PROJECT SUMMARY/ABSTRACT Inflammatory bowel disease (IBD) is a chronic inflammatory disorder that results from a loss in intestinal homeostasis between the commensal microbiota and the immune system. Importantly, the etiology of IBD remains unknown, although numerous studies have demonstrated a central role for CD4 T cells in the induction of IBD. A tremendous amount of research has been performed to understand the functional aspects of the CD4 T cells that mediate the pathology and symptoms associated with IBD, and many of the currently approved therapies, as well as a number of treatments in clinical trials, target these properties. Still, there is not a cure for either Crohn’s Disease (CD) or Ulcerative Colitis (UC), and this may be in part because little is known regarding how pathogenic CD4 T cells perpetuate the chronicity of disease. In recent years, the importance of T cell stemness has emerged as a critical parameter that sustains protective and pathogenic cell mediated immunity. During chronic intestinal inflammation, we observe that effector CD4 T cells with stem-like properties are able to both sustain and confer disease and that these cells preferentially express ST6Gal-I-dependent α2-6 linked sialic acids on the cell surface. In addition, we find that ST6Gal-I sialylation of N-glycans is important for expression of the stemness-associated transcription factor, TCF1. These preliminary findings lead us to hypothesize that effector CD4 T cell stemness promotes and sustains chronic intestinal inflammation in both mice and humans, and that this intestinal inflammation in turn operates to preserve this unique population of T cells. Furthermore, we postulate that ST6Gal-I mediated sialylation is central to effector CD4 T cell stemness and the chronicity of disease. To test these hypotheses, we propose the following specific aims: Aim 1. Determine the impact of ST6Gal-I mediated sialylation on CD4 T cell driven intestinal inflammation and CD4 T cell stemness. Aim 2. Determine the effect of the inflammatory intestinal environment on CD4 T cell stemness. Aim 3. Determine the contribution of stem-like effector CD4 T cells to chronic intestinal inflammation. Collectively, these studies are designed to provide new information regarding how CD4 T cell stemness impacts the chronicity of IBD. Moreover, findings that result from this application will illustrate how the composition of the commensal microbiota, as well as post-translational modification of cell surface proteins, specifically N-glycan sialylation, regulates CD4 T cell pathogenicity and stemness during chronic intestinal inflammation.