Objective: To determine transferrin receptor (TfR) expression of human mesenchymal stem cells (MSCs) in vitro and after transplantation in rabbit spinal cord,and to detect implanted MSCs by in vitro autoradiography.
Methods: Human mesenchymal stem cells (hMSCs) were isolated from fetal blood. Flow cytometry assay, immuno-fluorescent staining and receptor binding assay were used to determine TfR expression of hMSCs. Radioiodinated transferrin saturated with iron [(125)I-Tf(Fe)(2)] was used as tracer. The hMSCs transplanted in rabbit spinal cord was tracked by in vitro autoradiography. Diffusion of (125)I-Tf(Fe)(2) in spinal cord was examined with autoradiography.
Results: TfR expression of MSCs was demonstrated by flow cytometry assay, immuno-fluorescent staining and receptor binding assay in vitro. (125)I-Tf(Fe)(2) bound to hMSCs with a equilibrium dissociation constant (KD) of (0.98+/-0.12) nmol/L and a maximal density of binding sites (B(max)) of (107 702+/-6 226) sites per cell. Immuno-fluorescent staining showed that TfRs were expressed on hMSCs on the 2nd day but not be expressed on the 10th day post transplantation. Autoradiography showed distinct accumulation of (125)I-Tf(Fe)(2) but not (125)I-HSA at hMSCs implantation sites of spinal cord sections on the 2nd day post transplantation. (125)I-Tf(Fe)(2) had diffused into spinal cord 16 hours after incubation.
Conclusion: Implanted hMSCs could be detected by in vitro autoradiography with (125)I-Tf(Fe)(2) on the 2nd day after being transplanted in spinal cord. To track implanted hMSCs with radionuclide imaging techniques in vivo, TfR was a suitable target for imaging and radioiodinated Tf(Fe)(2) was a feasible tracer.