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ObjectiveWe investigated whether the normal human spinal enthesis contained resident myeloid cell populations, capable of producing pivotal proinflammatory cytokines including tumour necrosis factor (TNF) and interleukin (IL)-23 and determined whether these could be modified by PDE4 inhibition.MethodsNormal human enthesis soft tissue (ST) and adjacent perientheseal bone (PEB) (n=15) were evaluated using immunohistochemistry (IHC), digested for myeloid cell phenotyping, sorted and stimulated with different adjuvants (lipopolysaccharide and mannan). Stimulated enthesis fractions were analysed for inducible production of spondyloarthropathy disease-relevant mediators (IL-23 full protein, TNF, IL-1β and CCL20). Myeloid populations were also compared with matched blood populations for further mRNA analysis and the effect of PDE4 inhibition was assessed.ResultsA myeloid cell population (CD45+ HLADR+ CD14+ CD11c+) phenotype was isolated from both the ST and adjacent PEB and termed ‘CD14+ myeloid cells’ with tissue localisation confirmed by CD14+ IHC. The CD14− fraction contained a CD123+ HLADR+ CD11c− cell population (plasmacytoid dendritic cells). The CD14+ population was the dominant entheseal producer of IL-23, IL-1β, TNF and CCL20. IL-23 and TNF from the CD14+ population could be downregulated by a PDE4I and other agents (histamine and 8-Bromo-cAMP) which elevate cAMP. Entheseal CD14+ cells had a broadly similar gene expression profile to the corresponding CD14+ population from matched blood but showed significantly lower CCR2 gene expression.ConclusionsThe human enthesis contains a CD14+ myeloid population that produces most of the inducible IL-23, IL-1β, TNF and CCL20. This population has similar gene expression profile to the matched blood CD14+ population.

Background Neutrophils are present in the early phases of spondyloarthritis-related uveitis, skin and intestinal disease, but their role in enthesitis, a cardinal musculoskeletal lesion in spondyloarthritis, remains unknown. We considered the role of neutrophils in the experimental SKG mouse model of SpA and in human axial entheses. Methods Early inflammatory infiltrates in the axial and peripheral entheseal sites in SKG mice were evaluated using immunohistochemistry and laser capture microdissection of entheseal tissue. Whole transcriptome analysis was carried out using Affymetrix gene array MTA 1.0, and data was analyzed via IPA. We further isolated neutrophils from human peri-entheseal bone and fibroblasts from entheseal soft tissue obtained from the axial skeleton of healthy patients and determined the response of these cells to fungal adjuvant. Results Following fungal adjuvant administration, early axial and peripheral inflammation in SKG mice was characterized by prominent neutrophilic entheseal inflammation. Expression of transcripts arising from neutrophils include abundant mRNA for the alarmins S100A8 and S100A9. In normal human axial entheses, neutrophils were present in the peri-entheseal bone. Upon fungal stimulation in vitro, human neutrophils produced IL-23 protein, while isolated human entheseal fibroblasts produced chemokines, including IL-8, important in the recruitment of neutrophils. Conclusion Neutrophils with inducible IL-23 production are present in uninflamed human entheseal sites, and neutrophils are prominent in early murine spondyloarthritis-related enthesitis. We propose a role for neutrophils in the early development of enthesitis.

Background Bone marrow mesenchymal stem cells (BM-MSCs) can dampen inflammation in animal models of inflammatory rheumatisms and human osteoarthritis. They are expected to be a solution for numerous human conditions. However, in rheumatoid arthritis (RA) and spondyloarthritis (SpA), subsets of subchondral BM-MSCs might conversely fuel synovitis and enthesitis. Main text Abnormal behaviour of BM-MSCs and/or their progeny has been found in RA and SpA. BM-MSCs also contribute to the ossifying processes observed in ankylosing spondylitis. Some synovial fibroblastic stem cells probably derive from BM-MSCs, but some stem cells can also migrate through the bare zone area of joints, not covered by cartilage, into the synovium. BM-MSCs can also migrate in the synovium over tendons. Sub-populations of bone marrow stem cells also invade the soft tissue side of enthesis via small holes in the bone cortex. The present review aims (1) to make a focus on these two aspects and (2) to put forward the hypothesis that lasting epigenetic changes of some BM-MSCs, induced by transient infections of the bone marrow close to the synovium and/or entheses (i.e. trained immunity of BM-MSCs and/or their progeny), contribute to the pathogenesis of inflammatory rheumatisms. Such hypothesis would fit with (1) the uneven distribution and/or flares of arthritis and enthesitis observed at the individual level in RA and SpA (reminiscent of what is observed following reactive arthritis and/or in Whipple’s disease); (2) the subchondral bone marrow oedema and erosions occurring in many RA patients, in the bare zone area; and (3) the frequent relapses of RA and SpA despite bone marrow transplantation, whereas most BM-MSCs resist graft preconditioning. Conclusion Some BM-MSCs might be more the problem than the solution in inflammatory rheumatisms. Subchondral bone marrow BM-MSCs and their progeny trafficking through the bare zone area of joints or holes in the bone cortex of entheses should be thoroughly studied in RA and SpA respectively. This may be done first in animal models. Mini-arthroscopy of joints could also be used in humans to specifically sample tissues close to the bare zone and/or enthesis areas.

The severity of rotator cuff injury outcomes and a lack of tendon-to-bone enthesis regeneration strategies have inspired advances in biomaterials science to develop methods for interfacial tissue engineering. Here, we demonstrate a triphasic biomaterial comprising a non-mineralized, anisotropic collagen scaffold and a mineralized isotropic collagen scaffold linked via a continuous thiolated gelatin (Gel-SH) interface. This material provides a stratified environment in composition and porous architecture, and we report functional activity of human mesenchymal stem cells (hMSCs) across the scaffold. Notably, MSCs can be seeded onto the triphasic biomaterial and remain viable for up to 21 days. In addition, MSCs within the interfacial Gel-SH interfacial zone express markers associated with the rotator cuff fibrocartilaginous enthesis, including gene upregulation of COL1A1, COL3A1, SOX9, BMP4, TGFβ1 and functional secretion of TGF-β1. Altogether, these findings suggest that this triphasic scaffold design could create a permissive environment for fibrochondrogenic activity in support of eventual enthesis interfacial tissue engineering applications. [Display omitted] -A novel triphasic tendon-to-bone scaffold contains multiscale structural gradients-Human mesenchymal stem cells are viable and active in all regions over 3 weeks-Upregulation of enthesis-associated genes COL1A1, COL3A1, SOX9, BMP4-Functional secretion of TGF-β1, particularly in the first week of culture

Abstract Repairing injured tendon or ligament attachments to bones (enthesis) remains costly and challenging. Despite superb surgical management, the disorganized enthesis newly formed after surgery accounts for high recurrence rates after operations. Tissue engineering offers efficient alternatives to promote healing and regeneration of the specialized enthesis tissue. Load-transmitting functions thus can be restored with appropriate biomaterials and engineering strategies. Interestingly, recent studies have focused more on microstructure especially the arrangement of fibers since Rossetti successfully demonstrated the variability of fiber underspecific external force. In this review, we provide an important update on the current strategies for scaffold-based tissue engineering of enthesis when natural structure and properties are equally emphasized. We firstly described compositions, structures and features of natural enthesis with their special mechanical properties highlighted. Stimuli for growth, development and healing of enthesis widely used in popular strategies are systematically summarized. We discuss the fabrication of engineering scaffolds from the aspects of biomaterials, techniques and design strategies and comprehensively evaluate the advantages and disadvantages of each strategy. At last, this review pinpoints the remaining challenges and research directions to make breakthroughs in further studies. Graphical abstract

The firm integration of anterior cruciate ligament (ACL) grafts into bones remains the most demanding challenge in ACL reconstruction, since graft loosening means graft failure. For a functional-tissue-engineered ACL substitute to be realized in future, robust bone attachment sites (entheses) have to be re-established. The latter comprise four tissue compartments (ligament, non-calcified and calcified fibrocartilage, separated by the tidemark, bone) forming a histological and biomechanical gradient at the attachment interface between the ACL and bone. The ACL enthesis is surrounded by the synovium and exposed to the intra-articular micromilieu. This review will picture and explain the peculiarities of these synovioentheseal complexes at the femoral and tibial attachment sites based on published data. Using this, emerging tissue engineering (TE) strategies addressing them will be discussed. Several material composites (e.g., polycaprolactone and silk fibroin) and manufacturing techniques (e.g., three-dimensional-/bio-printing, electrospinning, braiding and embroidering) have been applied to create zonal cell carriers (bi- or triphasic scaffolds) mimicking the ACL enthesis tissue gradients with appropriate topological parameters for zones. Functionalized or bioactive materials (e.g., collagen, tricalcium phosphate, hydroxyapatite and bioactive glass (BG)) or growth factors (e.g., bone morphogenetic proteins [BMP]-2) have been integrated to achieve the zone-dependent differentiation of precursor cells. However, the ACL entheses comprise individual (loading history) asymmetric and polar histoarchitectures. They result from the unique biomechanical microenvironment of overlapping tensile, compressive and shear forces involved in enthesis formation, maturation and maintenance. This review should provide a road map of key parameters to be considered in future in ACL interface TE approaches.

Purpose of ReviewDiffuse Idiopathic Skeletal Hyperostosis (DISH) is considered a metabolic condition, characterized by new bone formation affecting mainly at entheseal sites. Enthesitis and enthesopathies occur not only in the axial skeleton but also at some peripheral sites, and they resemble to some extent the enthesitis that is a cardinal feature in spondyloarthritis (SpA), which is an inflammatory disease.Recent FindingsWe review the possible non-metabolic mechanism such as inflammation that may also be involved at some stage and help promote new bone formation in DISH. We discuss supporting pathogenic mechanisms for a local inflammation at sites typically affected by this disease, and that is also supported by imaging studies that report some similarities between DISH and SpA.SummaryLocal inflammation, either primary or secondary to metabolic derangements, may contribute to new bone formation in DISH. This new hypothesis is expected to stimulate further research in both the metabolic and inflammatory pathways in order to better understand the mechanisms that lead to new bone formation. This may lead to development of measures that will help in earlier detection and effective management before damage occurs.

Proper mechanical stimulation can improve rotator cuff enthesis injury repair. However, the underlying mechanism of mechanical stimulation promoting injury repair is still unknown. In this study, we found that Prrx1 + cell was essential for murine rotator cuff enthesis development identified by single-cell RNA sequence and involved in the injury repair. Proper mechanical stimulation could promote the migration of Prrx1 + cells to enhance enthesis injury repair. Meantime, TGF-β signaling and primary cilia played an essential role in mediating mechanical stimulation signaling transmission. Proper mechanical stimulation enhanced the release of active TGF-β1 to promote migration of Prrx1 + cells. Inhibition of TGF-β signaling eliminated the stimulatory effect of mechanical stimulation on Prrx1 + cell migration and enthesis injury repair. In addition, knockdown of Pallidin to inhibit TGF-βR2 translocation to the primary cilia or deletion of Ift88 in Prrx1 + cells also restrained the mechanics-induced Prrx1 + cells migration. These findings suggested that mechanical stimulation could increase the release of active TGF-β1 and enhance the mobilization of Prrx1 + cells to promote enthesis injury repair via ciliary TGF-β signaling.

ObjectiveTo analyse the frequency of power Doppler (PD) enthesitis in active axial spondyloarthritis (axSpA) and psoriatic arthritis (PsA) patients and its potential usefulness in clinical practice.MethodsA prospective multicentre cross-sectional study in patients with axSpA and PsA with active disease was undertaken. Patients underwent bilateral ultrasound (US) examination of the peripheral entheses according to the Madrid Sonographic Enthesis Index (MASEI). The MASEI and Outcome Measures in Rheumatology (OMERACT) PD enthesitis definitions were checked. An inter-reader analysis of recorded videos was performed to determine reliability.ResultsSixty-four consecutive patients were included. The mean DAS28 (3.9 ± 1.3) for peripheral involvement, mean BASDAI (5.6 ± 2.2) for axial involvement, and CRP values (10 ± 10.9) reflected moderate-high disease activity at baseline. The mean global MASEI score was 29.4 (± 11.4), and 55 patients (86%) scored ≥ 18 (proposed cut-off point to diagnose SpA). At the patient level, abnormal US findings consistent with at least one enthesis showing a PD signal were observed in 52 (81.3%) patients using the MASEI PD definition and 48 (75%) using the OMERACT PD definition, without significant variation between axSpA and PsA. The inter-reader reliability was excellent (kappa = 0.92 for MASEI PD and 0.86 for OMERACT PD).ConclusionsPD enthesitis was found in the majority of patients with active axSpA and PsA, independent of axial or peripheral affectation. Both MASEI and OMERACT PD definitions were useful in detecting active enthesitis. These findings support the usefulness of a PD US evaluation of entheses in the assessment of axSpA and PsA.Key Points• PD enthesitis is a very common finding in patients with active axSpA and PsA• Both MASEI and OMERACT PD definitions are useful to detect active enthesitis• US enthesitis may reveal information in axSpA and PsA

Purpose of ReviewThis review seeks to provide an overview of the role of inflammation and metabolism in tendon cell function, tendinopathy, and tendon healing. We have summarized the state of knowledge in both tendon and enthesis.Recent FindingsRecent advances in the field include a substantial improvement in our understanding of tendon cell biology, including the heterogeneity of the tenocyte environment during homeostasis, the diversity of the cellular milieu during in vivo tendon healing, and the effects of inflammation and altered metabolism on tendon cell function in vitro. In addition, the mechanisms by which altered systemic metabolism, such as diabetes, disrupts tendon homeostasis continue to be better understood.SummaryA central conclusion of this review is the critical need to better define fundamental cellular and signaling mechanisms of inflammation and metabolism during tendon homeostasis, tendinopathy, and tendon healing in order to identify therapies to enhance or maintain tendon function.