In a series of experiments in animals, researchers at
Johns Hopkins have successfully used a technique that
tracks mesenchymal stem cells via magnetic resonance
imaging to monitor the progress of the cells in repairing
tissue scarred by heart attack.
The Johns Hopkins findings, presented in November at
the American Heart Association's Scientific Sessions and
published in a supplement to the journal Circulation, are
believed to be the first demonstration of how the
technique, which labels the cells with minuscule iron oxide
particles, can be used to assess the clinical benefit, if
any, of cell-based therapies.
According to senior investigator and veterinary
radiologist Dara Kraitchman, the technique has potentially
broader applications and benefits for patient care because
MRI technology is widely available and avoids the
discomfort and risk of infection from biopsies, the
standard method used in therapy checkups.
In a related study, also presented at the meeting, the
Johns Hopkins team showed that a more advanced technique
used with MRI, called inversion recovery with on-resonant
water suppression, or IRON for short, could be used to
monitor iron-labeled stem cells and to guide deployment of
a stent, a device that widens arteries at risk of clogging
and prompting a heart attack.
Previous Johns Hopkins research on animals whose
hearts had been injected with adult stem cells showed that
heart function was restored to its original condition
within two months, and more than 75 percent of dead scar
tissue disappeared, having been replaced with
healthy-looking heart tissue. Clinical studies are now
under way at Johns Hopkins and elsewhere to find out if
similar benefits result in humans.
It is still a scientific puzzle as to whether adult
stem cells develop into new and healthy heart tissue, or
exactly how long their healing effects last, but MRI offers
the best chance for determining just how well the therapy
works at repairing damaged hearts, Kraitchman said.
The researchers made stem cells visibly distinct from
all others by labeling them with a metallic compound made
up of iron oxide nanoparticles, one-thousandth of a
millimeter in diameter, which can be permanently taken up
within cells and, unlike most other metals, seen by MRI.
In the latest study, 13 dogs underwent surgery to
create heart muscle damage similar to what happens in a
naturally occurring heart attack. Six were treated with
iron oxide-labeled stem cells and seven served as study
controls, receiving no stem-cell injections.
Mesenchymal bone marrrow stem cells, known to give
rise to a variety of cell types, were injected across three
regions of the hearts to find out if injecting one region
or another made a difference in how well the heart
recovered. The sites included heart attack-damaged muscle
consisting of mostly dead scar tissue, and normal undamaged
heart tissue, as well as tissue at the border area between
the scar and normal tissue, called the peri-infarction
Kraitchman, an associate professor at the
School of Medicine, said that in the peri-infarction
zone, some life remains in the tissue, including a working
vascular supply with some capillaries.
For two months after injection, the animals were
monitored by cardiac MRI at weekly intervals.
The damaged area decreased significantly, by 20
percent in both groups, showing that healing had occurred.
However, the size, or mass, of left ventricular tissue
decreased by 2.5 percent in the stem-cell group, while the
control group lost more than 20 percent of its mass,
indicating that the stem cell-treated hearts were
maintaining their muscle strength during the healing
process while control hearts were showing steady signs of
failure and reduced function.
MRI also showed that stem cells were incorporated into
the heart tissue itself, mostly in the peri-infarction
zone. Measurements of the heart's pumping function also
improved in the same region.
"Our results show that MRI tracking of mesenchymal
stem cells can be used--as a replacement to surgical
biopsy--to verify that such cell-based therapies reached
damaged areas of the heart and were able to effect repair
and improve heart function, said study senior
co-investigator Jeff Bulte, an associate professor of
radiology, who developed the labeling method.
In other experiments in rabbits and dogs, the
researchers successfully used the IRON method, which was
developed at Johns Hopkins, to track relatively small
numbers of stem cells in the body and to deploy a metallic
stent, a mesh-like device that opens blood vessels narrowed
by fat and calcium buildup.
Physicians must confirm that potential therapies,
whether they are cell-based or involve devices, are
delivered as planned to the targeted organ or other part of
the body, said lead investigator Wesley Gilson, a
postdoctoral research fellow. Gilson's work was recognized
at the heart meeting, where he was a finalist for the
prestigious Melvin Judkins Young Clinical Investigator
With IRON, conventional MRI technology is adapted to
reveal images of ever-smaller numbers of cells, avoiding
image artifacts that mimic the appearance of iron-labeled
Scientists were able to visualize metal objects, which
previously appeared as dark spots on the MRI screen, by
suppressing the vast majority of the conventional image
produced from water molecules (or so-called on-resonant
signal), the most common substance in the body.
By eliminating the water-based signals, the scientists
were left only with the signal produced from metal objects
(or so-called off-resonant signal), such as prosthetic
screws, metal clips or stents.
In effect, Gilson said, the machine was made sensitive
to iron molecules in the formerly unseen region. "We
effectively shut out what we could easily see, to focus on
what remained, and it worked. What formerly appeared on MRI
as signal voids, or hyperintense signals, became clearly
To validate the new technique, the Johns Hopkins team
successfully differentiated between concentrations of
iron-labeled stem cells, extracted from a dog's bone
marrow. In lab testing, the various concentrations, ranging
from hundreds of cells to 2 million mesenchymal stem cells
per 100 microliters of fluid, were injected into infarcted
heart tissue from a dog, and then made visible by MRI using
IRON. In another related experiment, injections of
iron-labeled stem cells were made into a leg of a rabbit to
see if they could be tracked in the body with the IRON-MRI
Results from both experiments showed that the new
method could distinguish each concentration from the other
and also track the labeled stem cells in the beating heart
To test other applications of MRI using IRON, the team
tracked delivery of an implantable cardiac device, a stent,
in a dog. The researchers successfully deployed a
conventional stainless steel stent, with a catheter, to one
of the animal's main arteries. In conventional MRI, the
stainless steel stent would produce a large signal void
that blurs the image of surrounding tissue. With MRI using
IRON, however, the stent appeared as a bright object,
similar to how it would appear if tracked by X-ray imaging,
the traditional method used to deploy the devices.
The ready availability of MRI machines means that IRON
could be widely introduced into clinical practice within a
relatively short time, Gilson said.
Funding for this study was provided by the National
Institutes of Health. Additional support came from Osiris
Therapeutics, of Baltimore, which developed the process for
preparing the adult mesenchymal stem-cell product extracted
from bone marrow that was used in the study.
Besides Kraitchman, Bulte and Gilson, researchers
involved in these studies were Matthias Stuber, Lawrence
Hofmann, Dorota Kedziorek and Parag Karmarkar.