Magnetic labeling of mammalian cells with use of ferumoxides and TAs is possible and may enable cellular MR imaging and tracking in experimental and clinical settings.

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Magnetic cellular labeling with the ferumoxides-PLL complex had no short- or long-term toxic effects on tumor or stem cells.

Demyelination is a common pathological finding in human neurological diseases and frequently persists as a result of failure of endogenous repair. Transplanted oligodendrocytes and their precursor cells can (re)myelinate axons, raising the possibility of therapeutic intervention. The migratory capacity of transplanted cells is of key importance in determining the extent of (re)myelination and can, at present, be evaluated only by using invasive and irreversible procedures. We have exploited the transferrin receptor as an efficient intracellular delivery device for magnetic nanoparticles, and transplanted tagged oligodendrocyte progenitor cells into the spinal cord of myelin-deficient rats. Cell migration could be easily detected by using three-dimensional magnetic resonance microscopy, with a close correlation between the areas of contrast enhancement and the achieved extent of myelination. The present results demonstrate that magnetic resonance tracking of transplanted oligodendrocyte progenitors is feasible; this technique has the potential to be easily extended to other neurotransplantation studies involving different precursor cell types. Demyelination is an important pathological component of multiple sclerosis and other neurological disorders. Recent research has suggested that it is possible to promote (re)myelination in animal models of abnormal myelination or demyelination (1-3), either by endogenous oligodendrocytes or exogenous myelinating cells. The latter repair mechanism has received particular attention, because it has been shown that transplanted oligodendrocyte precursor cells can myelinate large areas in the central nervous system (4). A similar therapeutic approach in humans may be pursued and is supported by the safety and effectiveness of human neurotransplantation studies (5, 6). The clinical outcome of such studies will be directly determined by the extent of myelination, and thus by the immediate dispersion, migratory capacity, and long-term survival of grafted cells. A technique that could monitor the grafted cell migration continuously and noninvasively is crucial to guide further advances in neurotransplantation research. We hypothesized that tagging grafted cells with a magnetic label might allow magnetic resonance (MR) tracking of their migratory capacity, not unlike an earlier MR microscopy study that used labeled progeny cells to follow cell movements and lineages in the developing embryo (7). Magnetically labeled cells previously have received interest to study neural grafting procedures (8, 9) and cellular trafficking (10-13) by MR imaging.Oligodendrocyte progenitors have a greater migratory and myelinating capacity than mature glial cells (14, 15). We therefore chose the rat oligodendrocyte progenitor cell line 17) as the graft, with the myelin-deficient (md) rat as a recipient, because the migration pattern of LacZ ϩ CG-4 cells has been documented in detail with this model (18). The md rat carries an X-linked recessive mutation in the proteolipid protein gene and is characteriz...

Ferumoxides, dextran-coated superparamagnetic iron oxide (SPIO) particles, form ferumoxide-transfection agent (FE-TA) complexes that are internalized into endosomes/lysosomes and have been used to label cells for in vivo MRI tracking and localization studies. A better understanding of the physical state of the FE-TA complexes during endocytosis could improve their use. The purpose of this study was to measure the rate of the degradation of iron particles under varying physiological conditions. FE-TA complexes were incubated in seven different buffers containing different chelates with different pH. Reducible iron concentrations, T2 relaxation rates and gradient echo (GRE) magnetic resonance images (MRI) were obtained from each condition immediately after incubation and at 6, 24, 48, 72 and 96 h and days 7, 14 and 21. The dynamics of FE-TA in the endosome/lysosomes within the cells were visualized with electron microscopy. Sodium citrate buffer at pH 4.5 rapidly dissolved FE-TA complexes. However, FE-TA complexes were less soluble in the same buffer at pH 5.5. Similarly, FE-TA complexes were not readily soluble in any of the other buffers with or without chelates, regardless of pH. Electron microscopic images showed degraded FE-TA in some intracellular endosome/lysosomes between days 3 and 5. In the cellular environment, some of the FE-TA-containing endosomes were found to fuse with lysosomes, causing rapid dissociation at low pH and exposing the iron core to chelates that resulted in soluble Fe(III) within the lysosomes. The studies presented represent a first step in identifying the important cellular environmental parameters affecting the integrity of FE-TA complexes.

We report on a novel and straightforward magnetic cell labeling approach that combines three FDA-approved drugs, ferumoxytol (F), heparin (H) and protamine (P) in serum free media to form self-assembling nanocomplexes that effectively label cells for in vivo MRI. We observed that the HPF nanocomplexes were stable in serum free cell culture media. HPF nanocomplexes exhibited a three-fold increase in T2 relaxivity compared to F. Electron Microscopy revealed internalized HPF within endosomes, confirmed by Prussian blue staining of labeled cells. There was no long-term effect or toxicity on cellular physiology or function of HPF-labeled hematopoietic stem cells, bone marrow stromal cells, neural stem cells, and T-cells when compared to controls. In vivo MRI detected 1000 HPF-labeled cells implanted in rat brains. HPF labeling method should facilitate the monitoring by MRI of infused or implanted cells in clinical trials.

This technique can be used to label cells for in vivo MRI tracking of stem cells and lymphocytes. FE at a concentration of 25 to 50 microg/mL with a ratio of SPIO to PLL of 1:0.03 to 1:0.06 would be sufficient to label cells for cellular MRI.

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