MHC-class I-restricted CD4 T cells: a nanomolar affinity TCR has improved anti-tumor efficacy in vivo compared to the micromolar wild-type TCR

What's the Catch? The Significance of Catch Bonds in T Cell Activation

One of the main goals in T cell biology has been to investigate how TCR recognition of peptide:MHC (pMHC) determines T cell phenotype and fate. Ag recognition is required to facilitate survival, expansion, and effector function of T cells. Historically, TCR affinity for pMHC has been used as a predictor for T cell fate and responsiveness, but there have now been several examples of nonfunctional high-affinity clones and low-affinity highly functional clones. Recently, more attention has been paid to the TCR being a mechanoreceptor where the key biophysical determinant is TCR bond lifetime under force. As outlined in this review, the fundamental parameters between the TCR and pMHC that control Ag recognition and T cell triggering are affinity, bond lifetime, and the amount of force at which the peak lifetime occurs.

Germline T cell receptor exchange results in physiological T cell development and function

T cell receptor (TCR) transgenic mice represent an invaluable tool to study antigen-specific immune responses. In the pre-existing models, a monoclonal TCR is driven by a non-physiologic promoter and randomly integrated into the genome. Here, we create a highly efficient methodology to develop T cell receptor exchange (TRex) mice, in which TCRs, specific to the self/tumor antigen mesothelin (Msln), are integrated into the Trac locus, with concomitant Msln disruption to circumvent T cell tolerance. We show that high affinity TRex thymocytes undergo all sequential stages of maturation, express the exogenous TCR at DN4, require MHC class I for positive selection and undergo negative selection only when both Msln alleles are present. By comparison of TCRs with the same specificity but varying affinity, we show that Trac targeting improves functional sensitivity of a lower affinity TCR and confers resistance to T cell functional loss. By generating P14 TRex mice with the same specificity as the widely used LCMV-P14 TCR transgenic mouse, we demonstrate increased avidity of Trac-targeted TCRs over transgenic TCRs, while preserving physiologic T cell development. Together, our results support that the TRex methodology is an advanced tool to study physiological antigen-specific T cell behavior.

Empirical and Rational Design of T Cell Receptor-Based Immunotherapies

The use of T cells reactive with intracellular tumor-associated or tumor-specific antigens has been a promising strategy for cancer immunotherapies in the past three decades, but the approach has been constrained by a limited understanding of the T cell receptor’s (TCR) complex functions and specificities. Newer TCR and T cell-based approaches are in development, including engineered adoptive T cells with enhanced TCR affinities, TCR mimic antibodies, and T cell-redirecting bispecific agents. These new therapeutic modalities are exciting opportunities by which TCR recognition can be further exploited for therapeutic benefit. In this review we summarize the development of TCR-based therapeutic strategies and focus on balancing efficacy and potency versus specificity, and hence, possible toxicity, of these powerful therapeutic modalities.

Why the Outcome of Anti-Tumor Immune Responses is Heterogeneous: A Novel Idea in the Context of Immunological Heterogeneity in Cancers

The question as to why some hosts can eradicate their tumors while others succumb to tumor-progression remains unanswered. Here, a provocative concept is proposed that intrinsic differences in the T cell receptor (TCR) repertoire of individuals may influence the outcome of anti-tumor immunity by affecting the frequency and/or variety of tumor-reactive CD8 and/or CD4 tumor-infiltrating lymphocytes. This idea implicates that the TCR repertoire in a given patient might not provide sufficiently different TCR clones that can recognize tumor antigens, namely, "a hole in the TCR repertoire" might exist. This idea may provide a novel perspective to further dissect the mechanisms underlying heterogeneous anti-tumor immune responses in different hosts. Besides tumor-intrinsic heterogeneity and host microbiome, the various factors that may constantly shape the dynamic TCR repertoire are also discussed. Elucidating mechanistic differences in different individuals' immune systems will allow to better harness immune system to design new personalized cancer immunotherapy.

Engineering CD4+ T Cells to Enhance Cancer Immunity

This review presents key advances in combining T cell receptor (TCR) gene transfer to redirect T-cell specificity with gene engineering in order to enhance cancer-protective immune function. We discuss how emerging insights might be applied to CD4+ T cells. Although much attention has been paid to the role of CD8+ cytotoxic T cells in tumour protection, we provide convincing evidence that CD4+ helper T cells play a critical role in cancer immune responses in animal models and also in patients. We demonstrate that genetic engineering technologies provide exciting opportunities to extend the specificity range of CD4+ T cells from MHC class-II-presented epitopes to include peptides presented by MHC class I molecules. Functional enhancement of tumour immunity can improve the sensitivity of T cells to cancer antigens, promote survival in a hostile tumour microenvironment, boost cancer-protective effector mechanisms and enable the formation of T-cell memory. Engineered cancer-specific CD4+ T cells may contribute to protective immunity by a direct pathway involving cancer cell killing, and by an indirect pathway that boosts the function, persistence and memory formation of CD8+ T cells.

MHC Class I-Restricted TCR-Transgenic CD4+ T Cells Against STEAP1 Mediate Local Tumor Control of Ewing Sarcoma In Vivo

In this study we report the functional comparison of T cell receptor (TCR)-engineered major histocompatibility complex (MHC) class I-restricted CD4⁺ versus CD8⁺ T cells targeting a peptide from six transmembrane epithelial antigen of the prostate 1 (STEAP1) in the context of HLA-A*02:01. STEAP1 is a tumor-associated antigen, which is overexpressed in many cancers, including Ewing sarcoma (EwS). Based on previous observations, we postulated strong antitumor potential of tumor-redirected CD4⁺ T cells transduced with an HLA class I-restricted TCR against a STEAP1-derived peptide. We compared CD4⁺ T cell populations to their CD8⁺ counterparts in vitro using impedance-based xCELLigence and cytokine/granzyme release assays. We further compared antitumor activity of STEAP¹³⁰-TCR transgenic (tg) CD4⁺ versus CD8⁺ T cells in tumor-bearing xenografted Rag2^-/-gc^-/- mice. TCR tgCD4⁺ T cells showed increased cytotoxic features over time with similar functional avidity compared to tgCD8⁺ cells after 5-6 weeks of culture. In vivo, local tumor control was equal. Assessing metastatic organotropism of intraveniously (i.v.) injected tumors, only tgCD8⁺ cells were associated with reduced metastases. In this analysis, EwS-redirected tgCD4⁺ T cells contribute to local tumor control, but fail to control metastatic outgrowth in a model of xenografted EwS.

Engineering Strategies to Enhance TCR-Based Adoptive T Cell Therapy

T cell receptor (TCR)-based adoptive T cell therapies (ACT) hold great promise for the treatment of cancer, as TCRs can cover a broad range of target antigens. Here we summarize basic, translational and clinical results that provide insight into the challenges and opportunities of TCR-based ACT. We review the characteristics of target antigens and conventional αβ-TCRs, and provide a summary of published clinical trials with TCR-transgenic T cell therapies. We discuss how synthetic biology and innovative engineering strategies are poised to provide solutions for overcoming current limitations, that include functional avidity, MHC restriction, and most importantly, the tumor microenvironment. We also highlight the impact of precision genome editing on the next iteration of TCR-transgenic T cell therapies, and the discovery of novel immune engineering targets. We are convinced that some of these innovations will enable the field to move TCR gene therapy to the next level.

A rare population of tumor antigen-specific CD4+CD8+ double-positive αβ T lymphocytes uniquely provide CD8-independent TCR genes for engineering therapeutic T cells

Background

High-affinity tumor antigen-specific T-cell receptor (TCR) gene is required to engineer potent T cells for therapeutic treatment of cancer patients. However, discovery of suitable therapeutic TCR genes is hampered by the fact that naturally occurring tumor antigen-specific TCRs are generally of low-affinity, and artificial modification of TCRs can mediate cross-reactivity to other antigens expressed in normal tissues. Here, we discovered a naturally occurring T-cell clone which expressed high-affinity HLA-A*02:01 (A*02)-restricted TCR against NY-ESO-1 from a patient who had NY-ESO-1-expressing ovarian tumor.

Methods

A*02-restricted NY-ESO-1-specific T-cell clones were established from peripheral blood of patients who had NY-ESO-1-expressing ovarian tumors. TCR α and β chain genes were retrovirally transduced into polyclonally activated T cells. Phenotype and function of the parental and TCR-transduced T cells were analyzed by flow cytometry, ELISA and cytotoxicity assay. In vivo therapeutic efficacy was investigated in a xenograft model using NOD/SCID/IL-2Rγ-deficient (NSG) mice.

Results

A rare population of NY-ESO-1-specific T cells, which we named 19305DP, expressed cell surface CD4, CD8α, and CD8β but not CD56 and recognized A*02⁺NY-ESO-1⁺ cancer cell lines in a CD4- and CD8-independent manner. 19305DP showed a gene expression profile that is consistent with a mixed profile of CD4⁺ and CD8⁺ single-positive T cells. Both CD4⁺ and CD8⁺ T cells that were retrovirally transduced with 19305DP-derived TCR gene (19305DP-TCR) showed strong reactivity against A*02⁺NY-ESO-1⁺ cancer cells, whereas TCR genes from the conventional A*02-restricted NY-ESO-1-specific CD8⁺ single-positive T-cell clones functioned only in CD8⁺ T cells. Both 19305DP-TCR gene-engineered CD4⁺ and CD8⁺ T cells eliminated A*02⁺NY-ESO-1⁺ tumor xenografts in NSG mice. Finally, based on reactivity against a series of alanine-substituted peptides and a panel of normal human tissue-derived primary cells, 19305DP-TCR was predicted to have no cross-reactivity against any human non-NY-ESO-1 proteins.

Conclusion

Together, our results indicate that the naturally occurring 19305DP-TCR derived from CD4⁺CD8⁺ double-positive αβ T cells, is a promising therapeutic TCR gene for effective and safe adoptive T-cell therapy in A*02⁺ patients with NY-ESO-1-expressing tumor.

Electronic supplementary material

The online version of this article (10.1186/s40425-018-0467-y) contains supplementary material, which is available to authorized users.

Human melanoma immunotherapy using tumor antigen-specific T cells generated in humanized mice

A major factor hindering the exploration of adoptive immunotherapy in preclinical settings is the limited availability of tumor-reactive human T cells. Here we developed a humanized mouse model that permits large-scale production of human T cells expressing the engineered melanoma antigen MART-1-specific TCR. Humanized mice, made by transplantation of human fetal thymic tissue and CD34⁺ cells virally-transduced with HLA class I-restricted melanoma antigen (MART-1)-specific TCR gene, showed efficient development of MART-1-TCR⁺ human T cells with predominantly CD8⁺ cells. Importantly, MART-1-TCR⁺CD8⁺ T cells developing in these mice were capable of mounting antigen-specific responses in vivo, as evidenced by their proliferation, phenotypic conversion and IFN-γ production following MART-1 peptide immunization. Moreover, these MART-1-TCR⁺CD8⁺ T cells mediated efficient killing of melanoma cells in an HLA/antigen-dependent manner. Adoptive transfer of in vitro expanded MART-1-TCR⁺CD8⁺ T cells induced potent antitumor responses that were further enhanced by IL-15 treatment in melanoma-bearing recipients. Finally, a short incubation of MART-1-specific T cells with rapamycin acted synergistically with IL-15, leading to significantly improved tumor-free survival in recipients with metastatic melanoma. These data demonstrate the practicality of using humanized mice to produce potentially unlimited source of tumor-specific human T cells for experimental and preclinical exploration of cancer immunotherapy. This study also suggests that pretreatment of tumor-reactive T cells with rapamycin in combination with IL-15 administration may be a novel strategy to improve the efficacy of adoptive T cell therapy.

Human leucocyte antigen class I-redirected anti-tumour CD4+ T cells require a higher T cell receptor binding affinity for optimal activity than CD8+ T cells

CD4⁺ T helper cells are a valuable component of the immune response towards cancer. Unfortunately, natural tumour‐specific CD4⁺ T cells occur in low frequency, express relatively low‐affinity T cell receptors (TCRs) and show poor reactivity towards cognate antigen. In addition, the lack of human leucocyte antigen (HLA) class II expression on most cancers dictates that these cells are often unable to respond to tumour cells directly. These deficiencies can be overcome by transducing primary CD4⁺ T cells with tumour‐specific HLA class I‐restricted TCRs prior to adoptive transfer. The lack of help from the co‐receptor CD8 glycoprotein in CD4⁺ cells might result in these cells requiring a different optimal TCR binding affinity. Here we compared primary CD4⁺ and CD8⁺ T cells expressing wild‐type and a range of affinity‐enhanced TCRs specific for the HLA A*0201‐restricted NY‐ESO‐1‐ and gp100 tumour antigens. Our major findings are: (i) redirected primary CD4⁺ T cells expressing TCRs of sufficiently high affinity exhibit a wide range of effector functions, including cytotoxicity, in response to cognate peptide; and (ii) optimal TCR binding affinity is higher in CD4⁺ T cells than CD8⁺ T cells. These results indicate that the CD4⁺ T cell component of current adoptive therapies using TCRs optimized for CD8⁺ T cells is below par and that there is room for substantial improvement.

Recent advances in T-cell engineering for use in immunotherapy

Adoptive T-cell therapies have shown exceptional promise in the treatment of cancer, especially B-cell malignancies. Two distinct strategies have been used to redirect the activity of ex vivo engineered T cells. In one case, the well-known ability of the T-cell receptor (TCR) to recognize a specific peptide bound to a major histocompatibility complex molecule has been exploited by introducing a TCR against a cancer-associated peptide/human leukocyte antigen complex. In the other strategy, synthetic constructs called chimeric antigen receptors (CARs) that contain antibody variable domains (single-chain fragments variable) and signaling domains have been introduced into T cells. Whereas many reviews have described these two approaches, this review focuses on a few recent advances of significant interest. The early success of CARs has been followed by questions about optimal configurations of these synthetic constructs, especially for efficacy against solid tumors. Among the many features that are important, the dimensions and stoichiometries of CAR/antigen complexes at the synapse have recently begun to be appreciated. In TCR-mediated approaches, recent evidence that mutated peptides (neoantigens) serve as targets for endogenous T-cell responses suggests that these neoantigens may also provide new opportunities for adoptive T-cell therapies with TCRs.

CD4(+) and CD8(+) TCRβ repertoires possess different potentials to generate extraordinarily high-avidity T cells

Recent high throughput sequencing analysis has revealed that the TCRβ repertoire is largely different between CD8(+) and CD4(+) T cells. Here, we show that the transduction of SIG35α, the public chain-centric HLA-A*02:01(A2)/MART127-35 TCRα hemichain, conferred A2/MART127-35 reactivity to a substantial subset of both CD8(+) and CD4(+) T cells regardless of their HLA-A2 positivity. T cells individually reconstituted with SIG35α and different A2/MART127-35 TCRβ genes isolated from CD4(+) or CD8(+) T cells exhibited a wide range of avidity. Surprisingly, approximately half of the A2/MART127-35 TCRs derived from CD4(+) T cells, but none from CD8(+) T cells, were stained by A2/MART127-35 monomer and possessed broader cross-reactivity. Our results suggest that the differences in the primary structure of peripheral CD4(+) and CD8(+) TCRβ repertoire indeed result in the differences in their ability to form extraordinarily high avidity T cells which would otherwise have been deleted by central tolerance.

New Strategies in Engineering T-cell Receptor Gene-Modified T cells to More Effectively Target Malignancies

The immune system, T cells in particular, have the ability to target and destroy malignant cells. However, antitumor immune responses induced from the endogenous T-cell repertoire are often insufficient for the eradication of established tumors, as illustrated by the failure of cancer vaccination strategies or checkpoint blockade for most tumors. Genetic modification of T cells to express a defined T-cell receptor (TCR) can provide the means to rapidly generate large numbers of tumor-reactive T cells capable of targeting tumor cells in vivo. However, cell-intrinsic factors as well as immunosuppressive factors in the tumor microenvironment can limit the function of such gene-modified T cells. New strategies currently being developed are refining and enhancing this approach, resulting in cellular therapies that more effectively target tumors and that are less susceptible to tumor immune evasion.

T cell receptor binding affinity governs the functional profile of cancer-specific CD8+ T cells

Antigen-specific T cell receptor (TCR) gene transfer via patient-derived T cells is an attractive approach to cancer therapy, with the potential to circumvent immune regulatory networks. However, high-affinity tumour-specific TCR clonotypes are typically deleted from the available repertoire during thymic selection because the vast majority of targeted epitopes are derived from autologous proteins. This process places intrinsic constraints on the efficacy of T cell-based cancer vaccines and therapeutic strategies that employ naturally generated tumour-specific TCRs. In this study, we used altered peptide ligands and lentivirus-mediated transduction of affinity-enhanced TCRs selected by phage display to study the functional properties of CD8(+) T cells specific for three different tumour-associated peptide antigens across a range of binding parameters. The key findings were: (i) TCR affinity controls T cell antigen sensitivity and polyfunctionality; (ii) supraphysiological affinity thresholds exist, above which T cell function cannot be improved; and (iii) T cells transduced with very high-affinity TCRs exhibit cross-reactivity with self-derived peptides presented by the restricting human leucocyte antigen. Optimal system-defined affinity windows above the range established for natural tumour-specific TCRs therefore allow the enhancement of T cell effector function without off-target effects. These findings have major implications for the rational design of novel TCR-based biologics underpinned by rigorous preclinical evaluation.

Unpredicted phenotypes of two mutants of the TcR DMF5

When a T-cell Receptor (TcR) interacts with its cognate peptide-MHC (pMHC), it triggers activation of a signaling cascade that results in the elicitation of a T cell effector function. Different models have been proposed to understand which parameters are needed to obtain an optimal activation of the signaling. It was speculated that improving the binding of a TcR could bring a stronger pMHC recognition, hence a stronger stimulation of the T cell. However, it was recently shown that an increase in affinity does not seem to be sufficient to guarantee improved functionality. A combination of factors is necessary to place the modified TcR in an optimal functional window. We here compared the binding parameters of two mutants of the melanoma antigen peptide MART-127-35 specific TcR DMF5. The first mutant was previously isolated by others in a screen for improved TcR. It was reported to have an increased CD8-independent activity. We confirmed these data and showed that the enhancement was neither due to change in half life (t1/2) nor Kd of the pMHC-TcR complex. The second mutant was designed based on a previous report claiming that a particular polymorphic residue in the TRAV12-2 chain was stabilizing the TcR. We created a DMF5 mutant for this residue and showed that, unexpectedly, this TcR had acquired a reduced overall activity although the TcR-pMHC complex was more stable when compared to the TcR wild type complex (increased t1/2). In addition, the soluble TcR form of this mutant bound target cells less efficiently. From this we concluded that kinetic parameters do not always predict the superior functionality of mutant TcRs.

TCR affinity for p/MHC formed by tumor antigens that are self-proteins: impact on efficacy and toxicity

Recent studies have shown that the range of affinities of T cell receptors (TCRs) against non-mutated cancer peptide/class I complexes are lower than TCR affinities for foreign antigens. Raising the affinity of TCRs for optimal activity of CD8 T cells, and for recruitment of CD4 T cell activity against a class I antigen, provides opportunities for more robust adoptive T cell therapies. However, TCRs with enhanced affinities also risk increased reactivity with structurally related self-peptides, and off-target toxicities. Careful selection of tumor peptide antigens, in silico proteome screens, and in vitro peptide specificity assays will be important in the development of the most effective, safe TCR-based adoptive therapies.

T Cell Receptor Engineering and Analysis Using the Yeast Display Platform

The αβ heterodimeric T cell receptor (TCR) recognizes peptide antigens that are transported to the cell surface as a complex with a protein encoded by the major histocompatibility complex (MHC). T cells thus evolved a strategy to sense these intracellular antigens, and to respond either by eliminating the antigen-presenting cell (e.g., a virus-infected cell) or by secreting factors that recruit the immune system to the site of the antigen. The central role of the TCR in the binding of antigens as peptide-MHC (pepMHC) ligands has now been studied thoroughly. Interestingly, despite their exquisite sensitivity (e.g., T cell activation by as few as 1-3 pepMHC complexes on a single target cell), TCRs are known to have relatively low affinities for pepMHC, with K D values in the micromolar range. There has been interest in engineering the affinity of TCRs in order to use this class of molecules in ways similar to now done with antibodies. By doing so, it would be possible to harness the potential of TCRs as therapeutics against a much wider array of antigens that include essentially all intracellular targets. To engineer TCRs, and to analyze their binding features more rapidly, we have used a yeast display system as a platform. Expression and engineering of a single-chain form of the TCR, analogous to scFv fragments from antibodies, allow the TCR to be affinity matured with a variety of possible pepMHC ligands. In addition, the yeast display platform allows one to rapidly generate TCR variants with diverse binding affinities and to analyze specificity and affinity without the need for purification of soluble forms of the TCRs. The present chapter describes the methods for engineering and analyzing single-chain TCRs using yeast display.

A novel T cell receptor single-chain signaling complex mediates antigen-specific T cell activity and tumor control

Adoptive transfer of genetically modified T cells to treat cancer has shown promise in several clinical trials. Two main strategies have been applied to redirect T cells against cancer: (1) introduction of a full-length T cell receptor (TCR) specific for a tumor-associated peptide-MHC, or (2) introduction of a chimeric antigen receptor, including an antibody fragment specific for a tumor cell surface antigen, linked intracellularly to T cell signaling domains. Each strategy has advantages and disadvantages for clinical applications. Here, we present data on the in vitro and in vivo effectiveness of a single-chain signaling receptor incorporating a TCR variable fragment as the targeting element (referred to as TCR-SCS). This receptor contained a single-chain TCR (Vα-linker-Vβ) from a high-affinity TCR called m33, linked to the intracellular signaling domains of CD28 and CD3ζ. This format avoided mispairing with endogenous TCR chains and mediated specific T cell activity when expressed in either CD4 or CD8 T cells. TCR-SCS-transduced CD8-negative cells showed an intriguing sensitivity, compared to full-length TCRs, to higher densities of less stable pepMHC targets. T cells that expressed this peptide-specific receptor persisted in vivo, and exhibited polyfunctional responses. Growth of metastatic antigen-positive tumors was significantly inhibited by T cells that expressed this receptor, and tumor cells that escaped were antigen-loss variants. TCR-SCS receptors represent an alternative targeting receptor strategy that combines the advantages of single-chain expression, avoidance of TCR chain mispairing, and targeting of intracellular antigens presented in complex with MHC proteins.

Role of T cell receptor affinity in the efficacy and specificity of adoptive T cell therapies

Over the last several years, there has been considerable progress in the treatment of cancer using gene modified adoptive T cell therapies. Two approaches have been used, one involving the introduction of a conventional αβ T cell receptor (TCR) against a pepMHC cancer antigen, and the second involving introduction of a chimeric antigen receptor (CAR) consisting of a single-chain antibody as an Fv fragment linked to transmembrane and signaling domains. In this review, we focus on one aspect of TCR-mediated adoptive T cell therapies, the impact of the affinity of the αβ TCR for the pepMHC cancer antigen on both efficacy and specificity. We discuss the advantages of higher-affinity TCRs in mediating potent activity of CD4 T cells. This is balanced with the potential disadvantage of higher-affinity TCRs in mediating greater self-reactivity against a wider range of structurally similar antigenic peptides, especially in synergy with the CD8 co-receptor. Both TCR affinity and target selection will influence potential safety issues. We suggest pre-clinical strategies that might be used to examine each TCR for possible on-target and off-target side effects due to self-reactivities, and to adjust TCR affinities accordingly.

Enhanced-affinity murine T-cell receptors for tumor/self-antigens can be safe in gene therapy despite surpassing the threshold for thymic selection

Many of the most promising tumor antigens for T-cell-based cancer immunotherapies are unmodified self-antigens. Unfortunately, the avidity of T cells specific for these antigens is limited by central tolerance during T-cell development in the thymus, resulting in decreased anti-tumor efficacy of these T cells. One approach to overcoming this obstacle is to mutate T-cell receptor (TCR) genes from naturally occurring T cells to enhance the affinity for the target antigen. These enhanced-affinity TCRs can then be developed for use in TCR gene therapy. Although TCRs with significantly enhanced affinity have been generated using this approach, it is not clear whether these TCRs, which bypass the affinity limits imposed by negative selection, remain unresponsive to the low levels of self-antigen generally expressed by some normal tissues. Here we show that 2 variants of a high-affinity WT1-specific TCR with enhanced affinity for WT1 are safe and do not mediate autoimmune tissue infiltration or damage when transduced into peripheral CD8 T cells and transferred in vivo. However, if expressed in developing T cells and subjected to thymic selection, the same enhanced-affinity TCRs signal tolerance mechanisms in the thymus, resulting in T cells with attenuated antigen sensitivity in the periphery.