HLA-B27 polymorphism at position 116 critically influences the association with TAP/tapasin, intracellular trafficking and conformational homodimers formation

HLA peptide-binding pocket diversity modulates immunological complications after cord blood transplant in acute leukaemia

Pocket motifs and their amino acid positions of HLA molecules are known to govern antigen presentation to effector cells. Our objective was to analyse their influence on the risk of graft-versus-host disease (GVHD) and relapse after umbilical cord blood transplant (UCBT). The transplant characteristics of 849 patients with acute leukaemia were obtained from the Eurocord/EBMT database. Higher acute (a) GVHD was associated with homozygosity of UCB HLA-C amino acid positions 77 and 80 (NN/KK) (p = 0.008). Severe aGVHD was associated with HLA-A pocket B YSAVMENVHY motif (p = 0.002) and NN and RR genotypes of the HLA-C amino acid positions 77 and 156 (p = 0.006 and p = 0.002). Such risk was also increased in case of recipient and UCB mismatches in P4 (p < 0.0001) and P9 (p = 0.003) pockets of HLA-DQB1 alleles. For chronic GVHD, the pocket B YYAVMEISNY motif of the HLA-B*15:01 allele and the absence of mismatch between recipient and UCB in the P6 pocket of HLA-DRB1 were associated with a lower risk (p = 0.0007 and p = 0.0004). In relapse, both UCB pocket B YFAVMENVHY belonging to HLA-A*32:01 and recipient pocket B YDSVGENYQY motif of the HLA-C*07:01 allele were associated with higher risk (p = 0.0026 and p = 0.015). We provide clues on HLA-mediated cellular interactions and their role in the development of GVHD and relapse.

EULAR study group on ‘MHC-I-opathy’: identifying disease-overarching mechanisms across disciplines and borders

The ‘MHC-I (major histocompatibility complex class I)-opathy’ concept describes a family of inflammatory conditions with overlapping clinical manifestations and a strong genetic link to the MHC-I antigen presentation pathway. Classical MHC-I-opathies such as spondyloarthritis, Behçet’s disease, psoriasis and birdshot uveitis are widely recognised for their strong association with certain MHC-I alleles and gene variants of the antigen processing aminopeptidases ERAP1 and ERAP2 that implicates altered MHC-I peptide presentation to CD8+T cells in the pathogenesis. Progress in understanding the cause and treatment of these disorders is hampered by patient phenotypic heterogeneity and lack of systematic investigation of the MHC-I pathway.

Here, we discuss new insights into the biology of MHC-I-opathies that strongly advocate for disease-overarching and integrated molecular and clinical investigation to decipher underlying disease mechanisms. Because this requires transformative multidisciplinary collaboration, we introduce the EULAR study group on MHC-I-opathies to unite clinical expertise in rheumatology, dermatology and ophthalmology, with fundamental and translational researchers from multiple disciplines such as immunology, genomics and proteomics, alongside patient partners. We prioritise standardisation of disease phenotypes and scientific nomenclature and propose interdisciplinary genetic and translational studies to exploit emerging therapeutic strategies to understand MHC-I-mediated disease mechanisms. These collaborative efforts are required to address outstanding questions in the etiopathogenesis of MHC-I-opathies towards improving patient treatment and prognostication.

The interplay between HLA-B27 and ERAP1/ERAP2 aminopeptidases: from anti-viral protection to spondyloarthritis

The human leukocyte antigen class I gene HLA-B27 is the strongest risk factor for ankylosing spondylitis (AS), a chronic inflammatory arthritic disorder. More recently, the Endoplasmic Reticulum Aminopeptidase (ERAP) 1 and 2 genes have been identified by genome wide association studies (GWAS) as additional susceptibility factors. In the ER, these aminopeptidases trim the peptides to a length suitable to fit into the groove of the major histocompatibility complex (MHC) class I molecules. It is noteworthy that an epistatic interaction between HLA-B27 and ERAP1, but not between HLA-B27 and ERAP2, has been highlighted. However, these observations suggest a paramount centrality for the HLA-B27 peptide repertoire that determines the natural B27 immunological function, i.e. the T cell antigen presentation and, as a by-product, elicits HLA-B27 aberrant behaviours: (i) the misfolding leading to ER stress responses and autophagy and (ii) the surface expression of homodimers acting as ligands for innate immune receptors. In this context, it has been observed that the HLA-B27 carriers, besides being prone to autoimmunity, display a far better surveillance to some viral infections. This review focuses on the ambivalent role of HLA-B27 in autoimmunity and viral protection correlating its functions to the quantitative and qualitative effects of ERAP1 and ERAP2 polymorphisms on their enzymatic activity.

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.

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.

Selector function of MHC I molecules is determined by protein plasticity

The selection of peptides for presentation at the surface of most nucleated cells by major histocompatibility complex class I molecules (MHC I) is crucial to the immune response in vertebrates. However, the mechanisms of the rapid selection of high affinity peptides by MHC I from amongst thousands of mostly low affinity peptides are not well understood. We developed computational systems models encoding distinct mechanistic hypotheses for two molecules, HLA-B*44:02 (B*4402) and HLA-B*44:05 (B*4405), which differ by a single residue yet lie at opposite ends of the spectrum in their intrinsic ability to select high affinity peptides. We used in vivo biochemical data to infer that a conformational intermediate of MHC I is significant for peptide selection. We used molecular dynamics simulations to show that peptide selector function correlates with protein plasticity, and confirmed this experimentally by altering the plasticity of MHC I with a single point mutation, which altered in vivo selector function in a predictable way. Finally, we investigated the mechanisms by which the co-factor tapasin influences MHC I plasticity. We propose that tapasin modulates MHC I plasticity by dynamically coupling the peptide binding region and α3 domain of MHC I allosterically, resulting in enhanced peptide selector function.

HLA-B27 subtype oligomerization and intracellular accumulation patterns correlate with predisposition to spondyloarthritis

OBJECTIVE

Mechanisms underlying the striking association of spondyloarthritis (SpA) with the class I major histocompatibility complex molecule HLA-B27 remain poorly understood. SpA-like disease develops spontaneously in B*2705-transgenic rats, in conjunction with high HLA-B27 expression levels. This study was undertaken to examine the effects of increased expression of HLA-B27 alleles that are differentially associated with SpA on oligomerization and intracellular redistribution.

METHODS

HeLa cells were transfected with complementary DNA encoding for HLA-B proteins fused to yellow fluorescent protein and/or Renilla luciferase and harvested at an early phase and a later phase of expression. We monitored HLA-B intracellular trafficking and localization by means of microscopy and live-cell imaging. Bioluminescence resonance energy transfer (BRET) and Western blotting were used to monitor HLA-B oligomerization.

RESULTS

At low expression levels, BRET signals were similarly elevated for all SpA-associated HLA-B27 alleles tested, but were lower for the nonassociated B*2706. Of note, at higher expression levels, HLA-B27 signals remained steady while signal for HLA-B7 decreased sharply, reaching the level observed for B*2706. This was due at least in part to a decreased oligomer proportion without unfolded protein response outbreak. Such differential behavior was not abrogated by proteasome inhibition. With increased expression, all HLA-B proteins accumulated to a high density in cytoplasmic vesicles with labile form and size. The extent of this phenomenon was closely correlated with the level of association with predisposition to SpA.

CONCLUSION

To our knowledge, this is the first report of a correlation between the level of predisposition to SpA conferred by HLA-B27 alleles and their biochemical behavior. These findings open new perspectives for understanding the pathogenicity of HLA-B27.

Recognition of distinct cross-reactive virus-specific CD8+ T cells reveals a unique TCR signature in a clinical setting

Human CMV still remains problematic in immunocompromised patients, particularly after solid organ transplantation. CMV primary disease and reactivation greatly increase the risks associated with incidences of chronic allograft rejection and decreased survival in transplant recipients. But whether this is due to direct viral effects, indirect viral effects including cross-reactive antiviral T cell immunopathology, or a combination of both remains undetermined. In this article, we report the novel TCR signature of cross-reactive HLA-A*02:01 (A2) CMV (NLVPMVATV [NLV])-specific CD8(+) T cells recognizing a specific array of HLA-B27 alleles using technical advancements that combine both IFN-γ secretion and multiplex nested RT-PCR for determining paired CDR3α/β sequences from a single cell. This study represents the first evidence, to our knowledge, of the same A2-restricted cross-reactive NLV-specific TCR-α/β signature (TRAV3TRAJ31_TRBV12-4TRBJ1-1) in two genetically distinct individuals. Longitudinal posttransplant monitoring of a lung transplant recipient (A2, CMV seropositive) who received a HLA-B27 bilateral lung allograft showed a dynamic expansion of the cross-reactive NLV-specific TCR repertoire before CMV reactivation. After resolution of the active viral infection, the frequency of cross-reactive NLV-specific CD8(+) T cells reduced to previremia levels, thereby demonstrating immune modulation of the T cell repertoire due to antigenic pressure. The dynamic changes in TCR repertoire, at a time when CMV reactivation was subclinical, illustrates that prospective monitoring in susceptible patients can reveal nuances in immune profiles that may be clinically relevant.

The biochemistry and immunology of non-canonical forms of HLA-B27

HLA-B27 (B27) is strongly associated with the spondyloarthritides. B27 is expressed at the cell surface of antigen presenting cells (APC) both as canonical β2m-associated and non-canonical β2m-free heavy chain (FHC) forms which include B27 dimers (termed B272). B27 FHC forms arise in an endosomal compartment from recycling β2m-associated B27. Formation of cell surface FHC dimers is critically dependent on an unpaired reactive cysteine 67 in the α1 helix of the class I heavy chain. HLA-B27 also form redox-inducible β2m-associated dimers on exosomes and apoptosing cells. By contrast with cell surface expressed cysteine 67-dependent heavy chain dimers these dimers are dependent on a cytoplasmic cysteine 325 for their formation. HLA-B27 binds to immunoregulatory receptors including members of the Killer cell Immunoglobulin-like (KIR) and Leukocyte Immunoglobulin-like receptor family. B27 FHC bind to different but overlapping sets of these immunoreceptors compared to classical β2m-associated HLA-B27. B27 FHC bind more strongly to KIR3DL2 and LILRB2 immune receptor than other β2m-associated HLA-class I ligands. Genetic studies have implicated genes which control production of the important proinflammatory cytokine IL-17 in the pathogenesis of spondyloarthritis. Cell surface HLA-B27 FHC binding to these immune receptors or acting through other mechanisms could impact on the pathogenesis of spondyloarthritis by promoting immune cell production of IL-17. Here we review the literature on these non-canonical forms of HLA-B27 and the immune receptors they bind to and discuss the possible relevance of these interactions to the pathogenesis of spondyloarthropathy.

The arthritis-associated HLA-B*27:05 allele forms more cell surface B27 dimer and free heavy chain ligands for KIR3DL2 than HLA-B*27:09

Objectives. HLA-B*27:05 is associated with AS whereas HLA-B*27:09 is not associated. We hypothesized that different interactions with KIR immune receptors could contribute to the difference in disease association between HLA-B*27:05 and HLAB*27:09. Thus, the objective of this study was to compare the formation of β2m-free heavy chain (FHC) including B27 dimers (B272) by HLA-B*27:05 and HLA-B*27:09 and their binding to KIR immunoreceptors.

Methods. We studied the formation of HLA-B*27:05 and HLA-B*27:09 heterotrimers and FHC forms including dimers in vitro and in transfected cells. We investigated HLA-B*27:05 and HLA-B*27:09 binding to KIR3DL1, KIR3DL2 and LILRB2 by FACS staining with class I tetramers and by quantifying interactions with KIR3DL2CD3ε-reporter cells and KIR3DL2-expressing NK cells. We also measured KIR expression on peripheral blood NK and CD4 T cells from 18 HLA-B*27:05 AS patients, 8 HLA-B27 negative and 12 HLA-B*27:05+ and HLA-B*27:09+ healthy controls by FACS staining.

Results. HLA-B*27:09 formed less B27₂ and FHC than HLA-B*27:05. HLA-B*27:05-expressing cells stimulated KIR3DL2CD3ε-reporter T cells more effectively. Cells expressing HLA-B*27:05 promoted KIR3DL2+ NK cell survival more strongly than HLA-B*27:09. HLA-B*27:05 and HLA-B*27:09 dimer tetramers stained KIR3DL1, KIR3DL2 and LILRB2 equivalently. Increased proportions of NK and CD4 T cells expressed KIR3DL2 in HLA-B*27:05+ AS patients compared with HLA-B*27:05+, HLA-B*27:09+ and HLA-B27− healthy controls.

Conclusion. Differences in the formation of FHC ligands for KIR3DL2 by HLA-B*27:05 and HLA-B*27:09 could contribute to the differential association of these alleles with AS.

Residue 81 confers a restricted C-terminal peptide binding motif in HLA-B*44:09

Knowledge about the magnitude of individual polymorphism is a critical part in understanding the complexity of comprehensive mismatching. HLA-B*44:09 differs from the highly frequent HLA-B*44:02 allele by amino acid exchanges at residues 77, 80, 81, 82 and 83. We aimed to identify the magnitude of these mismatches on the features of HLA-B*44:09 bound peptides since residues 77, 80 and 81 comprise part of the F pocket which determines sequence specificity at the pΩ position of the peptide. Using soluble HLA technology we determined >200 individual (nonduplicate) self-peptides from HLA-B*44:09 and compared their features with that of the published peptide features of HLA-B*44:02. Both alleles illustrate an anchor motif of E at p2. In contrast to the C-terminal peptide binding motif of B*44:02 (W, F, Y or L), B*44:09-derived peptides are restricted predominantly to L or F. The source of peptides for both alleles is identical (LCL 721.221 cells) allowing us to identify 23 shared peptides. The majority of these peptides however contained the restricted B*44:09 anchor motif of F or L at the pΩ position. Molecular modelling based on the B*44:02 structure highlights that the differences of the C-terminal peptide anchor between both alleles can be explained primarily by the B*44:02(81Ala) > B*44:09(81Leu) polymorphism which restricts the size of the amino acid that can be accommodated in the F pocket of B*44:09. These results highlight that every amino acid substitution has an impact of certain magnitude on the alleles function and demonstrate how surrounding residues orchestrate peptide specificity.

The oxidative folding and misfolding of human leukocyte antigen-b27

The major histocompatibility complex class I molecule human leukocyte antigen (HLA)-B27 is strongly associated with a group of inflammatory arthritic disorders known as the spondyloarthropathies. Many autoimmune diseases exhibit associations with major histocompatibility complex molecules encoded within the class II locus with defined immune responses either mediated by T or B-lymphocytes. Despite the association being known for over 30 years, no defined immune response and target autoantigens have been characterized for the spondyloarthropathies. Thus, the mechanism and role of HLA-B27 in disease pathogenesis remains undetermined. One hypothesis that has recently received much attention has focused around the enhanced propensity for HLA-B27 to misfold and the increased tendency of the heavy chain to dimerize. The misfolding of HLA-B27 has been associated with its redox status and this is postulated to be involved in disease development. Here we discuss the impact of the redox status on HLA-B27 biosynthesis and function.

Dimerization of soluble disulfide trap single-chain major histocompatibility complex class I molecules dependent on peptide binding affinity

Stable presentation of peptide epitope by major histocompatibility complex (MHC) class I molecules is a prerequisite for the efficient expansion of CD8(+) T cells. The construction of single-chain MHC class I molecules in which the peptide, β(2)-microglobulin, and MHC heavy chain are all joined together via flexible linkers increases peptide-MHC stability. We have expressed two T cell epitopes that may be useful in leukemia treatment as single-chain MHC class I molecules, aiming to develop a system for the expansion of antigen-specific CD8(+) T cells in vitro. Disulfide trap versions of these single-chain MHC molecules were also created to improve anchoring of the peptides in the MHC molecule. Unexpectedly, we observed that soluble disulfide trap single-chain molecules expressed in eukaryotic cells were prone to homodimerization, depending on the binding affinity of the peptide epitope. The dimers were remarkably stable and efficiently recognized by conformation-specific antibodies, suggesting that they consisted of largely correctly folded molecules. However, dimerization was not observed when the disulfide trap molecules were expressed as full-length, transmembrane-anchored molecules. Our results further emphasize the importance of peptide binding affinity for the efficient folding of MHC class I molecules.

Pathogenicity of Misfolded and Dimeric HLA-B27 Molecules

The association between HLA-B27 and the group of autoimmune inflammatory arthritic diseases, the spondyloarthropathies (SpAs) which include ankylosing spondylitis (AS) and Reactive Arthritis (ReA), has been well established and remains the strongest association between any HLA molecule and autoimmune disease. The mechanism behind this striking association remains elusive; however animal model and biochemical data suggest that HLA-B27 misfolding may be key to understanding its association with the SpAs. Recent investigations have focused on the unusual biochemical structures of HLA-B27 and their potential role in SpA pathogenesis. Here we discuss how these unusual biochemical structures may participate in cellular events leading to chronic inflammation and thus disease progression.

Th17 cells expressing KIR3DL2+ and responsive to HLA-B27 homodimers are increased in ankylosing spondylitis

CD4 Th cells producing the proinflammatory cytokine IL-17 (Th17) have been implicated in a number of inflammatory arthritides including the spondyloarthritides. Th17 development is promoted by IL-23. Ankylosing spondylitis, the most common spondyloarthritis (SpA), is genetically associated with both HLA-B27 (B27) and IL-23R polymorphisms; however, the link remains unexplained. We have previously shown that B27 can form H chain dimers (termed B27(2)), which, unlike classical HLA-B27, bind the killer-cell Ig-like receptor KIR3DL2. In this article, we show that B27(2)-expressing APCs stimulate the survival, proliferation, and IL-17 production of KIR3DL2(+) CD4 T cells. KIR3DL2(+) CD4 T cells are expanded and enriched for IL-17 production in the blood and synovial fluid of patients with SpA. Despite KIR3DL2(+) cells comprising a mean of just 15% of CD4 T in the peripheral blood of SpA patients, this subset accounted for 70% of the observed increase in Th17 numbers in SpA patients compared with control subjects. TCR-stimulated peripheral blood KIR3DL2(+) CD4 T cell lines from SpA patients secreted 4-fold more IL-17 than KIR3DL2(+) lines from controls or KIR3DL2(-) CD4 T cells. Strikingly, KIR3DL2(+) CD4 T cells account for the majority of peripheral blood CD4 T cell IL-23R expression and produce more IL-17 in the presence of IL-23. Our findings link HLA-B27 with IL-17 production and suggest new therapeutic strategies in ankylosing spondylitis/SpA.

Mutational analysis reveals a complex interplay of peptide binding and multiple biological features of HLA-B27

Molecular polymorphism influences the strong association of HLA-B27 with ankylosing spondylitis through an unknown mechanism. Natural subtypes and site-directed mutants were used to analyze the effect of altering the peptide-binding site of this molecule on its stability, interaction with tapasin, folding, and export. The disease-associated subtypes B*2705, B*2702, and B*2704 showed higher thermostability at 50 °C than all other subtypes and mutants, except some mimicking B*2702 polymorphism. The lowest values were found among pocket B mutants, most of which interacted strongly with tapasin, but otherwise there was no correlation between thermostability and tapasin interaction. Mutants resulting in increased hydrophobicity frequently acquired their maximal thermostability faster than those with increased polarity, suggesting that this process is largely driven by the thermodynamics of peptide binding. Folding, export, and tendency to misfold were influenced by polymorphism all along the peptide-binding site and were not specifically dependent on any particular region or structural feature. Frequent uncoupling of thermostability, folding/misfolding, and export can be explained by the distinct effect of mutations on the acquisition of a folded conformation, the optimization rate of B27-peptide complexes, and their quality control in the endoplasmic reticulum, all of which largely depend on the ways in which mutations alter peptide binding, without excluding additional effects on interactions with tapasin or other proteins involved in folding and export. The similarity of the generally disease-associated B*2707 to nondisease-associated subtypes in all the features analyzed suggests that molecular properties other than antigen presentation may not currently explain the relationship between HLA-B27 polymorphism and ankylosing spondylitis.