Magnetic labeling and in vitro MR imaging of rat bone marrow mesenchymal stem cells

Objective: To label rat bone marrow mesenchymal stem cells with feridex combined with poly-l-lysine (PLL), and to determine the feasibility of detection of magnetically labeled stem cells with MR imaging. Methods: Feridex were incubated with PLL for 1 hour to obtain a complex of feridex-PLL. Mesenchymal stem cells isolated from the bone marrows of Wistar rats were cultured and expanded. By the 4th passage, cells were co-incubated overnight with the feridex-PLL complex. Prussian blue staining for demonstrating intracytoplastic nanoparticles and trypan-blue exclusion test for cell viability were performed respectively at 24 h, 1 w, 2 w, 3 w after labeling. MR imaging of cell suspensions was performed by using T₁ WI, T₂ WI and T₂* WI sequences at a clinical 1.5 T MR system. Results: Numerous intracytoplastic iron particles were stained with Prussian blue. With division of stem cells, the stained particles were seen decreased gradually. Trypan blue exclusion test at 24 h, 1 w, 2 w and 3 w showed that the viability of the labeled cells was 91.00%, 93.00%, 91.75%, and 92.50%, not significantly different with that of nonlabeled cells (P > 0.05). For 10³, 10⁴ and 10⁵ cells, T₂ signal intensity decreased by 63.75%, 82.31% and 91.92% respectively, T₂* signal intensity decreased by 68.24%, 83.01%, and 93.94% respectively. For 10⁵ labeled cells, T₂* signal intensity decreased by 93.75%, 75.92%, 41.75% and 8.83% respectively at 24 h, 1 w, 2 w and 3 w after labeling. Conclusion: Magnetic labeling of rat bone marrow stem cells with feridex-PLL complex is feasible, efficient and safe. T₂* WI is the most sensitive sequence to detect the labeled cells. The degree of T₂ signal decreasing may be related to the cell count and division phase.