1 Summary: 2 CD4 T cell activation provides a model system for studying molecular mechanisms that coordinate a wide variety 3 of often competing physiological processes. When CD4 T cells encounter an antigen in the proper context, they 4 rapidly accumulate biomass, undergo extensive expansion, and differentiate into functional lineages that spe- 5 cialize on cytokine production. Robust and effective CD4 T cell-mediated immune responses require proper al- 6 location of metabolic resources through the central carbon metabolic pathways to sustain energetically costly 7 processes like growth, proliferation, and cytokine production. Also, ancillary metabolic pathways, such as amino 8 acid catabolism and polyamine (PA) biosynthesis, are critical to regulating T cell proliferation and inflammation. 9 The objective of this proposal is to understand how the arginine-polyamine metabolic axis is regulated during 10 T cell activation and ultimately contributes to inflammation and autoimmunity. Our lab recently revealed that the 11 transcription factor, c-Myc, controls an ancillary metabolic pathway that connects arginine (Arg) catabolism to 12 the biosynthesis of PAs, which are an essential class of polycationic metabolites ubiquitously present in all living 13 organisms. Unlike most other amino acids that are primarily used for anabolic protein synthesis during T cell 14 activation, most cellular Arg is catabolized and funneled into synthesizing PA (Arg-PA metabolic axis). Genetic 15 and pharmacological perturbation in the intracellular PA pool suppresses proliferation, suppresses TH1 and TH17 16 differentiation, but enhances iTreg differentiation. Hence, we hypothesize that the arginine-polyamine metabolic 17 axis orchestrates a metabolic checkpoint to optimize CD4 Teff cell proliferation and inflammatory re- 18 sponse. This checkpoint may be therapeutically exploited by polyamine blocking therapies. The aims of 19 this proposal are to 1) decipher the Arg-PA metabolic axis reprogramming and assess the impact of crucial 20 metabolic steps on Teff cells in the context of modulating Myc; 2) determine the outcomes of modulating Arg-PA 21 axis in regulating the effector function of T cells; 3) assess the contribution of PA de novo biosynthesis and the 22 PA salvage pathway to the intracellular PA-pool and T cell proliferation and effector function, and 4) develop and 23 test complementary enzymatic, genetic, and dietary strategies to exploit the Arg-PA axis to modulate inflamma- 24 tory response and autoimmunity in animal models of multiple sclerosis and rheumatoid arthritis. Collectively, the 25 expected outcomes of this project are significant as it will reveal the fundamental principles of the emerging 26 connections between cell metabolism, immune signaling, and T cell differentiation. These studies are critical to 27 developing novel approaches and therapeutic interventions that improve clinical outcomes of inflammatory and 28 autoimmune diseases.