The researchers from the Washington University School of Medicine in St. Louis have described a new technique that uses liquid perfluorocarbon nanoparticle labelling of stem cells to track their migration to tumour vasculatures after they are injected into mice models.
The nanoparticles easily incorporate into the stem cells which can then be rapidly imaged by magnetic resonance imaging (MRI) scanners that can be tuned to the 19F nucleus.
"The cells just take these particles in naturally - no special sauces have to be added to make them tasty to these cells," said Dr Samuel Wickline, the lead author of the study which will be published in the June 2007 edition of the journal 'FASEB'.
"Then the cells just go about their business and do what they're supposed to do by homing in on targeted regions of the body."
Current labelling techniques typically use iron oxide nanoparticles, which produce dark contrast effects in proton MRI studies.
However, these nanoparticles are not readily absorbed by the cells, and often require adjunctive treatments to incorporate the markers which can lead to the loss of cell viability.
The perfluorocarbon labelling of the cells was conducted by incubating the cells in an emulsion of the liquid nanoparticles for 12 hours. After incubation the cells were washed to remove any adhered particles from the surface of the cells.
The researchers found a high level of cell survival after labelling, with more than 90 per cent surviving 12 hours - a similar result to the control group.
Furthermore, cell sectioning indicated that the nanoparticles were located in the cell cytoplasm and not simply bound to the plasma membrane.
The researchers also showed that cells injected into an athymic mouse tumour model (MDA-435) migrated within five days to the tumour vasculature indicating the cells maintained their in vivo functionality.
"We can tune an MRI scanner to the specific frequency of the fluorine compound in the nanoparticles, and only the nanoparticle-containing cells will be visible in the scan," said Wickline.
"That eliminates any background signal, which often interferes with medical imaging. Moreover, the lack of interference means we can measure very low amounts of the labelled cells and closely estimate their number by the brightness of the image."
These results were confirmed by confocal microscopy of a fluorescent dye that had been co-internalised with the nanoparticles.
The cells were probed with monoclonal antibodies for characteristic stem and progenitor markers and their expression analysed with flow cytometry.
The cells expressed markers indicative of undifferentiated progenitor stem cells, blood-forming or hematopoietic stem cells, and blood vessel-forming or endothelial cells, including CD34, CD133, Tie-2 and CD31.
In addition, the researchers showed that at least two perfluorocarbon compounds could be used with the technique, both perfluorooctylbromide (PFOB) and perfluoro-15-crown-5-ether (PFCE).
This could potentially allow different types of cell to be labelled with different compounds and then detected separately by tuning the MRI scanner to each compounds specific resonance frequency.