PROJECT SUMMARY Programming of self-antigen specific CD4+ Tregs in the thymus is essential for suppression of aberrant immune responses and prevention of autoimmunity. Tregs also control inflammation during viral infection and following pathogen clearance. This is particularly relevant for some types of viral infections such as the 1918 influenza pandemic strain, and more recently, specific strains of coronaviruses, where the immune response itself can be pathogenic. In addition to viral infections, high levels of type I IFN are a defining feature of some autoimmune diseases, such as SLE or Sjögren’s syndrome; Tregs reduce the symptoms of such autoimmune diseases as well. However, we know relatively little about the Tregs involved in either of the above processes. For example, what types of Tregs are involved in these responses? Do Tregs that dampen anti-viral immune responses or suppress SLE-like disease develop in the thymus? If so, what is the thymic niche that controls differentiation of this Treg subset? What APCs/stromal cells are required for differentiation of this Treg subset? What specific functional roles does this Treg subset play during viral infections or SLE? Our preliminary data demonstrate that a unique subset of Tregs develops in the thymus characterized by a strong interferon-stimulated gene-signature (ISG-Tregs). We propose that this ISG-Treg subset plays a key role in governing antiviral immune responses and suppressing immune responses to autoimmune diseases characterized by high levels of IFN. We will explore this hypothesis in two specific aims. In aim 1, we will determine how IFN signaling in the thymus affects thymic selection and the development of ISG-Tregs. In aim 2, we will determine the functions of ISG-Tregs in response to viral infections and systemic lupus erythematosis. Successful completion of these aims will allow us to identify the mechanism by which ISG- Tregs arise in the thymus, and determine the role of ISG-Tregs in immune responses to viruses, and in autoimmune diseases associated with high levels of IFN, such as SLE or Sjögren’s syndrome.