Diversity of T cell repertoire shaped by a single peptide ligand is critically affected by its acid residue at a T cell receptor contact

T cell differentiation in the thymus is driven by positive selection through the interaction of αβ T cell receptors (TCRs) with selfpeptides bound to self-major histocompatibility complex molecules, yet the influence of the peptide sequence on this process remains unknown. To address this issue, we have compared CD4⁺ T cell differentiation between two sets of mouse lines in which MHC class II I-A^b molecules are occupied with either Eα chain-derived peptide (pEα) or its variant, (p)60k, with one amino acid substitution from leucine to lysine at P5 residue of TCR contacts. Here, we show that despite the comparable expression of I-A^b-peptide complex in the thymus, this substitution from leucine to lysine affects efficiency of positive selection, resulting in extremely small numbers of CD4⁺ T cells to be selected to mature on I-A^b-(p)60k complex. Furthermore, we show that, although I-A^b-pEα complex selects diverse T cells, T cell repertoire shaped by I-A^b-(p)60k complex is markedly constrained. Our findings thus suggest that positive selection is both specific and degenerate, depending on the amino acid residues at TCR contacts of the selecting self-peptides.