Introduction
Juvenile dermatomyositis (JDM) is a rare inflammatory disease of childhood affecting skin and muscle, frequently resulting in calcinosis, and sometimes with potentially life threatening complications including gut vasculitis and interstitial lung disease [
1]. Research into the disease pathogenesis has identified abnormalities of the immune system including circulating autoantibodies targeting nuclear antigens [
2], dysregulation of T helper cell subsets [
3] and a prominent interferon alpha (IFNα) gene signature in the peripheral blood of JDM patients [
4]. In JDM muscle, one of the earliest detectable changes is increased expression of Class I major histocompatibility complex (MHC) on skeletal muscle [
5]. Class I MHC can be upregulated non-specifically in response to muscle injury [
6], but in JDM is itself thought to perpetuate the disease process [
7]. In animal models, forced over-expression of Class I protein leads to muscle fibre damage and a marked myeloid infiltrate into the muscle [
8]. One of the mechanisms of cellular injury, known as endoplasmic reticulum (ER) stress, results from accumulation of Class I protein within the ER which then activates multiple pathways, including the unfolded protein response (UPR) culminating in muscle cell death. Gene expression profiling from patients with adult myositis, as well as animal models, confirm that the UPR is up-regulated in myositis [
7,
9]. Despite clear evidence that ER stress contributes to muscle cell death, it remains unclear if myocytes directly promote the inflammatory process and in particular the myeloid cell infiltrate frequently seen early in myositis, or whether this is merely a secondary response to tissue necrosis.
Once recruited, myeloid cells may potentiate the loss of muscle tissue by secreting a potent inflammatory heterodimeric protein, myeloid related protein (MRP) 8/14 (S100A8/A9) [
10] which signals in a Toll-like receptor (TLR) 4 dependent manner [
11] to induce apoptosis of skeletal muscle. MRP plays a pathogenic role in other childhood rheumatic diseases, including arthritis and vasculitis, and in these conditions it is a valuable biomarker of disease severity [
12,
13]. In JDM, the clinical and serological markers of skin and muscle disease currently available lack sensitivity [
14]. In this study we examined the role of MRP8/14 as a biomarker of myositis and the mechanisms by which MRP8/14 may contribute to muscle disease. Our results show a close correlation between MRP8/14 levels in JDM serum and disease activity scores. MRP8/14 was expressed by CD68+ macrophages within JDM muscle and led to release of inflammatory mediators from muscle, an effect that was accentuated by ER stress. Our study identifies a novel pathway by which macrophage-muscle crosstalk can perpetuate inflammatory myositis.
Discussion
The assessment of disease activity in JDM, and the distinction of disease flare from deconditioning or muscle atrophy, is still largely dependent on clinical evaluation. In this study, MRP8/14 has been identified as a novel biomarker of disease activity in JDM, and found to be superior to existing serological correlates of disease. By exploring the effects of MRP8 and MRP14 on skeletal muscle
in vitro and correlating results with
ex vivo JDM samples, we have identified a pro-inflammatory role of MRP8 for myositic muscle that we propose contributes to the enrichment of key inflammatory mediators MCP-1 and IL-6, seen in JDM muscle and serum, which then contribute to further recruitment of inflammatory cells to muscle. MRP8 has been shown to be the active component stimulating TLR4 in murine models of inflammation whereas MRP14 seems to have a regulatory function in the MRP8/14 complex. The exact mechanisms activating the MRP8/14 heterodimer
in vivo are currently not clear but co-stimulation may be required [
35].
MRP8/14 has been shown to be a highly sensitive marker of disease activity in a range of rheumatic disorders, including arthritis, vasculitis and autoinflammatory disease [
12,
13]. As a biomarker, MRP8/14 has many characteristics that make it suitable for both clinical and research use; it is easily detected in serum even at low levels, is already in clinical use to detect gut inflammation [
36] and is stable in clinical serum samples even when transported at room temperature. Most recently, we have successfully used MRP8/14 to identify patients with juvenile arthritis who are likely to remain in remission following withdrawal of immunosuppression by methotrexate [
37]. This proof of concept study confirms a role for MRP8/14 in the detection of sub-clinical disease activity in rheumatic disorders and could potentially apply to JDM to assist with the withdrawal of immunosuppression.
Other biomarkers have been identified in JDM, including the IFNα gene signature and IFN induced chemokines [
4,
38‐
40]. IFNα is produced by plasmacytoid dendritic cells (pDC), and induces transcriptional activators which bind downstream response elements in promoter sequences (IFN-stimulated response elements, ISRE), enhancing the transcription of many immune related genes including the chemokine MCP-1 [
41]. Our results suggest that MCP-1 can also be produced by a MRP-dependent pathway, by muscle fibres themselves in JDM. It is, therefore, of interest, that in a recent report MCP-1, an IFNα associated chemokine correlated better with disease activity than the IFNα gene signature itself [
4]. This may suggest that IFNα-independent production of MCP-1, including muscle derived MCP-1, may play a role in juvenile myositis.
It is striking that JDM biopsies that were taken relatively early in disease (median disease duration six months), already show a major infiltration of monocytes/macrophages, often in the absence of cells from the adaptive immune system. Previous studies have not clearly defined the role of such infiltrating myeloid cells in muscle cell damage and inflammation during myositis. Our results clearly show that MRP8 and MRP14 are secreted by CD68+ infiltrating cells and that these are largely CD163- suggesting that they are indeed pro-inflammatory in phenotype. We propose that the local secretion of MRP proteins by these cells has several downstream effects, including the stimulation of muscle to produce MCP-1 and IL-6. MCP-1 secreted by skeletal muscle, in response to MRP, may then play an important role in propagating the inflammatory infiltrate. Cells recruited into DM muscle, including monocytes and memory T cells, express high levels of CCR2, the sole receptor for MCP-1 [
42,
43]. MRP8 and MRP14 may be important in linking an initial innate immune response with a later adaptive one by recruiting CCR2+ memory T cells and supporting the differentiation of local B cells into plasma cells as this is dependent on signalling through CCR2 [
44]. As further evidence of the importance of MCP-1 in myositis, animal models have implicated this chemokine in a transgenic model of selective over expression of self MHC Class l in skeletal muscle, MHC over expression induced MCP-1 production [
8] and, in a model of viral induced myositis, blockade of MCP-1 significantly attenuated muscle inflammation [
45].
Our results demonstrating that MRP8 induced IL-6 and MCP-1 secretion by myoblasts add to a growing body of evidence that suggests that muscle itself contributes to the inflammatory process [
8,
46]. To test how muscle derived cytokine secretion would be altered by non-immune insults to the muscle, known to occur in DM [
7‐
9], we adopted a thapsigargin induced model of muscle ER stress. Using this system we have now identified ER stress as a mechanism for priming myoblasts to secrete IL-6 in response to a second signal, such as macrophage derived MRP8 binding TLR4. This is pertinent to myositis as IL-6 is known to correlate with disease activity in idiopathic inflammatory myositis (IIM) and IL-6 blockade attenuates muscle inflammation in some mouse models [
47]. Given that IL-6 is produced by a range of immune cells, including macrophages, B and T cells, it is difficult to discern the exact contribution made by skeletal muscle towards the enrichment of this cytokine in JDM serum. Nevertheless, MCP-1 and IL-6 appear to be tightly co-regulated in JDM serum [
4] and, given that both are expressed by inflamed muscle, it is possible that the close correlation between serum levels and muscle disease activity [
4,
40] is explained by the muscle itself being a key source of these cytokines in JDM.
One limitation of our study is that we are unable to exclude the action of MRP8 on other receptors apart from TLR4, such as the for advanced glycation end products (RAGE) [
48]. However, data from the murine system would suggest that TLR4 is the dominant receptor for MRP8 [
35].
This study contributes novel insights into the possible roles of macrophage derived MRP8 and MRP14 in driving production of chemokines and cytokines by muscle cells. These data emphasise the importance of skeletal muscle as an organ with the potential for immune functions and demonstrate how cross talk between muscle and the innate immune system can be instrumental in sustaining further adaptive responses as well as on-going inflammation in autoimmune muscle disease.
Acknowledgements
The Juvenile Dermatomyositis Research Group would like to thank all of the patients and their families who contributed to the Juvenile Dermatomyositis Cohort Study. We thank all local research coordinators and principal investigators who have made this research possible. The members who contributed are as follows: Dr Kate Armon and Mr Joe Ellis-Gage (Norfolk and Norwich University Hospitals), Dr Liza McCann, Mr Ian Roberts, Dr Eileen Baildam and Ms Louise Hanna (The Royal Liverpool Children’s Hospital, Alder Hey, Liverpool), Dr Phil Riley and Ms Ann McGovern (Royal Manchester Children’s Hospital, Manchester), Dr Clive Ryder and Mrs. Janis Scott (Birmingham Children’s Hospital, Birmingham), Dr Sue Wyatt, Mrs Gillian Jackson, Dr Tania Amin, Mark Wood and Vanessa Van Rooyen (Leeds General Infirmary, Leeds), Dr Joyce Davidson, Dr Janet Gardner-Medwin, Dr Neil Martin and Ms Sue Ferguson (The Royal Hospital for Sick Children, Yorkhill, Glasgow), Dr Mark Friswell, Professor Helen Foster, Mrs Alison Swift, Dr Sharmila Jandial, Ms Vicky Stevenson and Ms Debbie Wade (Great North Children’s Hospital, Newcastle), Dr Helen Venning, Mrs Elizabeth Stretton and Ms Mary Jordan (Queens Medical Centre, Nottingham), Professor Lucy Wedderburn, Dr Clarissa Pilkington, Dr N. Hasson, Mrs Sue Maillard, Ms Elizabeth Halkon, Ms Virginia Brown, Ms Audrey Juggins, Dr Sally Smith, Mrs Sian Lunt, Ms Elli Enayat, Mrs Hemlata Varsani, Miss Laura Beard and Miss Katie Arnold (Great Ormond Street Hospital, London), Dr Kevin Murray (Princess Margaret Hospital, Perth, Western Australia) Dr John Ioannou (University College London Hospital).
We thank Dr Vincent Mouly, Institut de Myologie, Paris for advice on muscle cell lines and Dr Jane Goodall, Cambridge University for providing XBP-1 primer sequences.
The JDM Cohort Study and this work have been supported by generous grants from the Wellcome Trust UK (085860), Action Medical Research UK, (SP4252), and The Henry Smith Charity. The JDM Cohort study has been adopted onto the Comprehensive Research Network through the Medicines for Children Research Network (
http://www.mcrn.org.uk). LW is supported in part by the Great Ormond Street Hospital Children’s Charity. KN is a Wellcome Trust Intermediate Clinical Fellow (097259). TV and JR were supported by grants from the Interdisciplinary Centre for Clinical Research at the University of Muenster (Vo2/014/09 to T.V. as well as grant Ro2/004/10 to J.R) and the Deutsche Forschungsgemeinschaft (DFG project RO 1190/9-1).
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
HV carried out immunohistochemistry, myoblast culture and PCR. HW, TV and JR assayed MRP and provided technical support. VS, PK and PB assayed MCP-1. KM provided the myoblast cell line. KN, KM and HV analysed data and performed statistical analyses. HV, KN, KM, JR and LW conceived of the study, designed experiments and participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.