Early-Phase Luciferase Signals of Islet Grafts Predicts Successful Subcutaneous Site Transplantation in Rats

Purpose The transplantation of pancreatic islets is a promising cell replacement therapy for type 1 diabetes. Subcutaneous islet transplantation is currently under investigation as a means to circumvent problems associated with standard intra-hepatic islet transplantation. As modifications are being developed to improve the efficacy of subcutaneous islet transplantation, it is important to have robust methods to assess engraftment. Experimentally, ATP-dependent bioluminescence imaging using luciferase reporter genes has been effective for non-invasively tracking engraftment. However, it was heretofore unknown if the bioluminescence of subcutaneously transplanted luciferase-expressing islet grafts correlates with diabetes reversal, a primary outcome of transplantation. Procedures A retrospective analysis was conducted using data obtained from subcutaneous islet transplantations in Lewis rats. The analysis included transplantations from our laboratory in which islet donors were transgenic rats ubiquitously expressing luciferase and recipients were wild type, streptozotocin-induced diabetic rats. Data from 79 bioluminescence scans were obtained from 27 islet transplantations during the post-transplant observation period (up to 6 weeks). The bioluminescence intensity of the subcutaneously transplanted grafts, captured after the intravenous administration of luciferin, was correlated with diabetes reversal. Results After subcutaneous transplantation, islet bioluminescence decreased over time, dropping > 50 % from 1 to 3 weeks post-transplant. Bioluminescence intensity in the early post-transplant phase (1–2 weeks) correlated with the subsequent reversal of diabetes; based on optimized bioluminescence cutoff values, the bioluminescence intensity of islets at 1 and 2 weeks predicted successful transplantations. However, intensity in the late post-transplant phase (≥ 4 weeks) did not reflect transplantation outcomes. Conclusions Early-phase bioluminescence imaging of luciferase-expressing islets could serve as a useful tool to predict the success of subcutaneous islet transplantations by preceding changes in glucose homeostasis.