Vasculogenic degradable hydrogel to enhance islet engraftment and function within an alternative transplant site

摘要

Introduction: Type 1 diabetes affects millions worldwide, and while exogenous insulin improves life expectancies, patients face high rates of secondary complications. Islet transplantation is a promising alternative therapy that can potentially restore native insulin signaling and eliminate secondary complications of diabetes, but widespread application is hindered in part by a non-ideal intrahepatic islet transplant site that contributes to massive early graft loss and instability. Various alternative sites for donor islets have been explored, including the subcutaneous (SUBQ), small bowel mesentery (SBM) and omentum, or equivalent murine epididymal fat pad (EFP), sites. Key characteristics of an ideal alternative transplant site include low mechanical forces, graft retrievability, and a high degree of vascularization. Recapitulation of the islet organoid's native vascular network in the host is pivotal to transplanted islet engraftment, with vasculogenic endothelial growth factor (VEGF) demonstrating a crucial role in islet revascularization. To that end, we developed a VEGF-bound proteolytically degradable polyethylene glycol (PEG) hydrogel system for islet delivery to diverse transplant sites, and we demonstrate that localized vascular bed enhancement correlates with improved islet engraftment and glycemic control. Methods: 4-arm PEG-Maleimide (20kDa) gels (50 μL) were modified with RGD adhesive peptide and VEGF and delivered in situ to the SUBQ, SBM, or EFP sites by crosslinking with a proteolytically degradable peptide. Recipients were lectin perfused and grafts evaluated via whole mount imaging, analysis via ImageJ. ～550 syngeneic IEQ were delivered to the same sites in gels in diabetic C57BL/6J mice. Blood glucose, body weight, and intraperitoneal glucose tolerance test (IPGTT) were metrics of graft function, with excised grafts evaluated by immunohistochemistry. Results and Discussion: To evaluate the vasculogenic potential of our hydrogel system at the site of implant, gels were delivered to three transplant sites (SUBQ, SBM, and EFP), with or without VEGF vasculogenic factor. Lectin perfused grafts were excised and whole mount imaged (Fig. 1 A, scale bars = 200 μm), and image analysis evaluated vessel fractional area (Fig. 1B) for control (blue symbols) and VEGF (green symbols) groups at 2 (open symbols) and 4 (closed symbols) week time points, where EFP + VEGF outperformed all other sites at both 2 and 4 weeks (†††† P ＜ 0.0001, ††† P ＜ 0.001 vs SUBQ + VEGF; $ P ＜ 0.05 vs. SBM + VEGF), and performed most similarly to pancreatic controls (dashed line). Evaluation of a minimal islet mass of -550 syngeneic IEQ, delivered by in situ gelation in the three sites in diabetic mice either with (solid lines) or without VEGF (dashed lines) demonstrates a positive relationship between degree of vascularization and islet engraftment (Fig. 1C). Conclusion: We demonstrate herein a relationship between degree of vascularization and islet engraftment within an alternative transplant site, as well as a facile method for delivering vasculogenic factors to the transplant site via an in situ, synthetic hydrogel islet delivery system.