The inflammatory milieu in idiopathic inflammatory myositis

Macrophage involvement in idiopathic inflammatory myopathy: pathogenic mechanisms and therapeutic prospects

Idiopathic inflammatory myopathies are a group of systemic autoimmune diseases characterized by chronic muscle inflammation and diverse clinical manifestations. Macrophages, pivotal components of innate immunity, are implicated in immune responses, inflammation resolution, and tissue repair. Distinct macrophage polarization states play vital roles in disease progression and resolution. Mechanistically, activated macrophages release proinflammatory cytokines, chemokines, and reactive oxygen species, perpetuating immune responses and tissue damage. Dysregulated macrophage polarization contributes to sustained inflammation. Here, we reviewed the intricate contributions of macrophages to IIM pathogenesis and explored novel therapeutic avenues. We discussed emerging strategies targeting macrophages, including receptor-based interventions and macrophage polarization modulation, for IIM treatment. This review underscores the multifaceted involvement of macrophages in IIM pathogenesis and offers insights into potential therapeutic approaches targeting these immune cells for disease management.

Major histocompatibility complex I-induced endoplasmic reticulum stress mediates the secretion of pro-inflammatory muscle-derived cytokines

Major histocompatibility complex (MHC) I is an important component of intracellular antigen presentation. However, improper expression of MHC I upon the cell surface has been associated with several autoimmune diseases. Myositis is a rare acquired autoimmune disease which targets skeletal muscle, and MHC I overexpression on the surface of muscle fibres and immune cell infiltration are clinical hallmarks. MHC I overexpression may have an important pathogenic role, mediated by the activation of the endoplasmic reticulum (ER) stress response. Given the evidence that muscle is a diverse source of cytokines, we aimed to investigate whether MHC I overexpression can modify the profile of muscle-derived cytokines and what role the ER stress pathway may play. Using C2C12 myoblasts we overexpressed MHC I with a H-2k^b vector in the presence or absence of salubrinal an ER stress pathway modifying compound. MHC I overexpression induced ER stress pathway activation and elevated cytokine gene expression. MHC I overexpression caused significant release of cytokines and chemokines, which was attenuated in the presence of salubrinal. Conditioned media from MHC I overexpressing cells induced in vitro T-cell chemotaxis, atrophy of healthy myotubes and modified mitochondrial function, features which were attenuated in the presence of salubrinal. Collectively, these data suggest that MHC I overexpression can induce pro-inflammatory cytokine/chemokine release from C2C12 myoblasts, a process which appears to be mediated in-part by the ER stress pathway.

Identification of hub biomarkers and immune cell infiltration characteristics of polymyositis by bioinformatics analysis

Background

Polymyositis (PM) is an acquirable muscle disease with proximal muscle involvement of the extremities as the main manifestation; it is a category of idiopathic inflammatory myopathy. This study aimed to identify the key biomarkers of PM, while elucidating PM-associated immune cell infiltration and immune-related pathways.

Methods

The gene microarray data related to PM were downloaded from the Gene Expression Omnibus database. The analyses using Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes, gene set enrichment analysis (GSEA), and protein-protein interaction (PPI) networks were performed on differentially expressed genes (DEGs). The hub genes of PM were identified using weighted gene co-expression network analysis (WGCNA) and least absolute shrinkage and selection operator (LASSO) algorithm, and the diagnostic accuracy of hub markers for PM was assessed using the receiver operating characteristic curve. In addition, the level of infiltration of 28 immune cells in PM and their interrelationship with hub genes were analyzed using single-sample GSEA.

Results

A total of 420 DEGs were identified. The biological functions and signaling pathways closely associated with PM were inflammatory and immune processes. A series of four expression modules were obtained by WGCNA analysis, with the turquoise module having the highest correlation with PM; 196 crossover genes were obtained by combining DEGs. Subsequently, six hub genes were finally identified as the potential biomarkers of PM using LASSO algorithm and validation set verification analysis. In the immune cell infiltration analysis, the infiltration of T lymphocytes and subpopulations, dendritic cells, macrophages, and natural killer cells was more significant in the PM.

Conclusion

We identified the hub genes closely related to PM using WGCNA combined with LASSO algorithm, which helped clarify the molecular mechanism of PM development and might have great significance for finding new immunotherapeutic targets, and disease prevention and treatment.

Using peripheral blood immune signatures to stratify patients with adult and juvenile inflammatory myopathies

OBJECTIVE

The inflammatory idiopathic myopathies (IIM) are a group of rare autoimmune diseases defined by muscle weakness and characterized by pro-inflammatory infiltrates in muscle. Little is known about the immunological profile in peripheral blood of these patients and how this relates to IIM subtypes. This study aimed to stratify adult and juvenile-onset IIM patients according to immune cell profile.

METHODS

Peripheral blood mononuclear cells from 44 patients with adult myositis (AM), 15 adolescent-onset juvenile dermatomyositis (a-JDM), and 40 age-matched healthy controls were analysed by flow cytometry to quantify 33 immune cell subsets. Adult myositis patients were grouped according to myositis subtype; DM and polymyositis; and also autoantibody specificity. Disease activity was determined by the myositis disease activity assessment tool and clinicians' decision on treatment.

RESULTS

Unique immune signatures were identified for DM, polymyositis and a-JDM compared with healthy controls. DM patients had a T-cell signature comprising increased CD4+ and TH17 cell frequencies and increased immune cell expression of IL-6. Polymyositis patients had a B-cell signature with reduced memory B cells. A-JDM had decreased naïve B cells and increased CD4+T cells. All patient groups had decreased CD8+central memory T-cell frequencies. The distinct immune signatures were also seen when adult myositis patients were stratified according to auto-antibody expression; patients with anti-synthetase-antibodies had reduced memory B cells and patients with autoimmune rheumatic disease overlap had an elevated Th17 profile.

CONCLUSION

Unique immune signatures were associated with adult vs juvenile disease. The Th17 signature in DM patients supports the potential use of IL-17 inhibitors in treatment of IIMs.

Expression of interleukin-18 in muscle tissue of patients with polymyositis or dermatomyositis and effects of conventional immunosuppressive treatment

Objectives

To investigate the expression of IL-18 in symptomatic and asymptomatic muscle tissues of patients with PM and DM and the effects of conventional immunosuppressive treatment on such expression.

Methods

Two cohorts of patients were included in this study. The first cohort consisted of 10 new-onset myositis patients. IL-18 expression was compared between symptomatic and asymptomatic muscle biopsies that were taken prior to treatment. The second cohort consisted of another 10 patients with repeated muscle biopsies before and after 8 months with conventional immunosuppressive treatment. Using immunohistochemistry, IL-18 expression in muscle tissues was compared before and after treatment. Biopsies from seven healthy individuals were included as controls.

Results

IL-18 expression was predominantly localized to inflammatory cells and capillaries in patients and mostly to capillaries in healthy controls. Total IL-18 expression in muscle tissues from the new-onset patients, at both symptomatic and asymptomatic sites, was significantly higher compared with healthy controls (P = 0.007 and P = 0.002) with no statistical difference in appearances between symptomatic and asymptomatic sites. The number of IL-18 positive capillaries was not different among symptomatic, asymptomatic and healthy muscles. Total IL-18 expression appeared lower in biopsies from patients receiving and improving with immunosuppressive treatment, particularly the number of IL-18 positive inflammatory cells but not the number of IL-18 positive capillaries, which was consistent with significantly decreased expression of CD68+ macrophages (P = 0.04).

Conclusion

IL-18 is highly expressed in muscle tissue in the context of inflammatory myopathies and based on its plausible effector functions could provide a novel therapeutic target in future.

Could Lymphocyte Profiling be Useful to Diagnose Systemic Autoimmune Diseases?

Considering the implications of B, T, and natural killer (NK) cells in the pathophysiology of systemic autoimmune diseases, the assessment of their distribution in the blood could be helpful for physicians in the complex process of determining a precise diagnosis. In primary Sjögren's syndrome, transitional and active naive B cells are increased and memory B cells are decreased compared to healthy controls and other systemic diseases. However, their utility to improve the accuracy of classification criteria has not been proven. In early untreated rheumatoid arthritis, proportions of regulatory T cells are constantly reduced, but other patterns are difficult to determine given the heterogeneity of published studies. In systemic lupus erythematosus, the lack of studies using large cohorts of patients and the diversity of the possible pathological mechanisms involved are also important impediments. Nevertheless, transitional B cell and plasma cell proportions are increased in most of the studies, the CD4/CD8 ratio is decreased, and the number of NK cells is reduced. Despite the low number of studies, anomalies of lymphocyte subset distribution was also described in ANCA-associated vasculitis, systemic scleroderma, and myositis. For now, flow cytometric analysis of lymphocyte subsets has focused mainly on specific subpopulations and is more useful for basic and translational research than for diagnostics in clinical practice. However, new modern methods such as mass cytometry and bioinformatics analyses may offer the possibility to simultaneously account for the relative proportions of multiple lymphocyte subsets and define a global profile in homogeneous groups of patients. The years to come will certainly incorporate such global lymphocyte profiling in reclassification of systemic autoimmune diseases.

Activated dendritic cells modulate proliferation and differentiation of human myoblasts

Idiopathic Inflammatory Myopathies (IIMs) are a heterogeneous group of autoimmune diseases affecting skeletal muscle tissue homeostasis. They are characterized by muscle weakness and inflammatory infiltration with tissue damage. Amongst the cells in the muscle inflammatory infiltration, dendritic cells (DCs) are potent antigen-presenting and key components in autoimmunity exhibiting an increased activation in inflamed tissues. Since, the IIMs are characterized by the focal necrosis/regeneration and muscle atrophy, we hypothesized that DCs may play a role in these processes. Due to the absence of a reliable in vivo model for IIMs, we first performed co-culture experiments with immature DCs (iDC) or LPS-activated DCs (actDC) and proliferating myoblasts or differentiating myotubes. We demonstrated that both iDC or actDCs tightly interact with myoblasts and myotubes, increased myoblast proliferation and migration, but inhibited myotube differentiation. We also observed that actDCs increased HLA-ABC, HLA-DR, VLA-5, and VLA-6 expression and induced cytokine secretion on myoblasts. In an in vivo regeneration model, the co-injection of human myoblasts and DCs enhanced human myoblast migration, whereas the absolute number of human myofibres was unchanged. In conclusion, we suggest that in the early stages of myositis, DCs may play a crucial role in inducing muscle-damage through cell–cell contact and inflammatory cytokine secretion, leading to muscle regeneration impairment.

The Immune Response and the Pathogenesis of Idiopathic Inflammatory Myositis: a Critical Review

The pathogenesis of idiopathic inflammatory myositis (IIMs, including polymyositis and dermatomyositis) remains largely enigmatic, despite advances in the study of the role played by innate immunity, adaptive immunity, genetic predisposition, and environmental factors in an orchestrated response. Several factors are involved in the inflammatory state that characterizes the different forms of IIMs which share features and mechanisms but are clearly different with respect to the involved sites and characteristics of the inflammation. Cellular and non-cellular mechanisms of both the immune and non-immune systems have been identified as key regulators of inflammation in polymyositis/dermatomyositis, particularly at different stages of disease, leading to the fibrotic state that characterizes the end stage. Among these, a special role is played by an interferon signature and complement cascade with different mechanisms in polymyositis and dermatomyositis; these differences can be identified also histologically in muscle biopsies. Numerous cellular components of the adaptive and innate immune response are present in the site of tissue inflammation, and the complexity of idiopathic inflammatory myositis is further supported by the involvement of non-immune mechanisms such as hypoxia and autophagy. The aim of this comprehensive review is to describe the major pathogenic mechanisms involved in the onset of idiopathic inflammatory myositis and to report on the major working hypothesis with therapeutic implications.

Factors Associated with Interstitial Lung Disease in Patients with Polymyositis and Dermatomyositis: A Systematic Review and Meta-Analysis

OBJECTIVES

Interstitial lung disease (ILD) is an extramuscular manifestation that results in increased morbidity and mortality from polymyositis (PM) and dermatomyositis (DM). The aim of this study was to systematically evaluate risk factors associated with the development of ILD in PM/DM.

METHODS

Observational studies were identified from searching PubMed, Medline, Embase, and the Cochrane Library. Pooled odds ratios (ORs) or standardized mean differences (SMDs) and corresponding 95% confidence intervals (CIs) were obtained for the relationships between risk factors and ILD in PM/DM using either fixed- or random-effects models, whichever were appropriate. Heterogeneity tests, sensitivity analyses, and publication bias assessments were also performed.

RESULTS

Twenty-three studies were selected for a meta-analysis that included 834 patients and 1245 control subjects. Risk factors that may have increased the risk of developing ILD in PM/DM patients included older age at diagnosis (SMD, 0.35; 95% CI, 0.18-0.52; P < 0.0001), arthritis/arthralgia (OR, 3.17; 95% CI, 1.99-5.04; P < 0.00001), fever (OR, 2.31; 95% CI, 1.42-3.76; P = 0.0007), presence of anti-Jo-1 antibodies (OR, 3.34; 95% CI, 2.16-5.16; P < 0.00001), elevated erythrocyte sedimentation rate (ESR; SMD, 0.48; 95% CI, 0.32-0.64; P < 0.00001), presence of anti-MDA5 antibodies (OR, 18.26; 95% CI, 9.66-34.51; P < 0.00001), and elevated C-reactive protein level (CRP; OR, 3.50; 95% CI, 1.48-8.28; P = 0.004). Meanwhile, malignancy (OR, 0.36; 95% CI, 0.18-0.72; P = 0.004) reduced the risk of developing ILD in PM/DM patients.

CONCLUSION

Our meta-analysis results suggest that the association between PM/DM and ILD may be due to such risk factors as older age at diagnosis, arthritis/arthralgia, fever, presence of anti-Jo-1 antibodies, elevated ESR, presence of anti-MDA5 antibodies, and elevated CRP level, while malignancy was associated with a reduced risk of developing ILD. Thus, these variables may be used to guide screening processes for ILD in patients with PM/DM.

An overview of polymyositis and dermatomyositis

Polymyositis and dermatomyositis are inflammatory myopathies that differ in their clinical features, histopathology, response to treatment, and prognosis. Although their clinical pictures differ, they both present with symmetrical, proximal muscle weakness. Treatment relies mainly upon empirical use of corticosteroids and immunosuppressive agents. A deeper understanding of the molecular pathways that drive pathogenesis, careful phenotyping, and accurate disease classification will aid clinical research and development of more efficacious treatments. In this review we address the current knowledge of the epidemiology, clinical characteristics, diagnostic evaluation, classification, pathogenesis, treatment, and prognosis of polymyositis and dermatomyositis.

Treatment with CA-074Me, a Cathepsin B inhibitor, reduces lung interstitial inflammation and fibrosis in a rat model of polymyositis

Cathepsin B (CB) is involved in the turnover of proteins and has various roles in maintaining the normal metabolism of cells. In our recent study, CB is increased in the muscles of polymyositis/dermatomyositis (PM/DM). However, the role of CB in interstitial lung disease (ILD) has not been reported. ILD is a frequent complication of PM/DM, which is the leading cause of death in PM/DM. It carries high morbidity and mortality in connective tissue diseases, characterized by an overproduction of inflammatory cytokines and induced fibrosis, resulting in respiratory failure. The etiology and pathogenesis of ILD remain incompletely understood. This study investigated whether treatment with CA-074Me, a specific inhibitor of CB, attenuates ILD in PM. CB expression, inflammation, and fibrosis were analyzed in the lung tissues from patients with PM/DM. The animal model of PM was induced in guinea pigs with Coxsackie virus B1 (CVB1). CA-074Me was given 24 h after CVB1 injection for 7 consecutive days. At the end of the experiment, the animals were killed and lung tissues were collected for the following analysis. Inflammation, fibrosis and apoptosis cells, and cytokines were assessed by histological examinations and immunohistochemical analyses, western blot analysis and transferase-mediated dUTP nick-end labeling assay. In patients with PM/DM, the protein levels of CB were significantly elevated in lung tissues compared with healthy controls, which correlated with increases in inflammation and fibrosis. Similarly, the expression of CB, inflammation and fibrosis, CD8(+) T cell, CD68(+) cell, tumor necrosis factor-alpha, transforming growth factor-beta1 infiltrations, and apoptotic cell death were significantly increased in lung tissues of the guinea-pig model of CVB1-induced PM. These changes were attenuated by the administration of CA-074Me. In conclusion, this study demonstrates that PM/DM increases CB expression in lung tissues and inhibition of CB reduces ILD in a guinea-pig model of CVB1-induced PM. This finding suggests that CB may be a potential therapeutic target for ILD.

Low serum levels of vitamin D in idiopathic inflammatory myopathies

OBJECTIVES

To evaluate serum levels of 25(OH) vitamin D in patients with idiopathic inflammatory myopathies (IIM) (polymyositis (PM), dermatomyosistis (DM), inclusion body myositis (IBM) and juvenile DM (JDM)) and to compare these with healthy controls.

METHODS

Serum samples from 149 patients with IIM and 290 healthy controls matched for gender and the month of blood sampling were analysed for 25(OH) vitamin D. ORs for vitamin D classes with 95% CI were calculated using a matched (conditional) logistic regression model. Groups were compared by the Kruskal-Wallis test and p values <0.05 were considered significant.

RESULTS

Patients with IIM had significantly lower serum levels of 25(OH) vitamin D than healthy controls (median 39 (10-168) nmol/l vs 68 (19-197) nmol/l; p=0.0001). There was no significant difference in vitamin D levels between the myositis subgroups. When vitamin D levels were subclassified into deficient (<50 nmol/l), insufficient (50-74 nmol/l) and normal (≥75 nmol/l), most of the patients with PM (68%), DM (65%) and IBM (53%) had deficient levels compared with only 60 (21%) healthy individuals. In patients with IIM the OR for deficient versus normal was 17.7 (95% CI 8.1 to 38.6) and the OR for insufficient versus normal was 2.4 (95% CI 1.2 to 4.7).

CONCLUSIONS

Low serum levels of vitamin D were found in most patients with IIM and may confer a risk factor for developing adult myositis, similar to some other autoimmune diseases.

Fascin-expressing Dendritic Cells Dominate in Polymyositis and Dermatomyositis

Objective.

Dendritic cells (DC) have been reported among inflammatory infiltrating cells in muscle tissue in idiopathic inflammatory myopathies (IIM), but to our knowledge no studies concerning the expression of langerin (CD207) or fascin (markers of immature and mature DC, respectively) in IIM have been published.

Methods.

Immunohistochemical analyses of langerin and fascin expression were performed on specimens from normal muscles, as well as those affected by polymyositis (PM) and dermatomyositis (DM). The results were analyzed by Mann-Whitney U test.

Results.

In PM and DM, fascin-positive cells were numerous in the majority of the studied samples in perimysial, endomysial, and perivascular cellular infiltrates. Sporadic langerin-positive cells were detected.

Conclusion.

Fascin-positive DC predominance in inflammatory infiltrates in myositic muscles confirms the prevalence of mature forms and indicates that there are conditions stimulating DC maturation in both PM and DM. The induction of immunological tolerance by inhibiting DC maturation may be a promising direction for studies of myositis treatment.

Clinical significance of peripheral blood lymphocyte subsets in patients with polymyositis and dermatomyositis

Peripheral blood lymphocyte subsets were determined by flow cytometry in 89 Chinese patients with polymyositis (PM) and dermatomyositis (DM). We aimed to investigate the clinical significance of peripheral blood lymphocyte subsets in PM/DM. Patients with active DM showed significant decreases in numbers of CD3(+) cells, CD3(+)CD4(+) cells, and CD3(+)CD8(+) cells, as compared to patients with inactive DM and healthy controls (P < 0.05). CD3(+) and CD3(+)CD4(+) cell counts were significantly lower in DM before treatment, compared with after treatment (t = -5.714 and -3.665, P < 0.05). Counts of CD3(+) cells, CD3(+)CD4(+) cells, CD3(+)CD8(+) cells, and CD19(+)CD5(-) cells were all correlated with the total disease activity score as determined by the Myositis Disease Activity Assessment Visual Analogue Scale (P < 0.05). The decreased number of CD3(+) cells and the decreased percentage of CD3(+)CD4(+) cells were additionally correlated with the presence of interstitial lung disease in PM/DM (P < 0.05). The presence of levels of CD3(+)CD8(+) cells was risk factor for death (b = -0.011, OR = 0.989, P < 0.05). The identification of peripheral blood T lymphocyte subsets in PM/DM appears to be useful as a reference marker in the evaluation of clinical disease activity, and be useful in the comprehensive assessment of clinical lung involvement. A decrease in CD8(+) T cells may predict a poor outcome in patients with PM/DM.

Effects of immunosuppressive treatment on interleukin-15 and interleukin-15 receptor α expression in muscle tissue of patients with polymyositis or dermatomyositis

OBJECTIVES

To investigate the expression of interleukin (IL)-15 and IL-15 receptor α (IL-15Rα) in muscle tissue from patients with polymyositis or dermatomyositis before and after conventional immunosuppressive (IS) treatment.

METHODS

Muscle biopsies from 17 patients before and after conventional IS treatment and seven healthy individuals were investigated by immunohistochemistry using antibodies against IL-15 and IL-15Rα. Quantification was performed by computerised image analysis. Cellular localisation of IL-15 was determined by double immunofluorescence. Clinical outcome was measured by the functional index and serum creatine kinase. Human myotubes were cultured and IL-15 staining was performed by immunocytochemistry.

RESULTS

IL-15 was observed in mononuclear inflammatory cells of muscle tissue while IL-15Rα was localised to mononuclear inflammatory cells, capillaries and large vessels. Double staining showed localisation of IL-15 to CD163+ macrophages. A significantly larger number of IL-15 and IL-15Rα-positive cells were seen in muscle tissue of patients compared with healthy individuals. Baseline IL-15 expression correlated negatively with improvement in muscle function. After conventional IS treatment, a significantly lower number of IL-15 and IL-15Rα-positive cells was found. However, compared with controls, eight of 17 patients still had more IL-15-positive cells and less muscle function improvement was shown in this group of patients, both in short-term and long-term observations. Human differentiated myotubes were negative for IL-15 staining.

CONCLUSIONS

IL-15 and its receptor are expressed in the muscle tissue of patients with myositis and IL-15 expression is correlated with improvement in muscle function. IL-15 may play a role in the pathogenesis of myositis and could be a biological treatment target, at least in a subgroup of patients with polymyositis or dermatomyositis.

Disseminated idiopathic myofasciitis in ferrets

First described in 2003, disseminated idiopathic myofasciitis (DIM) has emerged as a new disease in young, domestic ferrets. DIM is a severe inflammatory condition that affects primarily muscles and surrounding connective tissues. The disease is characterized by rapid onset of clinical signs, high fever, neutrophilic leukocytosis, and general lack of response to therapeutic intervention. Until recently DIM was considered fatal, but a few surviving ferrets indicate there may be an effective treatment protocol. DIM is suspected to be an immune-mediated disease, but the etiopathogenesis is not known. This article reviews clinical and pathologic findings in DIM patients, covers recommended diagnostic procedures and clinical management of ferrets with DIM, and discusses potential etiologies for this newly recognized disease in ferrets.

Lesional and nonlesional skin from patients with untreated juvenile dermatomyositis displays increased numbers of mast cells and mature plasmacytoid dendritic cells

OBJECTIVE

To investigate the distribution of mast cells and dendritic cell (DC) subsets in paired muscle and skin (lesional/nonlesional) from untreated children with juvenile dermatomyositis (DM).

METHODS

Muscle and skin biopsy samples (4 skin biopsy samples with active rash) from 7 patients with probable/definite juvenile DM were compared with muscle and skin samples from 10 healthy pediatric controls. Mast cell distribution and number were assessed by toluidine blue staining and analyzed by Student's t-test. Immunohistochemical analysis was performed to identify mature DCs, myeloid DCs (MDCs), and plasmacytoid DCs (PDCs) by using antibodies against DC-LAMP, blood dendritic cell antigen 1 (BDCA-1), and BDCA-2, respectively. Myxovirus resistance protein A (MxA) staining indicated active type I interferon (IFN) signaling; positive staining was scored semiquantitatively and analyzed using the Mann-Whitney U test.

RESULTS

Both inflamed and nonlesional skin from patients with juvenile DM contained more mast cells than did skin from pediatric controls (P = 0.029), and comparable numbers of mast cells were present in lesional and nonlesional skin. Interestingly, mast cell numbers were greater in skin than in paired muscle tissue from patients with juvenile DM (P = 0.014) and were not increased in muscle from patients with juvenile DM compared with control muscle. Both muscle and skin from patients with juvenile DM showed more mature PDCs and MxA staining than did their corresponding control tissues (P < 0.05). In both muscle and skin from patients with juvenile DM and in pediatric control muscle, there were fewer MDCs than PDCs, and the distributions of MDCs and PDCs were similar in pediatric control skin samples.

CONCLUSION

The identification of mast cells in skin (irrespective of rash) from patients with juvenile DM, but not in paired muscle tissue, suggests that they have a specific role in juvenile DM skin pathophysiology. In skin from patients with juvenile DM, increased numbers of PDCs and increased expression of type I IFN-induced protein suggest a selective influence on T cell differentiation and subsequent effector function.