Recent Updates in the Immunopathology of Type 3 Immunity-Mediated Enthesitis.

PURPOSE OF REVIEW: Enthesitis is a cardinal feature of spondyloarthritis (SpA). Despite increasing available treatments, challenges remain in adequately controlling inflammation and subsequent new bone formation (NBF) in entheses; thus, a better understanding of the immunopathogenesis is warranted. RECENT FINDINGS: Increasing evidence has identified immune cells playing key roles in enthesitis such as γδ T cells and group 3 innate lymphoid cells (ILC3), possibly with site-specific regulatory systems. The presence of T cells producing interleukin (IL)-17 independent of IL-23 in human spinal entheses was recently reported, which may corroborate the discrepancy between recent clinical trials and pre-clinical studies. In addition, the contribution of myeloid cells has also been focused in both human and pre-clinical SpA models. Moreover, not only the IL-23/IL-17 signaling, but other key type 3 immunity mediators, such as IL-22 and granulocyte-macrophage colony-stimulating factor (GM-CSF), have been reported as pivotal cytokines in inflammation and NBF of entheses. Immune cells demonstrating distinct features orchestrate entheses, leading to the complex landscape of enthesitis. However, recent advances in understanding the immunopathogenesis may provide new therapeutic targets and future research directions.

The transition from enthesis physiological responses in health to aberrant responses that underpin spondyloarthritis mechanisms.

PURPOSE OF REVIEW: Despite immunology and translational therapeutics advances in inflammatory arthritis over the past two decades, the enthesis, which is the epicentric of the spondyloarthritis family pathological process, retains many mysteries because of tissue inaccessibility that hampers direct immune study. As entheses are subject to almost continuous mechanical stress and spondyloarthritis is linked to microdamage or injury and joint stress, it is cardinal to understand the physiological changes occurring within the entheses not only to be able to differentiate disease from health but also to understand the transition normal physiology break down and its merges into spondyloarthritis-related disease. RECENT FINDINGS: Imaging has played a major role in understanding the enthesis in human. Remarkable insights from enthesis functioning and microdamage in normal and with ageing including those linked to body mass index is emerging. The impact of mechanical stress and degenerative conditions on the development of the secondary entheseal vascular changes is not understood. Of note, ultrasound studies in psoriasis have shown higher power Doppler changes compared to controls pointing towards a role for vascular changes in the development of enthesitis in psoriatic arthritis. SUMMARY: The literature pertaining to normal entheses changes with age, microdamage and vascular changes in health is providing a roadmap for understanding of the enthesis and its potential role in evolution of spondyloarthritis including psoriatic arthritis.

Intestinal and enthesis innate immunity in early axial spondyloarthropathy.

Axial SpA (axSpA), encompassing AS, is a multifactorial disease that localizes to sites of high spinal biomechanical stress. Much has been written on T cells and adaptive immunity in axSpA, which is understandable given the very strong HLA-B27 disease association. Extra-axial disease characteristically involves the anterior uveal tract, aortic root, lung apex and terminal ileum. Under recent classification, axSpA is classified as an intermediate between autoimmunity and autoinflammatory disease, with the latter term being synonymous with innate immune dysregulation. The purpose of this review is to evaluate the 'danger signals' from both the exogenous intestinal microbiotal adjuvants or pathogen-associated molecular patterns that access the circulation and endogenously derived damaged self-tissue or damage-associated molecular patterns derived from entheses and other sites of high biomechanical stress or damage that may serve as key drivers of axSpA onset, evolution, disease flares and eventual outcomes.

The management of enthesitis in clinical practice.

PURPOSE OF REVIEW: Enthesitis is a hallmark feature of the spondyloarthropathies (SpA). This review provides an overview of recent insights on diagnosis and management of enthesitis. RECENT FINDINGS: Recent studies support the use of imaging for diagnosis because of its higher sensitivity and specificity compared with clinical examination. Several new MRI and ultrasound scoring systems have been developed for enthesitis, which may facilitate the use of imaging in research. Enthesitis has been evaluated as a primary study outcome mainly in psoriatic arthritis (PsA); however, the use of different indices and definitions of improvement limits comparison across studies. There is very limited information about the efficacy of synthetic disease modifying antirheumatic drugs (DMARDs) for the treatment of enthesitis. In contrast, targeted and biologic DMARDs have all shown efficacy in treating enthesitis compared with placebo. There have been only a few head-to-head trials that compared two different cytokine inhibitors for the treatment of enthesitis. Preliminary data suggest that targeting IL-17 or IL12/23 may be more efficacious for controlling enthesitis than TNF inhibition. SUMMARY: Emerging data suggest interleukin-17 and 12/23 inhibitors may be the first choice in PsA patients with enthesitis. Further head-to-head studies are needed before making definitive recommendations.

Role of Microparticles in the Pathogenesis of Inflammatory Joint Diseases.

Rheumatoid arthritis (RA), juvenile idiopathic arthritis (JIA), ankylosing spondylitis (AS), and psoriatic arthritis (PsA) make up a group of chronic immune-mediated inflammatory diseases (IMIDs). The course of these diseases involves chronic inflammation of joints and enthesopathies, which can result in joint damage and disability. Microparticles (MPs) are a group of small spherical membranous vesicles. The structure and cellular origin of MPs, mechanisms that stimulate their secretion and the place of their production, determine their biological properties, which could become manifest in the pathogenesis of immune-mediated inflammatory diseases. Microparticles can stimulate synovitis with proinflammatory cytokines and chemokines. MPs may also contribute to the pathogenesis of rheumatic diseases by the formation of immune complexes and complement activation, pro-coagulation activity, activation of vascular endothelium cells, and stimulation of metalloproteinase production. It seems that in the future, microparticles can become a modern marker of disease activity, a response to treatment, and, possibly, they can be used in the prognosis of the course of arthritis. The knowledge of the complexity of MPs biology remains incomplete and it requires further comprehensive studies to explain how they affect the development of rheumatic diseases. This review focuses on the immunopathogenic and therapeutic role of MPs in chronic immune-mediated inflammatory joint diseases.

Evidence that tissue resident human enthesis γδT-cells can produce IL-17A independently of IL-23R transcript expression.

OBJECTIVES: Murine models of interleukin (IL)-23-driven spondyloarthritis (SpA) have demonstrated entheseal accumulation of γδT-cells which were responsible for the majority of local IL-17A production. However, IL-23 blockers are ineffective in axial inflammation in man. This study investigated γδT-cell subsets in the normal human enthesis to explore the biology of the IL-23/17 axis. METHODS: Human spinous processes entheseal soft tissue (EST) and peri-entheseal bone (PEB) were harvested during elective orthopaedic procedures. Entheseal γδT-cells were evaluated using immunohistochemistry and isolated and characterised using flow cytometry. RNA was isolated from γδT-cell subsets and analysed by qPCR. Entheseal γδT-cells were stimulated with phorbol 12-myristate 13-acetate (PMA) and ionomycin, anti-CD3/28 or IL-23 and IL-17A production was measured by high-sensitivity ELISA and qPCR. RESULTS: Entheseal γδT-cells were confirmed immunohistochemically with Vδ1 and Vδ2 subsets that are cytometrically defined. Transcript profiles of both cell populations suggested tissue residency and immunomodulatory status. Entheseal Vδ2 cells expressed high relative abundance of IL-23/17-associated transcripts including IL-23R, RORC and CCR6, whereas the Vδ1 subset almost completely lacked detectable IL-23R transcript. Following PMA stimulation IL-17A was detectable in both Vδ1 and Vδ2 subsets, and following CD3/CD28 stimulation both subsets showed IL-17A and IL-17F transcripts with neither transcript being detectable in the Vδ1 subset following IL-23 stimulation. CONCLUSION: Spinal entheseal Vδ1 and Vδ2 subsets are tissue resident cells with inducible IL-17A production with evidence that the Vδ1 subset does so independently of IL-23R expression.

The effect of anti-IL-6 receptor antibody for the treatment of McH-lpr/lpr-RA1 mice that spontaneously developed destructive arthritis and enthesitis.

BACKGROUND: McH-lpr/lpr-RA1 mice are a new strain of mice which spontaneously develop destructive arthritis and enthesitis in the ankle. There is no published data that drug treatment has been trialed on these mice. This study examined the effect of the mouse anti-IL-6 receptor antibody, MR16-1, for the treatment of arthritis and enthesitis in McH-lpr/lpr-RA1 mice. METHODS: Male McH-lpr/lpr-RA1 mice were randomly divided into control and treatment groups. MR16-1 was administered from 10 weeks of age for the treatment group. Saline was applied for the control group. The drug was administered once a week, at an initial dose of 2 mg, then maintained at 0.5 mg once per week thereafter. The effects were evaluated by the histopathological synovitis score, in vivo imaging using indocyanine green liposomes, and analysis of the gene expression of inflammatory cytokines. RESULTS: Tissue analyses were carried out at 14, 17 and 20 weeks of age. The synovitis scores of treated groups were significantly lower compared with those of the control group at 14 and 17 weeks of age. The kappa coefficient was 0.77. However, progression of entheseal ossification persisted in the MR16-1 treated group. In vivo imaging using indocyanine green liposomes showed significant decreases in signal intensities of treated groups at week 14, but no significant differences were observed at week 18. Blood serum amyloid A levels in treated groups were significantly lower at 17 weeks of age. The gene expression levels of Tnf and Il17 were also significantly lower in MR16-1 treated groups. CONCLUSIONS: Administration of the anti-IL-6 receptor antibody is effective for the treatment of synovitis and bone destruction of McH-lpr/lpr-RA1 mice. McH-lpr/lpr-RA1 mice may be a suitable experimental model for the development of new treatments for destructive arthritis and enthesitis. IL-6 signal blockade could contribute to the treatment of destructive arthritis, and further studies should be carried out to confirm its potential in the prevention of enthesopathy developed to ossification.

Enthesitis: Much More Than Focal Insertion Point Inflammation.

PURPOSE OF REVIEW: Recognition of the importance of enthesitis as the pivotal pathological process underpinning spondyloarthropathies (SpA) has increased in recent years. Thus, we summarized the current knowledge on the pathogenic role of enthesitis on SpA shown by both animal models and human studies in vivo. RECENT FINDINGS: Experimental models have shown several SpA-like diseases that commence at entheses and are linked to nail disease as well as dactylitis, two important entheseal-associated conditions in humans. Frequently, enthesitis is not the primary outcome measure in studies of peripheral PsA and SpA although arguably it is the key parameter being indirectly assessed in spinal disease in ankylosing spondylitis. The use of different agents including JAK, IL-17, and IL-23 inhibitors contributes significantly to our understanding of enthesitis in terms of involved immune pathways. Enthesitis and enthesis organ inflammation may be the primary pathological process underlying SpA associated skeletal inflammation. Emergent studies are beginning to elucidate the molecular basis for this type of joint inflammatory response.

Augmented Th17 Differentiation Leads to Cutaneous and Synovio-Entheseal Inflammation in a Novel Model of Psoriatic Arthritis.

OBJECTIVE: To introduce a novel preclinical animal model of psoriatic arthritis (PsA) in R26Stat3Cstopfl/fl CD4Cre mice, and to investigate the role of Th17 cytokines in the disease pathogenesis. METHODS: We characterized a novel murine model of Th17-driven cutaneous and synovio-entheseal disease directed by T cell-specific expression of a hyperactive Stat3 allele. By crossing R26Stat3Cstopfl/fl CD4Cre mice onto an interleukin-22 (IL-22)-knockout background or treating the mice with a neutralizing antibody against IL-17, we interrogated how these Th17 cytokines could contribute to the pathogenesis of PsA. RESULTS: R26Stat3Cstopfl/fl CD4Cre mice developed acanthosis, hyperkeratosis, and parakeratosis of the skin, as well as enthesitis/tendinitis and periarticular bone erosion in different joints, accompanied by osteopenia. T cell-specific expression of a hyperactive Stat3C allele was found to drive the augmented Th17 response in these animals. Careful characterization of the mouse bone marrow revealed an increase in osteoclast progenitor (OCP) and RANKL-producing cells, which contributed to the osteopenia phenotype observed in the mutant animals. Abrogation of the Th17 cytokines IL-17 or IL-22 improved both the skin and bone phenotype in R26Stat3Cstopfl/fl CD4Cre mice, revealing a central role of Th17 cells in the regulation of OCP and RANKL expression on stromal cells. CONCLUSION: Perturbation of the IL-23/Th17 axis instigates Th17-mediated inflammation in R26Stat3Cstopfl/fl CD4Cre mice, leading to cutaneous and synovio-entheseal inflammation and bone pathologic features highly reminiscent of human PsA. Both IL-17A and IL-22 produced by Th17 cells appear to play critical roles in promoting the cutaneous and musculoskeletal inflammation that characterizes PsA.

Interleukin-23-Dependent γ/δ T Cells Produce Interleukin-17 and Accumulate in the Enthesis, Aortic Valve, and Ciliary Body in Mice.

OBJECTIVE: The spondyloarthritides (SpA) are a group of rheumatic diseases characterized by ossification and inflammation of entheseal tissue, the region where tendon attaches to bone. Interleukin-23 (IL-23) is involved in the pathogenesis of SpA by acting on IL-23 receptor (IL-23R) expressed on enthesis-resident lymphocytes. Upon IL-23 binding, CD3+CD4-CD8- tissue-resident lymphocytes secrete IL-17A and IL-22, leading to inflammation, bone loss, and ossification. Knowledge about enthesis-resident lymphocytes remains fragmentary, and the contribution of entheseal γ/δ T cells in particular is not clear. This study was undertaken to investigate the presence of γ/δ T cells in the enthesis. METHODS: We used 2-photon microscopy and flow cytometry to analyze entheseal lymphocytes from C57BL/6, Tcrd-H2BeGFP, Rorc-GFP, and IL-23R-eGFP mice. To analyze entheseal γ/δ T cells in IL-23-induced inflammation, Tcrd-H2BeGFP mice were crossed with mice of the susceptible B10.RIII background. Hydrodynamic injection of IL-23 minicircle DNA was performed for overexpression of IL-23 and induction of inflammation. Light-sheet fluorescence microscopy was used to visualize arthritic inflammation. RESULTS: Activated Vγ6+CD27- γ/δ T cells were abundant in uninflamed entheseal tissue and constituted the large majority of retinoic acid receptor-related orphan nuclear receptor γt (RORγt)+IL-23R+ enthesis-resident lymphocytes. Fetal thymus-dependent γ/δ T cells were the main source of IL-17A at the enthesis. Under inflammatory conditions, γ/δ T cells increased in number at the Achilles tendon enthesis, aortic root, and adjacent to the ciliary body. CONCLUSION: Entheseal γ/δ T cells are derived from fetal thymus and are maintained as self-renewing tissue-resident cells. As main IL-17A producers within tissues exposed to mechanical stress including enthesis, γ/δ T cells are key players in the pathogenesis of IL-23-induced local inflammation.