Dipeptides catalyze rapid peptide exchange on MHC class I molecules

Preparation empty peptide-receptive MHC class I complex for large-scale detection through photolabile peptide ligands

Peptide-major histocompatibility complex (pMHC) multimers are wide recognized as the premier technique for detecting, characterizing, and isolating antigen-specific CD8+ T-cell subsets. These multimers are specifically useful in studying infections, autoimmune conditions, and cancer through single-cell analysis techniques such as flow cytometry and fluorescence microscopy. However, the development of high-throughput assays with commercially available pMHC tetramers can be expensive, while in-house production may pose challenges for most biology research laboratories. In this context, we introduce a cost-friendly and uncomplicated protocol to prepare empty MHC class I tetramers using disulfide-stabilized molecules and photolabile peptide ligands. Our method relies on disulfide bond-stabilized MHC-I molecules, which demonstrated stability when folded into stable monomers in the presence of a photolabile epitope. These monomers, upon ultraviolet irradiation and streptavidin binding, efficiently assemble into tetramers devoid of any peptide. Following a short incubation with the peptide of interest under gentle conditions, the resulting pMHC tetramer effectively detects patient-sourced, neoantigen-specific T cells. Our unique approach streamlines large-scale pMHC generation, thus paving the way for advancements in T cell-based diagnostics and personalized therapies.

Development of a Stable Peptide-Major Histocompatibility Complex (MHC) via Sortase and Click Chemistry

T cells play a crucial role in antitumor immune responses and the clearance of infected cells. They identify their targets through the binding of T-cell receptors (TCRs) to peptide-major histocompatibility complex (pMHC) molecules present in cancer cells, infected cells, and antigen-presenting cells. This interaction is often weak, requiring multimeric pMHC molecules to enhance the avidity for identifying antigen-specific T cells. Current exchangeable pMHC-I tetramerization methods may overlook TCRs recognizing less stable yet immunogenic peptides. In vivo applications targeting antigen-specific T cells demand the genetic synthesis of a pMHC fusion for each unique peptide antigen, which poses a significant challenge. To address these challenges, we developed a sortase and click chemistry-mediated approach for generating stable pMHC molecules. Leveraging sortase technology, we introduced an azide click-handle near the N-terminus of β2m, proximal to the MHC-peptide-binding groove. Simultaneously, the peptide was engineered with a multi glycine linker and a C-terminal alkyne click-handle. Azide-alkyne click reactions efficiently immobilized the peptide onto the MHC molecule, providing a versatile and efficient method for pMHC generation. The resulting peptide-clicked-MHC specifically binds to its cognate TCR and remains stable for over 3 months at 4 °C in the absence of any additional free peptide. The stability of the pMHC and its affinity to cognate TCRs are influenced by the linker's nature and length. Multi glycine linkers outperform poly(ethylene glycol) (PEG) linkers in this regard. This technology expands the toolkit for identifying and targeting antigen-specific T cells, enhancing our understanding of cancer-specific immune responses, and has the potential to streamline the development of personalized immunotherapies.

Fluorescence labeling of anchor-modified Mart-1 peptide for increasing its affinity for HLA-A*0201: Hit two targets with one arrow

One of the most successful strategies in designing peptide-based cancer vaccines is modifying natural epitope peptides to increase their binding strength to human leukocyte antigens (HLAs). Anchor-modified Mart-1 peptide (ELAGIGILTV) is among the artificial epitope peptides with the highest binding affinity for HLA-A*0201. In this study, by fluorescence labeling of its either C- or N-terminus with Nε -(5-carboxyfluorescein)-l-lysine, we not only made it traceable but also drastically increased its binding strength to HLA-A*0201. HLA streptamer, for the first time, is introduced for measuring the binding constants (Ka ) of the labeled peptides. The affinity of the labeled peptides for the HLA-A*201 of the MCF-7 cells was extraordinarily high and co-incubating them with the highest possible amount of the unlabeled peptide, as a competitor, did not significantly prohibit them from binding to the HLA. The reproducibility of the obtained results was confirmed by using the T2 cell line. The HLA-deficient K562 cell line was used as the negative control. With in silico simulations, we found two hydrophobic pockets on both sides of HLA-A*0201 for anchoring the C- or N-terminal 5-carboxyfluorescein probe, which can explain the extraordinary affinity of the labeled peptides for the HLA-A*0201.

Antigen-specific and cross-reactive T cells in protection and disease

Human T cells have a diverse T-cell receptor (TCR) repertoire that endows them with the ability to identify and defend against a broad spectrum of antigens. The universe of possible antigens that T cells may encounter, however, is even larger. To effectively surveil such a vast universe, the T-cell repertoire must adopt a high degree of cross-reactivity. Likewise, antigen-specific and cross-reactive T-cell responses play pivotal roles in both protective and pathological immune responses in numerous diseases. In this review, we explore the implications of these antigen-driven T-cell responses, with a particular focus on CD8+ T cells, using infection, neurodegeneration, and cancer as examples. We also summarize recent technological advances that facilitate high-throughput profiling of antigen-specific and cross-reactive T-cell responses experimentally, as well as computational biology approaches that predict these interactions.

Thermal-exchange HLA-E multimers reveal specificity in HLA-E and NKG2A/CD94 complex interactions

There is growing interest in HLA-E-restricted T-cell responses as a possible novel, highly conserved, vaccination targets in the context of infectious and malignant diseases. The developing field of HLA multimers for the detection and study of peptide-specific T cells has allowed the in-depth study of TCR repertoires and molecular requirements for efficient antigen presentation and T-cell activation. In this study, we developed a method for efficient peptide thermal exchange on HLA-E monomers and multimers allowing the high-throughput production of HLA-E multimers. We optimized the thermal-mediated peptide exchange, and flow cytometry staining conditions for the detection of TCR and NKG2A/CD94 receptors, showing that this novel approach can be used for high-throughput identification and analysis of HLA-E-binding peptides which could be involved in T-cell and NK cell-mediated immune responses. Importantly, our analysis of NKG2A/CD94 interaction in the presence of modified peptides led to new molecular insights governing the interaction of HLA-E with this receptor. In particular, our results reveal that interactions of HLA-E with NKG2A/CD94 and the TCR involve different residues. Altogether, we present a novel HLA-E multimer technology based on thermal-mediated peptide exchange allowing us to investigate the molecular requirements for HLA-E/peptide interaction with its receptors.

Partial peptide dissociation and binding groove plasticity in two major histocompatibility complex class I alleles - differences between alleles versus force field and sampling effects

Major histocompatibility complex class I (MHC I) reports a cell's health status by presenting antigenic peptides inside its binding groove. However, MHC I binding grooves can differ largely in their plasticity, from binding grooves that are conformationally stable by themselves to those that require a high-affinity peptide to be bound to attain conformational stability. These latter MHC I alleles are dependent on the C-terminus of the peptide that stabilizes the F-pocket region of their binding grooves. It has remained unclear to what extent a peptide-MHC I complex (pMHC I) can tolerate the (intermittent) partial dissociation of high-affinity peptides, especially of the peptide's N-terminus. Using bias exchange umbrella sampling (BEUS), a technique to achieve enhanced sampling in molecular dynamics (MD) simulations, we obtained the free-energy profiles of the N-terminal dissociation of a respective high-affinity peptide from HLA-B*35:01 and HLA-B*44:02, two alleles on opposite ends of the scale regarding binding groove plasticity. The potential of mean force (PMF) for HLA-B*35:01 was calculated for two different sets of starting structures and is compared with a PMF obtained previously with a different force field to disentangle allele differences from force field and sampling effects. For both alleles, the free-energy profiles indicate that the peptide N-terminus dissociates in a substantial fraction of the pMHC I, suggesting that their crystal structures with fully bound peptides only partially capture the dynamic conformational ensemble of pMHC I in solution, and thus in the cell.

In solution, high-affinity peptides are likely to dissociate partially from two alleles of major histocompatibility complex I. Despite very similar free-energy profiles, two molecular dynamics force fields predict different underlying mechanisms.

In vivo mRNA delivery to virus-specific T cells by light-induced ligand exchange of MHC class I antigen-presenting nanoparticles

Simultaneous delivery of mRNA to multiple populations of antigen (Ag)-specific CD8+ T cells is challenging given the diversity of peptide epitopes and polymorphism of class I major histocompatibility complexes (MHCI). We developed Ag-presenting nanoparticles (APNs) for mRNA delivery using pMHCI molecules that were refolded with photocleavable peptides to allow rapid ligand exchange by UV light and site-specifically conjugated with a lipid tail for postinsertion into preformed mRNA lipid nanoparticles. Across different TCR transgenic mouse models (P14, OT-1, and Pmel), UV-exchanged APNs bound and transfected their cognate Ag-specific CD8+ T cells equivalent to APNs produced using conventionally refolded pMHCI molecules. In mice infected with PR8 influenza, multiplexed delivery of UV-exchanged APNs against three immunodominant epitopes led to ~50% transfection of a VHH mRNA reporter in cognate Ag-specific CD8+ T cells. Our data show that UV-mediated peptide exchange can be used to rapidly produce APNs for mRNA delivery to multiple populations of Ag-specific T cells in vivo.

Peptide–MHC complexes: dressing up to manipulate T cells against autoimmunity and cancer

Antigen-specificity of T cells provides important clues to the pathogenesis of T cell-mediated autoimmune diseases and immune-evasion strategies of tumors. Identification of T cell clones involved in autoimmunity or cancer is achieved with soluble peptide–MHC (pMHC) complex multimers. Importantly, these complexes can also be used to manipulate disease-relevant T cells to restore homeostasis of T cell-mediated immune response. While auto-antigen-specific T cells can be deleted or anergized by T cell receptor engagement with cognate pMHC complexes in the absence of costimulation, integration of these complexes in artificial antigen-presenting systems can activate tumor antigen-specific T cells. Here the authors discuss the advancements in pMHC-complex-mediated immunotherapeutic strategies in autoimmunity and cancer and identify the lacunae in these strategies that need to be addressed to facilitate clinical implementation.

Homotypic and heterotypic in cis associations of MHC class I molecules at the cell surface

Through the presentation of peptide antigens to cytotoxic T lymphocytes, major histocompatibility complex (MHC) class I molecules mediate the adaptive immune response against tumors and viruses. Additional non-immunological functions include the heterotypic association of class I molecules with cell surface receptors, regulating their activities by unknown mechanisms. Also, homotypic associations resulting in class I dimers and oligomers - of unknown function - have been related to pathological outcomes. In this review, we provide an overview of the current knowledge about the occurrence, biochemical nature, and dynamics of homotypic and heterotypic associations of class I molecules at the cell surface with special focus on the molecular species that take part in the complexes and on the evidence that supports novel biological roles for class I molecules. We show that both heterotypic and homotypic class I associations reported in the literature describe not one but several kinds of oligomers with distinctive stoichiometry and biochemical properties.

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Major histocompatibility complex class I molecules form homotypic and heterotypic associations at the cell surface.

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Associations show distinctive stoichiometry and biochemical properties.

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Associations might regulate immunological and non-immunological processes.

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Heterotypic association with cell surface receptors might regulate receptor's activity.

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Homotypic associations have been related to pathological outcomes.

Rapid peptide exchange on MHC class I by small molecules elucidates dynamics of bound peptide

Complexes of peptides with recombinant major histocompatibility complex class I molecules (rpMHCs) are an important tool for T cell detection, isolation, and activation in cancer immunotherapy. The rapid preparation of rpMHCs is aided by peptide exchange, for which several technologies exist. Here, we show peptide exchange with small-molecule alcohols and demonstrate that they accelerate the dissociation of pre-bound peptides, creating a novel method for rapid production of rpMHCs and increasing the understanding of the conformational flexibility of the MHC-bound peptides.

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Small alcohols can catalyze peptide exchange on MHC-I.

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C-terminal peptide binding plays an important role in ethanol mediated exchange.

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MHC-I ethanol peptide exchange is allotype dependent.

Guidelines for the use of flow cytometry and cell sorting in immunological studies (third edition)

The third edition of Flow Cytometry Guidelines provides the key aspects to consider when performing flow cytometry experiments and includes comprehensive sections describing phenotypes and functional assays of all major human and murine immune cell subsets. Notably, the Guidelines contain helpful tables highlighting phenotypes and key differences between human and murine cells. Another useful feature of this edition is the flow cytometry analysis of clinical samples with examples of flow cytometry applications in the context of autoimmune diseases, cancers as well as acute and chronic infectious diseases. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid. All sections are written and peer-reviewed by leading flow cytometry experts and immunologists, making this edition an essential and state-of-the-art handbook for basic and clinical researchers.

A novel and efficient approach to high-throughput production of HLA-E/peptide monomer for T-cell epitope screening

Over the past two decades, there has been a great interest in the study of HLA-E-restricted αβ T cells during bacterial and viral infections, including recently SARS-CoV-2 infection. Phenotyping of these specific HLA-E-restricted T cells requires new tools such as tetramers for rapid cell staining or sorting, as well as for the identification of new peptides capable to bind to the HLA-E pocket. To this aim, we have developed an optimal photosensitive peptide to generate stable HLA-E/pUV complexes allowing high-throughput production of new HLA-E/peptide complexes by peptide exchange. We characterized the UV exchange by ELISA and improved the peptide exchange readout using size exclusion chromatography. This novel approach for complex quantification is indeed very important to perform tetramerization of MHC/peptide complexes with the high quality required for detection of specific T cells. Our approach allows the rapid screening of peptides capable of binding to the non-classical human HLA-E allele, paving the way for the development of new therapeutic approaches based on the detection of HLA-E-restricted T cells.

Peptide-MHC I complex stability measured by nanoscale differential scanning fluorimetry reveals molecular mechanism of thermal denaturation

Recombinant major histocompatibility complex class I molecules are used in diagnostic and therapeutic approaches in cancer immunotherapy, with many studies exploring their binding to antigenic peptides. Current techniques for kinetic peptide binding studies are hampered by high sample consumption, low throughput, interference with protein stability, and/or high background signal. Here, we validate nanoscale differential scanning fluorimetry (nanoDSF), a method using the tryptophan fluorescence of class I molecules, for class I/peptide binding, and we use it to determine the molecular mechanism of the thermal denaturation of HLA-A*02:01.

Major histocompatibility complex (MHC) class I and class II proteins: impact of polymorphism on antigen presentation

The major histocompatibility complex (MHC) loci are amongst the most polymorphic regions in the genomes of vertebrates. In the human population, thousands of MHC gene variants (alleles) exist that translate into distinct allotypes equipped with overlapping but unique peptide binding profiles. Understanding the differential structural and dynamic properties of MHC alleles and their interaction with critical regulators of peptide exchange bears the potential for more personalized strategies of immune modulation in the context of HLA-associated diseases.

Changing the landscape of tumor immunology: novel tools to examine T cell specificity

Immunotherapy has established itself as a stalwart arm in patient care and with precision medicine forms the new paradigm in cancer treatment. T cells are an important group of immune cells capable of potent cancer immune surveillance and immunity. The advent of bioinformatics, particularly more recent advances incorporating algorithms employing machine learning, provide a seemingly limitless ability for T cell analysis and hypothesis generation. Such endeavors have become indispensable to research efforts accelerating and evolving to such an extent that there exists an appreciable gap between knowledge and proof of function and application. Exciting new technologies such as DNA barcoding, cytometry by time-of-flight (CyTOF), and peptide-exchangeable pHLA multimers inclusive of rare and difficult HLA alleles offer high-throughput cell-by-cell analytical capabilities. These outstanding recent contributions to T cell research will help close this gap and potentially bring practical benefit to patients.

Needle in a Haystack: The Naïve Repertoire as a Source of T Cell Receptors for Adoptive Therapy with Engineered T Cells

T cell engineering with antigen-specific T cell receptors (TCRs) has allowed the generation of increasingly specific, reliable, and versatile T cell products with near-physiological features. However, a broad applicability of TCR-based therapies in cancer is still limited by the restricted number of TCRs, often also of suboptimal potency, available for clinical use. In addition, targeting of tumor neoantigens with TCR-engineered T cell therapy moves the field towards a highly personalized treatment, as tumor neoantigens derive from somatic mutations and are extremely patient-specific. Therefore, relevant TCRs have to be de novo identified for each patient and within a narrow time window. The naïve repertoire of healthy donors would represent a reliable source due to its huge diverse TCR repertoire, which theoretically entails T cells for any antigen specificity, including tumor neoantigens. As a challenge, antigen-specific naïve T cells are of extremely low frequency and mostly of low functionality, making the identification of highly functional TCRs finding a "needle in a haystack." In this review, we present the technological advancements achieved in high-throughput mapping of patient-specific neoantigens and corresponding cognate TCRs and how these platforms can be used to interrogate the naïve repertoire for a fast and efficient identification of rare but therapeutically valuable TCRs for personalized adoptive T cell therapy.

Mild Acid Elution and MHC Immunoaffinity Chromatography Reveal Similar Albeit Not Identical Profiles of the HLA Class I Immunopeptidome

To understand and treat immunology-related diseases, a comprehensive, unbiased characterization of major histocompatibility complex (MHC) peptide ligands is of key importance. Preceding the analysis by mass spectrometry, MHC class I peptide ligands are typically isolated by MHC immunoaffinity chromatography (MHC-IAC) and less often by mild acid elution (MAE). MAE may provide a cheap alternative to MHC-IAC for suspension cells but has been hampered by the high number of contaminating, MHC-unrelated peptides. Here, we optimized MAE, yielding MHC peptide ligand purities of more than 80%. When compared with MHC-IAC, obtained peptides were similar in numbers, identities, and to a large extent intensities, while the percentage of cysteinylated peptides was 5 times higher in MAE. The latter benefitted the discovery of MHC-allotype-specific, distinct cysteinylation frequencies at individual positions of MHC peptide ligands. MAE revealed many MHC ligands with unmodified, N-terminal cysteine residues which get lost in MHC-IAC workflows. The results support the idea that MAE might be particularly valuable for the high-confidence analysis of post-translational modifications by avoiding the exposure of the investigated peptides to enzymes and reactive molecules in the cell lysate. Our improved and carefully documented MAE workflow represents a high-quality, cost-effective alternative to MHC-IAC for suspension cells.

T cell antigen discovery

T cells respond to threats in an antigen-specific manner using T cell receptors (TCRs) that recognize short peptide antigens presented on major histocompatibility complex (MHC) proteins. The TCR-peptide-MHC interaction mediated between a T cell and its target cell dictates its function and thereby influences its role in disease. A lack of approaches for antigen discovery has limited the fundamental understanding of the antigenic landscape of the overall T cell response. Recent advances in high-throughput sequencing, mass cytometry, microfluidics and computational biology have led to a surge in approaches to address the challenge of T cell antigen discovery. Here, we summarize the scope of this challenge, discuss in depth the recent exciting work and highlight the outstanding questions and remaining technical hurdles in this field.

High throughput pMHC-I tetramer library production using chaperone-mediated peptide exchange

Peptide exchange technologies are essential for the generation of pMHC-multimer libraries used to probe diverse, polyclonal TCR repertoires in various settings. Here, using the molecular chaperone TAPBPR, we develop a robust method for the capture of stable, empty MHC-I molecules comprising murine H2 and human HLA alleles, which can be readily tetramerized and loaded with peptides of choice in a high-throughput manner. Alternatively, catalytic amounts of TAPBPR can be used to exchange placeholder peptides with high affinity peptides of interest. Using the same system, we describe high throughput assays to validate binding of multiple candidate peptides on empty MHC-I/TAPBPR complexes. Combined with tetramer-barcoding via a multi-modal cellular indexing technology, ECCITE-seq, our approach allows a combined analysis of TCR repertoires and other T cell transcription profiles together with their cognate antigen specificities in a single experiment. The new approach allows TCR/pMHC interactions to be interrogated easily at large scale.

Peptide-MHC (pMHC) tetramers are important tools for probing T cell repertoire and adaptive immune responses. Here the authors use a molecular chaperone, TAPBPR, to develop a high-throughput, multiplexible platform for pMHC tetramer generation to facilitate simultaneous assessments of T cell repertoire/antigen specificity and transcriptome.

Production of soluble pMHC-I molecules in mammalian cells using the molecular chaperone TAPBPR

Current approaches for generating major histocompatibility complex (MHC) Class-I proteins with desired bound peptides (pMHC-I) for research, diagnostic and therapeutic applications are limited by the inherent instability of empty MHC-I molecules. Using the properties of the chaperone TAP-binding protein related (TAPBPR), we have developed a robust method to produce soluble, peptide-receptive MHC-I molecules in Chinese Hamster Ovary cells at high yield, completely bypassing the requirement for laborious refolding from inclusion bodies expressed in E.coli. Purified MHC-I/TAPBPR complexes can be prepared for multiple human allotypes, and exhibit complex glycan modifications at the conserved Asn 86 residue. As a proof of concept, we demonstrate both HLA allele-specific peptide binding and MHC-restricted antigen recognition by T cells for two relevant tumor-associated antigens. Our system provides a facile, high-throughput approach for generating pMHC-I antigens to probe and expand TCR specificities present in polyclonal T cell repertoires.

Guidelines for the use of flow cytometry and cell sorting in immunological studies (second edition)

These guidelines are a consensus work of a considerable number of members of the immunology and flow cytometry community. They provide the theory and key practical aspects of flow cytometry enabling immunologists to avoid the common errors that often undermine immunological data. Notably, there are comprehensive sections of all major immune cell types with helpful Tables detailing phenotypes in murine and human cells. The latest flow cytometry techniques and applications are also described, featuring examples of the data that can be generated and, importantly, how the data can be analysed. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid, all written and peer-reviewed by leading experts in the field, making this an essential research companion.

Production and Thermal Exchange of Conditional Peptide-MHC I Multimers

Cytotoxic CD8+ T cells mediate cellular immunity through recognition of specific antigens presented by MHC class I on all nucleated cells. Studying T cell interactions and responses provides invaluable information on infection, autoimmunity and cancer. Fluorescently labeled multimers of MHC I can be used to quantify, characterize, and isolate specific CD8+ T cells by flow cytometry. Here we describe the production and use of conditional MHC I multimers that can be loaded with peptides of choice by incubating them at a defined temperature. Multimers are folded with a template peptide that forms a stable complex at low temperature, but dissociates at a defined elevated temperature. Using this technology multiple MHC I multimers can be generated in parallel, to allow staining and isolation of large sets of antigen-specific CD8+ T cells, especially when combined with barcoding technologies. © 2019 The Authors.

Empty peptide-receptive MHC class I molecules for efficient detection of antigen-specific T cells

The peptide-dependent stability of MHC class I molecules poses a substantial challenge for their use in peptide-MHC multimer–based approaches to comprehensively analyze T cell immunity. To overcome this challenge, we demonstrate the use of functionally empty MHC class I molecules stabilized by a disulfide bond to link the α1and α2helices close to the F pocket. Peptide-loaded disulfide-stabilized HLA-A*02:01 shows complete structural overlap with wild-type HLA-A*02:01. Peptide-MHC multimers prepared using disulfide-stabilized HLA-A*02:01, HLA-A*24:02, and H-2Kbcan be used to identify antigen-specific T cells, and they provide a better staining index for antigen-specific T cell detection compared with multimers prepared with wild-type MHC class I molecules. Disulfide-stabilized MHC class I molecules can be loaded with peptide in the multimerized form without affecting their capacity to stain T cells. We demonstrate the value of empty-loadable tetramers that are converted to antigen-specific tetramers by a single-step peptide addition through their use to identify T cells specific for mutation-derived neoantigens and other cancer-associated antigens in human melanoma.

High-throughput identification of human antigen-specific CD8+ and CD4+ T cells using soluble pMHC multimers

Peptide major histocompatibility complex (pMHC) multimers have been used since decades to identify, isolate and analyze antigen-specific T cells by flow (and more recently mass) cytometry. Yet well established as a standard technology, improvements are still required to face the growing needs of personalized immune monitoring. Here we review the latest developments about (i) the quality of pMHC class I and II monomers, (ii) the importance of the multimeric scaffold, (iii) the staining conditions and (iv) the high-throughput synthesis of pMHC monomers. Finally, innovative multiplexed, combinatorial strategies for parallel detection of antigen-specific T cells in a single sample are discussed.

Structural aspects of chaperone-mediated peptide loading in the MHC-I antigen presentation pathway

Recognition of foreign and dysregulated antigens by the cellular innate and adaptive immune systems is in large part dependent on the cell surface display of peptide/MHC (pMHC) complexes. The formation of such complexes requires the generation of antigenic peptides, proper folding of MHC molecules, loading of peptides onto MHC molecules, glycosylation, and transport to the plasma membrane. This complex series of biosynthetic, biochemical, and cell biological reactions is known as "antigen processing and presentation". Here, we summarize recent work, focused on the structural and functional characterization of the key MHC-I-dedicated chaperones, tapasin, and TAPBPR. The mechanisms reflect the ability of conformationally flexible molecules to adapt to their ligands, and are comparable to similar processes that are exploited in peptide antigen loading in the MHC-II pathway.

Dynamically Driven Allostery in MHC Proteins: Peptide-Dependent Tuning of Class I MHC Global Flexibility

T cell receptor (TCR) recognition of antigenic peptides bound and presented by class I major histocompatibility complex (MHC) proteins underlies the cytotoxic immune response to diseased cells. Crystallographic structures of TCR-peptide/MHC complexes have demonstrated how TCRs simultaneously interact with both the peptide and the MHC protein. However, it is increasingly recognized that, beyond serving as a static platform for peptide presentation, the physical properties of class I MHC proteins are tuned by different peptides in ways that are not always structurally visible. These include MHC protein motions, or dynamics, which are believed to influence interactions with a variety of MHC-binding proteins, including not only TCRs, but other activating and inhibitory receptors as well as components of the peptide loading machinery. Here, we investigated the mechanisms by which peptides tune the dynamics of the common class I MHC protein HLA-A2. By examining more than 50 lengthy molecular dynamics simulations of HLA-A2 presenting different peptides, we identified regions susceptible to dynamic tuning, including regions in the peptide binding domain as well as the distal α3 domain. Further analyses of the simulations illuminated mechanisms by which the influences of different peptides are communicated throughout the protein, and involve regions of the peptide binding groove, the β₂-microglobulin subunit, and the α3 domain. Overall, our results demonstrate that the class I MHC protein is a highly tunable peptide sensor whose physical properties vary considerably with bound peptide. Our data provides insight into the underlying principles and suggest a role for dynamically driven allostery in the immunological function of MHC proteins.

Successive crystal structure snapshots suggest the basis for MHC class I peptide loading and editing by tapasin

MHC-I epitope presentation to CD8⁺ T cells is directly dependent on peptide loading and selection during antigen processing. However, the exact molecular bases underlying peptide selection and binding by MHC-I remain largely unknown. Within the peptide-loading complex, the peptide editor tapasin is key to the selection of MHC-I-bound peptides. Here, we have determined an ensemble of crystal structures of MHC-I in complex with the peptide exchange-associated dipeptide GL, as well as the tapasin-associated scoop loop, alone or in combination with candidate epitopes. These results combined with mutation analyses allow us to propose a molecular model underlying MHC-I peptide selection by tapasin. The N termini of bound peptides most probably bind first in the N-terminal and middle region of the MHC-I peptide binding cleft, upon which the peptide C termini are tested for their capacity to dislodge the tapasin scoop loop from the F pocket of the MHC-I cleft. Our results also indicate important differences in peptide selection between different MHC-I alleles.

TAPBPR mediates peptide dissociation from MHC class I using a leucine lever

Tapasin and TAPBPR are known to perform peptide editing on major histocompatibility complex class I (MHC I) molecules; however, the precise molecular mechanism(s) involved in this process remain largely enigmatic. Here, using immunopeptidomics in combination with novel cell-based assays that assess TAPBPR-mediated peptide exchange, we reveal a critical role for the K22-D35 loop of TAPBPR in mediating peptide exchange on MHC I. We identify a specific leucine within this loop that enables TAPBPR to facilitate peptide dissociation from MHC I. Moreover, we delineate the molecular features of the MHC I F pocket required for TAPBPR to promote peptide dissociation in a loop-dependent manner. These data reveal that chaperone-mediated peptide editing on MHC I can occur by different mechanisms dependent on the C-terminal residue that the MHC I accommodates in its F pocket and provide novel insights that may inform the therapeutic potential of TAPBPR manipulation to increase tumour immunogenicity.

A two-hybrid antibody micropattern assay reveals specific in cis interactions of MHC I heavy chains at the cell surface

We demonstrate a two-hybrid assay based on antibody micropatterns to study protein-protein interactions at the cell surface of major histocompatibility complex class I (MHC I) proteins. Anti-tag and conformation-specific antibodies are used for individual capture of specific forms of MHC I proteins that allow for location- and conformation-specific analysis by fluorescence microscopy. The assay is used to study the in cis interactions of MHC I proteins at the cell surface under controlled conditions and to define the involved protein conformations. Our results show that homotypic in cis interactions occur exclusively between MHC I free heavy chains, and we identify the dissociation of the light chain from the MHC I protein complex as a condition for MHC I in cis interactions. The functional role of these MHC I protein-protein interactions at the cell surface needs further investigation. We propose future technical developments of our two-hybrid assay for further analysis of MHC I protein-protein interactions.

A flexible MHC class I multimer loading system for large-scale detection of antigen-specific T cells

Luimstra et al. describe a temperature-mediated peptide exchange method for generating many different epitope-specific MHC class I multimers in parallel. This simple and versatile technology allows fast and efficient production of MHC I reagents for immune monitoring of T cell responses.

Adaptive immunity is initiated by T cell recognition of specific antigens presented by major histocompatibility complexes (MHCs). MHC multimer technology has been developed for the detection, isolation, and characterization of T cells in infection, autoimmunity, and cancer. Here, we present a simple, fast, flexible, and efficient method to generate many different MHC class I (MHC I) multimers in parallel using temperature-mediated peptide exchange. We designed conditional peptides for HLA-A*02:01 and H-2K^b that form stable peptide–MHC I complexes at low temperatures, but dissociate when exposed to a defined elevated temperature. The resulting conditional MHC I complexes, either alone or prepared as ready-to-use multimers, can swiftly be loaded with peptides of choice without additional handling and within a short time frame. We demonstrate the ease and flexibility of this approach by monitoring the antiviral immune constitution in an allogeneic stem cell transplant recipient and by analyzing CD8⁺ T cell responses to viral epitopes in mice infected with lymphocytic choriomeningitis virus or cytomegalovirus.

Identification of Immunogenic Epitopes by MS/MS

The interrogation of cell surface-presented immunogenic epitopes is of great importance to differentiate diseased cells in consequence to malignant transformation or viral infections. On the basis of this knowledge, next-generation immunotherapies against cancers, autoimmunity, or infectious diseases can be developed. The identification of altered peptide repertoires of transformed cells renders mass spectrometry-based analysis indispensable. This is evident considering the low correlation of gene or protein expression alterations, respectively, with changes in the peptide repertoire rendering those analyses less informative. Nevertheless, immunogenicity of peptides appearing to be exclusively found on diseased cells has to be finally proven in T cell-based assays. This review highlights the capabilities and limitations of mass spectrometry in the identification of entire immunopeptidomes, as well as individual potential immunogenic epitopes with a strong focus on cancer. Furthermore, an overview of state-of-the-art immunogenicity screens is presented.

Evolution of MHC-based technologies used for detection of antigen-responsive T cells

T cell-mediated recognition of peptide-major histocompatibility complex (pMHC) class I and II molecules is crucial for the control of intracellular pathogens and cancer, as well as for stimulation and maintenance of efficient cytotoxic responses. Such interactions may also play a role in the development of autoimmune diseases. Novel insights into this mechanism are crucial to understanding disease development and establishing new treatment strategies. MHC multimers have been used for detection of antigen-responsive T cells since the first report by Altman et al. showed that tetramerization of pMHC class I molecules provided sufficient stability to T cell receptor (TCR)-pMHC interactions, allowing detection of MHC multimer-binding T cells using flow cytometry. Since this breakthrough the scientific community has aimed for expanding the capacity of MHC multimer-based detection technologies to facilitate large-scale epitope discovery and immune monitoring in limited biological material. Screening of T cell specificity using large libraries of pMHC molecules is suitable for analyses of T cell recognition potentially at genome-wide levels rather than analyses restricted to a selection of model antigens. Such strategies provide novel insights into the immune specificities involved in disease development and response to immunotherapy, and extend fundamental knowledge related to T cell recognition patterns and cross-recognition by TCRs. MHC multimer-based technologies have now evolved from detection of 1-2 different T cell specificities per cell sample, to include more than 1000 evaluable pMHC molecules using novel technologies. Here, we provide an overview of MHC multimer-based detection technologies developed over two decades, focusing primarily on MHC class I interactions.

Partial Dissociation of Truncated Peptides Influences the Structural Dynamics of the MHCI Binding Groove

Antigen processing on MHCI involves the exchange of low-affinity peptides by high-affinity, immunodominant ones. This peptide editing process is mediated by tapasin and ERAAP at the peptide C- and N-terminus, respectively. Since tapasin does not contact the peptide directly, a sensing mechanism involving conformational changes likely allows tapasin to distinguish antigen-loaded MHCI molecules from those occupied by weakly bound, non-specific peptides. To understand this mechanism at the atomic level, we performed molecular dynamics simulations of MHCI allele B*44:02 loaded with peptides truncated or modified at the C- or N-terminus. We show that the deletion of peptide anchor residues leads to reversible, partial dissociation of the peptide from MHCI on the microsecond timescale. Fluctuations in the MHCI α2-1 helix segment, bordering the binding groove and cradled by tapasin in the PLC, are influenced by the peptide C-terminus occupying the nearby F-pocket. Simulations of tapasin complexed with MHCI bound to a low-affinity peptide show that tapasin widens the MHCI binding groove near the peptide C-terminus and weakens the attractive forces between MHCI and the peptide. Our simulations thus provide a detailed, spatially resolved picture of MHCI plasticity, revealing how peptide loading status can affect key structural regions contacting tapasin.

Reprogramming MHC specificity by CRISPR-Cas9-assisted cassette exchange

The development of programmable nucleases has enabled the application of new genome engineering strategies for cellular immunotherapy. While targeted nucleases have mostly been used to knock-out or knock-in genes in immune cells, the scarless exchange of entire immunogenomic alleles would be of great interest. In particular, reprogramming the polymorphic MHC locus could enable the creation of matched donors for allogeneic cellular transplantation. Here we show a proof-of-concept for reprogramming MHC-specificity by performing CRISPR-Cas9-assisted cassette exchange. Using murine antigen presenting cell lines (RAW264.7 macrophages), we demonstrate that the generation of Cas9-induced double-stranded breaks flanking the native MHC-I H2-Kd locus led to exchange of an orthogonal H2-Kb allele. MHC surface expression allowed for easy selection of reprogrammed cells by flow cytometry, thus obviating the need for additional selection markers. MHC-reprogrammed cells were fully functional as they could present H2-Kd-restricted peptide and activate cognate T cells. Finally, we investigated the role of various donor template formats on exchange efficiency, discovering that templates that underwent in situ linearization resulted in the highest MHC-reprogramming efficiency. These findings highlight a potential new approach for the correcting of MHC mismatches in cellular transplantation.

Major Histocompatibility Complex (MHC) Class I and MHC Class II Proteins: Conformational Plasticity in Antigen Presentation

Antigen presentation by major histocompatibility complex (MHC) proteins is essential for adaptive immunity. Prior to presentation, peptides need to be generated from proteins that are either produced by the cell's own translational machinery or that are funneled into the endo-lysosomal vesicular system. The prolonged interaction between a T cell receptor and specific pMHC complexes, after an extensive search process in secondary lymphatic organs, eventually triggers T cells to proliferate and to mount a specific cellular immune response. Once processed, the peptide repertoire presented by MHC proteins largely depends on structural features of the binding groove of each particular MHC allelic variant. Additionally, two peptide editors-tapasin for class I and HLA-DM for class II-contribute to the shaping of the presented peptidome by favoring the binding of high-affinity antigens. Although there is a vast amount of biochemical and structural information, the mechanism of the catalyzed peptide exchange for MHC class I and class II proteins still remains controversial, and it is not well understood why certain MHC allelic variants are more susceptible to peptide editing than others. Recent studies predict a high impact of protein intermediate states on MHC allele-specific peptide presentation, which implies a profound influence of MHC dynamics on the phenomenon of immunodominance and the development of autoimmune diseases. Here, we review the recent literature that describe MHC class I and II dynamics from a theoretical and experimental point of view and we highlight the similarities between MHC class I and class II dynamics despite the distinct functions they fulfill in adaptive immunity.

Specific Capture of Peptide-Receptive Major Histocompatibility Complex Class I Molecules by Antibody Micropatterns Allows for a Novel Peptide-Binding Assay in Live Cells

Binding assays with fluorescently labeled ligands and recombinant receptor proteins are commonly performed in 2D arrays. But many cell surface receptors only function in their native membrane environment and/or in a specific conformation, such as they appear on the surface of live cells. Thus, receptors on live cells should be used for ligand binding assays. Here, it is shown that antibodies preprinted on a glass surface can be used to specifically array a peptide receptor of the immune system, i.e., the major histocompatibility complex class I molecule H-2K^b , into a defined pattern on the surface of live cells. Monoclonal antibodies make it feasible to capture a distinct subpopulation of H-2K^b and hold it at the cell surface. This patterned receptor enables a novel peptide-binding assay, in which the specific binding of a fluorescently labeled index peptide is visualized by microscopy. Measurements of ligand binding to captured cell surface receptors in defined confirmations apply to many problems in cell biology and thus represent a promising tool in the field of biosensors.

Major Histocompatibility Complex (MHC) Class I and MHC Class II Proteins: Conformational Plasticity in Antigen Presentation

Antigen presentation by major histocompatibility complex (MHC) proteins is essential for adaptive immunity. Prior to presentation, peptides need to be generated from proteins that are either produced by the cell’s own translational machinery or that are funneled into the endo-lysosomal vesicular system. The prolonged interaction between a T cell receptor and specific pMHC complexes, after an extensive search process in secondary lymphatic organs, eventually triggers T cells to proliferate and to mount a specific cellular immune response. Once processed, the peptide repertoire presented by MHC proteins largely depends on structural features of the binding groove of each particular MHC allelic variant. Additionally, two peptide editors—tapasin for class I and HLA-DM for class II—contribute to the shaping of the presented peptidome by favoring the binding of high-affinity antigens. Although there is a vast amount of biochemical and structural information, the mechanism of the catalyzed peptide exchange for MHC class I and class II proteins still remains controversial, and it is not well understood why certain MHC allelic variants are more susceptible to peptide editing than others. Recent studies predict a high impact of protein intermediate states on MHC allele-specific peptide presentation, which implies a profound influence of MHC dynamics on the phenomenon of immunodominance and the development of autoimmune diseases. Here, we review the recent literature that describe MHC class I and II dynamics from a theoretical and experimental point of view and we highlight the similarities between MHC class I and class II dynamics despite the distinct functions they fulfill in adaptive immunity.

Endoplasmic Reticulum Aminopeptidase 1 (ERAP1) Polymorphism Relevant to Inflammatory Disease Shapes the Peptidome of the Birdshot Chorioretinopathy-Associated HLA-A*29:02 Antigen

Birdshot chorioretinopathy is a rare ocular inflammation whose genetic association with HLA-A*29:02 is the highest between a disease and a major histocompatibility complex (MHC) molecule. It belongs to a group of MHC-I-associated inflammatory disorders, also including ankylosing spondylitis, psoriasis, and Behçet's disease, for which endoplasmic reticulum aminopeptidases (ERAP) 1 and/or 2 have been identified as genetic risk factors. Since both enzymes are involved in the processing of MHC-I ligands, it seems reasonable that common peptide-mediated mechanisms may underlie the pathogenesis of these diseases. In this study, comparative immunopeptidomics was used to characterize >5000 A*29:02 ligands and quantify the effects of ERAP1 polymorphism and expression on the A*29:02 peptidome in human cells. The peptides predominant in an active ERAP1 context showed a higher frequency of nonamers and bulkier amino acid side chains at multiple positions, compared with the peptides predominant in a less active ERAP1 background. Thus, ERAP1 polymorphism has a large influence, shaping the A*29:02 peptidome through length-dependent and length-independent effects. These changes resulted in increased affinity and hydrophobicity of A*29:02 ligands in an active ERAP1 context. The results reveal the nature of the functional interaction between A*29:02 and ERAP1 and suggest that this enzyme may affect the susceptibility to birdshot chorioretinopathy by altering the A*29:02 peptidome. The complexity of these alterations is such that not only peptide presentation but also other potentially pathogenic features could be affected.