Introduction: Subcutaneous islet transplantation is an attractive alternative approach for islet transplantation. However, the subcutaneous site is characterized by poor vascularization, thus hindering long-term engraftment of islet grafts. To address this, the use of basic fibroblast growth factor (bFGF) containing agarose rods placed subcutaneously has been proposed as a strategy to induce subcutaneous blood vessels and improve pancreatic islet engraftment. Nevertheless, the optimal duration for rod placement remains unclear. Here, we focus on the temporal change of subcutaneous inflammatory reaction caused by bFGF-containing agarose rod placement and determining the optimal placement period for subcutaneous islet transplantation.
Method: We implanted bFGF-containing agarose rods subcutaneously in C57BL/6 mice, and divided them into two groups based on the duration of rod implantation; the 7-day group and the 14-day group. We then transplanted 500 islets of C57BL/6 mice into the subcutaneous site of both groups. We compared the changes of inflammatory cytokines and cell infiltration in the transplant site were compared between both groups. According to the results, we subcutaneously transplanted pancreatic islets into the 7-day group treated with anti-TNF antibody or anti-IL6 antibody, and evaluated the transition of blood glucose levels.
Results: The 14-day group showed earlier glycemic normalization than the 7-day group (glycemic normalization rate at 30 days post-transplantation: 17% in the 7-day group: 83% in the 14-day group). The duration required for blood glucose levels to recover to a normal range after transplantation was 40.8 days for the 7-day group and 12 days for the 14-day group.  The proportion of macrophages in CD11b-positive cells that infiltrated subcutaneously was significantly higher in the 14-day group (7-day group: 7.8%; 14-day group: 24%; p=0.03), and the ratio of macrophage 1 to macrophage 2 was higher in the 7-day group (2.12 in the 7-day group: 0.10 in the 14-day group, p=0.001). Among the examined inflammatory cytokines, TNF and IL-6 levels in the 14-day group were significantly lower than those in the 7-day group (p<0.05 for both). The 7-day group treated with anti-TNF antibody or anti-IL6 antibody demonstrated improved engraftment efficiency (glycemic normalization rate at 30 days after transplantation: 83%). 
Conclusion: Our findings indicate that the duration of subcutaneous preparation can impact the efficiency of subcutaneous islet transplantation. We believe that optimizing the subcutaneous environment for islet transplantation will lead to greater success of subcutaneous islet transplantation.