Methods for quantifying T cell receptor binding affinities and thermodynamics

Structural basis of the TCR-pHLA complex provides insights into the unconventional recognition of CDR3β in TCR cross-reactivity and alloreactivity

Evidence shows that some class I human leucocyte antigen (HLA) alleles are related to durable HIV controls. The T18A TCR, which has the alloreactivity between HLA-B∗42:01 and HLA-B∗81:01 and the cross-reactivity with different antigen mutants, can sustain long-term HIV controls. Here the structural basis of the T18A TCR binding to the immunodominant HIV epitope TL9 (TPQDLNTML180-188) presented by HLA-B∗42:01 was determined and compared to T18A TCR binding to the TL9 presented by the allo-HLA-B∗81:01. For differences between HLA-B∗42:01 and HLA-B∗81:01, the CDR1α and CDR3α loops adopt a small rearrangement to accommodate them. For different conformations of the TL9 presented by different HLA alleles, not like the conventional recognition of CDR3s to interact with peptide antigens, CDR3β of the T18A TCR shifts to avoid the peptide antigen but intensively recognizes the HLA only, which is different with other conventional TCR structures. Featured sequence pairs of CDR3β and HLA might account for this and were additionally found in multiple other diseases indicating the popularity of the unconventional recognition pattern which would give insights into the control of diseases with epitope mutating such as HIV.

Best practice for improved accuracy: a critical reassessment of van't Hoff analysis of melt curves

Biomolecular thermodynamics, particularly for DNA, are frequently determined via van't Hoff analysis of optically-measured melt curves. Accurate and precise values of thermodynamic parameters are essential for the modelling of complex systems involving cooperative effects, such as RNA tertiary structure and DNA origami because the uncertainties associated with each motif in a folding energy landscape can compound, significantly reducing the power of predictive models. We follow the sources of uncertainty as they propagate through a typical van't Hoff analysis to derive best practices for melt experiments and subsequent data analysis, assuming perfect signal baseline correction. With appropriately designed experiments and analysis, a van't Hoff approach can provide surprisingly high precision, e.g., enthalpies may be determined with a precision as low as a 10^-2 kJ∙mol^-1 for an 8 base DNA oligomer.

Structural Basis for Unusual TCR CDR3β Usage Against an Immunodominant HIV-1 Gag Protein Peptide Restricted to an HLA-B*81:01 Molecule

In HIV infection, some closely associated human leukocyte antigen (HLA) alleles are correlated with distinct clinical outcomes although presenting the same HIV epitopes. The mechanism that underpins this observation is still unknown, but may be due to the essential features of HLA alleles or T cell receptors (TCR). In this study, we investigate how T18A TCR, which is beneficial for a long-term control of HIV in clinic, recognizes immunodominant Gag epitope TL9 (TPQDLTML180-188) from HIV in the context of the antigen presenting molecule HLA-B*81:01. We found that T18A TCR exhibits differential recognition for TL9 restricted by HLA-B*81:01. Furthermore, via structural and biophysical approaches, we observed that TL9 complexes with HLA-B*81:01 undergoes no conformational change after TCR engagement. Remarkably, the CDR3β in T18A complexes does not contact with TL9 at all but with intensive contacts to HLA-B*81:01. The binding kinetic data of T18A TCR revealed that this TCR can recognize TL9 epitope and several mutant versions, which might explain the correlation of T18A TCR with better clinic outcomes despite the relative high mutation rate of HIV. Collectively, we provided a portrait of how CD8⁺ T cells engage in HIV-mediated T cell response.

Recent developments in immunotherapy of cancers caused by human papillomaviruses

A subset of oncogenic human papillomaviruses (HPVs) is the main cause of genital cancers, most importantly cervical cancer and an increasing number of head and neck cancers. Despite the availability of prophylactic vaccines against the most prevalent oncogenic HPV types, HPV‐induced malignancies are still a major health and economic burden. Besides conventional treatment with surgery, chemotherapy and radiation, immunotherapy is emerging as an efficient adjuvant option. Here, we review relevant studies and ongoing clinical trials using immune checkpoint inhibitors, therapeutic vaccines, gene editing approaches and adoptive T cell therapies, with special focus on engineered TCR T cells, which are showing encouraging results and could lead to significant improvement in the treatment of HPV+‐infected cancer patients.

T-cell receptor affinity in the age of cancer immunotherapy

The strength of the interaction between T-cell receptors (TCRs) and their ligands, peptide/major histocompatibility complex complexes (pMHCs), is one of the most frequently discussed and investigated features of T cells in immuno-oncology today. Although there are many molecules on the surface of T cells that interact with ligands on other cells, the TCR/pMHC is the only receptor-ligand pair that offers antigen specificity and dictates the functional response of the T cell. The strength of the TCR/pMHC interaction, along with the environment in which this interaction takes place, is key to how the T cell will respond. The TCR repertoire of T cells that interact with tumor-associated antigens is vast, although typically of low affinity. Here, we focus on the low-affinity interactions between TCRs from CD8+ T cells and different models used in immuno-oncology.

Improving T Cell Receptor On-Target Specificity via Structure-Guided Design

T cell receptors (TCRs) have emerged as a new class of immunological therapeutics. However, though antigen specificity is a hallmark of adaptive immunity, TCRs themselves do not possess the high specificity of monoclonal antibodies. Although a necessary function of T cell biology, the resulting cross-reactivity presents a significant challenge for TCR-based therapeutic development, as it creates the potential for off-target recognition and immune toxicity. Efforts to enhance TCR specificity by mimicking the antibody maturation process and enhancing affinity can inadvertently exacerbate TCR cross-reactivity. Here we demonstrate this concern by showing that even peptide-targeted mutations in the TCR can introduce new reactivities against peptides that bear similarity to the original target. To counteract this, we explored a novel structure-guided approach for enhancing TCR specificity independent of affinity. Tested with the MART-1-specific TCR DMF5, our approach had a small but discernible impact on cross-reactivity toward MART-1 homologs yet was able to eliminate DMF5 cross-recognition of more divergent, unrelated epitopes. Our study provides a proof of principle for the use of advanced structure-guided design techniques for improving TCR specificity, and it suggests new ways forward for enhancing TCRs for therapeutic use.

T cell receptor cross-reactivity expanded by dramatic peptide-MHC adaptability

T cell receptor cross-reactivity allows a fixed T cell repertoire to respond to a much larger universe of potential antigens. Recent work has emphasized the importance of peptide structural and chemical homology, as opposed to sequence similarity, in T cell receptor cross-reactivity. Surprisingly though, T cell receptors can also cross-react between ligands with little physiochemical commonalities. Studying the clinically relevant receptor DMF5, we demonstrate that cross-recognition of such divergent antigens can occur through mechanisms that involve heretofore unanticipated rearrangements in the peptide and presenting MHC protein, including binding-induced peptide register shifts and extensions from MHC peptide binding grooves. Moreover, cross-reactivity can proceed even when such dramatic rearrangements do not translate into structural or chemical molecular mimicry. Beyond demonstrating new principles of T cell receptor cross-reactivity, our results have implications for efforts to predict and control T cell specificity and cross-reactivity, and highlight challenges associated with predicting T cell reactivities.

Critical biological parameters modulate affinity as a determinant of function in T-cell receptor gene-modified T-cells

T-cell receptor (TCR)-pMHC affinity has been generally accepted to be the most important factor dictating antigen recognition in gene-modified T-cells. As such, there is great interest in optimizing TCR-based immunotherapies by enhancing TCR affinity to augment the therapeutic benefit of TCR gene-modified T-cells in cancer patients. However, recent clinical trials using affinity-enhanced TCRs in adoptive cell transfer (ACT) have observed unintended and serious adverse events, including death, attributed to unpredicted off-tumor or off-target cross-reactivity. It is critical to re-evaluate the importance of other biophysical, structural, or cellular factors that drive the reactivity of TCR gene-modified T-cells. Using a model for altered antigen recognition, we determined how TCR-pMHC affinity influenced the reactivity of hepatitis C virus (HCV) TCR gene-modified T-cells against a panel of naturally occurring HCV peptides and HCV-expressing tumor targets. The impact of other factors, such as TCR-pMHC stabilization and signaling contributions by the CD8 co-receptor, as well as antigen and TCR density were also evaluated. We found that changes in TCR-pMHC affinity did not always predict or dictate IFNγ release or degranulation by TCR gene-modified T-cells, suggesting that less emphasis might need to be placed on TCR-pMHC affinity as a means of predicting or augmenting the therapeutic potential of TCR gene-modified T-cells used in ACT. A more complete understanding of antigen recognition by gene-modified T-cells and a more rational approach to improve the design and implementation of novel TCR-based immunotherapies is necessary to enhance efficacy and maximize safety in patients.

How an alloreactive T-cell receptor achieves peptide and MHC specificity

T-cell receptor (TCR) allorecognition is often presumed to be relatively nonspecific, attributable to either a TCR focus on exposed major histocompatibility complex (MHC) polymorphisms or the degenerate recognition of allopeptides. However, paradoxically, alloreactivity can proceed with high peptide and MHC specificity. Although the underlying mechanisms remain unclear, the existence of highly specific alloreactive TCRs has led to their use as immunotherapeutics that can circumvent central tolerance and limit graft-versus-host disease. Here, we show how an alloreactive TCR achieves peptide and MHC specificity. The HCV1406 TCR was cloned from T cells that expanded when a hepatitis C virus (HCV)-infected HLA-A2^- individual received an HLA-A2⁺ liver allograft. HCV1406 was subsequently shown to recognize the HCV nonstructural protein 3 (NS3):1406-1415 epitope with high specificity when presented by HLA-A2. We show that NS3/HLA-A2 recognition by the HCV1406 TCR is critically dependent on features unique to both the allo-MHC and the NS3 epitope. We also find cooperativity between structural mimicry and a crucial peptide "hot spot" and demonstrate its role, along with the MHC, in directing the specificity of allorecognition. Our results help explain the paradox of specificity in alloreactive TCRs and have implications for their use in immunotherapy and related efforts to manipulate TCR recognition, as well as alloreactivity in general.

Using Global Analysis to Extend the Accuracy and Precision of Binding Measurements with T cell Receptors and Their Peptide/MHC Ligands

In cellular immunity, clonally distributed T cell receptors (TCRs) engage complexes of peptides bound to major histocompatibility complex proteins (pMHCs). In the interactions of TCRs with pMHCs, regions of restricted and variable diversity align in a structurally complex fashion. Many studies have used mutagenesis to attempt to understand the "roles" played by various interface components in determining TCR recognition properties such as specificity and cross-reactivity. However, these measurements are often complicated or even compromised by the weak affinities TCRs maintain toward pMHC. Here, we demonstrate how global analysis of multiple datasets can be used to significantly extend the accuracy and precision of such TCR binding experiments. Application of this approach should positively impact efforts to understand TCR recognition and facilitate the creation of mutational databases to help engineer TCRs with tuned molecular recognition properties. We also show how global analysis can be used to analyze double mutant cycles in TCR-pMHC interfaces, which can lead to new insights into immune recognition.

The basis for limited specificity and MHC restriction in a T cell receptor interface

αβ T cell receptors (TCRs) recognize peptides presented by major histocompatibility complex (MHC) proteins using multiple complementarity determining region (CDR) loops. TCRs display an array of poorly understood recognition properties, including specificity, cross-reactivity, and MHC restriction. Here we report a comprehensive thermodynamic deconstruction of the interaction between the A6 TCR and the Tax peptide presented by the class I MHC HLA-A*0201, uncovering the physical basis for the receptor's recognition properties. Broadly, our findings are in conflict with widely-held generalities regarding TCR recognition, such as the relative contributions of central and peripheral peptide residues and the roles of the hypervariable and germline CDR loops in engaging peptide and MHC. Instead we find that the recognition properties of the receptor emerge from the need to engage the composite peptide/MHC surface, with the receptor utilizing its CDR loops in a cooperative fashion such that specificity, cross-reactivity, and MHC restriction are inextricably linked.

Loss of T cell antigen recognition arising from changes in peptide and major histocompatibility complex protein flexibility: implications for vaccine design

Modification of the primary anchor positions of antigenic peptides to improve binding to major histocompatibility complex (MHC) proteins is a commonly used strategy for engineering peptide-based vaccine candidates. However, such peptide modifications do not always improve antigenicity, complicating efforts to design effective vaccines for cancer and infectious disease. Here we investigated the MART-1(27-35) tumor antigen, for which anchor modification (replacement of the position two alanine with leucine) dramatically reduces or ablates antigenicity with a wide range of T cell clones despite significantly improving peptide binding to MHC. We found that anchor modification in the MART-1(27-35) antigen enhances the flexibility of both the peptide and the HLA-A*0201 molecule. Although the resulting entropic effects contribute to the improved binding of the peptide to MHC, they also negatively impact T cell receptor binding to the peptide·MHC complex. These results help explain how the "anchor-fixing" strategy fails to improve antigenicity in this case, and more generally, may be relevant for understanding the high specificity characteristic of the T cell repertoire. In addition to impacting vaccine design, modulation of peptide and MHC flexibility through changes to antigenic peptides may present an evolutionary strategy for the escape of pathogens from immune destruction.

TCRs used in cancer gene therapy cross-react with MART-1/Melan-A tumor antigens via distinct mechanisms

T cells engineered to express TCRs specific for tumor Ags can drive cancer regression. The first TCRs used in cancer gene therapy, DMF4 and DMF5, recognize two structurally distinct peptide epitopes of the melanoma-associated MART-1/Melan-A protein, both presented by the class I MHC protein HLA-A*0201. To help understand the mechanisms of TCR cross-reactivity and provide a foundation for the further development of immunotherapy, we determined the crystallographic structures of DMF4 and DMF5 in complex with both of the MART-1/Melan-A epitopes. The two TCRs use different mechanisms to accommodate the two ligands. Although DMF4 binds the two with a different orientation, altering its position over the peptide/MHC, DMF5 binds them both identically. The simpler mode of cross-reactivity by DMF5 is associated with higher affinity toward both ligands, consistent with the superior functional avidity of DMF5. More generally, the observation of two diverging mechanisms of cross-reactivity with the same Ags and the finding that TCR-binding orientation can be determined by peptide alone extend our understanding of the mechanisms underlying TCR cross-reactivity.