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Pitfalls in employing superparamagnetic iron oxide particles for stem cell labelling and in vivo MRI tracking

Superparamagnetic iron oxide (SPIO) particles, including Feridex® (Endorem®), Sinerem® and Resovist®, were developed initially as contrast agents for liver, spleen and lymph node MRI [1, 2]. Recently, there has been growing interest among the scientific community to use SPIO agents for stem cell labelling to allow in vivo MRI tracking of transplanted cells [3]. In addtion to numerous studies in animal models, off-label applications in humans have also been reported [4].

Although labelling stem cells with SPIO particles to render them visible by MRI is an important technique in the field of tissue engineering, it is crucially important not to overlook the following areas during experimental design and interpretation; otherwise, inaccurate or even wrong conclusions can be made.

Prussian blue staining has been commonly used to confirm that the stem cells are labelled with SPIO particles. However, it is not easy using Prussian blue stain to confirm whether SPIO particles are labelled intracellularly or whether they are merely attached to the surface of cells. It can be assumed that the SPIO particles that are merely attached to stem cells surfaces are more likely to fall off the cells during in vitro handling in the later stages and also after being transplanted into the in vivo system. This leads to the possibility that stem cells that initially appeared to have been properly labelled based on Prussian blue staining evidence become "un-labelled" later on. Electron microscopy can be used to overcome the shortfall of Prussian blue staining. However, electron microscopy is costly and time consuming, and it can only look at a small portion of labelled cells.

The fate of intracellularly labelled SPIO particles is not yet fully understood. This will in part depend upon the coating materials and the size of the SPIO particles. Some SPIO particles will eventually be biodegraded within the cells after a period of time. Our own transmission electronic microscope data suggested that active exocytosis activities might exist, i.e. the stem cells might "make an effort" to get rid of unwanted materials (in this case, SPIO particles) that do not appear to be essential for stem cells' survival and regeneration. Therefore, not all stem cells labelled with SPIO particles in vitro will have the presumed level of SPIO particles or a sufficient amount of such particles to render the stem cells trackable in vivo by means of MRI after transplantation.

Apart from the losses discussed above related to the labelling procedure, some SPIO-labelled stem cells may actually die after being transplanted into the in vivo system. Although these dead stem cells can be visualized with MRI, they no longer have the biological functions of migration, engraftment, regeneration and differentiation.

Dead stem cells and their debris will be engulfed by macrophages or other cells with a similar function, such as glial cells in the brain and spinal cord. The SPIO particles removed from stem cells via exocytosis and those that dropped off the surface of the stem cells are also likely be taken up by monocytes/macrophages, especially when stem cells are delivered intravascularly; SPIO particles will also be deposited in lymph nodes. In this case, MRI will also visualize these monocytes/macrophages and lymph nodes.

To conclude, employing superparamagnetic iron oxide particles for stem cell labelling and in vivo MRI tracking is a new and explicated technique. Further experience and knowledge remains to be gained on its proper application. It is prudent not to overlook the complex in vitro and in vivo environment during and after stem cell SPIO labelling for study design and interpretation.

Yours etc.,

Y-X J WANG ¹ H-H WANG ² D W T AU ³ B-S ZOU ⁴ and L-S TENG ²

¹ Department of Diagnostic Radiology and Organ Imaging The Chinese University of Hong Kong Prince of Wales Hospital Shatin Hong Kong E-mail: yixiang_wang@cuhk.edu.hk, ² Department of Oncological Surgery The First Affiliated Hospital College of Medicine Zhejiang University Hangzhou, ³ Department of Biology and Chemistry City University of Hong Kong Kowloon Hong Kong, ⁴ Micro-Nano Technologies Research Center and State Key Lab of CBSC Hunan University Changsha China

Received for publication June 18, 2008. Revision received August 4, 2008. Accepted for publication August 21, 2008.

References

1. Wang YXJ, Hussain SM, Krestin GP. Superparamagnetic iron oxide contrast agents: physiochemical characteristics and applications in MR imaging. European Radiology 2001;11:2319–31.[CrossRef][Medline]
2. Corot C, Robert P, Idée JM, Port M. Recent advances in iron oxide nanocrystal technology for medical imaging. Advanced Drug Delivery Reviews 2006;58:1471–1504.[CrossRef][Medline]
3. Bulte JW, Kraitchman DL. Monitoring cell therapy using iron oxide MR contrast agents. Curr Pharm Biotechnol 2004;5:567–84.[CrossRef][Medline]
4. Zhu JH, Zhou LF, FengGe XW. Tracking neural stem cells in patients with brain trauma. N Engl J Med 2006;355:2376–78.[Free Full Text]

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