Alanine scanning mutagenesis of an alphabeta T cell receptor: mapping the energy of antigen recognition.

The T cell receptor (TCR) from the alloreactive T lymphocyte 2C recognizes a nonamer peptide QL9 complexed with the MHC class I molecule H2-Ld. Forty-two single-site alanine substitutions of the 2C TCR were analyzed for binding to QL9/Ld and anti-TCR antibodies. The results provided a detailed energy map of T cell antigen recognition and indicated that the pMHC and clonotypic antibody epitopes on the TCR were similar. Although residues in each Valpha and Vbeta CDR are important in binding pMHC, the most significant energy for the TCR/QL9/Ld interaction was contributed by CDRs 1 and 2 of both alpha and beta chains. The extent to which the individual energy contributions are directed at class I helices or peptide was also assessed.

A residue in the center of peptide QL9 affects binding to both Ld and the T cell receptor.

The TCR from the alloreactive clone 2C recognizes p2C (LSPFPFDL)/Ld and QL9 (QLSPFPFDL)/Ld complexes with affinities of 2 x 10(6) and 10(7) M(-1). Recently, it was proposed that the Phe at position 4 of p2C is critical for recognition by the 2C TCR. To further characterize the role of this peptide position in binding to Ld and in recognition by the 2C TCR, we changed the corresponding peptide position in QL9 (position 5) to other amino acids. Binding affinities of these peptides for Ld and of the peptide/Ld complexes for a soluble single chain TCR were determined. Unexpectedly, it was shown that this peptide position has a significant effect on Ld binding (100-fold), with positively charged and hydrophobic residues having a beneficial effect and negatively charged residues having a detrimental effect. Measurements of the binding affinities of these peptide/Ld complexes for the 2C TCR showed that at 4 degrees C only a Tyr substitution at this position retained high affinity for the TCR. However, significant differences in TCR binding were observed among QL9 peptide variants at 4 degrees C compared with that at 37 degrees C. The influence of this peptide position on both Ld binding and TCR binding may suggest that the 2C TCR recognizes an Ld conformational determinant that is altered by interactions with the residue at position 5 of QL9. A strong correlation was also observed between peptide-Ld affinity and the ability of peptides to sensitize Ld target cells for lysis by CTL 2C. The results are considered in view of recent models on the relationship between T cell activity and TCR-peptide-MHC binding properties.

Binding properties and solubility of single-chain T cell receptors expressed in E. coli.

The diversity and domain structure of alpha beta T cell receptors (TCR) are similar to immunoglobulins based on sequence homologies, but the three-dimensional structure of the alpha beta-heterodimer has not been solved. To begin structure/function studies, we have compared the properties of a soluble single-chain V alpha V beta TCR (scTCR) expressed in three E. coli systems. The V alpha and V beta regions were expressed with pelB or ompA signal sequences or as a thioredoxin fusion protein. The scTCRs were detected only in the insoluble fraction of the cells and could be solubilized in guanidine and renatured to obtain properly folded scTCR from each system. Only a small fraction (1-5%) of the ompA and pelB scTCRs folded properly. In contrast, the thioredoxin fusion protein exhibited high total yields and a solubility that was ten times higher than the other scTCRs. The thioredoxin fusion protein also bound specifically to the peptide/MHC ligand with a KD of approximately 0.7 microM, as shown by a competitive inhibition assay with Fab fragments that recognize the MHC complex. Furthermore, estimates from saturation binding with antibodies that react with the native TCR indicated that up to 80% of the thioredoxin fusion protein was in the properly folded form. The improved yield, solubility, and binding activity of the thioredoxin-scTCR should make it useful for various structure/ function studies.

Specificity and binding properties of a single-chain T cell receptor.

The specificity of a T cell is dictated by an alpha beta T cell receptor (TCR) that recognizes a complex of peptide and a product of the major histocompatibility complex (MHC). Recent studies have begun to characterize the affinities and kinetics of these interactions, but details of the alpha beta TCR structure and function are not known. To examine some of these issues we focus in this report on a TCR derived from the T cell clone 2C. This TCR binds to a complex of the nonapeptide QL9 and the class I MHC product Ld with the highest affinity of any known TCR/ligand interaction (KD approximately 10 (-7) M). Circular dichroism showed that a single-chain TCR (scTCR) containing linked V alpha and V beta regions from T cell 2C and refolded from Escherichia coli inclusion bodies exhibited the characteristic beta-sheet structure of immunoglobulins. A sensitive assay that is capable of detecting the interaction of soluble scTCR with peptide /MHC ligand on the surface of target cells was used to demonstrate that the peptide specificity of this scTCR reflects that of the TCR found on the surface of 2C. Analysis of several scTCR V alpha region mutants confirmed that the V alpha domain is critical for the specificity of scTCR binding. Finally, we identified some notable differences in the complementarity determining regions (CDR) of the 2C TCR compared to the CDR of previously characterized, cytochrome- specific TCR. These differences are discussed in the light of what is known about antibody binding sites, the high affinity of the 2C TCR, and the nature of the residues on QL9 that are predicted to interact with the TCR.

Sequence diversity of T cell receptor alpha chain transcripts from BALB/c thymus.

Most of the diversity in T cell receptor subunits resides in the region that is the equivalent of the CDR3 of immunoglobulins. In order to learn more about the relative contributions of the various mechanisms that generate this diversity we have analyzed the sequences of alpha chain transcripts from BALB/c thymus. The J alpha repertoire of BALB/c mice was examined by comparison of new J alpha sequences and previously published sequences. Among the 41 J alpha genes examined, most of the diversity is located at the 5' end, consistent with the notion that this region contacts the antigen. VJ junctional diversity was examined by sequencing various V alpha J alpha combinations derived from different stages of development. Deletion of bases from the ends of V and J genes does not occur with equal frequency. A greater number of bases were deleted on average from the ends of J genes. Bases were added at junctions frequently in isolates from adult animals, consistent with the presence of terminal deoxynucleotidyl transferase. However, there were short stretches of sequences at junctions which were also present at the 5' end of J genes. These findings extend recent observations that alpha chain genes use multiple mechanisms for generating diversity.