Many pro-inflammatory pathways leading to arthritis have global effects on the immune system rather than only acting locally in joints. The reason behind the regional and patchy distribution of arthritis represents a longstanding paradox. Here we show that biomechanical loading acts as a decisive factor in the transition from systemic autoimmunity to joint inflammation. Distribution of inflammation and erosive disease is confined to mechano-sensitive regions with a unique microanatomy. Curiously, this pathway relies on stromal cells but not adaptive immunity. Mechano-stimulation of mesenchymal cells induces CXCL1 and CCL2 for the recruitment of classical monocytes, which can differentiate into bone-resorbing osteoclasts. Genetic ablation of CCL2 or pharmacologic targeting of its receptor CCR2 abates mechanically-induced exacerbation of arthritis, indicating that stress-induced chemokine release by mesenchymal cells and chemo-attraction of monocytes determines preferential homing of arthritis to certain hot spots. Thus, mechanical strain controls the site-specific localisation of inflammation and tissue damage in arthritis.

Mechanical loading is an important factor in musculoskeletal health and disease. Tendons and ligaments require physiological levels of mechanical loading to develop and maintain their tissue architecture, a process that is achieved at the cellular level through mechanotransduction-mediated fine tuning of the extracellular matrix by tendon and ligament stromal cells. Pathological levels of force represent a biological (mechanical) stress that elicits an immune system-mediated tissue repair pathway in tendons and ligaments. The biomechanics and mechanobiology of tendons and ligaments form the basis for understanding how such tissues sense and respond to mechanical force, and several mechanical stressrelated tendon and ligament disorders overlap anatomically with joints affected by chronic inflammatory arthritis. The role of mechanical stress in 'overuse' injuries, such as tendinopathy, has long been known, but mechanical stress is now also emerging as a possible trigger for some forms of chronic inflammatory arthritis, including spondyloarthritis and rheumatoid arthritis. Thus, seemingly diverse diseases of the musculoskeletal system might have similar mechanisms of immunopathogenesis owing to conserved responses to mechanical stress.

ObjectivesThe mechanisms driving onset of joint inflammation in arthritides such as rheumatoid arthritis and spondyloarthritis and the conversion to disease chronicity are poorly understood. We hypothesised mechanostrain could play an instrumental role herein by engaging local and/or systemic pathways, thereby attenuating disease course and outcome.MethodsThe development of collagen antibody-induced arthritis (CAIA) in C57BL/6 mice was evaluated both clinically and histologically under different loading regimens: control, voluntary running or hindpaw unloading. Bone surface porosity was quantified by high-resolution µ-CT. Gene expression analyses were conducted by microarrays and qPCR on microdissected entheses, murine and human synovial tissues (both normal and inflamed). Serum cytokines and chemokines were measured by ELISA. The influence of complement activation and T regulatory (Treg) cell function on the induction and resolution phase of disease was studied by respectively pharmacological modulation and conditional Treg depletion.ResultsVoluntary running strongly impacts the course of arthritis by impairing the resolution phase of CAIA, leading to more persistent inflammation and bone surface porosity. Mechanical strain induced local complement activation, increased danger-associated molecular pattern expression, activating Fcγ receptors as well as changes in fibroblast phenotype. Interestingly, complement C5a receptor blockade inhibited the enhanced joint pathology caused by voluntary running. Moreover, Treg depletion led to a loss of disease resolution in CAIA mice, which was not observed under voluntary running conditions.ConclusionsRunning promotes onset and chronicity of arthritis by local upregulation of complement activators and hampering regulatory T cell feedback loops.

Osteoporosis affects millions worldwide and is often caused by osteoclast induced bone loss. Here, we identify the cytoplasmic protein ELMO1 as an important ‘signaling node’ in osteoclasts. We note that ELMO1 SNPs associate with bone abnormalities in humans, and that ELMO1 deletion in mice reduces bone loss in four in vivo models: osteoprotegerin deficiency, ovariectomy, and two types of inflammatory arthritis. Our transcriptomic analyses coupled with CRISPR/Cas9 genetic deletion identify Elmo1 associated regulators of osteoclast function, including cathepsin G and myeloperoxidase. Further, we define the ‘ELMO1 interactome’ in osteoclasts via proteomics and reveal proteins required for bone degradation. ELMO1 also contributes to osteoclast sealing zone on bone-like surfaces and distribution of osteoclast-specific proteases. Finally, a 3D structure-based ELMO1 inhibitory peptide reduces bone resorption in wild type osteoclasts. Collectively, we identify ELMO1 as a signaling hub that regulates osteoclast function and bone loss, with relevance to osteoporosis and arthritis.

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Introduction Macrophage activation syndrome (MAS) is a severe condition, which can appear as a complication of inflammatory rheumatic diseases such as systemic juvenile idiopathic arthritis (sJIA) and adult onset Still's disease (AOSD), a viral infection, or a malignancy. Interleukin (IL)À18 is a proinflammatory cytokine of the IL-1 family, known as a strong interferon (IFN)-g inducer, that is naturally inhibited by IL-18 binding protein (IL-18BP). High levels of unbound biologically active IL-18 have been described in patients with sJIA, AOSD, and MAS, suggesting that IL-18 is involved in the pathogenesis of these diseases. Objectives To examine the effect of excessive IL-18 signalling in a mouse model of MAS induced by repetitive toll like receptor (TLR)9 stimulation, and explore the consequences of IL-18 or IFN-g blockade on MAS manifestations. Methods MAS was induced by repeated intraperitoneal CpG injections in IL-18BP deficient (IL-18BP-/-) mice and in wild type (WT) littermates. Anti-IL-18 receptor (IL-18R) or anti-IFN-g monoclonal antibodies were administered prior to CpG injections. Clinical and biological manifestations of MAS were studied. Plasma levels of free IL-18 were measured by ELISA. Expression levels of IFN-g and downstream IFN-g-induced effectors (CXCL9, CIITA) were explored by ELISA or Luminex in plasma, and by RT-qPCR in spleen and liver. Results Naïve IL-18BP-/mice had no spontaneous phenotype. After repeated CpG injections, IL-18BP-/mice displayed significantly more severe MAS phenotype than their WT littermates, including more pronounced weight loss, splenomegaly, anaemia, thrombocytopenia, hyperferritinemia and hepatitis. This phenotype was associated with elevated plasma levels of unbound IL-18 and the presence of bone marrow hemophagocytes in IL-18BP-/mice only. In addition, IL-18BP-/mice displayed higher plasma levels of IFN-g and CXCL9, as well as increased Ifng, Cxcl9 and CIIta mRNA expression in the spleen and liver. IL-18 blockade using an anti-IL-18R antibody attenuated MAS manifestations in IL-18BP-/mice and abrogated IFN-g production and downstream signalling. IFN-g blockade using an anti-IFN-g antibody also attenuated the MAS phenotype. Conclusions By using IL-18BP-/mice, we showed that unopposed IL-18 signalling was detrimental in the TLR9-induced MAS model. Importantly, blocking IL-18, as well as IFN-g, improved disease in IL-18BP-/mice. Altogether, our results suggest that IL-18 exerts a pathogenic role in this model of MAS, acting upstream of IFN-g.

BackgroundForce induced microdamage to joint tissue is hypothesized to trigger inflammatory events in the joint leading to arthritis. Patients with inflammatory arthritis, such as rheumatoid arthritis (RA) and spondyloarthritis (SpA), are found to have inflammation in “mechanical hotspots” and mechanical loading in mouse models of these diseases is pro-arthritogenic1,2. To date, the molecular mechanism involved in converting force to a biological signal that promotes arthritis is not known.ObjectivesThis study aims to identify stretch induced genes in synovial fibroblasts, and the effect of these “mechano-sensitive” genes on arthritis.MethodsHuman synovial fibroblasts were stretched in vitro for 4hrs using the FlexCell system and analysed by microarray. Top stretch induced genes were measured in RA, SpA and healthy synovial tissue by qPCR. Patient synovium was further analysed by immunohistochemistry. Bhlhe40 deficient mice were subjected to collagen induced arthritis (CIA) and KBxN serum transfer arthritis (STA). FACS was performed on ankle synovium. uCT was performed on whole ankles, with morphological changes scored by blinded readers, and calcaneus erosions by customs scripts in FIJI.Results600 genes were found to be differentially expressed in stretched synovial fibroblasts (fold change > +/-1.5, adjusted p<0.05). 25% of these genes were found to be transcription factors, which included BHLHE40. BHLHE40 mRNA was elevated in the synovial tissue of RA/SpA vs healthy subjects (1.56 fold change), and BHLHE40 protein was widely detectable in synovial fibroblasts and macrophages (Figure 1). Bhlhe40 deficient mice were completely protected against CIA (incidence: 0% vs 40%, n=30 per group), but Bhlhe40 did not block the generation of anti-collagen antibodies. Bhlhe40 deficient mice were partially protected against STA (peak clinical score at day 7; 5.2 vs 6.8, n=15 per group), with reduced synovial macrophage (CD11b+Ly6G-F4/80+) and neutrophil (CD11b+Ly6G+) frequency observed in the arthritic Bhlhe40 deficient mice compared to wildtype controls. Bhlhe40 had no impact on bone erosions with STA.Figure 1.BHLHE40 is widely expressed in human synovium. Synovium obtained from total knee replacement. FFPE samples were stained for synovial macrophages (HLADR+) and fibroblasts (FAP+). Images acquired with the Zeiss LSM 780.ConclusionBHLHE40 was identified as a force-induced gene in synovial fibroblasts and was found to be upregulated in patients with inflammatory arthritis. Importantly, Bhlhe40 strongly promotes joint inflammation in murine models of arthritis and uncouples systemic autoimmunity from joint tissue inflammation. Thus, we have identified BHLHE40 as a novel regulator of mechanical loading-associated inflammation.References[1]Cambré, I. et al. Mechanical strain determines the site-specific localization of inflammation and tissue damage in arthritis. Nat. Commun.9, 4613 (2018).[2]Jacques, P. et al. Proof of concept: enthesitis and new bone formation in spondyloarthritis are driven by mechanical strain and stromal cells. Ann. Rheum. Dis.73, 437–445 (2014).Disclosure of InterestsNone declared