STRONG SIMILARITIES IN ANTIGEN FINE SPECIFICITY AMONG DRB1-ASTERISK-1302-RESTRICTED TETANUS TOXIN TT830-843-SPECIFIC TCRS IN SPITE OF HIGHLY HETEROGENEOUS CDR3

We investigated the Ag fine specificity of four TCRs that shared the s ame VP segment but used Vas of three different subfamilies and display ed highly heterogeneous alpha and beta CD R3. The TCRs recognized the tetanus toxin tt830-843 (QYIKANSKFICITE) epitope presented by DRB1130 2. By using a large panel of monosubstituted peptide analogues, we fir st defined the requirements for tt830-843 binding to DRB11302. We fou nd that three residues, I-832, N-835, and G(840), were critical for th e interaction with DRB11302. Residues potentially contacted by the fo ur TCRs were functionally defined by measuring the IL-2 response to th e analogues. Except for the first and the last three residues, as well as I-832, and G(840), all of the others appeared to provide contacts with the four TCRs, indicating a considerable overlapping in the way t hese TCRs interact with the peptide. More importantly, and contrary to expectations, the two TCRs expressing the same V alpha/V beta germ-li ne segments showed a strikingly similar reactivity toward nearly all s ubstitutions; moreover, more pronounced differences were observed when comparing TCRs using different Va segments. These results indicate th at TCRs with entirely distinct CDR3s in the context of conserved V seg ments may not differ substantially in the way they recognize the ligan d, and may provide new insights into understanding the formation of TC R/peptide/MHC ternary complexes. During these studies, we noticed that analogues with nonconservative substitutions at I-832, which bound ve ry unstably to DRB11302, could effectively stimulate T cells, suggest ing a role of the TCR in contributing toward stabilization of peptide binding.