Tracking intrahepatically transplanted islets labeled with Feridex-polyethyleneimine complex using a clinical 3.0-T magnetic resonance imaging scanner

Magnetic Resonance Imaging of Transplanted Porcine Neonatal Pancreatic Cell Clusters Labeled with Exendin-4-Conjugated Manganese Magnetism-Engineered Iron Oxide Nanoparticles

Recently, we have shown that manganese magnetism-engineered iron oxide nanoparticles (MnMEIO NPs) conjugated with exendin-4 (Ex4) act as a contrast agent that directly trace implanted mouse islet β-cells by magnetic resonance imaging (MRI). Here we further advanced this technology to track implanted porcine neonatal pancreatic cell clusters (NPCCs) containing ducts, endocrine, and exocrine cells. NPCCs from one-day-old neonatal pigs were isolated, cultured for three days, and then incubated overnight with MnMEIO-Ex4 NPs. Binding of NPCCs and MnMEIO-Ex4 NPs was confirmed with Prussian blue staining in vitro prior to the transplantation of 2000 MnMEIO-Ex4 NP-labeled NPCCs beneath the left renal capsule of six nondiabetic nude mice. The 7.0 T MRI on recipients revealed persistent hypointense areas at implantation sites for up to 54 days. The MR signal intensity of the graft on left kidney reduced 62–88% compared to the mirror areas on the contralateral kidney. Histological studies showed colocalization of insulin/iron and SOX9/iron staining in NPCC grafts, indicating that MnMEIO-Ex4 NPs were taken up by mature β-cells and pancreatic progenitors. We conclude that MnMEIO-Ex4 NPs are excellent contrast agents for detecting and long-term monitoring implanted NPCCs by MRI.

Exendin-4-Conjugated Manganese Magnetism-Engineered Iron Oxide Nanoparticles as a Potential Magnetic Resonance Imaging Contrast Agent for Tracking Transplanted β-Cells

To specifically detect and trace transplanted islet β-cells by magnetic resonance imaging (MRI), we conjugated manganese magnetism-engineered iron oxide nanoparticles (MnMEIO NPs) with exendin-4 (Ex4) which specifically binds glucagon-like peptide-1 receptors on the surface of β-cells. The size distribution of MnMEIO and MnMEIO-Ex4 NPs were 67.8 ± 1.3 and 70.2 ± 2.3 nm and zeta potential 33.3 ± 0.5 and 0.6 ± 0.1 mV, respectively. MnMEIO and MnMEIO-Ex4 NPs with iron content ≤ 40 μg/mL did not affect MIN6 β-cell viability and insulin secretion. Positive iron staining was found in MIN6 β-cells loaded with MnMEIO-Ex4 NPs but not in those with MnMEIO NPs. A transmission electron microscope confirmed MnMEIO-Ex4 NPs were distributed in the cytoplasm of MIN6. In vitro MR images revealed a loss of signal intensity in MIN6 β-cells labeled with MnMEIO-Ex4 NPs but not with MnMEIO NPs. After transplantation of islets labeled with MnMEIO-Ex4, the graft under kidney capsule could be visualized on MRI as persistent hypointense areas up to 17 weeks. Moreover, histology of the islet graft showed positive staining for insulin, glucagon and iron. Our results indicate MnMEIO-Ex4 NPs are safe and effective for the detection and long-term monitoring of transplanted β-cells by MRI.

Magnetic Resonance Imaging of Transplanted Porcine Neonatal Pancreatic Cell Clusters Labeled with Chitosan-Coated Superparamagnetic Iron Oxide Nanoparticles in Mice

Neonatal pancreatic cell clusters (NPCCs) are potential tissues for the treatment of diabetes. Different from adult cells, they continuously proliferate and differentiate after transplantation. In this study, we utilized magnetic resonance imaging (MRI) to detect and monitor implanted NPCCs. NPCCs were isolated from one-day-old neonatal pigs, cultured for three days, and then incubated overnight with the contrast agent chitosan-coated superparamagnetic iron oxide (CSPIO) nanoparticles. In vitro, Prussian blue staining and MR scans of CSPIO-labeled NPCCs were performed. In vivo, we transplanted 2000 CSPIO-labeled NPCCs under the kidney capsule of nondiabetic nude mice. Recipients were scanned with 7.0T MRI. Grafts were removed for histology with insulin and Prussian blue staining. After being incubated overnight with CSPIO, NPCCs showed positive iron staining and appeared as dark spots on MR scans. After transplantation of CSPIO-labeled NPCCs, persistent hypointense areas were observed at recipients’ implant sites for up to 54 days. Moreover, histology showed colocalization of the insulin and iron staining in 15-, 51- and 55-day NPCC grafts. Our results indicate that transplanted NPCCs survived and differentiated to β cells after transplantation, and that MRI is a useful tool for the detection and monitoring of CSPIO-labeled NPCC grafts.

MRI tracking of autologous pancreatic progenitor-derived insulin-producing cells in monkeys

Insulin-producing cells (IPCs) derived from a patient’s own stem cells offer great potential for autologous transplantation in diabetic patients. However, the limited survival of engrafted cells remains a bottleneck in the application of this strategy. The present study aimed to investigate whether nanoparticle-based magnetic resonance (MR) tracking can be used to detect the loss of grafted stem cell-derived IPCs in a sensitive and timely manner in a diabetic monkey model. Pancreatic progenitor cells (PPCs) were isolated from diabetic monkeys and labeled with superparamagnetic iron oxide nanoparticles (SPIONs). The SPION-labeled cells presented as hypointense signals on MR imaging (MRI). The labeling procedure did not affect the viability or IPC differentiation of PPCs. Importantly, the total area of the hypointense signal caused by SPION-labeled IPCs on liver MRI decreased before the decline in C-peptide levels after autotransplantation. Histological analysis revealed no detectable immune response to the grafts and many surviving insulin- and Prussian blue-positive cell clusters on liver sections at one year post-transplantation. Collectively, this study demonstrates that SPIO nanoparticles can be used to label stem cells for noninvasive, sensitive, longitudinal monitoring of stem cell-derived IPCs in large animal models using a conventional MR imager.

Magnetic resonance imaging of mouse islet grafts labeled with novel chitosan-coated superparamagnetic iron oxide nanoparticles

OBJECT

To better understand the fate of islet isografts and allografts, we utilized a magnetic resonance (MR) imaging technique to monitor mouse islets labeled with a novel MR contrast agent, chitosan-coated superparamagnetic iron oxide (CSPIO) nanoparticles.

MATERIALS AND METHODS

After being incubated with and without CSPIO (10 µg/ml), C57BL/6 mouse islets were examined under transmission electron microscope (TEM) and their insulin secretion was measured. Cytotoxicity was examined in α (αTC1) and β (NIT-1 and βTC) cell lines as well as islets. C57BL/6 mice were used as donors and inbred C57BL/6 and Balb/c mice were used as recipients of islet transplantation. Three hundred islets were transplanted under the left kidney capsule of each mouse and then MR was performed in the recipients periodically. At the end of study, the islet graft was removed for histology and TEM studies.

RESULTS

After incubation of mouse islets with CSPIO (10 µg/mL), TEM showed CSPIO in endocytotic vesicles of α- and β-cells at 8 h. Incubation with CSPIO did not affect insulin secretion from islets and death rates of αTC1, NIT-1 and βTC cell lines as well as islets. After syngeneic and allogeneic transplantation, grafts of CSPIO-labeled islets were visualized on MR scans as persistent hypointense areas. At 8 weeks after syngeneic transplantation and 31 days after allogeneic transplantation, histology of CSPIO-labeled islet grafts showed colocalized insulin and iron staining in the same areas but the size of allografts decreased with time. TEM with elementary iron mapping demonstrated CSPIO distributed in the cytoplasm of islet cells, which maintained intact ultrastructure.

CONCLUSION

Our results indicate that after syngeneic and allogeneic transplantation, islets labeled with CSPIO nanoparticles can be effectively and safely imaged by MR.

In vivo imaging of transplanted islets labeled with a novel cationic nanoparticle

To monitor pancreatic islet transplantation efficiency, reliable noninvasive imaging methods, such as magnetic resonance imaging (MRI) are needed. Although an efficient uptake of MRI contrast agent is required for islet cell labeling, commercially-available magnetic nanoparticles are not efficiently transduced into cells. We herein report the in vivo detection of transplanted islets labeled with a novel cationic nanoparticle that allowed for noninvasive monitoring of islet grafts in diabetic mice in real time. The positively-charged nanoparticles were transduced into a β-cell line, MIN6 cells, and into isolated islets for 1 hr. MRI showed a marked decrease in the signal intensity on T1- and T2-weighted images at the implantation site of the labeled MIN 6 cells or islets in the left kidneys of mice. These data suggest that the novel positively-charged nanoparticle could be useful to detect and monitor islet engraftment, which would greatly aid in the clinical management of islet transplant patients.

Obstacles on the way to the clinical visualisation of beta cells: looking for the Aeneas of molecular imaging to navigate between Scylla and Charybdis

For more than a decade, researchers have been trying to develop non-invasive imaging techniques for the in vivo measurement of viable pancreatic beta cells. However, in spite of intense research efforts, only one tracer for positron emission tomography (PET) imaging is currently under clinical evaluation. To many diabetologists it may remain unclear why the imaging world struggles to develop an effective method for non-invasive beta cell imaging (BCI), which could be useful for both research and clinical purposes. Here, we provide a concise overview of the obstacles and challenges encountered on the way to such BCI, in both native and transplanted islets. We discuss the major difficulties posed by the anatomical and cell biological features of pancreatic islets, as well as the chemical and physical limits of the main imaging modalities, with special focus on PET, SPECT and MRI. We conclude by indicating new avenues for future research in the field, based on several remarkable recent results.

Current status of immunomodulatory and cellular therapies in preclinical and clinical islet transplantation

Clinical islet transplantation is a β-cell replacement strategy that represents a possible definitive intervention for patients with type 1 diabetes, offering substantial benefits in terms of lowering daily insulin requirements and reducing incidences of debilitating hypoglycemic episodes and unawareness. Despite impressive advances in this field, a limiting supply of islets, inadequate means for preventing islet rejection, and the deleterious diabetogenic and nephrotoxic side effects associated with chronic immunosuppressive therapy preclude its wide-spread applicability. Islet transplantation however allows a window of opportunity for attempting various therapeutic manipulations of islets prior to transplantation aimed at achieving superior transplant outcomes. In this paper, we will focus on the current status of various immunosuppressive and cellular therapies that promote graft function and survival in preclinical and clinical islet transplantation with special emphasis on the tolerance-inducing capacity of regulatory T cells as well as the β-cells regenerative capacity of stem cells.

Noninvasive imaging techniques in islet transplantation

Since the Edmonton trials, insulin independence can reproducibly be achieved after islet transplantation. However, a majority of patients resume insulin treatment in the first 5 years after transplantation. Several mechanisms have been proposed but are difficult to pinpoint in one particular patient. Current tools for the metabolic monitoring of islet grafts indicate islet dysfunction when it is too late to take action. Noninvasive imaging of transplanted islets could be used to study β-cell mass and β-cell function just after infusion, during vascularization or autoimmune and alloimmune attacks. This review will focus on the most recent advances in various imaging techniques (bioluminescence imaging, fluorescence optical imaging, MRI, and positron emission tomography). Emphasis will be placed on pertinent approaches for translation to human practice.

Improved quantification of islet transplants by magnetic resonance imaging with Resovist

OBJECTIVES

We aimed to develop an accurate and reproducible method to quantify transplanted islets and monitor their functional status in vivo. To support this aim, we investigated the cytotoxic effect of Resovist on islet function and survival.

METHODS

The average pixel number for a single Resovist-labeled islet was measured. To determine Resovist cytotoxicity, DNA fragmentation, adenosine diphosphate-adenosine triphosphate ratio, ion channel activity, and in vivo islet function were evaluated. To quantitatively monitor the fate of islet transplant, we transplanted Resovist-labeled islets into syngeneic C57BL/6 mice for magnetic resonance imaging analysis.

RESULTS

The average pixel volume for a medium-sized islet (100-150 μm in diameter) was determined from the contrast signal void of magnetic resonance image. Toxicological analysis showed that Resovist did not affect islet at concentrations up to 40 times the labeling dose. In the quantitative analysis, the number of contrast spots did not correlated with the number of transplanted islets, whereas our newly adopted measure showed a significant correlation.

CONCLUSIONS

Islet transplant survival may be safely and accurately monitored using magnetic resonance imaging with the Resovist. We found in this study that pixel number may correlate more closely than the number of contrast spots with the number of islets transplanted.

Target-specific agents imaging ectopic and orthotopic human pancreatic cancer xenografts

OBJECTIVES

This study aimed to develop target-specific binding agents for in vitro and in vivo imaging of human pancreatic cancer.

METHODS

A monoclonal neutrophil gelatinase-associated lipocalin (NGAL)-specific antibody and a peptide specific for matrix metalloproteinase (MMP) were labeled with a near-infrared dye for in vitro and in vivo imaging studies. Fluorescence or confocal microscopy was used to determine antibody or peptide binding and internalization of agents into human AsPC-1, Panc-1, and MiaPaCa pancreatic cancer cell lines and in mice bearing ectopic or orthotopic pancreatic tumor transplants.

RESULTS

Both the NGAL-specific antibody and MMP peptide bound to pancreatic cancer cells with high specificity; most NGAL-specific antibody localized to the cytosol. In vivo imaging results demonstrated high signal intensity of both agents bound to the tumor. The average tumortr-to-background ratio of antibody and peptide was 1.29 and 2.86, respectively. Signal was also detectable in the liver, kidneys, and bladder.

CONCLUSIONS

Both NGAL-specific antibody and MMP peptide bound to cancer cells, and the labeled antibody was internalized. These results demonstrate that both agents can be used to enhance detection of human pancreatic cancer xenografts. However, the biodistribution patterns of these agents might limit their use in research and clinical practice.

A novel method for quantitative monitoring of transplanted islets of langerhans by positive contrast magnetic resonance imaging

The Automatic Quantitative Ultrashort Echo Time imaging (AQUTE) protocol for serial MRI allows quantitative in vivo monitoring of iron labeled pancreatic islets of Langerhans transplanted into the liver, quantifying graft implantation and persistence in a rodent model. Rats (n = 14), transplanted with iron oxide loaded cells (0-4000 islet equivalents, IEQ), were imaged using a 3D radial ultrashort echo time difference technique (dUTE) on a Siemens MAGNETOM 3T clinical scanner up to 5 months postsurgery. In vivo 3D dUTE images gave positive contrast from labeled cells, suppressing liver signal and small vessels, allowing automatic quantification. Position of labeled islet clusters was consistent over time and quantification of hyperintense pixels correlated with the number of injected IEQs (R² = 0.898, p < 0.0001), and showed persistence over time (5 months posttransplantation). Automatic quantification was superior to standard imaging and manual counting methods, due to the uniform suppressed background and high contrast, resulting in significant timesavings, reproducibility and ease of quantification. Three-dimensional coverage of the whole liver in the absence of cardiac/respiratory artifact provided further improvement over conventional imaging. This imaging protocol reliably quantifies transplanted islet mass and has high translational potential to clinical studies of transplanted pancreatic islets.