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

Abstract

T cell differentiation in the thymus is driven by positive selection through the interaction of ␣␤ 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 E␣ chainderived peptide ( pE␣) or its variant, p60K, 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 bpeptide 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 -p60K complex. Furthermore, we show that, although I-A b -pE␣ complex selects diverse T cells, T cell repertoire shaped by I-A b -p60K 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.A lthough the diversity of ␣␤ T cell receptors (TCRs) theoretically reaches 10 15 by random rearrangement of five gene segments (V␣, J␣, V␤, D␤, and J␤) and random nucleotide addition (1), mature T cells express highly selected TCRs in that they exhibit tolerance to self-antigenic peptides and restriction by self-MHC molecules. This mainly results from two reciprocal selection processes, positive and negative selection, acting during T cell development in the thymus. Positive selection is the process that induces the differentiation of CD4 ϩ CD8 ϩ immature thymocytes into CD4 Ϫ CD8 ϩ or CD4 ϩ CD8 Ϫ mature thymocytes only when their TCRs recognize self-MHC class I or class II molecules with sufficient, albeit weak, affinity and ensures that the specificity of mature T cell repertoire is directed against antigenic peptides bound to self-MHC molecules. On the other hand, negative selection plays a central role in tolerance induction by eliminating immature thymocytes bearing TCRs specific for self-peptides bound to self-MHC molecules.Since the discovery that positive selection is influenced by the substitutions at amino acid residues of MHC molecules positioned toward the peptide-binding groove (2-5), the role of self-peptides in positive selection has been the subject of considerable debate. This was first directly addressed for CD8 ϩ T cells using fetal thymic organ cultures derived from mutant mouse strains where a particular MHC class I-peptide complex is expressed by exogenously adding a given peptide to the culture (6-8). Subsequently, several groups developed in vivo experimental system focusing on the role of self-peptides in positive selection of CD4 ϩ T cells by creating mouse strains in which all or a large fraction of the MHC class II molecules are occupied with a single peptide (9-13). With the use of in vitro and in vivo experimental systems, it wa...

CD8+ TCR Repertoire Formation Is Guided Primarily by the Peptide Component of the Antigenic Complex

“…These differences related neither to CD8 + T cell avidity for pMHC I nor to predicted peptide affinity for MHC I (A.I. Costa, unpublished observations) and is presumably caused by structural determinants of the MHC I-bound peptide, as has been shown previously in mice (40,41). These studies revealed that single amino acid mutants can have a profound impact on T cell repertoire diversity, both in an Ag-specific situation (40) and during selection of clonotypes in the thymus (41).…”

Organ-specific autoimmunity in mice whose T cell repertoire is shaped by a single antigenic peptide

“…Three lines of transgenic mice that express the I-Aβ b chain covalently bound to Eα52-68 but lack endogenous I-Aβ b , invariant chain, and/or β2-microglobulin have been described elsewhere (22)(23)(24). All mice were maintained under specific pathogen-free conditions, and mice hemizygous for the transgene were used in the present study.…”

“…The expression of I-A b -Eα52-68 complex was readily detected in the thymic cortex and medulla in one mouse line, B2L TKO, whereas the expression in another line, H3 TKO, was extremely low in the thymus (22,24). However, CD4 + T cell differentiation was observed in both B2L TKO and H3 TKO mice (22,24). Thus, these mice made it feasible to follow the immunological consequences of CD4 + T cells selected to mature on I-A b -Eα52-68 complex expressed at different levels in the thymus, without these T cells being influenced by an immune response against other self-or foreign antigenic peptides.…”

Organ-specific autoimmunity in mice whose T cell repertoire is shaped by a single antigenic peptide

“…Thus far, no evidence has been provided that the I-A b molecules expressed in such mice present self-or foreign antigenic peptides other than Eα52-68 to T cells (22,25,26), suggesting that I-A b -Eα52-68 complex is the only MHC-peptide complex expressed in these mice. The expression of I-A b -Eα52-68 complex was readily detected in the thymic cortex and medulla in one mouse line, B2L TKO, whereas the expression in another line, H3 TKO, was extremely low in the thymus (22,24). However, CD4 + T cell differentiation was observed in both B2L TKO and H3 TKO mice (22,24).…”

“…Earlier, we reported on transgenic mouse lines that have been developed by introducing the gene encoding the I-Aβ b chain covalently bound to Eα chain-derived peptide (Eα52-68) into mice lacking I-Aβ b , invariant chain, and β2-microglobulin (TKO mice) (22)(23)(24). Thus far, no evidence has been provided that the I-A b molecules expressed in such mice present self-or foreign antigenic peptides other than Eα52-68 to T cells (22,25,26), suggesting that I-A b -Eα52-68 complex is the only MHC-peptide complex expressed in these mice.…”