibility in the development of biomolecule-inspired nanomaterials, especially when zebrafish embryos are used as a preclinical screening model.