Visualization of exosomes from mesenchymal stem cells in vivo by magnetic resonance imaging

Background and purpose: We develop a method of imaging exosomes in vivo according to the vital role of exosomes in intercellular communication. This study aims to design a new label method that allows the visualization of labeled exosomes with magnetic resonance imaging (MRI). Methods: We designed a fusion protein consisting of two parts, namely, ferritin heavy chain (FTH1) and a truncated lactadherin. FTH1 is used as an MRI reporter. Lactadherin is a trans-membrane protein. The lactadherin protein are mostly located on the outer surface of exosomes. We replaced the outer membrane part of lactadherin with FTH1, infected mesenchymal stem cells with lentivirus carrying the fusion protein, and isolated exosomes from the labeled cells by ultracentrifugation. Labeled exosomes were validated by transmission electron microscopy images, Western blot, nanosight particle tracking, and visualized in vitro and in vivo by MRI. Results: FTH1 expression would suppress mesenchymal stem cell proliferation, whereas the characterization of labeled exosomes remains comparable with unlabeled exosomes. MR imaging shows that exosomes labeled with FTH1 can be visualized in vitro and in vivo. Conclusion: This innovative reporter-imaging approach to track and visualize exosomes with MRI can be utilized as a tool for the study of the role of exosomes under different conditions.