The origins of autoimmune diseases are poorly understood and standard treatments still rely heavily on immune suppressants that block immunity to infections and tumors despite the recent introduction of novel immune modulatory reagents. Better treatments will likely emerge from improved understanding of the processes that induce and maintain immune tolerance to suppress autoimmunity, and the genetic and environmental factors that cause tolerance breakdown. Our long-term goals are to elucidate the role of the natural regulatory enzyme indoleamine 2,3 dioxygenase (IDO) in suppressing autoimmunity, and to harness this pathway as a novel approach to treat autoimmune syndromes. Chronic type I interferon (IFN?) production by plasmacytoid dendritic cells (pDCs) that sense microbial infections is a feature of several autoimmune syndromes, implying that sustained IFN? release drives tolerance breakdown. However IFN? induces some dendritic cell (DC) subsets to express IDO, and DCs expressing IDO suppress T cell responses, in part by stabilizing the default regulatory phenotypes of Foxp3-lineage CD4 T cells (Tregs). Thus reagents that induce IDO and activate Tregs may ameliorate autoimmune diseases. Recently, we reported that treating mice with DNA nanoparticles suppressed T cell responses to immunization and alleviated immune-mediated joint injury in a mouse model of arthritis. Tolerogenic responses to nanoparticle cargo DNA were mediated via IFN? and IDO but were not dependent on TLR9 or IFN signaling accordingly, removing TLR9 ligands (CpG motifs) from cargo DNA did not affect tolerogenic responses but lowered potentially toxic release of IFN. In this revised application we identify the DNA sensing pathway in rare innate immune cells that drives tolerogenic responses to cargo DNA, and show that DNA nanoparticles inhibited autoimmune disease onset in mouse models of multiple sclerosis and type I diabetes. Our goals are to elucidate the tolerogenic pathway targeted by DNA nanoparticles and to engineer DNA nanoparticles as potential tolerogenic vaccines to treat and prevent autoimmune syndromes.