Knee Surgery: Iron Helps Track Stem Cells

07/18/2013

A new, noninvasive technique for tracking stem cells after transplantation developed at the Stanford University School of Medicine could help surgeons determine whether a procedure to repair injured or worn-out knees is successful.

The technique relies on an imaging agent already approved by the U.S. Food and Drug Administration for an entirely different purpose: anemia treatment. Although this study used rodents, the approach is likely to be adapted for use in humans this fall as part of a clinical trial in which mesenchymal stem cells will be delivered to the site of patients' knee injuries. Mesenchymal stem cells are capable of differentiating into bone and cartilage, as well as muscle, fat and tendon, but not into the other cell types that populate the body.

Mesenchymal stem cells have been used with some success in cartilage-repair procedures. "These cells can be easily derived from bone marrow of patients who are going to undergo the knee-repair procedure," said Heike Daldrup-Link, associate professor of radiology. "And they can differentiate into the real-life tissues that compose our joints. But here, too, things can go wrong. The newly transferred cells might fail to engraft, or die. They might migrate away. They could develop into tissues other than cartilage, most commonly fibrous scar tissue."

Relatively few transplanted cells go the distance. The ability to monitor the cells' engraftment after they are deposited at a patient's knee-injury site is therefore essential. With the new technique, magnetic resonance imaging can visualize stem cells for several weeks after they have been implanted, giving orthopaedic surgeons a better sense of whether the transplantation was successful.

Until now, the only ways of labeling mesenchymal stem cells so that they could be noninvasively imaged have required their manipulation in the laboratory. Upon extraction, the delicate cells have to be given to lab personnel, incubated with contrast agents, spun in a centrifuge and washed and returned to the surgeons, who then transplant the cells into a patient.

The new technique involves labeling the cells before extraction, while they reside in the donor's bone marrow. For the study, medical doctor Aman Khurana and Fanny Chapelin, a research associate, injected ferumoxytol, an FDA-licensed anemia treatment composed of iron-oxide nanoparticles, into rats prior to extracting bone marrow from them. Then, after enriching the mixture for mesenchymal stem cells, the investigators injected it into the sites of knee injuries in recipient rats. They followed the implanted cells' progress for up to four weeks, comparing the results with those obtained both from cells labeled in laboratory dishes and from unlabeled cells.

Daldrup-Link and others previously have used ferumoxytol for stem-cell labeling in a dish. However, mesenchymal stem cells in a laboratory dish take up very little of this substance. Interestingly, the researchers showed in a series of experiments that, ensconced in donor rats' bone marrow, the same cells are avid ferumoxytol absorbers. Even several weeks after transplantation into the recipient rats' knees, the mesenchymal stem cells retain enough iron to provide a strong MRI signal.

COMPAMED.de; Source: Stanford University