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Idiopathic inflammatory myopathies (IIM), also known as myositis, are a heterogeneous group of autoimmune disorders with varying clinical manifestations, treatment responses and prognoses. Muscle weakness is usually the classical clinical manifestation but other organs can be affected, including the skin, joints, lungs, heart and gastrointestinal tract, and they can even result in the predominant manifestations, supporting that IIM are systemic inflammatory disorders. Different myositis-specific auto-antibodies have been identified and, on the basis of clinical, histopathological and serological features, IIM can be classified into several subgroups — dermatomyositis (including amyopathic dermatomyositis), antisynthetase syndrome, immune-mediated necrotizing myopathy, inclusion body myositis, polymyositis and overlap myositis. The prognoses, treatment responses and organ manifestations vary among these groups, implicating different pathophysiological mechanisms in each subtype. A deeper understanding of the molecular pathways underlying the pathogenesis and identifying the auto-antigens of the immune reactions in these subgroups is crucial to improving outcomes. New, more homogeneous subgroups defined by auto-antibodies may help define disease mechanisms and will also be important in future clinical trials for the development of targeted therapies and in identifying biomarkers to guide treatment decisions for the individual patient. Idiopathic inflammatory myopathies, also known as myositis, are a heterogeneous group of autoimmune disorders usually characterized by chronic inflammation of the muscle. This Primer reviews the epidemiology, pathophysiology, diagnosis, treatment, patient quality of life and future research needs of this group of diseases.

Patients with idiopathic inflammatory myopathies (IIMs) suffer an increased burden of comorbidities, but data on mortality in recently diagnosed IIM are conflicting. Also, little is known when, if ever, in relation to IIM diagnosis, mortality is increased.MethodsA population-based IIM cohort of patients diagnosed between 2002 and 2011 and general population comparators were identified using healthcare registers. They were linked to the cause of death register for follow-up.Results224 (31%) of the 716 patients with IIM and 870 (12%) of the 7100 general population died during follow-up. This corresponded to a mortality rate of 60/1000 person-years in IIM and 20/1000 person-years in the general population. The cumulative mortality at 1 year after diagnosis was 9% in IIM and 1% in the general population, and increased in both IIM and the general population with time. The overall hazard ratio (HR) 95%CI of death comparing IIM with the general population was 3.7 (3.2 to 4.4). When we stratified on time since diagnosis, we noted an increase in mortality already within the first year of diagnosis compared with the general population, HR 9.6 (95% CI 6.9 to 13.5). This HR then plateaued around 2 after >10 years with the disease, although the estimates were not statistically significant. Malignancies, diseases of the circulatory and respiratory system were common causes of death.ConclusionMortality is increased in patients with contemporary IIM. The increased mortality was noted within a year of diagnosis, which calls for extra vigilance during the first year of IIM diagnosis.

Anti-synthetase syndrome (ASSD) is an autoimmune disease characterized by the presence of autoantibodies targeting one of several aminoacyl t-RNA synthetases (aaRSs) along with clinical features including interstitial lung disease, myositis, Raynaud’s phenomenon, arthritis, mechanic’s hands, and fever. The family of aaRSs consists of highly conserved cytoplasmic and mitochondrial enzymes, one for each amino acid, which are essential for the RNA translation machinery and protein synthesis. Along with their main functions, aaRSs are involved in the development of immune responses, regulation of transcription, and gene-specific silencing of translation. During the last decade, these proteins have been associated with cancer, neurological disorders, infectious responses, and autoimmune diseases including ASSD. To date, several aaRSs have been described to be possible autoantigens in different diseases. The most commonly described are histidyl (HisRS), threonyl (ThrRS), alanyl (AlaRS), glycyl (GlyRS), isoleucyl (IleRS), asparaginyl (AsnRS), phenylalanyl (PheRS), tyrosyl (TyrRS), lysyl (LysRS), glutaminyl (GlnRS), tryptophanyl (TrpRS), and seryl (SerRS) tRNA synthetases. Autoantibodies against the first eight autoantigens listed above have been associated with ASSD while the rest have been associated with other diseases. This review will address what is known about the function of the aaRSs with a focus on their autoantigenic properties. We will also describe the anti-aaRSs autoantibodies and their association to specific clinical manifestations, and discuss their potential contribution to the pathogenesis of ASSD.

ObjectivesTo investigate the association between infection or respiratory tract disease and future risk of developing idiopathic inflammatory myopathy (IIM).MethodsA case–control study was performed using Swedish nationwide registers. Adults with newly diagnosed IIM were identified (2002–2011) from the National Patient Register (NPR) and the Swedish Rheumatology Register (n=957). Controls were matched by age, sex and place of residence (n=9476). Outpatient visits and hospitalisations preceding IIM diagnosis indicating infection or respiratory disease were identified from NPR. Conditional logistic regression models were used to calculate OR and 95% CI. Sensitivity analyses were performed by varying the exposure definition, adjusting for previous healthcare consumption and excluding individuals with connective tissue disease, IIM lung phenotype or IIM-associated cancer.ResultsPreceding infections were more common in IIM cases compared with controls (13% vs 9%) and were associated with an increased risk of IIM (OR 1.5, 95% CI 1.2 to 1.9). Gastrointestinal and respiratory tract infections were associated with an increased risk of IIM while cutaneous infections were not.Preceding respiratory tract disease was present in 10% of IIM cases and 4% of controls (OR 2.3, 95% CI 1.8 to 3.0). Both upper and lower respiratory tract diseases were associated with an increased risk of IIM.Variations in exposure and outcome definitions did not greatly affect the results.ConclusionsInfections and respiratory tract diseases are associated with an increased risk of IIM which suggests that the triggering of the immune system may take place outside the skeletal muscle.

Stratifying patients with myositis into clinically meaningful subtypes would be ideal for enabling research into pathogenic mechanisms and targeted therapies. Incomplete knowledge of the molecular pathways that underlie myositis, inappropriate classification criteria and a lack of specific agents have all been mutually hindering progress in treating these diseases, but, as the authors explain in this Review, insights into the mechanisms—immune and nonimmune—involved in myositis are precipitating wider progress in the field. The inflammatory myopathies—collectively, myositis—are a heterogeneous group of chronic muscle disorders that differ in response to immunosuppressive treatment. Insufficient knowledge of the molecular pathways that drive pathogenesis (and underlie the clinical differences between subtypes) has hindered accurate classification, which in turn has been detrimental for clinical research. Nevertheless, new insights into pathogenesis are paving the way for improvements in diagnosis, classification and treatment. Accumulating data suggest that both immune and nonimmune mechanisms cause muscle weakness. Phenotyping of the T cells that accumulate in muscle tissue has identified proinflammatory, apoptosis resistant and cytotoxic CD4 + and CD8 + CD28 null populations. Several myositis-specific autoantibodies have been identified, associated with distinct clinical phenotypes. Thus, adaptive immunity is involved in pathogenesis, and both T and B cells are interesting targets for therapy. Furthermore, genotyping has revealed activation of the type I interferon pathway in patients with dermatomyositis or with expression of particular autoantibodies. Decreased release of Ca 2+ from the sarcoplasmic reticulum, as a consequence of release of proinflammatory cytokines and high mobility group protein B1, might contribute to muscle weakness, and nonimmune mechanisms potentially include a role for endoplasmic reticulum stress, autophagy and hypoxia. Deeper understanding, careful phenotyping of patients—and new classification criteria—will expedite clinical research. Key Points Myositis is a heterogeneous group of chronic inflammatory muscle disorders, the origins of which are not yet clear and for which efficient treatment is largely lacking Recent findings suggest that both immune and nonimmune mechanisms are involved in the pathogenesis of myositis, and that different molecular pathways might predominate in different subsets of myositis The immune mechanisms involve immune cells—T cells, B cells, dendritic cells and macrophages—and their products, such as cytokines and antibodies Myositis-specific autoantibodies are helpful in the diagnosis of myositis, they identify different clinical subsets of myositis, and they might be important for differentiating pathogenic mechanisms between patients with myositis Among nonimmune pathogenic mechanisms, there seem to be roles for endoplasmic reticulum stress, hypoxia and autophagy, contributing to the cause of muscle weakness New classification criteria are needed to identify more homogenous subsets of patients, and subsets that are likely to share molecular pathways Studying the outcomes of targeted therapies will facilitate understanding of disease mechanisms

Correspondence to Professor Ingrid E Lundberg, Rheumatology Unit, Karolinska University Hospital, Stockholm S-171 76, Sweden; ingrid.lundberg@ki.se We read with interest the letter titled ‘Performance of the 2017 European League Against Rheumatism/American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies in clinical practice’ by Hočevar et al published in the Annals of the Rheumatic Diseases.1 In the letter the authors report the sensitivity and specificity of the recently published ‘2017 European League Against Rheumatism/American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies and their major subgroups’2 observed in a retrospective cohort of 95 patients with idiopathic inflammatory myopathies (IIM) collected between 2010 and 2017 in a Slovenian rheumatology centre. The report in the letter emphasises the need to validate the criteria in external cohorts including all aspects of the heterogeneous IIM population and also the need to revise the ‘2017 European League Against Rheumatism/American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies and their major subgroups’, including the newly identified subgroups of IIM like the immune-mediated necrotising myopathy and the patients with predominating extramuscular manifestations such as interstitial lung disease associated with the newly identified MSAs using validated antibody assays. Performance of the 2017 European League Against Rheumatism/American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies in clinical practice.

ObjectivesThe diagnosis of inclusion body myositis (IBM) can be challenging as it can be difficult to clinically distinguish from other forms of myositis, particularly polymyositis (PM). Recent studies have shown frequent presence of autoantibodies directed against cytosolic 5′-nucleotidase 1A (cN-1A) in patients with IBM. We therefore, examined the autoantigenicity and disease specificity of major epitopes of cN-1A in patients with sporadic IBM compared with healthy and disease controls.MethodsSerum samples obtained from patients with IBM (n=238), PM and dermatomyositis (DM) (n=185), other autoimmune diseases (n=246), other neuromuscular diseases (n=93) and healthy controls (n=35) were analysed for the presence of autoantibodies using immunodominant cN-1A peptide ELISAs.ResultsAutoantibodies directed against major epitopes of cN-1A were frequent in patients with IBM (37%) but not in PM, DM or non-autoimmune neuromuscular diseases (<5%). Anti-cN-1A reactivity was also observed in some other autoimmune diseases, particularly Sjögren's syndrome (SjS; 36%) and systemic lupus erythematosus (SLE; 20%).ConclusionsIn summary, we found frequent anti-cN-1A autoantibodies in sera from patients with IBM. Heterogeneity in reactivity with the three immunodominant epitopes indicates that serological assays should not be limited to a distinct epitope region. The similar reactivities observed for SjS and SLE demonstrate the need to further investigate whether distinct IBM-specific epitopes exist.

Background We aimed to evaluate the 1-year and 2-year outcome of a health-enhancing physical activity (HEPA) support program on global pain, pressure pain sensitivity, and exercise-induced segmental and plurisegmental hypoalgesia (EIH) in persons with rheumatoid arthritis (RA). Methods Thirty participants (27 women and 3 men) were recruited from a larger intervention cohort that engaged in strength training and moderate-intensity aerobic activity. Assessments were performed before the HEPA intervention and at 1-year and 2-year follow-ups. Global pain was assessed on a visual analogue scale (0–100). Pressure pain thresholds (PPTs) and suprathreshold pressure pain at rest corresponding to 4/10 (medium pain) (SP4) and 7/10 (strong pain) (SP7) on Borg CR 10 scale were assessed by algometry. In a subsample ( n = 21), segmental and plurisegmental EIH were assessed during standardized submaximal static contraction (30% of the individual maximum), by algometry, alternately at the contracting right M. quadriceps and the resting left M. deltoideus . Results Global pain decreased from before the intervention to 2-year follow-up (median 11 to median 6, P = 0.040). PPTs and SP4 pressure pain at rest did not change from before the intervention to 2-year follow-up, while SP7 decreased from mean 647 kPa to mean 560 kPa ( P = 0.006). Segmental EIH during static muscle contraction increased from the assessment before the intervention (from mean 1.02 to mean 1.42, P = 0.001), as did plurisegmental EIH (from mean 0.87 to mean 1.41, P <0.001). There were no statistically significant changes in segmental or plurisegmental EIH from before the intervention to 2-year follow-up. Conclusion Participation in a long-term HEPA support program was associated with reduced global pain, whereas pressure pain sensitivity at rest was not reduced and EIH did not change. Thus, our results do not favor the hypothesis that long-term HEPA reduces pain by improving descending pain inhibition in persons with RA. Trial registration ISRCTN25539102 , ISRCTN registry, date assigned March 4, 2011. The trial was retrospectively registered.

Background Idiopathic inflammatory myopathies (IIM) are autoimmune disorders characterized by muscle inflammation and autoreactive B cell responses. The Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway is essential for B cell functions, making it a promising therapeutic target. This study explores the potential of tofacitinib, a JAK1/JAK3 inhibitor, to modulate B cell activity in IIM. Methods Peripheral B cell populations from dermatomyositis (DM), anti-synthetase syndrome (ASyS) and overlap myositis (OM) patients were analyzed by flow cytometry. Peripheral blood mononuclear cells (PBMC) or sorted memory B cells were cultured with tofacitinib and stimulated with combinations of CD40, IL-21, IL-2, BAFF and CpG. B cell proliferation, differentiation and (auto)antibody, cytokine/chemokine production were assessed by flow cytometry, Luminex, and ELISA/ELiA assays. Results The IIM peripheral B cell compartment had elevated transitional and naive B cells, with reduced Bmem frequencies compared to healthy donors. Tofacitinib significantly inhibited CD40/IL-21-induced B cell proliferation, plasmablast formation and function in PBMC and B cell-only cultures across all IIM subgroups, predominantly affecting the IL-21-induced differentiation and antibody production. Remarkably, tofacitinib reduced the levels of anti-Jo1 autoantibodies, as well as of CXCL10 and CXCL13 in ASyS memory B cell cultures. Conclusions These findings highlight the B cell involvement in IIM, evidenced by altered peripheral B cell composition in active disease and the effective inhibition of essential B cell responses, including proliferation, differentiation, and (auto)antibody production, by tofacitinib in vitro. This positions the JAK/STAT pathway as a promising new therapeutic target to modulate B cell activity in IIM.