Genome-wide analyses of MS susceptibility loci have emphasized the importance of MHC class II genes, strongly implicating antigen presentation to CD4 T cells as a key process in the pathogenesis of MS. However, it has been very difficult to study myelin-specific CD4 T cells from MS patients ex vivo, due to low T cell receptor affinities for the relevant peptide-MHC complexes. Therefore, two fundamental questions remain unresolved: First, which molecular properties distinguish myelin-specific T cells in patients with MS from those in healthy individuals? Second, are there distinguishing properties of myelin-specific and virusspecific T cells that could be exploited for therapeutic gain? In Aim 1, we will study TCR recognition by myelin-specific T cells from MS patients. Imaging experiments identified alterations in immunological synapse formation by myelin-specific compared to virus-specific T cells, and structural studies demonstrated unusual binding topologies by some myelin-specific TCRs isolated from MS patients. The Garcia lab recently developed a yeast peptide-MHC display approach to identify entirely novel peptide ligands for a given TCR and showed that one ofthe peptides was recognized with a non-conventional TCR topology and failed to induce signaling. This means that TCR binding topology can have a profound effect on signaling. This approach will be used to examine the irnpact of TCR topology on the function of myelin-specific T cells and determine whether administration of such peptides can prevent spontaneous disease in human TCR/MHC transgenic mice. In Aim 2, we will collaborate with Drs. Love and Hafler to study myelin and virus-specific T cells from MS patients and control subjects identified with the nanowell device. Antigenresponsive T cells will be isolated from nanowells with a robotic device, which enables detailed molecular characterization of myelin-specific T cell populations. The transcriptome of myelin-specific T cells from MS patients and control subjects will be examined, with an emphasis on the expression of genes associated with susceptibility to MS, transcription factors and cytokine signaling molecules. Clonal expansion of myelinspecific will be assessed by sequencing of TCR chains, and the frequency of myelin-reactive TCR sequences in peripheral blood and CSF T cells will be determined by lllumina sequencing ofthe TCR repertoire. These studies will determine whether myelin-specific T cells with particular cytokine proflles and/or gene expression programs are preferentially expanded in MS patients compared to healthy subjects.