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Rheumatoid arthritis is a chronic autoimmune disease that causes progressive articular damage, functional loss, and comorbidity. The development of effective biologics and small-molecule kinase inhibitors in the past two decades has substantially improved clinical outcomes. Just as understanding of pathogenesis has led in large part to the development of drugs, so have mode-of-action studies of these specific immune-targeted agents revealed which immune pathways drive articular inflammation and related comorbidities. Cytokine inhibitors have definitively proven a critical role for tumour necrosis factor α and interleukin 6 in disease pathogenesis and possibly also for granulocyte–macrophage colony-stimulating factor. More recently, clinical trials with Janus kinase (JAK) inhibitors have shown that cytokine receptors that signal through the JAK/STAT signalling pathway are important for disease, informing the pathogenetic function of additional cytokines (such as the interferons). Finally, successful use of costimulatory blockade and B-cell depletion in the clinic has revealed that the adaptive immune response and the downstream events initiated by these cells participate directly in synovial inflammation. Taken together, it becomes apparent that understanding the effects of specific immune interventions can elucidate definitive molecular or cellular nodes that are essential to maintain complex inflammatory networks that subserve diseases like rheumatoid arthritis.

Inflammation and its resolution is under-studied in medicine despite being essential for understanding the development of chronic inflammatory disease. In this review article, we discuss the resolution of inflammation in both a biological and translational context. We introduce the concept of impaired resolution leading to diseases like rheumatoid arthritis, Crohn's disease, and asthma, as well as the cellular and molecular components that contribute to resolution of joint, gut, and lung inflammation, respectively. Finally, we discuss potential intervention strategies for fostering the resolution process, and their implications for the therapy of inflammatory diseases. Inflammation is a component of many chronic inflammatory diseases and yet it is understudied in medicine. Here, the authors review novel insights in to inflammation and how impairment of its resolution can lead to diseases.

The increased understanding of the immune mechanisms of rheumatoid arthritis has led to the development of a considerable number of new therapeutic agents that alter the natural history of the disease and reduce mortality. Rheumatoid arthritis is a common autoimmune disease that is associated with progressive disability, systemic complications, early death, and socioeconomic costs. 1 The cause of rheumatoid arthritis is unknown, and the prognosis is guarded. However, advances in understanding the pathogenesis of the disease have fostered the development of new therapeutics, with improved outcomes. The current treatment strategy, which reflects this progress, is to initiate aggressive therapy soon after diagnosis and to escalate the therapy, guided by an assessment of disease activity, in pursuit of clinical remission. However, several unmet needs remain. Current conventional and biologic disease-modifying therapies sometimes fail or produce only . . .

In this Review, the authors summarize the substantial progress that has been made in understanding the pathophysiology of bone erosions and discuss the improvements in the diagnosis, monitoring and treatment of such lesions. Bone erosion is a central feature of rheumatoid arthritis and is associated with disease severity and poor functional outcome. Erosion of periarticular cortical bone, the typical feature observed on plain radiographs in patients with rheumatoid arthritis, results from excessive local bone resorption and inadequate bone formation. The main triggers of articular bone erosion are synovitis, including the production of proinflammatory cytokines and receptor activator of nuclear factor κB ligand (RANKL), as well as antibodies directed against citrullinated proteins. Indeed, both cytokines and autoantibodies stimulate the differentiation of bone-resorbing osteoclasts, thereby stimulating local bone resorption. Although current antirheumatic therapy inhibits both bone erosion and inflammation, repair of existing bone lesions, albeit physiologically feasible, occurs rarely. Lack of repair is due, at least in part, to active suppression of bone formation by proinflammatory cytokines. This Review summarizes the substantial progress that has been made in understanding the pathophysiology of bone erosions and discusses the improvements in the diagnosis, monitoring and treatment of such lesions. Key Points Articular bone erosions are a central clinical feature of rheumatoid arthritis Imaging techniques enable early detection of bone erosions and provide insights into disease pathogenesis Bone erosion is a result of enhanced osteoclast differentiation and inhibition of osteoblast-mediated bone repair Autoantibodies and cytokines, including proinflammatory cytokines and receptor activator of nuclear factor κB ligand, are the major precipitating factors in bone erosion in rheumatoid arthritis Antirheumatic therapies block progression of bone erosion by mitigating synovial inflammation and restoring bone balance

Over the past several years, a pathophysiological role for the IL-23–IL-17 pathway in human disease has been defined. A subset of rheumatic diseases, including psoriatic arthritis (PsA) and ankylosing spondylitis (AS), are now acknowledged to be triggered by dysregulated IL-23–IL-17 pathway activation. Genetic evidence links the IL-23–IL-17 pathway to inflammation in these rheumatic diseases, and mechanistic data from mice support a functional role for IL-23–IL-17 pathway activation in the development of enthesitis and in entheseal bone formation. Furthermore, analysis of human tissue samples, as well as data from clinical trials, also supports a role for activation of the IL-23–IL-17 pathway in these diseases. The unique bone phenotype that occurs in PsA and AS is a surprising coexistence of both systemic bone loss and periosteal and entheseal bone formation and is likely to be the result of the actions of IL-23 and/or IL-17 on bone. However, the effects of these cytokines on bone cells are complex, and controversy remains regarding their exact roles in the specific bone microenvironments relevant to PsA and AS.

Most rheumatic and musculoskeletal diseases (RMDs) can be placed along a spectrum of disorders, with autoinflammatory diseases (including monogenic systemic autoinflammatory diseases) and autoimmune diseases (such as systemic lupus erythematosus and antiphospholipid syndrome) representing the two ends of this spectrum. However, although most autoinflammatory diseases are characterized by the activation of innate immunity and inflammasomes and classical autoimmunity typically involves adaptive immune responses, there is some overlap in the features of autoimmunity and autoinflammation in RMDs. Indeed, some ‘mixed-pattern’ diseases such as spondyloarthritis and some forms of rheumatoid arthritis can also be delineated. A better understanding of the pathogenic pathways of autoinflammation and autoimmunity in RMDs, as well as the preferential cytokine patterns observed in these diseases, could help us to design targeted treatment strategies.Most rheumatic and musculoskeletal diseases (RMDs) fall along a spectrum of disorders from autoinflammatory diseases to autoimmune diseases, with ‘mixed-pattern’ RMDs having features of autoinflammation and autoimmunity. A better understanding of the pathogenic pathways of autoinflammation and autoimmunity in RMDs should enhance targeted treatment strategies.

The gut microbiome is a key regulator of bone health that affects postnatal skeletal development and skeletal involution. Alterations in microbiota composition and host responses to the microbiota contribute to pathological bone loss, while changes in microbiota composition that prevent, or reverse, bone loss may be achieved by nutritional supplements with prebiotics and probiotics. One mechanism whereby microbes influence organs of the body is through the production of metabolites that diffuse from the gut into the systemic circulation. Recently, short-chain fatty acids (SCFAs), which are generated by fermentation of complex carbohydrates, have emerged as key regulatory metabolites produced by the gut microbiota. This Review will focus on the effects of SCFAs on the musculoskeletal system and discuss the mechanisms whereby SCFAs regulate bone cells.

Rheumatoid arthritis (RA) is a chronic autoimmune inflammatory disorder that primarily affects the joints. One hypothesis for the pathogenesis of RA is that disease begins at mucosal sites as a consequence of interactions between the mucosal immune system and an aberrant local microbiota, and then transitions to involve the synovial joints. Alterations in the composition of the microbial flora in the lungs, mouth and gut in individuals with preclinical and established RA suggest a role for mucosal dysbiosis in the development and perpetuation of RA, although establishing whether these alterations are the specific consequence of intestinal involvement in the setting of a systemic inflammatory process, or whether they represent a specific localization of disease, is an ongoing challenge. Data from mouse models of RA and investigations into the preclinical stages of disease also support the hypothesis that these alterations to the microbiota predate the onset of disease. In addition, several therapeutic options widely used for the treatment of RA are associated with alterations in intestinal microbiota, suggesting that modulation of intestinal microbiota and/or intestinal barrier function might be useful in preventing or treating RA.Intestinal dysbiosis is thought to be involved in the early stages of rheumatoid arthritis (RA). In this Review, the authors discuss the gut–joint axis in RA and the potentially pathogenic role of gut-derived immune cells in the joints.

Treatment of giant-cell arteritis with tocilizumab, an interleukin-6 receptor alpha inhibitor, while prednisone was tapered over a 26-week period resulted in higher rates of sustained remission than prednisone tapering plus placebo and reduced the total prednisone dose.

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 the anatomical extent of several mechanical stress-related disorders in tendons and ligaments overlaps with that of chronic inflammatory arthritis in joints. 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.Mechanical loading is an important factor in the development of tendon and ligament disorders. In this Review, the authors discuss the evidence for the known role of mechanical loading in tendinopathy and its potential role in inflammatory arthritis.