Pathogenicity of Misfolded and Dimeric HLA-B27 Molecules

RNA sequencing and bioinformatics analysis of differentially expressed genes in the peripheral serum of ankylosing spondylitis patients

BACKGROUND

Ankylosing spondylitis (AS) is a chronic progressive autoimmune disease characterized by spinal and sacroiliac arthritis, but its pathogenesis and genetic basis are largely unclear.

METHODS

We randomly selected three serum samples each from an AS and a normal control (NC) group for high-throughput sequencing followed by using edgeR to find differentially expressed genes (DEGs). Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes, Reactome pathway analyses, and Gene Set Enrichment Analysis were used to comprehensively analyze the possible functions and pathways involved with these DEGs. Protein-protein interaction (PPI) networks were constructed using the STRING database and Cytoscape. The modules and hub genes of these DEGs were identified using MCODE and CytoHubba plugins. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was used to validate the expression levels of candidate genes in serum samples from AS patients and healthy controls.

RESULTS

We successfully identified 100 significant DEGs in serum. When we compared them with the NC group, 49 of these genes were upregulated in AS patients and 51 were downregulated. GO function and pathway enrichment analysis indicated that these DEGs were mainly enriched in several signaling pathways associated with endoplasmic reticulum stress, including protein processing in the endoplasmic reticulum, unfolded protein response, and ubiquitin-mediated proteolysis. We also constructed a PPI network and identified the highly connected top 10 hub genes. The expression levels of the candidate hub genes PPARG, MDM2, DNA2, STUB1, UBTF, and SLC25A37 were then validated by RT-qPCR analysis. Finally, receiver operating characteristic curve analysis suggested that PPARG and MDM2 may be the potential biomarkers of AS.

CONCLUSIONS

These findings may help to further elucidate the pathogenesis of AS and provide valuable potential gene biomarkers or targets for the diagnosis and treatment of AS.

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.

Measuring Synthesis and Degradation of MHC Class I Molecules

Major histocompatibility complex (MHC) class I molecules function to present pathogen derived peptides to cytotoxic T cells and act as ligands for Natural Killer cells, thus alerting the immune system to the presence of invading pathogens. However, some MHC class I molecules, most notably HLA-B27, can be strongly associated with autoimmune diseases. In addition, the MHC class I pathway is a target for numerous viral evasion strategies Understanding not only the antigen presenting functions, but also the biosynthesis and the degradation pathways of MHC class I molecules has therefore become important in determining their role in pathogen and autoimmune related diseases. Here, we describe how using epitope tagged MHC class I molecules can aid in the analysis of MHC class I molecule biosynthesis and degradation as well as complementary studies using conventional conformationally specific antibodies. Coupled together with pharmacological manipulation which can target both biosynthetic and degradative pathways, this offers a powerful tool in analyzing MHC class I molecules.

Ankylosing spondylitis: etiology, pathogenesis, and treatments

Ankylosing spondylitis (AS), a common type of spondyloarthropathy, is a chronic inflammatory autoimmune disease that mainly affects spine joints, causing severe, chronic pain; additionally, in more advanced cases, it can cause spine fusion. Significant progress in its pathophysiology and treatment has been achieved in the last decade. Immune cells and innate cytokines have been suggested to be crucial in the pathogenesis of AS, especially human leukocyte antigen (HLA)‑B27 and the interleukin‑23/17 axis. However, the pathogenesis of AS remains unclear. The current study reviewed the etiology and pathogenesis of AS, including genome-wide association studies and cytokine pathways. This study also summarized the current pharmaceutical and surgical treatment with a discussion of future potential therapies.

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

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

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.

Role of HLA-B27 in the pathogenesis of ankylosing spondylitis (Review)

The study of ankylosing spondylitis (AS) has made significant progress over the last decade. Genome-wide association studies have identified and further substantiated the role of susceptibility genes outside the major histocompatibility complex locus. However, human leukocyte antigen (HLA)-B27 has been suggested to be important in the pathogenesis of AS, contributing to ~20.1% of AS heritability. The current review will present the classical and non-classical forms of HLA-B27, as well as their pathogenic roles, and further discuss the hypotheses regarding the potential pathogenesis of AS. In addition, the association between the pathogenic role of HLA-B27 and inflammatory indexes, including the interleukin-23/−17 axis will be investigated to provide novel insights into the pathogenesis of AS. The aim of the present review is to provide an update of the current research into the pathogenesis of AS, and provide a comprehensive description of the pathogenic role of HLA-B27 in AS.

Disease-associated polymorphisms in ERAP1 do not alter endoplasmic reticulum stress in patients with ankylosing spondylitis

The mechanism by which human leukocyte antigen B27 (HLA-B27) contributes to ankylosing spondylitis (AS) remains unclear. Genetic studies demonstrate that association with and interaction between polymorphisms of endoplasmic reticulum aminopeptidase 1 (ERAP1) and HLA-B27 influence the risk of AS. It has been hypothesised that ERAP1-mediated HLA-B27 misfolding increases endoplasmic reticulum (ER) stress, driving an interleukin (IL) 23-dependent, pro-inflammatory immune response. We tested the hypothesis that AS-risk ERAP1 variants increase ER-stress and concomitant pro-inflammatory cytokine production in HLA-B27(+) but not HLA-B27(-) AS patients or controls. Forty-nine AS cases and 22 healthy controls were grouped according to HLA-B27 status and AS-associated ERAP1 rs30187 genotypes: HLA-B27(+)ERAP1(risk), HLA-B27(+)ERAP1(protective), HLA-B27(-)ERAP1(risk) and HLA-B27(-)ERAP1(protective). Expression levels of ER-stress markers GRP78 (8 kDa glucose-regulated protein), CHOP (C/EBP-homologous protein) and inflammatory cytokines were determined in peripheral blood mononuclear cell and ileal biopsies. We found no differences in ER-stress gene expression between HLA-B27(+) and HLA-B27(-) cases or healthy controls, or between cases or controls stratified by carriage of ERAP1 risk or protective alleles in the presence or absence of HLA-B27. No differences were observed between expression of IL17A or TNF (tumour necrosis factor) in HLA-B27(+)ERAP1(risk), HLA-B27(+)ERAP1(protective) and HLA-B27(-)ERAP1(protective) cases. These data demonstrate that aberrant ERAP1 activity and HLA-B27 carriage does not alter ER-stress levels in AS, suggesting that ERAP1 and HLA-B27 may influence disease susceptibility through other mechanisms.

Endoplasmic reticulum degradation-enhancing α-mannosidase-like protein 1 targets misfolded HLA-B27 dimers for endoplasmic reticulum-associated degradation

OBJECTIVE

HLA-B27 forms misfolded heavy chain dimers, which may predispose individuals to inflammatory arthritis by inducing endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). This study was undertaken to define the role of the UPR-induced ER-associated degradation (ERAD) pathway in the disposal of HLA-B27 dimeric conformers.

METHODS

HeLa cell lines expressing only 2 copies of a carboxy-terminally Sv5-tagged HLA-B27 were generated. The ER stress-induced protein ER degradation-enhancing α-mannosidase-like protein 1 (EDEM1) was overexpressed by transfection, and dimer levels were monitored by immunoblotting. EDEM1, the UPR-associated transcription factor X-box binding protein 1 (XBP-1), the E3 ubiquitin ligase hydroxymethylglutaryl-coenzyme A reductase degradation 1 (HRD1), and the degradation-associated proteins derlin 1 and derlin 2 were inhibited using either short hairpin RNA or dominant-negative mutants. The UPR-associated ERAD of HLA-B27 was confirmed using ER stress-inducing pharamacologic agents in kinetic and pulse chase assays.

RESULTS

We demonstrated that UPR-induced machinery can target HLA-B27 dimers and that dimer formation can be controlled by alterations to expression levels of components of the UPR-induced ERAD pathway. HLA-B27 dimers and misfolded major histocompatibility complex class I monomeric molecules bound to EDEM1 were detected, and overexpression of EDEM1 led to inhibition of HLA-B27 dimer formation. EDEM1 inhibition resulted in up-regulation of HLA-B27 dimers, while UPR-induced ERAD of dimers was prevented in the absence of EDEM1. HLA-B27 dimer formation was also enhanced in the absence of XBP-1, HRD1, and derlins 1 and 2.

CONCLUSION

The present findings indicate that the UPR ERAD pathway can dispose of HLA-B27 dimers, thus presenting a potential novel therapeutic target for modulation of HLA-B27-associated inflammatory disease.

Incidence of Toxoplasma retinochoroiditis in patients with ankylosing spondylitis after using TNF-α blockers

TNF-α blockers are associated with reactivation of latent granulomatous infections and almost 6% of the world population has some chorioretinitis (CR) caused by Toxoplasma gondii. Thus, the blockade of TNF-α could reactivate a latent toxoplasmosis infection (LTxI). This study was conducted to evaluate the prevalence and incidence of chronic and active CR related to T. gondii in patients with ankylosing spondylitis (AS). A total of 74 eyes from 37 active AS outpatients starting TNFα blockers were compared with 35 AS patients, matched to age and sex, under conventional therapy in a prospective and controlled trial. All patients underwent serological tests for T. gondii, as well as periodic ophthalmologic examination during 12months. Active CR was defined if a white, focal retinochoroidal lesion with overlying vitreous inflammation had been found. Retinochoroidal lesions with sharp edges, hyperpigmented borders and atrophic center were defined as CR scars. At baseline, no patient had active CR. From the 144 eyes examined, almost 6% had CR scars and only 2.1% had a typical toxoplasmic CR scar and all of them were negative for HLA-B27. During 12months of follow-up, no recurrence or new CR were observed. AS patients using TNF-α blockers do not have a higher risk of acute or chronic CR caused by T. gondii.

Measuring synthesis and degradation of MHC class I molecules

Major histocompatibility complex (MHC) class I molecules function to present pathogen-derived peptides to cytotoxic T cells or act as ligands for Natural Killer cells, thus alerting the immune system to the presence of invading pathogens. Furthermore MHC class I molecules can be strongly associated with autoimmune diseases. Therefore understanding not only the biosynthesis and the degradation pathways of MHC class I molecules has become important in determining their role in pathogen and autoimmune-related diseases. Here we describe how using epitope-tagged MHC class I molecules can aid in the analysis of MHC class I molecule biosynthesis and degradation and also complement studies using conventional conformationally specific antibodies. Coupled together with pharmacological manipulation which can target both biosynthetic and degradative pathways, this offers a powerful tool in analyzing MHC class I molecules.