Expression, refolding and crystallization of murine MHC class I H-2Db in complex with human beta2-microglobulin

Non-classical HLA-E restricted CMV 15-mer peptides are recognized by adaptive NK cells and induce memory responses

Introduction

Human cytomegalovirus (HCMV) reactivation causes complications in immunocompromised patients after hematopoietic stem cell transplantation (HSCT), significantly increasing morbidity and mortality. Adaptive Natural Killer (aNK) cells undergo a persistent reconfiguration in response to HCMV reactivation; however, the exact role of aNK cell memory in HCMV surveillance remains elusive.

Methods

We employed mass spectrometry and computational prediction approaches to identify HLA-E-restricted HCMV peptides that can elucidate aNK cell responses. We also used the K562 cell line transfected with HLA-E0*0103 for specific peptide binding and blocking assays. Subsequently, NK cells were cocultured with dendritic cells (DCs) loaded with each of the identified peptides to examine aNK and conventional (c)NK cell responses.

Results

Here, we discovered three unconventional HLA-E-restricted 15-mer peptides (SEVENVSVNVHNPTG, TSGSDSDEELVTTER, and DSDEELVTTERKTPR) derived from the HCMV pp65-protein that elicit aNK cell memory responses restricted to HCMV. aNK cells displayed memory responses towards HMCV-infected cells and HCMV-seropositive individuals when primed by DCs loaded with each of these peptides and predicted 9-mer versions. Blocking the interaction between HLA-E and the activation NKG2C receptor but not the inhibitory NKG2A receptor abolished these specific recall responses. Interestingly, compared to the HLA-E complex with the leader peptide VMAPRTLIL, HLA-E complexes formed with each of the three identified peptides significantly changed the surface electrostatic potential to highly negative. Furthermore, these peptides do not comprise the classical HLA-E-restriction motifs.

Discussion

These findings suggest a differential binding to NKG2C compared to HLA-E complexes with classical leader peptides that may result in the specific activation of aNK cells. We then designed six nonameric peptides based on the three discovered peptides that could elicit aNK cell memory responses to HCMV necessary for therapeutic inventions. The results provide novel insights into HLA-E-mediated signaling networks that mediate aNK cell recall responses and maximize their reactivity.

Challenges and opportunities in animal models of psoriatic arthritis

OBJECTIVE

To review the preparation, characteristics and research progress of different PsA animal models.

METHODS

Computerized searches were conducted in CNKI, PubMed and other databases to classify and discuss the relevant studies on PsA animal models. The search keywords were "PsA and animal model(s), PsA and animal(s), PsA and mouse, PsA and mice, PsA and rat(s), PsA and rabbit(s), PsA and dog(s)" RESULTS: The experimental animals currently used to study PsA are mainly rodents, including mice and rats. According to the different methods of preparing the models, the retrieved animal models were classified into spontaneous or genetic mutation, transgenic and induced animal models. These PsA animal models involve multiple pathogenesis, some experimental animals' lesions appear in a short and comprehensive cycle, some have a high success rate in molding, and some are complex and less reproducibility. This article summarizes the preparation methods, advantages and disadvantages of different models.

CONCLUSIONS

The animal models of PsA aim to mimic the clinicopathological alterations of PsA patients through gene mutation, transgenesis or targeted proinflammatory factor and to reveal new pathogenic pathways and therapeutic targets by exploring the pathological features and clinical manifestations of the disease. This work will have very far-reaching implications for the in-depth understanding of PsA and the development of new drugs.

l- to d-Amino Acid Substitution in the Immunodominant LCMV-Derived Epitope gp33 Highlights the Sensitivity of the TCR Recognition Mechanism for the MHC/Peptide Structure and Dynamics

Presentation of pathogen-derived epitopes by major histocompatibility complex I (MHC-I) can lead to the activation and expansion of specific CD8⁺ T cell clones, eventually resulting in the destruction of infected target cells. Altered peptide ligands (APLs), designed to elicit immunogenicity toward a wild-type peptide, may affect the overall stability of MHC-I/peptide (pMHC) complexes and modulate the recognition by T cell receptors (TCR). Previous works have demonstrated that proline substitution at position 3 (p3P) of different MHC-restricted epitopes, including the immunodominant LCMV-derived epitope gp33 and escape variants, may be an effective design strategy to increase epitope immunogenicity. These studies hypothesized that the p3P substitution increases peptide rigidity, facilitating TCR binding. Here, molecular dynamics simulations indicate that the p3P modification rigidifies the APLs in solution predisposing them for the MHC-I loading as well as once bound to H-2D^b, predisposing them for TCR binding. Our results also indicate that peptide position 6, key for interaction of H-2D^b/gp33 with the TCR P14, takes a suboptimal conformation before as well as after binding to the TCR. Analyses of H-2D^b in complex with APLs, in which position 6 was subjected to an l- to d-amino acid modification, revealed small conformational changes and comparable pMHC thermal stability. However, the l- to d-modification reduced significantly the binding to P14 even in the presence of the p3P modification. Our combined data highlight the sensitivity of the TCR for the conformational dynamics of pMHC and provide further tools to dissect and modulate TCR binding and immunogenicity via APLs.

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.

The Immunogenicity of a Proline-Substituted Altered Peptide Ligand toward the Cancer-Associated TEIPP Neoepitope Trh4 Is Unrelated to Complex Stability

Human cancers frequently display defects in Ag processing and presentation allowing for immune evasion, and they therefore constitute a significant challenge for T cell-based immunotherapy. We have previously demonstrated that the antigenicity of tumor-associated Ags can be significantly enhanced through unconventional residue modifications as a novel tool for MHC class I (MHC-I)-based immunotherapy approaches. We have also previously identified a novel category of cancer neo-epitopes, that is, T cell epitopes associated with impaired peptide processing (TEIPP), that are selectively presented by MHC-I on cells lacking the peptide transporter TAP. In this study, we demonstrate that substitution of the nonanchoring position 3 into a proline residue of the first identified TEIPP peptide, the murine Trh4, results in significantly enhanced recognition by antitumor CTLs toward the wild-type epitope. Although higher immunogenicity has in most cases been associated with increased MHC/peptide complex stability, our results demonstrate that the overall stability of H-2D^b in complex with the highly immunogenic altered peptide ligand Trh4-p3P is significantly reduced compared with wild-type H-2D^b/Trh4. Comparison of the crystal structures of the H-2D^b/Trh4-p3P and H-2D^b/Trh4 complexes revealed that the conformation of the nonconventional methionine anchor residue p5M is altered, deleting its capacity to form adequate sulfur-π interactions with H-2D^b residues, thus reducing the overall longevity of the complex. Collectively, our results indicate that vaccination with Thr4-p3P significantly enhances T cell recognition of targets presenting the wild-type TEIPP epitope and that higher immunogenicity is not necessarily directly related to MHC/peptide complex stability, opening for the possibility to design novel peptide vaccines with reduced MHC/peptide complex stability.

Crystal structures of H-2Db in complex with the LCMV-derived peptides GP92 and GP392 explain pleiotropic effects of glycosylation on antigen presentation and immunogenicity

Post-translational modifications significantly broaden the epitope repertoire for major histocompatibility class I complexes (MHC-I) and may allow viruses to escape immune recognition. Lymphocytic choriomeningitis virus (LCMV) infection of H-2b mice generates CD8+ CTL responses directed towards several MHC-I-restricted epitopes including the peptides GP92 (CSANNSHHYI) and GP392 (WLVTNGSYL), both with a N-glycosylation site. Interestingly, glycosylation has different effects on the immunogenicity and association capacity of these two epitopes to H-2Db. To assess the structural bases underlying these functional results, we determined the crystal structures of H-2Db in complex with GP92 (CSANNSHHYI) and GP392 (WLVTNGSYL) to 2.4 and 2.5 Å resolution, respectively. The structures reveal that while glycosylation of GP392 most probably impairs binding, the glycosylation of the asparagine residue in GP92, which protrudes towards the solvent, possibly allows for immune escape and/or forms a neo-epitope that may select for a different set of CD8 T cells. Altogether, the presented results provide a structural platform underlying the effects of post-translational modifications on epitope binding and/or immunogenicity, resulting in viral immune escape.

The MHC Class I Cancer-Associated Neoepitope Trh4 Linked with Impaired Peptide Processing Induces a Unique Noncanonical TCR Conformer

MHC class I downregulation represents a significant challenge for successful T cell-based immunotherapy. T cell epitopes associated with impaired peptide processing (TEIPP) constitute a novel category of immunogenic Ags that are selectively presented on transporter associated with Ag processing-deficient cells. The TEIPP neoepitopes are CD8 T cell targets, derived from nonmutated self-proteins that might be exploited to prevent immune escape. In this study, the crystal structure of H-2D(b) in complex with the first identified TEIPP Ag (MCLRMTAVM) derived from the Trh4 protein has been determined to 2.25 Å resolution. In contrast to prototypic H-2D(b) peptides, Trh4 takes a noncanonical peptide-binding pattern with extensive sulfur-π interactions that contribute to the overall complex stability. Importantly, the noncanonical methionine at peptide position 5 acts as a main anchor, altering only the conformation of the H-2D(b) residues Y156 and H155 and thereby forming a unique MHC/peptide conformer that is essential for recognition by TEIPP-specific T cells. Substitution of peptide residues p2C and p5M to the conservative α-aminobutyric acid and norleucine, respectively, significantly reduced complex stability, without altering peptide conformation or T cell recognition. In contrast, substitution of p5M to a conventional asparagine abolished recognition by the H-2D(b)/Trh4-specific T cell clone LnB5. We anticipate that the H-2D(b)/Trh4 complex represents the first example, to our knowledge, of a broader repertoire of alternative MHC class I binders.

Proline substitution independently enhances H-2D(b) complex stabilization and TCR recognition of melanoma-associated peptides

The immunogenicity of H-2D(b) (D(b)) restricted epitopes can be significantly increased by substituting peptide position 3 to a proline (p3P). The p3P modification enhances MHC stability without altering the conformation of the modified epitope allowing for T-cell cross-reactivity with the native peptide. The present study reveals how specific interactions between p3P and the highly conserved MHC heavy chain residue Y159 increase the stability of D(b) in complex with an optimized version of the melanoma-associated epitope gp10025-33 . Furthermore, the p3P modification directly increased the affinity of the D(b)/gp10025-33 -specific T-cell receptor (TCR) pMel. Surprisingly, the enhanced TCR binding was independent from the observed increased stability of the optimized D(b)/gp10025-33 complex and from the interactions formed between p3P and Y159, indicating a direct effect of the p3P modification on TCR recognition.

Unexpected T-cell recognition of an altered peptide ligand is driven by reversed thermodynamics

The molecular basis underlying T-cell recognition of MHC molecules presenting altered peptide ligands is still not well-established. A hierarchy of T-cell activation by MHC class I-restricted altered peptide ligands has been defined using the T-cell receptor P14 specific for H-2D(b) in complex with the immunodominant lymphocytic choriomeningitis virus peptide gp33 (KAVYNFATM). While substitution of tyrosine to phenylalanine (Y4F) or serine (Y4S) abolished recognition by P14, the TCR unexpectedly recognized H-2D(b) in complex with the alanine-substituted semiagonist Y4A, which displayed the most significant structural modification. The observed functional hierarchy gp33 > Y4A > Y4S = Y4F was neither due to higher stabilization capacity nor to differences in structural conformation. However, thermodynamic analysis demonstrated that while recognition of the full agonist H-2D(b) /gp33 was strictly enthalpy driven, recognition of the weak agonist H-2D(b) /Y4A was instead entropy driven with a large reduction in the favorable enthalpy term. The fourfold larger negative heat capacity derived for the interaction of P14 with H-2D(b) /gp33 compared with H-2D(b) /Y4A can possibly be explained by higher water entrapment at the TCR/MHC interface, which is also consistent with the measured opposite entropy contributions for the interactions of P14 with both MHCs. In conclusion, this study demonstrates that P14 makes use of different strategies to adapt to structural modifications in the MHC/peptide complex.

Inflammation-associated nitrotyrosination affects TCR recognition through reduced stability and alteration of the molecular surface of the MHC complex

Nitrotyrosination of proteins, a hallmark of inflammation, may result in the production of MHC-restricted neoantigens that can be recognized by T cells and bypass the constraints of immunological self-tolerance. Here we biochemically and structurally assessed how nitrotyrosination of the lymphocytic choriomeningitis virus (LCMV)-associated immunodominant MHC class I-restricted epitopes gp33 and gp34 alters T cell recognition in the context of both H-2D^b and H-2K^b. Comparative analysis of the crystal structures of H-2K^b/gp34 and H-2K^b/NY-gp34 demonstrated that nitrotyrosination of p3Y in gp34 abrogates a hydrogen bond interaction formed with the H-2K^b residue E152. As a consequence the conformation of the TCR-interacting E152 was profoundly altered in H-2K^b/NY-gp34 when compared to H-2K^b/gp34, thereby modifying the surface of the nitrotyrosinated MHC complex. Furthermore, nitrotyrosination of gp34 resulted in structural over-packing, straining the overall conformation and considerably reducing the stability of the H-2K^b/NY-gp34 MHC complex when compared to H-2K^b/gp34. Our structural analysis also indicates that nitrotyrosination of the main TCR-interacting residue p4Y in gp33 abrogates recognition of H-2D^b/gp33-NY complexes by H-2D^b/gp33-specific T cells through sterical hindrance. In conclusion, this study provides the first structural and biochemical evidence for how MHC class I-restricted nitrotyrosinated neoantigens may enable viral escape and break immune tolerance.