Multiple Sclerosis (MS) is an autoimmune disease of the central nervous system that results in multi-focal demyelination of axons and associated axonal damage. Genetic linkage analysis has identified a number of candidate loci associated with MS, but non-MHC genes having a strong association have yet to be identified. The animal model for MS, Experimental Autoimmune Encephalomyelitis (EAE), is a T cell mediated disease. beta1,6N-acetylglucosaminyltransferase V (Mgat5), an enzyme in the Asn (N)-Iinked protein glycosylation pathway, is a potent negative regulator of T cell activation and autoimmunity in mice. Mgat5 deficiency lowers T cell activation thresholds by directly enhancing TCR clustering and signaling at the site of antigen presentation. Mgat5-/- T Cells hyperproliferate in response to TCR agonists and Mgat5 deficient mice display increased susceptibility to EAE and develop kidney autoimmune disease. The GIcNAc a1,6 branching initiated by Mgat5 is preferentially extended by two or more N-acetyllactosamine units, the ligand for galectins, a family of carbohydrate binding proteins that regulate T cell signaling, proliferation and apoptosis. TCR associated Mgat5 modified glycans are bound to galectin-3, participating in a putative cell surface galectin-glycoprotein lattice that inhibits TCR recruitment to the site of antigen presentation. To further elucidate the role of Mgat5 and the galectin-glycoprotein lattice in the regulation of T cell function and autoimmunity, three specific aims are proposed. Specific Aim #1 will identify additional protein and carbohydrate components of the T cell galectin-glycoprotein lattice and their relevance to T cell function and adhesion. T cells isolated from mice with targeted mutations in glycosylation will be analyzed for alterations in cell function and receptor - galectin interactions by florescence microscopy, co-immunoprecipitation, mass spectroscopy, signaling and proliferation assays. Specific Aim #2 will explore the regulation and function of Mgat5 glycans in TH1and TH2 CD4+, CD8+ and memory T cell subsets. The influence of Mgat5 deficiency on the balance between TH1 promotion and TH2 inhibition of autoimmune disease will be determined. Specific Aim #3 will use EAE induction by adoptive transfer to determine the roles of altered T cell adhesion and non-T cells to the Mgat5 deficient autoimmune phenotype.