Different Strategies Adopted by Kb and Ld to Generate T Cell Specificity Directed against Their Respective Bound Peptides

Abstract

Mouse T cell clone 2C recognizes two different major histocompatibility (MHC) ligands, the self MHC K b and the allogeneic MHC L d . Two distinct peptides, SIY (SIYRYYGL) and QL9 (QLSPFPFDL), act as strong and specific agonists when bound to K b and L d , respectively. To explore further the mechanisms involved in peptide potency and specificity, here we examined a collection of single amino acid peptide variants of SIY and QL9 for 1) T cell activity, 2) binding to their respective MHC, and 3) binding to the 2C T cell receptor (TCR) and high affinity TCR mutants. Characterization of SIY binding to MHC K b revealed significant effects of three SIY residues that were clearly embedded within the K b molecule. In contrast, QL9 binding to MHC L d was influenced by the majority of peptide side chains, distributed across the entire length of the peptide. Binding of the SIY-K b complex to the TCR involved three SIY residues that were pointed toward the TCR, whereas again the majority of QL9 residues influenced binding of TCRs, and thus the QL9 residues had impacts on both L d and TCR binding. In general, the magnitude of T cell activity mediated by a peptide variant was influenced more by peptide binding to MHC than by binding the TCR, especially for higher affinity TCRs. Findings with both systems, but QL9-L d in particular, suggest that many single-residue substitutions, introduced into peptides to improve their binding to MHC and thus their vaccine potential, could impair T cell reactivity due to their dual impact on TCR binding. Elimination of virus-infected cells or cancer cells by cytotoxic T lymphocytes is governed by interactions between an␣␤ heterodimeric T cell receptor (TCR) 2 and a short, processed, peptide that is bound to a product of the major histocompatibility complex (peptide-MHC) (1). It is well established that a single TCR is capable of recognizing multiple distinct peptide-MHC ligands while maintaining exquisite specificity for each (2, 3).Ultimately, both the specificity and potency of a peptide derives from its interactions with the MHC product and with the TCR. The importance of understanding the role of each peptide residue in eliciting T cell activity can be seen in the considerable effort toward the development of peptide vaccines for the treatment of infection diseases or cancer (4). These efforts have often attempted to enhance the activity of a peptide by generating peptide variants with improved binding for the MHC product (5-7). This approach is absolutely dependent on the ability of T cells to react well not only with the peptide variant but with the native peptide that is presented by infected cells or by the cancer cells. In this regard, it is important to understand whether changes to a peptide impact not only MHC product binding but also binding by the TCR.Multiple genes and extensive polymorphism in the MHC enable different MHC products to bind and present a distinct set of peptides (8). For example, the mouse MHC products K b and L d differ by 30 amino acid residues in the peptide...

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T Cell Antigen Receptor Recognition of Antigen-Presenting Molecules

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