Quantitative Analysis of Tissue Distribution of the B16BL6-Derived Exosomes Using a Streptavidin-Lactadherin Fusion Protein and Iodine-125-Labeled Biotin Derivative After Intravenous Injection in Mice

We previously succeeded in the visualization of tissue distribution of B16BL6 cells-derived exosomes by labeling with Gaussia luciferase (gLuc)-LA, a fusion protein of gLuc (a reporter protein) and lactadherin (LA; an exosome-tropic protein). However, total amount of B16BL6-derived exosomes delivered to each organ could not be evaluated because of the reduction of luminescent signal from gLuc-LA. The aim of the present study was to quantitatively evaluate the tissue distribution of B16BL6-derived exosomes. To this end, we labeled B16BL6-derived exosomes with iodine-125 (125I) based on streptavidin (SAV)-biotin system. A plasmid vector encoding fusion protein, SAV-LA, was constructed, and B16BL6 cells were transfected with the plasmid to obtain SAV-LA-coupled exosomes. SAV-LA-coupled exosomes were incubated with (3-125I-iodobenzoyl) norbiotinamide (125I-IBB) to obtain 1251-labeled B16BL6 exosomes. After intravenous injection of 125I-labeled B16BL6 exosomes into mice, radioactivity quickly disappeared from the blood circulation. At 4 h, 28%, 1.6%, and 7% of the injected radioactivity/organ was detected in the liver, spleen, and lung, respectively. These results indicate that 125 I-labeling of exosomes using SAV-biotin system is a useful method to quantitatively evaluate the amount of exogenously administered exosomes delivered to each organ and that the liver is the major organ in the clearance of exogenously administered B16BL6-derived exosomes.