Suppression of Islet Xenogeneic Immune Responses by local Indoleamine 2, 3 Dioxygense Expression

Background: Type 1 diabetes (T1D) is characterized by destruction of pancreatic β cells leading to severe medical complications. Successful islet transplantation rising hope that islet transplant would become a new therapeutic approach for treating T1D. One of the primary complications of transplantation is the necessity for lifelong immunosuppression. To address this concern, our research group developed a novel local immunosuppressive therapy based on Indoleamine 2,3 Dioxygense (IDO) expression that prevents islet allograft immune rejection. IDO, catalyzing tryptophan to N-formylkynurenine, is mainly involved in peripheral tolerance induction by suppressing adaptive immune responses. To further complicate efforts of organ donation, human organ shortages have fueled the concept of xeno-organ transplantation. In spite of several attempts to develop an effective immunosuppressive therapy, delayed xenograft rejection, mediated by macrophage infiltration, is still the main issue preventing the widespread usage of animal organs for transplantation. Hypothesis: Local IDO expression can prevent islet xenograft immuno-rejection via modulation of macrophage inflammatory responses and inducing apoptosis in these cells. Methods: For in vivo studies composite three-dimensional islet grafts, engineered by embedding xenogeneic rat islets within the normal or IDO–expressing fibroblasts populated collagen gel matrix, were transplanted into renal subcapsular space of mice. Then, macrophage infiltration and xenogeneic islet transplant uptake were determined. For in vitro studies, Raw264.7cells were cultured in tryptophan deficient media with increasing concentrations of kynurenine (kyn). Raw264.7cells viability and their nitric oxide production in response to Lipopolysaccharide stimulation were evaluated. Results: In vivo studies showed that macrophage infiltration was significantly reduced in IDO-expressing grafts without administration of systemic immunosuppressive agents. Islet structure and functionality remained intact in IDO-expressing grafts while macrophages infiltrated islets in control mice. In vitro studies show that tryptophan deficiency but not kyn enriched environment has significant inhibitory effect on the nitric oxide production by Raw264.7 cells. It was shown that tryptophan deficiency but not high levels of kyn reduce Raw264.7 cells viability by activating apoptosis. Conclusions: Findings of this study supports the feasibility of using local IDO expression as an alternative for systemic immunosuppressive treatments for protecting islet xenotransplantation.