882 Selective affinity-enhanced T cell receptor bispecific targeting of KRAS G12D neoantigen driven cancers

Background

KRAS is the most frequently mutated oncogene, yet mutant KRAS has historically been a challenging target for conventional small molecule drug development. Tumour specific neoantigen peptides derived from KRAS are presented by cell surface human leucocyte antigens (HLA) and form a class of shared, tumour-specific antigens that are attractive targets for immunotherapy.

Methods

A T cell clone that specifically recognizes the most common KRAS G12D mutant presented as a peptide in the context of HLA-A*11:01 was isolated from healthy donor PBMCs. The affinity of the respective T cell receptor (TCR) was enhanced by phage display and the x-ray crystal structures of the affinity-enhanced TCR bound to HLA presenting mutant KRAS G12D and wildtype (KRAS WT) peptides were solved. We used structural, biochemical, and computational approaches to investigate the molecular interactions underlying TCR selectivity for mutant KRAS G12D. Finally, the high affinity TCR was engineered into a soluble T cell engaging ImmTAC (Immune mobilizing monoclonal TCR Against Cancer) molecule, IMC-KRAS-G12D, and in vitro cell-based assays were performed to evaluate its potency and selectivity.

Results

The affinity of the engineered TCR was enhanced by a million-fold and demonstrated remarkable ability to distinguish between KRAS G12D and KRAS WT peptide presented by HLA-A*11:01. X-ray crystal structures demonstrate that TCR binding is almost identical between KRAS G12D and KRAS WT despite a binding affinity difference of >4000 fold. The mutant residue G12D is buried into the HLA peptide binding groove and acts as a secondary anchor, making it inaccessible to the TCR. Thermodynamic analysis of TCR-HLA interaction combined with molecular dynamics simulations indicates a novel mechanism of peptide selectivity, mediated by an indirect energetic mechanism driven by an induced fit in the peptide upon TCR binding. In functional assays, this molecular differentiation translated into biological specificity with IMC-KRAS-G12D mediating T cell activation in response to cells pulsed with or expressing KRAS G12D but not KRAS WT. Furthermore, IMC-KRAS-G12D was able to redirect T cell cytotoxicity towards target KRAS G12D presenting colon cancer cells, while sparing normal colon epithelial cells

Conclusions

We developed a high affinity TCR bispecific with exquisite specificity towards a common shared neoantigen, KRAS G12D, that is a relevant therapeutic target in a wide range of cancers. These findings reveal a novel molecular mechanism for TCR selectivity for a neoantigen that differs from self-antigen by only a single amino acid, with attendant implications for therapeutic development.

Specificity of bispecific T cell receptors and antibodies targeting peptide-HLA

Tumor-associated peptide-human leukocyte antigen complexes (pHLAs) represent the largest pool of cell surface-expressed cancer-specific epitopes, making them attractive targets for cancer therapies. Soluble bispecific molecules that incorporate an anti-CD3 effector function are being developed to redirect T cells against these targets using 2 different approaches. The first achieves pHLA recognition via affinity-enhanced versions of natural TCRs (e.g., immune-mobilizing monoclonal T cell receptors against cancer [ImmTAC] molecules), whereas the second harnesses an antibody-based format (TCR-mimic antibodies). For both classes of reagent, target specificity is vital, considering the vast universe of potential pHLA molecules that can be presented on healthy cells. Here, we made use of structural, biochemical, and computational approaches to investigate the molecular rules underpinning the reactivity patterns of pHLA-targeting bispecifics. We demonstrate that affinity-enhanced TCRs engage pHLA using a comparatively broad and balanced energetic footprint, with interactions distributed over several HLA and peptide side chains. As ImmTAC molecules, these TCRs also retained a greater degree of pHLA selectivity, with less off-target activity in cellular assays. Conversely, TCR-mimic antibodies tended to exhibit binding modes focused more toward hot spots on the HLA surface and exhibited a greater degree of crossreactivity. Our findings extend our understanding of the basic principles that underpin pHLA selectivity and exemplify a number of molecular approaches that can be used to probe the specificity of pHLA-targeting molecules, aiding the development of future reagents.

Abstract A24: Antigen-targeted soluble bispecific T-cell receptor (ImmTACTM) molecules in immunotherapy

Cancer immunotherapies function to harness the body’s own immune system to eradicate cancer cells. Critical to this process is the T cell, capable of directing potent and antigen-specific immune responses. T-cell receptor (TCR) antigens are short intracellularly processed peptides presented on the cell surface. As the majority of the proteome is processed and presented on the surface of cells and hence can be targeted by a TCR, TCR-based therapies offer distinct advantages over antibody-based therapies that are limited to targeting only secreted or cell surface proteins. In this study, we describe our approach to address the limitations and utilize the natural advantages of the human TCR to target a diverse range of tumor indications. Immune mobilizing monoclonal TCRs Against Cancer (ImmTAC) molecules are a novel class of bispecific biologic formed from an affinity-engineered, soluble, targeted TCR fused to a T cell redirecting anti-CD3 scFv. We describe our process to engineer high-affinity ImmTAC molecules towards novel cancer antigens that have been identified and validated in house. Additionally, we discuss the challenges and opportunities in the preclinical testing of an exquisitely human-specific biologic that led to the development of an entirely in vitro preclinical package that is proven to support the clinical development of ImmTAC molecules to clinical development. In summary, we present a novel approach to cancer immunotherapy, detailing the engineering, optimization, preclinical testing and clinical development of ImmTAC molecules, a new bispecific biologic.

Note: This abstract was not presented at the conference.

Citation Format: Annelise Vuidepot. Antigen-targeted soluble bispecific T-cell receptor (ImmTACTM) molecules in immunotherapy [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr A24.

Abstract B44: ImmTAC molecules: Beyond HLA-A*02:01—the identification and isolation of dual-HLA-specific T cell receptors

ImmTAC (Immune mobilizing monoclonal TCRs against cancer) molecules are a new class of bi-specific biologic that combine a T-cell receptor (TCR)-targeting system with an anti-CD3 effector function to activate highly potent and specific T-cell responses to cancer cells. ImmTAC molecules have the potential to overcome many of the limitations of other immuno-oncology agents; using TCRs to target intracellularly processed peptides presented by HLA allows access to a vast landscape of cancer-specific antigens that antibody-based therapies cannot currently target. Soluble agents also have a number of benefits over cellular therapies in terms of ease of manufacture and supply. At Immunocore we are constantly striving for innovation and looking for new opportunities to advance the ImmTAC platform. One limitation of TCR-targeting is HLA-restriction, which means that a single ImmTAC molecule is highly specific towards a single HLA-subtype. Lead ImmTAC molecules target HLA-A2, which has the highest coverage in the Caucasian population of approximately 55%. Our desire is to cover all patients in need; therefore, we are developing ImmTAC molecules that target peptides on other HLA alleles with an aim to increase patient access across a range of ethnic groups. In addition, the ability to target peptides presented in multiple HLA alleles would significantly increase the number of patients eligible for this treatment. HLA-A3 and HLA-A11 are part of the same superfamily and share largely overlapping peptide repertoires. We have identified peptides from gp100, a melanoma associated-antigen, which are presented by both HLA-A3 and HLA-A11. Identifying a TCR that is able to recognize a peptide in both these HLA alleles is the first step in generating our first dual-HLA-specific ImmTAC. Here we describe the generation of new TCRs suitable for ImmTAC generation, using our integrated in-house TCR discovery process. TCRs were isolated using multiple methods from HLA-A3 or HLA-A11 donors. We were able to identify antigen-specific T cells from multiple donors that recognized the gp100-peptide-HLA complex. The majority of these T cells were specific to the HLA-allele of the donor and failed to recognize the same peptide in the alternative HLA allele. However, we were able to identify TCRs that had a measurable binding affinity to the gp100 peptide presented in both HLA-A3 and HLA-A11. These data illustrate the successful isolation of wild-type TCRs suitable for affinity maturation and the first step in generating our first dual-HLA-restricted ImmTAC molecule.

Citation Format: Vanessa L. Clark, Linda Hibbert, Debbie Wright, Sarah Bailey, Alice Barkell, Kathy Hale, Elizabeth Evans, Alasdair Gunn, Maria Busz, Karolina Lech, Michelle Teng, Luke Williams, Joseph Dukes, Annelise Vuidepot, Bent Jakobsen. ImmTAC molecules: Beyond HLA-A*02:01—the identification and isolation of dual-HLA-specific T cell receptors [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B44.

TCRs with Distinct Specificity Profiles Use Different Binding Modes to Engage an Identical Peptide-HLA Complex

The molecular rules driving TCR cross-reactivity are poorly understood and, consequently, it is unclear the extent to which TCRs targeting the same Ag recognize the same off-target peptides. We determined TCR-peptide-HLA crystal structures and, using a single-chain peptide-HLA phage library, we generated peptide specificity profiles for three newly identified human TCRs specific for the cancer testis Ag NY-ESO-1_157-165-HLA-A2. Two TCRs engaged the same central peptide feature, although were more permissive at peripheral peptide positions and, accordingly, possessed partially overlapping peptide specificity profiles. The third TCR engaged a flipped peptide conformation, leading to the recognition of off-target peptides sharing little similarity with the cognate peptide. These data show that TCRs specific for a cognate peptide recognize discrete peptide repertoires and reconciles how an individual's limited TCR repertoire following negative selection in the thymus is able to recognize a vastly larger antigenic pool.

Abstract B001: Generation of ImmTACTM molecules: Engineering high-affinity soluble T-cell receptors for the treatment of cancer

Immunotherapeutic strategies are centered on harnessing the human immune system to recognise and destroy cancer cells. At Immunocore, we have developed bi-specific Immune-mobilising monoclonal TCRs Against Cancer (ImmTAC) molecules comprising a soluble TCR fused to an anti-CD3 effector function that redirect T-cells to destroy cancer cells. The TCR-targeting domain overcomes a key limitation of traditional antibody-based therapies by targeting short peptide fragments derived from intracellularly processed proteins presented on the cell surface by human leukocyte antigens (HLAs), offering up to nine-fold more potential targets than are accessible to antibodies. To mediate efficient T-cell-mediated tumor clearance, ImmTAC molecules are engineered to overcome the weak affinities of natural TCRs imposed by thymic selection through a complex multistep engineering process described herein.T-cell clones recognising in-house validated cancer-specific peptides are isolated and affinity-enhanced up to a million fold by introducing mutations to the complementarity determining regions, resulting in picomolar affinity TCRs capable of recognizing exceptionally low numbers of target on the cancer cell surface. The TCR is fused to an anti-CD3 scFv to generate an ImmTAC molecule and is made soluble through the inclusion of a non-native disulphide bond. The efficacy and specificity of the ImmTAC molecule is scrutinized using a range of cellular and molecular assays. The potential application of the ImmTAC platform is exemplified by the expanding portfolio of ImmTAC molecules targeting diverse disease indications. Our lead candidate, IMCgp100, recognizes the melanoma-associated gp100 peptide and is in pivotal trials for the treatment of patients with metastatic uveal melanoma.

Citation Format: Izabela Bombik, Alessio Bortoluzzi, Nicole Mai, Andrew Preston, Annelise Vuidepot, Bent K. Jakobsen, Nathaniel Liddy. Generation of ImmTACTM molecules: Engineering high-affinity soluble T-cell receptors for the treatment of cancer [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr B001.

An approved in vitro approach to preclinical safety and efficacy evaluation of engineered T cell receptor anti-CD3 bispecific (ImmTAC) molecules

Robust preclinical testing is essential to predict clinical safety and efficacy and provide data to determine safe dose for first-in-man studies. There are a growing number of examples where the preclinical development of drugs failed to adequately predict clinical adverse events in part due to their assessment with inappropriate preclinical models. Preclinical investigations of T cell receptor (TCR)-based immunotherapies prove particularly challenging as these biologics are human-specific and thus the conventional testing in animal models is inadequate. As these molecules harness the full force of the immune system, and demonstrate tremendous potency, we set out to design a preclinical package that would ensure adequate evaluation of these therapeutics. Immune Mobilising Monoclonal TCR Against Cancer (ImmTAC) molecules are bi-specific biologics formed of an affinity-enhanced TCR fused to an anti-CD3 effector function. ImmTAC molecules are designed to activate human T lymphocytes and target peptides within the context of a human leukocyte antigen (HLA), thus require an intact human immune system and peptidome for suitable preclinical screening. Here we draw upon the preclinical testing of four ImmTAC molecules, including IMCgp100, the first ImmTAC molecule to reach the clinic, to present our comprehensive, informative and robust approach to in vitro preclinical efficacy and safety screening. This package comprises a broad range of cellular and molecular assays using human tissues and cultured cells to test efficacy, safety and specificity, and hence predict human responses in clinical trials. We propose that this entirely in vitro package offers a potential model to be applied to screening other TCR-based biologics.

Abstract 4873: ImmTACs re-direct the immune system efficiently to eradicate cancer

Immunotherapy strategies that are able to induce T cell infiltration into tumors and activate a cytotoxic T cell response have the potential to destroy the tumor. Although T cells can mediate clearance of a tumor, thymic selection and the suppressive microenvironment limit their effectiveness. To overcome poor tumor immunogenicity, we have developed a unique platform that enables the generation of ImmTACs (Immune-mobilising monoclonal TCRs Against Cancer); these are comprised of an affinity enhanced TCR specific for a cancer antigen fused to an anti-CD3 scFv. The TCR end targets and binds MHC class I/peptide complex displayed on cancerous cells while the anti-CD3 scFv end engages polyclonal T cells to mediate a potent anti-tumor response.

The most advanced ImmTAC, IMCgp100, targets the HLA-A2/gp100280-288 epitope presented by melanoma cells. IMCgp100 is currently in a Phase I/IIa clinical trial for advance malignant melanoma and is showing promising clinical efficacy in some patients. Here we report a series of in vitro experiments evaluating IMCgp100 mechanism of action. IMCgp100 is able to redirect T cells from healthy donors or from melanoma patients to destroy cancer cells and secrete a range of inflammatory cytokines and chemokines associated with T cell trafficking into tumors; some of these cytokines also induce upregulation of inhibitory pathway molecules. We show that effector memory cells in the CD8+ and CD4+ T cell compartments are very efficient in eliminating melanoma cells and in expanding upon IMCgp100 engagement. The effects of combining IMCgp100 with agents that relieve the suppression imposed by immune check-point molecules have also been investigated.

Citation Format: Giovanna Bossi, Rupert Kenefeck, Caroline Caroline Boudousquie, Jane Harper, Joseph Dukes, Nathaniel Liddy, Samantha Paston, Tara Mahon, Peter Molloy, Malkit Sami, Emma Baston, Brian Cameron, Annelise Vuidepot, Namir Hassan, Bent K. Jakobsen. ImmTACs re-direct the immune system efficiently to eradicate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4873.

Abstract 4872: Developing high affinity, soluble T cell receptors for the treatment of cancer

Immunotherapeutic strategies that drive activation of cytotoxic T cells possess significant potential to eradicate tumours. Whereas monoclonal antibodies are restricted to targeting secreted or cell surface proteins, T cell receptors (TCRs) are able to recognise a wider range of targets. This is achieved through binding to short peptide fragments derived from proteins that are degraded intracellularly and presented at the cell surface by human leukocyte antigens (HLAs). Natural cancer specific TCRs however, have weak affinities and cancer cells often develop escape mechanisms to avoid destruction by T cells.

To overcome this, we have developed Immune mobilising monoclonal TCRs Against Cancer (ImmTACs); a new class of soluble bi-specific molecules comprising affinity-enhanced, monoclonal T cell receptors (mTCRs) fused to an anti-CD3 scFv. ImmTACs target peptides presented by HLA, and through the anti-CD3 effector, re-direct cytotoxic T cells to achieve highly specific and potent tumour cell killing.

At Immunocore, we have developed an integrated in-house process for the generation of ImmTACs and here describe the critical engineering steps involved. T-cell clones that specifically recognise validated cancer antigens are isolated from peripheral blood lymphocytes and the TCR-encoding sequences are identified by RACE. To confirm antigen binding, TCR á and â chains are expressed as inclusion bodies in bacteria, co-refolded in vitro, and their binding to the target peptide:HLA tested by Surface Plasmon Resonance (SPR). The affinity of the TCR is then enhanced up to a million-fold through directed evolution, utilising phage display. Individual mutants are screened by SPR and combined to generate ImmTACs with pM affinities (KD) and binding half-lives of many hours. A range of biochemical and cellular assays are then performed to assess the potency and specificity of each ImmTAC generated.

This process has been successfully applied to produce ImmTACs for a wide range of targets, demonstrating the robustness of the platform. Our lead candidate, IMCgp100, is undergoing Phase IIa clinical trials in patients with advanced malignant melanoma. This reagent, which specifically targets the gp100 (280-288) peptide presented by HLA-A2 on melanoma cells, is well tolerated and shows very promising therapeutic potential.

Citation Format: Andrew Knox, Fiona Chester, Frayne Bianchi, Sarah Bailey, Lucie Bouard, Nathaniel Liddy, Giovanna Bossi, Jane Harper, Joseph Dukes, Samantha Paston, Tara Mahon, Jessie Gavarret, Peter Molloy, Malkit Sami, Emma Baston, Brian Cameron, Alex Powlesland, Penio Todorov, Andrew Johnson, Martin Ebner, Yvonne McGrath, Namir Hassan, Annelise Vuidepot, Bent Jakobsen. Developing high affinity, soluble T cell receptors for the treatment of cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4872.

Direct molecular mimicry enables off-target cardiovascular toxicity by an enhanced affinity TCR designed for cancer immunotherapy

Natural T-cell responses generally lack the potency to eradicate cancer. Enhanced affinity T-cell receptors (TCRs) provide an ideal approach to target cancer cells, with emerging clinical data showing significant promise. Nevertheless, the risk of off target reactivity remains a key concern, as exemplified in a recent clinical report describing fatal cardiac toxicity, following administration of MAGE-A3 specific TCR-engineered T-cells, mediated through cross-reactivity with an unrelated epitope from the Titin protein presented on cardiac tissue. Here, we investigated the structural mechanism enabling TCR cross-recognition of MAGE-A3 and Titin, and applied the resulting data to rationally design mutants with improved antigen discrimination, providing a proof-of-concept strategy for altering the fine specificity of a TCR towards an intended target antigen. This study represents the first example of direct molecular mimicry leading to clinically relevant fatal toxicity, mediated by a modified enhanced affinity TCR designed for cancer immunotherapy. Furthermore, these data demonstrate that self-antigens that are expressed at high levels on healthy tissue should be treated with extreme caution when designing immuno-therapeutics.

Abstract A037: IMCgp100 ImmTAC: A new immunotherapeutic reagent for the treatment of malignant melanoma

For years cancer therapies were mainly concentrated around surgery, chemotherapy and radiotherapy. Recently, a new domain has emerged. Immunotherapies aim to exploit, enhance and optimise the patient's immune system to be more potent at eradicating tumours. T-cell immunotherapies, however, often encounter challenges with low cell surface expression of tumour-associated-antigens (TAAs), down-regulation of class I MHC molecules and low T-cell avidity for TAAs, all contributing to tumour escape.

To address these challenges, Immunocore Ltd. has developed ImmTAC (Immune Mobilising mTCR Against Cancer) technology that aims to recruit and redirect circulating T-cells to target and kill cancer cells with high specificity. ImmTACs are composed of an affinity enhanced monoclonal T-cell receptor (mTCR) combined with an anti-CD3 specific antibody fragment (CD3-scFv). The mTCR targets cancer cells that express TAAs displayed on class I MHC molecules, with high affinity. The CD3-scFv effector domain then drives the recruitment and activation of T-cells. ImmTACs enable an immune synapse to form, leading to the destruction of cancer cells by T-cells.

TAA candidates are selected according to their level of gene expression by measuring their mRNA levels by RT-PCR in healthy and cancer cells, in order to reduce off-target toxicity, as well as by measuring their cell surface expression on class I MHCs by mass spectrometry, to verify target presentation.

We present here our current clinical ImmTAC candidate, IMCgp100, which targets the melanoma-associated gp100 antigen presented on HLA-A2. IMCgp100 has been extensively studied in vitro and is undergoing a phase IIa clinical trial in advanced melanoma patients in the UK and US. The maximum tolerated dose has been established and a dose expansion phase is currently underway. T-cell mobilisation, relevant cytokine release as well as tumour shrinkage have been monitored. The very encouraging results observed so far in our clinical trials show ImmTAC technology to be a promising efficient immunotherapy for cancer.

Citation Format: Johanne Pentier, Mary Connolly, Martina Canestraro, Ruth Ryan, Luise Weigand, Namir Hassan, Annelise Vuidepot, Bent Jakobsen. IMCgp100 ImmTAC: A new immunotherapeutic reagent for the treatment of malignant melanoma. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A037.

Abstract 670: ImmTACs: Bi-specific TCR-anti-CD3 fusions for potent re-directed killing of cancer cells

Immunotherapeutic strategies that are able to drive activation of T cells, and in particular CD8+ cells, possess significant potential to eradicate tumors. Although it is well-established that CD8+ T cells can mediate potent destruction of tumor tissue, the immunosuppressive tumor microenvironment and thymic selection serve to limit the availability and effectiveness of tumor-specific T cells. These factors typically prevent an effective immune response in cancer patients. Despite some successful attempts to counter immune-suppression using bi-specific, immune-activating antibodies, the paucity of tumor-specific antibody targets limits this approach. ImmTACs (immune mobilising monoclonal TCRs against cancer) are a new class of soluble bi-specific molecules comprising, pico-molar affinity T cell receptors (TCRs) fused to an anti-CD3 specific scFv. Unlike antibodies, TCRs target peptide epitopes, derived from intracellular antigens, and presented on the cell surface by Class I MHC, thus providing access to a large pool of tumor-associated antigens. The high affinity recognition mediated by the engineered TCR portion of ImmTACs overcomes the problems posed by thymic selection, as well as HLA down-regulation on tumor cells, while the anti-CD3 moiety re-directs T cells to mediate a potent anti-tumor response.

The most advanced ImmTAC reagent to-date, IMCgp100, targets the HLA-A2 presented gp100280-288 epitope. This reagent represents a first-in-class treatment for malignant melanoma, and is currently in Phase I clinical testing, with maximum tolerated dose having been established. Here we examine the processes which underpin the mechanism of action of IMCgp100, as well as the broader ImmTAC platform. These studies demonstrate that IMCgp100 is able to redirect T cells from healthy donors and from melanoma patients to destroy cancer cells with low cell-surface epitope densities (25 to 70 epitopes per cell), and to secrete a range of inflammatory cytokines. Effector and central memory cells in the CD8+ and CD4+ repertoire rapidly respond to ImmTAC engagement on melanoma cells, while the naïve cells provide a slower but nonetheless potent response. ImmTACs can induce an anti-tumor T cell response despite the presence of immunosuppressive regulatory T cells and expression of inhibitory molecules such as PD-L1. In addition, ImmTAC activated cytotoxic T cells are capable of serially killing a number of target cells. Finally, we have been able to demonstrate that IMCgp100 can trigger DC X presentation of melanoma associated antigens providing a potential mechanism for inducing a long-term self-sustained anti-cancer response. Taken together these data demonstrate the potential of the ImmTAC platform for targeted cancer therapy.

Citation Format: Giovanna Bossi, Debbie Baker, Katherine Adams, Jane Harper, Joseph Dukes, Nathaniel Liddy, Samantha Paston, Yvonne McGrath, Tara Mahon, Pater Molloy, Malkit Sami, Emma Baston, Brian Cameron, Andrew Johnson, Annelise Vuidepot, Namir Hassan, Bent Jakobsen. ImmTACs: Bi-specific TCR-anti-CD3 fusions for potent re-directed killing of cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 670. doi:10.1158/1538-7445.AM2014-670

Abstract 2900: IMCgp100: A novel bi-specific biologic for the treatment of malignant melanoma

Despite significant advances in the treatment of metastatic melanoma, long-term remission for the majority of patients remains elusive. Kinase inhibitors provide potent but short-term responses for a significant proportion of patients and immunotherapy elicits long-term responses with the prospect of cure, but only in a minority.

IMCgp100 is a novel bi-specific immunotherapy comprising a soluble, affinity enhanced, T cell receptor (TCR) specific for the melanoma-associated antigen gp100, fused to an anti-CD3 specific antibody fragment (scFv). The engineered TCR portion of the drug targets and binds the gp100 peptide 280-288 antigen, which is over-expressed and presented by HLA-A2 on the surface of melanoma cells. The anti-CD3 scFv portion captures and redirects T cells to kill the melanoma cells, while normal antigen negative tissues are unaffected. Here, we present data which provides the foundation for the clinical observations. In vitro, IMCgp100 is demonstrated to potently redirect T cells from late stage cancer patients to target melanoma tumors exhibiting HLA-down regulation, even in the presence of high numbers of regulatory T cells. Target cell killing is observed within hours and is specific for gp100. In addition killing is associated with the release of various pro-inflammatory cytokines and chemokines as well as cross-presentation of gp100 and other melanoma-associated antigens by dendritic cells. Thus, IMCgp100 demonstrates the potential to elicit potent short-term responses and trigger longer-term anti melanoma activity in vivo.

IMCgp100 is undergoing Phase I clinical testing in patients with advanced malignant melanoma; with the maximum tolerated dose having been established. The drug is well tolerated with evidence of tumor shrinkage. Analyses of serum samples obtained from patients on the trial demonstrate T cell mobilisation and transient drug mediated increases in various cytokines and chemokines, some of which are reported to play a key role in anti-melanoma responses. These data support the potential of IMCgp100 as an effective treatment for malignant melanoma.

Citation Format: Namir J. Hassan, Giovanna Bossi, Debbie Baker, Katherine Adams, Jane Harper, Joseph Dukes, Nathaniel Liddy, Samantha Paston, Yvonne McGrath, Tara Mahon, Peter Molloy, Malkit Sami, Emma Baston, Brian Cameron, Andrew Johnson, Annelise Vuidepot, Gerry Linette, Michael Kalos, Carl June, Bent Jakobsen. IMCgp100: A novel bi-specific biologic for the treatment of malignant melanoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2900. doi:10.1158/1538-7445.AM2014-2900

Identification of a Titin-derived HLA-A1-presented peptide as a cross-reactive target for engineered MAGE A3-directed T cells

MAGE A3, which belongs to the family of cancer-testis antigens, is an attractive target for adoptive therapy given its reactivation in various tumors and limited expression in normal tissues. We developed an affinity-enhanced T cell receptor (TCR) directed to a human leukocyte antigen (HLA)-A*01-restricted MAGE A3 antigen (EVDPIGHLY) for use in adoptive therapy. Extensive preclinical investigations revealed no off-target antigen recognition concerns; nonetheless, administration to patients of T cells expressing the affinity-enhanced MAGE A3 TCR resulted in a serious adverse event (SAE) and fatal toxicity against cardiac tissue. We present a description of the preclinical in vitro functional analysis of the MAGE A3 TCR, which failed to reveal any evidence of off-target activity, and a full analysis of the post-SAE in vitro investigations, which reveal cross-recognition of an off-target peptide. Using an amino acid scanning approach, a peptide from the muscle protein Titin (ESDPIVAQY) was identified as an alternative target for the MAGE A3 TCR and the most likely cause of in vivo toxicity. These results demonstrate that affinity-enhanced TCRs have considerable effector functions in vivo and highlight the potential safety concerns for TCR-engineered T cells. Strategies such as peptide scanning and the use of more complex cell cultures are recommended in preclinical studies to mitigate the risk of off-target toxicity in future clinical investigations.

Bi-specific TCR-anti CD3 redirected T-cell targeting of NY-ESO-1- and LAGE-1-positive tumors

NY-ESO-1 and LAGE-1 are cancer testis antigens with an ideal profile for tumor immunotherapy, combining up-regulation in many cancer types with highly restricted expression in normal tissues and sharing a common HLA-A*0201 epitope, 157–165. Here, we present data to describe the specificity and anti-tumor activity of a bifunctional ImmTAC, comprising a soluble, high-affinity T-cell receptor (TCR) specific for NY-ESO-1_157–165 fused to an anti-CD3 scFv. This reagent, ImmTAC-NYE, is shown to kill HLA-A2, antigen-positive tumor cell lines, and freshly isolated HLA-A2- and LAGE-1-positive NSCLC cells. Employing time-domain optical imaging, we demonstrate in vivo targeting of fluorescently labelled high-affinity NYESO-specific TCRs to HLA-A2-, NY-ESO-1_157–165-positive tumors in xenografted mice. In vivo ImmTAC-NYE efficacy was tested in a tumor model in which human lymphocytes were stably co-engrafted into NSG mice harboring tumor xenografts; efficacy was observed in both tumor prevention and established tumor models using a GFP fluorescence readout. Quantitative RT-PCR was used to analyze the expression of both NY-ESO-1 and LAGE-1 antigens in 15 normal tissues, 5 cancer cell lines, 10 NSCLC, and 10 ovarian cancer samples. Overall, LAGE-1 RNA was expressed at a greater frequency and at higher levels than NY-ESO-1 in the tumor samples. These data support the clinical utility of ImmTAC-NYE as an immunotherapeutic agent for a variety of cancers.

Different affinity windows for virus and cancer-specific T-cell receptors: implications for therapeutic strategies

T-cell destiny during thymic selection depends on the affinity of the TCR for autologous peptide ligands presented in the context of MHC molecules. This is a delicately balanced process; robust binding leads to negative selection, yet some affinity for the antigen complex is required for positive selection. All TCRs of the resulting repertoire thus have some intrinsic affinity for an MHC type presenting an assortment of peptides. Generally, TCR affinities of peripheral T cells will be low toward self-derived peptides, as these would have been presented during thymic selection, whereas, by serendipity, binding to pathogen-derived peptides that are encountered de novo could be stronger. A crucial question in assessing immunotherapeutic strategies for cancer is whether natural TCR repertoires have the capacity for efficiently recognizing tumor-associated peptide antigens. Here, we report a comprehensive comparison of TCR affinities to a range of HLA-A2 presented antigens. TCRs that bind viral antigens fall within a strikingly higher affinity range than those that bind cancer-related antigens. This difference may be one of the key explanations for tumor immune escape and for the deficiencies of T-cell vaccines against cancer.

Abstract 3528: IMCmage1: A novel bi-specific biologic re-directing T cells to kill MAGE-A3/A6 presenting cancers

In a minority of cancer patients immunotherapy has shown the capacity to eradicate tumours leading to clinical remission and the promise of a cure. In the majority of patients however, a cure remains elusive due to active immune evasion by cancers; HLA-down-regulation and immunosuppression are two of the known mechanisms adopted by cancers to promote their survival and proliferation. To overcome these challenges we have developed bi-specific soluble biologics termed ImmTACs (Immune mobilising mTCR against cancer) to re-direct the immune system to recognise and kill cancers. Antigenic peptide fragments presented by HLA molecules on the surface of cancer cells constitute the largest class of cancer associated targets. T cells scan the HLA-peptide (pHLA) antigens being presented to them; sufficient recognition by the harboured T Cell Receptor (TCR) will result in T cell activation and killing of the antigen presenting cell. In cancer patients this process is inefficient partly due to the low affinity TCRs expressed by tumour specific T cells and the low abundance of pHLA on cancers. ImmTACs comprise a soluble TCR with an enhanced affinity for cancer associated pHLA (targeting end) fused to an anti-CD3 scfv, enabling potent T cell re-direction (effector end). Our pipeline constitutes a number of ImmTACs targeting various antigen pHLA complexes relevant to numerous cancer indications. IMCmage1 is a novel ImmTAC targeting MAGE-A3168-176 in the context of HLA-A1. MAGE-A3 is a well validated cancer testis antigen expressed in a variety of cancers including myeloma, NSCLC, prostate cancer, melanoma, bladder cancer, oesophageal cancer and others. IMCmage1 re-directs T cells from cancer patients or healthy donors to kill a range of MAGE positive cell-lines in vitro; this activity is observed against cells presenting as few as 40 epitopes per cell and is coupled with the release of pro-inflammatory cytokines including IFNα, TNFα, IL-2, MIP1α and others. We also demonstrate that IMCmage1 specifically targets and kills the myeloma associated population within CD138+ cells extracted from the marrow of a stage III myeloma patient. IMCmage1 specificity was confirmed by exposure to a panel of HLA-A1 MAGE negative primary cells derived from various organs such as the heart, skin, lung and others; no significant activity was detected. A phase I clinical trial in multiple myeloma to assess tolerability and establish a maximum tolerated dose is planned to commence in Q2 2012.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3528. doi:1538-7445.AM2012-3528

Abstract 4744: Soluble, high affinity TCRs fused to anti-CD3 redirect T cells to kill cancer cells presenting MAGE-A3 and NY-ESO antigens

In the last decade, major efforts in the fight against cancer have focused on galvanizing the adaptive immune system to kill tumors. Many of these endeavors are based on the development and clinical use of monoclonal antibodies (mAb) which are the most successful class of immune modulating agent identified to date. While mAbs show promise against certain cancers, their specificity is limited to integral membrane proteins; this hinders their extensive development for the purposes of targeting cancer cells. In contrast to mAbs, T cell receptors (TCRs) recognize peptides bound to major histocompatibility complex class I (MHC I) molecules. These peptides are derived from endogenously processed proteins, and therefore represent a different repertoire of targets to those recognized by mAbs. This alternate spectrum of antigens provides the potential to target cancers using an untapped source of well-validated epitopes. Naturally occurring TCRs, however, have relatively low affinities for their antigen compared to antibody binding. Advances in engineering techniques have allowed the generation of high affinity monoclonal TCRs (mTCRs) with picomolar affinities for their antigen. Using targeted mutagenesis and phage display, we have generated a number of soluble, high affinity mTCRs specific for several reported tumor-associated antigens. Through mTCR fusion to an anti-CD3 single chain variable fragment (scfv), we produced bifunctional proteins that redirect T cell immune specificity. These novel proteins are termed ImmTACs (Immune-mobilizing mTCRs Against Cancer).

We present data showing the potential of two such ImmTACs, NY-ESO-ImmTAC and MAGE-A3-ImmTAC, to treat certain cancers. NY-ESO1 and MAGE-A3 are cancer testes antigens and therefore represent potentially very clean molecular targets. We demonstrate that both NY-ESO- and MAGE-A3-ImmTACs are capable of potently redirecting unstimulated CD8+ T cells against multiple myeloma, colorectal carcinoma and non-small cell lung cancer cell lines despite the presentation of extremely low antigen numbers (<100 epitopes/cell) on the cell surface. ImmTAC-redirected T cells respond with multiple effector functions including production of granzyme B, IFNγ and IL-2. Using the NY-ESO-ImmTAC we observed significant redirected degranulation of T cells against a primary lung tumor sample which was shown to express NY-ESO. We also present data from an established tumor model using the OV-79 cell line derived from an ovarian tumor which is shown to express MAGE. Administration of MAGE-A3-ImmTAC to animals with established tumors resulted in inhibition of tumor growth in all ImmTAC treated animals with regression or cure in some. Thus both MAGE-A3 and NY-ESO-ImmTACs possess the potential to be highly specific, potent cancer immunotherapies offering a targeting and therapeutic approach distinct from any other biologic in development.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4744. doi:10.1158/1538-7445.AM2011-4744

Abstract 1787: ImmTACs: Bi-functional reagents for redirected tumour cell killing

The human immune system can theoretically identify malignant cells by inspecting cell surface Class I HLA -peptide complexes for the presence of disease-associated epitopes. Indeed, many cancer patients generate CD8 cyto-toxic T cell responses to tumour-associated antigens; the majority of patients, however, fail to clear tumours since T cell avidity for self-antigens tends to be weak, and cancer cells employ escape mechanisms for avoiding destruction by T cells. To overcome these issues, we have engineered novel, bi-functional protein therapeutics termed ImmTACs (Immune Mobilising mTCR Against Cancer) which re-direct the immune system to target and destroy tumour cells with a high degree of potency and specificity. An ImmTAC comprises a high affinity ‘monoclonal’ T cell Receptor (mTCR) targeting a cancer-associated HLA-peptide complex, fused to an anti-CD3 scFv domain which activates an anti-tumour T cell response.

We demonstrate that ImmTACs against a number of different cancer-associated antigens can target and kill tumour cells expressing as few as 10-20 epitopes per cell with pico-Molar potency. ImmTACs preferentially activate effector memory CD8 T cells, resulting in secretion of multiple cytokines and tumour cell killing; a single activated T cell can kill multiple antigen positive tumour cells. Furthermore, we demonstrate that the reagents are able to inhibit tumour growth in mouse xenograft models. In vitro ImmTAC potency translates to a dose of less than 1mg in humans, representing a significant advance over existing targeted anti-cancer therapies. Currently we are conducting a phase I dose-escalation trial and Phase 0 exploratory trial using ImmTAC-gp100 in late stage melanoma patients. In summary, ImmTACs offers a novel therapeutic approach for the treatment of various cancers with the potential to provide major benefits over current treatments including reduction in dose to sub-mg quantities and an improved safety profile.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1787. doi:10.1158/1538-7445.AM2011-1787

High affinity soluble ILT2 receptor: a potent inhibitor of CD8(+) T cell activation

Using directed mutagenesis and phage display on a soluble fragment of the human immunoglobulin super-family receptor ILT2 (synonyms: LIR1, MIR7, CD85j), we have selected a range of mutants with binding affinities enhanced by up to 168,000-fold towards the conserved region of major histocompatibility complex (MHC) class I molecules. Produced in a dimeric form, either by chemical cross-linking with bivalent polyethylene glycol (PEG) derivatives or as a genetic fusion with human IgG Fc-fragment, the mutants exhibited a further increase in ligand-binding strength due to the avidity effect, with resident half-times (t(1/2)) on the surface of MHC I-positive cells of many hours. The novel compounds antagonized the interaction of CD8 co-receptor with MHC I in vitro without affecting the peptide-specific binding of T-cell receptors (TCRs). In both cytokine-release assays and cell-killing experiments the engineered receptors inhibited the activation of CD8(+) cytotoxic T lymphocytes (CTLs) in the presence of their target cells, with subnanomolar potency and in a dose-dependent manner. As a selective inhibitor of CD8(+) CTL responses, the engineered high affinity ILT2 receptor presents a new tool for studying the activation mechanism of different subsets of CTLs and could have potential for the development of novel autoimmunity therapies.

Abstract 5616: Soluble, high affinity TCRs fused to anti-CD3 redirect T cells to kill cancer cells presenting MAGE-A3 and NY-ESO antigens

In the last decade, major efforts in the fight against cancer have focussed on galvanising the adaptive immune system to kill tumours. Many of these endeavours are based on the development and clinical use of monoclonal antibodies (mAb) which are the most successful class of immune modulating agent identified to date. However, while mAbs show promise against certain cancers, their specificity is limited to integral membrane proteins; this hinders their extensive development for the purposes of targeting cancer cells.

In contrast to mAbs, T cell receptors (TCRs) recognise peptides bound to major histocompatibility complex class I (MHC I) molecules. These peptides are derived from endogenously processed proteins, and therefore represent a different repertoire of targets to those recognised by mAbs. This alternate spectrum of antigens provides the potential to target cancers using an untapped source of well-validated epitopes. Naturally occurring TCRs, however, have relatively low affinities for their antigen compared to antibody binding. Advances in engineering techniques have allowed the generation of high affinity monoclonal TCRs (mTCRs) with picomolar affinities for their antigen. Using targeted mutagenesis and phage display, we have generated a number of soluble, high affinity mTCRs specific for several reported tumour-associated antigens. Through mTCR fusion to an anti-CD3 single chain variable fragment (scfv), we produced bifunctional proteins that redirect T cell immune specificity. These novel proteins are termed ImmTACs (Immune-mobilising mTCRs Against Cancer). We present data demonstrating the potential of two such ImmTAC molecules, NY-ESO-ImmTAC and MAGE-A3-ImmTAC, to treat certain cancers. NY-ESO1 and MAGE-A3 are both cancer testes antigens and therefore represent potentially very clean molecular targets. We demonstrate that both NY-ESO-ImmTAC and MAGE-A3-ImmTAC are capable of potently redirecting unstimulated peripheral blood mononuclear cells (PBMC) or CD8+ T cells against multiple myeloma, colorectal carcinoma and non-small cell lung cancer cell lines despite the presentation of extremely low antigen numbers (<100 epitopes/cell) on the surface of these cells. ImmTAC-redirected T cells respond with multiple effector functions including the production of granzyme B, IFNγ and IL-2, resulting in a high level of antigen-specific cancer cell killing over the course of 24 hours. The mechanism of killing is at least partly contributed by the caspase-3/7 apoptotic pathway. We also show that NY-ESO-ImmTAC and MAGE-A3-ImmTAC were inert by all our measures in the presence of normal primary cells including melanocytes, hepatocytes and astrocytes. Thus both MAGE-A3 and NY-ESO-ImmTACs possess the potential to be highly specific, potent cancer immunotherapies offering a targeting and therapeutic approach distinct from any other biologic in development.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5616.

Control of HIV-1 immune escape by CD8 T cells expressing enhanced T-cell receptor

HIV's considerable capacity to vary its HLA-I-restricted peptide antigens allows it to escape from host cytotoxic T lymphocytes (CTLs). Nevertheless, therapeutics able to target HLA-I-associated antigens, with specificity for the spectrum of preferred CTL escape mutants, could prove effective. Here we use phage display to isolate and enhance a T-cell antigen receptor (TCR) originating from a CTL line derived from an infected person and specific for the immunodominant HLA-A(*)02-restricted, HIVgag-specific peptide SLYNTVATL (SL9). High-affinity (K(D) < 400 pM) TCRs were produced that bound with a half-life in excess of 2.5 h, retained specificity, targeted HIV-infected cells and recognized all common escape variants of this epitope. CD8 T cells transduced with this supraphysiologic TCR produced a greater range of soluble factors and more interleukin-2 than those transduced with natural SL9-specific TCR, and they effectively controlled wild-type and mutant strains of HIV at effector-to-target ratios that could be achieved by T-cell therapy.

Crystallization and preliminary X-ray structural studies of a Melan-A pMHC-TCR complex

Melanocytes are specialized pigmented cells that are found in all healthy skin tissue. In certain individuals, diseased melanocytes can form malignant tumours, melanomas, which cause the majority of skin-cancer-related deaths. The melanoma-associated antigenic peptides are presented on cell surfaces via the class I major histocompatibility complex (MHC). Among the melanoma-associated antigens, the melanoma self-antigen A/melanoma antigen recognized by T cells (Melan-A/MART-1) has attracted attention because of its wide expression in primary and metastatic melanomas. Here, a preliminary X-ray crystal structural study of a soluble cognate T-cell receptor (TCR) in complex with a pMHC presenting the Melan-A peptide (ELAGIGILTV) is reported. The TCR and pMHC were refolded, purified and mixed together to form complexes, which were crystallized using the sitting-drop vapour-diffusion method. Single TCR-pMHC complex crystals were cryocooled and used for data collection. Diffraction data showed that these crystals belonged to space group P4(1)/P4(3), with unit-cell parameters a = b = 120.4, c = 81.6 A. A complete data set was collected to 3.1 A and the structure is currently being analysed.

Cutting Edge: Evidence of Direct TCR α-Chain Interaction with Superantigen

Superantigens are known to activate a large number of T cells. The SAg is presented by MHC class II on the APC and its classical feature is that it recognizes the variable region of the beta-chain of the TCR. In this article, we report, by direct binding studies, that staphylococcal enterotoxin (SE) H (SEH), a bacterial SAg secreted by Staphylococcus aureus, instead recognizes the variable alpha-chain (TRAV27) of TCR. Furthermore, we show that different SAgs (e.g., SEH and SEA) can simultaneously bind to one TCR by binding the alpha-chain and the beta-chain, respectively. Theoretical three-dimensional models of the penta complexes are presented. Hence, these findings open up a new dimension of the biology of the staphylococcal enterotoxins.

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.

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.