This project focuses on the recognition of HLA-DR2/peptide complexes by myelin basic protein (MBP) specific T cell receptors (TCRs) that have been isolated from multiple sclerosis (MS) patients. It was previously thought that TCRs are highly specific for particular foreign or self-peptides. However, a number of studies performed over the past several years have demonstrated that there is a substantial degree of degeneracy in TCR recognition of MHC/peptide complexes. Microbial peptides can activate autoreactive T cell clones, despite having relatively little sequence homology with the self-peptide. The structural and functional basis of degenerate peptide recognition by TCRs will be examined in the proposed experiments. Combinatorial peptide libraries will be used to examine the sequence diversity of peptides that activate autoreactive T cell clones specific for myelin basic protein (Aim 1). MBP specific TCRs will be expressed as soluble proteins in the Baculovirus system and the binding affinity of these TCRs for different HLA-DR2/peptide complexes will be examined (Aim 2). A soluble MBP specific TCR will be co-crystallized with the HLA-DR2/MBP peptide complex and a crossreactive HLA-DR2/peptide complex. The structures of these TCR/HLA-DR2/peptide complexes will be determined by X-ray crystallography (Aim 3). Analysis of the structural requirements for peptide binding and TCR recognition may have important implications for understanding the pathogenesis of MS and for designing peptide-based therapies.