Cleavage of transaldolase by granzyme B causes the loss of enzymatic activity with retention of antigenicity for multiple sclerosis patients.

Multiple sclerosis (MS) is an autoimmune demyelinating disease of the CNS resulting from a progressive loss of oligodendrocytes. Transaldolase (TAL) is expressed at selectively high levels in oligodendrocytes of the brain, and postmortem sections show concurrent loss of myelin basic protein and TAL from sites of demyelination. Infiltrating CD8(+) CTLs are thought to play a key role in oligodendrocyte cell death. Cleavage by granzyme B (GrB) is predictive for autoantigenicity of self-proteins, thereby further implicating CTL-induced death in the initiation and propagation of autoimmunity. The precursor frequency and CTL activity of HLA-A2-restricted TAL 168-176-specific CD8(+) T cells is increased in MS patients. In this paper, we show that TAL, but not myelin basic protein, is specifically cleaved by human GrB. The recognition site of GrB that resulted in the cleavage of a dominant TAL fragment was mapped to a VVAD motif at aa residue 27 by N-terminal sequencing and confirmed by site-directed mutagenesis. The major C-terminal GrB cleavage product, residues 28-337, had no enzymatic activity but retained the antigenicity of full-length TAL, effectively stimulating the proliferation and CTL activity of PBMCs and of CD8(+) T cell lines from patients with MS. Sera of MS patients exhibited similar binding affinity to wild-type and GrB-cleaved TAL. Because GrB mediates the killing of target cells and cleavage by GrB is predictive of autoantigen status of self proteins, GrB-cleaved TAL-specific T cell-mediated cytotoxicity may contribute to the progressive destruction of oligodendrocytes in patients with MS.

Human major histocompatibility complex (MHC) class I molecules with disulfide traps secure disease-related antigenic peptides and exclude competitor peptides.

The ongoing discovery of disease-associated epitopes detected by CD8 T cells greatly facilitates peptide-based vaccine approaches and the construction of multimeric soluble recombinant proteins (e.g. tetramers) for isolation and enumeration of antigen-specific CD8 T cells. Related to these outcomes of epitope discovery is the recent demonstration that MHC class I/peptide complexes can be expressed as single chain trimers (SCTs) with peptide, beta(2)m and heavy chain connected by linkers to form a single polypeptide chain. Studies using clinically relevant mouse models of human disease have shown that SCTs expressed by DNA vaccination are potent stimulators of cytotoxic T lymphocytes. Their vaccine efficacy has been attributed to the fact that SCTs contain a preprocessed and preloaded peptide that is stably displayed on the cell surface. Although SCTs of HLA class I/peptide complexes have been previously reported, they have not been characterized for biochemical stability or susceptibility to exogenous peptide binding. Here we demonstrate that human SCTs remain almost exclusively intact when expressed in cells and can incorporate a disulfide trap that dramatically excludes the binding of exogenous peptides. The mechanistic and practical applications of these findings for vaccine development and T cell isolation/enumeration are discussed.

T cell receptor recognition via cooperative conformational plasticity.

Although T cell receptor cross-reactivity is a fundamental property of the immune system and is implicated in numerous autoimmune pathologies, the molecular mechanisms by which T cell receptors can recognize and respond to diverse ligands are incompletely understood. In the current study we examined the response of the human T cell lymphotropic virus-1 (HTLV-1) Tax-specific T cell receptor (TCR) A6 to a panel of structurally distinct haptens coupled to the Tax 11-19 peptide with a lysine substitution at position 5 (Tax5K, LLFG[K-hapten]PVYV). The A6 TCR could cross-reactively recognize one of these haptenated peptides, Tax-5K-4-(3-Indolyl)-butyric acid (IBA), presented by HLA-A*0201. The crystal structures of Tax5K-IBA/HLA-A2 free and in complex with A6 reveal that binding is mediated by a mechanism of cooperative conformational plasticity involving conformational changes on both sides of the protein-protein interface, including the TCR complementarity determining region (CDR) loops, Valpha/Vbeta domain orientation, and the hapten-modified peptide. Our findings illustrate the complex role that protein dynamics can play in TCR cross-reactivity and highlight that T cell receptor recognition of ligand can be achieved through diverse and complex molecular mechanisms that can occur simultaneously in the interface, not limited to molecular mimicry and CDR loop shifts.

Extensive T cell receptor cross-reactivity on structurally diverse haptenated peptides presented by HLA-A2.

Previous studies have shown that individual TCRs are able to effectively recognize multiple peptide/MHC complexes that have varying degrees of structural diversity. These TCR cross-reactivities have usually been demonstrated by using peptides that have different amino acid sequences. To further examine the extent to which TCRs can accommodate structurally diverse ligands, we analyzed human TCR cross-reactivity to eight structurally distinct haptens that are coupled to the HLA-A2-binding Tax peptide with a lysine substitution at position 5 (Tax-5K, LLFG[K-hapten]PVYV). The results demonstrate that 71% percent of the haptenated-peptide-induced CTL lines could cross-react on at least one other hapten. We compared the effects of HLA-A2 mutants with substitutions at known TCR contact sites for recognition by hapten-cross-reactive CTL. Recognition of the A2 mutants was remarkably similar whether they were presenting the immunizing or the cross-reactive peptide, indicating that similar amino acid contacts are made by the TCR during recognition of both complexes. Thus, hapten cross-reactivity is apparently accomplished without major adjustments to the interaction between the TCR and the surface of the HLA-A2 molecule. Collectively, these results suggest that TCRs possess the molecular flexibility to accommodate very structurally diverse ligands while retaining conserved interactions with the surface of the MHC molecule.

CD8+ T cell-mediated HLA-A*0201-restricted cytotoxicity to transaldolase peptide 168-176 in patients with multiple sclerosis.

Transaldolase (TAL) is expressed at selectively high levels in oligodendrocytes and targeted by autoreactive T cells of patients with multiple sclerosis (MS). Among 14 TAL peptides with predicted HLA-A2 binding, TAL 168-176 (LLFSFAQAV, TALpep) exhibited high affinity for HLA-A2. Prevalence of HLA-A2-restricted CD8+ T cells specific for TALpep was increased in PBMC of HLA-A2+ MS patients, as compared with HLA-A2- MS patients, HLA-A2+ other neurological disease patients, and HLA-A2+ healthy donors. HLA-A*0201/TALpep tetramers detected increased frequency of TAL-specific CD8+ T cells, and precursor frequency of TAL-specific IFN-gamma-producing T cells was increased in each of seven HLA-A2+ MS patients tested. Stimulation by TALpep or rTAL of PBMC from HLA-A2+ MS patients elicited killing of TALpep-pulsed HLA-A2-transfected HmyA2.1 lymphoma cells, but not HLA-A3-transfected control HmyA3.1 targets. Without peptide pulsing of targets, HLA-A2-transfected, but not control MO3.13 oligodendroglial cells, expressing high levels of endogenous TAL, were also killed by CD8+ CTL of MS patients, indicating recognition of endogenously processed TAL. TCR Vbeta repertoire analysis revealed use of the TCR Vbeta14 gene by T cell lines (TCL) of MS patients generated via stimulation by TAL- or TALpep-pulsed APCs. All TAL-specific TCL-binding HLA-A*0201/TALpep tetramers expressed TCR Vbeta14 on the cell surface. Moreover, Ab to TCR Vbeta14 abrogated cytotoxicity by HLA-A2-restricted TAL-specific TCL. Therefore, TAL-specific CTL may serve as a novel target for therapeutic intervention in patients with MS.

Unraveling a hotspot for TCR recognition on HLA-A2: evidence against the existence of peptide-independent TCR binding determinants.

T cell receptor (TCR) recognition of peptide takes place in the context of the major histocompatibility complex (MHC) molecule, which accounts for approximately two-thirds of the peptide/MHC buried surface. Using the class I MHC HLA-A2 and a large panel of mutants, we have previously shown that surface mutations that disrupt TCR recognition vary with the identity of the peptide. The single exception is Lys66 on the HLA-A2 alpha1 helix, which when mutated to alanine disrupts recognition for 93% of over 250 different T cell clones or lines, independent of which peptide is bound. Thus, Lys66 could serve as a peptide-independent TCR binding determinant. Here, we have examined the role of Lys66 in TCR recognition of HLA-A2 in detail. The structure of a peptide/HLA-A2 molecule with the K66A mutation indicates that although the mutation induces no major structural changes, it results in the exposure of a negatively charged glutamate (Glu63) underneath Lys66. Concurrent replacement of Glu63 with glutamine restores TCR binding and function for T cells specific for five different peptides presented by HLA-A2. Thus, the positive charge on Lys66 does not serve to guide all TCRs onto the HLA-A2 molecule in a manner required for productive signaling. Furthermore, electrostatic calculations indicate that Lys66 does not contribute to the stability of two TCR-peptide/HLA-A2 complexes. Our findings are consistent with the notion that each TCR arrives at a unique solution of how to bind a peptide/MHC, most strongly influenced by the chemical and structural features of the bound peptide. This would not rule out an intrinsic affinity of TCRs for MHC molecules achieved through multiple weak interactions, but for HLA-A2 the collective mutational data place limits on the role of any single MHC amino acid side-chain in driving TCR binding in a peptide-independent fashion.

Strategic mutations in the class I major histocompatibility complex HLA-A2 independently affect both peptide binding and T cell receptor recognition.

Mutational studies of T cell receptor (TCR) contact residues on the surface of the human class I major histocompatibility complex (MHC) molecule HLA-A2 have identified a "functional hot spot" that comprises Arg(65) and Lys(66) and is involved in recognition by most peptide-specific HLA-A2-restricted TCRs. Although there is a significant amount of functional data on the effects of mutations at these positions, there is comparatively little biochemical information that could illuminate their mode of action. Here, we have used a combination of fluorescence anisotropy, functional assays, and Biacore binding experiments to examine the effects of mutations at these positions on the peptide-MHC interaction and TCR recognition. The results indicate that mutations at both position 65 and position 66 influence peptide binding by HLA-A2 to various extents. In particular, mutations at position 66 result in significantly increased peptide dissociation rates. However, these effects are independent of their effects on TCR recognition, and the Arg(65)-Lys(66) region thus represents a true "hot spot" for TCR recognition. We also made the observation that in vitro T cell reactivity does not scale with the half-life of the peptide-MHC complex, as is often assumed. Finally, position 66 is implicated in the "dual recognition" of both peptide and TCR, emphasizing the multiple roles of the class I MHC peptide-binding domain.

A correlation between TCR Valpha docking on MHC and CD8 dependence: implications for T cell selection.

T cell receptors (TCR) adopt a similar orientation when binding with major histocompatibility complex (MHC) molecules, yet the biological mechanism that generates this similar TCR orientation remains obscure. We show here the cocrystallographic structure of a mouse TCR bound to a human MHC molecule not seen by the TCR during thymic development. The orientation of this xenoreactive murine TCR atop human MHC deviates from the typical orientation more than any previously determined TCR/MHC structure. This unique orientation is solely due to the placement of the TCR Valpha domain on the MHC. In light of new information provided by this structure, we have reanalyzed the existing TCR/MHC cocrystal structures and discovered unique features of TCR Valpha domain position on class I MHC that correlate with CD8 dependence. Finally, we propose that the orientation seen in TCR recognition of MHC is a consequence of selection during T cell development.

Tax and M1 peptide/HLA-A2-specific Fabs and T cell receptors recognize nonidentical structural features on peptide/HLA-A2 complexes.

Both TCRs and Ab molecules are capable of MHC-restricted recognition of peptide/MHC complexes. However, such MHC restriction is the predominant mode of recognition by T cells, but is extremely rare for B cells. The present study asks whether the dichotomy in Ag recognition modes of T and B cells could be due to fundamental differences in the methods by which TCRs and Abs recognize peptide/MHC complexes. We have compared MHC and peptide recognition by panels of CTL lines specific for the Tax and M1 peptides presented by HLA-A2 plus Tax and M1 peptide/HLA-A2-specific human Fabs that were selected from a naive phage display library. Collectively, the results indicate both striking similarities and important differences between Fab and TCR recognition of MHC and peptide components of the Tax and M1/HLA-A2 complexes. These findings suggest that these two classes of immunoreceptors have solved the problem of specific recognition of peptide/MHC complexes by nonidentical mechanisms. This conclusion is important in part because it indicates that Ab engineering approaches could produce second-generation Ab molecules that more closely mimic TCR fine specificity. Such efforts may produce more efficacious diagnostic and therapeutic agents.

MHC recognition by hapten-specific HLA-A2-restricted CD8+ CTL.

T cell recognition by peptide-specific alphabeta TCRs involves not only recognition of the peptide, but also recognition of multiple molecular features on the surface of the MHC molecule to which the peptide has been bound. We have previously shown that TCRs that are specific for five different peptides presented by HLA-A2 recognize similar molecular features on the surface of the alpha1 and alpha2 helices of the HLA-A2 molecule. We next asked whether these same molecular features of the HLA-A2 molecule would be recognized by hapten-specific HLA-A2-restricted TCRs, given that hapten-specific T cells frequently show reduced MHC dependence/restriction. The results show that a panel of CD8+ CTL that are specific for the hapten DNP bound to two different peptides presented by HLA-A2 do the following: 1) show stringent MHC restriction, and 2) are largely affected by the same mutations on the HLA-A2 molecule that affected recognition by peptide-specific CTL. A small subset of this panel of CD8+ CTL can recognize a mutant HLA-A2 molecule in the absence of hapten. These data suggest that TCR recognition of a divergent repertoire of ligands presented by HLA-A2 is largely dependent upon common structural elements in the central portion of the peptide-binding site.

Thermodynamic and kinetic analysis of a peptide-class I MHC interaction highlights the noncovalent nature and conformational dynamics of the class I heterotrimer.

The class I major histocompatibility (MHC) molecule is a heterotrimer composed of a heavy chain, the small subunit beta(2)-microglobulin (beta(2)m), and a peptide. Fluorescence anisotropy has been used to assay the interaction of a labeled peptide with a recombinant, soluble form of the class I MHC HLA-A2. Consistent with earlier work, peptide binding is shown to be a two-step process limited by a conformational rearrangement in the heavy chain/beta(2)m heterodimer. However, we identify two pathways for peptide dissociation from the heterotrimer: (1) initial peptide dissociation leaving a heavy chain/beta(2)m heterodimer and (2) initial dissociation of beta(2)m, followed by peptide dissociation from the heavy chain. Eyring analyses of rate constants measured as a function of temperature permit for the first time a complete thermodynamic characterization of peptide binding. We find that in this case peptide binding is mostly entropically driven, likely reflecting the hydrophobic character of the peptide binding groove and the peptide anchor residues. Thermodynamic and kinetic analyses of peptide-MHC interactions as performed here may be of practical use in the engineering of peptides with desired binding properties and will aid in the interpretation of the effects of MHC and peptide substitutions on peptide binding and T cell reactivity. Finally, our data suggest a role for beta(2)m in dampening conformational dynamics in the heavy chain. Remaining conformational variability in the heavy chain once beta(2)m has bound may be a mechanism to promote promiscuity in peptide binding.

TCR ligand discrimination is enforced by competing ERK positive and SHP-1 negative feedback pathways.

Functional discrimination between structurally similar self and foreign antigens is a main attribute of adaptive immunity. Here we describe two feedback mechanisms in T lymphocytes that together sharpen and amplify initial signaling differences related to the quality of T cell receptor (TCR) engagement. Weakly binding ligands predominantly trigger a negative feedback loop leading to rapid recruitment of the tyrosine phosphatase SHP-1, followed by receptor desensitization through inactivation of Lck kinase. In contrast, strongly binding ligands efficiently activate a positive feedback circuit involving Lck modification by ERK, preventing SHP-1 recruitment and allowing the long-lasting signaling necessary for gene activation. The characteristics of these pathways suggest that they constitute an important part of the mechanism allowing T cells to discriminate between self and foreign ligands.

MHC allele-specific molecular features determine peptide/HLA-A2 conformations that are recognized by HLA-A2-restricted T cell receptors.

The structures of alphabeta TCRs bound to complexes of class I MHC molecules and peptide show that the TCRs make multiple contacts with the alpha1 and alpha2 helixes of the MHC. Previously we have shown that the A6 TCR in complex with the HLA-A2/Tax peptide has 15 contact sites on HLA-A2. Single amino acid mutagenesis of these contact sites demonstrated that mutation of only three amino acids clustered on the alpha1 helix (R65, K66, A69) disrupted recognition by the A6 TCR. In the present study we have asked whether TCRs that recognize four other peptides presented by HLA-A2 interact with the MHC in identical, similar, or different patterns as the A6 TCR. Mutants K66A and Q155A had the highest frequency of negative effects on lysis. A subset of peptide-specific CTL also selectively recognized mutants K66A or Q155A in the absence of exogenous cognate peptides, indicating that these mutations affected the presentation of endogenous peptide/HLA-A2 complexes. These findings suggest that most HLA-A2-restricted TCRs recognize surfaces on the HLA-A2/peptide complex that are dependent upon the side chains of K66 and Q155 in the central portion of the peptide binding groove. Crystallographic structures of several peptide/HLA-A2 structures have shown that the side chains of these critical amino acids that make contact with the A6 TCR also contact the bound peptide. Collectively, our results indicate that the generalized effects of changes at these critical amino acids are probably due to the fact that they can be directly contacted by TCRs as well as influence the binding and presentation of the bound peptides.