In vivo tracking of stem cells in brain and spinal cord injury.

Cellular magnetic resonance (MR) imaging is a rapidly growing field that aims to visualize and track cells in living organisms. Superparamagnetic iron oxide (SPIO) nanoparticles offer a sufficient signal for T2 weighted MR images. We followed the fate of embryonic stem cells (ESCs) and bone marrow mesenchymal stem cells (MSCs) labeled with iron oxide nanoparticles (Endorem) and human CD34+ cells labeled with magnetic MicroBeads (Miltenyi) in rats with a cortical or spinal cord lesion, models of stroke and spinal cord injury (SCI), respectively. Cells were either grafted intracerebrally, contralaterally to a cortical photochemical lesion, or injected intravenously. During the first post-transplantation week, grafted MSCs or ESCs migrated to the lesion site in the cortex as well as in the spinal cord and were visible in the lesion on MR images as a hypointensive signal, persisting for more than 30 days. In rats with an SCI, we found an increase in functional recovery after the implantation of MSCs or a freshly prepared mononuclear fraction of bone marrow cells (BMCs) or after an injection of granulocyte colony stimulating factor (G-CSF). Morphometric measurements in the center of the lesions showed an increase in white matter volume in cell-treated animals. Prussian blue staining confirmed a large number of iron-positive cells, and the lesions were considerably smaller than in control animals. Additionally, we implanted hydrogels based on poly-hydroxypropylmethacrylamide (HPMA) seeded with nanoparticle-labeled MSCs into hemisected rat spinal cords. Hydrogels seeded with MSCs were visible on MR images as hypointense areas, and subsequent Prussian blue histological staining confirmed positively stained cells within the hydrogels. To obtain better results with cell labeling, new polycation-bound iron oxide superparamagnetic nanoparticles (PC-SPIO) were developed. In comparison with Endorem, PC-SPIO demonstrated a more efficient intracellular uptake into MSCs, with no decrease in cell viability. Our studies demonstrate that magnetic resonance imaging (MRI) of grafted adult as well as ESCs labeled with iron oxide nanoparticles is a useful method for evaluating cellular migration toward a lesion site.