The quantity, accessibility, and abundance of subcutaneous adipose tissue in humans make it an attractive alternative to bone marrow as a source of adult stem cells for therapeutic purposes. Adult adipose-derived mesenchymal stem cells can differentiate into a variety of lineages including adipose, bone, cartilage, and muscle. In addition, the use of adult stem cells for regenerative medicine rather than those from embryos avoids concerns with ethics, safety, and immunology. One important issue is the ability to track the transplanted stem cells during the regeneration process to evaluate the stem cell-mediated healing. The objective of this study was to compare the efficiency, longevity, and intensity of carboxyfluorescein diacetate, succinimidyl ester (CFDA SE) and quantum dot nanocrystal (Qtracker™, Invitrogen, Carlsbad, CA, USA) labeled adipose-derived stem cells (ADSC) over an in vitro culture period of 4 weeks. Adipose-derived stem cells (6 x 106) previously isolated and frozen at -196°C were thawed and cultured in 75-cm3 flasks with 14 mL of DMEM. Cells were grown to 80% confluence and trypsinized. After trypsinization, the cells were divided into 4 treatments (3 x 106 cells per treatment). The treatments were (1) unlabeled control, (2) labeled with 30 μM CFDA SE, (3) labeled with 15 nM Qtracker™, and (4) labeled with 15 nM Qtracker™, following the Invitrogen Qtracker™ protocol. Cells (1 x 106) were removed from each treatment every week for 4 weeks and fixed in formalin for later analysis. When all the samples were collected, they were analyzed using flow cytometry. Data were analyzed via chi-square test. The percentage of cells labeled with CFDA SE and Qtracker™ was 99.35 and 98.46%, respectively, immediately after labeling. By 1 wk, the percentage of cells labeled with CFDA SE and Qtracker™ had deceased (P < 0.01) to 0.11 and 1.48%, respectively. The CFDA SE-labeled cell percentages had decreased (P < 0.01) to 0% at 2, 3, and 4 wk, respectively. The Qtracker™-labeled cells also decreased (P < 0.01) to 0.745, 1.69 and 0.45% at 2, 3, and 4 wk, respectively. The high rate of cell division of these cells in vitro might be responsible for the rapid loss of both labels during the first week of culture. Previous results from our lab have shown that the CFDA SE is retained in the cells for up to 6 wk in vivo (Lima AS et al. 2006 Reprod. Fertil. Dev. 18, 208). Similar studies need to be done with the quantum dot-labeled cells to determine the Qtracker™ label’s longevity in vivo. In conclusion, quantum dots can be used to label ADSC, in vitro, for at least 4 wk, albeit at much lower levels than those observed during the week following labeling. Determination of a suitable label for high-percentage porcine ADSC labeling during long-term in vitro culture remains to be completed.
This research was supported by the Intel Scholar’s Program and the Illinois Regenerative Medicine Institute.
In Vivo Imaging Technology of Transplanted Stem Cells Using Quantum Dots for Regenerative Medicine
