Improved islet labeling for longitudinal monitoring by MRI using cationic magnetoliposomes

The transplantation of pancreatic islets (PIs) containing insulin producing beta cells is considered an alternative approach for the treatment of type 1 diabetes mellitus (T1DM) [1]. Currently, different transplantation sites are assessed in clinical trials to improve outcome [2, 3]. To better monitor and validate the success of transplantation, it is desirable to monitor the location of engrafted islets noninvasively [4]. In vivo MR imaging of transplanted islets is one of the most suitable cell tracking methods, however, this requires labeling of the islets with a suitable contrast agent prior to transplantation [5-8]. In this study, we have tested the feasibility of cationic magnetoliposomes (MLs) compared to commercial contrast agents (Endorem and Resovist) by labeling insulinoma cells (INS1E) and freshly isolated rat PIs. It was possible to incorporate MR detectable amounts of MLs in a much shorter time (2 hours) without addition of transfection agents when compared to Endorem and Resovist. MLs did not show negative effects on the PIs viability and functional parameters in vitro. Further, labeled islets were transplanted in the renal sub-capsular region of healthy mice. Hypointense contrast in MR images due to the labeled PIs was detected in vivo upon transplantation. However MRI based detection was not possible when islets were labeled with Endorem and Resovist alone, but after addition of the transfection agent Poly-L-Lysin (PLL). The findings of this study indicate that MLs provides a satisfactory means to image pancreatic islets and have no deleterious effect on their function, which is promising for future pre-clinical and clinical studies involving the assessment of islet transplantation. Magnetoliposomes (MLs) are built up of nm-sized magnetite (Fe3O4) core, in which the original stabilizing lauric acid coat is first replaced by a phospholipid bilayer during incubation and dialysis of the fatty acid coated particle with performed small unilamellar phospholipid vesicles. Mechanistically, it has been proven that the process of ML formation is controlled by spontaneous transfer of phospholipids according to the so-called aqueous transfer model [9,10]. As MLs can be categorized as ‘ultrasmall superparamagnetic iron oxides’ (commonly designated as USPIO ́s), a high gradient magnetic field (HGM) is needed to withdraw the MLs from the incubation mixture. For a 14 nmdiameter iron oxide core covered with an intact phospholipid bilayer, typically, a phospholipid/Fe3O4 (mmol/g) ratio between 0.7 and 0.8 is calculated. The ligand exchange reaction can also be used to incorporate fluorophores (fluorescent labels for microscopic validation) or targeting molecules (small molecules targeting the GLUT2, monoclonal antibodies and potentially nanobodies) to the MLs double-layer.