Project Summary Giant Cell Arteritis (GCA) is an autoimmune and autoinflammatory disease which targets the aorta and its major branch vessels. GCA causes vaso-occlusive disease, leading to blindness and stroke. About half of the patients develop GCA aortitis, a potentially life-threatening complication due to aortic dissection and aneurysm formation. The underlying disease process is a granulomatous arteritis, with CD4 T cells, macrophages and multinucleated giant cells infiltrating into the vessel wall, eliciting maladaptive wall remodeling with neoangiogenesis and lumen- occlusive intimal hyperplasia. We have identified aberrant expression of the oncogene NOTCH1 in CD4 T cells as a key abnormality in the immune system of GCA patients. Here, we will examine the hypothesis that NOTCH signaling transforms protective immunity into pathogenic immunity by suppressing the mitochondrial enzyme succinate dehydrogenase (SDH) and truncating the tricarboxylic acid (TCA) cycle. Fragmentation of the TCA cycle then leads to the accumulation of the metabolic intermediate succinate, which is released into the tissue site and functions as a second messenger. We propose that succinate secreted by NOTCH1hi SDHlo CD4 T cells targets surrounding cells to redirect T effector cell differentiation, to induce multinucleated macrophages and to promote microvascular neoangiogenesis. We have assembled key enabling resources to mechanistically study how NOTCH-instructed succinate release enhances vascular inflammation; including a large cohort of clinically well phenotyped GCA patients and a chimeric mouse model in which vasculitis is induced in engrafted human arteries to corroborate in vitro data by in vivo studies. Aim 1 will define the molecular mechanisms leading to NOTCH-dependent SDH loss-of-function, building on preliminary studies that implicate RNA-binding proteins in regulating SDH mRNA stability through N6-methyladenosine modifications. Aim 2A examines mechanistically how succinate reprograms T effector cell differentiation. Experiments are designed to investigate how succinate paralyzes the NF-kappaB inhibitor A20/TNFAIP3 to unleash NF-kappaB signaling and induce polyfunctional effector T cells (Thpoly), including T cells that co-produce IFN-, IL-17, TNF-α, IL-21 and IL-22. Aim 2B will determine how NOTCH-instructed succinate alters macrophage function, specifically by driving formation of tissue-destructive multinucleated giant cells. We will delineate how succinate elicits a robust DNA damage response and how it promotes nuclear division and halts cytokinesis by interfering with the spindle assembly checkpoint. Aim 2C is focused on succinate’s role in inducing a pro-angiogenic endothelial cell (EC) phenotype and will explore how succinate-trained EC migrate, proliferate, and lose their barrier function. Aim 3 will bridge from the bench to the bedside and will test whether the suppression of succinate production by blocking the upstream enzyme a-ketoglutarate...