Systemic lupus erythematosus (SLE) is an immune complex-mediated disease in which clinical disease manifests after an innate immune response. Endothelial cells (EC) are conditional effector cells activated by inflammatory stimuli to produce intracellular reactive oxygen species (ROS) and signal for influx of inflammatory cells into tissue. We have published that endothelial nitric oxide synthase (eNOS) modulates inflammation in lupus nephritis (LN) and that uncoupling of EC eNOS (which reduces production of anti-inflammatory nitric oxide (NO) and increases production of pro-inflammatory ROS) is a potential therapeutic target in SLE. We hypothesize that sepiapterin, a compound that couples eNOS, will improve LN outcomes and vascular function in murine LN and work by modulating the PI3Kinase/Akt pathway to reduce expression of inflammation and fibrosis genes in mice and in cultured glomerular endothelial cells. We further hypothesize that the proposed approach may have off target effects, particularly in T cells, monocyte/macrophages, and podocytes. To address this overarching hypothesis, the following specific aims are proposed: Specific Aim 1) Determine the effect of sepiapterin in murine LN on clinical and histologic LN outcomes, large vessel endothelial function, and pharmacodynamic markers of treatment response. We hypothesize that sepiapterin will improve LN outcomes and vascular function in murine LN. To determine the effect of and optimal therapeutic indication for ECD targeted therapy, we will use the NZM2410 model of spontaneous LN and compare groups with and without sepiapterin treatment to determine effect on three different endpoints. 1) preventing/prolonging LN onset with sepiapterin monotherapy, 2) in mycophenolate-treated mice, improving/hastening induction of remission with sepiapterin as an adjunctive therapy, and 3) after inducing remission, prolonging/preventing LN flare with sepiapterin monotherapy after mycophenolate withdrawal. LN clinical and histologic parameters and large vessel endothelial function will be the primary endpoints. Biomarkers of systemic oxidative stress will serve as exploratory pharmacodynamic indicators. Specific Aim 2. Determine differential gene expression and phosphokinase signaling events, with a focus on the PI3K/Akt pathway, induced in human glomerular endothelial cells (HRGECs) by LN serum and modulated by sepiapterin. Based on our published and preliminary studies, we hypothesize that inflammatory gene expression and PDGF receptor/PI3K/Akt/FoxO1 signaling induced by LN flare serum can be modulated by sepiapterin in HRGECs. Knowledge of pathways activated by LN flare serum that both initiate redox-regulated pathways and that can be modulated by restoring coupling to eNOS will be important for rational development of therapies to target ECD in LN. We will culture HRGECs with serum from SLE patients with active nephritis or a cytokine mix and perturb signaling through the PDGFR, PI3K, and Akt to dete...