The Role of Autophagy in the Degradation of Misfolded HLA-B27 Heavy Chains

METTL14-m6A-FOXO3a axis regulates autophagy and inflammation in ankylosing spondylitis

The role of m6A in ankylosing spondylitis (AS) remains largely obscure. In this study, we found that m6A modification was decreased in T cells of AS, and the abnormal m6A modification was attributed to the downregulation of methyltransferase-like 14 (METTL14). METTL14 exerted a critical role in regulating autophagy activity and inflammation via targeting Forkhead box O3a (FOXO3a). Mechanistically, the loss of METTL14 decreased the expression of FOXO3a, leading to the damage of autophagic flux and the aggravation of inflammation. Inversely, the forced expression of METTL14 upregulated the expression of FOXO3a, thereby activating autophagy and alleviating inflammation. Furthermore, our results revealed that METTL14 targeted FOXO3a mRNA and regulated its expression and stability in a m6A-dependent manner. These findings uncovered the functional importance of m6A methylation mechanisms in the regulation of autophagy and inflammation, which expanded our understanding of this interaction and was critical for the development of therapeutic strategies for AS.

Comprehensive profiling of extracellular vesicles in uveitis and scleritis enables biomarker discovery and mechanism exploration

BACKGROUND

Uveitis and posterior scleritis are sight-threatening diseases with undefined pathogenesis and accurate diagnosis remains challenging.

METHODS

Two plasma-derived extracellular vesicle (EV) subpopulations, small and large EVs, obtained from patients with ankylosing spondylitis-related uveitis, Behcet's disease uveitis, Vogt-Koyanagi-Harada syndrome, and posterior scleritis were subjected to proteomics analysis alongside plasma using SWATH-MS. A comprehensive bioinformatics analysis was performed on the proteomic profiles of sEVs, lEVs, and plasma. Candidate biomarkers were validated in a new cohort using ELISA. Pearson correlation analysis was performed to analyze the relationship between clinical parameters and proteomic data. Connectivity map database was used to predict therapeutic agents.

RESULTS

In total, 3,668 proteins were identified and over 3000 proteins were quantified from 278 samples. When comparing diseased group to healthy control, the proteomic profiles of the two EV subgroups were more correlated with disease than plasma. Comprehensive bioinformatics analysis highlighted potential pathogenic mechanisms for these diseases. Potential biomarker panels for four diseases were identified and validated. We found a negative correlation between plasma endothelin-converting enzyme 1 level and mean retinal thickness. Potential therapeutic drugs were proposed, and their targets were identified.

CONCLUSIONS

This study provides a proteomic landscape of plasma and EVs involved in ankylosing spondylitis-related uveitis, Behcet's disease uveitis, Vogt-Koyanagi-Harada syndrome, and posterior scleritis, offers insights into disease pathogenesis, identifies valuable biomarker candidates, and proposes promising therapeutic agents.

Uncovering the Underworld of Axial Spondyloarthritis

Axial spondyloarthritis (axial-SpA) is a multifactorial disease characterized by inflammation in sacroiliac joints and spine, bone reabsorption, and aberrant bone deposition, which may lead to ankylosis. Disease pathogenesis depends on genetic, immunological, mechanical, and bioenvironmental factors. HLA-B27 represents the most important genetic factor, although the disease may also develop in its absence. This MHC class I molecule has been deeply studied from a molecular point of view. Different theories, including the arthritogenic peptide, the unfolded protein response, and HLA-B27 homodimers formation, have been proposed to explain its role. From an immunological point of view, a complex interplay between the innate and adaptive immune system is involved in disease onset. Unlike other systemic autoimmune diseases, the innate immune system in axial-SpA has a crucial role marked by abnormal activity of innate immune cells, including γδ T cells, type 3 innate lymphoid cells, neutrophils, and mucosal-associated invariant T cells, at tissue-specific sites prone to the disease. On the other hand, a T cell adaptive response would seem involved in axial-SpA pathogenesis as emphasized by several studies focusing on TCR low clonal heterogeneity and clonal expansions as well as an interindividual sharing of CD4/8 T cell receptors. As a result of this immune dysregulation, several proinflammatory molecules are produced following the activation of tangled intracellular pathways involved in pathomechanisms of axial-SpA. This review aims to expand the current understanding of axial-SpA pathogenesis, pointing out novel molecular mechanisms leading to disease development and to further investigate potential therapeutic targets.

Paradoxical Effects of Endoplasmic Reticulum Aminopeptidase 1 Deficiency on HLA-B27 and Its Role as an Epistatic Modifier in Experimental Spondyloarthritis

OBJECTIVE

We undertook this study to examine the functional basis for epistasis between endoplasmic reticulum aminopeptidase 1 (ERAP1) and HLA-B27 in experimental spondyloarthritis (SpA).

METHODS

ERAP1-knockout rats were created using genome editing and bred with HLA-B27/human β2 -microglobulin-transgenic (HLA-B27-Tg) rats and HLA-B7-Tg rats. The effects of ERAP1 deficiency on HLA allotypes were determined using immunoprecipitation and immunoblotting, flow cytometry, allogeneic T cell proliferation assays, and gene expression analyses. Animals were examined for clinical features of disease, and tissue was assessed by histology.

RESULTS

ERAP1 deficiency increased the ratio of folded to unfolded (β2 m-free) HLA-B27 heavy chains, while having the opposite effect on HLA-B7. Furthermore, in rats with ERAP1 deficiency, HLA-B27 misfolding was reduced, while free HLA-B27 heavy chain dimers on the cell surface and monomers were increased. The effects of ERAP1 deficiency persisted during up-regulation of HLA-B27 and led to a reduction in endoplasmic reticulum stress. ERAP1 deficiency reduced the prevalence of arthritis in HLA-B27-Tg rats by two-thirds without reducing gastrointestinal inflammation. Dendritic cell abnormalities attributed to the presence of HLA-B27, including reduced allogeneic T cell stimulation and loss of CD103-positive/major histocompatibility complex class II-positive cells, were not rescued by ERAP1 deficiency, while excess Il23a up-regulation was mitigated.

CONCLUSION

ERAP1 deficiency reduced HLA-B27 misfolding and improved folding while having opposing effects on HLA-B7. The finding that HLA-B27-Tg rats had partial protection against SpA in this study is consistent with genetic evidence that loss-of-function and/or reduced expression of ERAP1 reduces the risk of ankylosing spondylitis. Functional studies support the concept that the effects of ERAP1 on HLA-B27 and SpA may be a consequence of how peptides affect the biology of this allotype rather than their role as antigenic determinants.

Natural product-based PROteolysis TArgeting Chimeras (PROTACs)

Covering: upto 2022Natural products have an embedded recognition of protein surfaces. They possess this property as they are produced by biosynthetic enzymes and are substrates for one or more enzymes in the biosynthetic pathway. The inherent advantages, compared to synthetic compound libraries, is this ligand-protein binding which is, in many cases, a function of the 3-dimensional properties. Protein degradation is a recent novel therapeutic approach with several compounds now in the clinic. This review highlights the potential of PROteolysis TArgeting Chimeras (PROTACs) in the area of natural products. The approach will complement existing approaches such as the direct use of a bioactive natural product or its analogues, pharmacophore development and drug-antibody conjugates. The chemical synthesis and challenges of using natural product-based PROTACs are summarised. The review also highlights methods to detect the ternary complexes necessary for PROTAC mechanism of action.

Joint together: The etiology and pathogenesis of ankylosing spondylitis

Spondyloarthritis (SpA) refers to a group of diseases with inflammation in joints and spines. In this family, ankylosing spondylitis (AS) is a rare but classic form that mainly involves the spine and sacroiliac joint, leading to the loss of flexibility and fusion of the spine. Compared to other diseases in SpA, AS has a very distinct hereditary disposition and pattern of involvement, and several hypotheses about its etiopathogenesis have been proposed. In spite of significant advances made in Th17 dynamics and AS treatment, the underlying mechanism remains concealed. To this end, we covered several topics, including the nature of the immune response, the microenvironment in the articulation that is behind the disease's progression, and the split between the hypotheses and the evidence on how the intestine affects arthritis. In this review, we describe the current findings of AS and SpA, with the aim of providing an integrated view of the initiation of inflammation and the development of the disease.

Resistance to unfolding by acidic pH and resistance to lysosomal degradation explains disease-association of HLA-B27 subtypes

Several hypotheses have been proposed to explain the high rate of disease association of HLA-B27 with ankylosing spondylitis (AS), including formation of disulfide-bonded dimers and misfolding of the heavy chain (HC), involving formation of high molecular weight (HMW) multimers. Recently, we have shown that the HMW entities of non-disease associated (non-DA) subtypes cause activation of endosomal-lysosomal pathways, while disease-associated (DA) subtypes of HLA-B27 cause activation of autophagy and unfolded protein response (UPR) pathways. In this paper, we seek an explanation for the failure of these pathways to degrade the HMW entities of DA subtypes of HLA-B27, using a combination of in vitro assays, using extracellular domains of heavy chains (EDHC), as well as in vivo assays, using stable transfectants of the full lengths of heavy chains (FLHC) of DA and non-DA subtypes. Our data shows that both DA and non-DA subtypes form HMW entities. However, non-DA HMW entities display far greater levels of degradation than DA HMW species. Non-DA EDHC display greater loss of structure at lysosomal pH in vitro. This was confirmed by experiments showing that (i) DA FLHCs co-localize with LAMP1, and (ii) induction of autophagy by rapamycin causes significant decrease in levels of non-DA HMW entities, but not that of DA HMW entities. These results point towards lack of facile lysosomal clearance of FLHCs of DA subtypes, suggesting that disease association of HLA-B27 subtypes is correlated with higher persistence of HMW entities in the low pH of lysosomes, with higher potential to trigger immune response.

Differences in Cellular Clearing Mechanisms of Aggregates of Two Subtypes of HLA-B27

Ankylosing spondylitis (AS) belongs to a group of diseases, called spondyloarthropathies (SpA), that are strongly associated with the genetic marker HLA-B27. AS is characterized by inflammation of joints and primarily affects the spine. Over 160 subtypes of HLA-B27 are known, owing to high polymorphism. Some are strongly associated with disease (e.g., B*2704), whereas others are not (e.g., B*2709). Misfolding of HLA-B27 molecules [as dimers, or as high-molecular-weight (HMW) oligomers] is one of several hypotheses proposed to explain the link between HLA-B27 and AS. Our group has previously established the existence of HMW species of HLA-B27 in AS patients. Still, very little is known about the mechanisms underlying differences in pathogenic outcomes of different HLA-B27 subtypes. We conducted a proteomics-based evaluation of the differential disease association of HLA B*2704 and B*2709, using stable transfectants of genes encoding the two proteins. A clear difference was observed in protein clearance mechanisms: whereas unfolded protein response (UPR), autophagy, and aggresomes were involved in the degradation of B*2704, the endosome–lysosome machinery was primarily involved in B*2709 degradation. These differences offer insights into the differential disease association of B*2704 and B*2709.

Monocytes and Macrophages in Spondyloarthritis: Functional Roles and Effects of Current Therapies

Spondyloarthritis (SpA) is a family of chronic inflammatory diseases, being the most prevalent ankylosing spondylitis (AS) and psoriatic arthritis (PsA). These diseases share genetic, clinical and immunological features, such as the implication of human leukocyte antigen (HLA) class I molecule 27 (HLA-B27), the inflammation of peripheral, spine and sacroiliac joints and the presence of extra-articular manifestations (psoriasis, anterior uveitis, enthesitis and inflammatory bowel disease). Monocytes and macrophages are essential cells of the innate immune system and are the first line of defence against external agents. In rheumatic diseases including SpA, the frequency and phenotypic and functional characteristics of both cell types are deregulated and are involved in the pathogenesis of these diseases. In fact, monocytes and macrophages play key roles in the inflammatory processes characteristics of SpA. The aim of this review is analysing the characteristics and functional roles of monocytes and macrophages in these diseases, as well as the impact of different current therapies on these cell types.

The gut-enthesis axis and the pathogenesis of Spondyloarthritis

Subclinical inflammation is associated with Spondylarthritis (SpA). SpA patients show features of dysbiosis, altered gut barrier function, and local expansion of innate and innate-like cells involved in type 3 immune response. The recirculation of intestinal primed immune cells into the bloodstream and, in some cases, in the joints and the inflamed bone marrow of SpA patients gave the basis of the gut-joint axis theory. In the light of the critical role of enthesis in the pathogenesis of SpA and the identification of mucosal-derived immune cells residing into the normal human enthesis, a gut-enthesis axis is also likely to exist. This work reviews the current knowledge on enthesis-associated innate immune cells' primary involvement in enthesitis development, questions their origin, and critically discusses the clues supporting the existence of a gut-enthesis axis contributing to SpA development.

Immunopathophysiology of Juvenile Spondyloarthritis (jSpA): The "Out of the Box" View on Epigenetics, Neuroendocrine Pathways and Role of the Macrophage Migration Inhibitory Factor (MIF)

Juvenile spondyloarthritis (jSpA) is a an umbrella term for heterogeneous group of related seronegative inflammatory disorders sharing common symptoms. Although it mainly affects children and adolescents, it often remains active during adulthood. Genetic and environmental factors are involved in its occurrence, although the exact underlying immunopathophysiology remains incompletely elucidated. Accumulated evidence suggests that, in affected patients, subclinical gut inflammation caused by intestinal dysbiosis, is pivotal to the future development of synovial-entheseal complex inflammation. While the predominant role of IL17/23 axis, TNF-α, and IL-7 in the pathophysiology of SpA, including jSpA, is firmly established, the role of the cytokine macrophage migration inhibitory factor (MIF) is generally overlooked. The purpose of this review is to discuss and emphasize the role of epigenetics, neuroendocrine pathways and the hypothalamic-pituitary (HPA) axis, and to propose a novel hypothesis of the role of decreased NLRP3 gene expression and possibly MIF in the early phases of jSpA development. The decreased NLRP3 gene expression in the latter, due to hypomethylation of promotor site, is (one of) the cause for inflammasome malfunction leading to gut dysbiosis observed in patients with early jSpA. In addition, we highlight the role of MIF in the complex innate, adaptive cellular and main effector cytokine network, Finally, since treatment of advanced bone pathology in SpA remains an unmet clinical need, I suggest possible new drug targets with the aim to ultimately improve treatment efficacy and long-term outcome of jSpA patients.

MicroRNAs in Axial Spondylarthritis: an Overview of the Recent Progresses in the Field with a Focus on Ankylosing Spondylitis and Psoriatic Arthritis

Purpose of Review

To highlight the recent discoveries and lines of evidence on the role of microRNAs in ankylosing spondylitis (AS) and psoriatic arthritis (PsA), focusing on their expression profiling and mechanisms of action.

Recent Findings

AS and PsA are chronic inflammatory musculoskeletal diseases with axial manifestations and represent an excellent model for studying microRNAs contribution to the disease pathogenesis, particularly through immunomodulation, inflammation, and bone remodelling, or their value as candidate diagnostic and prognostic biomarkers.

Summary

MicroRNAs are single-stranded nucleotides able to regulate gene expression. They are a key component of the epigenetic machinery, involved in physiological and pathological processes. The contribution of microRNAs in AS and PsA (such as miR-29a in regulating bone metabolism) is highlighted by several works in the field but their utility as possible markers must be still confirmed, particularly in larger patients’ cohorts.

Ankylosing spondylitis: an autoimmune or autoinflammatory disease?

Ankylosing spondylitis (AS) is a chronic inflammatory disorder of unknown aetiology. Unlike other systemic autoimmune diseases, in AS, the innate immune system has a dominant role characterized by aberrant activity of innate and innate-like immune cells, including γδ T cells, group 3 innate lymphoid cells, neutrophils, mucosal-associated invariant T cells and mast cells, at sites predisposed to the disease. The intestine is involved in disease manifestations, as it is at the forefront of the interaction between the mucosal-associated immune cells and the intestinal microbiota. Similarly, biomechanical factors, such as entheseal micro-trauma, might also be involved in the pathogenesis of the articular manifestation of AS, and sentinel immune cells located in the entheses could provide links between local damage, genetic predisposition and the development of chronic inflammation. Although these elements might support the autoinflammatory nature of AS, studies demonstrating the presence of autoantibodies (such as anti-CD74, anti-sclerostin and anti-noggin antibodies) and evidence of activation and clonal expansion of T cell populations support an autoimmune component to the disease. This Review presents the evidence for autoinflammation and the evidence for autoimmunity in AS and, by discussing the pathophysiological factors associated with each, aims to reconcile the two hypotheses.

Aberrant antigen processing and presentation: Key pathogenic factors leading to immune activation in Ankylosing spondylitis

The strong association of HLA-B*27 with ankylosing spondylitis (AS) was first reported nearly 50 years ago. However, the mechanistic link between HLA-B*27 and AS has remained an enigma. While 85-90% of AS patients possess HLA-B*27, majority of HLA-B*27 healthy individuals do not develop AS. This suggests that additional genes and genetic regions interplay with HLA-B*27 to cause AS. Previous genome-wide association studies (GWAS) identified key genes that are distinctively expressed in AS, including the Endoplasmic Reticulum Aminopeptidase (ERAP) 1 and ERAP2. As these gene-encoding molecules are primarily implicated in the process of peptide processing and presentation, potential pathological interaction of these molecules with HLA-B*27 may operate to cause AS by activating downstream immune responses. The aberrant peptide processing also gives rise to the accumulation of unstable protein complex in endoplasmic reticulum (ER), which drives endoplasmic reticulum-associated protein degradation (ERAD) and unfolded protein response (UPR) and activates autophagy. In this review, we describe the current hypotheses of AS pathogenesis, focusing on antigen processing and presentation operated by HLA-B*27 and associated molecules that may contribute to the disease initiation and progression of AS.

The enigmatic role of HLA-B*27 in spondyloarthritis pathogenesis

Establishing a clear role for HLA-B*27 in the pathogenesis of spondyloarthritis continues to be challenging. Aberrant properties of the heavy chain as well as a potential role presenting arthritogenic peptides continue to be pursued as plausible mechanisms. Recent studies implicate HLA-B*27 in aberrant bone formation. An unanticipated cell surface interaction between HLA-B*27 and the bone morphogenetic protein pathway receptor subunit ALK2 may augment TGFβ superfamily signaling pathways, increasing responsiveness to Activin A and TGFβ. This has the potential to increase bone formation as well as Th17 T cell development, presenting an attractive model to explain several aspects of axial and peripheral spondyloarthritis. In a separate study, intracellular effects of misfolded HLA-B*27 implicate this mechanism in increased osteoblast mineralization and bone formation. HLA-B*27 expression in early osteoblasts activates unfolded protein response-mediated X-box binding protein-1 mRNA splicing and induction of the retinoic acid receptor-β gene, with downstream increases in expression of tissue non-specific alkaline phosphatase. Increased TNAP expression in osteoblasts was linked to increased mineralization in vitro and bone formation in vivo. In the ongoing search for evidence of arthritogenic peptides, high-throughput TCR (T cell receptor) sequencing has provided evidence for reduced clonal expansion and increased TCR diversity in ankylosing spondylitis. In addition to two common CD8+ TCR sequences identified in one study, similar CD8 and CD4 TCR motifs were found in another study. Further work will be needed to shed light on the nature of the peptide-HLA class I complex recognized by these T cells and its role in disease.

Lessons on SpA pathogenesis from animal models

Understanding the complex mechanisms underlying a disorder such as spondyloarthritis (SpA) may benefit from studying animal models. Several suitable models have been developed, in particular to investigate the role of genetic factors predisposing to SpA, including HLA-B27, ERAP1, and genes related to the interleukin (IL)-23/IL-17 axis. One of the best examples of such research is the HLA-B27 transgenic rat model that fostered the emergence of original theories regarding HLA-B27 pathogenicity, including dysregulation of innate immunity, contribution of the adaptive immune system to chronic inflammation, and influence of the microbiota on disease development. Very recently, a new model of HLA-B27 transgenic Drosophila helped to expand further some of those theories in an unexpected direction involving the TGFβ/BMP family of mediators. On the other hand, several spontaneous, inducible, and/or genetically modified mouse models-including SKG mouse, TNF^ΔARE mouse and IL-23-inducible mouse model of SpA-have highlighted the importance of TNFα and IL-23/IL-17 axis in the development of SpA manifestations. Altogether, those animal models afford not only to study disease mechanism but also to investigate putative therapeutic targets.

Understanding the Pathogenesis of Spondyloarthritis

Spondyloarthritis comprises a group of inflammatory diseases of the joints and spine, with various clinical manifestations. The group includes ankylosing spondylitis, reactive arthritis, psoriatic arthritis, arthritis associated with inflammatory bowel disease, and undifferentiated spondyloarthritis. The exact etiology and pathogenesis of spondyloarthritis are still unknown, but five hypotheses explaining the pathogenesis exist. These hypotheses suggest that spondyloarthritis is caused by arthritogenic peptides, an unfolded protein response, HLA-B*27 homodimer formation, malfunctioning endoplasmic reticulum aminopeptidases, and, last but not least, gut inflammation and dysbiosis. Here we discuss the five hypotheses and the evidence supporting each. In all of these hypotheses, HLA-B*27 plays a central role. It is likely that a combination of these hypotheses, with HLA-B*27 taking center stage, will eventually explain the development of spondyloarthritis in predisposed individuals.

HLA-B27 Subtypes Predisposing to Ankylosing Spondylitis Accumulate in an Endoplasmic Reticulum-Derived Compartment Apart From the Peptide-Loading Complex

OBJECTIVE

It was previously shown that HLA-B27 subtypes predisposing to spondyloarthritis (SpA), i.e., B*27:02, B*27:05, and B*27:07, displayed an increased propensity to form intracellular oligomers and to accumulate at a high density in cytoplasmic vesicles, as compared to the non-SpA-associated HLA-B*07:02 and HLA-B*27:06. This study was undertaken to characterize the nature and content of HLA-B-containing vesicles and to further examine their relevance to SpA predisposition.

METHODS

Vesicles containing HLA-B proteins were detected in transfected HeLa cells and in cells from SpA patients or HLA-B27/human β₂ -microglobulin (hβ₂ m)-transgenic rats, by microscopy. The nature and content of HLA-B-containing vesicles were characterized in colocalization experiments with appropriate markers.

RESULTS

The SpA-associated HLA-B*27:04 subtype accumulated at higher levels (P < 10^-5 ) in cytoplasmic vesicles compared to HLA-B*27:06, from which it differs only by 2 substitutions, reinforcing the correlation between vesicle formation and SpA predisposition. Colocalization studies showed that those vesicles contained misfolded HLA-B heavy chain along with β₂ m and endoplasmic reticulum (ER) chaperones (calnexin, calreticulin, BiP, glucose-regulated protein 94-kd) and belonged to the ER but were distinct from the peptide-loading complex (PLC). Similar vesicles were observed in immune cells from HLA-B27+ SpA patients, in greater abundance than in healthy controls (P < 0.01), and in dendritic cells from HLA-B27/hβ₂ m transgenic rats, correlating with SpA susceptibility.

CONCLUSION

Accumulation of misfolded HLA-B heavy chain along with β₂ m and ER chaperones into ER-derived vesicles distinct from the PLC is a characteristic feature of HLA-B27 subtypes predisposing to SpA. This phenomenon could contribute to HLA-B27 pathogenicity, via a noncanonical mechanism.

mTOR Blockade by Rapamycin in Spondyloarthritis: Impact on Inflammation and New Bone Formation in vitro and in vivo

Introduction: Spondyloarthritis (SpA) is characterized by inflammation, articular bone erosions and pathologic new bone formation. Targeting TNFα or IL-17A with current available therapies reduces inflammation in SpA, however, treatment of the bone pathology in SpA remains an unmet clinical need. Activation of the mammalian target Of rapamycin (mTOR) promotes IL-17A expression and osteogenesis. Therefore, the inhibition of mTOR (with rapamycin) could be a promising therapeutic avenue in SpA.

Objectives: To investigate the effect of blocking mTOR on inflammation, bone erosions and new bone formation in SpA.

Methods: Peripheral blood mononuclear cells (PBMCs) from patients with SpA were stimulated with anti-CD3/CD28 in the presence or absence of rapamycin and the resulting cytokine expression was assessed. Fibroblast-like synoviocytes (FLS) from SpA patients were assessed for osteogenic differentiation potential in conditions with TNFα, IL-17A, or TNFα plus IL-17A, in the presence or absence of rapamycin. HLA-B27/Huβ2m transgenic rats were immunized with low dose heat-inactivated Mycobacterium tuberculosis (M. tub), treated with 1.5 mg/kg rapamycin prophylactically or therapeutically and monitored for arthritis and spondylitis. Histology and mRNA analysis were performed after 5 weeks of treatment to assess inflammation and bone pathology.

Results:In vitro TNFα and IL-17A protein production by SpA PBMCs was inhibited in the presence of rapamycin. Rapamycin also inhibited osteogenic differentiation of human SpA FLS. Ex vivo analysis of SpA synovial biopsies indicated activation of the mTOR pathway in the synovial tissue of SpA patients. In vivo, prophylactic treatment of HLA-B27/Huβ2m transgenic rats with rapamycin significantly inhibited the development and severity of inflammation in peripheral joints and spine (arthritis and spondylitis), with histological evidence of reduced bone erosions and new bone formation around peripheral joints. In addition, therapeutic treatment with rapamycin significantly decreased severity of arthritis and spondylitis, with peripheral joint histology showing reduced inflammation, bone erosions and new bone formation. IL-17A mRNA expression was decreased in the metacarpophalangeal joints after rapamycin treatment.

Conclusion: mTOR blockade inhibits IL-17A and TNFα production by PBMCs, and osteogenic differentiation of FLS from patients with SpA in vitro. In the HLA-B27 transgenic rat model of SpA, rapamycin inhibits arthritis and spondylitis development and severity, reduces articular bone erosions, decreases pathologic new bone formation and suppresses IL-17A expression. These results may support efforts to evaluate the efficacy of targeting the mTOR pathway in SpA patients.

Degradation of proteins by PROTACs and other strategies

Blocking the biological functions of scaffold proteins and aggregated proteins is a challenging goal. PROTAC proteolysis-targeting chimaera (PROTAC) technology may be the solution, considering its ability to selectively degrade target proteins. Recent progress in the PROTAC strategy include identification of the structure of the first ternary eutectic complex, extra-terminal domain-4-PROTAC-Von-Hippel-Lindau (BRD4-PROTAC-VHL), and PROTAC ARV-110 has entered clinical trials for the treatment of prostate cancer in 2019. These discoveries strongly proved the value of the PROTAC strategy. In this perspective, we summarized recent meaningful research of PROTAC, including the types of degradation proteins, preliminary biological data in vitro and in vivo, and new E3 ubiquitin ligases. Importantly, the molecular design, optimization strategy and clinical application of candidate molecules are highlighted in detail. Future perspectives for development of advanced PROTAC in medical fields have also been discussed systematically.

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.