In vivo dynamic cell tracking with long-wavelength excitable and near-infrared fluorescent polymer dots

Development of cell-based therapeutic systems has attracted great interest in biomedicine. In vivo cell tracking by fluorescence provides indispensable information for further advancing cell therapy in clinical applications. However, it is still challenging in many cases because of the limited light penetration depth as well as the variations in fluorescent probes, cell lines, and labeling brightness. Here, we designed highly fluorescent polymer dots (Pdots) with far-red-light absorption and near-infrared (NIR) emission for cell tracking. The Pdots consisted of a donor-acceptor polymer blending system where intra-particle energy transfer yielded a narrow-band emission at 800 nm with a high quantum yield of ~0.22. We investigated biocompatibility and cell labeling brightness of the Pdots coated with cell penetrating peptides. Flow cytometry indicated that the cell-labeling brightness of both stem cells and cancer cells increased as much as ~4 orders of magnitude comparing the intensity measurements of labeled cells and controls. Yet, in vivo cell tracking results revealed distinctive fluorescence distribution for the same number of cells that were administered into mice through the tail vein. The stem cells initially accumulated in the lung and remained for seven days, whereas the cancer cells tended to be cleared by the liver in four days. The difference is likely due to the fact that cancer cells are easily attacked by the immune system, whereas stem cells have low immunogenicity. Results obtained herein confirm that NIR-fluorescent Pdots are promising platforms for in vivo cell tracking in small animals.