PKH Dyes Should Be Avoided in the EVs Biodistribution Study of the Brain: A Call for Caution.

INTRODUCTION: Extracellular vesicles (EVs) are nanosized membrane vesicles that are naturally secreted by almost all cells and have gained considerable attention. Many studies have applied EVs to the treatment of brain diseases and validated their effectiveness. Although only a few EVs can penetrate the blood‒brain barrier (BBB) into the brain after administration, it has been proven that EVs and their cargos exert their effects by interacting with brain cells. PKH dyes are commonly used to stain EVs for distribution studies. However, systematic investigations of imaging characteristics of the PKH-labeled EVs distributed in the brain are still scarce. METHODS: We stained EVs derived from mesenchymal stem cells with PKH26 or PKH67. PKH26-labeled EVs and PKH67-labeled EVs were administered at the same time into each mouse while PKH26-labeled EVs were given through tail veins and PKH67-labeled EVs were given through intraperitoneal injection. Confocal microscopy was used to explore the distribution difference of two types of EVs given via different routes in the brain. RESULTS: The fluorescence of PKH26 and PKH67 had nearly identical distributions in brain slices after 1 h, 6 h, 12 h and 1 day of EV administration. Under the same confocal parameters, brain slices without EVs administration demonstrated the same result. However, liver slices from mice administered with labeled EVs showed obviously different distributions of two types PKH fluorescence. DISCUSSION: These findings raise questions about the ability of PKH dyes as labels for EVs when explore the EV brain distribution observed via confocal microscopy.