In vitro labeling and MRI of mesenchymal stem cells from human umbilical cord blood

OBJECTIVE: The aim of this study was to label human umbilical cord blood mesenchymal stem cells (MSCs) with poly-l-lysine (PLL)-conjugated superparamagnetic iron oxide particles and to obtain magnetic resonance (MR) images of the labeled MSCs' suspension at 1.5 T. MATERIAL AND METHODS: PLL was conjugated with iron oxide to form superparamagnetic particles called Fe(2)O(3)-PLL. Human umbilical cord blood MSCs were isolated, purified, expanded and incubated with Fe(2)O(3)-PLL. Prussian blue stain was performed to show intracellular iron; spectrometry was used to quantify iron uptake within cells. Tetrazolium salt (MTT) assay was applied to evaluate toxicity and proliferation of MSCs labeled with various concentrations of Fe(2)O(3)-PLL. The cell apoptosis rate was determined by annexin V/propichium iodide (PI) double staining method. Vials containing cells underwent MR imaging (MRI) with T(1), T(2) and T(2)* weighted MRI. RESULTS: Iron-containing intracytoplasmatic vesicles could be observed clearly with Prussian blue staining in all samples except the unlabeled control. The iron content per cell determined by spectrometry was 64.51+/-10.32 pg. Among MSCs with and without labeling of various concentrations of Fe(2)O(3)-PLL, MTT values of light absorption had no statistically significant difference (Kruskal-Wallis test, chi(2)=10.35, P=.17). A concentration at 20 mug/ml of iron appeared most suitable for incubating cells. Of labeled and unlabeled MSCs, the early annexin V-fluorescein isothiocyanate (FITC)-positive/PI-negative] and late (annexin V-FITC-positive/PI-positive) apoptotic cells were 10.34+/-0.43%/11.36+/-1.30% and 4.01+/-1.76%/2.98+/-1.37%, respectively, and there were no significant differences between them (P>.05). T(2) weighted image (WI) and T(2)*WI demonstrated significant decrease of signal intensity (SI) in vials containing 1 x 10(6) (1 day), 1x10(6) (8 days) and 5 x 10(5) labeled cells, in comparison with unlabeled cells (P<.05). The percentage change of SI (DeltaSI) was significantly higher in 10(6) labeled cells after 1-day culture than that in the same number of labeled cells after 8-day culture and that in 5 x 10(5) labeled cells, particularly on T(2)*WI (P<.05). Among pulse sequences, T(2)*WI demonstrated the highest DeltaSI (P<.05). CONCLUSION: The human umbilical cord blood MSCs can be labeled with Fe(2)O(3)-PLL without significant change in viability and apoptosis. The suspension of labeled MSCs can be imaged with standard 1.5-T MR equipment.