Relationship between inflammation, bone destruction, and osteoproliferation in the HLA-B27/human β2 -microglobulin-transgenic rat model of spondylarthritis

Evaluation of β2-microglobulin in the condition and prognosis of psoriasis patients

BACKGROUND

Numerous studies have linked the inflammatory pathway in psoriasis and metabolic disease, while no specific marker defined it. It is worth exploring the association of β2-microglobulin (β2M) in psoriasis severity and comorbidities.

OBJECTIVES

To investigate the correlation between blood β2M level and psoriasis severity, to explore the inflammatory factors influencing the occurrence of psoriasis comorbidities such as arthritis, diabetes, and hypertension.

METHODS

Ninety-seven psoriasis patients were analyzed in the cohort retrospective study during 12 weeks.

RESULTS

Significantly higher levels of blood β2M and ESR were observed in the group that patients' PASI ≥10 than in the group that PASI <10. Blood β2M level had strong significantly positive correlations with the PASI in Pearson's correlation analysis. In the model that systemic inflammatory factors to find psoriasis comorbidity risk factors, logistic regression analysis showed that blood β2M level was the significant risk factor associated with diabetes and hypertension. High-sensitivity C-reactive protein (hsCRP) was the significant risk factor associated with arthritis.

CONCLUSIONS

Patients with a severer psoriasis tended to have higher blood β2M levels and severer inflammatory state. In the systemic inflammation indexes, the level of blood β2M affected the risk of hypertension and diabetes, and hsCRP affected the risk of arthritis in patients with psoriasis.

Bone Involvement in Rheumatoid Arthritis and Spondyloartritis: An Updated Review

Several rheumatologic diseases are primarily distinguished by their involvement of bone tissue, which not only serves as a mere target of the condition but often plays a pivotal role in its pathogenesis. This scenario is particularly prominent in chronic inflammatory arthritis such as rheumatoid arthritis (RA) and spondyloarthritis (SpA). Given the immunological and systemic nature of these diseases, in this review, we report an overview of the pathogenic mechanisms underlying specific bone involvement, focusing on the complex interactions that occur between bone tissue's own cells and the molecular and cellular actors of the immune system, a recent and fascinating field of interest defined as osteoimmunology. Specifically, we comprehensively elaborate on the distinct pathogenic mechanisms of bone erosion seen in both rheumatoid arthritis and spondyloarthritis, as well as the characteristic process of aberrant bone formation observed in spondyloarthritis. Lastly, chronic inflammatory arthritis leads to systemic bone involvement, resulting in systemic bone loss and consequent osteoporosis, along with increased skeletal fragility.

Role of neutrophil interleukin-23 in spondyloarthropathy spectrum disorders

Neutrophilic inflammation is a pervasive characteristic common to spondyloarthropathies and related disorders. This inflammation manifests as Munro's microabscesses of the skin and osteoarticular neutrophilic inflammation in patients with psoriatic arthritis, intestinal crypt abscesses in patients with inflammatory bowel disease, ocular hypopyon in anterior uveitis, and neutrophilic macroscopic and microscopic inflammation in patients with Behçet's disease. Strong MHC class I associations are seen in these diseases, which represent so-called MHC-I-opathies, and these associations indicate an involvement of CD8 T-cell immunopathology that is not yet well understood. In this Personal View, we highlight emerging data suggesting that the T-cell-neutrophil axis involves both a T-cell-mediated and interleukin (IL)-17-mediated (type 17) recruitment and activation of neutrophils, and also a sequestration of activated neutrophils at disease sites that might directly amplify type 17 T-cell responses. This amplification likely involves neutrophilic production of IL-23 and proteases as well as other feedback mechanisms that could be regulated by local microbiota, pathogens, or tissue damage. This crosstalk between innate and adaptive immunity offers a novel explanation for how bacterial and fungal microbes at barrier sites could innately control type 17 T-cell development, with the aim of restoring tissue homoeostasis, and could potentially explain features of clinical disease and treatment response, such as the fast-onset action of the IL-23 pathway blockade in certain patients. This axis could be crucial to understanding non-response to IL-23 inhibitors among patients with ankylosing spondylitis, as the axial skeleton is a site rich in neutrophils and a site of haematopoiesis with myelopoiesis in adults.

CXCL12/CXCR4-Rac1-mediated migration of osteogenic precursor cells contributes to pathological new bone formation in ankylosing spondylitis

Ankylosing spondylitis (AS) is a chronic inflammatory disease characterized by inflammatory back pain and spinal ankylosis due to pathological new bone formation. Here, we identified CXCL12 as a critical contributor to pathological new bone formation through recruitment of osteogenic precursor cells (OPCs). CXCL12 was found highly expressed in the regions that would potentially develop pathological new bone. OPCs were recruited to the regions where CXCL12 was up-regulated. Inhibition of CXCL12/CXCR4 axis with AMD3100 or conditional knockout of CXCR4 attenuated OPCs migration and subsequent pathological new bone formation in animal models of AS. By contrast, a genetically engineered animal model with CXCL12 overexpression developed a joint ankylosis phenotype. Furthermore, Rac1 was found essential for OPCs migration and pathological new bone formation. These findings ravel the novel role of CXCL12 in AS and indicate a potential strategy for targeting the CXCL12/CXCR4-Rac1 axis to prevent progression of axial skeleton ankylosis.

Dual Blockade of TNF and IL-17A Inhibits Inflammation and Structural Damage in a Rat Model of Spondyloarthritis

The tumor necrosis factor (TNF) and IL-23/IL-17 axes are the main therapeutic targets in spondyloarthritis. Despite the clinical efficacy of blocking either pathway, monotherapy does not induce remission in all patients and its effect on new bone formation remains unclear. We aimed to study the effect of TNF and IL-17A dual inhibition on clinical disease and structural damage using the HLA-B27/human β2-microglobulin transgenic rat model of SpA. Immunized rats were randomized according to arthritis severity, 1 week after arthritis incidence reached 50%, to be treated twice weekly for a period of 5 weeks with either a dual blockade therapy of an anti-TNF antibody and an anti-IL-17A antibody, a single therapy of either antibody, or PBS as vehicle control. Treatment-blinded observers assessed inflammation and structural damage clinically, histologically and by micro-CT imaging. Both single therapies as well as TNF and IL-17A dual blockade therapy reduced clinical spondylitis and peripheral arthritis effectively and similarly. Clinical improvement was confirmed for all treatments by a reduction of histological inflammation and pannus formation (p < 0.05) at the caudal spine. All treatments showed an improvement of structural changes at the axial and peripheral joints on micro-CT imaging, with a significant decrease for roughness (p < 0.05), which reflects both erosion and new bone formation, at the level of the caudal spine. The effect of dual blockade therapy on new bone formation was more prominent at the axial than the peripheral level. Collectively, our study showed that dual blockade therapy significantly reduces inflammation and structural changes, including new bone formation. However, we could not confirm a more pronounced effect of dual inhibition compared to single inhibition.

Paradoxical Augmentation of Experimental Spondyloarthritis by RORC Inhibition in HLA-B27 Transgenic Rats

Objective

IL-17A plays a major role in the pathogenesis of spondyloarthritis (SpA). Here we assessed the impact of inhibition of RAR related orphan receptor-γ (RORC), the key transcription factor controlling IL-17 production, on experimental SpA in HLA-B27 transgenic (tg) rats.

Methods

Experimental SpA was induced by immunization of HLA-B27 tg rats with heat-inactivated Mycobacterium tuberculosis. Splenocytes obtained at day 7, 14 and 21 after immunization were restimulated ex vivo to assess the induction of pro-inflammatory cytokines. Rats were then prophylactically treated with a RORC inhibitor versus vehicle control. The biologic effect of RORC inhibition was assessed by pro-inflammatory cytokine expression in draining lymph nodes. Arthritis and spondylitis were monitored clinically, and the degree of peripheral and axial inflammation, destruction and new bone formation was confirmed by histology.

Results

Ex vivo mRNA and protein analyses revealed the rapid and selective induction of IL-17A and IL-22 production by a variety of lymphocyte subsets upon disease induction in HLA-B27 tg rats. Prophylactic RORC inhibition in vivo suppressed the expression of IL-17A, IL17F, and IL-22 without affecting the expression of other T helper cell subset related genes. This biological effect did not translate into clinical efficacy as RORC inhibition significantly accelerated the onset of arthritis and spondylitis, and aggravated the clinical severity of arthritis. This worsening of experimental SpA was confirmed by histopathological demonstration of increased inflammation, destruction, and new bone formation.

Conclusion

Despite a significant suppression of the IL-17 axis, RORC inhibitor treatment accelerates and aggravates experimental SpA in the HLA-B27 tg rat model.

To move or not to move: the paradoxical effect of physical exercise in axial spondyloarthritis

In the last years, new researches focused on the role of biomechanical stress and microdamage in the pathogenesis of inflammatory arthritis and, in particular, in axial spondyloarthritis (axSpA). Animal models showed how entheseal stress and physical exercise could contribute to the development of inflammation and new bone formation at entheseal and articular sites, by activating innate immune system and the release of cytokines. Furthermore, clues of the involvement of biomechanical stress in the development of axSpA are present in clinical experiences. However, rehabilitation and exercise programmes are the cornerstone of treatment for axSpA, reducing disease activity and improving spinal function and quality of life. The concept of mechanical stress as a contributor to disease development and progression represents, potentially, a conceptual challenge for this approach. The aim of this review is to discuss the current evidence on the intriguing contribution of the biomechanical stress to the pathogenesis of inflammation and new bone formation and to evaluate and reflect on the role of exercise in the treatment and in the management of the disease, considering both the beneficial effects and its possible paradoxical action.

Ixekizumab: an IL-17A inhibitor for the treatment of axial Spondylarthritis

INTRODUCTION

Axial spondyloarthritis (axSpA) is an inflammatory arthritis which affects primarily the entheses of the spine and sacroiliac joints with peripheral joint synovitis and extra-articular manifestations. In 2017, the first IL-17A inhibitor (IL-17Ai) secukinumab was approved for the treatment of radiographic axSpA not responding adequately to conventional therapies, and this was followed in 2019 by a second IL-17Ai, ixekizumab. These agents represent the first alternative class of biological treatments after the TNF inhibitor which dominated the therapeutic landscape of axSpA for over a decade.

AREAS COVERED

This review discusses the role of IL-17Ais in the treatment in axSpA focusing on the newest IL-17Ai ixekizumab. It provides a detailed overview of the drug pharmacodynamic, pharmacokinetics, and clinical trial data, including areas of future research needed in the post-marketing era.

EXPERT OPINION

Early trials of ixekizumab for axSpA have shown encouraging results and an acceptable safety profile. Future phase IV trials should focus on direct head-to-head comparisons between ixekizumab and other biologic drugs, and stratify patients according to important disease characteristics known to affect treatment response including sex, HLA-B27 status, presence of MRI bone marrow edema at baseline, disease duration and any extra-articular manifestations.

Transmembrane TNF drives osteoproliferative joint inflammation reminiscent of human spondyloarthritis

TNF plays a key role in immune-mediated inflammatory diseases including rheumatoid arthritis (RA) and spondyloarthritis (SpA). It remains incompletely understood how TNF can lead to different disease phenotypes such as destructive peripheral polysynovitis in RA versus axial and peripheral osteoproliferative inflammation in SpA. We observed a marked increase of transmembrane (tm) versus soluble (s) TNF in SpA versus RA together with a decrease in the enzymatic activity of ADAM17. In contrast with the destructive polysynovitis observed in classical TNF overexpression models, mice overexpressing tmTNF developed axial and peripheral joint disease with synovitis, enthesitis, and osteitis. Histological and radiological assessment evidenced marked endochondral new bone formation leading to joint ankylosis over time. SpA-like inflammation, but not osteoproliferation, was dependent on TNF-receptor I and mediated by stromal tmTNF overexpression. Collectively, these data indicate that TNF can drive distinct inflammatory pathologies. We propose that tmTNF is responsible for the key pathological features of SpA.

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.

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.

The role of IL-17A in axial spondyloarthritis and psoriatic arthritis: recent advances and controversies

Although the pathogenic mechanisms underlying axial spondyloarthritis (axSpA) and psoriatic arthritis (PsA) are not fully elucidated, several lines of evidence suggest that immune responses mediated by interleukin 17A (IL-17A) play a pivotal role in both diseases. This is best highlighted by the significant clinical efficacy shown with inhibitors of IL-17A in treating axSpA and PsA. Nevertheless, a number of knowledge gaps exist regarding the role of IL-17A in the pathophysiology of spondyloarthritis in man, including its cellular origin, its precise role in discrete disease processes such enthesitis, bone erosion, and bone formation, and the reasons for the discrepant responses to IL-17A inhibition observed in certain other spondyloarthritis manifestations. In this review, we focus on the latest data from studies investigating the role of IL-17A in ankylosing spondylitis (AS) and PsA that build on existing and emerging scientific knowledge in the field. Key remaining research questions are also highlighted to guide future research.

Translating GWAS in rheumatic disease: approaches to establishing mechanism and function for genetic associations with ankylosing spondylitis

Ankylosing spondylitis (AS) is a highly heritable chronic inflammatory arthritis characterized by osteoproliferation, fusion of affected joints and systemic manifestations. Many disease associations for AS have been reported through genome-wide association studies; however, identifying modulated genes and functional mechanism remains challenging. This review summarizes current genetic associations involving AS and describes strategic approaches for functional follow-up of disease-associated variants. Fine mapping using methods leveraging Bayesian approaches are outlined. Evidence highlighting the importance of context specificity for regulatory variants is reviewed, noting current evidence in AS for the relevant cell and tissue type to conduct such analyses. Technological advances for understanding the regulatory landscape within which functional variants may act are discussed using exemplars. Approaches include defining regulatory elements based on chromatin accessibility, effects of variants on genes at a distance through evidence of physical interactions (chromatin conformation capture), expression quantitative trait loci mapping and single-cell methodologies. Opportunities for mechanistic studies to investigate the function of specific variants, regulatory elements and genes enabled by genome editing using clustered regularly interspaced short palindromic repeats/Cas9 are also described. Further progress in our understanding of the genetics of AS through functional genomic and epigenomic approaches offers new opportunities to understand mechanism and develop innovative treatments.

Unmet Needs in Axial Spondyloarthritis

During the past decade, the well-known disease called ankylosing spondylitis has come to be considered as a subset of the broader entity referred as axial spondyloarthritis (axSpA), which also includes non-radiographic axSpA. The need of this new classification was aimed to improve the sensitivity for an early diagnosis, to reduce diagnostic delay, and to allow an early treatment. Although there is improvement in the recognition, the management of patients, and the treatment strategies of axSpA, unmet needs persist. There is still a substantial gap of 5-8 years between the onset of symptoms and the diagnosis of axSpA and, even in patients diagnosed early, 20-40% of them do not respond or have a loss of response to anti-TNF treatment. Moreover, the pathogenesis of the disease and, in particular, the mechanisms of new bone formation are far to be completely understood. Nevertheless, the discovery of IL-23/IL-17 axis with the development of biologic inhibitors, the identification of new subsets of effector cells, together with the interest in the detection of potential biomarkers of bone formation brought the approach to axSpA into a new era. This review is intended to enhance awareness and understanding of axSpA and to identify and discuss the current unmet needs in axSpA, including diagnosis, classification, biomarkers, pathogenesis, management, and treatment strategies.

The Initiation, but Not the Persistence, of Experimental Spondyloarthritis Is Dependent on Interleukin-23 Signaling

IL-17A is a central driver of spondyloarthritis (SpA), its production was originally proposed to be IL-23 dependent. Emerging preclinical and clinical evidence suggests, however, that IL-17A and IL-23 have a partially overlapping but distinct biology. We aimed to assess the extent to which IL-17A-driven pathology is IL-23 dependent in experimental SpA. Experimental SpA was induced in HLA-B27/Huβ2m transgenic rats, followed by prophylactic or therapeutic treatment with an anti-IL23R antibody or vehicle control. Spondylitis and arthritis were scored clinically and hind limb swelling was measured. Draining lymph node cytokine expression levels were analyzed directly ex vivo, and IL-17A protein was measured upon restimulation with PMA/ionomycin. Prophylactic treatment with anti-IL23R completely protected against the development of both spondylitis and arthritis, while vehicle-treated controls did develop spondylitis and arthritis. In a therapeutic study, anti-IL23R treatment failed to reduce the incidence or decrease the severity of experimental SpA. Mechanistically, expression of downstream effector cytokines, including IL-17A and IL-22, was significantly suppressed in anti-IL23R versus vehicle-treated rats in the prophylactic experiments. Accordingly, the production of IL-17A upon restimulation was reduced. In contrast, there was no difference in IL-17A and IL-22 expression after therapeutic anti-IL23R treatment. Targeting the IL-23 axis during the initiation phase of experimental SpA-but not in established disease-inhibits IL-17A expression and suppresses disease, suggesting the existence of IL-23-independent IL-17A production. Whether IL-17A can be produced independent of IL-23 in human SpA remains to be established.

A spontaneous model of spondyloarthropathies that develops bone loss and pathological bone formation: A process regulated by IL27RA-/- and mutant-p53

Spondyloarthropathies, the second most frequently occurring form of chronic inflammatory arthritis, affects young adults in particular. However, a proper model with which to study the biology of this disease and to develop therapeutics is lacking. One of the most accepted animal models for this disease uses HLA-B27/Hu-β2m transgenic rats; however, only 30%-50% of male HLA-B27/Hu-β2m rats develop spontaneous, clinically apparent spondylitis and have a variable time until disease onset. Here, we report a high-incidence, low-variation spontaneous mouse model that delineates how the combination of inflammatory cytokine interleukin-27 (IL-27) signaling deficiency and mitogenic signaling (mutant p53^R172H) in vivo, leads to bone loss in the vertebral bodies and ossification of the cartilage in the intervertebral discs. In this human disease–like mouse model, bone loss and pathogenic bone development are seen as early as 4 months of age in the absence of inflammatory aggregates in the enthesis or intervertebral disc.

New concept of 3D printed bone clip (polylactic acid/hydroxyapatite/silk composite) for internal fixation of bone fractures

Open reduction with internal fixation is commonly used for the treatment of bone fractures. However, postoperative infection associated with internal fixation devices (intramedullary nails, plates, and screws) remains a significant complication, and it is technically difficult to fix multiple fragmented bony fractures using internal fixation devices. In addition, drilling in the bone to install devices can lead to secondary fracture, bone necrosis associated with postoperative infection. In this study, we developed bone clip type internal fixation device using three- dimensional (3D) printing technology. Standard 3D model of the bone clip was generated based on computed tomography (CT) scan of the femur in the rat. Polylacticacid (PLA), hydroxyapatite (HA), and silk were used for bone clip material. The purpose of this study was to characterize 3D printed PLA, PLA/HA, and PLA/HA/Silk composite bone clip and evaluate the feasibility of these bone clips as an internal fixation device. Based on the results, PLA/HA/Silk composite bone clip showed similar mechanical property, and superior biocompatibility compared to other types of the bone clip. PLA/HA/Silk composite bone clip demonstrated excellent alignment of the bony segments across the femur fracture site with well-positioned bone clip in an animal study. Our 3D printed bone clips have several advantages: (1) relatively noninvasive (drilling in the bone is not necessary), (2) patient-specific design (3) mechanically stable device, and (4) it provides high biocompatibility. Therefore, we suggest that our 3D printed PLA/HA/Silk composite bone clip is a possible internal fixation device.

Innate Immune Activation Can Trigger Experimental Spondyloarthritis in HLA-B27/Huβ2m Transgenic Rats

Spondyloarthritis (SpA) does not display the typical features of auto-immune disease. Despite the strong association with MHC class I, CD8⁺ T cells are not required for disease induction in the HLA-B27/Huβ2m transgenic rats. We used Lewis HLA-B27/Huβ2m transgenic rats [21-3 × 283-2]F1, HLA-B7/Huβ2m transgenic rats [120-4 × 283-2]F1, and wild-type rats to test our hypothesis that SpA may be primarily driven by the innate immune response. In vitro, splenocytes were stimulated with heat-inactivated Mycobacterium tuberculosis and cytokine expression and production was measured. In vivo, male and female rats were immunized with 30, 60, or 90 µg of heat-inactivated M. tuberculosis and clinically monitored for spondylitis and arthritis development. After validation of the model, we tested whether prophylactic and therapeutic TNF targeting affected spondylitis and arthritis. In vitro stimulation with heat-inactivated M. tuberculosis strongly induced gene expression of pro-inflammatory cytokines such as TNF, IL-6, IL-1α, and IL-1β, in the HLA-B27 transgenic rats compared with controls. In vivo immunization induced an increased spondylitis and arthritis incidence and an accelerated and synchronized onset of spondylitis and arthritis in HLA-B27 transgenic males and females. Moreover, immunization overcame the protective effect of orchiectomy. Prophylactic TNF targeting resulted in delayed spondylitis and arthritis development and reduced arthritis severity, whereas therapeutic TNF blockade did not affect spondylitis and arthritis severity. Collectively, these data indicate that innate immune activation plays a role in the initiation of HLA-B27-associated disease and allowed to establish a useful in vivo model to study the cellular and molecular mechanisms of disease initiation and progression.

Tumor Necrosis Factor Alpha Overexpression Induces Mainly Osteoclastogenesis at the Vertebral Site

Syndesmophyte occurrence and axial bone loss were investigated in the heterozygous Tg187 tumor necrosis factor (TNF) transgenic mouse model (Tg-huTNF) of arthritis. Female and male Tg-huTNF mice were compared to wild-type mice (WT) at 2, 4, 6, 8, and 10 weeks. Syndesmophytes, intervertebral disc space, osteoclasts, osteoid surface, and vertebra microarchitecture were assessed by histomorphometry and microcomputed tomography. No spontaneous syndesmophyte formation was detected in Tg-huTNF compared to WT mice. However, increased porosity was observed mainly in peridiscal lumbar vertebra. Accordingly, bone microarchitecture parameters were altered in Tg-huTNF mice, with decrease in bone volume fraction, and trabecular number and thickness after 6 weeks compared to WT (p < 0.05). Osteoclast count and surface were increased (p < 0.01). Moreover, the non-mineralized (osteoid) surface was also increased in Tg-huTNF after 6 weeks (p < 0.01). Despite increased osteoclast and osteoid surfaces, an imbalance between both was observed in favour of osteoid surface at the early phase and then to osteoclast surface. These results demonstrated an axial bone loss in the Tg-huTNF model, additional to the common limb arthritis, related to overexpression of TNF. However, the absence of syndesmophyte and the increase of osteoid surface suggested that chronic inflammation might block bone mineralisation. Finally, the relative increased osteoid surface was not enough to compensate the high osteoclast activity.

Pathogenesis of ligaments ossification in spondyloarthritis: insights and doubts

Despite intensive research in spondyloarthritis pathogenesis, some important questions still remain unanswered, particularly concerning enthesis new bone formation. Several evidences suggest that it prevalently occurs by endochondral ossification, however it remains to identify factors that can induce and influence its initiation and progression. Recent progress, achieved in animal models and in vitro and genetic association studies, has led us to hypothesize that several systemic factors (adipokines and gut hormones) and local factors (BMP and Wnt signaling) as well as angiogenesis and mechanical stress are involved. We critically review and summarize the available data and delineate the possible mechanisms involved in enthesis ossification, particularly at spinal ligament level. KEY MESSAGES Complete understanding of spondyloarthritis pathophysiology requires insights into inflammation, bone destruction and bone formation, which are all located in entheses and lead all together to ankylosis and functional disability. Several factors probably play a role in the pathogenesis of bone formation in entheses including not only cytokines but also several systemic factors such as adipokines and gut hormones, and local factors, such as BMP and Wnt signaling, as well as angiogenesis and mechanical stress. Data available about pathophysiology of new bone formation in spondyloarthritis are limited and often conflicting and future studies are needed to better delineate it and to develop new therapeutic approaches.

The Human Leukocyte Antigen (HLA)-B27 Peptidome in Vivo, in Spondyloarthritis-susceptible HLA-B27 Transgenic Rats and the Effect of Erap1 Deletion

HLA-B27 is a class I major histocompatibility (MHC-I) allele that confers susceptibility to the rheumatic disease ankylosing spondylitis (AS) by an unknown mechanism. ERAP1 is an aminopeptidase that trims peptides in the endoplasmic reticulum for binding to MHC-I molecules. ERAP1 shows genetic epistasis with HLA-B27 in conferring susceptibility to AS. Male HLA-B27 transgenic rats develop arthritis and serve as an animal model of AS, whereas female B27 transgenic rats remain healthy. We used large scale quantitative mass spectrometry to identify over 15,000 unique HLA-B27 peptide ligands, isolated after immunoaffinity purification of the B27 molecules from the spleens of HLA-B27 transgenic rats. Heterozygous deletion of Erap1, which reduced the Erap1 level to less than half, had no qualitative or quantitative effects on the B27 peptidome. Homozygous deletion of Erap1 affected approximately one-third of the B27 peptidome but left most of the B27 peptidome unchanged, suggesting the possibility that some of the HLA-B27 immunopeptidome is not processed in the presence of Erap1. Deletion of Erap1 was permissive for the AS-like phenotype, increased mean peptide length and increased the frequency of C-terminal hydrophobic residues and of N-terminal Ala, Ser, or Lys. The presence of Erap1 increased the frequency of C-terminal Lys and Arg, of Glu and Asp at intermediate residues, and of N-terminal Gly. Several peptides of potential interest in AS pathogenesis, previously identified in human cell lines, were isolated. However, rats susceptible to arthritis had B27 peptidomes similar to those of non-susceptible rats, and no peptides were found to be uniquely associated with arthritis. Whether specific B27-bound peptides are required for AS pathogenesis remains to be determined. Data are available via ProteomeXchange with identifier PXD005502.

Insulin-Like Growth Factor I Does Not Drive New Bone Formation in Experimental Arthritis

Introduction

Insulin like growth factor (IGF)-I can act on a variety of cells involved in cartilage and bone repair, yet IGF-I has not been studied extensively in the context of inflammatory arthritis. The objective of this study was to investigate whether IGF-I overexpression in the osteoblast lineage could lead to increased reparative or pathological bone formation in rheumatoid arthritis and/or spondyloarthritis respectively.

Methods

Mice overexpressing IGF-I in the osteoblast lineage (Ob-IGF-I^+/-) line 324–7 were studied during collagen induced arthritis and in the DBA/1 aging model for ankylosing enthesitis. Mice were scored clinically and peripheral joints were analysed histologically for the presence of hypertrophic chondrocytes and osteocalcin positive osteoblasts.

Results

90–100% of the mice developed CIA with no differences between the Ob-IGF-I^+/- and non-transgenic littermates. Histological analysis revealed similar levels of hypertrophic chondrocytes and osteocalcin positive osteoblasts in the ankle joints. In the DBA/1 aging model for ankylosing enthesitis 60% of the mice in both groups had a clinical score 1<. Severity was similar between both groups. Histological analysis revealed the presence of hypertrophic chondrocytes and osteocalcin positive osteoblasts in the toes in equal levels.

Conclusion

Overexpression of IGF-I in the osteoblast lineage does not contribute to an increase in repair of erosions or syndesmophyte formation in mouse models for destructive and remodeling arthritis.

Data showing non-conventional HLA-B27 expression in axial joints and gut tissue from B27 transgenic rats, and in frozen and paraffin-fixed synovial SpA tissue

Data is presented showing expression of non-conventional (NC) heavy chain forms of B27 in synovial tissues from SpA patients. Data is presented showing the expression patterns of NC-B27 in joint, gastrointestinal and lymphoid tissues from B27 transgenic (TG¹) rats with M. tuberculosis-induced SpA. Expression of NC-B27 was determined by immunohistochemistry and flow cytometry using HC10 and HD6 antibodies. These data are the extension of the data presented and discussed in “Non-conventional forms of HLA-B27 are expressed in Spondyloarthritis joints and gut tissue” (O. Rysnik, K. McHugh, L. M. van Duivenvoorde, M. N. van Tok, G. Guggino, J. D. Taurog, S. Kollnberger, F. Ciccia, D. L. Baeten, P. Bowness, 2016) [1].

Non-conventional forms of HLA-B27 are expressed in spondyloarthritis joints and gut tissue

Objectives

Human leukocyte antigen (HLA)-B27 (B27) is the strongest genetic factor associated with development of Ankylosing Spondylitis and other spondyloarthropathies (SpA), yet the role it plays in disease pathogenesis remains unclear. We investigated the expression of potentially pathogenic non-conventional heavy chain forms (NC) of B27 in synovial and intestinal tissues obtained from SpA patients. We also determined the presence of NC-B27 in joints, lymphoid and gastrointestinal tissue from B27 transgenic (TG¹) rats with M.tuberculosis-induced SpA.

Methods

Expression of NC-B27 in human SpA joints and gut and in (21-3 × 283-2)F₁ HLA-B27/Huβ2m rat tissue was determined by immunohistochemistry, flow cytometry and confocal microscopy analysis using HC10 and HD6 antibodies.

Results

Both HC10- and HD6-reactive HLA molecules were present in synovial tissue from SpA patients. Both NC-B27 and KIR3DL2, a ligand for NC-B27, were expressed in inflamed terminal ileal tissues in patients with early SpA. Infiltrating cells in inflamed joint tissues isolated from B27 TG¹ rats expressed high levels of NC-B27. NC-B27 were also expressed in joint-resident cells from ankle and tail joints of B27 TG¹ rats prior to clinical arthritis. The expression of NC-B27 on B27 TG¹ rat CD11b/c⁺, CD8α⁺, cells from spleens and LNs increased with animal age and disease progression.

Conclusions

Non-conventional HLA class 1 molecules are expressed on resident and infiltrating cells in both synovial and GI tissues in human SpA. NC-B27 expression in joints and lymphoid tissues from B27 TG¹ rats prior to the onset of arthritis is consistent with the hypothesis that they play a pathogenic role in SpA.

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HLA-B27 predisposes to Ankylosing Spondylitis and related Spondyloarthritis (SpA).

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Non-conventional HLA-B27 free heavy chains (NC-B27) bind innate immune receptors.

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NC HLA class I molecules are expressed in joint and gut tissues in human and rat SpA.

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B27 transgenic rats express NC-B27 prior to arthritis onset.

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These data support a pathogenic role for non-conventional B27 in SpA.

Inflammation-driven bone formation in a mouse model of ankylosing spondylitis: sequential not parallel processes

Background

Ankylosing spondylitis (AS) is an immune-mediated arthritis particularly targeting the spine and pelvis and is characterised by inflammation, osteoproliferation and frequently ankylosis. Current treatments that predominately target inflammatory pathways have disappointing efficacy in slowing disease progression. Thus, a better understanding of the causal association and pathological progression from inflammation to bone formation, particularly whether inflammation directly initiates osteoproliferation, is required.

Methods

The proteoglycan-induced spondylitis (PGISp) mouse model of AS was used to histopathologically map the progressive axial disease events, assess molecular changes during disease progression and define disease progression using unbiased clustering of semi-quantitative histology. PGISp mice were followed over a 24-week time course. Spinal disease was assessed using a novel semi-quantitative histological scoring system that independently evaluated the breadth of pathological features associated with PGISp axial disease, including inflammation, joint destruction and excessive tissue formation (osteoproliferation). Matrix components were identified using immunohistochemistry.

Results

Disease initiated with inflammation at the periphery of the intervertebral disc (IVD) adjacent to the longitudinal ligament, reminiscent of enthesitis, and was associated with upregulated tumor necrosis factor and metalloproteinases. After a lag phase, established inflammation was temporospatially associated with destruction of IVDs, cartilage and bone. At later time points, advanced disease was characterised by substantially reduced inflammation, excessive tissue formation and ectopic chondrocyte expansion. These distinct features differentiated affected mice into early, intermediate and advanced disease stages. Excessive tissue formation was observed in vertebral joints only if the IVD was destroyed as a consequence of the early inflammation. Ectopic excessive tissue was predominantly chondroidal with chondrocyte-like cells embedded within collagen type II- and X-rich matrix. This corresponded with upregulation of mRNA for cartilage markers Col2a1, sox9 and Comp. Osteophytes, though infrequent, were more prevalent in later disease.

Conclusions

The inflammation-driven IVD destruction was shown to be a prerequisite for axial disease progression to osteoproliferation in the PGISp mouse. Osteoproliferation led to vertebral body deformity and fusion but was never seen concurrent with persistent inflammation, suggesting a sequential process. The findings support that early intervention with anti-inflammatory therapies will be needed to limit destructive processes and consequently prevent progression of AS.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0805-0) contains supplementary material, which is available to authorized users.

Immunopathology of synovitis: from histology to molecular pathways

Increased knowledge about pathological processes active in inflammatory joint diseases is needed to initiate personalized medicine based on targeted treatments in the future. The molecular and cellular pathways that are active during joint inflammation may differ between the various inflammatory joint diseases, between different patient subgroups within one disease, or even between different stages of the disease in a single patient. In this review, we evaluate synovial inflammation in terms of descriptive histopathology through to more functional studies on human synovial tissue inflammation in RA and SpA, in phenotypic subgroups of RA and SpA patients, and during the disease course of both diseases.

Axial spondyloarthritis

The term axial spondyloarthritis covers both non-radiographic disease and radiographic disease (also known as ankylosing spondylitis). Some studies have been performed to investigate the prevalence of axial spondyloarthritis, although most are limited to patients with radiographic disease. A strong genetic association has been shown between axial spondyloarthritis and human leukocyte antigen-B27 (HLA-B27), but the pathogenetic role of HLA-B27 has not yet been clarified. Tumour necrosis factor (TNF), IL-17, IL-23 and downstream pathways also seem to be important - based on the good results of therapies directed against these molecules - but their exact role in the inflammatory process is also not yet clear. Elucidating the interaction between osteoproliferation and inflammation will be crucial for the prevention of long-term structural damage of the bone. The development of new criteria for classification, diagnosis and screening of patients with axial spondyloarthritis will enable earlier intervention for this chronic inflammatory disease. MRI has become an important tool for the early detection of axial spondyloarthritis. NSAIDs and TNF blockers are effective therapies, including in the early non-radiographic stage. Therapeutic blockade of IL-17 or IL-23 seems to be a promising new treatment option. Tools for measuring quality of life in axial spondyloarthritis have become relevant to assess the impact that the disease has on patients. These diagnostic and therapeutic advances will continue to change the management of axial spondyloarthritis, and new insights into the disease pathogenesis will hopefully accelerate this process. For an illustrated summary of this Primer, visit: http://go.nature.com/51b1af.

Osteoclast activators are elevated in intervertebral disks with Modic changes among patients operated for herniated nucleus pulposus

PURPOSE

Modic changes (MC) are associated with low back pain (LBP). Inflammation is considered as a key factor that triggers symptoms in especially type I MC, but so far of the potential inflammatory candidates only TNFα has been linked to MC. The objective of the study was to analyze a set of inflammatory mediators in human surgical disk samples and quantify their association with MC in the adjacent vertebral bodies.

METHODS

The study sample consisted of 51 intervertebral disk tissue specimens; 20 'No MC' disks, 19 'Type I MC' disks, and 12 'Type II MC' disks. mRNA expression of 46 cytokines was quantified from isolated RNA. Tissue samples were stained using hematoxylin and eosin, toluidine blue, Herovici, CD68 and CD163.

RESULTS

No significant differences were found in the amount of macrophages or presence of chondrocyte conglomerates between the MC groups. Of the multiple genes tested, statistically significant associations were observed for M-CSF1 (p = 0.028), RANKL (p = 0.035), RUNX1 (p = 0.032), and RUNX2 (p = 0.047) that were increased in 'Type II MC,' while OSCAR (p = 0.042) was increased in 'Type I MC' group compared to 'No MC.'

CONCLUSIONS

Since these cytokines are related to differentiation and proliferation of osteoclasts, our data suggest that the stimulation of vertebral osteoclasts by factors secreted by disk tissue is involved in the pathophysiology of MC.

Bone Lineage Proteins in the Entheses of the Midfoot in Patients with Spondyloarthritis

Objective.

Patients with juvenile-onset spondyloarthritis (SpA) may develop ankylosis of the midfoot resembling the spinal changes seen in patients with ankylosing spondylitis (AS). The study of the histopathology of the feet of patients with tarsitis could help us understand the pathogenesis of bone formation in affected structures in the SpA. The objective of our study was to describe the histopathologic characteristics of the midfoot in patients with tarsitis associated with SpA.

Methods.

We obtained synovial sheaths, entheses, and bone samples from 20 patients with SpA with midfoot pain/tenderness and swelling. Tissue samples underwent H&E staining; immunohistochemistry for CD3, CD4, CD8, CD68, and CD20 cell identification; and immunofluorescence for bone lineage proteins, including osteocalcin, osteopontin, parathyroid hormone-related protein, bone sialoprotein, and alkaline phosphatase.

Results.

Slight edema and hyalinization were found in some tendon sheaths, and few inflammatory cells were detected in the entheses. In bones, we found some changes suggesting osteoproliferation, including endochondral and intramembranous ossification, but no inflammatory cells. In entheses showing bone proliferation, we detected osteocalcin and osteopontin in cells with a fibroblast-mesenchymal phenotype, suggesting the induction of entheseal cells toward an osteoblast phenotype.

Conclusion.

Osteoproliferation and abnormal expression of bone lineage proteins, but no inflammatory infiltration, characterize midfoot involvement in patients with SpA. In this sense, tarsitis (or ankylosing tarsitis) resembles the involvement of the spine in patients with AS. Ossification may be in part explained by the differentiation of mesenchymal entheseal cells toward the osteoblastic lineage.

Role of protein phosphatase magnesium-dependent 1A and anti-protein phosphatase magnesium-dependent 1A autoantibodies in ankylosing spondylitis

OBJECTIVE

Although ankylosing spondylitis (AS) is driven by immune-mediated processes, little is known about the presence and role of autoantibodies in this disease. This study was undertaken to investigate whether autoantibodies occur in and are involved in AS.

METHODS

We performed human protein microarray analysis of sera derived from patients with AS or other autoimmune disorders to identify autoantibodies associated specifically with AS, and identified autoantibody targeting of protein phosphatase magnesium-dependent 1A (PPM1A) in AS. We performed enzyme-linked immunosorbent assay (ELISA) analysis of sera from 2 independent AS cohorts to confirm autoantibody targeting of PPM1A, and to assess associations between levels of anti-PPM1A antibodies and AS disease severity or response to anti-tumor necrosis factor (anti-TNF) therapy (as measured by Bath AS Disease Activity Index [BASDAI] score). Levels of anti-PPM1A antibodies were also evaluated in sera from rats transgenic for HLA-B27 and human β2 -microglobulin. The expression of PPM1A was assessed by immunohistochemistry in synovial tissue samples from patients with AS, rheumatoid arthritis, or osteoarthritis. The role of PPM1A in osteoblast differentiation was investigated by gene knockdown and overexpression.

RESULTS

AS was associated with autoantibody targeting of PPM1A, and levels of anti-PPM1A autoantibodies were significantly higher in patients with more advanced sacroiliitis and correlated positively with BASDAI score after treatment with anti-TNF agents. The levels of anti-PPM1A autoantibodies were also higher in the sera of transgenic rats that are prone to develop spondyloarthritis than in those that are not. PPM1A was expressed in AS synovial tissue, and PPM1A overexpression promoted osteoblast differentiation, whereas PPM1A knockdown suppressed it.

CONCLUSION

Anti-PPM1A autoantibodies are present in AS, and our findings suggest that PPM1A may contribute to the pathogenic bone ankylosis characteristic of AS.

Altered bone material properties in HLA-B27 rats include reduced mineral to matrix ratio and altered collagen cross-links

Spondyloarthropathy and inflammatory bowel disease (IBD), which includes ulcerative colitis and Crohn's disease, are often associated with severe osteopenia/osteoporosis in both children and adults. HLA-B27 transgenic rats present a phenotype that includes severe colitis and severely accelerated alveolar bone loss. The purpose of this study was to evaluate long bone density status, systemic bone metabolic markers, and intrinsic bone material properties in HLA-B27 transgenic (TG) rats, and compare them with those of age- and sex-matched wild-type (WT) animals. The results indicate that in the HLA-B27 rat, an animal susceptible to both alveolar bone loss (ABL) and long bone osteopenia, there is a statistically significant negative correlation between ABL and long bone bone mineral density (BMD), as well as mineral/matrix ratio at active bone-forming trabecular surfaces. The TG animals had a lower mineral/matrix ratio and higher relative proteoglycan and advanced glycation end product (ϵ-N-Carboxymethyl-L-lysine) content and pyridinoline/divalent collagen cross-link ratio compared with WT. These results may provide better understanding of the interrelationship between osteoporosis and oral bone loss, the underlying causes of the inferior bone strength in the HLA-B27 transgenic animals, and could prove to be a useful model in the elucidation of the pathophysiology of spondyloarthropathy and IBD-associated osteopenia/osteoporosis and in the evaluation of pharmacological intervention(s) against such conditions.

Autoimmunity: break-through in the diagnosis and treatment of immune-mediated inflammatory diseases

The study of fundamental mechanisms of autoimmunity has been instrumental to clinical progress in the diagnosis and treatment of a range of immune-mediated inflammatory disorders. Dutch immunology has made major contributions to these developments, ranging from fundamental studies on immune cells, antibodies and cytokines to translational and clinical studies with targeted therapies in patients. In this paper we illustrate the progress made in our understanding of autoimmunity and the translational implications for human disease management by focusing on three areas: the autoantibody response in rheumatoid arthritis (RA), T-B cell interactions in Sjögren's syndrome (SS), and cytokine targeting in spondylarthritis (SpA).

Histomorphologic and histomorphometric characteristics of zygapophyseal joint remodeling in ankylosing spondylitis

OBJECTIVE

To unravel the mechanisms that control bony ankylosis in ankylosing spondylitis (AS).

METHODS

Histomorphologic and histomorphometric analyses were performed on zygapophyseal joints obtained from 18 patients with AS, 9 patients with osteoarthritis (OA), and 10 cadaver donors without a rheumatic disease (controls). The proteoglycan content of the cartilage was determined by Safranin O staining and the chondrocyte apoptosis according to caspase 3 expression.

RESULTS

AS joints were categorized into 3 groups according to the morphology of the joint surfaces and joint space: group 1 were joints with an open joint space, group 2 were joints with cartilaginous fusion, and group 3 were joints with bony fusion of the joint surfaces. Progressive loss of the joint space from group 1 joints to group 3 joints suggests that this grouping corresponds to sequential stages of joint remodeling. Cartilage thickness and subchondral bone plate thickness declined from group 1 to group 3 (P < 0.01). Increased chondrocyte apoptosis rates were found in groups 1 and 2 (P < 0.05), while in group 3, a reduction in the proteoglycan content was found (P < 0.001). Bone marrow replacement and invasion of the subchondral bone plate by fibrous tissue was found predominantly in AS joints in group 2.

CONCLUSION

Cartilage degeneration, indicated by cartilage thinning, enhanced chondrocyte apoptosis, and proteoglycan loss, and subchondral bone thinning, promoted by invasion of the subchondral bone plate by a fibrous tissue originating from the bone marrow, are hallmarks of joint remodeling in AS.

Reverse interferon signature is characteristic of antigen-presenting cells in human and rat spondyloarthritis

OBJECTIVE

In HLA-B27-transgenic rats, the development of a disorder that mimics spondyloarthritis (SpA) is highly correlated with dendritic cell (DC) dysfunction. The present study was undertaken to analyze the underlying mechanisms of this via transcriptome analysis.

METHODS

Transcriptome analysis of ex vivo-purified splenic CD103+CD4+ DCs from B27-transgenic rats and control rats was performed. Transcriptional changes in selected genes were confirmed by quantitative reverse transcriptase-polymerase chain reaction. A meta-analysis of our rat data and published data on gene expression in macrophages from ankylosing spondylitis (AS) patients was further performed.

RESULTS

Interferon (IFN) signaling was the most significantly affected pathway in DCs from B27-transgenic rats; the majority of genes connected to IFN were underexpressed in B27-transgenic rats as compared to controls. This pattern was already present at disease onset, persisted over time, and was conserved in 2 disease-prone B27-transgenic rat lines. In DCs from B27-transgenic rats, we further found an up-regulation of suppressor of cytokine signaling 3 (which may account for reverse IFN signaling) and a down-regulation of interleukin-27 (a cytokine that opposes Th17 differentiation and promotes Treg cells). The meta-analysis of data on conventional DCs from rats and data on monocyte-derived macrophages from humans revealed 7 IFN-regulated genes that were negatively regulated in both human and rat SpA (i.e., IRF1, STAT1, CXCL9, CXCL10, IFIT3, DDX60, and EPSTI1).

CONCLUSION

Our results suggest that expression of HLA-B27 leads to a defect in IFNγ signaling in antigen-presenting cells in both B27-transgenic rats and SpA patients, which may result in Th17 expansion and Treg cell alteration (as shown in B27-transgenic rats) and contribute to disease pathogenesis.

Inactivation of patched1 in murine chondrocytes causes spinal fusion without inflammation

OBJECTIVE

During development of the vertebrate skeleton, chondrocytes form a cartilage template that is gradually replaced by bone. Hormones of the Hedgehog (HH) family have been implicated in the ossification process, but their exact relationship to normal or pathogenic bone formation is unclear. This study was undertaken to establish a genetic tool that allows the discrete inactivation of genes in spinal chondrocytes, and to investigate in vivo how chondrocyte-specific ablation of the inhibitory HH receptor Patched 1 (Ptch1) affects skeleton integrity.

METHODS

A Cre-deleter mouse strain, mb1-Cre, for selective gene recombination in spinal chondrocytes was identified by in situ hybridization and histologic analysis. The mb1-Cre(+/-) animals were crossed with mice that harbor a loxP-flanked Ptch1 gene (Ptch1(flox/flox) ) to abrogate the inhibition of the HH signaling pathway in chondrocytes. The skeletal integrity of F1 mice was characterized by high-resolution flat-panel-based volume computed tomography and histologic staining procedures.

RESULTS

During the first weeks after birth, all mb1-Cre(+/-) /Ptch1(flox/flox) mice developed progressive spinal fusion with malformation of the vertebrae. This phenotype was caused by aberrant chondrocyte proliferation in the intervertebral discs that blocked endochondral ossification. Importantly, the disease pattern occurred in an inflammation-independent manner.

CONCLUSION

Our findings indicate that chronic activation of the HH signal pathway in spinal chondrocytes can trigger an ankylosing spine morphology without immune cell contributions. Hence, the destruction of cartilage and loss of axial joint integrity can result from chondrocyte-intrinsic defects of monogenic origin.

Autoimmune epididymoorchitis is essential to the pathogenesis of male-specific spondylarthritis in HLA-B27-transgenic rats

OBJECTIVE

Male rats transgenic for HLA-B27 and human β(2) -microglobulin (hβ(2) m) spontaneously develop epididymoorchitis (EO) preceding the development of spondylarthritis (SpA). In the specific B27/hβ(2) m-transgenic rat cross-strain (21-3 × 382-2)F(1) , only the males develop SpA, and neither sex develops gut inflammation. This study was undertaken to determine whether EO and SpA in male (21-3 × 382-2)F(1) rats are causally related. In addition, the primary characteristics of EO in this rat arthritis model were assessed.

METHODS

Male B27/hβ(2) m-transgenic (21-3 × 382-2)F(1) rats underwent bilateral, unilateral, or sham epididymoorchiectomy between ages 36 and 125 days. The castrated rats were given testosterone replacement. Alternatively, the 21-3 and 283-2 transgene loci were crossed with a transgene inducing aspermatogenesis. Rats were observed for the development of EO, arthritis, and spondylitis.

RESULTS

In unmanipulated transgenic rats, inflammation was first evident in the ductuli efferentes (DE; ducts linking the rete testis to epididymis) as early as age 30 days. The inflammation was initially neutrophilic, and later became granulomatous. Antisperm and anti-testis cell antibodies appeared in the rat serum after age 70 days. Cells infiltrating the testes were predominantly CD4+ T cells and CD68+ or CD163+ macrophages. Quantitative polymerase chain reaction of the DE, epididymis, and testis showed elevations in the levels of interferon-γ, interleukin-10 (IL-10), and IL-17A. In addition, levels of IL-12A, IL-22, IL-23A, and IL-23 receptor were found to be elevated in the DE. Remarkably, castration of the rats before age 91 days completely prevented the subsequent onset of arthritis and spondylitis, as did transgene-induced azospermia.

CONCLUSION

Autoimmune EO develops spontaneously in HLA-B27/hβ(2) m-transgenic (21-3 × 283-2)F(1) rats at age 30 days, the age when antigen-positive meiotic germ cells first exit the testis. Persistent testicular inflammation and/or antigenic stimulation are essential prerequisites for the subsequent development of SpA. Thus, dysregulated innate immunity at immune-privileged sites may be an essential mechanism triggering the onset of SpA.