Carbon dots for in vivo fluorescence imaging of adipose tissue-derived mesenchymal stromal cells

Tissue regeneration based on stem cell therapy is one of the most rapidly developing fields of modern medicine. Several properties of human mesenchymal stromal cells (MSCs), such as tropism toward a tumor or injury site, make them promising candidates for regenerative medicine, targeted therapy, or treating injured tissues. However, to fully understand the role of stem cells in therapeutic function, their visualization in vivo is essential. Here, we describe, for the first time, the use of biocompatible quaternized carbon dots (QCDs) as a novel stem-cell tracking probe for in vivo fluorescence imaging of transplanted human MSCs. By studying the in vitro cytotoxicity, intracellular distribution, and precise uptake mechanism, we showed that QCDs had a high biocompatibility and excellent fluorescence properties after 24 h incubation with MSCs. Further to demonstrate the in vivo feasibility of the system, QCD-labeled MSCs (100 μg/mL of QCDs, 24 h incubation time) were transplanted subcutaneously into an immunodeficient mouse and visualized by optical in vivo imaging. The labeled cells were strongly fluorescent, allowing their semi-quantitative detection. Moreover, the homing of intravenously transplanted QCD-labeled MSCs into the solid tumor was clearly shown. The results demonstrated that QCD-labeling of human MSCs is a highly promising approach for in vivo tracking during stem cell therapy.