Magnetic Resonance Imaging Study of Mouse Islet Allotransplantation

Although only 10% of islet transplant recipients maintain insulin independence, 80% of them are C-peptide positive at 5 years. To better understand the fate of transplanted islets, a magnetic resonance imaging (MRI) technique has been used to detect superparamagnetic iron oxide (SPIO)–labeled transplanted islets. Recently, we successfully used a novel MRI contrast agent, chitosan-coated SPIO (CSPIO) nanoparticles, to monitor mouse islet isografts for 18 weeks after transplantation. In the present study, we tested whether CSPIO could be applied to monitor islet allografts, which are supposedly rejected without immune interventions. Male C57BL/6 and Balb/c mice were used as donors and recipients of islet transplantation, respectively. After overnight incubation with or without CSPIO (10 μg/mL), 300 C57BL/6 islets were transplanted under the left kidney capsule of each Balb/c mouse. Starting from day 10 after transplantation, 3.0-Tesla MRI of the recipients was performed weekly. Four mice were followed for ≥38 days. At 38 and 45 days, 1 islet graft was removed for insulin and Prussian blue staining, respectively. From days 10 to 45 after transplantation, CSPIO-labeled islet grafts were visualized on MRI scans as sustained distinct hypointense spots homogeneously located at the upper pole of left kidney, the site of transplantation. At days 38 and 45, the histology of CSPIO-labeled islet grafts revealed insulin and iron staining colocalized in the same areas. Our results in a mouse allotransplantation model indicated that CSPIO-labeled islets survived as long as 45 days with positive MRI.