Thermodynamic and Structural Analysis of Peptide- and Allele-dependent Properties of Two HLA-B27 Subtypes Exhibiting Differential Disease Association

Pathophysiology and immunolgical basis of axial spondyloarthritis

Over the recent years the wider availability and application of state-of-the-art immunological technologies greatly advanced the insight into the mechanisms that play an important role in axial spondyloarthritis (axSpA) pathophysiology. This increased understanding has facilitated the development of novel treatments that target disease relevant pathways, hereby improving outcome for axSpA patients. In axSpA pathophysiology genetic and environmental factors as well as immune activation by mechanical or bacterial stress resulting in a chronic inflammatory response have a central role. The TNF and IL-23/IL-17 immune pathways play a pivotal role in these disease mechanisms. This review provides an outline of the immunological basis of axSpA with a focus on key genetic risk factors and their link to activation of the pathological immune response, as well as on the role of the gut and entheses in the initiation of inflammation with subsequent new bone formation in axSpA.

Intrinsic Folding Properties of the HLA-B27 Heavy Chain Revealed by Single Chain Trimer Versions of Peptide-Loaded Class I Major Histocompatibility Complex Molecules

Peptide-loaded Major Histocompatibility Complex (pMHC) class I molecules can be expressed in a single chain trimeric (SCT) format, composed of a specific peptide fused to the light chain beta-2 microglobulin (β2m) and MHC class I heavy chain (HC) by flexible linker peptides. pMHC SCTs have been used as effective molecular tools to investigate cellular immunity and represent a promising vaccine platform technology, due to their intracellular folding and assembly which is apparently independent of host cell folding pathways and chaperones. However, certain MHC class I HC molecules, such as the Human Leukocyte Antigen B27 (HLA-B27) allele, present a challenge due to their tendency to form HC aggregates. We constructed a series of single chain trimeric molecules to determine the behaviour of the HLA-B27 HC in a scenario that usually allows for efficient MHC class I molecule folding. When stably expressed, a pMHC SCT incorporating HLA-B27 HC formed chaperone-bound homodimers within the endoplasmic reticulum (ER). A series of HLA-B27 SCT substitution mutations revealed that the F pocket and antigen binding groove regions of the HLA-B27 HC defined the folding and dimerisation of the single chain complex, independently of the peptide sequence. Furthermore, pMHC SCTs can demonstrate variability in their association with the intracellular antigen processing machinery.

Epitope length variants balance protective immune responses and viral escape in HIV-1 infection

Cytotoxic T lymphocyte (CTL) and natural killer (NK) cell responses to a single optimal 10-mer epitope (KK10) in the human immunodeficiency virus type-1 (HIV-1) protein p24Gag are associated with enhanced immune control in patients expressing human leukocyte antigen (HLA)-B∗27:05. We find that proteasomal activity generates multiple length variants of KK10 (4-14 amino acids), which bind TAP and HLA-B∗27:05. However, only epitope forms ≥8 amino acids evoke peptide length-specific and cross-reactive CTL responses. Structural analyses reveal that all epitope forms bind HLA-B∗27:05 via a conserved N-terminal motif, and competition experiments show that the truncated epitope forms outcompete immunogenic epitope forms for binding to HLA-B∗27:05. Common viral escape mutations abolish (L136M) or impair (R132K) production of KK10 and longer epitope forms. Peptide length influences how well the inhibitory NK cell receptor KIR3DL1 binds HLA-B∗27:05 peptide complexes and how intraepitope mutations affect this interaction. These results identify a viral escape mechanism from CTL and NK responses based on differential antigen processing and peptide competition.

Dynamic allostery controls the peptide sensitivity of the Ly49C natural killer receptor

Using a variety of activating and inhibitory receptors, natural killer (NK) cells protect against disease by eliminating cells that have downregulated class I major histocompatibility complex (MHC) proteins, such as in response to cell transformation or viral infection. The inhibitory murine NK receptor Ly49C specifically recognizes the class I MHC protein H-2K^b. Unusual among NK receptors, Ly49C exhibits a peptide-dependent sensitivity to H-2K^b recognition, which has not been explained despite detailed structural studies. To gain further insight into Ly49C peptide sensitivity, we examined Ly49C recognition biochemically and through the lens of dynamic allostery. We found that the peptide sensitivity of Ly49C arises through small differences in H-2K^b-binding affinity. Although molecular dynamics simulations supported a role for peptide-dependent protein dynamics in producing these differences in binding affinity, calorimetric measurements indicated an enthalpically as opposed to entropically driven process. A quantitative linkage analysis showed that this emerges from peptide-dependent dynamic tuning of electrostatic interactions across the Ly49C–H-2K^b interface. We propose a model whereby different peptides alter the flexibility of H-2K^b, which in turn changes the strength of electrostatic interactions across the protein–protein interface. Our results provide a quantitative assessment of how peptides alter Ly49C-binding affinity, suggest the underlying mechanism, and demonstrate peptide-driven allostery at work in class I MHC proteins. Lastly, our model provides a solution for how dynamic allostery could impact binding of some, but not all, class I MHC partners depending on the structural and chemical composition of the interfaces.

Conformational Plasticity of HLA-B27 Molecules Correlates Inversely With Efficiency of Negative T Cell Selection

The development of autoimmune disorders is incompletely understood. Inefficient thymic T cell selection against self-peptides presented by major histocompatibility antigens (HLA in humans) may contribute to the emergence of auto-reactive effector cells, and molecular mimicry between foreign and self-peptides could promote T cell cross-reactivity. A pair of class I subtypes, HLA-B2705 and HLA-B2709, have previously been intensely studied, because they are distinguished from each other only by a single amino acid exchange at the floor of the peptide-binding groove, yet are differentially associated with the autoinflammatory disorder ankylosing spondylitis. Using X-ray crystallography in combination with ensemble refinement, we find that the non-disease-associated subtype HLA-B2709, when presenting the self-peptide pGR (RRRWHRWRL), exhibits elevated conformational dynamics, and the complex can also be recognized by T cells. Both features are not observed in case of the sequence-related self-peptide pVIPR (RRKWRRWHL) in complex with this subtype, and T cell cross-reactivity between pGR, pVIPR, and the viral peptide pLMP2 (RRRWRRLTV) is only rarely observed. The disease-associated subtype HLA-B2705, however, exhibits extensive conformational flexibility in case of the three complexes, all of which are also recognized by frequently occurring cross-reactive T cells. A comparison of the structural and dynamic properties of the six HLA-B27 complexes, together with their individual ability to interact with T cells, permits us to correlate the flexibility of HLA-B27 complexes with effector cell reactivity. The results suggest the existence of an inverse relationship between conformational plasticity of peptide-HLA-B27 complexes and the efficiency of negative selection of self-reactive cells within the thymus.

How ERAP1 and ERAP2 Shape the Peptidomes of Disease-Associated MHC-I Proteins

Four inflammatory diseases are strongly associated with Major Histocompatibility Complex class I (MHC-I) molecules: birdshot chorioretinopathy (HLA-A^*29:02), ankylosing spondylitis (HLA-B^*27), Behçet's disease (HLA-B^*51), and psoriasis (HLA-C^*06:02). The endoplasmic reticulum aminopeptidases (ERAP) 1 and 2 are also risk factors for these diseases. Since both enzymes are involved in the final processing steps of MHC-I ligands it is reasonable to assume that MHC-I-bound peptides play a significant pathogenetic role. This review will mainly focus on recent studies concerning the effects of ERAP1 and ERAP2 polymorphism and expression on shaping the peptidome of disease-associated MHC-I molecules in live cells. These studies will be discussed in the context of the distinct mechanisms and substrate preferences of both enzymes, their different patterns of genetic association with various diseases, the role of polymorphisms determining changes in enzymatic activity or expression levels, and the distinct peptidomes of disease-associated MHC-I allotypes. ERAP1 and ERAP2 polymorphism and expression induce significant changes in multiple MHC-I-bound peptidomes. These changes are MHC allotype-specific and, without excluding a degree of functional inter-dependence between both enzymes, reflect largely separate roles in their processing of MHC-I ligands. The studies reviewed here provide a molecular basis for the distinct patterns of genetic association of ERAP1 and ERAP2 with disease and for the pathogenetic role of peptides. The allotype-dependent alterations induced on distinct peptidomes may explain that the joint association of both enzymes and unrelated MHC-I alleles influence different pathological outcomes.

Polymorphisms in the F Pocket of HLA-B27 Subtypes Strongly Affect Assembly, Chaperone Interactions, and Heavy-Chain Misfolding

OBJECTIVE

HLA-B27 is associated with the inflammatory spondyloarthritides (SpA), although subtypes HLA-B*27:06 and HLA-B*27:09 are not. These subtypes differ from the HLA-B*27:05 disease-associated allele primarily at residues 114 and 116 of the heavy chain, part of the F pocket of the antigen-binding groove. Dimerization of HLA-B27 during assembly has been implicated in disease onset. The purpose of this study was to investigate the factors that influence differences in dimerization between disease-associated and non-disease-associated HLA-B27 alleles.

METHODS

HLA-B*27:05 and mutants resembling the HLA-B*27:06 and 09 subtypes were expressed in the rat C58 T cell line, the human CEM T cell line and its calnexin-deficient variant CEM.NKR. Immunoprecipitation, pulse-chase experiments, flow cytometry, and immunoblotting were performed to study the assembly kinetics, heavy-chain dimerization, and chaperone associations.

RESULTS

By expressing HLA-B*27:05, 06-like, and 09 alleles on a restrictive rat transporter associated with antigen processing background, we demonstrate that a tyrosine expressed at p116, either alone or together with an aspartic acid residue at p114, inhibited HLA-B27 dimerization and increased the assembly rate. F-pocket residues altered the associations with chaperones of the early major histocompatibility complex class I folding pathway. Calnexin was demonstrated to participate in endoplasmic reticulum (ER) stress-mediated degradation of dimers, whereas the oxidoreductase ERp57 does not appear to influence dimerization.

CONCLUSION

Residues within the F pocket of the peptide-binding groove, which differ between disease-associated and non-disease-associated HLA-B27 subtypes, can influence the assembly process and heavy-chain dimerization, events which have been linked to the initiation of disease pathogenesis.

Exploration of the Conformational Dynamics of Major Histocompatibility Complex Molecules

Major histocompatibility complex (MHC) molecules are loaded with a wide variety of self- and non-self-peptides in their binding grooves and present these to T cell receptors (TCRs) in order to activate the adaptive immune system. A large number of crystal structures of different MHC alleles with different bound peptides have been determined, and they have been found to be quite similar to one another regardless of the bound peptide sequence. The structures do not change markedly even when forming complexes with TCRs. Nonetheless, the degree of TCR activation does differ markedly depending on the peptide presented by the MHC. Recent structural studies in solution rather than as crystals have suggested that the conformational dynamics of MHC molecules may be responsible for the MHC stability differences. Furthermore, it was shown that the conformational dynamics of MHC molecules is important for peptide loading and presentation to TCR. Here, we describe the static and dynamic structures of MHC molecules and appropriate methods to analyze them. We focus particularly on nuclear magnetic resonance (NMR), one of the most powerful tools to study dynamic properties of proteins. The number of such studies in the literature is limited, but in this review, we show that NMR is valuable for elucidating the structural dynamics of MHC molecules.

Separate effects of the ankylosing spondylitis associated ERAP1 and ERAP2 aminopeptidases determine the influence of their combined phenotype on the HLA-B*27 peptidome

Ankylosing spondylitis (AS) is an inflammatory disease strongly associated with the Major Histocompatibility Complex class I (MHC-I) allotype HLA-B*27. The endoplasmic reticulum aminopeptidases (ERAP)1 and 2, which trim peptides to their optimal length for MHC-I binding, are also susceptibility factors for this disease. Both highly active ERAP1 variants and ERAP2 expression favor AS, whereas loss-of-function ERAP1 and loss-of-expression ERAP2 variants are protective. Yet, only ERAP1 is in epistasis with HLA-B*27. We addressed two issues concerning the functional interaction of ERAP1 and ERAP2 with the HLA-B*27 peptidome in human cells: 1) distinguishing the effects of ERAP1 from those of ERAP2, and 2) determining the influence of ERAP2 in distinct ERAP1 contexts. Quantitative comparisons of the HLA-B*27:05 peptidomes from cells with various ERAP1/ERAP2 phenotypes were carried out. When cells expressing ERAP2 and either high or low activity ERAP1 variants were compared, increased amounts of nonamers, relative to longer ligands, and decreased amounts of peptides with Ala1, were observed in the more active ERAP1 context. When cells expressing ERAP2 in a low activity ERAP1 context or lacking ERAP2 but expressing a highly active ERAP1 variant were compared, the same effects on peptide length and Ala1, but also significantly lower amounts of peptides with N-terminal basic residues and lower affinity of the peptidome, were observed in the ERAP2-positive context. Thus, ERAP1 and ERAP2 have significant and distinct effects on the HLA-B*27 peptidome, suggesting that both enzymes largely act as separate entities in vivo. This may explain their different patterns of association with AS.

Peptide-dependent conformational fluctuation determines the stability of the human leukocyte antigen class I complex

In immune-mediated control of pathogens, human leukocyte antigen (HLA) class I presents various antigenic peptides to CD8(+) T-cells. Long-lived peptide presentation is important for efficient antigen-specific T-cell activation. Presentation time depends on the peptide sequence and the stability of the peptide-HLA complex (pHLA). However, the determinant of peptide-dependent pHLA stability remains elusive. Here, to reveal the pHLA stabilization mechanism, we examined the crystal structures of an HLA class I allomorph in complex with HIV-derived peptides and evaluated site-specific conformational fluctuations using NMR. Although the crystal structures of various pHLAs were almost identical independent of the peptides, fluctuation analyses identified a peptide-dependent minor state that would be more tightly packed toward the peptide. The minor population correlated well with the thermostability and cell surface presentation of pHLA, indicating that this newly identified minor state is important for stabilizing the pHLA and facilitating T-cell recognition.

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.

Comparative biophysical characterization of chicken β2-microglobulin

β(2)-microglobulin (β(2)m) is the smallest building block of molecules belonging to the immunoglobulin superfamily. By comparing thermodynamic and structural characteristics of chicken β(2)m with those of other species, we seek to elucidate whether it is possible to pinpoint features that set the avian protein apart from other β(2)m. The thermodynamic assays revealed that chicken β(2)m exhibits a lower melting temperature than human β(2)m, and the H/D exchange behavior observed by infrared spectroscopy indicates a more flexible structure of the former protein. To understand these differences at a molecular level, we determined the structure of free chicken β(2)m by X-ray crystallography to a resolution of 2.0 Å. Our comparisons indicate that certain biophysical characteristics of the chicken protein, particularly its conformational flexibility, diverge considerably from those of the other β(2)m analyzed, although basic structural features have been retained through evolution.

Interaction pattern of Arg 62 in the A-pocket of differentially disease-associated HLA-B27 subtypes suggests distinct TCR binding modes

The single amino acid replacement Asp116His distinguishes the two subtypes HLA-B*2705 and HLA-B*2709 which are, respectively, associated and non-associated with Ankylosing Spondylitis, an autoimmune chronic inflammatory disease. The reason for this differential association is so far poorly understood and might be related to subtype-specific HLA:peptide conformations as well as to subtype/peptide-dependent dynamical properties on the nanoscale. Here, we combine functional experiments with extensive molecular dynamics simulations to investigate the molecular dynamics and function of the conserved Arg62 of the α1-helix for both B27 subtypes in complex with the self-peptides pVIPR (RRKWRRWHL) and TIS (RRLPIFSRL), and the viral peptides pLMP2 (RRRWRRLTV) and NPflu (SRYWAIRTR). Simulations of HLA:peptide systems suggest that peptide-stabilizing interactions of the Arg62 residue observed in crystal structures are metastable for both B27 subtypes under physiological conditions, rendering this arginine solvent-exposed and, probably, a key residue for TCR interaction more than peptide-binding. This view is supported by functional experiments with conservative (R62K) and non-conservative (R62A) B*2705 and B*2709 mutants that showed an overall reduction in their capability to present peptides to CD8+ T cells. Moreover, major subtype-dependent differences in the peptide recognition suggest distinct TCR binding modes for the B*2705 versus the B*2709 subtype.

HLA class I-associated diseases with a suspected autoimmune etiology: HLA-B27 subtypes as a model system

Although most autoimmune diseases are connected to major histocompatibility complex (MHC) class II alleles, a small number of these disorders exhibit a variable degree of association with selected MHC class I genes, like certain human HLA-A and HLA-B alleles. The basis for these associations, however, has so far remained elusive. An understanding might be obtained by comparing functional, biochemical, and biophysical properties of alleles that are minimally distinct from each other, but are nevertheless differentially associated to a given disease, like the HLA-B*27:05 and HLA-B*27:09 antigens, which differ only by a single amino acid residue (Asp116His) that is deeply buried within the binding groove. We have employed a number of approaches, including X-ray crystallography and isotope-edited infrared spectroscopy, to investigate biophysical characteristics of the two HLA-B27 subtypes complexed with up to ten different peptides. Our findings demonstrate that the binding of these peptides as well as the conformational flexibility of the subtypes is greatly influenced by interactions of the C-terminal peptide residue. In particular, a basic C-terminal peptide residue is favoured by the disease-associated subtype HLA-B*27:05, but not by HLA-B*27:09. This property appears also as the only common denominator of distinct HLA class I alleles, among them HLA-B*27:05, HLA-A*03:01 or HLA-A*11:01, that are associated with diseases suspected to have an autoimmune etiology. We postulate here that the products of these alleles, due to their unusual ability to bind with high affinity to a particular peptide set during positive T cell selection in the thymus, are involved in shaping an abnormal T cell repertoire which predisposes to the acquisition of autoimmune diseases.

Loss of recognition by cross-reactive T cells and its relation to a C-terminus-induced conformational reorientation of an HLA-B*2705-bound peptide

The human major histocompatibility complex class I antigen HLA-B*2705 binds several sequence-related peptides (pVIPR, RRKWRRWHL; pLPM2, RRRWRRLTV; pGR, RRRWHRWRL). Cross-reactivity of cytotoxic T cells (CTL) against these HLA-B*2705:peptide complexes seemed to depend on a particular peptide conformation that is facilitated by the engagement of a crucial residue within the binding groove (Asp116), associated with a noncanonical bulging-in of the middle portion of the bound peptide. We were interested whether a conformational reorientation of the ligand might contribute to the lack of cross-reactivity of these CTL with a peptide derived from voltage-dependent calcium channel α1 subunit (pCAC, SRRWRRWNR), in which the C-terminal peptide residue pArg9 could engage Asp116. Analyses of the HLA-B*2705:pCAC complex by X-ray crystallography at 1.94 Å resolution demonstrated that the peptide had indeed undergone a drastic reorientation, leading it to adopt a canonical binding mode accompanied by the loss of molecular mimicry between pCAC and sequence-related peptides such as pVIPR, pLMP2, and pGR. This was clearly a consequence of interactions of pArg9 with Asp116 and other F-pocket residues. Furthermore, we observed an unprecedented reorientation of several additional residues of the HLA-B*2705 heavy chain near the N-terminal region of the peptide, including also the presence of double conformations of two glutamate residues, Glu63 and Glu163, on opposing sides of the peptide binding groove. Together with the Arg-Ser exchange at peptide position 1, there are thus multiple structural reasons that may explain the observed failure of pVIPR-directed, HLA-B*2705-restricted CTL to cross-react with HLA-B*2705:pCAC complexes.

Structure of a classical MHC class I molecule that binds "non-classical" ligands

The chicken MHC YF1*7.1 X-ray structures reveal that this protein binds lipids and thus represents a "hybrid" class I complex with features of classical as well as non-classical MHC molecules.

Chicken YF1 genes share a close sequence relationship with classical MHC class I loci but map outside of the core MHC region. To obtain insights into their function, we determined the structure of the YF1*7.1/β₂-microgloblin complex by X-ray crystallography at 1.3 Å resolution. It exhibits the architecture typical of classical MHC class I molecules but possesses a hydrophobic binding groove that contains a non-peptidic ligand. This finding prompted us to reconstitute YF1*7.1 also with various self-lipids. Seven additional YF1*7.1 structures were solved, but only polyethyleneglycol molecules could be modeled into the electron density within the binding groove. However, an assessment of YF1*7.1 by native isoelectric focusing indicated that the molecules were also able to bind nonself-lipids. The ability of YF1*7.1 to interact with hydrophobic ligands is unprecedented among classical MHC class I proteins and might aid the chicken immune system to recognize a diverse ligand repertoire with a minimal number of MHC class I molecules.

Proteins encoded by the major histocompatibility complex (MHC) play crucial roles in vertebrate immune systems, presenting pathogen-derived protein fragments to receptors on effector cells. In contrast, some non-classical MHC class I proteins such as CD1 molecules possess a hydrophobic groove that allows them to display lipids. Chicken MHC-Y is a genetic region outside the core MHC that harbors several immune-related genes, among them YF1*7.1, which encodes a protein whose structure we solved in this study. YF1*7.1 is an MHC class I molecule that exhibits the architecture typical of classical MHC class I antigens but possesses a hydrophobic binding groove that binds non-peptidic ligands. By using lipid-binding assays, we show that this molecule can indeed bind lipids. Therefore, YF1*7.1 bridges, at least in structural terms, the traditional gap between classical and non-classical MHC class I molecules. Lipid-binding YF1 proteins might serve the chicken to enlarge its otherwise very small repertoire of antigen-presenting MHC class I molecules. Furthermore, comparative analyses of the two protein subunits of classical MHC molecules revealed a structural feature in chickens that appears to be shared by non-mammalian but not by mammalian vertebrates. This unique feature is indicative of a structure-dependent co-evolution of two genetically unlinked genes in non-mammalian species.

HLA-B27 and ankylosing spondylitis: tales from China

The two most frequent HLA-B27 subtypes worldwide are B*2704 and B*2705. In the Han population of China B*2704 and, to a lower extent, B*2705 are found with significant frequency, and both are associated to ankylosing spondylitis (AS). Two articles in this issue report that the association to AS in this ethnic group is stronger for B*2704 than for B*2705. Thus, at least among the Han, B*2704 would be the strongest known susceptibility factor for AS.

Dynamic interplay between viral adaptation and immune recognition during HIV-1 infection

Untreated human immunodeficiency virus (HIV) infections usually lead to death from AIDS, although the rate of the disease progression varies widely among individuals. The cytotoxic T lymphocyte (CTL) response, which is restricted by highly polymorphic MHC class I alleles, plays a central role in controlling HIV replication. It is now recognized that the antiviral efficacy of CTLs at the single cell level is dependent on their antigen specificity and is important in determining the quality of host response to viruses so that the individual will remain asymptomatic. However, because of the extreme mutational plasticity of HIV, HIV-specific CTL responses are continuously and dynamically changing. In order to rationally design an effective vaccine, the questions as to what constitutes an effective antiviral CTL response and what characterizes a potent antigenic peptide to induce such responses are becoming highlighted as needing to be answered.

CD8+ T-cell mediated self-reactivity in HLA-B27 context as a consequence of dual peptide conformation

HLA-B2709 does not predispose for Ankylosing Spondylitis although it differs from B2705, the most common and AS-associated subtype in different ethnic groups, only for the substitution His116Asp. Therefore, a productive approach to elucidate the molecular mechanisms of the disease could be the comparison of these alleles. B2705 has been shown to display certain self-peptides enriched in basic residues i.e., pVIPR and pGR, in a dual conformation and this is accompanied by the presence of specific cytotoxic T cells in patients with AS. In this study, we convalidate our previous observation that B2709 healthy subjects do not possess primary reactivity towards pVIPR while showing a prompt CD8+ T cell response driven by pGR. Notably, in the B2709 context of presentation, pVIPR assumes only a single conformation in contrast with pGR which is dimorphic. These results suggest a possible general connection between the occurrence of double peptide conformation and the property of inducing specific autoimmune responses.

Structural basis for T cell alloreactivity among three HLA-B14 and HLA-B27 antigens

The existence of cytotoxic T cells (CTL) cross-reacting with the human major histocompatibility antigens HLA-B14 and HLA-B27 suggests that their alloreactivity could be due to presentation of shared peptides in similar binding modes by these molecules. We therefore determined the crystal structures of the subtypes HLA-B*1402, HLA-B*2705, and HLA-B*2709 in complex with a proven self-ligand, pCatA (peptide with the sequence IRAAPPPLF derived from cathepsin A (residues 2-10)), and of HLA-B*1402 in complex with a viral peptide, pLMP2 (RRRWRRLTV, derived from latent membrane protein 2 (residues 236-244) of Epstein-Barr virus). Despite the exchange of 18 residues within the binding grooves of HLA-B*1402 and HLA-B*2705 or HLA-B*2709, the pCatA peptide is presented in nearly identical conformations. However, pLMP2 is displayed by HLA-B*1402 in a conformation distinct from those previously found in the two HLA-B27 subtypes. In addition, the complexes of HLA-B*1402 with the two peptides reveal a nonstandard, tetragonal mode of the peptide N terminus anchoring in the binding groove because of the exchange of the common Tyr-171 by His-171 of the HLA-B*1402 heavy chain. This exchange appears also responsible for reduced stability of HLA-B14-peptide complexes in vivo and slow assembly in vitro. The studies with the pCatA peptide uncover that CTL cross-reactive between HLA-B14 and HLA-B27 might primarily recognize the common structural features of the bound peptide, thus neglecting amino acid replacements within the rim of the binding grooves. In contrast, structural alterations between the three complexes with the pLMP2 peptide indicate how heavy chain polymorphisms can influence peptide display and prevent CTL cross-reactivity between HLA-B14 and HLA-B27 antigens.

HLA-B27-bound peptide repertoires: their nature, origin and pathogenetic relevance

Peptide binding is a central biological property of HLA-B27. The availability of HLA-B27 subtypes differentially associated to ankylosing spondylitis provides a unique tool to explore the relationship between peptide specificity and pathogenetic potential. Many studies have focused on defining the nature of subtype-bound repertoires, aiming to identify peptide features that may correlate with association to disease and to find constitutive self-ligands with sequence homology to microbial epitopes. These studies were pursued on the assumption that molecular mimicry between self and foreign ligands of HLA-B27 might trigger autoimmunity. A second level of involvement ofpeptide repertoires in the biology and immunopathology of HLA-B27 is through their critical influence on folding, maturation and cell surface expression and stability. Recent studies have emphasized the mechanisms ofpeptide loading and optimization, the interactions ofHLA-B27 with other components of the peptide-loading complex and the contribution of these interactions to shaping HLA-B27-bound peptide repertoires. A novel, more comprehensive and integrative, view is emerging in which the peptide binding specificity is a critical determinant of the whole HLA-B27 biology. A proper understanding of the relationships between peptide specificity and other molecular and functional features of HLA-B27 should provide the key to unveiling its pathogenetic role in spondyloarthritis.

T-cell tolerance for variability in an HLA class I-presented influenza A virus epitope

To escape immune recognition, viruses acquire amino acid substitutions in class I human leukocyte antigen (HLA)-presented cytotoxic T-lymphocyte (CTL) epitopes. Such viral escape mutations may (i) prevent peptide processing, (ii) diminish class I HLA binding, or (iii) alter T-cell recognition. Because residues 418 to 426 of the hypervariable influenza A virus nucleoprotein (NP(418-426)) epitope are consistently bound by class I HLA and presented to CTL, we assessed the impact that intraepitope sequence variability has upon T-cell recognition. CTL elicited by intranasal influenza virus infection were tested for their cross-recognition of 20 natural NP(418-426) epitope variants. Six of the variant epitopes, of both H1N1 and H3N2 origin, were cross-recognized by CTL while the remaining NP(418-426) epitope variants escaped targeting. A pattern emerged whereby variability at position 5 (P5) within the epitope reduced T-cell recognition, changes at P4 or P6 enabled CTL escape, and a mutation at P8 enhanced T-cell recognition. These data demonstrate that substitutions at P4 and/or P6 facilitate influenza virus escape from T-cell recognition and provide a model for the number, nature, and location of viral mutations that influence T-cell cross-recognition.

Impact of intrinsic cooperative thermodynamics of peptide-MHC complexes on antiviral activity of HIV-specific CTL

The antiviral activity of HIV-specific CTL is not equally potent but rather is dependent on their specificity. But what characteristic of targeted peptides influences CTL antiviral activity remains elusive. We addressed this issue based on HLA-B35-restricted CTLs specific for two overlapping immunodominant Nef epitopes, VY8 (VPLRPMTY) and RY11 (RPQVPLRPMTY). VY8-specific CTLs were more potently cytotoxic toward HIV-infected primary CD4(+) cells than RY11-specific CTLs. Reconstruction of their TCR revealed no substantial difference in their functional avidity toward cognate Ags. Instead, the decay analysis of the peptide-MHC complex (pMHC) revealed that the VY8/HLA-B35 complex could maintain its capacity to sensitize T cells much longer than its RY11 counterpart. Corroboratively, the introduction of a mutation in the epitopes that substantially delayed pMHC decay rendered Nef-expressing target cells more susceptible to CTL killing. Moreover, by using differential scanning calorimetry and circular dichroism analyses, we found that the susceptible pMHC ligands for CTL killing showed interdependent and cooperative, rather than separate or sequential, transitions within their heterotrimer components under the thermally induced unfolding process. Collectively, our results highlight the significant effects of intrinsic peptide factors that support cooperative thermodynamics within pMHC on the efficient CTL killing of HIV-infected cells, thus providing us better insight into vaccine design.

Low-affinity peptides and T-cell selection

The dual requirement for T cells to recognize a particular major histocompatibility complex (MHC) antigen presenting a foreign peptide and to lack strong reactivity with a complex of the same molecule when bound to a self-peptide, is attained by thymic positive and negative selection processes, the molecular details of which are currently only partially understood. However, the discovery of the thymoproteasome and our improved understanding of the dynamics of peptide presentation permit us to suggest that the biophysical properties of the MHC:peptide class I complexes engaged in positive T-cell selection will be distinct from those involved in negative selection, hence imposing differential barriers for T cells.

Subtypes of HLA-B27: history and implications in the pathogenesis of ankylosing spondylitis

HLA-B27 represents a family of 38 closely related cell surface proteins (encoded by the alleles HLA-B*2701-39) called subtypes of HLA-B27, most of which have evolved from the ubiquitous HLA-B*2705 (specifically the B*27052 allele). HLA-B27 subtypes are largely characterized by nucleotide substitutions (mostly nonsynonymous) in exons 2 and 3 which encode alpha1 and alpha2 domains ofthe peptide binding groove respectively. Table 1 shows the description of sequences of HLA-B27 allele sequences. The subtypes could have arisen from B*2705 by point mutation (B*2703, B*2709, B*2704), gene conversion (B*2701, B*2702, B*2708) and reciprocal recombination (B*2707) B*2706 could have arisen by interlocus gene conversion. Studies from different parts of the world reveal differences in the population distribution.

Peptides: the cornerstone of HLA-B27 biology and pathogenetic role in spondyloarthritis

The association of human leukocyte antigen (HLA)-B27 to ankylosing spondylitis is one of the strongest between a major histocompatibility complex molecule and a disease. Yet, the basis for this association remains unknown. Several hypotheses, each based on a particular feature of HLA-B27, guide much of the current research on the pathogenesis of this disease, but none has yet satisfactorily explained its mechanism and the differential association of B27 subtypes to it. In this review, the pathogenetic role of HLA-B27 will be analyzed from a global perspective of its biology, emphasizing the interdependency of multiple molecular features and the likely influence of disease-modifying gene products. From this perspective, peptide binding emerges as the cornerstone of all other biological properties.

Citrullination-dependent differential presentation of a self-peptide by HLA-B27 subtypes

Inflammatory processes are accompanied by the posttranslational modification of certain arginine residues within proteins to yield citrulline, although it is largely unknown how this modification influences antigen presentation. We employed crystallographic and functional studies to investigate whether the exchange of arginine to citrulline affects the display of a peptide by two human major histocompatibility antigen class I subtypes, HLA-B(*)2705 and HLA-B(*)2709. Both differ only in residue 116 within the peptide binding groove despite their differential association with ankylosing spondylitis, an inflammatory rheumatic disorder. The crystal structures described here show that a modified self-peptide, pVIPR-U5 (RRKWURWHL; U = citrulline), is presented by the two HLA-B27 molecules in distinct conformations. These binding modes differ not only drastically from each other but also from the conformations exhibited by the non-citrullinated peptide in a given subtype. The differential reactivity of HLA-B27-restricted cytotoxic T cells with modified or unmodified pVIPR supports the structural findings and shows that the presentation of citrullinated peptides has the potential to influence immune responses.

Molecular determinants of major histocompatibility complex class I complex stability: shaping antigenic features through short and long range electrostatic interactions

A single amino acid exchange between the major histocompatibility complex molecules HLA-B(*)2705 and HLA-B(*)2709 (Asp-116/His) is responsible for the emergence of distinct HLA-B27-restricted T cell repertoires in individuals harboring either of these two subtypes and could correlate with their differential association with the autoimmune disease ankylosing spondylitis. By using fluorescence depolarization and pK(a) calculations, we investigated to what extent electrostatic interactions contribute to shape antigenic differences between these HLA molecules complexed with viral, self, and non-natural peptide ligands. In addition to the established main anchor of peptides binding to HLA-B27, arginine at position 2 (pArg-2), and the secondary anchors at the peptide termini, at least two further determinants contribute to stable peptide accommodation. 1) The interaction of Asp-116 with arginine at peptide position 5, as found in pLMP2 (RRRWRRLTV; viral) and pVIPR (RRKWRRWHL; self), and with lysine in pOmega, as found in gag (KRWIILGLNK; viral), additionally stabilizes the B(*)2705 complexes by approximately 5 and approximately 27 kJ/mol, respectively, in comparison with B(*)2709. 2) The protonation state of the key residues Glu-45 and Glu-63 in the B-pocket, which accommodates pArg-2, affects peptide binding strength in a peptide- and subtype-dependent manner. In B(*)2705/pLMP2, protonation of Glu-45/Glu-63 reduces the interaction energy of pArg-2 by approximately 24 kJ/mol as compared with B(*)2705/pVIPR. B(*)2705/pVIPR is stabilized by a deprotonated Glu-45/Glu-63 pair, evoked by allosteric interactions with pHis-8. The mutual electrostatic interactions of peptide and HLA molecule, including peptide- and subtype-dependent protonation of key residues, modulate complex stability and antigenic features of the respective HLA-B27 subtype.

Natural MHC class I polymorphism controls the pathway of peptide dissociation from HLA-B27 complexes

Analysis of antigen dissociation provides insight into peptide presentation modes of folded human leukocyte antigen (HLA) molecules, which consist of a heavy chain, beta2-microglobulin (beta2m), and an antigenic peptide. Here we have monitored peptide-HLA interactions and peptide dissociation kinetics of two HLA-B27 subtypes by fluorescence depolarization techniques. A single natural amino-acid substitution distinguishes the HLA-B*2705 subtype that is associated with the autoimmune disease ankylosing spondylitis from the non-disease-associated HLA-B*2709 subtype. Peptides with C-terminal Arg or Lys represent 27% of the natural B*2705 ligands. Our results show that dissociation of a model peptide with a C-terminal Lys (GRFAAAIAK) follows a two-step mechanism. Final peptide release occurs in the second step for both HLA-B27 subtypes. However, thermodynamics and kinetics of peptide-HLA interactions reveal different molecular mechanisms underlying the first step, as indicated by different activation energies of 95+/-8 kJ/mol (HLA-B*2705) and 150+/-10 kJ/mol (HLA-B*2709). In HLA-B*2709, partial peptide dissociation probably precedes fast final peptide release, while in HLA-B*2705 an allosteric mechanism based on long-range interactions between beta2m and the peptide binding groove controls the first step. The resulting peptide presentation mode lasts for days at physiological temperature, and determines the peptide-HLA-B*2705 conformation, which is recognized by cellular ligands such as T-cell receptors.

Interaction of HLA-B27 homodimers with KIR3DL1 and KIR3DL2, unlike HLA-B27 heterotrimers, is independent of the sequence of bound peptide

HLA-B27 can form beta-2 microglobulin (beta2m)-associated heterotrimers (HLA-B27) and beta2m-free homodimers (B27(2)). Here, we study the role of complexed peptide in the interaction of these forms of B27 with the killer cell immunoglobulin (Ig)-like receptors KIR3DL1 and KIR3DL2 and with Ig-like transcripts LILRB1 and LILRB2. HLA-B27 tetramers complexed with three of five different naturally processed self peptides and three of seven pathogen-derived epitopes bound to KIR3DL1-expressing transfectants and NK cells. Heterotrimeric complexes containing peptides with charged amino acids at position 8 did not bind to KIR3DL1; however, studies with analogue peptides demonstrated that these are not the only peptide residues involved in binding. KIR3DL1 ligation by HLA-B27 inhibited NK cell IFN-gamma production in a peptide-dependent fashion. B27 but not HLA-A2, B7 or B57 heavy chains formed homodimers in the presence of peptide epitopes. B27(2) bound to KIR3DL1, KIR3DL2 and LILRB2 but not LILRB1. KIR3DL2 ligation by B27(2) inhibited NK and T cell IFN-gamma production. By contrast with HLA heterotrimers, B27(2) binding to KIR did not depend on the sequence of the bound peptide. Differences in KIR binding to classical HLA and B27(2) could be involved in the pathogenesis of spondyloarthritis.

B*2707 differs in peptide specificity from B*2705 and B*2704 as much as from HLA-B27 subtypes not associated to spondyloarthritis

HLA-B*2707 is associated with ankylosing spondylitis in most populations. Like the non-associated allotypes B*2706 and B*2709, it lacks Asp116 and shows preference for peptides with nonpolar C-terminal residues. The relationships between the peptide specificity of B*2707 and those of the disease-associated B*2705 and the non-associated subtypes were analyzed by determining the overlap between the corresponding peptide repertoires, the sequence of shared and differential ligands, and by comparing allospecific T cell epitopes with peptide sharing. The B*2707-bound repertoire was as different from that of B*2705 as from those of B*2706, B*2709, or the two latter subtypes from each other. Differences between B*2707 and B*2705 were based on their C-terminal residue specificity and a subtle modulation at other positions. Differential usage of secondary anchor residues explained the disparity between the B*2707-, B*2706-, and B*2709-bound repertoires. Similar differences in residue usage were found between B*2707 and both B*2704 and B*2706, as expected from the high peptide overlap between the two latter subtypes. T cell cross-reaction paralleled peptide sharing, suggesting that many shared ligands conserve their alloantigenic features on distinct subtypes. Our results indicate that association of HLA-B27 subtypes with ankylosing spondylitis does not correlate with higher peptide sharing among disease-associated subtypes or with obvious peptide motifs.

Cutting Edge: HLA-B27 Acquires Many N-Terminal Dibasic Peptides: Coupling Cytosolic Peptide Stability to Antigen Presentation

Ag presentation by MHC class I is a highly inefficient process because cytosolic peptidases destroy most peptides after proteasomal generation. Various mechanisms shape the MHC class I peptidome. We define a new one: intracellular peptide stability. Peptides with two N-terminal basic amino acids are more stable than other peptides. Such peptides should be overrepresented in the peptidome of MHC class I-associated peptides. HLA-B27 binding peptides use anchor residue R at P2 and, although most amino acids are allowed, particular amino acids are overrepresented at P1, including R and K. We show that such N-terminal dibasic peptides are indeed more efficiently presented by HLA-B27. This suggests that HLA-B27 can present peptides from Ags present in fewer copies than required for successful peptide generation for other MHC class I molecules.

Conformational dimorphism of self-peptides and molecular mimicry in a disease-associated HLA-B27 subtype

An interesting property of certain peptides presented by major histocompatibility complex (MHC) molecules is their acquisition of a dual binding mode within the peptide binding groove. Using x-ray crystallography at 1.4 A resolution, we show here that the glucagon receptor-derived self-peptide pGR ((412)RRRWHRWRL(420)) is presented by the disease-associated human MHC class I subtype HLA-B*2705 in a dual conformation as well, with the middle of the peptide bent toward the floor of the peptide binding groove of the molecule in both binding modes. The conformations of pGR are compared here with those of another self-peptide (pVIPR, RRKWRRWHL) that is also displayed in two binding modes by HLA-B*2705 antigens and with that of the viral peptide pLMP2 (RRRWRRLTV). Conserved structural features suggest that the N-terminal halves of the peptides are crucial in allowing cytotoxic T lymphocyte (CTL) cross-reactivity. In addition, an analysis of T cell receptors (TCRs) from pGR- or pVIPR-directed, HLA-B27-restricted CTL clones demonstrates that TCR from distinct clones but with comparable reactivity may share CDR3alpha but not CDR3beta regions. Therefore, the cross-reactivity of these CTLs depends on TCR-CDR3alpha, is modulated by TCR-CDR3beta sequences, and is ultimately a consequence of the conformational dimorphism that characterizes binding of the self-peptides to HLA-B*2705. These results lend support to the concept that conformational dimorphisms of MHC class I-bound peptides might be connected with the occurrence of self-reactive CTL.

Spondyloarthritis: update on pathogenesis and management

A great deal of progress has occurred in the past few years in elucidating the causes and designing new treatments for ankylosing spondylitis and other types of spondyloarthritis. In addition to the human leukocyte antigen (HLA)-B27 and other major histocompatibility complex (MHC) genes, chromosomal regions and genes elsewhere in the genome are being implicated both in disease susceptibility and severity. The various ways HLA-B27 may function in causing spondyloarthritis now are better understood to encompass not only antigen presentation but also other mechanisms, possibly all being operative in pathogenesis (misfolding of the HLA-B27 molecule, impaired intracellular killing of bacteria, and HLA-B27 itself serving as an autoantigen). Specific enteric and sexually acquired infections can trigger reactive arthritis, though no specific microbe has been identified in other forms of spondyloarthritis. Intestinal inflammation with impairment of the gut:blood barrier may be operative in driving ankylosing spondylitis and enteropathic arthritis. A number of treatments have been tried in spondyloarthritis, including older agents such as methotrexate and sulfasalazine but also newer drugs such as pamindronate. The recent introduction of tumor necrosis factor (TNF) blockers in the treatment of spondyloarthritis has offered the most hope in not only relieving symptoms and signs of both peripheral arthritis and enthesitis but also spinal disease, which often has been refractory to other agents. Their high cost and considerable side effect profile, however, have necessitated the establishment of guidelines for their use in these diseases in order to target the patient in whom they are likely to have the most benefit.

Thermodynamic and Structural Equivalence of Two HLA-B27 Subtypes Complexed with a Self-peptide

The F pocket of major histocompatibility complex (in humans HLA) class I molecules accommodates the C terminus of the bound peptide. Residues forming this pocket exhibit considerable polymorphism, and a single difference (Asp116 in HLA-B*2705 and His116 in HLA-B*2709 heavy chains) confers differential association of these two HLA-B27 subtypes to the autoimmune disease ankylosing spondylitis. As peptide presentation by HLA molecules is of central importance for immune responses, we performed thermodynamic (circular dichroism, differential scanning calorimetry, fluorescence polarization) and X-ray crystallographic analyses of both HLA-B27 subtypes complexed with the epidermal growth factor response factor 1-derived self-peptide TIS (RRLPIFSRL) to understand the impact of the Asp116His exchange on peptide display. This peptide is known to be presented in vivo by both subtypes, and as expected for a self-peptide, TIS-reactive cytotoxic T lymphocytes are absent in the respective individuals. The thermodynamic analyses reveal that both HLA-B27:TIS complexes exhibit comparable, relatively high thermostability (Tm approximately 60 degrees C) and undergo multi-step unfolding reactions, with dissociation of the peptide in the first step. As shown by X-ray crystallography, only subtle structural differences between the subtypes were observed regarding the architecture of their F pockets, including the presence of distinct networks of water molecules. However, no consistent structural differences were found between the peptide presentation modes. In contrast to other peptides displayed by the two HLA-subtypes which show either structural or dynamical differences in their peptide presentation modes, the TIS-complexed HLA-B*2705 and HLA-B*2709 subtypes are an example for thermodynamic and structural equivalence, in agreement with functional data.

HLA-B27: portraying immunodominant viral epitopes

Although the crystal structure of HLA-B27 has been known for a long time, only recently have X-ray diffraction studies of this molecule in complex with individual peptides become available. The report of three such structures involving viral epitopes that are immunodominant in HLA-B27-restricted T cell responses against influenza, Epstein-Barr and HIV viruses significantly improves our perception of critical aspects of the immunological and pathogenetic roles of HLA-B27, including (1) the molecular basis of its peptide-binding specificity and how this is modulated by subtype polymorphism, (2) the relationship between the structural and the antigenic features of immunodominant viral epitopes, (3) the basis for long term non-progression to AIDS of HIV-infected HLA-B27+ individuals, and (4) the structural features of microbial peptides influencing NK receptor engagement. Here, I discuss the implications of this and related studies for the relevance of HLA-B27 in host defense and as a pathogenetic molecule in spondyloarthritis.

Similar cell surface expression of β2-microglobulin-free heavy chains by HLA-B27 subtypes differentially associated with ankylosing spondylitis

OBJECTIVE

To determine whether the cell surface features of HLA-B27 subtypes reported to be differentially associated with ankylosing spondylitis (AS) differ in a way that correlates with disease susceptibility.

METHODS

Human cell transfectants expressing or lacking the transporter associated with antigen processing were used to determine the cell surface expression of B27 subtypes by flow cytometry with antibodies recognizing the B27 heterodimer or beta2-microglobulin (beta2m)-free heavy chains.

RESULTS

In lymphoid cells with an intact peptide-loading complex, all B27 subtypes, irrespective of their association with disease, showed similar ratios of free heavy chain to heterodimer, suggesting similar surface stability. A substantial decrease in dissociated heavy chains, which never reached 100%, was observed upon addition of a B27 ligand, with no significant differences among subtypes. This is compatible with similar surface expression of irreversible beta2m-free heavy chain forms among subtypes differentially associated with disease. In cells lacking the transporter associated with antigen processing, both disease-associated and non-disease-associated subtypes expressed a population of heterodimers at 26 degrees C that was less stable than the population expressed at 37 degrees C. In the presence of exogenous peptide, the expression of heterodimers increased, without a concomitant decrease in beta2m-free heavy chains. This suggests that in these cells, and for all subtypes tested, most of the dissociated heavy chains at the cell surface are in irreversible forms. At 37 degrees C, the expression of beta2m-free B27 heavy chains was very low on T2 transfectant cells.

CONCLUSION

HLA-B27 subtypes showing differential associations with AS are similar in their extent of beta2m dissociation and surface expression of free heavy chains.

High-affinity, peptide-specific T cell receptors can be generated by mutations in CDR1, CDR2 or CDR3

The third complementarity-determining regions (CDR3s) of antibodies and T cell receptors (TCRs) have been shown to play a major role in antigen binding and specificity. Consistent with this notion, we demonstrated previously that high-affinity, peptide-specific TCRs could be generated in vitro by mutations in the CDR3alpha region of the 2C TCR. In contrast, it has been argued that CDR1 and CDR2 are involved to a greater extent than CDR3s in the process of MHC restriction, due to their engagement of MHC helices. Based on this premise, we initiated the present study to explore whether higher affinity TCRs generated through mutations in these CDRs or other regions would lead to significant reductions in peptide specificity (i.e. the result of greater binding energy gained through interactions with major histocompatibility complex (MHC) helices). Yeast-display technology and flow sorting were used to select high-affinity TCRs from libraries of CDR mutants or random mutants. High-affinity TCRs with mutations in the first residue of the Valpha, CDR1, CDR2, or CDR3 were isolated. Unexpectedly, every TCR mutant, including those in CDR1 and CDR2, retained remarkable peptide specificity. Molecular modeling of various mutants suggested that such exquisite specificity may be due to: (1) enhanced electrostatic interactions with key peptide or MHC residues; or (2) stabilization of CDRs in specific conformations. The results indicate that the TCR is positioned so that virtually every CDR can contribute to the antigen-specificity of a T cell. The conserved diagonal docking of TCRs could thus orient each CDR loop to sense the peptide directly or indirectly through peptide-induced effects on the MHC.

The Solvent-Inaccessible Cys67Residue of HLA-B27 Contributes to T Cell Recognition of HLA-B27/Peptide Complexes

Crystallographic studies have suggested that the cysteine at position 67 (Cys(67)) in the B pocket of the MHC molecule HLA-B*2705 is of importance for peptide binding, and biophysical studies have documented altered thermodynamic stability of the molecule when Cys(67) was mutated to serine (Ser(67)). In this study, we used HLA-B27.Cys(67) and HLA-B27.Ser(67) tetramers with defined T cell epitopes to determine the contribution of this polymorphic, solvent-inaccessible MHC residue to T cell recognition. We generated these HLA-B27 tetramers using immunodominant viral peptides with high binding affinity to HLA-B27 and cartilage-derived peptides with lower affinity. We demonstrate that the yield of refolding of HLA-B27.Ser(67) molecules was higher than for HLA-B27.Cys(67) molecules and strongly dependent on the affinity of the peptide. T cell recognition did not differ between HLA-B27.Cys(67) and HLA.B27.Ser(67) tetramers for the viral peptides that were investigated. However, an aggrecan peptide-specific T cell line derived from an HLA-B27 transgenic BALB/c mouse bound significantly stronger to the HLA-B27.Cys(67) tetramer than to the HLA-B27.Ser(67) tetramer. Modeling studies of the molecular structure suggest the loss of a SH ... pi hydrogen bond with the Cys-->Ser substitution in the HLA-B27 H chain which reduces the stability of the HLA-B27/peptide complex. These results demonstrate that a solvent-inaccessible residue in the B pocket of HLA-B27 can affect TCR binding in a peptide-dependent fashion.

Dual, HLA-B27 subtype-dependent conformation of a self-peptide

The products of the human leukocyte antigen subtypes HLA-B*2705 and HLA-B*2709 differ only in residue 116 (Asp vs. His) within the peptide binding groove but are differentially associated with the autoimmune disease ankylosing spondylitis (AS); HLA-B*2705 occurs in AS-patients, whereas HLA-B*2709 does not. The subtypes also generate differential T cell repertoires as exemplified by distinct T cell responses against the self-peptide pVIPR (RRKWRRWHL). The crystal structures described here show that pVIPR binds in an unprecedented dual conformation only to HLA-B*2705 molecules. In one binding mode, peptide pArg5 forms a salt bridge to Asp116, connected with drastically different interactions between peptide and heavy chain, contrasting with the second, conventional conformation, which is exclusively found in the case of B*2709. These subtype-dependent differences in pVIPR binding link the emergence of dissimilar T cell repertoires in individuals with HLA-B*2705 or HLA-B*2709 to the buried Asp116/His116 polymorphism and provide novel insights into peptide presentation by major histocompatibility antigens.