Folding, heterodimeric association and specific peptide recognition of a murine alphabeta T-cell receptor expressed in Escherichia coli

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.

Tebentafusp: T Cell Redirection for the Treatment of Metastatic Uveal Melanoma

Metastatic disease from uveal melanoma occurs in almost 50% of patients suffering from this ocular tumour, with median survival from development of symptoms being around 1 year. In contrast to cutaneous melanoma, kinase inhibitors and immune checkpoint inhibitors are usually ineffective in patients with metastatic uveal melanoma. Tebentafusp is a novel form of immunotherapy based on the immune-mobilising monoclonal T cell receptor against cancer (ImmTAC) platform, which comprises a soluble T cell receptor that is fused to an anti-CD3 single-chain variable fragment. The T cell receptor domain of tebentafusp targets cells present a human leukocyte antigen-A*02:01 complexed with a peptide derived from the melanoma-associated antigen gp100, which is expressed strongly by melanoma cells, weakly by normal melanocytes and minimally by other tissues. The anti-CD3 domain recruits CD3+ T cells (and, indirectly, other immune cells), redirecting these to the melanoma cells. The most common adverse events with tebentafusp are manageable and usually transient. Early survival data in patients with metastatic uveal melanoma are promising when considered alongside historical data. Based on these encouraging results, a randomised study comparing tebentafusp to investigator’s choice of therapy in metastatic uveal melanoma is ongoing.

Soluble T-cell receptor design influences functional yield in an E. coli chaperone-assisted expression system

There is a quest for production of soluble protein of high quality for the study of T-cell receptors (TCRs), but expression often results in low yields of functional molecules. In this study, we used an E. coli chaperone-assisted periplasmic production system and compared expression of 4 different soluble TCR formats: single-chain TCR (scTCR), two different disulfide-linked TCR (dsTCR) formats, and chimeric Fab (cFab). A stabilized version of scTCR was also included. Additionally, we evaluated the influence of host (XL1-Blue or RosettaBlue^TM) and the effect of IPTG induction on expression profiles. A celiac disease patient-derived TCR with specificity for gluten was used, and we achieved detectable expression for all formats and variants. We found that expression in RosettaBlue^TM without IPTG induction resulted in the highest periplasmic yields. Moreover, after large-scale expression and protein purification, only the scTCR format was obtained in high yields. Importantly, stability engineering of the scTCR was a prerequisite for obtaining reliable biophysical characterization of the TCR-pMHC interaction. The scTCR format is readily compatible with high-throughput screening approaches that may enable both development of reagents allowing for defined peptide MHC (pMHC) characterization and discovery of potential novel therapeutic leads.

Tumor-targeting domains for chimeric antigen receptor T cells

Immunotherapy with chimeric antigen receptor (CAR) T cells has been advancing steadily in clinical trials. Since the ability of engineered T cells to recognize intended tumor-associated targets is crucial for the therapeutic success, antigen-binding domains play an important role in shaping T-cell responses. Single-chain antibody and T-cell receptor fragments, natural ligands, repeat proteins, combinations of the above and universal tag-specific domains have all been used in the antigen-binding moiety of chimeric receptors. Here we outline the advantages and disadvantages of different domains, discuss the concepts of affinity and specificity, and highlight the recent progress of each targeting strategy.

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.

Chaperone-assisted thermostability engineering of a soluble T cell receptor using phage display

We here report a novel phage display selection strategy enabling fast and easy selection of thermostabilized proteins. The approach is illustrated with stabilization of an aggregation-prone soluble single chain T cell receptor (scTCR) characteristic of the murine MOPC315 myeloma model. Random mutation scTCR phage libraries were prepared in E. coli over-expressing the periplasmic chaperone FkpA, and such over-expression during library preparation proved crucial for successful downstream selection. The thermostabilized scTCR^mut variants selected were produced in high yields and isolated as monomers. Thus, the purified scTCRs could be studied with regard to specificity and equilibrium binding kinetics to pMHC using surface plasmon resonance (SPR). The results demonstrate a difference in affinity for pMHCs that display germ line or tumor-specific peptides which explains the tumor-specific reactivity of the TCR. This FkpA-assisted thermostabilization strategy extends the utility of recombinant TCRs and furthermore, may be of general use for efficient evolution of proteins.

Re-targeting T-cells against cancer by gene-transfer of tumor-reactive receptors

BACKGROUND

Adoptive transfer of T-lymphocytes is a promising treatment for a variety of malignancies, but is often not feasible due to difficulties in generating T-cells reactive with the targeted antigen from patients. To facilitate rapid generation of cells for therapy, T-cells can be programmed with genes encoding for an antigen-specific T-cell receptor (TCR) or chimeric receptors.

OBJECTIVE

To discuss the molecular design and selected pitfalls of TCR gene modified T-cells and T-cells expressing chimeric receptors, so called T-bodies.

METHODS

A selected review of the recent literature.

CONCLUSION

Clinical trials report so far only limited efficacy of adoptively transferred genetically modified T-cells. However, the recent progress in engineering tumor-reactive T cells is providing a promising basis to further explore this treatment modality.

Structural features of T cell receptor variable regions that enhance domain stability and enable expression as single-chain ValphaVbeta fragments

The variable (V) domains of antibodies and T cell receptors (TCRs) share sequence homology and striking structural similarity. Single-chain antibody V domain constructs (scFv) are routinely expressed in a variety of heterologous systems, both for production of soluble protein as well as for in vitro engineering. In contrast, single-chain T cell receptor V domain constructs (scTCR) are prone to aggregation and misfolding and are refractory to display on phage or yeast in their wild-type form. However, through random mutagenesis and yeast display engineering, it has been possible to isolate scTCR mutants that are properly folded and displayed on the yeast surface. These displayed mutants can serve not only as a scaffold for further engineering but also as scTCR variants that exhibit favorable biophysical properties in Escherichia coli expression. Thus, a more comprehensive understanding of the V domain mutations that allowed display would be beneficial. Our goal here was to identify generalizable patterns of important mutations that can be applied to different TCRs. We compared five different scTCRs, four from mice and one from a human, for yeast surface display. Analysis of a collection of mutants revealed four distinct regions of TCR V domains that were most important for enabling surface expression: the Valpha-Vbeta interface, the HV4 of Vbeta, and the region of the Valpha and Vbeta domains normally apposed against the constant (C) domains. Consistent with the role of the V-C interface in surface display, reconstitution of this interface, by including the constant domains of each chain, allowed V domain display and alphabeta chain association on the yeast surface, thus providing an alternative TCR scaffold. However, the surface levels of TCR achieved with engineered scTCR mutants were superior to that of the ValphaCalpha/VbetaCbeta constructs. Therefore, we describe further optimization of the current strategy for surface display of the single-chain format in order to facilitate yeast display engineering of a broader range of scTCRs.

An oncolytic adenovirus redirected with a tumor-specific T-cell receptor

To improve safety and specificity of oncolytic adenoviruses, we introduced T-cell receptors (TCR) specific for a unique class of truly tumor-specific antigens into the adenoviral fiber protein. The adenoviral fiber knob responsible for attachment to the coxsackie-adenoviral receptor (CAR) on target cells was replaced by a single-chain TCR (scTCR) molecule with specificity for the melanoma-associated cancer-testis antigen MAGE-A1, presented by HLA-A1, and an extrinsic trimerization motif in a replicating Ad5 vector (Ad5.R1-scTCR). The production of the recombinant virus was initiated in a novel producer cell line that expressed an antibody-based hexon-specific receptor (293T-AdR) in the cell membrane. This new production system allowed CAR-independent and target antigen-independent propagation of Ad5.R1-scTCR. Infection with adenovirus bearing the scTCR-based fiber resulted in an efficient killing of target tumor cells. The infection was cell type specific because only HLA-A1(+)/MAGE-A1(+) melanoma cells were killed, and thus, this retargeting strategy provides a versatile tool for future clinical application.

Facilitating matched pairing and expression of TCR chains introduced into human T cells

Adoptive transfer of T lymphocytes is a promising treatment for a variety of malignancies but often not feasible due to difficulties generating T cells that are reactive with the targeted antigen from patients. To facilitate rapid generation of cells for therapy, T cells can be programmed with genes encoding the alpha and beta chains of an antigen-specific T-cell receptor (TCR). However, such exogenous alpha and beta chains can potentially assemble as pairs not only with each other but also with endogenous TCR alpha and beta chains, thereby generating alphabetaTCR pairs of unknown specificity as well as reducing the number of exogenous matched alphabetaTCR pairs at the cell surface. We demonstrate that introducing cysteines into the constant region of the alpha and beta chains can promote preferential pairing with each other, increase total surface expression of the introduced TCR chains, and reduce mismatching with endogenous TCR chains. This approach should improve both the efficacy and safety of ongoing efforts to use TCR transfer as a strategy to generate tumor-reactive T cells.

Expression and purification of human placenta lactogen in Escherichia coli

There are many growth factors secreted by placenta including growth hormone, placenta lactogen (PL), prolactin, follicle stimulating hormone, luteinizing hormone, thyroid stimulating hormone, and chorionic gonadotropin. For a systematic study of how these growth factors work together to result in the various biological functions and future clinical applications, it is needed to produce enough quantities of each protein. In this paper, we report the cloning of human PL (hPL) and expression by Escherichia coli (E. coli). Four kinds of expression vectors containing the hPL gene were transformed into several kinds of suitable host strains and grown at 37 and/or 30 degrees C. Determination of the yield of recombinant hPL by SDS-PAGE reveals that among the various conditions, pQE30-PL in E. coli strain M15[pREP4] expressed the largest amount of recombinant hPL at 37 degrees C. However, the expressed recombinant hPL was accumulated in inclusion body forms. The inclusion bodies were solubilized in 8M urea and purified by a His6 tagged affinity column under denaturing condition and the final yield of hPL was determined to be 48 mg/L. Intra-chain disulfide bonds could be formed either by oxidation in the refolding buffer or by air oxidation in the presence of urea. The biological activity was examined by the fact that hPL could stimulate erythroid maturation by the formation of hemoglobin in K-562 cells in the presence of erythropoietin. Initial optimization studies resulted in the production of 282.4 mg/L of hPL.

High-level bacterial secretion of single-chain alphabeta T-cell receptors

While numerous antibody-antigen systems have been structurally characterized, studies of structurally analogous T-cell receptor MHC systems have lagged behind largely due to the lack of a general TCR expression system. Efforts to develop bacterial systems have resulted in low yields (< 0.5 mg/l) of active material which is prone to proteolysis and aggregation. Here we report a strategy to secrete folded, soluble single chain T-cell receptors (scTCR) in the Escherichia coli periplasm using three representative alphabeta TCRs (172.10, 1934.4/c19 and 2B4). Shake flask yields between 0.5 and 30 mg/l active, purified material were attained for all TCRs studied and found to depend on the introduction of solubility-increasing amino acid substitutions, skp chaperone co-expression and C-terminal fusion to a human kappa constant domain in the context of a tightly regulated expression vector. This system will greatly enable crystallographic, thermodynamic and other biophysical analyses of TCRs which require large quantities of homogeneous material.

Two different T cell receptors use different thermodynamic strategies to recognize the same peptide/MHC ligand

A6 and B7 are two alphabeta T cell receptors (TCRs) that recognize the Tax peptide presented by the class I major histocompatibility molecule HLA-A2 (Tax/HLA-A2). Despite the fact that the two TCRs have different CDR loops and use different amino acid residues to contact their ligand, both receptors bind ligand with similar diagonal orientations. Here we show that they also bind with very similar binding affinities and kinetics (the DeltaDeltaG degrees for binding is approximately 0.3kcal/mol at 25 degrees C). The two receptors respond similarly to alterations in the MHC molecule, yet differ dramatically in their responses to ionic strength and temperature. The different responses to temperature indicate markedly different binding thermodynamics, which are not predictable from the surface area buried in the interfaces. A6 and B7 thus represent two TCRs that are both compatible with Tax/HLA-A2, although compatibility has been achieved through the use of different thermodynamic strategies. Finally, neither A6 nor B7 are predicted to undergo large conformational adaptations upon binding, distinguishing them from a number of other TCRs whose structure, thermodynamics, and kinetics have been characterized.

Production, characterization, and immunogenicity of a soluble rat single chain T cell receptor specific for an encephalitogenic peptide

The encephalitogenic rat T cell clone C14 recognizes the myelin basic protein 69-89 peptide in the context of the RT1B major histocompatibility complex (MHC) class II molecule. Modeling of the C14 TCR molecule indicated that previously identified CDR3 motifs are likely to be central to interaction with MHC class II-presented peptide. Here we report the cloning and expression of C14-derived single chain TCR (scTCR) molecules in an Escherichia coli expression system. The recombinant molecule consists of the Valpha2 domain connected to the Vbeta8.2 domain via a 15-residue linker. Soluble C14 scTCR was purified using conventional chromatography techniques and refolded by a rapid dilution procedure. C14 scTCR was able to bind soluble rat MHC class II molecules bearing covalently coupled Gp-BP-(69-89) peptide, as analyzed using surface plasmon resonance. Immune recognition of the C14 scTCR protein as an antigen revealed that limited regions of the TCR may be more likely to induce responsiveness.

Structural analysis of mycobacterial and murine hsp60 epitopes in complex with the class I MHC molecule H-2Db

The decameric peptide SALQNAASIA from the Mycobacterium bovis heat shock protein (hsp) 60 is recognized by the murine T-cell receptor UZ-3-4 in complex with the murine class I major histocompatibility complex molecule H-2D(b). This T-cell receptor cross-reacts with the H-2D(b)-bound non-homologous decameric peptide KDIGNIISDA from the murine hsp60, but does not recognize the nonameric mycobacterial peptide SALQNAASI. Cross-recognition of the KDIGNIISDA/H-2D(b) complex induces autoimmune pathology in immunodeficient mice. We solved the X-ray crystal structure of the SALQNAASIA/H-2D(b) complex at 3.0 A resolution, and we modelled the KDIGNIISDA and SALQNAASI peptides in the H-2D(b) binding site. The structural analysis of the H-2D(b)-bound hsp60 epitopes offers insight into T-cell receptor cross-reactivity.

Production of a soluble gammadelta T-cell receptor to identify ligands for the murine intestinal intraepithelial gammadelta T cell population

Although the functions and antigen recognition requirements of alphabeta T cells are well characterised, the antigens recognised by gammadelta T cells and the consequences of this recognition are unclear. gammadelta T cells are enriched within epithelia, where they eradicate transformed epithelial cells and regulate inflammation. To understand how this occurs, we need to understand the cellular ligands recognised by the gammadelta cell through the gammadelta T-cell receptor (TCR). We have therefore generated a soluble TCR (sTCR) to identify ligands for the murine gammadelta intestinal intraepithelial lymphocyte (IEL) population. sTCR was produced in the baculovirus expression system and purified by affinity chromatography on an anti-TCRdelta affinity column. sTCR was recognised by a panel of conformation-specific anti-TCRgammadelta antibodies. We will now use our sTCR to directly test the binding of putative ligands to the TCR using surface plasmon resonance, and to isolate the ligand biochemically.

Genetic retargeting of adenovirus vectors: functionality of targeting ligands and their influence on virus viability

BACKGROUND

We studied the ability of adenovirus type 5 (Ad5) to encapsidate new cellular ligands carried by their fibers to yield functional retargeted vectors for gene therapy. Recombinant Ad5 fibers containing shaft repeats 1 to 7 and an extrinsic trimerization motif, and terminated by its native knob or amino acid motifs containing RGD, have been rescued into infectious virions.

METHODS

Polypeptide ligands of cell surface molecules, including single-chain antibodies or epidermal growth factor, were cloned into recombinant fibers. Phenotypic analysis of fiber constructs and rescuing into the Ad5 genome were performed. Recombinant viruses were characterized with reference to fiber content, growth rate and infectivity.

RESULTS

A major limiting factor for recovering viable recombinant Ad5 carrying fiber-fused polypeptide ligands was apparently the ability of the ligand to fold correctly within the cellular cytoplasm. This constraint has previously not been systematically evaluated in the literature. Phenotypic analysis of the fiber-ligand fusions showed that their degree of cytoplasmic solubility correlated with their ability to yield viable Ad5 vectors. Our results suggested that the fiber manipulations diminish virus growth rate, probably through different, opposing effects: (i) the reduced shaft length increases fiber solubility in the absence of the knob but (ii) diminishes virus entry, and (iii) the absence of the knob alters the overall protein composition of the virion and decreases its fiber copy number.

CONCLUSIONS

Based on our findings, cytoplasmic solubility and cytoplasmic ligand reactivity of fiber-ligand fusion proteins are the best prediction criterion for viability and recovery of genetically retargeted Ad vectors.

Promiscuous peptide recognition of an autoreactive CD8(+) T-cell clone is responsible for autoimmune intestinal pathology

We have recently described a CD8(+) T-cell clone recognizing defined epitopes of both mycobacterial and murine hsp60 that are not sequence homologues. Adoptive transfer of this T-cell clone into T-cell deficient mice induced an autoimmune intestinal pathology. TCR analysis revealed the productive in frame rearrangement of two TCRa genes in this clone. Expression of two different TCR alpha chains by one T cell (dual TCR) is discussed as a potential mechanism underlying T-cell mediated autoimmunity. Here we addressed the question of whether hsp60 crossrecognition of self and non-self origin is directly linked to the surface expression of two TCRs by the same cell. Consequently, the potentially dual TCR of the hsp60 reactive T-cell clone was dissected into two single TCRs by double retroviral transduction of TCR deficient cell lines. Our data show that only one of the two TCR alpha/beta combinations formed a functional cell surface TCR and that post-translational allelic exclusion of the second alpha chain was achieved by the inability to pair with the TCR beta chain. Thus a single TCR is not only sufficient for crossrecognition with peptides that share minimal sequence homology, moreover this promiscuous TCR reactivity accounts also for immunopathology as recently shown.

A response calculus for immobilized T cell receptor ligands

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

Polyionic fusion peptides function as specific dimerization motifs

The de novo design of a molecular adapter for directed association and covalent linkage of two polypeptides is presented. Using peptides containing charged amino acid residues and an additional cysteine residue (AlaCysLys(8) and AlaCysGlu(8)) we demonstrate that the electrostatic interaction promotes the association of two synthetic peptides and, subsequently, disulfide bond formation. The reaction depends on both the redox potential and on the ionic strength of the buffer. Varying the redox potential, the interaction of the peptides was quantified by a Delta G(0') of 6.6 +/- 0.2 kcal/mol. Heterodimerization of the peptides is highly specific, a competition of association by other cysteine containing compounds could not be observed. Two proteins comprising cysteine-containing polyionic fusion peptides, a modified Fab fragment and an alpha-glucosidase fusion, could be specifically conjugated by directed association and subsequent disulfide bond formation. Both proteins retain their functional characteristics within the bifunctional conjugate: enzymatic activity of the alpha-glucosidase and antigen-binding capacity of the Fab fragment are equivalent to the non-conjugated components.

Survey of the 1999 surface plasmon resonance biosensor literature

The application of surface plasmon resonance biosensors in life sciences and pharmaceutical research continues to increase. This review provides a comprehensive list of the commercial 1999 SPR biosensor literature and highlights emerging applications that are of general interest to users of the technology. Given the variability in the quality of published biosensor data, we present some general guidelines to help increase confidence in the results reported from biosensor analyses.

Structure of a covalently stabilized complex of a human alphabeta T-cell receptor, influenza HA peptide and MHC class II molecule, HLA-DR1

An alphabeta T-cell receptor (alphabetaTCR)/hemagglutinin (HA) peptide/human leukocyte antigen (HLA)-DR1 complex was stabilized by flexibly linking the HA peptide with the human HA1.7 alphabetaTCR, to increase the local concentration of the interacting proteins once the peptide has been loaded onto the major histocompatibility complex (MHC) molecule. The structure of the complex, determined by X-ray crystallography, has a binding mode similar to that of the human B7 alphabetaTCR on a pMHCI molecule. Twelve of the 15 MHC residues contacted are at the same positions observed earlier in class I MHC/peptide/TCR complexes. One contact, to an MHC loop outside the peptide-binding site, is conserved and specific to pMHCII complexes. TCR gene usage in the response to HA/HLA-DR appears to conserve charged interactions between three lysines of the peptide and acidic residues on the TCR.

Characterizing the functionality of recombinant T-cell receptors in vitro: a pMHC tetramer based approach

The very low affinity of the T-cell receptor (TCR) for the peptide-major histocompatibility complex (pMHC) has made it very challenging to design assays for testing the functionality of these molecules on small scales, which in turn has severely hampered the progress in developing expression and refolding methodologies for the TCR. We have now developed an ELISA assay for detecting pMHC binding to functional recombinant TCRs. It uses tetramers of biotinylated pMHCs bound to a neutravidin-horseradish peroxidase conjugate and detects the presence of functional TCR, bound in a productive orientation to an immobilized anti-Cbeta antibody. Specificity can be stringently demonstrated by inhibition with monomeric pMHCs. The assay is very sensitive and specific, and requires only very small amounts of protein. It has allowed us to study the unstable recombinant TCR P14, which we expressed and refolded from Escherichia coli. The TCR P14 is directed against the most abundant epitope of LCMV. We have confirmed the specificity of the interaction by BIAcore, and were able to determine the dissociation constant of the interaction of the P14 TCR and of the gp33-pMHC as 6 microM. This affinity ranks it among the tighter ones of TCR-pMHC interactions, and unusually low affinity thus does not seem to be the cause of the modest protective power of these T-cells, compared to others elicited in the anti-LCMV response. This strategy of multimerizing one partner and immobilizing the other in both a native form and productive orientation should be generally useful for characterizing the weak interactions of cell-surface molecules.