Project Abstract The goal of this proposal is to define the mechanisms of immunodeficiency caused by a STING gain-of-function mutation in mice. STING is a cytosolic sensor of viral and host DNA. Activation of STING upon detection of cytosolic DNA triggers up-regulation of antiviral interferon (IFN)-stimulated genes (ISGs). Autosomal dominant STING gain-of-function mutations cause STING-associated vasculopathy with onset in infancy (SAVI), a rheumatic disease characterized by vasculopathy, skin lesions, interstitial lung disease, and up-regulation of type I IFN and ISGs. We previously described a mouse model of SAVI (heterozygous STING N153S mice) that exhibited some similarities to patients with SAVI, but also some important differences. Some of these differences may be due to the fact that the STING N153S mice are housed in a pathogen-free environment. Although STING detects pathogens and commensal microbes, whether viruses and microbes contribute to STING gain-of- function disease pathogenesis has not previously been tested. Unexpectedly, we discovered that STING gain-of-function mutant mice fail to develop lymph nodes and Peyer's patches, exhibit impaired antigen-specific CD8+ T cell responses, and are severely vulnerable to infections. We crossed our STING N153S mice to mice lacking an upstream regulator and downstream effectors of STING, as well as Rorγt-GFP mice. Additionally, we are generating STING N153S mice with a floxed-stop in the promoter. This will permit us to define the cell type-specific effects of STING N153S on lymphoid tissue organogenesis (Aim 1). Since STING N153S mice are severely vulnerable to infection, we will define mechanisms of immunodeficiency in studies of viral pathogenesis, including studies of bone marrow chimeric mice as well as adoptive transfer studies into Rag1-/- and Rag1-/- STING N153S mice. Additionally, we will test whether wild-type bone marrow transplantation into STING N153S recipient animals prolongs survival and prevents spontaneous disease and death in older adult mice (Aim 2). Finally, we will determine whether cyclic dinucleotides from the host (cGAMP) or commensal microbes (c-di-GMP) are required for spontaneous disease in STING N153S mice. (Aim 3). Collectively, these proposed studies will define ways in which developmental defects and STING- mediated detection of cyclic dinucleotides may contribute to immunodeficiency and spontaneous disease pathogenesis associated with STING gain-of-function.