Conversion of a T cell antagonist into an agonist by repairing a defect in the TCR/peptide/MHC interface: implications for TCR signaling

Structural comparison of allogeneic and syngeneic T cell receptor-peptide-major histocompatibility complex complexes: a buried alloreactive mutation subtly alters peptide presentation substantially increasing V(beta) Interactions

The crystal structures of the 2C/H-2K(bm3)-dEV8 allogeneic complex at 2.4 A and H-2K(bm3)-dEV8 at 2.15 A, when compared with their syngeneic counterparts, elucidate structural changes that induce an alloresponse. The Asp77Ser mutation that imbues H-2K(bm3)-dEV8 with its alloreactive properties is located beneath the peptide and does not directly contact the T cell receptor (TCR). However, the buried mutation induces local rearrangement of the peptide itself to preserve hydrogen bonding interactions between the peptide and the alpha(1) 77 residue. The COOH terminus of the peptide main chain is tugged toward the alpha(1)-helix such that its presentation to the TCR is altered. These changes increase the stability of the allogeneic peptide-major histocompatibility complex (pMHC) complex and increase complementarity in the TCR-pMHC interface, placing greater emphasis on recognition of the pMHC by the TCR beta-chain, evinced by an increase in shape complementarity, buried surface area, and number of TCR-pMHC contacting residues. A nearly fourfold increase in the number of beta-chain-pMHC contacts is accompanied by a concomitant 64% increase in beta-chain-pMHC shape complementarity. Thus, the allogeneic mutation causes the same peptide to be presented differently, temporally and spatially, by the allogeneic and syngeneic MHCs.

The specificity of TCR/pMHC interaction

Crystal structures of 11 complexes of TCRs with peptide/MHC (pMHC), that represent 6 independent TCRs, constitute the current structural database for deriving general insights into how alphabeta TCRs recognise peptide-bound MHC class I or class II. The TCRs adopt a roughly diagonal orientation on top of the pMHCs, but the identification of a set of conserved interactions that dictate this orientation is not apparent. Furthermore, the specific interaction of each TCR with its cognate pMHC partner is quite variable and also involves bound water molecules at the TCR/pMHC interface. In two of the systems, the structural basis for binding of altered peptide ligands has illustrated that the only significant conformational changes occur in the TCR/pMHC interface, but their small magnitude is inconsistent with the enormous variation in signalling outcomes. The TCRs adjust to different agonist, partial agonist and antagonist peptides by subtle conformational changes in their complementarity-determining regions, as previously observed in induced-fit mechanisms of antibody/antigen recognition. Alloreactive-complex structures determined or modelled so far indicate increased interactions of the TCR beta-chain with the pMHC compared with their syngeneic counterparts.

A response calculus for immobilized T cell receptor ligands

To address the molecular mechanism of T cell receptor (TCR) signaling, we have formulated a model for T cell activation, termed the 2D-affinity model, in which the density of TCR on the T cell surface, the density of ligand on the presenting surface, and their corresponding two-dimensional affinity determine the level of T cell activation. When fitted to T cell responses against purified ligands immobilized on plastic surfaces, the 2D-affinity model adequately simulated changes in cellular activation as a result of varying ligand affinity and ligand density. These observations further demonstrated the importance of receptor cross-linking density in determining TCR signaling. Moreover, it was found that the functional two-dimensional affinity of TCR ligands was affected by the chemical composition of the ligand-presenting surface. This makes it possible that cell-bound TCR ligands, despite their low affinity in solution, are of optimal two-dimensional affinity thereby allowing effective TCR binding under physiological conditions, i.e. at low ligand densities in cellular interfaces.

T cell activity correlates with oligomeric peptide-major histocompatibility complex binding on T cell surface

Recognition of virally infected cells by CD8+ T cells requires differentiation between self and nonself peptide-class I major histocompatibility complexes (pMHC). Recognition of foreign pMHC by host T cells is a major factor in the rejection of transplanted organs from the same species (allotransplant) or different species (xenotransplant). AHIII12.2 is a murine T cell clone that recognizes the xenogeneic (human) class I MHC HLA-A2.1 molecule (A2) and the syngeneic murine class I MHC H-2 D(b) molecule (D(b)). Recognition of both A2 and D(b) are peptide-dependent, and the sequences of the peptides recognized have been determined. Alterations in the antigenic peptides bound to A2 cause large changes in AHIII12.2 T cell responsiveness. Crystal structures of three representative peptides (agonist, null, and antagonist) bound to A2 partially explain the changes in AHIII12.2 responsiveness. Using class I pMHC octamers, a strong correlation is seen between T cell activity and the affinity of pMHC complexes for the T cell receptor. However, contrary to previous studies, we see similar half-lives for the pMHC multimers bound to the AHIII12.2 cell surface.

Manipulation of avidity to improve effectiveness of adoptively transferred CD8(+) T cells for melanoma immunotherapy in human MHC class I-transgenic mice

The adoptive transfer of tumor-reactive CD8(+) T cells into tumor-bearing hosts provides an attractive alternative to vaccination-based active immunotherapy of melanoma. The development of techniques that result in the preferential expansion of tumor-reactive T cells is therefore of great importance. In this study, we report the generation of HLA-A*0201-restricted CD8(+) T cell populations that recognize either tyrosinase(369-376) or gp100(209-217) from tolerant human class I MHC-transgenic mice by using single amino acid-substituted variant peptides. Low peptide concentration or restimulation with the parent peptide was used to enhance the functional avidity, defined by stimulation of IFN-gamma accumulation, and cross-reactivity of the resulting T cell populations. We found a direct correlation between the ability of a T cell population to respond in vitro to low concentrations of the precise peptide expressed on the tumor and its ability to delay the outgrowth of B16 melanoma after adoptive transfer. Surprisingly, we found that some T cells that exhibited high functional avidity and were effective in controlling tumor outgrowth exhibited low structural avidity, as judged by MHC-tetramer staining. Our results establish strategies for the development and selection of CD8(+) T cell populations that persist despite peripheral tolerance, and that can control melanoma outgrowth. Furthermore, they support the use of human MHC class I-transgenic mice as a preclinical model for developing effective immunotherapies that can be rapidly extended into therapeutic settings.

CD8(-) T cell transfectants that express a high affinity T cell receptor exhibit enhanced peptide-dependent activation

T cells are activated by binding of the T cell receptor (TCR) to a peptide-major histocompatibility complex (MHC) complex (pMHC) expressed on the surface of antigen presenting cells. Various models have predicted that activation is limited to a narrow window of affinities (or dissociation rates) for the TCR-pMHC interaction and that above or below this window, T cells will fail to undergo activation. However, to date there have not been TCRs with sufficiently high affinities in order to test this hypothesis. In this report we examined the activity of a CD8-negative T cell line transfected with a high affinity mutant TCR (K(D) = 10 nM) derived from cytotoxic T lymphocyte clone 2C by in vitro engineering. The results show that despite a 300-fold higher affinity and a 45-fold longer off-rate compared with the wild-type TCR, T cells that expressed the mutant TCRs were activated by peptide. In fact, activation could be detected at significantly lower peptide concentrations than with T cells that expressed the wild-type TCR. Furthermore, binding and functional analyses of a panel of peptide variants suggested that pMHC stability could account for apparent discrepancies between TCR affinity and T cell activity observed in several prior studies.

Zooming in on the hydrophobic ridge of H-2D(b): implications for the conformational variability of bound peptides

Class I major histocompatibility complex (MHC) molecules, which display intracellularly processed peptides on the cell surface for scanning by T-cell receptors (TCRs), are extraordinarily polymorphic. MHC polymorphism is believed to result from natural selection, since individuals heterozygous at the corresponding loci can cope with a larger number of pathogens. Here, we present the crystal structures of the murine MHC molecule H-2D(b) in complex with the peptides gp276 and np396 from the lymphocytic choriomeningitis virus (LCMV), solved at 2.18 A and 2.20 A resolution, respectively. The most prominent feature of H-2D(b) is a hydrophobic ridge that cuts across its antigen-binding site, which is conserved in the L(d)-like family of class I MHC molecules. The comparison with previously solved crystal structures of peptide/H-2D(b) complexes shows that the hydrophobic ridge focuses the conformational variability of the bound peptides in a "hot-spot", which could allow optimal TCR interaction and discrimination. This finding suggests a functional reason for the conservation of this structural element.

T cell selection and differential activation on structurally related HLA-DR4 ligands

Plasticity of TCR interactions during CD4(+) T cell activation by an MHC-peptide complex accommodates variation in the peptide or MHC contact sites in which recognition of an altered ligand by the T cell can modify the T cell response. To explore the contribution of this form of TCR cross-recognition in the context of T cell selection on disease-associated HLA molecules, we have analyzed the relationship between TCR recognition of the DRB1*0401- and DRB1*0404-encoded HLA class II molecules associated with rheumatoid arthritis. Thymic reaggregation cultures demonstrated that CD4(+) T cells selected on either DRB1*0401 or DRB1*0404 could be subsequently activated by the other MHC molecule. Using HLA tetramer technology we identify hemagglutinin residue 307-319-specific T cells restricted by DRB1*0401, but activated by hemagglutinin residues 307-319, in the context of DRB1*0404. One such clone exhibits an altered cytokine profile upon activation with the alternative MHC ligand. This altered phenotype persists when both class II molecules are present. These findings directly demonstrate that T cells selected on an MHC class II molecule carry the potential for activation on altered self ligands when encountering Ags presented on a related class II molecule. In individuals heterozygous for these alleles the possibility of TCR cross-recognition could lead to an aberrant immune response.

The impact of duration versus extent of TCR occupancy on T cell activation: a revision of the kinetic proofreading model

The widely accepted kinetic proofreading theory proposes that rapid TCR dissociation from a peptide/MHC ligand allows for stimulation of early but not late T cell activation events, explaining why low-affinity TCR ligands are poor agonists. We identified a low-affinity TCR ligand which stimulated late T cell responses but, contrary to predictions from kinetic proofreading, inefficiently induced early activation events. Furthermore, responses induced by this ligand were kinetically delayed compared to its high-affinity counterpart. Using peptide/MHC tetramers, we showed that activation characteristics could be dissociated from TCR occupancy by the peptide/MHC ligands. Our data argue that T cell responses are triggered by a cumulative signal which is reached at different time points for different TCR ligands.

alpha beta T cell receptor ligand-specific oligomerization revisited

The mechanism of T cell receptor signaling is unclear. Included among models for TCR signaling is ligand-induced oligomerization in a fashion analogous to other cell surface receptors. Published kinetic, saturation binding, and light scattering experiments have been interpreted to suggest a propensity for soluble alpha beta TCR/peptide/MHC ectodomain complexes to oligomerize. Upon performing these experiments with soluble ectodomains of human class I and class II restricted alpha beta TCRs, we find no evidence for dimerization or oligomerization of complexes. Apparently, oligomerization in solution to a detectable extent is not a general property of soluble alpha beta TCRs or their complexes with ligand. Our results suggest that membrane-anchored, fully assembled TCRs should be studied to determine the role oligomerization plays in T cell signaling.

Identification of a crucial energetic footprint on the alpha1 helix of human histocompatibility leukocyte antigen (HLA)-A2 that provides functional interactions for recognition by tax peptide/HLA-A2-specific T cell receptors

Structural studies have shown that class I major histocompatibility complex (MHC)-restricted peptide-specific T cell receptor (TCR)-alpha/betas make multiple contacts with the alpha1 and alpha2 helices of the MHC, but it is unclear which or how many of these interactions contribute to functional binding. We have addressed this question by performing single amino acid mutagenesis of the 15 TCR contact sites on the human histocompatibility leukocyte antigen (HLA)-A2 molecule recognized by the A6 TCR specific for the Tax peptide presented by HLA-A2. The results demonstrate that mutagenesis of only three amino acids (R65, K66, and A69) that are clustered on the alpha1 helix affected T cell recognition of the Tax/HLA-A2 complex. At least one of these three mutants affected T cell recognition by every member of a large panel of Tax/HLA-A2-specific T cell lines. Biacore measurements showed that these three HLA-A2 mutations also altered A6 TCR binding kinetics, reducing binding affinity. These results show that for Tax/HLA-A2-specific TCRs, there is a location on the central portion of the alpha1 helix that provides interactions crucial to their function with the MHC molecule.

Efficient T cell activation requires an optimal dwell-time of interaction between the TCR and the pMHC complex

Cytotoxic T cell (CTL) activation by antigen requires the specific detection of peptide-major histocompatibility class I (pMHC) molecules on the target-cell surface by the T cell receptor (TCR). We examined the effect of mutations in the antigen-binding site of a Kb-restricted TCR on T cell activation, antigen binding and dissociation from antigen.These parameters were also examined for variants derived from a Kd-restricted peptide that was recognized by a CTL clone. Using these two independent systems, we show that T cell activation can be impaired by mutations that either decrease or increase the binding half-life of the TCR-pMHC interaction. Our data indicate that efficient T cell activation occurs within an optimal dwell-time range of TCR-pMHC interaction. This restricted dwell-time range is consistent with the exclusion of either extremely low or high affinity T cells from the expanded population during immune responses.