Idd loci synergize to prolong islet allograft survival induced by costimulation blockade in NOD mice

OBJECTIVE—NOD mice model human type 1 diabetes and are used to investigate tolerance induction protocols for islet transplantation in a setting of autoimmunity. However, costimulation blockade–based tolerance protocols have failed in prolonging islet allograft survival in NOD mice.RESEARCH DESIGN AND METHODS—To investigate the underlying mechanisms, we studied the ability of costimulation blockade to prolong islet allograft survival in congenic NOD mice bearing insulin-dependent diabetes (Idd) loci that reduce the frequency of diabetes.RESULTS—The frequency of diabetes is reduced in NOD.B6 Idd3 mice and is virtually absent in NOD.B6/B10 Idd3 Idd5 mice. Islet allograft survival in NOD.B6 Idd3 mice treated with costimulation blockade is prolonged compared with NOD mice, and in NOD.B6/B10 Idd3 Idd5, mice islet allograft survival is similar to that achieved in C57BL/6 mice. Conversely, some Idd loci were not beneficial for the induction of transplantation tolerance. Alloreactive CD8 T-cell depletion in (NOD × CBA)F1 mice treated with costimulation blockade was impaired compared with similarly treated (C57BL/6.H2^g7 × CBA)F1 mice. Injection of exogenous interleukin (IL)-2 into NOD mice treated with costimulation prolonged islet allograft survival. NOD.B6 Idd3 mice treated with costimulation blockade deleted alloreactive CD8 T-cells and exhibited prolonged islet allograft survival.CONCLUSIONS—Il2 is the Idd3 diabetes susceptibility gene and can influence the outcome of T-cell deletion and islet allograft survival in mice treated with costimulation blockade. These data suggest that Idd loci can facilitate induction of transplantation tolerance by costimulation blockade and that IL-2/Idd3 is a critical component in this process.The NOD mouse is a model of type 1–like autoimmune diabetes and is used to study costimulation blockade–based transplantation tolerance within the context of autoimmunity (1–4). However, costimulation blockade protocols fail in NOD mice. To investigate further the cellular and genetic control of costimulation blockade–induced transplantation tolerance, we used NOD Idd congenic mice that have small introgressed regions of genetic intervals derived from diabetes-resistant C57 stocks. These mice exhibit varying degrees of protection from autoantibodies, insulitis, and diabetes (5). Using Idd congenic NOD mice, we have observed that islet allograft survival is improved by the addition of the diabetes-protective Idd3 locus (6,7).Idd3 modulates infiltration of autoreactive lymphocytes into the islets (8), and there is compelling evidence that Idd3 is the interleukin (IL)-2 gene (9). In vivo stimulated NOD T-cells produce twofold less IL-2 mRNA than cells from NOD congenic mice having protective alleles at Idd3 (9,10). Neutralizing antibodies to IL-2 lead to accelerated disease in NOD mice (11), and targeted genetic disruption of IL-2 accelerates type 1–like autoimmune diabetes (9). Treatment with exogenous IL-2 inhibits diabetes development in NOD mice and improves T regulatory (Treg) function (12). IL-2 is also known to have a nonredundant role in CD8 T-cell activation–induced cell death via the CD95 (Fas) pathway (13), is required for the development of self-tolerance (14), and is essential for the induction of allograft tolerance by costimulation blockade (15). However, IL-2 is a double-edged sword, since administration of IL-2 in vivo can either enhance or depress a cytotoxic T lymphocyte (CTL) response (16).In this study, we show that costimulation blockade fails to delete alloreactive CD8 T-cells in NOD mice. Genetic replacement of IL-2 in NOD.B6 Idd3 mice enhances alloreactive CD8 T-cell deletion and improves islet allograft survival. Finally, we show that Idd3 synergizes with genes within the Idd5 interval, leading to permanent islet allograft survival in a majority of NOD.B6/B10 Idd3 Idd5 mice treated with costimulation blockade.