MRI technique visualizes and protects cell transplants in diabetic animal models
Gwynne D. Koch
In type I diabetes, commonly known as juvenile diabetes, the immune system destroys the pancreatic beta cells that produce insulin, potentially leading to complications such as heart disease, blindness or kidney failure. Although the condition can be managed with careful monitoring of blood-sugar levels and insulin injections, transplanting healthy beta cells provides better insulin regulation. Traditional transplantation of cells must be accompanied by immune-suppressing drugs to prevent the body from attacking the foreign cells.
Aravind Arepally, Jeff W.M. Bulte and Brad P. Barnett, researchers at Johns Hopkins University in Baltimore, have developed a technique that encapsulates insulin-producing human islet cells in magnetic microcapsules, simultaneously protecting the transplanted cells from immunorejection and enabling magnetic resonance-guided delivery and tracking of the capsules using conventional clinical scanners.
Researchers use an MRI technique to guide the transplantation of magnetic microcapsules containing human pancreatic islet cells into the liver of diabetic swine. The white arrow indicates the needle used to puncture the portal vein, denoted by the black arrow (A). In vivo images of the liver before (B) and 5 min after (C) infusion of the capsules, which were clearly visualized as hypointensities, show the distribution of capsules throughout the liver. Follow-up MRI at three weeks showed no changes in magnetic resonance appearance, indicating that the encapsulated cells retain their functionality in vivo (D, E). Reprinted with permission of Nature Medicine.
The magnetocapsules, which are intended for intravascular delivery, were synthesized by mixing cells with an FDA-approved iron compound called Feridex and two alginate substances used as coating agents in oral medications and in food products for human consumption. The magnetic Feridex particles, which enable tracking by MRI, are biodegradable and have limited toxic side effects. The alginate substances create a thin membrane permeable to insulin but not to antibodies that surround the transplanted cells. Individual clusters containing between 500 and 1000 insulin-producing beta cells were encapsulated using an electrostatic droplet generator, which produces smaller, stronger and more uniform capsules as compared with the older air-jet technique.
The capsules measure about 350 μm in diameter and contain 81 ng of iron — three orders of magnitude higher than typically used — enabling clear visualization of single capsules using three-dimensional, inversion-recovery on-resonance MRI. They employed this positive-contrast method because it provides good capsule surface and soft-tissue contrast, high resolution, whole-body imaging capability and the ability to track magnetically labeled cells in vivo.
In vitro comparisons of stained and conventional unlabeled capsules did not reveal significant differences in capsule permeability, islet cell viability or insulin secretory response. Encapsulated cells were examined using an Olympus epifluorescence microscope equipped with an Olympus digital acquisition system and were macroscopically imaged with a Nikon digital SLR camera.
To assess the magnetocapsules in vivo, the scientists transplanted 6000 capsules containing 500 beta cells each into the abdomens of 15 diabetic mice. MRI was performed at 9.4 T with a Bruker horizontal bore scanner, using a custom-built animal holder and a whole-body volume transmitting/receiving coil. Blood glucose levels, which were measured every two to three days, returned to normal within one week and remained constant for two months, whereas nine out of 15 control mice transplanted with empty capsules died. The scientists concluded that the capsules retain their curative properties in vivo.
To mimic human transplantation, the researchers infused 40,000 magnetocapsules into the livers of 10 healthy swine. They inserted an MR-trackable catheter into the portal vein, a large central vein of the liver, using MRI to provide real-time monitoring of correct catheter positioning and capsule transplantation. Their method enabled access to the portal vein through an approach different from the current technique, x-ray fluoroscopy.
MRI and blood tests performed three weeks after transplantation indicated that the capsules remained intact in the liver, that the cells were secreting insulin at functional levels, and that there were no health complications. In vivo imaging was performed on a 1.5-T clinical MRI scanner from GE Medical Systems of Waukesha, Wis., using a four-channel phased-array coil and a real-time, steady-state free precession sequence. Ex vivo MRI of the explanted livers confirmed the in vivo findings.
The results of the studies on swine, an animal whose larger vasculature closely resembles that of humans, demonstrate the clinical applicability of this method in ongoing improvements in cell transplantation for the treatment of diabetes. Magnetic-resonance-guided islet transplantation might reduce the complication rates associated with x-ray fluoroscopy and might reduce or avoid the need for immunosuppressive therapies.
Labeling semipermeable microcapsules rather than directly labeling cells offers several advantages, including greater labeling uniformity and consistency as well as immunoprotection of the transplanted cells. The team is performing longer-term survival studies in swine and evaluating the use of these methods for other clinical cellular therapies involving liver and stem cells.
Nature Medicine, August 2007, pp. 986–991.
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