Possible protective role of HLA-B*2706 for ankylosing spondylitis

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.

Genetics of Ankylosing Spondylitis-Focusing on the Ethnic Difference Between East Asia and Europe

Ankylosing spondylitis (AS) is a common, highly heritable inflammatory arthritis affecting the mainly axial joints in both East Asia and Europe. To date, the pathogenesis of AS is still unknown, although we know that genetics play a vital role in it. The HLA-B27 allele is found in over 85% of AS patients. However, strong evidence suggests that other major histocompatibility complex (MHC) and non-MHC genes are also involved in the pathogenesis. In addition, current data showed that there were significant differences in both genomics and metagenomics among the different ethnic populations. The investigation of the key role of the microbiome in AS pathogenesis also highlighted the host–microbiome genetic interactions. Here, we systematically review current AS genetic research data and further compare genetic differences, especially between East Asian and European groups, which may highlight the challenge in future genetic studies.

Prevalence of HLA-B*27 subtypes in the Tamil population of India with Ankylosing spondylitis and its correlation with clinical features

INTRODUCTION

HLA-B*27 is strongly associated with Ankylosing spondylitis (AS). Its subtypes show considerable geographic and ethnic difference. The main aim of this study was to assess the frequency of subtypes of HLA-B*27 in the Indian Tamil AS patients.

METHODS AND MATERIALS

Adult AS patients positive for HLA-B*27 were considered for the study. The high-resolution typing to define HLA-B*27 subtypes were done using Invitrogen B kits from One Lambda (SeCore® Sequencing Kits, Thermo Fisher, United States).

RESULTS AND CONCLUSION

Prevalence of subtypes identified were HLA-B*27:04 (52.2%), HLA-B*27:05 (41.6%), HLA-B*27:07 (3.5%) and HLA-B*27:02 (2.7%). All subtypes showed disease predisposition for males. The most common extra articular manifestation seen was enthesitis in HLA-B*27:04 and HLA-B*27:05. Uveitis was mainly associated with HLA-B*27:05 and dactylitis with HLA-B*27:04. A significant peripheral joints involvement for female and axial joint involvement for males was seen in HLA-B*27:04. Our study establishes the prevalence of HLA-B*27 subtypes and the associated clinical phenotypes among the Indian Tamil population. Considering the variability of presentation, organ involvement, and disease course in different subtypes and across ethnicities it is critical to define these associations in the ethnic populations we treat for their appropriate care considering the significant negative health and socioeconomic effects of AS.

Enthesitis Related Arthritis in a Longitudinal Southeast Asian Registry: High Prevalence of HLA-B27, Different Sacroiliitis Risk Factors and Less Common Drug-Free Remission

Objective. To describe the clinical characteristics, predictors and treatment of children with Enthesitis Related Arthritis (ERA) in a Singapore longitudinal cohort over 11 years. Methods. ERA patients were recruited from our registry (2009–2019). Nonparametric descriptive statistics including median (interquartile range, IQR) were used to describe data. Kaplan–Meier survival and logistic/Cox regression analyses were used to estimate the probabilities and determine predictors of clinical variables, respectively. The significance level was set at <0.05. Results. One hundred and forty-six ERA patients (87% male, 82% Chinese) were included. Median onset age was 11.9 years (IQR 9.4–14.0) and median disease duration was 4.9 years (IQR 2.6–8.3). Family history of Human Leukocyte Antigen (HLA)-B27 associated diseases was positive in 7.5%. Acute uveitis occurred in 3.4%. Oligoarthritis was present in 89.7%. Hip, knee and ankle joints were among the most common joints involved. One-fourth had enthesitis at diagnosis (Achilles tendon entheses, 82.9%). Sacroiliitis occurred in 61%. Probabilities of sacroiliitis development were 0.364, 0.448 and 0.578 at 1, 2 and 5 years after onset, respectively. Negative HLA-B27, female, older age at onset and hip arthritis at diagnosis were associated with shorter time for sacroiliitis development (p = 0.001–0.049). Methotrexate (MTX) remained the most common disease modifying anti-rheumatic drug (DMARD) used (77.4%). However, 77.9% required anti-TNF (aTNF) therapy secondary to MTX failure. Among MTX-treated sacroiliitis patients, 85.3% failed, requiring aTNF, as compared to 63.2%patients without axial disease. Longer duration to diagnosis (p = 0.038) and MTX use (p = 0.007) predicted aTNF therapy. None had joint deformity. Conclusions. This study underscores differences in ERA clinical characteristics, predictors and treatment responses. Our ERA population had many unique findings but good functional outcomes.

Comparison of HLA-B*27 subtypes between Chinese patients with ankylosing spondylitis and non-ankylosing spondylitis carriers

Objective

To investigate the distribution of subtypes between HLA-B*27 (+) patients with ankylosing spondylitis (AS) and carriers.

Methods

This case–control study recruited Chinese Han patients with HLA-B*27 (+) AS from six hospitals in Zhejiang Province, China between 2013 and 2018. Patients who were examined for HLA-B*27 because of back pain or arthralgia but who did not have AS or arthritis were recruited as controls. HLA-B*27 target DNA was amplified by amplification refractory mutation systems and HLA-B*27 subtypes were determined by sequencing.

Results

The positive rate of HLA-B*27 was significantly higher in the AS group than in the control group. In AS patients, HLA-B*2704 was predominant at 86.4%, followed by HLA-B*2705 at 12.6%; HLA-B*2704 and HLA-B*2705 were found in 70.0% and 10.0% of controls, respectively. HLA-B*2702 and HLA-B*2706 were detected at low frequencies in the control group, while the rare subtype HLA-B*2715 was only observed in two (1.0%) patients with AS. HLA-B*2707 was not detected in AS or control groups.

Conclusion

HLA-B*2704 is the predominant subtype among patients with AS and carriers in southeast China.

Human Leukocyte Antigen (HLA) B27 Allotype-Specific Binding and Candidate Arthritogenic Peptides Revealed through Heuristic Clustering of Data-independent Acquisition Mass Spectrometry (DIA-MS) Data

Expression of HLA-B27 is strongly associated with ankylosing spondylitis (AS) and other spondyloarthropathies. While this is true for the majority of HLA-B27 allotypes, HLA-B*27:06 and HLA-B*27:09 are not associated with AS. These two subtypes contain polymorphisms that are ideally positioned to influence the bound peptide repertoire. The existence of disease-inducing peptides (so-called arthritogenic peptides) has therefore been proposed that are exclusively presented by disease-associated HLA-B27 allotypes. However, we have recently demonstrated that this segregation of allotype-bound peptides is not the case and that many peptides that display sequence features predicted to favor binding to disease-associated subtypes are also capable of being presented naturally by protective alleles. To further probe more subtle quantitative changes in peptide presentation, we have used a combination of data-independent acquisition (DIA) and multiple reaction monitoring (MRM) mass spectrometry to quantify the abundance of 1646 HLA-B27 restricted peptides across the eight most frequent HLA-B27 allotypes (HLA-B*27:02-HLA-B*27:09). We utilized K means cluster analysis to group peptides with similar allelic binding preferences across the eight HLA-B27 allotypes, which enabled us to identify the most-stringent binding characteristics for each HLA-B27 allotype and further refined their existing consensus-binding motifs. Moreover, a thorough analysis of this quantitative dataset led to the identification of 26 peptides, which are presented in lower abundance by HLA-B*27:06 and HLA-B*27:09 compared with disease-associated HLA-B27 subtypes. Although these differences were observed to be very subtle, these 26 peptides might encompass the sought-after arthritogenic peptide(s).

Revisiting the arthritogenic peptide theory: quantitative not qualitative changes in the peptide repertoire of HLA-B27 allotypes

OBJECTIVE

The association of HLA-B27 with spondyloarthropathy is one of the strongest documented for any autoimmune disease. A common hypothesis for this association is the arthritogenic peptide concept. This dictates that differences in the peptide binding preferences of disease-associated and non-disease-associated HLA-B27 allotypes underlie the presentation of bacterial and self-peptides, leading to cross-reactive T cell immunity and subsequent autoimmune attack of affected tissues. The aim of this study was to analyze and compare self-peptides from 8 HLA-B27 allotypes, to increase existing data sets of HLA-B27 ligands, to refine and compare their consensus-binding motifs, and to reveal similarities and differences in the peptide repertoire of the HLA-B27 subtypes.

METHODS

Qualitative differences in the peptides bound to the 8 most frequent HLA-B27 subtypes were determined by tandem mass spectrometry, and quantitative changes in allelic binding specificities were determined by highly sensitive and targeted multiple reaction monitoring mass spectrometry.

RESULTS

We identified >7,500 major histocompatibility complex class I peptides derived from the 8 most common HLA-B27 allotypes (HLA-B*27:02 to HLA-B*27:09). We describe individual binding motifs for these alleles for the 9-12-mer ligands. The peptide repertoires of these closely related alleles showed significant overlap. Allelic polymorphisms resulting in changes in the amino acid composition of the antigen-binding cleft manifested largely as quantitative changes in the peptide cargo of these molecules.

CONCLUSION

Absolute binding preferences of HLA-B27 allotypes do not explain disease association. The arthritogenic peptide theory needs to be reassessed in terms of quantitative changes in self-peptide presentation, T cell selection, and altered conformation of bound peptides.

Association of HLA-B27 genetic polymorphisms with ankylosing spondylitis susceptibility worldwide: a meta-analysis

OBJECTIVES

Many publications have evaluated the correlation between HLA-B27 polymorphisms and ankylosing spondylitis (AS), with conflicting results. We carried out this new meta-analysis in order to collect all the relevant studies to further clarify the association of HLA-B27 polymorphisms with AS susceptibility.

METHODS

Relevant published data were retrieved through Medline, PubMed, Web of Science, CNKI, and the Chinese BioMedical Literature Database on disc. The statistical analysis was conducted using Review Manager Version 5.0 and STATA 11.0. From these data, the odds ratio (OR) with a 95 % confidence interval (95 % CI) was calculated.

RESULTS

(1) A total of 38 studies, including 3,410 AS cases and 1,735 healthy controls, were collected in this meta-analysis. (2) Our results showed that B2704 was a risk factor but B2703, B2706, B2707, B2727, B2729, and B2747 may be protective factors for AS worldwide. (3) These subtypes, such as B2701, B2702, B2705, B2708-15, B2717-20, B2723-24, B2733, B2735, B2740, B2746, B2749, and B2767, showed no association with susceptibility to AS. There was a huge difference with previous reports for B2702 and B2705. (4) The B2702, B2704 and B2705 subtypes have existed high heterogeneity but no publication bias.

CONCLUSIONS

This meta-analysis in our study suggested that B2704 might be a potential risk factor, however, B2703, B2706, and B2707 might be potential protective factors of AS, especially in Asia.

Association of HLA-B27 status and gender with sacroiliitis in patients with ankylosing spondylitis

Objective: To observe the influence of human leucocyte antigen B27 (HLA-B27) status and gender on sacroiliitis on computed tomography (CT) in ankylosingspondylitis (AS).

Methods: We reviewed the archived medical records of the AS inpatients admitted in the Rheumatology Department of the First Affiliated Hospital of Wenzhou Medical University during the period from January 2007 through January 2013 and finally 386 patients were included in the study. The severity of sacroiliitis on CT was evaluated according to the grading used in the modified New York criteria for AS. Two-way classification analysis of variance (ANOVA) was employed to examine the effect of HLA-B27 status and gender on age at disease onset. The impact of HLA-B27 and gender on sacroiliitis on CT was tested by univariate and multivariate logistic regression analyses.

Results: There were 350 HLA-B27 positive patients (90.7%) and 36 HLA-B27 negative patients (9.3%). The ANOVA test indicated that HLA-B27 positive patients and male patients respectively had an earlier age at disease onset than HLA-B27 negative patients and female patients. The logistic regression analysis indicated that positive HLA-B27 status (OR 2.601, p=0.004) and male gender (OR 1.923, p=0.004) were significant predictors of worse sacroiliitis. In addition, elevated ESR (OR 2.181, p=0.013) and longer disease duration (OR 1.100, p<0.001) contributed to worse sacroiliitis likewise.

Conclusion: Positive HLA-B27 status and male gender are associated with worse sacroiliitis on CT, acting as predictors of sacroiliitis. Elevated ESR and longer disease duration also contribute to worse sacroiliitis. Meanwhile, positive HLA-B27 status and male gender are associated with earlier age at disease onset.

Old and new HLA associations with ankylosing spondylitis

Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease that primarily involves the axial skeleton and the sacroiliac joint, but may also affect peripheral joints and entheses. AS susceptibility is clearly attributable to genetic factors and the link between human leukocyte antigen (HLA)-B27 and AS is the strongest association between an HLA class I molecule and a disease. However, there is evidence for the involvement of other, non-B27 factors within the major histocompatibility complex (MHC) in AS susceptibility. MHC class I is clearly the most significant genetic region for the disease, although most of the genetic association of this region is driven by HLA-B27. Moreover, several studies have investigated the MHC class II region and its association with AS. This review summarizes the current findings concerning the MHC genetics of the disease, focusing in particular on the associations of HLA with AS found in different ethnic populations throughout the world, and the possible mechanisms underlying them.

Does HLA-B*2706 protect against ankylosing spondylitis? A meta-analysis

The human leukocyte antigen (HLA)-B*2706 is a relatively rare subtype of HLA-B27. In contrast to most HLA-B27 subtypes, some studies have reported HLA-B*2706 to be protective against ankylosing spondylitis (AS). A systematic review and a meta-analysis of available studies was performed to investigate the association of HLA-B*2706 with AS. After literature review a random effect meta-analysis was performed. No studies were found comparing the frequency of HLA-B*2706 in AS patients and controls. Meta-analysis of seven studies using HLA-B27-positive AS patients and controls showed a protective effect of HLA-B*2706 on development of AS in HLA-B27 individuals (odds ratio = 0.128, 95% CI = 0.043-0.378, P < 0.001). The results of the meta-analysis of HLA-B*2706 in HLA-B27-positive patients and controls is preliminary evidence of a protective effect of HLA-B*2706 against AS in the population. There is a clear need for additional studies on HLA-B*2706 in AS. Due to the fact that HLA-B*2706 is more or less restricted to Southeast Asia, researchers in this part of the world may have an essential role in performing these studies.

HLA B27 allele types in homogeneous groups of juvenile idiopathic arthritis patients in Latvia

Juvenile idiopathic arthritis (JIA) is a heterogeneous condition and therapeutic strategies vary in different JIA types. The routinely accepted practice to start with Sulphasalazine (SS) as the first line treatment in patients with HLA B27 positive JIA proves to be ineffective in a large proportion of children.

OBJECTIVE

to investigate HLA B27 positive JIA patients clinical characteristics, determined HLA B27 allele types and their connection with antirheumatic treatment in homogenous patient groups.

MATERIALS AND METHODS

56 patients diagnosed with JIA and observed over the period 2006 to 2009 included in the study. HLAB27 allele types were determined using PCR method.

RESULTS

In HLA B27 positive JIA patients mean disease onset was 12.34 ± 3.3 years. Most common (44%) JIA type was enthesitis related arthritis. Positive response to the treatment with SS was found in 32% of patients, Methotrexate (MTX) - in 43%, combined treatment - SS with MTX was effective in 12.5%. 12.5% of patients required combination MTX with Enbrel.Eight HLA B27 allele types were found in JIA patients in Latvia: *2702, *2703, *2704, *2705, *2710, *2715, *2717, *2728. The most common was *2705 - in 55% of cases. Among all the patients enthesitis related arthritis most commonly occurred in patients with HLAB*2705 allele (OR = 2.01, p < 0.02), oligoarthritis in patients with *2710 allele (OR = 3.0, p < 0.04) and polyarthritis with *2717 allele (OR = 3.0, p < 0.05). In patients with *2705 allele effective treatment was MTX (OR = 1.13, p < 0.03) and MTX with SS (OR = 2.02, p < 0.05), but in patients having *2703 allele - MTX with Enbrel (OR = 2.94, p < 0.02).

CONCLUSIONS

There are 8 different HLA B27 alleles in JIA patients in Latvia and the most common is *2705, but in order to assert them to be disease associated alleles, more extensive studies are needed, including control group of HLA B27 positive healthy individuals. Standard treatment approach with SS proves to be unsatisfactory in the majority of JIA patients. To improve children's quality of life achieving rapid disease control, the first line treatment in HLA B27 positive patients should be MTX. In order to start with the most appropriate drug it is necessary to determine HLAB 27 type at the onset of disease.

Predominant association of HLA-B*2704 with ankylosing spondylitis in Chinese Han patients

The HLA-B27 subtypes have a varied racial and ethnic prevalence throughout the world. However, the association of B27-subtypes with ankylosing spondylitis (AS) in the mainland China is unknown. To determine the association of B27-subtypes with AS in the Mainland Chinese Han population, a total of unrelated 153 patients with AS were enrolled in a large case-control association study, and 1545 unrelated, healthy, ethnically matched blood donors were included as controls. The genotyping of B27 and its subtypes was performed using the polymerase chain reaction with sequence specific primers (PCR-SSP). A total of 130 (84.97%) AS patients and 61 (3.95%) healthy controls were B27 positive. Three B27-subtypes, B*2704, B*2705 and B*2710, were further identified, of which both B*2704 and B*2705 were strongly AS associated. B*2710 was only detected in one AS patient and two other healthy controls. Considering only B27-positive cases and controls, a statistically different frequency of B27-subtypes was observed, with an over-representation of B*2704 (P = 0.018). B*2704 was clearly more strongly associated than B*2705 with AS [odds ratio (OR ) = 2.4, P = 0.011]. Furthermore, a combined analysis including three previous studies of B27-subtype distributions in Chinese AS cases confirmed the stronger association of B*2704 with AS than B*2705 (OR = 2.5, P = 0.00094).

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.

The association of HLA-B*27 subtypes with ankylosing spondylitis in Wuhan population of China

The aim of this study was to investigate the association of the B27 subtypes with ankylosing spondylitis (AS) in the Wuhan population of China. We selected 317 HLA-B27-positive individuals (145 controls and 172 patients with ankylosing spondylitis). The B27 subtypes were characterized using a PCR-SSP method. Six B27 subtypes were determined: B*2702, 03, 04, 05, 06 and B*13. HLA-B*2704 and HLA-B*2705 were the two high frequency genotypes in controls and patients. Compared with the controls, the AS patients had high frequency of B*2704 (patients 69.2% vs. controls 53.8%) and low frequency of B*2705 (patients 23.8% vs. controls 33.1%). B*2703 was detected in 10 (5.8%) patients and in 13 (8.9%) controls. B*2702, 06 and B*2713 were relatively rare. Our results show that the allele conferring risk to AS in the Wuhan population of China was B*2704 and B*2705. B*2704 is strongly associated with AS.

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.

Usefulness of human leucocyte antigen-B27 subtypes in predicting ankylosing spondylitis: Taiwan experience

BACKGROUND

Genetic factors are clearly attributed to the susceptibility of ankylosing spondylitis (AS). The human leucocyte antigen (HLA)-B27 proved to be the very useful marker for diagnosing AS. The aim of this study was to determine the prevalence of HLA-B27 subtypes in Taiwan and to investigate whether these subtypes may be of help in predicting the diagnosis of AS.

METHODS

A total of 314 patients with AS and a control group of 71 subjects positive for HLA-B27 detected by flow cytometry analysis were recruited for the study. HLA-B27 subtypes were confirmed by the polymerase chain reaction-sequence-specific primers and sequence-specific oligonucleotide probing.

RESULTS

Four B27 alleles were identified: B*2704, B*2705, B*2706 and B*2707. HLA-B*2704 was the predominant allele. There were significant differences in the distribution of HLA-B27 subtypes between patients with AS and controls. Five of them who were homozygous for the B*2704 allele were solely found in AS group but not in controls. Statistical analysis showed that B*2704 was positively associated with AS, which suggested an increased possibility of having AS. Other HLA-B27 subtypes showed no strong correlation with AS.

CONCLUSION

In the Taiwanese population, susceptibility to AS was determined by the presence of HLA-B*2704. Although B*2706 was reported to have a negative association with AS in Taiwanese, Thai and Chinese Singaporean populations, we report, in our study, two AS patients with B*2706 (0.6%). Disease heterogeneity suggests that other than genetic background, many pathogenic factors could be associated with AS. This may need to be investigated with a larger group of patients with AS and controls.

Treatment of ankylosing spondylitis with moxifloxacin

Ankylosing spondylitis (AS) is a human leukocyte antigen (HLA)-B27-associated chronic inflammatory disease of unknown etiology. There are few effective treatments for ankylosing spondylitis, which causes substantial morbidity. The relationship between AS and enterobacteria, especially Klebsiella pneumoniae, has been reported from several groups in several countries. We performed an open-label trial of moxifloxacin, a fluoroquinolone antibiotic, in patients with ankylosing spondylitis. Treatment with moxifloxacin resulted in significant and sustained improvement. At 12 weeks, patients treated with moxifloxacin had significantly greater improvement in primary outcome measures (P < 0.001). The moxifloxacin group also had significantly greater improvement in many of the secondary outcome measures (P < 0.001). In this twelve-week trial, moxifloxacin was safe, well tolerated, and associated with improvement in the inflammatory symptoms of AS.

Seronegative Spondyloarthropathy – Studies from the Asia Pacific Region

Recent therapeutic advances, in particular the use of anti-tumour necrosis factor (anti-TNF) agents, have revived interest in the seronegative spondyloarthropathies (SpA), a group of arthritides characterised by axial skeletal involvement and the absence of rheumatoid factor. The purpose of this article is to review the studies that have been done in the Asia Pacific region, as a broad understanding of the scope and severity of this group of diseases would enable rheumatologists and physicians in this part of the world to better manage their patients. The majority of genetic studies have focused on the associations of HLA-B27 with ankylosing spondylitis (AS) and SpA, while a few studies examined the associations of the CARD, IL-1, LMP2, TAP and TGF with AS. There are a handful of studies on the immunological responses to bacteria and cytokine levels in AS. The onset and clinical features of SpA have been reported from most countries in the region, but no data on patient outcomes, using current measurement tools such as the Bath Ankylosing Spondylitis Disease Activity index (BASDAI), is available. Validation of these instruments of measurement as well as classification criteria in different ethnic populations is necessary where no prior data exist. Future studies will likely be focused on better clinical characterisation of patient cohorts, particularly with regard to the use of currently used measurement tools for disease activity and spinal function and mobility, and the identification of the need for biologic therapy in each country. Key words: ESSG criteria, Genetics, Immunological and clinical features

HLA-B27 and the pathogenesis of spondyloarthropathies

The association of HLA-B27 with ankylosing spondylitis, a chronic inflammatory disease of the axial skeleton, and other spondyloarthropathies, is among the strongest of an MHC antigen and any disease. Yet, the basis for this association remains unknown. In this review the main current hypotheses concerning the pathogenetic role of HLA-B27 will be discussed. They focus on three molecular properties of the molecule: (1) its peptide-presenting specificity, (2) its slow folding and tendency to misfold, and (3) its capacity to form covalent heavy chain homodimers amenable to recognition by leukocyte receptors. On the basis of the peptide specificity spondyloarthropathies would be triggered through T-cell autoimmunity against a self-ligand of HLA-B27 elicited by a cross-reactive foreign antigen. HLA-B27 misfolding would trigger disease through activation of inflammatory pathways following induction of endoplasmic reticulum stress, thus independently of antigen presentation. Recognition of heavy chain homodimers by leukocyte receptors might be involved in disease through immunomodulation of both innate and adaptive responses to arthritogenic pathogens. None of these hypotheses can yet satisfactorily account for the pathogenesis of spondyloarthritides. It is proposed that the pathogenetic role of HLA-B27 will eventually be explained through a global understanding of its biology, in which the various features of this molecule are envisaged as inter-dependent in their contribution to disease.

HLA-B*2704, an Allotype Associated with Ankylosing Spondylitis, Is Critically Dependent on Transporter Associated with Antigen Processing and Relatively Independent of Tapasin and Immunoproteasome for Maturation, Surface Expression, and T Cell Recognition: Relationship to B*2705 and B*2706

B*2704 is strongly associated to ankylosing spondylitis in Asian populations. It differs from the main HLA-B27 allotype, B*2705, in three amino acid changes. We analyzed the influence of tapasin, TAP, and immunoproteasome induction on maturation, surface expression, and T cell allorecognition of B*2704 and compared some of these features with B*2705 and B*2706, allotypes not associated to disease. In the tapasin-deficient .220 cell line, this chaperone significantly influenced the extent of folding of B*2704 and B*2705, but not their egress from the endoplasmic reticulum. In contrast, B*2706 showed faster folding and no accumulation in the endoplasmic reticulum in the absence of tapasin. Surface expression of B*2704 was more tapasin dependent than B*2705. However, expression of free H chain decreased in the presence of this chaperone for B*2705 but not B*2704, suggesting that more suboptimal ligands were loaded on B*2705 in the absence of tapasin. Despite its influence on surface expression, tapasin had little effect on allorecognition of B*2704. Both surface expression and T cell recognition of B*2704 were critically dependent on TAP, as established with TAP-deficient and TAP-proficient T2 cells. Both immunoproteasome and surface levels of B*2704 were induced by IFN-gamma, but this had little effect on allorecognition. Thus, except for the differential effects of tapasin on surface expression, the tapasin, TAP, and immunoproteasome dependency of B*2704 for maturation, surface expression, and T cell recognition are similar to B*2705, indicating that basic immunological features are shared by the two major HLA-B27 allotypes associated to ankylosing spondylitis in human populations.

Mechanisms of disease: infection and spondyloarthritis

There is compelling evidence that some infections can initiate a chronic nonseptic arthritis. This has proved to be an important area of investigation into gene-environment interactions, particularly since HLA-B27 confers increased susceptibility to reactive arthritis. This research has investigated the microbiology of these events, and the strategies used by pathogens to induce chronic joint inflammation. Insights into the HLA-orchestrated immune response in this context have also shed light on the impact of HLA-B27 on immunity, which might provide insights into the mechanism of other HLA-B27-associated diseases. Despite the genetic link to reactive arthritis, there is no proven relationship between ankylosing spondylitis and an inciting infection. In general, most trials have found antibiotics to be ineffective in modifying the course of spondyloarthritis.

Diversity of human leukocyte antigen-B27 alleles in Han population of Hunan province, southern China

This study was to investigate the frequency of HLA-B27 and its subtypes in the Han population of Hunan province, southern China. One hundred and sixty-nine healthy unrelated donors were tested for HLA-B27 by polymerase chain reaction-sequence-specific primer (PCR-SSP). One hundred and twenty-eight B27-positive spondyloarthropathy patients and 18 B27-positive healthy controls were subtyped using the high-resolution PCR-SSP. The phenotype frequency of human leukocyte antigen (HLA)-B27 was found to be 2.36% in healthy population. Five B27 alleles were identified: B*2704, B*2705, B*2706, B*2707, and B*2724. No significant difference was found in the distribution of HLA-B27 subtypes between the patients and controls studied. Notably, B*2724 was observed in a juvenile patient with ankylosing spondylitis. This subtype has not been previously reported in Chinese ankylosing spondylitis (AS) patients and other ethnic groups.

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.

X-ray diffraction analysis of crystals from the human major histocompatibility antigen HLA-B*2706 in complex with a viral peptide and with a self-peptide

The human leukocyte antigen (HLA) alleles HLA-B*2704 and HLA-B*2706 show an ethnically restricted distribution and are differentially associated with ankylosing spondylitis, with HLA-B*2706 lacking association with this autoimmune disease. However, the products of the two alleles differ by only two amino acids, at heavy-chain residues 114 (His in HLA-B*2704; Asp in HLA-B*2706) and 116 (Asp in HLA-B*2704; Tyr in HLA-B*2706). Both residues could be involved in contacting amino acids of a bound peptide, suggesting that peptides presented by these subtypes play a role in disease pathogenesis. Two HLA-B*2706-peptide complexes were crystallized using the hanging-drop vapour-diffusion method with PEG as precipitant. Data sets were collected to resolutions of 2.70 A (viral peptide pLMP2, RRRWRRLTV; space group P2(1)2(1)2(1)) and 1.83 A (self-peptide pVIPR, RRKWRRWHL; space group P2(1)). Using HLA-B*2705 complexed with the pGR peptide (RRRWHRWRL) as a search model, unambiguous molecular-replacement solutions were found for both HLA-B*2706 complexes.

Preliminary X-ray diffraction analysis of crystals from the recombinantly expressed human major histocompatibility antigen HLA-B*2704 in complex with a viral peptide and with a self-peptide

The product of the human leukocyte antigen (HLA) gene HLA-B*2704 differs from that of the prototypical subtype HLA-B*2705 by three amino acids at heavy-chain residues 77 (Ser instead of Asp), 152 (Glu instead of Val) and 211 (Gly instead of Ala). In contrast to the ubiquitous HLA-B*2705 subtype, HLA-B*2704 occurs only in orientals. Both subtypes are strongly associated with spondyloarthropathies and the peptides presented by these subtypes are suspected to play a role in disease pathogenesis. HLA-B*2704 was crystallized in complex with a viral peptide and with a self-peptide using the hanging-drop vapour-diffusion method with PEG as a precipitant. Both crystals belong to space group P2(1)2(1)2(1). Data sets were collected to 1.60 A (complex with the self-peptide pVIPR) or to 1.90 A (complex with the viral peptide pLMP2) resolution using synchrotron radiation. With HLA-B*2705 complexed with pVIPR as a search model, unambiguous molecular-replacement solutions were found for the complexes of HLA-B*2704 with both peptides.

HLA-B27 in the Greek Cypriot population: distribution of subtypes in patients with ankylosing spondylitis and other HLA-B27-related diseases. The possible protective role of B*2707

The major purpose of the present study was to investigate the frequency of human leukocyte antigen (HLA)-B27 alleles in healthy controls and in patients with ankylosing spondylitis (AS) and other HLA-B27-related diseases in the Greek Cypriot population. We selected 102 HLA-B27-positive individuals (60 controls and 42 patients). Typing of the HLA-B27 alleles was performed by polymerase chain reaction amplification with sequence-specific primers. Only two alleles were detected in the patient group: B*2702 (n = 31, 73.8%) and B*2705 (n = 11, 26.2%). The HLA-B*2707 allele was detected (n = 10, 16.7%) only in the healthy controls in addition to the B*2702 (n = 31, 51.7%) and B*2705 (n = 19, 31.7%) alleles. Our results show a restricted number of HLA-B27 subtypes associated with AS and other B27-related diseases and an elevated frequency of the B*2702 allele in the AS patients. The allele B*2707 seems to have a protective role in the population studied because it was found only in the healthy controls.

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.

Does genotype mask the relationship between psychological factors and immune function?

This paper examined the interaction between genetic influences of the polymorphic human leukocyte antigens (DRB1 and DQB1) and psychological distress on the development of cellular immunity to the novel antigen, keyhole limpet hemocyanin (KLH). Participants (n = 227) were immunized with KLH and the development of cutaneous delayed-type hypersensitivity (DTH) against KLH was examined 3 weeks later. Distress was assessed using the Profile of Mood States. DNA was typed for the serologically defined DRB1 and DQB1 antigens. There was a significant correlation between distress at immunization and the development of DTH skin test responses to KLH (n = 214, r = .24, p = .003). HLA DQ2 was weakly associated with a decreased likelihood of developing a cutaneous delayed-type hypersensitivity response against KLH (odds ratio [OR] = 1.6; confidence interval [CI] 0.9-2.7). HLA DQ5 was weakly associated with an increased likelihood of responding to the antigen (OR=0.6; CI=0.3-1.0). The correlation between distress and immune function in HLA DQ2 negative individuals was .34 (n = 136, p = .00) and in HLA DQ2 positive individuals it was .06 (n = 74, p =. 64). For HLA DQ5 negative individuals the correlation was .26 (n = 140, p = .00) and for HLA DQ5 positive individuals it was .22 (n = 70, p = .07). These results suggest that the distress/immune relationship in genetically susceptible or protected individuals may be underestimated in psychoneuroimmunology research.

HLA-B27-restricted CD8+ T cell response to cartilage-derived self peptides in ankylosing spondylitis

OBJECTIVE

Several hypotheses have been proposed to explain the strong association between HLA-B27 and ankylosing spondylitis (AS). Among these, the arthritogenic peptide theory proposes that certain B27 subtype alleles bind specific arthritogenic peptide(s) due to their unique amino acid anchor residues. Cartilage antigens have been discussed as candidate targets for the immune response in AS. The recognition of HLA-B27-peptide complexes by self-reactive CD8+ T cells might contribute to joint-specific tissue damage. Therefore, we investigated the presence of autoreactive CD8+ T cells specific for cartilage-derived peptides in patients with AS.

METHODS

An HLA-B27-binding prediction program and a proteasome-cutting prediction program for the human 20S proteasome were used to screen 18 human cartilage proteins for potentially immunogenic nonamer peptides. The peptides identified were used to stimulate peripheral blood mononuclear cells from 20 HLA-B27-positive patients with AS and synovial fluid (SF) mononuclear cells from 7 HLA-B27-positive patients with AS. Activation of T cells was measured by antigen-specific intracellular cytokine staining and quantified by flow cytometry.

RESULTS

From the screening analysis, we identified 121 nonamer peptides. Of these, 1 peptide derived from type II collagen and 1 from type VI collagen were stimulatory for peripheral blood CD8+ T cells in only 1 of 20 patients. However, in 4 of 7 SF samples the same type VI collagen-derived nonamer peptide stimulated SF CD8+ T cells, but none of the other peptides was stimulatory. This CD8+ T cell response could be blocked by an anti-HLA-B27 antibody, confirming an HLA-B27-restricted immune response.

CONCLUSION

Our findings suggest that cartilage-directed cellular autoimmunity might play an important role in joint-specific tissue damage in patients with AS. Future research is necessary to determine whether the identified peptide is of pathogenetic relevance.

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.

Major histocompatibility complex class I peptide presentation after Salmonella enterica serovar typhimurium infection assessed via stable isotope tagging of the B27-presented peptide repertoire

Reactive arthritis (ReA) induced by infection with several gram-negative bacteria is strongly associated with expression of the major histocompatibility complex class I molecule HLA-B27. It is thought that due to the intracellular lifestyle of ReA-inducing bacteria, bacterial fragments can be presented by HLA-B27. Cytotoxic T cells recognizing such bacterial peptides or other induced host peptides could cross-react with self peptides presented in the joints, giving rise to disease. Studies to analyze the B27 peptide repertoire in relation to infection were severely hampered, as complex peptide profiles obtained from separate infected and noninfected cell preparations had to be compared. For this study, we applied a new approach to examine the effect of Salmonella enterica serovar Typhimurium infection on the B27 peptide repertoire presented by the HLA-B*2704 subtype associated with disease. Firstly, we showed that both host cell and S. enterica serovar Typhimurium proteins can be tagged metabolically with stable-isotope-labeled arginine. We then designed experiments so that either the tagged endogenous or tagged bacterial B*2704-presented peptide repertoires from infected cells could be analyzed by mass spectrometry from single peptide preparations that included uninfected controls. Using this new approach, we found no evidence for significant changes in endogenous B*2704 peptide presentation after infection or for any S. enterica serovar Typhimurium-derived B27-bound peptide. In conclusion, the hypothesis that S. enterica serovar Typhimurium induces changes in B27 peptide presentation could not be supported.

Polymorphism of HLA-B27 in Taiwanese Chinese

Heterogeneity of HLA-B27 is represented by a family of 24 closely related alleles/subtypes. Frequency and disease association of these alleles with spondyloarthropathies differ among ethnic groups. Accurate investigation of frequencies of alleles is often hindered by the size and demographic region of sample tested. With an ever-increasing number of B27 alleles being discovered, it is becoming imperative to establish disease association of each individual alleles and its biological importance. In a large number of normal healthy Taiwanese Chinese individuals (75,777) tested nationally, over a period of five years, in a single immunogenetics centre, we found additional B27 subtypes not revealed in a previous Taiwanese Chinese population study. The subtypes found in Taiwanese Chinese and the frequencies of each of the subtypes are reported.

HLA-B27 polymorphism in the Malays

The frequency of HLA-B27 and its subtypes was determined in 878 Malay subjects. Thirty-five of the subjects typed for HLA-A, -B and -DR were found to be positive for HLA-B27. The frequency of this allele in the Malay population was found to be 3.99%. The subtypes observed and their frequencies are: HLA-B*2704 (19.4%), HLA-B*2705 (5.6%), HLA-B*2706 (72.2%) and HLA-B*2707 (2.8%).

Identification of HLA-B27-restricted peptides in reactive arthritis and other spondyloarthropathies: computer algorithms and fluorescent activated cell sorting analysis as tools for hunting of HLA-B27-restricted chlamydial and autologous crossreactive peptides involved in reactive arthritis and ankylosing spondylitis

The illustrated clinical and experimental results demonstrate the strong relationship between the MHC class I antigen HLA-B27 and synovial CD8+ T cells with specificity for bacterial and possible self-antigen in SpA. These new aspects obtained in recent experimental and clinical studies might also provide clues to the pathomechanisms of joint inflammation in SpA. In particular, the newly developed techniques will be of great relevance in the near future. New and more precise bioalgorithms reflecting new insights in the biology and biochemistry of proteins as recently presented [98, 99] can be helpful (e.g., a program with an improved prediction of the features of immunoproteasomes). Intracellular and secreted cytokine staining by FACScan allows examination of a great number of cells expressing certain antigens in response to certain stimuli. The analysis of T-cell responses with tetramer/peptide complexes can be useful to screen tissue sections for TCR, recognizing foreign or self-derived epitopes on those complexes loaded with selected (e.g., bacterial) peptides. Identification of arthritogenic peptides and a further understanding of the immunology of the pathomechanisms in SpA might open ways to design new peptide vaccines to prevent inflammation, autoimmunity, and other diseases by early intervention [100].

Allospecific T cell epitope sharing reveals extensive conservation of the antigenic features of peptide ligands among HLA-B27 subtypes differentially associated with spondyloarthritis

HLA-B*2702, B*2704, and B*2705 are strongly associated with spondyloarthritis, whereas B*2706 is not. Subtypes differ among each other by a few amino acid changes and bind overlapping peptide repertoires. In this study we asked whether differential subtype association with disease is related to differentially bound peptides or to altered antigenicity of shared ligands. Alloreactive CTL raised against B*2704 were analyzed for cross-reaction with B*2705, B*2702, B*2706, and mutants mimicking subtype changes. These CTL are directed against many alloantigen-bound peptides and can be used to analyze the antigenicity of HLA-B27 ligands on different subtypes. Cross-reaction of anti-B*2704 CTL with B*2705 and B*2702 correlated with overlap of their peptidic anchor motifs, suggesting that many shared ligands have similar antigenic features on these three subtypes. Moreover, the percent of anti-B*2704 CTL cross-reacting with B*2706 was only slightly lower than the overlap between the corresponding peptide repertoires, suggesting that most shared ligands have similar antigenic features on these two subtypes. Cross-reaction with B*2705 or mutants mimicking changes between B*2704 and B*2705 was donor-dependent. In contrast, cross-reaction with B*2702 or B*2706 was less variable among individuals. Conservation of antigenic properties among subtypes has implications for allorecognition, as it suggests that shared peptides may determine cross-reaction across exposed amino acid differences in the MHC molecules and that the antigenic distinctness of closely related allotypes may differ among donors. Our results also suggest that differential association of HLA-B27 subtypes with spondyloarthritis is more likely related to differentially bound peptides than to altered antigenicity of shared ligands.

Molecular mimicry of an HLA-B27-derived ligand of arthritis-linked subtypes with chlamydial proteins

HLA-B27 is strongly associated with spondyloarthropathies, including ankylosing spondylitis and reactive arthritis. The latter disease is triggered by various Gram-negative bacteria. A dodecamer derived from the intracytoplasmic tail of HLA-B27 was a natural ligand of three disease-associated subtypes (B*2702, B*2704, and B*2705) but not of two (B*2706 and B*2709), weakly or not associated to spondyloarthropathy. This peptide was strikingly homologous to protein sequences from arthritogenic bacteria, particularly to a region of the DNA primase from Chlamydia trachomatis. A synthetic peptide with this bacterial sequence bound in vitro disease-associated subtypes equally as the natural B27-derived ligand. The chlamydial peptide was generated by the 20 S proteasome from a synthetic 28-mer with the sequence of the corresponding region of the bacterial DNA primase. Molecular modeling suggested that the B27-derived and chlamydial peptides adopt very similar conformations in complex with B*2705. The results demonstrate that an HLA-B27-derived peptide mimicking arthritogenic bacterial sequences is a natural ligand of disease-associated HLA-B27 subtypes and suggest that the homologous chlamydial peptide might be presented by HLA-B27 on Chlamydia-infected cells.

Low frequency of HLA-B*2706 in Taiwanese patients with ankylosing spondylitis

The presence of HLA-B27 in patients affected with ankylosing spondylitis (AS) was well established prior to the advent of DNA typing of various genes within the major histocompatibility complex (MHC) in humans. However, molecular typing of the MHC genes revealed that B27 comprises a motley assortment of alleles, some of which are strongly positively associated with the disease and some of which are negatively associated with the disease. B*2706 was reported to have a negative association with AS in the Thai population and in Chinese Singaporeans. We report here our finding of an absence of B*2706 in 184 Taiwanese AS patients.

HLA-B27 and the pathogenesis of spondyloarthritis

The association of HLA-B27 with ankylosing spondylitis and other spondyloarthropathies ranks among the strongest between any HLA antigen and a human disease. Yet, in spite of intense research and advanced knowledge of the biochemistry and biology of major histocompatibility complex molecules, the mechanism of this association remains unknown. This review attempts a critical assessment of current pathogenetic hypotheses from evidence concerning the epidemiology of HLA-B27 association with disease, its peptide-binding specificity, and other aspects of the molecular biology and immunology of this molecule.

Differential association of HLA-B*2705 and B*2709 to ankylosing spondylitis correlates with limited peptide subsets but not with altered cell surface stability

In contrast to HLA-B*2705, B*2709 is weakly or not associated to ankylosing spondylitis. Both allotypes differ by a single D116H change. We compared the B*2705- and B*2709-bound peptide repertoires by mass spectrometry to quantify the effect of B*2709 polymorphism on peptide specificity. In addition, shared and differentially bound ligands were sequenced to define the structural features of the various peptide subsets. B*2705 shared 79% of its peptide repertoire with B*2709. Shared ligands accounted for 88% of the B*2709-bound repertoire. All B*2705 ligands not bound to B*2709 had C-terminal basic or Tyr residues. Most B*2709-bound peptides had C-terminal aliphatic and Phe residues, but two showed C-terminal Arg or Tyr. The B*2709-bound repertoire included 12% of peptides not found in B*2705. These had aliphatic C-terminal residues, which are also favored in B*2705. However, these peptides bound weakly B*2705 in vitro, indicating distinct contribution of secondary anchor residues in both subtypes. Differences in peptide binding did not affect the ratio of native to beta2-microglobulin-free HLA-B27 heavy chain at the cell surface. Our results suggest that weaker association of B*2709 with ankylosing spondylitis is based on differential binding of a limited subset of natural ligands by this allotype.