Increased Peptide Contacts Govern High Affinity Binding of a Modified TCR Whilst Maintaining a Native pMHC Docking Mode.

Natural T cell receptors (TCRs) generally bind to their cognate pMHC molecules with weak affinity and fast kinetics, limiting their use as therapeutic agents. Using phage display, we have engineered a high affinity version of the A6 wild-type TCR (A6wt), specific for the human leukocyte antigen (HLA-A(∗)0201) complexed with human T cell lymphotropic virus type 111-19 peptide (A2-Tax). Mutations in just 4 residues in the CDR3ß loop region of the A6wt TCR were selected that improved binding to A2-Tax by nearly 1000-fold. Biophysical measurements of this mutant TCR (A6c134) demonstrated that the enhanced binding was derived through favorable enthalpy and a slower off-rate. The structure of the free A6c134 TCR and the A6c134/A2-Tax complex revealed a native binding mode, similar to the A6wt/A2-Tax complex. However, concordant with the more favorable binding enthalpy, the A6c134 TCR made increased contacts with the Tax peptide compared with the A6wt/A2-Tax complex, demonstrating a peptide-focused mechanism for the enhanced affinity that directly involved the mutated residues in the A6c134 TCR CDR3ß loop. This peptide-focused enhanced TCR binding may represent an important approach for developing antigen specific high affinity TCR reagents for use in T cell based therapies.

Production of a soluble disulfide bond-linked TCR in the cytoplasm of Escherichia coli trxB gor mutants.

Previously, we have described the use of phage display to generate high affinity disulfide bond-linked T cell receptors (TCRs). The affinities of the mutant TCRs were analysed after refolding of separately expressed alpha and beta chains from Escherichia coli inclusion bodies. This approach is only suitable for the analysis of small numbers of TCR variants. An attractive alternative would be soluble expression within the bacterial periplasm, but the generic production of TCRs within the E. coli periplasm has so far not proved successful. Here we show that functional, soluble TCR can be produced within the cytoplasm of trxB gor mutant E. coli strains, with maximum yields of 3.4 mg/l. We also investigated the effect of coexpressing the folding modulators Skp and DsbC finding that the TCR expression levels were largely unaffected by these chaperones. Importantly, we demonstrated that the amount of protein purified from 50 ml starter cultures was sufficient to show functionality of the TCR by specific antigen binding in both ELISA and surface plasmon resonance (SPR) assays. This TCR production method has the potential to allow rapid and medium throughput analysis of affinity-matured TCRs selected from TCR phage display libraries.

T cell receptor engagement of peptide-major histocompatibility complex class I does not modify CD8 binding.

Activation of cytotoxic T cells is initiated by engagement of the T-cell receptor (TCR) with peptide-major histocompatibility class I complexes (pMHCI). The CD8 co-receptor also binds to pMHCI, but at a distinct site, and allows the potential for tripartite TCR/pMHCI/CD8 interactions, which can increase T cell antigen sensitivity. There has been a substantial interest in the effect of the pMHCI/CD8 interaction upon TCR/pMHCI engagement, and several conflicting studies have examined this event, using the soluble extracellular domains of CD8 and the TCR, by surface plasmon resonance. However, the evidence to date suggests that the TCR engages cognate pMHCI before CD8 recruitment, so the question of whether TCR engagement alters CD8 binding is likely to be more relevant to the biological order of T cell antigen encounter. Here, we have examined the binding of CD8 to several variants of the HLA A2-restricted telomerase(540-548) antigen (ILAKFLHWL) and the HLA A2-restricted NY-ESO-1(157-165) antigen (SLLMWITQC) that bind to their cognate TCRs with distinct affinities and kinetics. These interactions represent a range of agonists that exhibit different CD8 dependency for activation of their respective T cells. By using engineered affinity enhanced TCRs to these ligands, which have extended off-rates of approximately 1h compared to seconds for the wildtype TCRs, we have examined pMHCI/CD8 binding before and during TCR-engagement. Here we show that the binding of the extracellular domain of the TCR to pMHCI does not transmit structural changes to the pMHCI-CD8 binding site that would alter the subsequent pMHCI/CD8 interaction.

Crystal structures of high affinity human T-cell receptors bound to peptide major histocompatibility complex reveal native diagonal binding geometry.

Naturally selected T-cell receptors (TCRs) are characterised by low binding affinities, typically in the range 1-100 microM. Crystal structures of syngeneic TCRs bound to peptide major histocompatibility complex (pMHC) antigens exhibit a conserved mode of binding characterised by a distinct diagonal binding geometry, with poor shape complementarity (SC) between receptor and ligand. Here, we report the structures of three in vitro affinity enhanced TCRs that recognise the pMHC tumour epitope NY-ESO(157-165) (SLLMWITQC). These crystal structures reveal that the docking mode for the high affinity TCRs is identical to that reported for the parental wild-type TCR, with only subtle changes in the mutated complementarity determining regions (CDRs) that form contacts with pMHC; both CDR2 and CDR3 mutations act synergistically to improve the overall affinity. Comparison of free and bound TCR structures for both wild-type and a CDR3 mutant reveal an induced fit mechanism arising from restructuring of CDR3 loops which allows better peptide binding. Overall, an increased interface area, improved SC and additional H-bonding interactions are observed, accounting for the increase in affinity. Most notably, there is a marked increase in the SC for the central methionine and tryptophan peptide motif over the native TCR.

Enhanced immunogenicity of CTL antigens through mutation of the CD8 binding MHC class I invariant region.

CD8(+) cytotoxic T lymphocytes (CTL) are key determinants of immunity to intracellular pathogens and neoplastic cells. Recognition of specific antigens in the form of peptide-MHC class I complexes (pMHCI) presented on the target cell surface is mediated by T cell receptor (TCR) engagement. The CD8 coreceptor binds to invariant domains of pMHCI and facilitates antigen recognition. Here, we investigate the biological effects of a Q115E substitution in the alpha2 domain of human leukocyte antigen (HLA)-A*0201 that enhances CD8 binding by approximately 50% without altering TCR/pMHCI interactions. Soluble and cell surface-expressed forms of Q115E HLA-A*0201 exhibit enhanced recognition by CTL without loss of specificity. These CD8-enhanced antigens induce greater CD3 zeta chain phosphorylation in cognate CTL leading to substantial increases in cytokine production, proliferation and priming of naive T cells. This effect provides a fundamental new mechanism with which to enhance cellular immunity to specific T cell antigens.

Human TCR-binding affinity is governed by MHC class restriction.

T cell recognition is initiated by the binding of TCRs to peptide-MHCs (pMHCs), the interaction being characterized by weak affinity and fast kinetics. Previously, only 16 natural TCR/pMHC interactions have been measured by surface plasmon resonance (SPR). Of these, 5 are murine class I, 5 are murine class II, and 6 are human class I-restricted responses. Therefore, a significant gap exists in our understanding of human TCR/pMHC binding due to the limited SPR data currently available for human class I responses and the absence of SPR data for human class II-restricted responses. We have produced a panel of soluble TCR molecules originating from human T cells that respond to naturally occurring disease epitopes and their cognate pMHCs. In this study, we compare the binding affinity and kinetics of eight class-I-specific TCRs (TCR-Is) to pMHC-I with six class-II-specific TCRs (TCR-IIs) to pMHC-II using SPR. Overall, there is a substantial difference in the TCR-binding equilibrium constants for pMHC-I and pMHC-II, which arises from significantly faster on-rates for TCRs binding to pMHC-I. In contrast, the off-rates for all human TCR/pMHC interactions fall within a narrow window regardless of class restriction, thereby providing experimental support for the notion that binding half-life is the principal kinetic feature controlling T cell activation.

Potent T cell agonism mediated by a very rapid TCR/pMHC interaction.

The interaction between T cell receptors (TCR) and peptide-major histocompatibility complex (pMHC) antigens can lead to varying degrees of agonism (T cell activation), or antagonism. The P14 TCR recognises the lymphocytic choriomeningitis virus (LCMV)-derived peptide, gp33 residues 33-41 (KAVYNFATC), presented in the context of H-2D(b). The cellular responses to various related H-2D(b) peptide ligands are very well characterised, and P14 TCR-transgenic mice have been used extensively in models of virus infection, autoimmunity and tumour rejection. Here, we analyse the binding of the P14 soluble TCR to a broad panel of related H-2D(b)-peptide complexes by surface plasmon resonance, and compare this with their diverse cellular responses. P14 TCR binds H-2D(b)-gp33 with a KD of 3 microM (+/-0.5 microM), typical of an immunodominant antiviral TCR, but with unusually fast kinetics (k(off) = 1 s(-1)), corresponding to a half-life of 0.7 s at 25 degrees C, outside the range previously observed for murine agonist TCR/pMHC interactions. The most striking feature of these data is that a very short half-life does not preclude the ability of a TCR/pMHC interaction to induce antiviral immunity, autoimmune disease and tumour rejection.

Functional and biophysical characterization of an HLA-A*6801-restricted HIV-specific T cell receptor.

HLA-A*6801 exhibits several unusual features. First, it is known to bind weakly to CD8 due to the presence of an A245V substitution in the alpha3 domain. Second, it is able to accommodate unusually long peptides as a result of peptide 'kinking' in the binding groove. Third, CD8+ cytotoxic T lymphocytes that recognise HLA-A*6801-restricted antigens can tolerate substantial changes in the peptide sequence without apparent loss of recognition. In addition, it has been suggested that HLA-A68-restricted TCR might bind with higher affinity than other TCR due to their selection in the presence of a decreased contribution from CD8. Here we (1) examine monoclonal T cell recognition of an HLA-A*6801-restricted HIV-1 Tat-derived 11-amino acid peptide (ITKGLGISYGR) and natural variant sequences thereof; (2) measure the affinity and kinetics of a TCR/pHLA-A68 interaction biophysically for the first time, showing that equilibrium binding occurs within the range previously determined for non-HLA-A68-restricted TCR (KD approx. 7 microM); and (3) show that "normalization" of the non-canonical HLA-A*6801 CD8-binding domain enhances recognition of agonist peptides without inducing non-specific activation. This latter effect may provide a fundamental new mechanism with which to enhance T cell immunity to specific antigens.

Computational design and crystal structure of an enhanced affinity mutant human CD8 alphaalpha coreceptor.

Human CD8 is a T cell coreceptor, which binds to pHLA I and plays a pivotal role in the activation of cytotoxic T lymphocytes. Soluble recombinant CD8 alphaalpha has been shown to antagonize T cell activation, both in vitro and in vivo. However, because of a very low affinity for pHLA I, high concentrations of soluble CD8 alphaalpha are required for efficient inhibition. Based upon our knowledge of the wild-type CD8/pHLA I structure, we have designed and produced a mutated form of soluble CD8 alphaalpha that binds to pHLA I with approximately fourfold higher affinity. We have characterized the binding of the high affinity CD8 mutant using surface plasmon resonance and determined its structure at 2.1 A resolution using X-ray crystallography. The analysis of this structure suggests that the higher affinity is achieved by providing a larger side chain that allows for an optimal contact to be made between the HLA alpha3 loop and the mutated CDR-like loops of CD8.

Crystal structure of HLA-A*2402 complexed with a telomerase peptide.

HLA-A*2402 is the most commonly expressed HLA allele in oriental populations. It is also widely expressed in the Caucasian population, making it one of, if not the most abundant HLA I types. In order to study its structure in terms of overall fold and peptide presentation, a soluble form of this HLA I (alpha1, alpha2, alpha3 and beta(2)m domains) has been expressed, refolded and crystallized in complex with a cancer-related telomerase peptide (VYGFVRACL), and its structure has been solved to 2.8 A resolution. The overall structure of HLA-A*2402 is virtually identical to other reported peptide-HLA I structures. However, there are distinct features observable from this structure at the HLA I peptide binding pockets. The size and depth of pocket B makes it highly suitable for binding to large aromatic side chains, which explains the high prevalence of tyrosine at peptide position 2. Also, for HLA binding at peptide position 5, there is an additional anchor point, which allows the proximal amino acids to protrude out, providing a prominent feature for TCR interaction. Finally, pocket F allows the anchor residue at position 9 to be bound unusually deeply within the HLA structure.

Structural and kinetic basis for heightened immunogenicity of T cell vaccines.

Analogue peptides with enhanced binding affinity to major histocompatibility class (MHC) I molecules are currently being used in cancer patients to elicit stronger T cell responses. However, it remains unclear as to how alterations of anchor residues may affect T cell receptor (TCR) recognition. We correlate functional, thermodynamic, and structural parameters of TCR-peptide-MHC binding and demonstrate the effect of anchor residue modifications of the human histocompatibility leukocyte antigens (HLA)-A2 tumor epitope NY-ESO-1(157-165)-SLLMWITQC on TCR recognition. The crystal structure of the wild-type peptide complexed with a specific TCR shows that TCR binding centers on two prominent, sequential, peptide sidechains, methionine-tryptophan. Cysteine-to-valine substitution at peptide position 9, while optimizing peptide binding to the MHC, repositions the peptide main chain and generates subtly enhanced interactions between the analogue peptide and the TCR. Binding analyses confirm tighter binding of the analogue peptide to HLA-A2 and improved soluble TCR binding. Recognition of analogue peptide stimulates faster polarization of lytic granules to the immunological synapse, reduces dependence on CD8 binding, and induces greater numbers of cross-reactive cytotoxic T lymphocyte to SLLMWITQC. These results provide important insights into heightened immunogenicity of analogue peptides and highlight the importance of incorporating structural data into the process of rational optimization of superagonist peptides for clinical trials.

Directed evolution of human T-cell receptors with picomolar affinities by phage display.

Peptides derived from almost all proteins, including disease-associated proteins, can be presented on the cell surface as peptide-human leukocyte antigen (pHLA) complexes. T cells specifically recognize pHLA with their clonally rearranged T-cell receptors (TCRs), whose natural affinities are limited to approximately 1-100 muM. Here we describe the display of ten different human TCRs on the surface of bacteriophage, stabilized by a nonnative interchain disulfide bond. We report the directed evolution of high-affinity TCRs specific for two different pHLAs: the human T-cell lymphotropic virus type 1 (HTLV-1) tax(11-19) peptide-HLA-A(*)0201 complex and the NY-ESO-1(157-165) tumor-associated peptide antigen-HLA-A(*)0201 complex, with affinities of up to 2.5 nM and 26 pM, respectively, and we demonstrate their high specificity and sensitivity for targeting of cell-surface pHLAs.

Crystallization and preliminary X-ray structural studies of a high-affinity CD8alphaalpha co-receptor to pMHC.

The class I CD8 positive T-cell response is involved in a number of conditions in which artificial down-regulation and control would be therapeutically beneficial. Such conditions include a number of autoimmune diseases and graft rejection in transplant patients. Although the CD8 T-cell response is dominated by the TCR-pMHC interaction, activation of T cells is in most cases also dependent on a number of associated signalling molecules. Previous work has demonstrated the ability of one such molecule (CD8) to act as an antagonist to T-cell activation if added in soluble form. Therefore, a high-affinity mutant CD8 (haCD8) has been developed with the aim of developing a therapeutic immunosuppressor. In order to fully understand the nature of the haCD8 interaction, this protein was crystallized using the sitting-drop vapour-diffusion method. Single haCD8 crystals were cryocooled and used for data collection. These crystals belonged to space group P6(4)22 (assumed by similarity to the wild type), with unit-cell parameters a = 101.08, c = 56.54 A. VM calculations indicated one molecule per asymmetric unit. A 2 A data set was collected and the structure is currently being determined using molecular replacement.

Design of soluble recombinant T cell receptors for antigen targeting and T cell inhibition.

The use of recombinant T cell receptors (TCRs) to target therapeutic interventions has been hindered by the naturally low affinity of TCR interactions with peptide major histocompatibility complex ligands. Here, we use multimeric forms of soluble heterodimeric alphabeta TCRs for specific detection of target cells pulsed with cognate peptide, discrimination of quantitative changes in antigen display at the cell surface, identification of virus-infected cells, inhibition of antigen-specific cytotoxic T lymphocyte activation, and identification of cross-reactive peptides. Notably, the A6 TCR specific for the immunodominant HLA A2-restricted human T cell leukemia virus type 1 Tax(11-19) epitope bound to HLA A2-HuD(87-95) (K(D) 120 microm by surface plasmon resonance), an epitope implicated as a causal antigen in the paraneoplastic neurological degenerative disorder anti-Hu syndrome. A mutant A6 TCR that exhibited dramatically increased affinity for cognate antigen (K(D) 2.5 nm) without enhanced cross-reactivity was generated; this TCR demonstrated potent biological activity even as a monomeric molecule. These data provide insights into TCR repertoire selection and delineate a framework for the selective modification of TCRs in vitro that could enable specific therapeutic intervention in vivo.

TCR affinity and negative regulation limit autoimmunity.

Autoimmune diseases are often mediated by self-reactive T cells, which must be activated to cause immunopathology. One mechanism, known as molecular mimicry, proposes that self-reactive T cells may be activated by pathogens expressing crossreactive ligands. Here we have developed a model to investigate how the affinity of the T-cell receptor (TCR) for the activating agent influences autoimmunity. Our model shows that an approximately fivefold difference in the TCR affinity for the activating ligand results in a 50% reduction in the incidence of autoimmunity. A reduction in TCR-ligand affinity to approximately 20 times lower than normal does not induce autoimmunity despite the unexpected induction of cytotoxic T lymphocytes (CTLs) and insulitis. Furthermore, in the absence of a key negative regulatory molecule, Cbl-b, 100% of mice develop autoimmunity upon infection with viruses encoding the lower-affinity ligand. Therefore, autoimmune disease is sensitive both to the affinity of the activating ligand and to normal mechanisms that negatively regulate the immune response.

Stable, soluble T-cell receptor molecules for crystallization and therapeutics.

Antibody and T-cell receptors (TCRs) are the primary recognition molecules of the adaptive immune system. Antibodies have been extensively characterized and are being developed for a large number of therapeutic applications. This has been possible because of the ability to manufacture stable, soluble, monoclonal antibodies which retain the antigen specificity of B cells. Unlike antibodies, TCRs are not expressed in a soluble form, but are anchored to the T-cell surface by an insoluble trans-membrane domain. Characterization and development of TCRs has been hampered by the lack of suitable methods for producing them as soluble and stable proteins. Here we report the engineering of soluble human TCRs suitable for crystallization studies and potentially for in vivo therapeutic use.

The CD8 T cell coreceptor exhibits disproportionate biological activity at extremely low binding affinities.

T lymphocytes recognize peptides presented in the context of major histocompatibility complex (MHC) molecules on the surface of antigen presenting cells. Recognition specificity is determined by the alphabeta T cell receptor (TCR). The T lymphocyte surface glycoproteins CD8 and CD4 enhance T cell antigen recognition by binding to MHC class I and class II molecules, respectively. Biophysical measurements have determined that equilibrium binding of the TCR with natural agonist peptide-MHC (pMHC) complexes occurs with KD values of 1-50 microm. The pMHCI/CD8 and pMHCII/CD4 interactions are significantly weaker than this (KD >100 microm), and the relative roles of TCR/pMHC and pMHC/coreceptor affinity in T cell activation remain controversial. Here, we engineer mutations in the MHCI heavy chain and beta2-microglobulin that further reduce or abolish the pMHCI/CD8 interaction to probe the significance of pMHC/coreceptor affinity in T cell activation. We demonstrate that the pMHCI/CD8 coreceptor interaction retains the vast majority of its biological activity at affinities that are reduced by over 15-fold (KD > 2 mm). In contrast to previous reports, we observe that the weak interaction between HLA A68 and CD8, which falls within this spectrum of reduced affinities, retains substantial functional activity. These findings are discussed in the context of current concepts of coreceptor dependence and the mechanism by which TCR coreceptors facilitate T cell activation.

Complete mapping of a novel HLA A*6801-restricted HIV-1 Tat epitope directly with a rapid modified enzyme-linked immunospot assay.

We identified a novel HLA A*6801-restricted HIV-1 Tat-derived cytotoxic T lymphocyte (CTL) epitope using an adapted enzyme-linked immunospot assay that allows the rapid ex vivo identification of CTL epitopes together with their associated HLA Class I restriction elements. The optimal 11 amino acid residue Tat epitope efficiently stabilized the refolding of monomeric peptide-HLA A6801 complexes in vitro and fluorochrome-labelled, tetrameric peptide-HLA A6801 complexes stained CD8 T cells specific for this epitope directly ex vivo.