Susceptibility to ankylosing spondylitis but not disease outcome is influenced by the level of HLA-B27 expression, which shows moderate variability over time

Exploring the HLA complex in autoimmunity: From the risk haplotypes to the modulation of expression

The genes mapping at the HLA region show high density, strong linkage disequilibrium and high polymorphism, which affect the association of HLA class I and class II genes with autoimmunity. We focused on the HLA haplotypes, genomic structures consisting of an array of specific alleles showing some degrees of genetic association with different autoimmune disorders. GWASs in many pathologies have identified variants in either the coding loci or the flanking regulatory regions, both in linkage disequilibrium in haplotypes, that are frequently associated with increased risk and may influence gene expression. We discuss the relevance of the HLA gene expression because the level of surface heterodimers determines the number of complexes presenting self-antigen and, thus, the strength of pathogenic autoreactive T cells immune response.

A Review of Laboratory Practices Using the HLA-B27 Survey by the College of American Pathologists: How Important Is Allele-Level Typing?

CONTEXT.—

Ankylosing spondylitis (AS) is an autoimmune disorder with a strong genetic risk, especially with HLA-B27. Clinical testing for HLA-B27 has been used to help diagnose patients with signs and symptoms of AS. Testing methods used by clinical laboratories for HLA-B27 fall into the broad categories of serologic/antibody- or molecular-based methods and have evolved over time. The College of American Pathologists (CAP) offers a proficiency testing survey for HLA-B27.

OBJECTIVE.—

To analyze HLA-B27 testing trends and their performance in the past decade, using the proficiency testing survey data submitted to CAP.

DESIGN.—

We analyzed the HLA-B27 CAP proficiency testing data from 2010 to 2020 for the method used, participant concordance, and error rates. Results from case scenarios to understand evolving scientific data around HLA-B27 risk alleles were also analyzed.

RESULTS.—

Antibody-based flow cytometry is the most common method, though it has decreased from 60% in 2010 to 52% in 2020, with a corresponding increase in molecular methods. Among the molecular methods, real-time polymerase chain reaction has increased from 2% to 15%. Flow cytometry had the highest error rate (5.33%), and sequence-specific oligonucleotide (0%) is the most accurate (0%). Results of case scenarios demonstrated that most participants understood that allele-level HLA-B27 typing results inform clinical interpretation, for example HLA-B*27:06 is not associated with AS.

CONCLUSIONS.—

These data demonstrated the changing trends for HLA-B27 testing during the past decade. HLA-B27 allelic typing provides a better understanding of AS association. This is possible by testing for the second field with methods like next-generation sequencing.

Antibodies for β2-Microglobulin and the Heavy Chains of HLA-E, HLA-F, and HLA-G Reflect the HLA-Variants on Activated Immune Cells and Phases of Disease Progression in Rheumatoid Arthritis Patients under Treatment

Rheumatoid arthritis (RA) is a progressive, inflammatory, autoimmune, symmetrical polyarticular arthritis. It is characterized by synovial infiltration and activation of several types of immune cells, culminating in their apoptosis and antibody generation against “altered” autoantigens. β2-microglobulin (β2m)-associated heavy chains (HCs) of HLA antigens, also known as closed conformers (Face-1), undergo “alteration” during activation of immune cells, resulting in β2m-free structural variants, including monomeric open conformers (Face-2) that are capable of dimerizing as either homodimers (Face-3) or as heterodimers (Face-4). β2m-free HCs uncover the cryptic epitopes that can elicit antibodies (Abs). We report here the levels of IgM and IgG Abs against both β2m and HCs of HLA-E, HLA-F, and HLA-G in 74 RA patients receiving immunosuppressive drugs. Anti-β2m IgM was present in 20 of 74 patients, whereas anti-β2m IgG was found in only 8 patients. Abs against β2m would be expected if Abs were generated against β2m-associated HLA HCs. The majority of patients were devoid of either anti-β2m IgM or IgG but had Abs against HCs of different HLA-Ib molecules. The paucity of anti-β2m Abs in this cohort of patients suggests that Abs were developed against β2m-free HLA HCs, such as Face-2, Face-3, and Face-4. While 63 of 68 patients had IgG Abs against anti-HLA-F HCs, 36 and 50 patients showed IgG Ab reactivity against HLA-E and anti-HLA-G HCs, respectively. Evidently, anti-HLA-F HC Abs are the most predominant anti-HLA-Ib HC IgG Abs in RA patients. The incidence and intensity of Abs against HLA-E, HLA-F, and HLA-G in the normal control group were much higher than those observed in RA patients. Evidently, the lower level of Abs in RA patients points to the impact of the immunosuppressive drugs on these patients. These results underscore the need for further studies to unravel the nature of HLA-F variants on activated immune cells and synoviocytes of RA patients.

The Impact of the 'Mis-Peptidome' on HLA Class I-Mediated Diseases: Contribution of ERAP1 and ERAP2 and Effects on the Immune Response

The strong association with the Major Histocompatibility Complex (MHC) class I genes represents a shared trait for a group of autoimmune/autoinflammatory disorders having in common immunopathogenetic basis as well as clinical features. Accordingly, the main risk factors for Ankylosing Spondylitis (AS), prototype of the Spondyloarthropathies (SpA), the Behçet's disease (BD), the Psoriasis (Ps) and the Birdshot Chorioretinopathy (BSCR) are HLA-B*27, HLA-B*51, HLA-C*06:02 and HLA-A*29:02, respectively. Despite the strength of the association, the HLA pathogenetic role in these diseases is far from being thoroughly understood. Furthermore, Genome-Wide Association Studies (GWAS) have highlighted other important susceptibility factors such as Endoplasmic Reticulum Aminopeptidase (ERAP) 1 and, less frequently, ERAP2 that refine the peptidome presented by HLA class I molecules to CD8⁺ T cells. Mass spectrometry analysis provided considerable knowledge of HLA-B*27, HLA-B*51, HLA-C*06:02 and HLA-A*29:02 immunopeptidome. However, the combined effect of several ERAP1 and ERAP2 allelic variants could generate an altered pool of peptides accounting for the "mis-immunopeptidome" that ranges from suboptimal to pathogenetic/harmful peptides able to induce non-canonical or autoreactive CD8⁺ T responses, activation of NK cells and/or garbling the classical functions of the HLA class I molecules. This review will focus on this class of epitopes as possible elicitors of atypical/harmful immune responses which can contribute to the pathogenesis of chronic inflammatory diseases.

A Possible Role of Intestinal Microbiota in the Pathogenesis of Ankylosing Spondylitis

Ankylosing spondylitis (AS) is a chronic inflammatory disease primarily affecting the sacroiliac joints and the spine, for which the pathogenesis is thought to be a result of the combination of host genetic factors and environmental triggers. However, the precise factors that determine one’s susceptibility to AS remain to be unraveled. With 100 trillion bacteria residing in the mammalian gut having established a symbiotic relation with their host influencing many aspects of host metabolism, physiology, and immunity, a growing body of evidence suggests that intestinal microbiota may play an important role in AS. Several mechanisms have been suggested to explain the potential role of the microbiome in the etiology of AS, such as alterations of intestinal permeability, stimulation of immune responses, and molecular mimicry. In this review, the existing evidence for the involvement of the microbiome in AS pathogenesis was discussed and the potential of intestinal microbiome-targeting strategies in the prevention and treatment of AS was evaluated.

Current perspective on the role of the interleukin-23/interleukin-17 axis in inflammation and disease (chronic arthritis and psoriasis)

TH17 is a lymphocyte subset, which is characterized by its polarization to secrete interleukin (IL)-17. IL-23 is the pivotal mediator responsible for TH17 differentiation and the IL-23/IL-17 axis has been strongly implicated in the pathogenesis of several immune mediated diseases, in particular chronic arthritis and skin psoriasis. This review will summarize the basic immunology and the new monoclonal antibodies, which antagonize this pathway allowing a new therapeutic approach.

Killer-cell immunoglobulin-like receptors (KIR) and HLA-class I heavy chains in ankylosing spondylitis

HLA-B27 (B27) interactions with the killer-cell immunoglobulin-like receptors (KIR) have been implicated in the pathogenesis of ankylosing spondylitis (AS), with consistent differences among populations. KIR3DL1 and possibly KIR3DS1 interact with classical B27, whereas KIR3DL2 binds B27 heavy chain dimers. The aim of this review is to summarize data from recent studies performed in our laboratory and from the literature, which provide support for a possible role of KIR3DL2/B27 dimer interactions in the pathogenesis of AS. Recent studies in cells from AS patients and from health controls carrying the predisposing B*2705 and the nonpredisposing B*2709 haplotypes, have shown a higher percentage of positive cells and a higher surface expression of KIR3DL2 receptors on natural killer (NK) and CD4+ T cells in B*2705 AS patients compared with B*2705, B*2709 and B27-negative healthy controls. Increased expression of HC10-reactive molecules on AS monocytes was seen, supporting the possible role of the KIR3DL2/B272 pair in the pathogenesis of AS. These results underline the importance of NK cells and innate immunity, and of CD4+ T cells in the inflammatory pathogenesis of AS.

Increased expression of human leucocyte antigen class I free heavy chains on monocytes of patients with spondyloarthritis and cells transfected with HLA-B27

Human leucocyte antigen (HLA)-B27 expression is correlated with spondyloarthritis (SpA), but its role in disease pathogenesis remains unclear. The aim of the study was to determine whether HLA-B27 free heavy chain (FHC) contributes to SpA pathogenesis. Flow cytometry was used to analyse the FHC expression on CD3+ and CD14+ cells in the peripheral blood (PB) and synovial fluid (SF) from SpA patients, healthy controls, and rheumatoid arthritis (RA) patients. Human monocytic U937 cell lines stably expressing enhanced green fluorescence protein (EGFP)/HLA-B27, EGFP/HLA-A2 or EGFP alone were created to further investigate the relation between HLA-B27 and FHC expression. The relative FHC level on CD14+ PB cells was significantly higher in SpA patients than in controls, but lower than on the SF cells of SpA patients. No significant correlation was found for relative FHC expression with HLA-B27 or β2-microglobulin expression. HLA-B27-transfected U937 cells expressed higher FHC levels than either EGFP/HLA-A2- or EGFP-transfected cells. HLA class I FHC expression was significantly increased on monocytes of SpA patients and HLA-B27-transfected cells, implying that FHC, perhaps mostly derived from HLA-B27, plays an important role in SpA pathogenesis.

HLA-B27 misfolding and ankylosing spondylitis

Understanding how HLA-B27 contributes to the pathogenesis of spondyloarthritis continues to be an important goal. Current efforts are aimed largely on three areas of investigation; peptide presentation to CD8T cells, abnormal forms of the HLA-B27 heavy chain and their recognition by leukocyte immunoglobulin-like receptors on immune effector cells, and HLA-B27 heavy chain misfolding and intrinsic biological effects on affected cells. In this chapter we review our current understanding of the causes and consequences of HLA-B27 misfolding, which can be defined biochemically as a propensity to oligomerize and form complexes in the endoplasmic reticulum (ER) with the chaperone BiP (HSPA5/GRP78). HLA-B27 misfolding is linked to an unusual combination of polymorphisms that identify this allele, and cause the heavy chain to fold and load peptides inefficiently. Misfolding can result in ER-associated degradation (ERAD) of heavy chains, which is mediated in part by the E3 ubiquitin ligase HRD1 (SYVN1), and the ubiquitin conjugating enzyme UBE2JL. Upregulation of HLA-B27 and accumulation of misfolded heavy chains can activate ER stress signaling pathways that orchestrate the unfolded protein response. In transgenic rats where HLA-B27 is overexpressed, UPR activation is prominent. However, it is specific for heavy chain misfolding, since overexpression of HLA-B7, an allele that does not misfold, fails to generate ER stress. UPR activation has been linked to cytokine dysregulation, promoting lL-23, IFNβ, and lL-1α production, and may activate the IL-23/IL-17 axis in these rats. IL-1α and IFNβ are pro- and anti-osteoclastogenic cytokines, respectively, that modulate osteoclast development in HLA-B27-expressing transgenic rat monocytes. Translational studies of patient derived cells expressing HLA-B27 at physiologic levels have provided evidence that ER stress and UPR activation can occur in peripheral blood, but this has not been reported to date in isolated macrophages. Inflamed gastrointestinal tissue reveals evidence for HLA-B27 misfolding, ERAD, and autophagy, without acute UPR activation. A more complete picture of conditions that impact HLA-B27 folding and misfolding, the full spectrum and time course of consequences of ER stress, and critical cell types involved is needed to understand the role of HLA-B27 misfolding in spondyloarthritis pathogenesis.

The link between ankylosing spondylitis, Crohn's disease, Klebsiella, and starch consumption

Both ankylosing spondylitis (AS) and Crohn's disease (CD) are chronic and potentially disabling interrelated conditions, which have been included under the group of spondyloarthropathies. The results of a large number of studies support the idea that an enteropathic pathogen, Klebsiella pneumoniae, is the most likely triggering factor involved in the initiation and development of these diseases. Increased starch consumptions by genetically susceptible individuals such as those possessing HLA-B27 allelotypes could trigger the disease in both AS and CD by enhancing the growth and perpetuation of the Klebsiella microbes in the bowel. Exposure to increased levels of these microbes will lead to the production of elevated levels of anti-Klebsiella antibodies as well as autoantibodies against cross-reactive self-antigens with resultant pathological lesions in the bowel and joints. Hence, a decrease of starch-containing products in the daily dietary intake could have a beneficial therapeutic effect on the disease especially when used in conjunction with the currently available medical therapies in the treatment of patients with AS and CD.