Expression of human type 1 diabetes requires both genetic predisposition and an environmental, probably viral, "trigger." Our laboratory has expertise in both diabetes pathogenesis and islet transplantation in rodent models, and here we propose studies of costimulation-based strategies to prevent, reverse, and cure diabetes. Our goal is to understand in the rat how costimulatory pathways control the balance between T effector and T regulatory (Treg) cells using: 1) unique new reagents and siRNA technology; 2) a novel in vitro system with which to study Treg and T effector cell development; and 3) new non-lymphopenic rat models that express diabetes in response to environmental agents including toll-like receptor (TLR) ligation and viral infection, immunological perturbants thought to be important in humans. We will use these resources to test the hypothesis that blockade of both initiating and effector/memory costimulatory pathways will be required to prevent and reverse autoimmune diabetes. Our overall approach will be to investigate T effector cell modulation by costimulation-blockade at 3 well-defined stages of diabetes development: 1) before disease expression begins, 2) as insulitis develops but before overt diabetes occurs, and 3) during autoimmune recurrence in islet graft recipients. Because our models are based on environmentally induced autoimmune diabetes, we will be able to identify the costimulatory pathways functional at each of these clinically important disease stages. Specific Aim No. 1 is to identify mechanisms by which different costimulatory pathways control generation of Treg and T effector cells. We will use a novel in vitro system and siRNA technology to test the hypothesis that costimulation blockade can modulate the intrathymic generation of T effector or Treg cells. Specific Aim No. 2 is to identify the mechanisms by which different costimulatory pathways modulate T effector cells during the early, prodromal phase of disease. We will use new reagents and our rat models to test the hypothesis that rationally designed combinations of reagents that block costimulation will prevent autoimmune diabetes. Specific Aim No. 3 is to investigate the mechanisms by which blockade of different costimulatory pathways prevents recurrent autoimmunity. We will use our extensive experience in islet transplantation and costimulation-based transplantation tolerance induction to test the hypothesis that rationally designed costimulatory blockade protocols will preclude autoimmune recurrence in diabetic islet graft recipients. Our belief and ultimate hope is that the data we generate will result in the design of therapeutic strategies that can be translated into clinical trial protocols.