Human TCR as antigen: Homologies and potentially cross-reactive HLA-DR2-restricted epitopes within the AV and BVCDR2 loops

The major function of the T-cell receptor is to confer antigen specificity to T cells. However, nascent TCR proteins that are not assembled into funct ional heterodimers may be processed and displayed with self MHC molecules o n the T-cell surface, and are thought to be the genesis of autoregulatory T cells that can limit inflammatory responses through T-T network interactio ns. In previous work, we and others have exploited this natural regulatory system using TCR peptides to amplify regulatory T cells that potentially ca n treat human autoimmune diseases such as multiple sclerosis (MS) and arthr itis. The development of this approach is limited by the diversity of human TCR V gene sequences, and by lack of knowledge of exactly which regions of the V gene proteins are immunogenic in association with various MHC allele s. To identify similar amino acid sequences within and among human V gene f amilies that might have immunologic cross reactivity, we aligned 74 known A V and 109 known BV protein sequences into homologous groups using the Clust alX program. Moreover, with a focus on CDR2 peptides that have previously b een used to induce regulatory T cells in clinical trials, we established ho mologous peptide groups, and then identified the optimal amino acid motifs for binding to two alleles, HLA-DRB1*1501 and DRB5*0101, that have been ass ociated with susceptibility to MS. From this analysis, > 75% of AV and BV C DR2 sequences were predicted to bind with at least moderate avidity to each of the DR2 alleles, thus enhancing the likelihood that they could be antig enic. Further ordering of putative TCR contact residues revealed a differen t set of homology groupings, including many intrafamily sequence matches an d some interfamily matches that might allow immunological cross reactivity. Particularly striking were DRB1*1501-restricted M-S and IY-S motifs shared by BV11, BV12, and BV13 and BV3, BV12, BV13, and BV17 family members, resp ectively, and DRB5*0101-restricted RL-H and RL-Y motifs shared by BV11, BV1 2, and BV13 and BV13 and BV17 family members, respectively. This analysis m ay be useful in designing an array of clinically useful homologous peptides with optimal MHC binding properties and highly cross-reactive TCR binding motifs.