An MHC interaction site maps to the amino-terminal half of the T cell receptor alpha chain variable domain

Functional analysis of the antigen binding sites on the MTB/HIV-1 peptide bispecific T-cell receptor complementarity determining region 3α

OBJECTIVE

Mycobacterium tuberculosis /human immunodeficiency virus (MTB/HIV) coinfection has become an urgent problem in the field of prevention and control of infectious diseases in recent years. Adoptive cellular immunotherapy using antigen-specific T-cell receptor (TCR) engineered T cells which recognize the specific antigen artificially may have tremendous potential in anti-MTB/HIV coinfection. We have previously successfully identified a MTB Ag85B 199-207 and HIV-1 Env 120-128 peptide-bispecific TCR screened out from peripheral blood mononuclear cells of a HLA-A∗0201 + healthy individual and have further studied that how residues on the predicted complementarity determining region (CDR) 3 of the β chain contribute to the bispecific TCR contact with the peptide-MHC. However, it is not clear which amino acids in the predicted CDR3α of the bispecific TCR play a crucial role in ligand recognition.

METHODS

The variants in the CDR3α of the bispecific TCR were generated using alanine substitution. We then evaluated the immune effects of the five variants on T-cell recognition upon encounter with the MTB or HIV-1 antigen.

RESULTS

Mutation of two amino acids (E112A, Y115A) in CDR3α of the bispecific TCR caused a markedly diminished T-cell response to antigen, whereas mutation of the other three amino acids (S113A, P114A, S116A) resulted in completely eliminated response.

CONCLUSION

This study demonstrates that Ser 113 , Pro 114 and Ser 116 in CDR3α of the bispecific TCR are especially important for antigen recognition. These results will pave the way for the future development of an improved high-affinity bispecific TCR for use in adoptive cellular immunotherapy for MTB/HIV coinfected patients.

Structure of MHC-Independent TCRs and Their Recognition of Native Antigen CD155

During normal T cell development in the thymus, αβ TCRs signal immature thymocytes to differentiate into mature T cells by binding to peptide-MHC ligands together with CD4/CD8 coreceptors. Conversely, in MHC and CD4/CD8 coreceptor-deficient mice, the thymus generates mature T cells expressing MHC-independent TCRs that recognize native conformational epitopes rather than linear antigenic-peptides presented by MHC. To date, no structural information of MHC-independent TCRs is available, and their structural recognition of non-MHC ligand remains unknown. To our knowledge in this study, we determined the first structures of two murine MHC-independent TCRs (A11 and B12A) that bind with high nanomolar affinities to mouse adhesion receptor CD155. Solution binding demonstrated the Vαβ-domain is responsible for MHC-independent B12A recognition of its ligand. Analysis of A11 and B12A sequences against various MHC-restricted and -independent TCR sequence repertoires showed that individual V-genes of A11 and B12A did not exhibit preference against MHC-restriction. Likewise, CDR3 alone did not discriminate against MHC binding, suggesting VDJ recombination together with Vα/Vβ pairing determine their MHC-independent specificity for CD155. The structures of A11 and B12A TCR are nearly identical to those of MHC-restricted TCR, including the conformations of CDR1 and 2. Mutational analysis, together with negative-staining electron microscopy images, showed that the CDR regions of A11 and B12A recognized epitopes on D1 domain of CD155, a region also involved in CD155 binding to poliovirus and Tactile in human. Taken together, MHC-independent TCRs adopt canonical TCR structures to recognize native Ags, highlighting the importance of thymic selection in determining TCR ligand specificity.

Broad cross-reactivity of the T-cell repertoire achieves specific and sufficiently rapid target searching

The molecular recognition of T-cell receptors is the hallmark of the adaptive immunity. Given the finiteness of the T-cell repertoire, individual T-cell receptors are necessary to be cross-reactive to multiple antigenic peptides. In this study, we quantify the variability of the cross-reactivity by using a string model that estimates the binding affinity between two sequences of amino acids. We examine sequences of 10,000 human T-cell receptors and 10,000 antigenic peptides, and obtain a full spectrum of cross-reactivity of the receptor-peptide binding. Then, we find that the cross-reactivity spectrum is broad. Some T-cells are reactive to 1000 peptides, but some T-cells are reactive to only one or two peptides. Since the degree of cross-reactivity has a correlation with the (un)binding affinity of receptors, we further investigate how the broad cross-reactivity affects the target searching of T-cells. High cross-reactive T-cells may not require many trials for searching correct targets, but they may spend long time to unbind from incorrect targets. In contrast, low cross-reactive T-cells may not spend long time to ignore incorrect targets, but they require many trials for screening correct targets. We evaluate this hypothesis, and show that the broad cross-reactivity of the natural T-cell repertoire can balance the trade-off between the rapid screening and unbinding penalty.

TCRα-TCRβ pairing controls recognition of CD1d and directs the development of adipose NKT cells

The interaction between the T cell antigen receptor (TCR) expressed by natural killer T cells (NKT cells) and the antigen-presenting molecule CD1d is distinct from interactions between the TCR and major histocompatibility complex (MHC). Our molecular modeling suggested that a hydrophobic patch created after TCRα-TCRβ pairing has a role in maintaining the conformation of the NKT cell TCR. Disruption of this patch ablated recognition of CD1d by the NKT cell TCR but not interactions of the TCR with MHC. Partial disruption of the patch, while permissive to the recognition of CD1d, significantly altered NKT cell development, which resulted in the selective accumulation of adipose-tissue-resident NKT cells. These results indicate that a key component of the TCR is essential for the development of a distinct population of NKT cells.

αβ T cell receptors that do not undergo major histocompatibility complex-specific thymic selection possess antibody-like recognition specificities

Major histocompatibility complex (MHC) restriction is the cardinal feature of T cell antigen recognition and is thought to be intrinsic to αβ T cell receptor (TCR) structure because of germline-encoded residues that impose MHC specificity. Here, we analyzed αβTCRs from T cells that had not undergone MHC-specific thymic selection. Instead of recognizing peptide-MHC complexes, the two αβTCRs studied here resembled antibodies in recognizing glycosylation-dependent conformational epitopes on a native self-protein, CD155, and they did so with high affinity independently of MHC molecules. Ligand recognition was via the αβTCR combining site and involved the identical germline-encoded residues that have been thought to uniquely impose MHC specificity, demonstrating that these residues do not only promote MHC binding. This study demonstrates that, without MHC-specific thymic selection, αβTCRs can resemble antibodies in recognizing conformational epitopes on MHC-independent ligands.

On the logic of restrictive recognition of peptide by the T-cell antigen receptor

This essay provides an analysis of the inadequacy of the current view of restrictive recognition of peptide by the T-cell antigen receptor. A competing model is developed, and the experimental evidence for the prevailing model is reinterpreted in the new framework. The goal is to contrast the two models with respect to their consistency, coverage of the data, explanatory power, and predictability.

What does the T-cell receptor recognize when it docks on an MHC-encoded restricting element?

The postulate is analyzed that single V-gene segments encode recognition of the allele-specific determinants (a) required for the restrictive response of the alphabeta TCR to peptide. The consequence of this is that the positively selected V-domain, Valpha or Vbeta, engages an allele-specific determinant (a) on one subunit or domain of the MHC-encoded restricting element. The entrained V-domain docks on an invariant determinant (i) on the complementing subunit or domain. Consequently, each functional V-domain expresses an anti-a site and an anti-i site, and all subunits or domains of MHC-encoded restricting elements express an a- and i-determinant. The evidence, both biological and structural, discussed here strongly supports this postulate which has far reaching consequences.

Identical alpha-chain T-cell receptor transcripts are present on T cells infiltrating coronary arteries of human cardiac allografts with chronic rejection

Chronic cardiac allograft rejection is characterized by graft arteriopathy and is a major obstacle of graft survival. We investigated T-cell receptor (TCR) alpha-chain transcripts of T cells infiltrating human epicardial coronary arteries from cardiac allografts with chronic rejection. The non-palindromic adaptor-polymerase chain reaction (NPA-PCR) was used to specifically amplify TCR alpha-chain transcripts from five explanted cardiac allografts with chronic rejection. The amplified products were cloned and sequenced to obtain the entire ValphaJalpha region. Immuno-histochemistry was used to identify the mononuclear cell infiltrates in the coronary arteries. All the five coronary artery specimens exhibited large populations of infiltrating mononuclear cells, which were primarily comprised of T cells and macrophages. In three specimens, high proportions ( approximately 80%) of identical alpha-chain TCR transcripts were detected. In peripheral blood mononuclear cells from a healthy individual, alpha-chain TCR transcripts were unique when compared to each other. Endomyocardial biopsies collected from one patient six months before the allograft was explanted, contained identical alpha-chain TCR transcripts to those found to be clonally expanded in the coronary arteries from this patient. These results indicate that T cells infiltrating the epicardial arteries of cardiac allografts with chronic rejection undergo proliferation and clonal expansion in response to a specific antigen, which very likely is an (allo)antigen(s).

T cell recognition of distinct peptide:I-Au conformers in murine experimental autoimmune encephalomyelitis

We have used T cells bearing TCRs that are closely related in sequence as probes to detect conformational variants of peptide-MHC complexes in murine experimental autoimmune encephalomyelitis in H-2(u) mice. The N-terminal epitope of myelin basic protein (MBP) is immunodominant in this model. Our studies have primarily focused on T cell recognition of a position 4 analog of this peptide (MBP1-9[4Y]) complexed with I-A(u). Using site-directed mutagenesis, we have mapped the functionally important complementarity determining region residues of the 1934.4 TCR Valpha domain. One of the resulting mutants (Tyr(95) to alanine in CDR3alpha, Y95A) has interesting properties: relative to the parent wild-type TCR, this mutant poorly recognizes Ag complexes generated by pulsing professional APCs (PL-8 cells) with MBP1-9[4Y] while retaining recognition of MBP1-9[4Y]-pulsed unconventional APCs or insect cell-expressed complexes of I-A(u) containing tethered MBP1-9[4Y]. Insect cell expression of recombinant I-A(u) with covalently tethered class II-associated invariant chain peptide or other peptides which bind relatively weakly, followed by proteolytic cleavage of the peptide linker and replacement by MBP1-9[4Y] in vitro, results in complexes that resemble peptide-pulsed PL-8 cells. Therefore, the distinct conformers can be produced in recombinant form. T cells that can distinguish these two conformers can also be generated by the immunization of H-2(u) mice, indicating that differential recognition of the conformers is observed for responding T cells in vivo. These studies have relevance to understanding the molecular details of T cell recognition in murine experimental autoimmune encephalomyelitis. They are also of particular importance for the effective use of multimeric peptide-MHC complexes to characterize the properties of Ag-specific T cells.

Recognition of a specific self-peptide: self-MHC class II complex is critical for positive selection of thymocytes expressing the D10 TCR

We examined the specificity of positive and negative selection by using transgenic mice carrying a variant of the D10 TCR. We demonstrate that a point mutation at position 51 within the CDR2alpha segment significantly reduces the avidity of this TCR for its cognate ligand, but does not impact recognition of nonself MHC class II molecules. Although structural studies have suggested that this TCR site interacts with the MHC class II beta-chain, the avidity of this TCR for its ligand and the function of the T cell can be reconstituted by a point mutation in the bound antigenic peptide. These data demonstrate that the bound peptide can indirectly alter TCR interactions by influencing MHC structure. Remarkably, reducing the avidity of this TCR for a specific antigenic peptide-MHC ligand has a dramatic impact on thymic selection. Positive selection of thymocytes expressing this TCR is nearly completely blocked, whereas negative selection on allogenic MHC class II molecules remains intact. Therefore, the recognition of self that promotes positive selection of the D10 TCR is highly peptide-specific.

Cutting edge: dynamic redistribution of tetraspanin CD81 at the central zone of the immune synapse in both T lymphocytes and APC

The tetraspanin CD81 has been involved in T-dependent B cell-mediated immune responses. However, the behavior of CD81 during immune synapse (IS) formation has not been elucidated. We determined herein that CD81 redistributed to the contact area of T cell-B cell and T cell-dendritic cell conjugates in an Ag-dependent manner. Confocal microscopy showed that CD81 colocalized with CD3 at the central supramolecular activation complex. Videomicroscopy studies with APC or T cells transiently expressing CD81-green fluorescent protein (GFP) revealed that in both cells CD81 redistributed toward the central supramolecular activation complex. In T lymphocytes, CD81-GFP rapidly redistributed to the IS, whereas, in the APC, CD81-GFP formed a large accumulation in the contact area that later concentrated in a discrete cluster and waves of CD81 accumulated at the IS periphery. These results suggest a relevant role for CD81 in the topography of the IS that would explain its functional implication in T cell-B cell collaboration.

The correlation between ovomucoid-derived peptides, human leucocyte antigen class II molecules and T cell receptor-complementarity determining region 3 compositions in patients with egg-white allergy

BACKGROUND

Food allergies are more prevalent in children, due to the immature gastrointestinal epithelial membrane barrier allowing more proteins through the barrier and into circulation. Ovomucoid (OM) is one of the major allergens that is found in egg white.

OBJECTIVE

The aim of this study was to determine T cell epitopes, antigen-presenting human leucocyte antigen (HLA) class II molecules of the T cell lines (TCLs) and T cell clones (TCCs), and complementarity determining region (CDR) 3 loops of the T cell receptor (TCR) alpha and beta chains of the TCCs specific to OM.

METHODS

We established TCLs and TCCs specific to OM from peripheral blood mononuclear cells (PBMCs) of four atopic patients with egg-white allergy using a mixture of a panel of overlapping synthetic peptides corresponding to the amino acid sequence of the entire OM. We identified the T cell epitopes by antigen-induced proliferative responses, antigen-presenting molecules using allogeneic PBMCs and CDR3 loops of the TCR alpha and beta chains by cloning and sequence analysis.

RESULTS

The TCLs and TCCs responded to seven different peptides, and their antigen-presenting molecules were different from each other. Sequence analysis of the TCR alpha and beta gene usage of the TCCs showed marked heterogeneity, and the usage of the CDR3 loop of the TCCs involved heterogenous amino acid residues. Interestingly, TCCs 'IH3.3' and 'YT6.1' recognized the same OM peptides, and had the same TCR Vbeta-Jbeta gene usage. Considering that peptide motifs bind to HLA class II molecules, the electrically charged residue (positive or negative) on the CDR3alpha and the CDR3beta loops of TCR of TCC may form ionic bonds with a charged residue on the HLA class II molecules-peptide complex.

CONCLUSIONS

TCCs that have the same TCR gene usage were established from patients who had shown similar hypersensitivity-type, indicating that antigen recognition by a specific TCR is closely associated with the characteristics of each patient's symptoms.

Negative selection of thymocytes expressing the D10 TCR

We have analyzed the patterns of positive and negative selection of thymocytes expressing the T cell antigen receptor (TCR) from the D10.G4.1 T cell clone. This TCR confers reactivity to several non-self MHC class II alleles with a remarkably broad range of avidities. Therefore, negative selection can be studied when induced by high-, intermediate-, or low-avidity interactions with endogenous peptide-MHC complexes, all within the same TCR transgenic system. These data directly demonstrate that MHC class II-peptide ligands that fail to activate mature T cells can promote negative selection of immature thymocytes. Additionally, we show that negative selection of thymocytes can occur at two distinct "time points" during development depending on the avidity of the TCR for the MHC-peptide complex. Finally, we show that the self-peptide repertoire plays a significant role in selection because alteration of the self-peptide repertoire by disruption of the H2-Ma gene drastically alters selection of D10 TCR-expressing thymocytes.

Activation of a T cell hybridoma by an alloligand results in differential effects on IL-2 secretion and activation-induced cell death

The molecular nature of the interaction of T cell receptors (TCR) with alloligands is not well understood. Although a role for groove-bound peptide(s) has been clearly demonstrated for major histocompatibility complex (MHC) class I alloreactivity, this has not been established for MHC class II-induced alloresponses. In the present study, we have analyzed the interaction of a nominal peptide-self MHC complex and of an alloligand with their cognate TCR (1934.4 TCR for autoantigen recognition and qCII85.33 TCR for allorecognition). Our results demonstrate that 1934.4 TCR recognition of the N-terminal epitope of myelin basic protein (Ac1-11, Ac=acetylated at position 1) complexed with the MHC class II molecule I-A(u) involves contacts with both chains of the MHC molecule. In contrast, qCII85.33 TCR recognition of an allopeptide:I-A(u) complex appears to predominantly involve the beta chain of the MHC molecule. Thus, the two TCR appear to have different footprints on the I-A(u) molecules. Unexpectedly, this differential involvement of the two chains of the I-A(u) molecule affects activation induced cell death, with allostimulation resulting in poor induction of FasL expression and relatively low levels of apoptosis. Significantly, stimulation of cognate T cells with alloantigen or autoantigen results in similar levels of IL-2 secretion. The reduced apoptosis of T cells in response to allostimulation may be one of the mechanisms that favors the expansion of a relatively large repertoire of alloreactive T cells.

Heart allograft survival in rats following immunization with soluble peptide MHC class I donor antigens: evidence for the role of indirect recognition in rejection

The current series of experiments addressed the question of whether indirect priming with donor MHC antigens affects heart allograft survival. LEW (RT-1(1)) rats were immunized with a mixture of two peptides corresponding to the variable region of MHC class I locus Aa antigen (alpha1 and alpha2 domain). The recipients were transplanted with a DA (RT1-1a) heart 1 month after immunization, and graft survival was closely monitored by ECG. All peptide-treated recipients presented with anti-peptide antibodies at the time of transplantation and developed a strongly accelerated graft rejection. These findings indicated that indirect recognition of MHC I donor antigens promotes heart allograft rejection.

Differential sensitivity to mutations in a single peptide by two TCRs having identical beta-chains and closely related alpha-chains

The TCR on CD4 T cells binds to and recognizes MHC class II:antigenic peptide complexes through molecular contacts with the peptide amino acid residues that face up and out of the peptide-binding groove. This interaction primarily involves the complementarity-determining regions (CDR) of the TCR alpha- and ss-chains contacting up to five residues of the peptide. We have used two TCRs that recognize the same antigenic peptide and have identical Vss8.2 chains, but differ in all three CDR of their related Valpha2 chains, to examine the fine specificity of the TCR:peptide contacts that lead to activation. By generating a peptide library containing all 20 aa residues in the five potential TCR contact sites, we were able to demonstrate that the two similar TCRs responded differentially when agonist, nonagonist, and antagonist peptide functions were examined. Dual substituted peptides containing an agonist residue at the N terminus, which interacts with CDR2alpha, and an antagonist residue at the C terminus, which interacts with the CDR3ss, were used to show that the nature of the overall signal through the TCR is determined by a combination of the type of signal received through both the TCR alpha- and ss-chains.

A NK1.1+ thymocyte-derived TCR beta-chain transgene promotes positive selection of thymic NK1.1+ alpha beta T cells

As a consequence of the peptide specificity of intrathymic positive selection, mice transgenic for a rearranged TCR beta-chain derived from conventional alphabeta T lymphocytes frequently carry mature T cells with significant skewing in the repertoire of the companion alpha-chain. To assess the generality of such an influence, we generated transgenic (Tg) mice expressing a beta-chain derived from nonclassical, NK1.1+ alphabeta T cells, the thymus-derived, CD1. 1-specific DN32H6 T cell hybridoma. Results of the sequence analysis of genomic DNA from developing DN32H6 beta Tg thymocytes revealed that the frequency of the parental alpha-chain sequence, in this instance the Valpha14-Jalpha281 canonical alpha-chain, is specifically and in a CD1.1-dependent manner, increased in the postselection thymocyte population. In accordance, we found phenotypic and functional evidence for an increased frequency of thymic, but interestingly not peripheral, NK1.1+ alphabeta T cells in DN32H6 beta Tg mice, possibly indicating a thymic determinant-dependent maintenance. Thus, in vivo expression of the rearranged TCR beta-chain from a thymus-derived NK1.1+ Valpha14+ T cell hybridoma promotes positive selection of thymic NK1.1+ alphabeta T cells. These observations indicate that the strong influence of productive beta-chain rearrangements on the TCR sequence and specificity of developing thymocytes, which operates through positive selection on self-determinants, applies to both classical and nonclassical alphabeta T cells and therefore represents a general phenomenon in intrathymic alphabeta T lymphocyte development.

Alteration at a single amino acid residue in the T cell receptor alpha chain complementarity determining region 2 changes the differentiation of naive CD4 T cells in response to antigen from T helper cell type 1 (Th1) to Th2

To study whether changes in the structure of a T cell receptor (TCR) at a single peptide-contacting residue could affect T cell priming with antigenic peptide, we made transgenic mice with a point mutation in the TCR alpha chain of the D10.G4.1 (D10) TCR and bred them to D10 beta chain transgenic mice. The mutation consisted of a leucine to serine substitution at position 51 (L51S), which we had already established contacted the second amino acid of the peptide such that the response to the reference peptide was reduced by approximately 100-fold. A mutation in the reference peptide CA134-146 (CA-WT) from the arginine at peptide position 2 to glycine (R2G) restored full response to this altered TCR. When we examined in vitro priming of naive CD4 T cells, we observed that the response to doses of CA-WT that induced T helper cell type 1 (Th1) responses in naive CD4 T cells from mice transgenic for the D10 TCR gave only Th2 responses in naive CD4 T cells derived from the L51S. However, when we primed the same T cells with the R2G peptide, we observed Th1 priming in both D10 and L51S naive CD4 T cells. We conclude from these data that a mutation in the TCR at a key position that contacts major histocompatibility complex-bound peptide is associated with a shift in T cell differentiation from Th1 to Th2.

T cell receptor V genes in multiple sclerosis: increased use of TCRAV8 and TCRBV5 in MBP-specific clones

It is probable that myelin-reactive T cells, including those specific for myelin basic protein (MBP) contribute to the pathogenesis of multiple sclerosis (MS). Although many studies have characterized the specificity, MHC restriction, and V gene use of MBP-specific T cells, there is little agreement as to whether there are differences between MS and controls, and how HLA-DR2, a risk factor for MS, might influence selection of MBP-specific T cells. We here discuss models in which MHC class II alleles could help shape the TCR repertoire, and then review more than 750 clones reported in the literature. The major finding from our analysis is that both TCRAV8 and BV5, but not BV6 were utilized more frequently in MS patients than non-MS patients in response to MBP, although no differences were found between DR2+ versus DR2- donors. These data indicate HLA-independent differences in the T cell repertoire between MS patients and controls that may be important for targeted TCR-based therapy. Moreover, we conclude that (1) HLA-DR alleles preferentially restrict MBP responses, although MS patients tend to use HLA-DQ and -DP alleles more often than control donors; (2) HLA-DR2 alleles are used to restrict only about half the MBP responses in MS patients, significantly less than in control patients; (3) the DRB1*1501 and DRB5*0101 subtypes within the Dw2 haplotype are used relatively equally to restrict MBP responses. In this context, we review the results of our previous clinical trials in progressive MS patients, demonstrating the ability of TCRBV5S2 peptides to induce clinically relevant regulatory responses that inhibit MBP-specific Th1 cells through a bystander suppression mechanism.

Systematic Mutagenesis of TCR Complementarity-Determining Region 3 Residues: A Single Conservative Substitution Dramatically Improves Response to Both Multiple HLA-DR Alleles and Peptide Variants

To define the relative contributions of HLA and peptide contacts with TCR complementarity-determining region (CDR) 3 residues in T cell recognition, systematic mutagenesis and domain swapping was conducted on two highly similar TCRs that both respond to the influenza hemagglutinin (HA) peptide, HA307–319, but with different HLA restrictions. Despite the primary sequence similarity of these TCRs, exchange of as little as two CDR3 residues between them completely abrogated responsiveness. At position 95 within CDR3α, various substitutions still allowed for some degree of recognition. One modest substitution, alanine for glycine (essentially the addition of a methyl group), significantly broadened the specificity of the TCR. Transfectants expressing this mutant TCR responded strongly in the context of multiple HLA-DR alleles and to HA peptide variants with substitutions at each TCR contact residue. These results suggest that the conformations of CDR3 loops are crucial to TCR specificity and that it may not be reliable to extrapolate from primary sequence similarities in TCRs to similarities in specificity. The ease with which a broad specificity is induced in this mutant TCR has implications for the mechanisms and frequency of alloreactivity and promiscuity in T cell responses.

TCR Reactivity in Human Nickel Allergy Indicates Contacts with Complementarity-Determining Region 3 but Excludes Superantigen-Like Recognition

Nickel is the most common inducer of contact sensitivity in humans. We previously found that overrepresentation of the TCRBV17 element in Ni-induced CD4+ T cell lines of Ni-allergic patients relates to the severity of the disease. Amino acid sequences of these β-chains suggested hypothetical contact points for Ni2+ ions in complementarity-determining region (CDR) 1 and CDR3. To specifically address the molecular requirements for Ni recognition by TCR, human TCR α- and β-chains of VB17+ Ni-reactive T cell clones were functionally expressed together with the human CD4 coreceptor in a mouse T cell hybridoma. Loss of CD4 revealed complete CD4 independence for one of the TCR studied. Putative TCR/Ni contact points were tested by pairing of TCR chains from different clones, also with different specificity. TCRBV17 chains with different J regions, but similar CDR3 regions, could be functionally exchanged. Larger differences in the CDR3 region were not tolerated. Specific combinations of α- and β-chains were required, excluding a superantigen-like activation by Ni. Mutation of amino acids in CDR1 of TCRBV17 did not affect Ag recognition, superantigen activation, or HLA restriction. In contrast, mutation of Arg95 or Asp96, conserved in many CDR3B sequences of Ni-specific, VB17+ TCR, abrogated Ni recognition. These results define specific amino acids in the CDR3B region of a VB17+ TCR to be crucial for human nickel recognition. CD4 independence implies a high affinity of such receptor types for the Ni/MHC complex. This may point to a dominant role of T cells bearing such receptors in the pathology of contact dermatitis.

Characterization of the interaction of a TCR alpha chain variable domain with MHC II I-A molecules

The alphabeta TCR recognizes peptides bound to MHC molecules. In the present study, we analyzed the interaction of a soluble TCR alpha chain variable domain (Valpha4.2-Jalpha40; abbreviated to Valpha4.2) with the MHC class II molecule I-Au. Valpha4.2 bound specifically to I-Au expressed on the surface of a transfected thymoma cell line. Modifications in the amino acid residues located within the three complementarity-determining regions (CDRs) of the Valpha domain did not markedly affect this interaction. However, mutation of glutamic acid to alanine at position 69 of the fourth hypervariable region (HV4alpha) significantly increased the binding. Antibody inhibition studies suggested that the binding site was partly contributed by a region of the beta chain of I-Au. Furthermore, the binding of Valpha4.2 to the MHC molecule was dependent on the nature of the peptide bound in the groove. Soluble Valpha4.2 specifically inhibited the activation of TCR transfectants by I-Au-expressing cells pulsed with an N-terminal peptide of myelin basic protein. Valpha4.2 also bound to MHC class II-expressing spleen cell populations from mice of the H-2(u) and H-2(d) haplotypes. The binding of Valpha4.2 to I-A molecules might explain the immunoregulatory effects reported previously for TCR alpha chains. This Valpha4.2 interaction may also be relevant to models of antigen presentation involving the binding of intact proteins to MHC class II molecules followed by their processing to generate epitopes suitable for T cell recognition.

Molecular requirements for T cell recognition by a major histocompatibility complex class II-restricted T cell receptor: the involvement of the fourth hypervariable loop of the Valpha domain

The role of two central residues (K68, E69) of the fourth hypervariable loop of the Valpha domain (HV4alpha) in antigen recognition by an MHC class II-restricted T cell receptor (TCR) has been analyzed. The TCR recognizes the NH2-terminal peptide of myelin basic protein (Ac1-11, acetylated at NH2 terminus) associated with the class II MHC molecule I-Au. Lysine 68 (K68) and glutamic acid 69 (E69) of HV4alpha have been mutated both individually and simultaneously to alanine (K68A, E69A). The responsiveness of transfectants bearing wild-type and mutated TCRs to Ac1-11-I-Au complexes has been analyzed in the presence and absence of expression of the coreceptor CD4. The data demonstrate that in the absence of CD4 expression, K68 plays a central role in antigen responsiveness. In contrast, the effect of mutating E69 to alanine is less marked. CD4 coexpression can partially compensate for the loss of activity of the K68A mutant transfectants, resulting in responses that, relative to those of the wild-type transfectants, are highly sensitive to anti-CD4 antibody blockade. The observations support models of T cell activation in which both the affinity of the TCR for cognate ligand and the involvement of coreceptors determine the outcome of the T cell-antigen-presenting cell interaction.

Highly restricted T cell repertoire shaped by a single major histocompatibility complex-peptide ligand in the presence of a single rearranged T cell receptor beta chain

The T cell repertoire is shaped by positive and negative selection of thymocytes through the interaction of alpha/beta-T cell receptors (TCR) with self-peptides bound to self-major histocompatibility complex (MHC) molecules. However, the involvement of specific TCR-peptide contacts in positive selection remains unclear. By fixing TCR-beta chains with a single rearranged TCR-beta irrelevant to the selecting ligand, we show here that T cells selected to mature on a single MHC-peptide complex express highly restricted TCR-alpha chains in terms of Valpha usage and amino acid residue of their CDR3 loops, whereas such restriction was not observed with those selected by the same MHC with diverse sets of self-peptides including this peptide. Thus, we visualized the TCR structure required to survive positive selection directed by this single ligand. Our findings provide definitive evidence that specific recognition of self-peptides by TCR could be involved in positive selection of thymocytes.

Subtle conformational changes induced in major histocompatibility complex class II molecules by binding peptides

Intracellular trafficking of major histocompatibility complex (MHC) class II molecules is characterized by passage through specialized endocytic compartment(s) where antigenic peptides replace invariant chain fragments in the presence of the DM protein. These changes are accompanied by structural transitions of the MHC molecules that can be visualized by formation of compact SDS-resistant dimers, by changes in binding of mAbs, and by changes in T cell responses. We have observed that a mAb (25-9-17) that is capable of staining I-Ab on the surface of normal B cells failed to interact with I-Ab complexes with a peptide derived from the Ealpha chain of the I-E molecule but bound a similar covalent complex of I-Ab with the class II binding fragment (class II-associated invariant chain peptides) of the invariant chain. Moreover, 25-9-17 blocked activation of several I-Ab-reactive T cell hybridomas but failed to block others, suggesting that numerous I-Ab-peptide complexes acquire the 25-9-17(+) or 25-9-17(-) conformation. Alloreactive T cells were also able to discriminate peptide-dependent variants of MHC class II molecules. Thus, peptides impose subtle structural transitions upon MHC class II molecules that affect T cell recognition and may thus be critical for T cell selection and autiommunity.

Structural basis of 2C TCR allorecognition of H-2Ld peptide complexes

MHC class I H-2Ld complexed with peptide QL9 (or p2Ca) is a high-affinity alloantigen for the 2C TCR. We used the crystal structure of H-2Ld with a mixture of bound peptides at 3.1 A to construct a model of the allogeneic 2C-Ld/QL9 complex for comparison with the syngeneic 2C-Kb/dEV8 structure. A prominent ridge on the floor of the Ld peptide-binding groove, not present in Kb, creates a C-terminal bulge in Ld peptides that greatly increases interactions with the 2C beta-chain. Furthermore, weak electrostatic complementarity between Asp77 on the alpha1 helix of Kb and 2C is enhanced in the allogeneic complex by closer proximity of QL9 peptide residue AspP8 to the 2C HV4 loop.

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.

Structural basis of plasticity in T cell receptor recognition of a self peptide-MHC antigen

The T cell receptor (TCR) inherently has dual specificity. T cells must recognize self-antigens in the thymus during maturation and then discriminate between foreign pathogens in the periphery. A molecular basis for this cross-reactivity is elucidated by the crystal structure of the alloreactive 2C TCR bound to self peptide-major histocompatibility complex (pMHC) antigen H-2Kb-dEV8 refined against anisotropic 3.0 angstrom resolution x-ray data. The interface between peptide and TCR exhibits extremely poor shape complementarity, and the TCR beta chain complementarity-determining region 3 (CDR3) has minimal interaction with the dEV8 peptide. Large conformational changes in three of the TCR CDR loops are induced upon binding, providing a mechanism of structural plasticity to accommodate a variety of different peptide antigens. Extensive TCR interaction with the pMHC alpha helices suggests a generalized orientation that is mediated by the Valpha domain of the TCR and rationalizes how TCRs can effectively "scan" different peptides bound within a large, low-affinity MHC structural framework for those that provide the slight additional kinetic stabilization required for signaling.

Promiscuous presentation and recognition of nucleosomal autoepitopes in lupus: role of autoimmune T cell receptor alpha chain

T cells specific for nucleosomal autoepitopes are selectively expanded in lupus mice and these Th cells drive autoimmune B cells to produce pathogenic antinuclear antibodies. We transfected the TCR-alpha and -beta chain genes of a representative, pathogenic autoantibody-inducing Th clone specific for the nucleosomal core histone peptide H471-94 into TCR-negative recipient cells. Although the autoimmune TCRs were originally derived from SNF1 (I-Ad/q) mice, the transfectants could recognize the nucleosomal autoepitope presented by APC-bearing I-A molecules of all haplotypes tested, as well as human DR molecules. Competition assays indicated that the autoepitopes bound to the MHC class II groove. Most remarkably, MHC-unrestricted recognition of the nucleosomal peptide epitope was conferred by the lupus TCR-alpha chain even when it paired with a TCR-beta chain of irrelevant specificity. Several other disease-relevant Th clones and splenic T cells of lupus mice had similar properties. The TCR-alpha chains of these murine lupus Th clones shared related motifs and charged residues in their CDRs, and similar motifs were apparent even in TCR-alpha chains of human lupus Th clones. The lupus TCR-alpha chains probably contact the nucleosomal peptide complexed with MHC with relatively high affinity/avidity to sustain TCR signaling, because CD4 coreceptor was not required for promiscuous recognition. Indeed, pathogenic autoantibody-inducing, CD4-negative, TCR-alphabeta+ Th cells are expanded in systemic lupus erythematosus. These results have implications regarding thymic selection and peripheral expansion of nucleosome-specific T cells in lupus. They also suggest that universally tolerogenic epitopes could be designed for therapy of lupus patients with diverse HLA alleles. We propose to designate nucleosomes and other antigens bearing universal epitopes "Pantigens" (for promiscuous antigens).

Signalling initiated with CD4-TCR or TCR-TCR interactions: comparison of tyrosine phosphorylation patterns and CD45 effects

Antigen-triggered response in T cells is mediated by the T cell receptor (TCR)/CD3-complex. This signalling, however, is modulated by a number of other surface molecules. Among the most important of these is the CD4/CD8 molecule which associates with the TCR/CD3-complex and binds to the MHC complex. The molecular mechanisms involved in interactions between TCR-TCR and TCR-CD4 are not fully understood. We have earlier described an experimental model that allows us to dissect signals involving CD4-TCR interactions and those involving TCR-TCR interactions using a mouse CD4-CD8- T cell hybridoma cell-line transfected either with the TCR from a mouse T-helper 2 cell-line (D10) alone or with both the TCR and the CD4 molecule. To further characterize these two different modes of signalling in T lymphocytes we have studied the tyrosine phosphorylation patterns resulting from these interactions. In addition, we have studied the modulatory effect of the CD45 molecule on these interactions. In contrast to some earlier reports, we found that both the patterns of induced tyrosine phosphorylation and the effects of CD45 modulation were essentially similar in the CD4-TCR and the TCR-TCR signal transduction cascades. The results are consistent with a purely synergistically amplifying function for CD4 on the TCR-mediated signalling.

The imprint of intrathymic self-peptides on the mature T cell receptor repertoire

The analysis of T cell receptor alpha (TCR alpha) chains in mice transgenic for a TCR beta chain has allowed us to demonstrate a central role for self-peptides in the positive intrathymic selection of major histocompatibility complex (MHC) class II-restricted T cells. Analysis of specific V alpha-J alpha joins in mature CD4+ TCRhigh thymocytes and in peripheral CD4+ T cells revealed a limitation in amino-acid sequences. By analysis of immature thymocytes, we could show that this limited repertoire was selected from a more diverse repertoire. By analysis of the same beta chain-transgenic mice bred to H-2Ma-deficient mice that express one or a very limited number of peptides, we could demonstrate that the V alpha-J alpha join repertoire was now altered and much more limited. Together, these data provide molecular and genetic evidence that the intrathymic positive selection of the TCR repertoire is critically affected by self-peptides presented by MHC class II molecules, most likely on thymic cortical epithelial cells.

Use of T cell receptor/HLA-DRB1*04 molecular modeling to predict site-specific interactions for the DR shared epitope associated with rheumatoid arthritis

OBJECTIVE

To use molecular modeling tools to analyze the potential structural basis for the genetic association of rheumatoid arthritis (RA) with the major histocompatibility complex (MHC) "shared epitope," a set of conserved amino acid residues in the third hypervariable region of the DRbeta chain.

METHODS

Homology model building techniques were used to construct molecular models of the arthritis-associated DRB1*0404 molecule and a T cell receptor (TCR) from T cell clone EM025, which is specific for DR4 molecules containing the shared epitope sequence. Interactive graphics techniques were used to orient the TCR on the DR molecule, guided by surface complementarity analysis.

RESULTS

The predicted TCR-MHC-peptide complex involved multiple interactions and specificity for the shared epitope. TCR residues CDR1beta D30, CDR2beta N51, and CDR3beta Q97 were positioned to potentially participate in hydrogen bond interactions with the shared epitope DRbeta residues Q70 and R71.

CONCLUSION

These results suggest a structural mechanism in which specific TCR recognition and possibly Vbeta selection are directly influenced by the disease-associated MHC polymorphisms.

The MHC reactivity of the T cell repertoire prior to positive and negative selection

T cell antigen receptors (TCRs) on mature T cells react with peptide antigens presented by self-MHC proteins and also frequently cross-react with foreign MHC proteins. The fundamental question whether MHC reactivity is inherent in the germline TCR sequences or is imposed by thymic selection was addressed here by inducing nonselective maturation of immature thymocytes in the absence of MHC molecules. MHC reactivity in the preselection repertoire is very high, but no higher than in the normal repertoire. Cross-reactivity of clones with multiple MHC molecules occurred to a similar extent in the preselection and MHC-selected repertoires. The results establish the MHC reactivity of the germline TCR repertoire, indicate the minimum fraction of immature thymocytes that must undergo negative selection, and suggest that some TCR-MHC contacts may be conserved.

Biophysical and structural studies of TCRs and ligands: implications for T cell signaling

The availability of soluble alphabeta TCRs and the individual chains has now made it possible to carry out structural studies of these molecules and analyze their molecular interactions with peptide-MHC ligands. Recent X-ray crystallographic structures of TCR alpha and beta chains have finally established their structural similarity with the lg molecules. Kinetic measurements of the interaction between TCRs and their ligands have provided strong evidence in favour of an affinity/avidity model for T cell activation in the periphery as well as during development in the thymus.

Major histocompatibility complex recognition by immune receptors: differences among T cell receptor versus antibody interactions with the VSV8/H-2Kb complex

The surface residues of the VSV8/Kb complex important for recognition by N15 and N26 alphabeta T cell receptors (TCR) were mapped by mutational analysis and compared to each other and with epitopes of well-characterized Kb specific monoclonal antibodies (mAb). Three features of immune receptor recognition emerge. First, the footprints of the two TCR on VSV8/Kb are similar with more than 80 % overlap between sites. Given that only 8 of 14 surface exposed VSV8/Kb residues identified as critical for TCR interaction are in common, the chemical basis of the N15 and N26 interactions is nevertheless distinct. Second, the cognate peptide is a major focus of TCR recognition: mutation at any of the three exposed side chains (at p1, p4 or p6) abrogates interaction of both TCR as measured by functional T cell activation. Third, in contrast to TCR, mAb bind to discrete segments on the periphery of the alpha1 and/or alpha2 helices without orientational restriction. These findings suggest that unlike soluble antibodies, surface membrane receptor-ligand interactions on opposing cells (i.e. TCR-peptide/ MHC, CD8-MHC) limit the orientational freedom of the TCR in the immune recognition process.

Alloreactive anti-HLA-B7 cytolytic T cell clones use restricted T cell receptor genes

Most alloreactive T cells specifically recognize peptides bound to donor MHC molecules. In addition to peptides, solvent accessible MHC residues also may stimulate alloreactive T cells. We studied T cell receptor (TCR) usage by 16 independent anti-HLA-B7 alloreactive cytolytic T lymphocyte (CTL) clones. Most or all of these CTL clones recognized unique peptides bound to HLA-B7. Despite the diversity of peptides recognized, 11 out of 15 CTL clones analyzed expressed TCR V(alpha) gene segment (AV) subgroups 1 and 3. Within AV subgroup 1, four of six clones expressed AV2; within AV subgroup 3, three clones used AV6. Ten of 14 CTL clones analyzed expressed V(beta) gene segment (BV) subgroups 4 and 1. In subgroup 4, BV14 was expressed by four of five alloreactive CTL clones. Similar AV and BV usage restriction was not found in mitogen-stimulated peripheral blood T cells from the major donor of the CTL clones. TCR A and TCR B junctional region sequences were quite diverse in length and sequence, although two CTL clones expressed nearly identical TCR B chains. We found no correlation between TCR AV or TCR BV usage and CTL recognition of 81 HLA-B7 variants. These results are consistent with models of TCR structure, in which very diverse TCR CDR3 regions contact very diverse peptides, and moderately diverse TCR CDR1 and CDR2 regions contact moderately diverse MHC alpha-helices.

Analysis of T-cell hybridomas with an unusual MHC class II-dependent ligand specificity

We have characterized two unusual T-cell hybridomas, 1E3 and 3B8, from H-2k mice immunized with I-Ab-transfected L cells (H-2k), that are stimulated by L cells transfected with I-Ab, I-Ak or I-Eb, but not by non-transfected L cells. These hybridomas could not be stimulated by spleen cells from H-2i3, H-2k, H-2b or H-2d mice. Monoclonal anti-I-A antibodies did not block their responses, suggesting that mouse major histocompatibility complex (MHC) class II molecules may be peptide donors rather than restriction elements for them. The stimulation of these hybridomas by fibroblast targets was not blocked by an anti-H-2kk, Dk-specific monoclonal antibody. Lipopolysaccharide (LPS)-activated splenic and peritoneal exudate cells from H-2k, H-2d, H-2i3, H-2b as well as beta 2-microglobulin-deficient, TAP-1-deficient and I-A alpha-deficient H-2b mice stimulated these hybridomas. LPS could also activate a macrophage cell line, but not a B-cell line, to become stimulatory for 1E3. A rat antiserum against untransfected L cells specifically and significantly blocked the response of 1E3. Thus, 1E3 may recognize a conserved murine MHC class II peptide loaded in a TAP-1-independent fashion on a non-classical, monomorphic, beta 2-microglobulin-independent restriction element.

Shared fine specificity between T-cell receptors and an antibody recognizing a peptide/major histocompatibility class I complex

Cytotoxic T cells recognize mosaic structures consisting of target peptides embedded within self-major histocompatibility complex (MHC) class I molecules. This structure has been described in great detail for several peptide-MHC complexes. In contrast, how T-cell receptors recognize peptide-MHC complexes have been less well characterized. We have used a complete set of singly substituted analogs of a mouse MHC class I, Kk-restricted peptide, influenza hemagglutinin (Ha)255-262, to address the binding specificity of this MHC molecule. Using the same peptide-MHC complexes we determined the fine specificity of two Ha255-262-specific, Kk-restricted T cells, and of a unique antibody, pSAN, specific for the same peptide-MHC complex. Independently, a model of the Ha255-262-Kk complex was generated through homology modeling and molecular mechanics refinement. The functional data and the model corroborated each other showing that peptide residues 1, 3, 4, 6, and 7 were exposed on the MHC surface and recognized by the T cells. Thus, the majority, and perhaps all, of the side chains of the non-primary anchor residues may be available for T-cell recognition, and contribute to the stringent specificity of T cells. A striking similarity between the specificity of the T cells and that of the pSAN antibody was found and most of the peptide residues, which could be recognized by the T cells, could also be recognized by the antibody.

A functionally significant allelic polymorphism in a T cell receptor V beta gene segment

The effect of an allelic polymorphism in the BV1S1 gene segment on recognition of major histocompatibility complex (MHC)-peptide complexes by a specific T cell receptor (TCR) was studied using RBL 2H3 cells transfected with TCR-CD3 zeta chimeric receptors. An HLA-A2-restricted human immunodeficiency virus (HIV) pol-specific cytotoxic T lymphocyte (CTL) clone utilizing the BV1S1A2 gene in combination with AV2S1A2 was identified and the extracellular domains of the TCR were fused to CD3 zeta. In degranulation assays RBL 2H3 transfectants expressing this receptor maintained the specificity of the parental CTL clone. The allelic variant BV1S1A1N1 containing a glutamine for histidine substitution at position 48 in the loop of the second complementarity-determining region was generated by site-directed mutagenesis. Transfection of this molecule as a CD3 zeta chimera together with the original AV2S1A2 CD3 zeta molecule resulted in cell surface expression of both chains but a loss of recognition of HLA-A2 HIV pol peptide-pulsed targets. The effect of this polymorphism on MHC-peptide recognition supports current models of TCR MHC-peptide interaction and provides evidence for a functional role for polymorphism in the TCRV genes.

Mitogens, superantigens, and nominal antigens elicit distinctive patterns of TCRB CDR3 diversity

The third complementarity-determining region (CDR3) is the only nongermline-encoded hypervariable region of the T cell receptor beta (TCRB) chain, and it is the region that has been predicted to confer fine specificity of the TCR for peptide-MHC complexes. For this reason analysis of TCRB CDR3 heterogeneity may provide insight into immune mechanisms operative in infectious and autoimmune diseases. PBMC stimulated with either mitogen (PHA), superantigen (TSST-1), or nominal antigen (tetanus toxoid) have been compared with unstimulated PBMC using a two-dimensional approach. Analysis of the expressed TCRBV gene repertoire CDR3 length profile coupled with SSCP methodology enabled the discrimination of sequences with the same CDR3 length. For both freshly isolated and PHA stimulated PBMC, a normally distributed spectrum of CDR3 lengths (five or more products) was observed. These products differed by 3 bp (1 amino acid) due to the strict requirement for in-frame rearrangements in the CDR3 region of TCR. By contrast, tetanus toxoid stimulated PBMC had restricted profiles for most TCRBV families after as few as 7 days of incubation. The oligoclonal nature of samples showing CDR3 length restriction was revealed by SSCP analysis and confirmed by sequence determination. Superantigen stimulation resulted in unique patterns of diversity, which included polyclonal expansion of specific TCRBV families as well as oligoclonal expansion of most other TCRBV families. These data reveal complex yet distinct patterns of TCR diversity in response to different T cell activation stimuli.

H-2Ld-alloreactive T cell hybridomas utilize diverse V alpha and V beta T cell receptor chains

We have sequenced the TCRs from Ld-specific alloreactive T cell hybridomas, whose reactivities we have found to be quite representative of those of a primary dm2 anti-BALB/cJ mixed lymphocyte reaction. We find V beta 6, V beta 7, V beta 8 and V beta 10 gene segments. V alpha usage is diverse, although closely related to that from peptide-specific Ld-restricted CTLs. V alpha-V beta selection provides evidence of preferential pairing. Amino acid frequency analysis shows that the alpha CDR2 region is rich in charged amino acids, in contrast to the beta CDR2 region. Our data suggests the beta chain may be more immunoglobulin-like than the alpha chain, and that charge complementarity may be important in TCR-MHC interactions. We do not consider our results to be contradictory to those previously reported but rather they may represent an early, more diverse response.

Modulation of promiscuous T cell receptor recognition by mutagenesis of CDR2 residues

The T cell receptor (TCR) recognizes a ligand composed of a major histocompatibility complex (MHC) molecule and a peptide antigen. Prior studies of murine T cell clones have demonstrated that residues in the CDR3 region of TCR interact with amino acids in the peptide during MHC-restricted antigen recognition. However, the questions of whether direct TCR MHC contacts are made and where such contact sites might map in the TCR have not been resolved. In this study, we have taken advantage of the promiscuous recognition of a peptide from influenza virus (HA 307-319) by human T cell clones to map sites in the TCR that mediate differences in human leukocyte antigen-D related (HLA-DR) restriction in the presence of a common peptide antigen. Site-specific mutagenesis of cloned TCR genes and transfection into Jurkat cells were used to demonstrate that single amino acid substitutions in CDR2 of the TCR-alpha chain controlled whether a T cell was restricted by the product of a single DR allele (DR7) or would respond to the HA 307-319 peptide when presented by the products of one of several different DR alleles (DR1, DR4, DR5, or DR7). Because the relevant DR alleles are defined by polymorphism in the DR-beta chain, these results also suggest a rotational orientation for recognition in which TCR-alpha interacts with DR beta.

The specificity and orientation of a TCR to its peptide-MHC class II ligands

A T cell-mediated immune response is mainly determined by the 3-5 aa residues that protrude upwards from a peptide bound to an MHC molecule. Alterations of these peptide residues can diminish, eliminate or radically alter the signal that the T cell receives through its T cell receptor (TCR). We have used peptide immunizations of normal mice and mice carrying alpha or beta chain TCR transgenes to identify three distinct peptide contact points. One, near the carboxyl terminus of the peptide, involves the beta chain CDR3 region; the second was centrally located and interacted with both the alpha and beta chain CDR3 loops; the third was near the amino terminus of the peptide, and affected V alpha gene usage, but not the structure of CDR3 of either TCR chain. Based on these results, we propose an orientation for the TCR of this cloned line and argue for its generality.

Different superantigens interact with distinct sites in the Vbeta domain of a single T cell receptor

CD4 T cell receptors (TCRs) recognize antigenic peptides presented by self major histocompatibility complex (MHC) class II molecules as well as non-self MHC class II molecules. The TCRs can also recognize endogenous retroviral gene products and bacterial toxins known collectively as superantigens (SAGs) that act mainly on the Vbeta gene segment-encoded portion of the Vbeta domain; most SAGs also require MHC II class for presentation. We have studied the interaction of the TCR from a well-characterized CD4 T cell line with SAGs by mutational analysis of its Vbeta domain. This appears to separate viral (v)SAG from bacterial (b)SAG recognition. T cells having a TCR with glycine to valine mutation in amino acid residue 51 (G51V) in complementarity determining region 2 of the TCR Vbeta domain fail to respond the bSAGs staphylococcal enterotoxin B (SEB), SEC1, SEC2, and SEC3, whereas they retain the ability to respond to non-self MHC class II molecules and to foreign peptides presented by self MHC class II molecules. It is interesting to note that T cells expressing mutations of both G51V and G53D of V beta regain the response to SEB and partially that to SEC1, but do not respond to SEC2, and SEC3, suggesting that different bacterial SAGs are viewed differently by the same TCR. These results are surprising, because it has been generally believed that SAG recognition by T cells is mediated exclusively by hypervariable region 4 on the exposed, lateral face of the TCR Vbeta domain. Response to the vSAG Mtv-7 was generated by mutation in Vbeta residue 24 (N24H), confirming previously published data. These data show that the vSAG Mtv-7 and bSAGs are recognized by different regions of the TCR Vbeta domain. In addition, various bSAGs are recognized differently by the same TCR. Thus, these mutational data, combined with the crystal structure of the TCR beta chain, provide evidence for distinct recognition sites for vSAG and bSAG.

T-cell receptor alpha locus V(D)J recombination by-products are abundant in thymocytes and mature T cells

In addition to the assembled coding regions of immunoglobulin and T-cell receptor (TCR) genes, the V(D)J recombination reaction can in principle generate three types of by-products in normal developing lymphocytes: broken DNA molecules that terminate in a recombination signal sequence or a coding region (termed signal or coding end molecules, respectively) and DNA molecules containing fused recombination signal sequences (termed reciprocal products). Using a quantitative Southern blot analysis of the murine TCR alpha locus, we demonstrate that substantial amounts of signal end molecules and reciprocal products, but not coding end molecules, exist in thymocytes, while peripheral T cells contain substantial amounts of reciprocal products. At the 5' end of the J alpha locus, 20% of thymus DNA exists as signal end molecules. An additional 30 to 40% of the TCR alpha/delta locus exists as remarkably stable reciprocal products throughout T-cell development, with the consequence that the TCR C delta region is substantially retained in alpha beta committed T cells. The disappearance of the broken DNA molecules occurs in the same developmental transition as termination of expression of the recombination activating genes, RAG-1 and RAG-2. These findings raise important questions concerning the mechanism of V(D)J recombination and the maintenance of genome integrity during lymphoid development.

Biophysical studies of T-cell receptors and their ligands

Recently developed methodologies for the production of the soluble extracellular domains of alpha beta TCRs have allowed several biophysical characterizations. The thermodynamic and kinetic parameters associated with specific ligand interactions between the TCR and MHC-peptide complexes, as well as superantigens, are now being established. Crystallographic studies of isolated TCR fragments have yielded the structures of a V alpha domain and the two extracellular domains of a beta-chain. These investigations are beginning to allow a new visualization of antigen recognition and T-cell activation processes.

Structure of HLA molecules and immunosuppressive effects of HLA derived peptides

Elucidation of the structure of MHC molecules has provided profound new insights into their function in antigen presentation. In addition, structural studies have implicated certain regions of MHC molecules in specific functions. Although much of MHC biology has concentrated on the extensive polymorphism among these molecules, there is also evolutionary pressure to maintain the relatively monomorphic portions of these molecules. Drs. Krensky and Clayberger have found that synthetic peptides corresponding to linear sequences of HLA molecules have immunomodulatory effects both in vitro and in vivo. In this paper, they review the structure of HLA molecules and their studies of HLA derived peptides as novel immunotherapeutics. Members of the heat shock protein 70 family are implicated in the HLA derived peptide immunosuppressive pathway.

Increased peptide promiscuity provides a rationale for the lack of N regions in the neonatal T cell repertoire

Making use of mice deficient for terminal deoxynucleotidyl transferase (TdT) expression and a random peptide library, we have examined the diversity and peptide specificity of the neonatal T cell repertoire specific for a single H-2Db-restricted peptide. Consistent with the predicted decrease in repertoire diversity, polyclonal CTL lines and individual clones from different TdTo mice are more similar to each other than those from different wild-type mice in terms of their fingerprints of cross-reactivity to the library and their TCR sequences. We have also found that several TdTo CTL clones cross-react with many more library peptides than wild-type CTL clones. In a few instances, the degree of peptide promiscuity correlates with TCR sequence characteristics such as N region addition and homology-directed recombination, but not CDR3 loop length. Based on epitope titrations for each clone, TCR affinity for antigen is consistently high; thus, this reduced specificity for peptide may coincide with an accentuated affinity for the alpha helices of the MHC. Peptide promiscuity in the neonate may allow the relatively small numbers of T cells in the periphery to protect against a broader range of pathogens.

Evidence that the antigen receptors of cytotoxic T lymphocytes interact with a common recognition pattern on the H-2Kb molecule

Recognition of class I MHC antigens involves interaction between TCRs of cytotoxic T lymphocytes (CTL) and the two alpha helices of MHC molecules. Using a combined panel of H-2Kb mutants selected by either a CTL clone or MAbs, we have shown evidence that the TCRs of 59 Kb-specific CTL clones shared a common binding pattern on the H-2Kb molecule. Mutations of amino acid residues at the C-terminal regions, but not the N-terminal regions, of the alpha helices abrogated the recognition by the majority of the clones. The data suggests that TCRs predominantly recognize the class I MHC molecule with an orientation that is parallel to the beta-pleated strands and diagonal to the alpha helices.