The HLA-B*2709 subtype in a patient with undifferentiated spondarthritis

The role of HLA-B*27 in spondyloarthritis

The mechanism by which HLA-B*27 predisposes to spondyloarthritis remains unresolved. Arthritogenic peptides have not been defined in humans and are not involved in experimental models of spondyloarthritis. Aberrant properties of HLA-B*27 can activate the IL-23/IL-17 axis in HLA-B*27 transgenic rats and humans. In HLA-B*27-independent rodent models, spondyloarthritis can be driven by IL-23 triggering entheseal-resident CD4^-/CD8^- T cells or CD4⁺ Th17 T cells. These findings point toward noncanonical mechanisms linking HLA-B*27 to the disease and provide a potential explanation for HLA-B*27-negative spondyloarthritis. Gut microbial dysbiosis may be important in the development of spondyloarthritis. HLA-B*27-induced changes in gut microbiota are complex and suggest an ecological model of dysbiosis in rodents. The importance of the IL-23/IL-17 axis in ankylosing spondylitis has been demonstrated by studies showing efficacy of IL-17. Although deciphering the precise role(s) of HLA-B*27 in disease requires further investigation, considerable progress has been made in understanding this complex relationship.

HLA-B*27 subtypes and their implications in the pathogenesis of ankylosing spondylitis

Ankylosing spondylitis (AS) is a highly heritable kind of arthritis that affects the vertebral column. AS risk has been associated strongly with Human leukocyte antigen (HLA)-B*27. In fact, some HLA-B*27 subtypes have been associated with the increased disease risk, although some specific subtypes have not shown such associations. It is supposed that HLA-B*27 plays a major role in the etiopathogenesis of the disease. However, the difference in pathogenic outcomes of HLA-B*27 certain subtypes needs to be clarified. The purpose of this review article is to overview on the detailed implications of the HLA-B*27 subtypes in the etiopathogenesis of AS. Moreover, the role of ERAP1 in AS and its epistasis with HLA-B*27 have been reviewed.

From HLA-B27 to spondyloarthritis: a journey through the ER

Almost four decades of research into the role of human leukocyte antigen-B27 (HLA-B27) in susceptibility to spondyloarthritis has yet to yield a convincing answer. New results from an HLA-B27 transgenic rat model now demonstrate quite convincingly that CD8(+) T cells are not required for the inflammatory phenotype. Discoveries that the HLA-B27 heavy chain has a tendency to misfold during the assembly of class I complexes in the endoplasmic reticulum (ER) and to form aberrant disulfide-linked dimers after transport to the cell surface have forced the generation of new ideas about its role in disease pathogenesis. In transgenic rats, HLA-B27 misfolding generates ER stress and leads to activation of the unfolded protein response, which dramatically enhances the production of interleukin-23 (IL-23) in response to pattern recognition receptor agonists. These findings have led to the discovery of striking T-helper 17 cell activation and expansion in this animal model, consistent with results emerging from humans with spondyloarthritis and the discovery of IL23R as an additional susceptibility gene for ankylosing spondylitis. Together, these results suggest a novel link between HLA-B27 and the T-helper 17 axis through the consequences of protein misfolding and open new avenues of investigation as well as identifying new targets for therapeutic intervention in this group of diseases.

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.

HLA-B27 misfolding and spondyloarthropathies

HLA-B27 plays a central role in the pathogenesis of many spondyloarthropathies and in particular ankylosing spondylitis. The observation that the HLA-B27 heavy chain has a tendency to misfold has raised the possibility that associated diseases may belong in a rapidly expanding category of protein misfolding disorders. The synthesis of the HLA-B27 heavy chain, assembly with beta(2)m and the loading of peptide cargo, occurs in the endoplasmic reticulum (ER) before transport to the cell surface. The evidence indicates that misfolding occurs in the ER prior to beta(2)m association and peptide optimization and is manifested in the formation of aberrant inter- and intra-chain disulfide bonds and accumulation of heavy chain bound to the chaperone BiP. Enhanced accumulation of misfolded heavy chains during the induction of class I expression by cytokines, can cause ER stress resulting in activation of the unfolded protein response (UPR). Effects of UPR activation on cytokine production are beginning to emerge and may provide important missing links between HLA-B27 misfolding and spondyloarthritis. In this chapter we will review what has been learned about HLA-B27 misfolding in human cells and in the transgenic rat model of spondyloarthritis-like disease, considering it in the context of other protein misfolding disorders. These studies provide a framework to support much needed translational work assessing HLA-B27 misfolding and UPR activation in patient-derived material, its consequences for disease pathogenesis and ultimately how and where to focus intervention strategies.

HLA-B27 misfolding and spondyloarthropathies

HLA-B27 plays a central role in the pathogenesis of many spondyloarthropathies and in particular ankylosing spondylitis. The observation that the HLA-B27 heavy chain has a tendency to misfold has raised the possibility that associated diseases may belong in a rapidly expanding category of protein misfolding disorders. The synthesis of the HLA-B27 heavy chain, assembly with beta2m and the loading of peptide cargo, occurs in the endoplasmic reticulum (ER) before transport to the cell surface. The evidence indicates that misfolding occurs in the ER prior to b2m association and peptide optimization and is manifested in the formation of aberrant inter- and intra-chain disulfide bonds and accumulation of heavy chain bound to the chaperone BiP. Enhanced accumulation ofmisfolded heavy chains during the induction of class I expression by cytokines, can cause ER stress resulting in activation of the unfolded protein response (UPR). Effects of UPR activation on cytokine production are beginning to emerge and may provide important missinglinks between HLA-B27 misfolding and spondyloarthritis. In this chapter we will review what has been learned about HLA-B27 misfolding in human cells and in the transgenic rat model of spondyloarthritis-like disease, considering it in the context of other protein misfolding disorders. These studies provide a framework to support much needed translational work assessing HLA-B27 misfolding and UPR activation in patient-derived material, its consequences for disease pathogenesis and ultimately how and where to focus intervention strategies.

Pharmacological management of undifferentiated spondyloarthropathies

Among undifferentiated spondyloarthropathies (uSpA) are included incomplete forms or early phases of definite seronegative spondyloarthritides and cases of spondyloarthritides that remain undifferentiated. The treatment of all spondyloarthritides is more conditioned by the disease localisation, number of involved districts and severity of inflammation rather than by the subtype of the spondyloarthritides itself. Specific studies focused on the pharmacological approach to uSpA are scarce. As a consequence, many drugs are used on the basis of the experiences that have been gained in the treatment of other spondyloarthritides. At present, non-steroidal anti-inflammatory drugs/COX-2 inhibitors and disease-modifying antirheumatic drugs are the main therapeutic agents for the involvement of peripheral joints in uSpA. In those cases in which severe symptoms persist despite these treatments or when there is a severe axial involvement, antitumour necrosis factor agents represent an effective choice. In these patients, antitumour necrosis factor treatment raises the important possibility of blocking a shift from uSpA to differentiated severe forms of spondyloarthritides such as ankylosing spondylitis. Local administration of corticosteroids is useful when a small number of joints are involved, as is also the case for the extra-articular localisations of soft tissues.

Seronegative spondyloarthritides

Epidemiological studies on the spondyloarthritides have been hindered in the past by the lack of adequate classification criteria for the whole group of these diseases. Using the Amor and the European Spondyloathropathy Study Group (ESSG) criteria the total prevalence of such diseases has been found to be higher than estimated in the past. The prevalence of ankylosing spondylitis varies across populations, but closely parallels the frequency of HLA B27-associated subtypes. The lack of well established criteria for reactive arthritis and the varying expression of its clinical manifestations are the principal reasons for the under-reporting of the true prevalence and incidence of this type of spondyloarthritis. Few data exist on the prevalence and incidence of psoriatic arthritis. A recent European study on an inception cohort of patients having inflammatory bowel disease has evaluated the prevalence of spondyloarthritis using the ESSG criteria. Of the patients studied, 18% met these criteria. Undifferentiated spondyloarthritis is one of the most frequent spondyloarthritides. It also includes a number of different subtypes.

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.

Predisposing factors in the spondyloarthropathies: new insights into the role of HLA-B27

Spondyloarthropathies represent complex genetic diseases whose development is influenced by environmental factors. Estimates suggest that three to nine loci may be responsible for the majority of the genetic susceptibility to ankylosing spondylitis. The only susceptibility locus identified to date in multiple populations is HLA-B, where several HLA-B27 alleles (subtypes) are strongly associated with disease. Recent evidence implicates cytochrome P450 2D6 as a second locus, although its influence on overall risk appears small. Despite considerable efforts to define how HLA-B27 contributes to disease, its role remains enigmatic. Increasing evidence suggests it has effects that are unrelated to its physiologic function. The basis for this is unknown but may be a consequence of the unusual tendency of this allele to misfold.

HLA-B27 polymorphism

HLA-B27 is a serologic specificity that encompasses 25 different alleles that encode 23 different products (proteins): HLA-B*2701 to B*2723. These alleles are also called subtypes of HLA-B27, and they may have evolved from the most widespread subtype, B*2705. These subtypes are distinguished from changes mostly in exons 2 and 3, which encode the alpha 1 and alpha 2 domains of the B27 molecule, respectively. Occurrence of ankylosing spondylitis (AS) or related spondyloarthropathy (SpA) has thus far been documented in subjects possessing any one of the first ten (B*2701 to B*2710) subtypes studied. However, B*2706 in Southeast Asian and B*2709 in the Italian island population of Sardinia seem not to be associated with AS. The 13 most recent subtypes have not yet been studied for disease association. It is important to investigate which of them are and are not associated with AS and related SpA, and whether certain subtypes show any preferential association with some of the clinical features or forms of these diseases among the various ethnic/racial populations and geographic regions of the world. This is expected to provide clues as to the mechanism of disease association, and one of the strongest reasons to study the B27 subtypes is to learn the effects of the sequence variations on the peptide-binding specificity of the molecule. Among these peptides may be the putative arthritogenic peptide(s).

HLA-B27 and genetic predisposing factors in spondyloarthropathies

With the mapping of the human genome having been completed, our ability to investigate and ideally better understand the genetic basis of rheumatic diseases is advancing at a rapid pace. Substantial evidence strongly favors a direct role for HLA-B27 in genetic susceptibility to ankylosing spondylitis and related spondyloarthropathies, although the underlying molecular basis has yet to be identified. HLA-B27 contributes only 16 to 50% of the total genetic risk for the disease, clearly indicating that other genes must be involved. However, no other putative disease genes have yet been absolutely proven. Potential genes include MHC (HLA class II, low molecular weight proteasome [LMP], transporter associated with antigen processing (TAP), tumor necrosis factor [TNF]-alpha, and major histocompatibility complex class I chain-related gene A (MICA), as well as non-MHC genes (IL-1RA, IL-6, IL-10, and CYP2D6). Genome-wide screens have identified other chromosomal areas of interest: 1p, 2q, 6p, 9q, 10q, 16q, and 19q. However, different studies have given conflicting results. HLA-B27 itself is a serologic specificity, which encompasses 25 different alleles that encode 23 different products (proteins): HLA-B*2701 to B*2723. These alleles may have evolved from the most widespread subtype, B*2705, and two of them, B*2706 in Southeast Asia and B*2709 in Sardinia, seem not to be associated with ankylosing spondylitis. The distinction between the disease associated and nonassociated subtypes may provide clues to the actual role of B27 in disease pathogenesis.