Display, engineering, and applications of antigen-specific T cell receptors

Rapid screening of TCR-pMHC interactions by the YAMTAD system

Personalized immunotherapy, such as cancer vaccine and TCR-T methods, demands rapid screening of TCR-pMHC interactions. While several screening approaches have been developed, their throughput is limited. Here, the Yeast Agglutination Mediated TCR antigen Discovery system (YAMTAD) was designed and demonstrated to allow fast and unbiased library-on-library screening of TCR-pMHC interactions. Our proof-of-principle study achieved high sensitivity and specificity in identifying antigens for a given TCR and identifying TCRs recognizing a given pMHC for modest library sizes. Finally, the enrichment of high-affinity TCR-pMHC interactions by YAMTAD in library-on-library screening was demonstrated. Given the high throughput (10⁶–10⁸ × 10⁶–10⁸ in theory) and simplicity (identifying TCR-pMHC interactions without purification of TCR and pMHC) of YAMTAD, this study provides a rapid but effective platform for TCR-pMHC interaction screening, with valuable applications in future personalized immunotherapy.

Application of phage display for T-cell receptor discovery

The immune system is tasked to keep our body unharmed and healthy. In the immune system, B- and T-lymphocytes are the two main components working together to stop and eliminate invading threats like virus particles, bacteria, fungi and parasite from attacking our healthy cells. The function of antibodies is relatively more direct in target recognition as compared to T-cell receptors (TCR) which recognizes antigenic peptides being presented on the major histocompatibility complex (MHC). Although phage display has been widely applied for antibody presentation, this is the opposite in the case of TCR. The cell surface TCR is a relatively large and complex molecule, making presentation on phage surfaces challenging. Even so, recombinant versions and modifications have been introduced to allow the growing development of TCR in phage display. In addition, the increasing application of TCR for immunotherapy has made it an important binding motif to be developed by phage display. This review will emphasize on the application of phage display for TCR discovery as well as the engineering aspect of TCR for improved characteristics.

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.

Human T-cell receptor V gene segment of alpha and beta families: A revised primer design strategy

The application of human TCR in cancer immunotherapy has gained momentum with developments in tumor killing strategies using endogenous adaptive immune responses. The successful coverage of a diverse TCR repertoire is mainly attributed to the primer design of the human TCR V genes. Here, we present a refined primer design strategy of the human TCR V gene by clustering V gene sequence homolog for degenerate primer design based on the data from IMGT. The primers designed were analyzed and the PCR efficiency of each primer set was optimized. A total of 112 alpha and 160 beta sequences were aligned and clustered using a phylogram yielding 32 and 27 V gene primers for the alpha and beta family. The new primer set was able to provide 93.75% and 95.63% coverage for the alpha and beta family, respectively. A semi-qualitative approach using the designed primer set was able to provide a relative view of the TCR V gene diversity in different populations. Taken together, the new primers provide a more comprehensive coverage of the TCR gene diversity for improved TCR library generation and TCR V gene analysis studies.

Improving TCR affinity on 293T cells

This study presents an efficient method to improve TCR affinity, comprising 1) CDR-directed saturation mutation of TCR cDNA, 2) transient TCR display on CD3-expressing HEK293T (CD3-293T) cells by simple plasmid transfection, 3) staining with HLA-tetramers, and 4) multi-round sorting of cells with CD8-independent tetramer binding on a flow cytometer. Using these procedures, we successfully identified mutant TCRs with enhanced binding from an HLA-A*24:02-restricted, human telomerase reverse transcriptase (hTERT)-specific TCR. Two such clones, 2A7A and 2D162, harboring mutations in CDR1 and CDR2 of TCRβ, respectively, were isolated with both showing sequential four amino acid substitutions. When expressed on CD3-293T cells along with wild-type TCRα, the TCR molecules of these mutants as well as their combinatory mutation, bound to HLA-A24/hTERT-tetramers more strongly than the wild-type TCRs, without binding to control tetramers. Besides, in order to facilitate a functional study of TCR, we established an artificial T cell line, designated as CD8I-J2, which expresses a human CD8 and IFN-γ producing cassette by modifying Jurkat-derived J.RT3-T3.5 cells. CD8I-J2 cells expressing wild-type or affinity-enhanced hTERT-specific TCRs were analyzed for their recognition of serially diluted cognate peptide on HLA-A*24:02-transduced T2 cells. CD8I-J2 cells expressing each mutant TCR recognized the hTERT peptide at lower concentrations than wild-type TCR. The hierarchy of peptide recognition is concordant with tetramer binding on CD3-293T cells and none of these mutant TCRs were cross-reactive with irrelevant peptides reported to be present on HLA-A*24:02 molecules as far as tested. These methods might thus be useful for obtaining high affinity mutants from other TCRs of interest.

Generation of higher affinity T cell receptors by antigen-driven differentiation of progenitor T cells in vitro

Many promising targets for T-cell-based cancer immunotherapies are self-antigens. During thymic selection, T cells bearing T cell receptors (TCRs) with high affinity for self-antigen are eliminated. The affinity of the remaining low-avidity TCRs can be improved to increase their antitumor efficacy, but conventional saturation mutagenesis approaches are labor intensive, and the resulting TCRs may be cross-reactive. Here we describe the in vitro maturation and selection of mouse and human T cells on antigen-expressing feeder cells to develop higher-affinity TCRs. The approach takes advantage of natural Tcrb gene rearrangement to generate diversity in the length and composition of CDR3β. In vitro differentiation of progenitors transduced with a known Tcra gene in the presence of antigen drives differentiation of cells with a distinct agonist-selected phenotype. We purified these cells to generate TCRβ chain libraries pre-enriched for target antigen specificity. Several TCRβ chains paired with a transgenic TCRα chain to produce a TCR with higher affinity than the parental TCR for target antigen, without evidence of cross-reactivity.

In-vitro blockade of the CD4 receptor co-signal in antigen-specific T-cell stimulation cultures induces the outgrowth of potent CD4 independent T-cell effectors

T-cell receptor (TCR) redirected T cells are promising tools for adoptive cancer immunotherapy. Since not only CD8 but also CD4 T cells are key players for efficient antitumor responses, the targeted redirection of both subsets with the same antigen-specific TCR comes more and more into focus. Although rapidly evolving technologies enable the reliable genetic re-programming of T cells, the limited availability of TCRs that induce T-cell activation in both T-cell subsets without CD4/CD8 co-receptor contribution hampers the broad application of this approach. We developed a novel stimulation approach, which drives the activation and proliferation of CD4 T-cell populations capable of inducing effector functions in a CD4-independent manner. Naive-enriched CD4 T cells were stimulated against dendritic cells (DC) expressing allogeneic HLA-DP antigens upon RNA transfection and CD4/HLA interactions were blocked by the addition of CD4 binding antibody. Evolving CD4 T-cell populations were specifically activated independent of the CD4 co-signal and induced strong TCR-mediated IFN-γ secretion as well as cytolysis upon recognition of leukemia cells expressing HLA-DP antigen. Our novel stimulation approach may facilitate the generation of CD4 T cells as source for co-receptor independent TCRs for future immunotherapies.

Biomolecular engineering for nanobio/bionanotechnology

Biomolecular engineering can be used to purposefully manipulate biomolecules, such as peptides, proteins, nucleic acids and lipids, within the framework of the relations among their structures, functions and properties, as well as their applicability to such areas as developing novel biomaterials, biosensing, bioimaging, and clinical diagnostics and therapeutics. Nanotechnology can also be used to design and tune the sizes, shapes, properties and functionality of nanomaterials. As such, there are considerable overlaps between nanotechnology and biomolecular engineering, in that both are concerned with the structure and behavior of materials on the nanometer scale or smaller. Therefore, in combination with nanotechnology, biomolecular engineering is expected to open up new fields of nanobio/bionanotechnology and to contribute to the development of novel nanobiomaterials, nanobiodevices and nanobiosystems. This review highlights recent studies using engineered biological molecules (e.g., oligonucleotides, peptides, proteins, enzymes, polysaccharides, lipids, biological cofactors and ligands) combined with functional nanomaterials in nanobio/bionanotechnology applications, including therapeutics, diagnostics, biosensing, bioanalysis and biocatalysts. Furthermore, this review focuses on five areas of recent advances in biomolecular engineering: (a) nucleic acid engineering, (b) gene engineering, (c) protein engineering, (d) chemical and enzymatic conjugation technologies, and (e) linker engineering. Precisely engineered nanobiomaterials, nanobiodevices and nanobiosystems are anticipated to emerge as next-generation platforms for bioelectronics, biosensors, biocatalysts, molecular imaging modalities, biological actuators, and biomedical applications.

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.

Clonotypically similar hybrid αβ T cell receptors can exhibit markedly different surface expression, antigen specificity and cross-reactivity

Emerging data indicate that particular major histocompatibility complex (MHC)‐bound antigenic peptides can be recognized by identical or near‐identical αβ T cell receptors (TCRs) in different individuals. To establish the functional relevance of this phenomenon, we artificially paired α and β chains from closely related TCRs specific for the human leucocyte antigen (HLA)‐B*35:01‐restricted HIV‐1 negative regulatory factor (Nef)‐derived epitope VY8 (VPLRPMTY, residues 74–81). Several hybrid TCRs generated in this manner failed to express at the cell surface, despite near homology with naturally isolated αβ chain combinations. Moreover, a substantial proportion of those αβ TCRs that did express lost specificity for the index VY8 peptide sequence. One such hybrid αβ pair gained neo‐variant specificity in the context of the VY8 backbone. Collectively, these data show that clonotypically similar TCRs can display profound differences in surface expression, antigen specificity and cross‐reactivity with potential relevance for the control of mutable viruses.

Soluble T-cell receptors produced in human cells for targeted delivery

Recently, technology has become available to generate soluble T-cell receptors (sTCRs) that contain the antigen recognition part. In contrast to antibodies, sTCRs recognize intracellular in addition to extracellular epitopes, potentially increasing the number of applications as reagents for target detection and immunotherapy. Moreover, recent data show that they can be used for identification of their natural peptide ligands in disease. Here we describe a new and simplified expression method for sTCRs in human cells and show that these sTCRs can be used for antigen-specific labeling and elimination of human target cells. Four different TCRs were solubilized by expression of constructs encoding the TCR alpha (α) and beta (β) chains lacking the transmembrane and intracellular domains, linked by a ribosomal skipping 2A sequence that facilitates equimolar production of the chains. Cell supernatants containing sTCRs labeled target cells directly in a peptide (p)-human leukocyte antigen (HLA)-specific manner. We demonstrated that a MART-1p/HLA-A*02:01-specific sTCR fused to a fluorescent protein, or multimerized onto magnetic nanoparticles, could be internalized. Moreover, we showed that this sTCR and two sTCRs recognizing CD20p/HLA-A*02:01 could mediate selective elimination of target cells expressing the relevant pHLA complex when tetramerized to streptavidin-conjugated toxin, demonstrating the potential for specific delivery of cargo. This simple and efficient method can be utilized to generate a wide range of minimally modified sTCRs from the naturally occurring TCR repertoire for antigen-specific detection and targeting.

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.

Structure-Based, Rational Design of T Cell Receptors

Adoptive cell transfer using engineered T cells is emerging as a promising treatment for metastatic melanoma. Such an approach allows one to introduce T cell receptor (TCR) modifications that, while maintaining the specificity for the targeted antigen, can enhance the binding and kinetic parameters for the interaction with peptides (p) bound to major histocompatibility complexes (MHC). Using the well-characterized 2C TCR/SIYR/H-2K(b) structure as a model system, we demonstrated that a binding free energy decomposition based on the MM-GBSA approach provides a detailed and reliable description of the TCR/pMHC interactions at the structural and thermodynamic levels. Starting from this result, we developed a new structure-based approach, to rationally design new TCR sequences, and applied it to the BC1 TCR targeting the HLA-A2 restricted NY-ESO-1_157–165 cancer-testis epitope. Fifty-four percent of the designed sequence replacements exhibited improved pMHC binding as compared to the native TCR, with up to 150-fold increase in affinity, while preserving specificity. Genetically engineered CD8⁺ T cells expressing these modified TCRs showed an improved functional activity compared to those expressing BC1 TCR. We measured maximum levels of activities for TCRs within the upper limit of natural affinity, K_D = ∼1 − 5 μM. Beyond the affinity threshold at K_D < 1 μM we observed an attenuation in cellular function, in line with the “half-life” model of T cell activation. Our computer-aided protein-engineering approach requires the 3D-structure of the TCR-pMHC complex of interest, which can be obtained from X-ray crystallography. We have also developed a homology modeling-based approach, TCRep 3D, to obtain accurate structural models of any TCR-pMHC complexes when experimental data is not available. Since the accuracy of the models depends on the prediction of the TCR orientation over pMHC, we have complemented the approach with a simplified rigid method to predict this orientation and successfully assessed it using all non-redundant TCR-pMHC crystal structures available. These methods potentially extend the use of our TCR engineering method to entire TCR repertoires for which no X-ray structure is available. We have also performed a steered molecular dynamics study of the unbinding of the TCR-pMHC complex to get a better understanding of how TCRs interact with pMHCs. This entire rational TCR design pipeline is now being used to produce rationally optimized TCRs for adoptive cell therapies of stage IV melanoma.

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.

Engineering improved T cell receptors using an alanine-scan guided T cell display selection system

T cell receptors (TCRs) on T cells recognize peptide-major histocompatibility complex (pMHC) molecules on the surface of antigen presenting cells and this interaction determines the T cell immune response. Due to negative selection, naturally occurring TCRs bind self (tumor) peptides with low affinity and have a much higher affinity for foreign antigens. This complicates isolation of naturally occurring, high affinity TCRs that mediate more effective tumor rejection for therapeutic purposes. An attractive approach to resolve this issue is to engineer high affinity TCRs in vitro using phage, yeast or mammalian TCR display systems. A caveat of these systems is that they rely on a large library by random mutagenesis due to the lack of knowledge regarding the specific interactions between the TCR and pMHC. We have focused on the mammalian retroviral display system because it uniquely allows for direct comparison of TCR-pMHC-binding properties with T-cell activation outcomes. Through an alanine-scanning approach, we are able to quickly map the key amino acid residues directly involved in TCR-pMHC interactions thereby significantly reducing the library size. Using this method, we demonstrate that for a self-antigen-specific human TCR (R6C12) the key residues for pMHC binding are located in the CDR3β region. This information was used as a basis for designing an efficacious TCR CDR3α library that allowed for selection of TCRs with higher avidity than the wild-type as evaluated through binding and activation experiments. This is a direct approach to target specific TCR residues in TCR library design to efficiently engineer high avidity TCRs that may potentially be used to enhance adoptive immunotherapy treatments.

Phage display of functional αβ single-chain T-cell receptor molecules specific for CD1b:Ac₂SGL complexes from Mycobacterium tuberculosis-infected cells

The development of molecules specific for M. tuberculosis-infected cells has important implications, as these tools may facilitate understanding of the mechanisms regulating host pathogen interactions in vivo. In addition, development of new tools capable to targeting M. tuberculosis-infected cells may have potential applications to diagnosis, treatment, and prevention of tuberculosis (TB). Due to the lack of CD1b polymorphism, M. tuberculosis lipid-CD1b complexes could be considered as universal tuberculosis infection markers. The aim of the present study was to display on the PIII surface protein of m13 phage, a human αβ single-chain T-cell receptor molecule specific for CD1b:2-stearoyl-3-hydroxyphthioceranoyl-2´-sulfate-α-α´-D-trehalose (Ac₂SGL) which is a complex presented by human cells infected with M. tuberculosis. The results showed the pIII fusion particle was successfully displayed on the phage surface. The study of the recognition of the recombinant phage in ELISA and immunohistochemistry showed the recognition of CD1b:Ac₂SGL complexes and cells in human lung tissue from a tuberculosis patient respectively, suggesting the specific recognition of the lipid-CD1b complex.

Design and characterization of a protein superagonist of IL-15 fused with IL-15Rα and a high-affinity T cell receptor

To avoid high systemic doses, strategies involving antigen-specific delivery of cytokine via linked antibodies or antibody fragments have been used. Targeting cancer-associated peptides presented by major histocompatibility complex (MHC) molecules (pepMHC) increases the number of potential target antigens and takes advantage of cross-presentation on tumor stroma and in draining lymph nodes. Here, we use a soluble, high-affinity single-chain T cell receptor Vα-Vβ (scTv), to deliver cytokines to intracellular tumor-associated antigens presented as pepMHC. As typical wild-type T cell receptors (TCRs) exhibit low affinity (K(d) = 1-100 μM or more), we used an engineered TCR, m33, that binds its antigenic peptide SIYRYYGL (SIY) bound to the murine class I major histocompatability complex protein H2-K(b) (SIY/K(b) ) with nanomolar affinity (K(d) = 30 nM). We generated constructs consisting of m33 scTv fused to murine interleukin 2 (IL-2), interleukin 15 (IL-15), or IL-15/IL-15Rα (IL-15 linked to IL-15Rα sushi domain, called "superfusion"). The fusions were purified with good yields and bound specifically to SIY/K(b) with high affinity. Proper cytokine folding and binding were confirmed, and the fusions were capable of stimulating proliferation of cytokine-dependent cells, both when added directly and when presented in trans, bound to cells with the target pepMHC. The m33 superfusion was particularly potent and stable and represents a promising design for targeted antitumor immunomodulation.

Human antitumor CD8+ T cells producing Th1 polycytokines show superior antigen sensitivity and tumor recognition

Adoptive transfer of T cells expressing transgenic TCR with antitumor specificity provides a hopeful new therapy for patients with advanced cancer. To fulfill a large need for TCR with high affinity and specificity for various tumor entities, we sought to identify parameters for rapid selection of CTL clones with suitable characteristics. Twelve CTL clones displaying different Ag sensitivities for the same peptide-MHC epitope of the melanoma-associated Ag tyrosinase were analyzed in detail. Better MHC-multimer binding and slower multimer release are thought to reflect stronger TCR-peptide-MHC interactions; thus, these parameters would seem well suited to identify higher avidity CTL. However, large disparities were found comparing CTL multimer binding with peptide sensitivity. In contrast, CD8(+) CTL with superior Ag sensitivity mediated good tumor cytotoxicity and also secreted the triple combination of IFN-γ, IL-2, and TNF-α, representing a Th1 pattern often missing in lower avidity CTL. Furthermore, recipient lymphocytes were imbued with high Ag sensitivity, superior tumor recognition, as well as capacity for Th1 polycytokine secretion after transduction with the TCR of a high-avidity CTL. Thus, Th1 polycytokine secretion served as a suitable parameter to rapidly demark cytotoxic CD8(+) T cell clones for further TCR evaluation.

Challenges in T cell receptor gene therapy

The function of T lymphocytes as orchestrators and effectors of the adaptive immune response is directed by the specificity of their T cell receptors (TCRs). By transferring into T cells the genes encoding antigen-specific receptors, the functional activity of large populations of T cells can be redirected against defined targets including virally infected or cancer cells. The potential of therapeutic T cells to traffic to sites of disease, to expand and to persist after a single treatment remains a major advantage over the currently available immunotherapies that use monoclonal antibodies. Here we review recent progress in the field of TCR gene therapy, outlining challenges to its successful implementation and the strategies being used to overcome them. We detail strategies used in the optimization of affinity and surface expression of the introduced TCR, the choice of T cell subpopulations for gene transfer, and the promotion of persistence of gene-modified T cells in vivo. We review the safety concerns surrounding the use of gene-modified T cells in patients, discussing emerging solutions to these problems, and describe the increasingly positive results from the use of gene-modified T cells in recent clinical trials of adoptive cellular immunotherapy. The increasing sophistication of measures to ensure the safety of engineered T cells is accompanied by an increasing number of clinical trials: these will be essential to guide the effective translation of cellular immunotherapy from the laboratory to the bedside.

The binding affinity of a soluble TCR-Fc fusion protein is significantly improved by crosslinkage with an anti-Cβ antibody

The identification and cloning of tumor antigen-specific T cell receptors (TCRs) and the production of the soluble form of the TCR (sTCR) contributed to the development of diagnostic and therapeutic tools for cancer. Recently, several groups have reported the development of technologies for the production of sTCRs. The native sTCR has a very low binding affinity for the antigenic peptide/MHC (p/MHC) complex. In this study, we established a technology to produce high affinity, functional sTCRs. We generated a novel sTCR-Fc fusion protein composed of the TCR V and C regions of the TCR linked to the immunoglobulin (Ig) Fc region. A Western blot analysis revealed that the molecular weight of the fusion protein was approximately 60 kDa under reducing conditions and approximately 100-200 kDa under non-reducing conditions. ELISAs using various antibodies showed that the structure of each domain of the TCR-Fc protein was intact. The TCR-Fc protein immobilized by an anti-Cβ antibody effectively bound to a p/MHC tetramer. An SPR analysis showed that the TCR-Fc protein had a low binding affinity (KD; 1.1 × 10(-5)M) to the p/MHC monomer. Interestingly, when the TCR-Fc protein was pre-incubated with an anti-Cβ antibody, its binding affinity for p/MHC increased by 5-fold (2.2 × 10(-6)M). We demonstrated a novel method for constructing a functional soluble TCR using the Ig Fc region and showed that the binding affinity of the functional sTCR-Fc was markedly increased by an anti-Cβ antibody, which is probably due to the stabilization of the Vα/Vβ region of the TCR. These findings provide new insights into the binding of sTCRs to p/MHCs and will hopefully be instrumental in establishing functional sTCR as a diagnostic and therapeutic tool for cancer.

Cell surface display of functional human MHC class II proteins: yeast display versus insect cell display

Reliable and robust systems for engineering functional major histocompatibility complex class II (MHCII) proteins have proved elusive. Availability of such systems would enable the engineering of peptide-MHCII (pMHCII) complexes for therapeutic and diagnostic applications. In this paper, we have developed a system based on insect cell surface display that allows functional expression of heterodimeric DR2 molecules with or without a covalently bound human myelin basic protein (MBP) peptide, which is amenable to directed evolution of DR2-MBP variants with improved T cell receptor (TCR)-binding affinity. This study represents the first example of functional display of human pMHCII complexes on insect cell surface. In the process of developing this pMHCII engineering system, we have also explored the potential of using yeast surface display for the same application. Our data suggest that yeast display is a useful system for analysis and engineering of peptide binding of MHCII proteins, but not suitable for directed evolution of pMHC complexes that bind with low affinity to self-reactive TCRs.

Single-chain VαVβ T-cell receptors function without mispairing with endogenous TCR chains

Transduction of exogenous T-cell receptor (TCR) genes into patients' activated peripheral blood T cells is a potent strategy to generate large numbers of specific T cells for adoptive therapy of cancer and viral diseases. However, the remarkable clinical promise of this powerful approach is still being overshadowed by a serious potential consequence: mispairing of the exogenous TCR chains with endogenous TCR chains. These 'mixed' heterodimers can generate new specificities that result in graft-versus-host reactions. Engineering TCR constant regions of the exogenous chains with a cysteine promotes proper pairing and reduces the mispairing, but, as we show here, does not eliminate the formation of mixed heterodimers. By contrast, deletion of the constant regions, through use of a stabilized Vα/Vβ single-chain TCR (scTv), avoided mispairing completely. By linking a high-affinity scTv to intracellular signaling domains, such as Lck and CD28, the scTv was capable of activating functional T-cell responses in the absence of either the CD3 subunits or the co-receptors, and circumvented mispairing with endogenous TCRs. Such transduced T cells can respond to the targeted antigen independent of CD3 subunits via the introduced scTv, without the transduced T cells acquiring any new undefined and potentially dangerous specificities.

Identification and engineering of human variable regions that allow expression of stable single-chain T cell receptors

Single-chain antibody fragments (scFv), consisting of two linked variable regions (V(H) and V(L)), are a versatile format for engineering and as potential antigen-specific therapeutics. Although the analogous format for T cell receptors (TCRs), consisting of two linked V regions (Vα and Vβ; referred to here as scTv), could provide similar opportunities, all wild-type scTv proteins examined to date are unstable. This obstacle has prevented scTv fragments from being widely used for engineering or therapeutics. To further explore whether some stable human scTv fragments could be expressed, we used a yeast system in which display of properly folded domains correlates with ability to express the folded scTv in soluble form. We discovered that, unexpectedly, scTv fragments that contained the human Vα2 region (IMGT: TRAV12 family) were displayed and properly associated with different Vβ regions. Furthermore, a single polymorphic residue (Ser(α49)) in the framework region conferred additional thermal stability. These stabilized Vα2-containing scTv fragments could be expressed at high levels in Escherichia coli, and used to stain target cells that expressed the specific pep-HLA-A2 complexes. Thus, the scTv fragments can serve as a platform for engineering TCRs with diverse specificities, and possibly for therapeutic or diagnostic applications.

Rational design of T cell receptors with enhanced sensitivity for antigen

Enhancing the affinity of therapeutic T cell receptors (TCR) without altering their specificity is a significant challenge for adoptive immunotherapy. Current efforts have primarily relied on empirical approaches. Here, we used structural analyses to identify a glycine-serine variation in the TCR that modulates antigen sensitivity. A G at position 107 within the CDR3β stalk is encoded within a single mouse and human TCR, TRBV13-2 and TRBV12-5 respectively. Most TCR bear a S107. The S hydroxymethyl side chain intercalates into the core of the CDR3β loop, stabilizing it. G107 TRBV possess a gap in their CDR3β where this S hydroxymethyl moiety would fit. We predicted based on modeling and molecular dynamics simulations that a G107S substitution would increase CDR3β stability and thereby augment receptor sensitivity. Experimentally, a G107S replacement led to an ∼10–1000 fold enhanced antigen sensitivity in 3 of 4 TRBV13-2⁺ TCR tested. Analysis of fine specificity indicated a preserved binding orientation. These results support the feasibility of developing high affinity antigen specific TCR for therapeutic purposes through the identification and manipulation of critical framework residues. They further indicate that amino acid variations within TRBV not directly involved in ligand contact can program TCR sensitivity, and suggest a role for CDR3 stability in this programming.

T-cell engineering for cancer immunotherapy

The adoptive transfer of tumor-reactive cells is a promising approach for the treatment of melanoma and some other cancers. To remedy the difficulties associated with the isolation and expansion of tumor-reactive T cells in most cancer patients, peripheral blood T cells can be retargeted to any chosen tumor antigen by the genetic transfer of an antigen-specific receptor. The transduced receptors may be human leukocyte antigen-restricted, heterodimeric T-cell antigen receptor (TCRs), or chimeric antigen receptors (CARs), which typically recognize native cell-surface antigens. Considerable progress has been made in recent years to address the challenges posed by the transfer of either receptor type. Vector and protein modifications enable the expression of TCR chains in human T cells at functional levels and with a reduced risk of mis-pairing with endogenous TCR chains. The combinatorial inclusion of activating and costimulatory domains in CARs has dramatically enhanced the signaling properties of the chimeric receptors described over a decade ago. Based on the effective T-cell transduction and expansion procedures now available to support clinical investigation, improved designer TCRs and second generation CARs targeting an array of antigens are being evaluated in a range of hematological malignancies and solid tumors.

Rebalancing immune specificity and function in cancer by T-cell receptor gene therapy

Adoptive immunotherapy with tumor-specific T lymphocytes has demonstrated clinical benefit in some cancers, particularly melanoma. Yet isolating and expanding tumor-specific cells from patients is challenging and there is limited ability to control T-cell affinity and response characteristics. T-cell receptor (TCR) gene therapy, in which T lymphocytes for immunotherapy are redirected using an introduced rearranged TCR, has emerged as an important alternative. Successful TCR gene therapy requires consideration of a number of issues, including TCR specificity and affinity, optimal gene therapy constructs, types of T cells administered, and the survival and activity of the modified cells. In this review we highlight the rationale for and experience with TCR gene therapy as well as new approaches to enhancing it.

Combinations of affinity-enhancing mutations in a T cell receptor reveal highly nonadditive effects within and between complementarity determining regions and chains

Understanding the energetic and structural response to multiple mutations in a protein-protein interface is a key aspect of rational protein design. Here we investigate the cooperativity of combinations of point mutations of a T cell receptor (TCR) that binds in vivo to HLA-A2 MHC and a viral peptide. The mutations were obtained from two sources: a structure-based design study on the TCR alpha chain (nine mutations) and an in vitro selection study on the TCR beta chain (four mutations). In addition to combining the highest-affinity variants from each chain, we tested other combinations of mutations within and among the chains, for a total of 23 TCR mutants that we measured for binding kinetics to the peptide and major histocompatibility complex. A wide range of binding affinities was observed, from 2- to 1000-fold binding improvement versus that of the wild type, with significant nonadditive effects observed within and between TCR chains. This included an amino acid-dependent cooperative interaction between CDR1 and CDR3 residues that are separated by more than 9 A in the wild-type complex. When analyzing the kinetics of the mutations, we found that the association rates were primarily responsible for the cooperativity, while the dissociation rates were responsible for the anticooperativity (less-than-additive energetics). On the basis of structural modeling of anticooperative mutants, we determined that side chain clash between proximal mutants likely led to nonadditive binding energies. These results highlight the complex nature of TCR association and binding and will be informative in future design efforts that combine multiple mutant residues.

Enhanced T cell receptor gene therapy for cancer

IMPORTANCE OF THE FIELD

Adoptive therapy with T cell receptor- (TCR-) redirected T cells has shown efficacy in mouse tumor models and first responses in cancer patients. One prerequisite to elicit effective anti-tumor reactivity is the transfer of high-avidity T cells. Their generation, however, faces several technical difficulties. Target antigens are often expressed at low levels and their recognition requires the use of high-affine receptors. Yet, mainly low-affinity TCRs have been isolated from tumor-infiltrating lymphocytes. Furthermore, upon transfer into a T cell the introduced receptor has to compete with the endogenous TCR.

AREAS COVERED IN THIS REVIEW

This review discusses how the functional avidity of TCR-modified T cells can be enhanced by i) increasing the amount of introduced TCR heterodimers on the cell surface; and ii) generating receptors with high affinity. Risks of TCR gene therapy and possible safety mechanisms are discussed.

WHAT THE READER WILL GAIN

The reader will gain an overview of the technical developments in TCR and T cell engineering.

TAKE HOME MESSAGE

Despite technical obstacles, many advances have been made in the generation of high-avidity T cells expressing enhanced TCRs. Mouse studies and clinical trials will evaluate the effect of these improvements.

Chimeric antigen receptor-engineered T cells for immunotherapy of cancer

CD4⁺ and CD8⁺ T lymphocytes are powerful components of adaptive immunity, which essentially contribute to the elimination of tumors. Due to their cytotoxic capacity, T cells emerged as attractive candidates for specific immunotherapy of cancer. A promising approach is the genetic modification of T cells with chimeric antigen receptors (CARs). First generation CARs consist of a binding moiety specifically recognizing a tumor cell surface antigen and a lymphocyte activating signaling chain. The CAR-mediated recognition induces cytokine production and tumor-directed cytotoxicity of T cells. Second and third generation CARs include signal sequences from various costimulatory molecules resulting in enhanced T-cell persistence and sustained antitumor reaction. Clinical trials revealed that the adoptive transfer of T cells engineered with first generation CARs represents a feasible concept for the induction of clinical responses in some tumor patients. However, further improvement is required, which may be achieved by second or third generation CAR-engrafted T cells.

High-affinity T cell receptor differentiates cognate peptide-MHC and altered peptide ligands with distinct kinetics and thermodynamics

Interactions between the T cell receptor and cognate peptide-MHC are crucial initiating events in the adaptive immune response. These binding events are highly specific yet occur with micromolar affinity. Even weaker interactions between TCR and self-pMHC complexes play critical regulatory roles in T cell development, maintenance and coagonist activity. Due to their low-affinity, the kinetics and thermodynamics of such weak interactions are difficult to study. In this work, we used M15, a high-affinity TCR engineered from the 3.L2 TCR system, to study the binding properties, thermodynamics, and specificity of two altered peptide ligands (APLs). Our affinity measurements of the high-affinity TCR support the view that the wild type TCR binds these APLs in the millimolar affinity range, and hence very low affinities can still elicit biological functions. Finally, single methylene differences among the APLs gave rise to strikingly different binding thermodynamics. These minor changes in the pMHC antigen were associated with significant and unpredictable changes in both the entropy and enthalpy of the reaction. As the identical TCR was analyzed with several structurally similar ligands, the distinct thermodynamic binding profiles provide a mechanistic perspective on how exquisite antigen specificity is achieved by the T cell receptor.

TCR transgenes and transgene cassettes for TCR gene therapy: status in 2008

The genetic introduction of T cell receptor genes into T cells has been developed over the past decade as a strategy to induce defined antigen-specific T cell immunity. With the potential value of TCR gene therapy well-established in murine models and the feasibility of infusion of TCR-modified autologous T cells shown in a first phase I trial, the next key step will be to transform TCR gene transfer from an experimental technique into a robust clinical strategy. In this review, we discuss the different properties of the TCR transgene and transgene cassette that can strongly affect both the efficacy and the safety of TCR gene transfer.

Subtle affinity-enhancing mutations in a myelin oligodendrocyte glycoprotein-specific TCR alter specificity and generate new self-reactivity

We describe a simple iterative approach to augment TCR affinity, which we studied using a myelin oligodendrocyte glycoprotein-specific TCR. We hypothesized that single amino acid modifications in TCR CDR3 could enhance TCR sensitivity through focal interactions with antigenic peptide while minimizing the risk of cross-reactivity observed previously in TCR more broadly mutagenized using in vitro evolution techniques. We show that this iterative method can indeed generate TCR with Ag sensitivity 100-fold greater than the parental receptor and can endow TCR with coreceptor independence. However, we also find that single amino acid mutations in the CDR3 can alter TCR fine specificity, affecting recognition requirements for Ag residues over most of the length of the MHC binding groove. Furthermore, minimal changes in surface-exposed CDR3 amino acids, even the addition of a single hydroxyl group or conversion of a methyl or sulfhydryl moiety to a hydroxyl, can confer modified Ag-specific TCR with new self-reactivity. In vivo modeling of modified TCR through retroviral TCR gene transfer into Rag(-/-) mice confirmed the biological significance of these altered reactivities, although it also demonstrated the feasibility of producing Ag-specific, positively selecting, coreceptor-independent receptors with markedly increased Ag sensitivity. These results affirm the possibility of readily generating affinity-enhanced TCR for therapeutic purposes but demonstrate that minimal changes in TCR CDR3 structure can promote self reactivity and thereby emphasize the importance of caution in validating receptors with even subtle alterations before clinical application.

The promise and potential pitfalls of chimeric antigen receptors

One important purpose of T cell engineering is to generate tumor-targeted T cells through the genetic transfer of antigen-specific receptors, which consist of either physiological, MHC-restricted T cell receptors (TCRs) or non MHC-restricted chimeric antigen receptors (CARs). CARs combine antigen-specificity and T cell activating properties in a single fusion molecule. First generation CARs, which included as their signaling domain the cytoplasmic region of the CD3zeta or Fc receptor gamma chain, effectively redirected T cell cytotoxicity but failed to enable T cell proliferation and survival upon repeated antigen exposure. Receptors encompassing both CD28 and CD3zeta are the prototypes for second generation CARs, which are now rapidly expanding to a diverse array of receptors with different functional properties. First generation CARs have been tested in phase I clinical studies in patients with ovarian cancer, renal cancer, lymphoma, and neuroblastoma, where they have induced modest responses. Second generation CARs, which are just now entering the clinical arena in the B cell malignancies and other cancers, will provide a more significant test for this approach. If the immunogenicity of CARs can be averted, the versatility of their design and HLA-independent antigen recognition will make CARs tools of choice for T cell engineering for the development of targeted cancer immunotherapies.

Signal integration: a framework for understanding the efficacy of therapeutics targeting the human EGFR family

The human EGFR (HER) family is essential for communication between many epithelial cancer cell types and the tumor microenvironment. Therapeutics targeting the HER family have demonstrated clinical success in the treatment of diverse epithelial cancers. Here we propose that the success of HER family-targeted monoclonal antibodies in cancer results from their ability to interfere with HER family consolidation of signals initiated by a multitude of other receptor systems. Ligand/receptor systems that initiate these signals include cytokine receptors, chemokine receptors, TLRs, GPCRs, and integrins. We further extrapolate that improvements in cancer therapeutics targeting the HER family are likely to incorporate mechanisms that block or reverse stromal support of malignant progression by isolating the HER family from autocrine and stromal influences.

Biosensor detection systems: engineering stable, high-affinity bioreceptors by yeast surface display

Over the past two decades, the field of biosensors has been developing fast, portable, and convenient detection tools for various molecules of interest, both biological and environmental. Although much attention is paid to the transduction portion of the sensor, the actual bioreceptor that binds the ligand is equally critical. Tight, specific binding by the bioreceptor is required to detect low levels of the relevant ligand, and the bioreceptor must be stable enough to survive immobilization, storage, and in ideal cases, regeneration on the biosensing device. Often, naturally-occurring bioreceptors or antibodies that are specific for a ligand either express affinities that may be too low to detect useful levels, or the proteins are too unstable to be used and reused as a biosensor. Further engineering of these receptors can improve their utility. Here, we describe in detail the use of yeast surface display techniques to carry out directed evolution of bioreceptors to increase both the stability of the molecules and their affinity for the ligands. This powerful technique has enabled the production of stabilized, single-chain antibodies, T cell receptors, and other binding molecules that exhibit affinity increases for their ligands of up to 1 million-fold and expression of stable molecules in E. coli.

Engineering higher affinity T cell receptors using a T cell display system

The T cell receptor (TCR) determines the cellular response to antigens, which are presented on the surface of target cells in the form of a peptide bound to a product of the major histocompatibility complex (pepMHC). The response of the T cell depends on the affinity of the TCR for the pepMHC, yet many TCRs have been shown to be of low affinity, and some naturally occurring T cell responses are poor due to low affinities. Accordingly, engineering the TCR for increased affinity for pepMHC, particularly tumor-associated antigens, has become an increasingly desirable goal, especially with the advent of adoptive T cell therapies. For largely technical reasons, to date there have been only a handful of TCRs engineered in vitro for higher affinity using well established methods of protein engineering. Here we report the use of a T cell display system, using a retroviral vector, for generating a high-affinity TCR from the mouse T cell clone 2C. The method relies on the display of the TCR, in its normal, signaling competent state, as a CD3 complex on the T cell surface. A library in the CDR3alpha of the 2C TCR was generated in the MSCV retroviral vector and transduced into a TCR-negative hybridoma. Selection of a high-affinity, CD8-independent TCR was accomplished after only two rounds of flow cytometric sorting using the pepMHC SIYRYYGL/Kb (SIY/Kb). The selected TCR contained a sequence motif in the CDR3alpha with characteristics of several other TCRs previously selected by yeast display. In addition, it was possible to directly use the selected T cell hybridoma in functional assays without the need for sub-cloning, revealing that the selected TCR was capable of mediating CD8-independent activity. The method may be useful in the direct isolation and characterization of TCRs that could be used in therapies with adoptive transferred T cells.

Primer sets for cloning the human repertoire of T cell Receptor Variable regions

Background

Amplification and cloning of naïve T cell Receptor (TR) repertoires or antigen-specific TR is crucial to shape immune response and to develop immuno-based therapies. TR variable (V) regions are encoded by several genes that recombine during T cell development. The cloning of expressed genes as large diverse libraries from natural sources relies upon the availability of primers able to amplify as many V genes as possible.

Results

Here, we present a list of primers computationally designed on all functional TR V and J genes listed in the IMGT^®, the ImMunoGeneTics information system^®. The list consists of unambiguous or degenerate primers suitable to theoretically amplify and clone the entire TR repertoire. We show that it is possible to selectively amplify and clone expressed TR V genes in one single RT-PCR step and from as little as 1000 cells.

Conclusion

This new primer set will facilitate the creation of more diverse TR libraries than has been possible using currently available primer sets.