V. We-thank-dr and . Mouly, Institut de Myologie, Paris for advice on muscle cell lines and Dr Jane Goodall, Cambridge University for providing XBP-1 primer sequences

L. Wedderburn and L. Rider, Juvenile dermatomyositis: new developments in pathogenesis, assessment and treatment, Best Practice & Research Clinical Rheumatology, vol.23, issue.5, pp.665-678, 2009.
DOI : 10.1016/j.berh.2009.07.007

H. Gunawardena, L. Wedderburn, J. North, Z. Betteridge, J. Dunphy et al., Clinical associations of autoantibodies to a p155/140 kDa doublet protein in juvenile dermatomyositis, Rheumatology, vol.47, issue.3, pp.324-328, 2008.
DOI : 10.1093/rheumatology/kem359

R. Morita, N. Schmitt, S. Bentebibel, R. Ranganathan, L. Bourdery et al., Human Blood CXCR5+CD4+ T Cells Are Counterparts of T Follicular Cells and Contain Specific Subsets that Differentially Support Antibody Secretion, Immunity, vol.34, issue.1, pp.108-121, 2011.
DOI : 10.1016/j.immuni.2010.12.012

H. Bilgic, S. Ytterberg, S. Amin, K. Mcnallan, J. Wilson et al., Interleukin-6 and type I interferon-regulated genes and chemokines mark disease activity in dermatomyositis, Arthritis & Rheumatism, vol.6, issue.Suppl 1, pp.3436-3446, 2009.
DOI : 10.1002/art.24936

C. Li, H. Varsani, J. Holton, B. Gao, P. Woo et al., MHC Class I overexpression on muscles in early juvenile dermatomyositis

E. Bartoccioni, S. Gallucci, F. Scuderi, E. Ricci, S. Servidei et al., MHC class I, MHC class II and intercellular adhesion molecule-1 (ICAM-1) expression in inflammatory myopathies, Clinical & Experimental Immunology, vol.136, issue.Suppl., pp.166-172, 1994.
DOI : 10.1111/j.1365-2249.1994.tb06032.x

K. Nagaraju, L. Casciola-rosen, I. Lundberg, R. Rawat, S. Cutting et al., Activation of the endoplasmic reticulum stress response in autoimmune myositis: Potential role in muscle fiber damage and dysfunction, Arthritis & Rheumatism, vol.289, issue.6, pp.1824-1835, 2005.
DOI : 10.1002/art.21103

K. Nagaraju, N. Raben, L. Loeffler, T. Parker, P. Rochon et al., Conditional up-regulation of MHC class I in skeletal muscle leads to self-sustaining autoimmune myositis and myositis-specific autoantibodies, Proceedings of the National Academy of Sciences, vol.97, issue.16, pp.9209-9214, 2000.
DOI : 10.1073/pnas.97.16.9209

C. Li, P. Knopp, H. Moncrieffe, B. Singh, S. Shah et al., Overexpression of MHC Class I Heavy Chain Protein in Young Skeletal Muscle Leads to Severe Myositis, The American Journal of Pathology, vol.175, issue.3, pp.1030-1040, 2009.
DOI : 10.2353/ajpath.2009.090196

S. Seeliger, T. Vogl, I. Engels, J. Schroder, C. Sorg et al., Expression of Calcium-Binding Proteins MRP8 and MRP14 in Inflammatory Muscle Diseases, The American Journal of Pathology, vol.163, issue.3, pp.947-956, 2003.
DOI : 10.1016/S0002-9440(10)63454-2

K. Hirono, D. Foell, Y. Xing, S. Miyagawa-tomita, F. Ye et al., Expression of Myeloid-Related Protein-8 and -14 in Patients With Acute Kawasaki Disease, Journal of the American College of Cardiology, vol.48, issue.6, pp.1257-1264, 2006.
DOI : 10.1016/j.jacc.2006.02.077

M. Frosch, A. Strey, T. Vogl, N. Wulffraat, W. Kuis et al., Myeloid-related proteins 8 and 14 are specifically secreted during interaction of phagocytes and activated endothelium and are useful markers for monitoring disease activity in pauciarticular-onset juvenile rheumatoid arthritis, Arthritis & Rheumatism, vol.43, issue.3, pp.628-637, 2000.
DOI : 10.1002/1529-0131(200003)43:3<628::AID-ANR20>3.0.CO;2-X

C. Pilkington and L. Wedderburn, Paediatric Idiopathic Inflammatory Muscle Disease, Drugs, vol.65, issue.10, pp.1355-1365, 2005.
DOI : 10.2165/00003495-200565100-00004

N. Martin, P. Krol, S. Smith, K. Murray, C. Pilkington et al., A national registry for juvenile dermatomyositis and other paediatric idiopathic inflammatory myopathies: 10 years' experience; the Juvenile Dermatomyositis National (UK and Ireland) Cohort Biomarker Study and Repository for Idiopathic Inflammatory Myopathies, Rheumatology, vol.50, issue.1, pp.137-145, 2011.
DOI : 10.1093/rheumatology/keq261

A. Bohan and J. Peter, Polymyositis and Dermatomyositis, New England Journal of Medicine, vol.292, issue.8, pp.403-407, 1975.
DOI : 10.1056/NEJM197502202920807

H. Varsani, K. Newton, C. Li, B. Harding, J. Holton et al., Quantification of normal range of inflammatory changes in morphologically normal pediatric muscle, Muscle & Nerve, vol.18, issue.2, pp.259-261, 2008.
DOI : 10.1002/mus.20898

D. Lovell, C. Lindsley, R. Rennebohm, S. Ballinger, S. Bowyer et al., Development of validated disease activity and damage indices for the juvenile idiopathic inflammatory myopathies: II. The childhood myositis assessment scale (CMAS): a quantitative tool for the evaluation of muscle function, Arthritis & Rheumatism, vol.292, issue.10, pp.2213-2219, 1999.
DOI : 10.1002/1529-0131(199910)42:10<2213::AID-ANR25>3.0.CO;2-8

L. Rider, E. Giannini, M. Harris-love, G. Joe, D. Isenberg et al., Defining clinical improvement in adult and juvenile myositis, J Rheumatol, vol.30, pp.603-617, 2003.

W. De-jager, B. Prakken, J. Bijlsma, W. Kuis, and G. Rijkers, Improved multiplex immunoassay performance in human plasma and synovial fluid following removal of interfering heterophilic antibodies, Journal of Immunological Methods, vol.300, issue.1-2, pp.124-135, 2005.
DOI : 10.1016/j.jim.2005.03.009

L. Wedderburn, H. Varsani, C. Li, K. Newton, A. Amato et al., International consensus on a proposed score system for muscle biopsy evaluation in patients with juvenile dermatomyositis: A tool for potential use in clinical trials, Arthritis & Rheumatism, vol.168, issue.7, pp.1192-1201, 2007.
DOI : 10.1002/art.23012

G. Zwadlo, R. Schlegel, and C. Sorg, A monoclonal antibody to a subset of human monocytes found only in the peripheral blood and inflammatory tissues, J Immunol, vol.137, pp.512-518, 1986.

J. Roth, M. Goebeler, U. Erpenstein, and C. Sorg, Differential regulation of the macrophage-specific surface antigen RM3/1 by cyclosporine, azathioprine, and dexamethasone, Transplantation, vol.57, pp.127-133, 1994.

C. Zhu, V. Mouly, R. Cooper, K. Mamchaoui, A. Bigot et al., Cellular senescence in human myoblasts is overcome by human telomerase reverse transcriptase and cyclin-dependent kinase 4: consequences in aging muscle and therapeutic strategies for muscular dystrophies, Aging Cell, vol.20, issue.4, pp.515-523, 2007.
DOI : 10.1111/j.1474-9726.2007.00306.x

T. Vogl, N. Leukert, K. Barczyk, K. Strupat, and J. Roth, Biophysical characterization of S100A8 and S100A9 in the absence and presence of bivalent cations, Biochimica et Biophysica Acta (BBA) - Molecular Cell Research, vol.1763, issue.11, pp.1298-1306, 2006.
DOI : 10.1016/j.bbamcr.2006.08.028

B. Price, L. Mannheim-rodman, and S. Calderwood, Brefeldin A, thapsigargin, and AlF4? stimulate the accumulation of GRP78 mRNA in a cycloheximide dependent manner, whilst induction by hypoxia is independent of protein synthesis, Journal of Cellular Physiology, vol.258, issue.3, pp.545-552, 1992.
DOI : 10.1002/jcp.1041520314

A. Liprandi, C. Bartoli, D. Figarella-branger, J. Pellissier, and H. Lepidi, Local expression of monocyte chemoattractant protein-1 (MCP-1) in idiopathic inflammatory myopathies, Acta Neuropathologica, vol.97, issue.6, pp.642-648, 1999.
DOI : 10.1007/s004010051041

Y. Komohara, J. Hirahara, T. Horikawa, K. Kawamura, E. Kiyota et al., AM-3K, an Anti-macrophage Antibody, Recognizes CD163, a Molecule Associated with an Anti-inflammatory Macrophage Phenotype, Journal of Histochemistry & Cytochemistry, vol.178, issue.7, pp.763-771, 2006.
DOI : 10.1354/vp.37-2-168

B. Przybyla, C. Gurley, J. Harvey, E. Bearden, P. Kortebein et al., Aging alters macrophage properties in human skeletal muscle both at rest and in response to acute resistance exercise, Experimental Gerontology, vol.41, issue.3, pp.320-327, 2006.
DOI : 10.1016/j.exger.2005.12.007

K. Nagaraju, N. Raben, G. Merritt, L. Loeffler, K. Kirk et al., A variety of cytokines and immunologically relevant surface molecules are expressed by normal human skeletal muscle cells under proinflammatory stimuli, Clinical and Experimental Immunology, vol.143, issue.3, pp.407-414, 1998.
DOI : 10.1016/0165-5728(95)00045-4

G. Vattemi, W. Engel, J. Mcferrin, and V. Askanas, Endoplasmic Reticulum Stress and Unfolded Protein Response in Inclusion Body Myositis Muscle, The American Journal of Pathology, vol.164, issue.1
DOI : 10.1016/S0002-9440(10)63089-1

M. Hirota, M. Kitagaki, H. Itagaki, and S. Aiba, QUANTITATIVE MEASUREMENT OF SPLICED XBP1 mRNA AS AN INDICATOR OF ENDOPLASMIC RETICULUM STRESS, The Journal of Toxicological Sciences, vol.31, issue.2, pp.149-156, 2006.
DOI : 10.2131/jts.31.149

F. Martinon, X. Chen, A. Lee, and L. Glimcher, TLR activation of the transcription factor XBP1 regulates innate immune responses in macrophages, Nature Immunology, vol.67, issue.5, pp.411-418, 2010.
DOI : 10.1196/annals.1359.012

U. Ozcan, Q. Cao, E. Yilmaz, A. Lee, N. Iwakoshi et al., Endoplasmic Reticulum Stress Links Obesity, Insulin Action, and Type 2 Diabetes, Science, vol.306, issue.5695, pp.457-461, 2004.
DOI : 10.1126/science.1103160

T. Vogl, K. Tenbrock, S. Ludwig, N. Leukert, C. Ehrhardt et al., Mrp8 and Mrp14 are endogenous activators of Toll-like receptor 4, promoting lethal, endotoxin-induced shock, Nature Medicine, vol.267, issue.9, pp.1042-1049, 2007.
DOI : 10.1038/nm1638

J. Tibble, K. Teahon, B. Thjodleifsson, A. Roseth, G. Sigthorsson et al., A simple method for assessing intestinal inflammation in Crohn's disease, Gut, vol.47, issue.4, pp.506-513, 2000.
DOI : 10.1136/gut.47.4.506

D. Foell, N. Wulffraat, L. Wedderburn, H. Wittkowski, M. Frosch et al., Methotrexate Withdrawal at 6 vs 12 Months in Juvenile Idiopathic Arthritis in Remission<subtitle>A Randomized Clinical Trial</subtitle>, JAMA, vol.303, issue.13, pp.1266-1273, 2010.
DOI : 10.1001/jama.2010.375

M. Salajegheh, S. Kong, J. Pinkus, R. Walsh, A. Liao et al., ) conjugates proteins in dermatomyositis muscle with perifascicular atrophy, Annals of Neurology, vol.197, issue.1, pp.53-63, 2010.
DOI : 10.1002/ana.21805

Z. Tezak, E. Hoffman, J. Lutz, T. Fedczyna, D. Stephan et al., Gene Expression Profiling in DQA1*0501+ Children with Untreated Dermatomyositis: A Novel Model of Pathogenesis, The Journal of Immunology, vol.168, issue.8, pp.4154-4163, 2002.
DOI : 10.4049/jimmunol.168.8.4154

A. Reed, E. Peterson, H. Bilgic, S. Ytterberg, S. Amin et al., Changes in novel biomarkers of disease activity in juvenile and adult dermatomyositis are sensitive biomarkers of disease course, Arthritis & Rheumatism, vol.56, issue.12, pp.4078-4086, 2012.
DOI : 10.1002/art.34659

J. Banchereau and V. Pascual, Type I Interferon in Systemic Lupus Erythematosus and Other Autoimmune Diseases, Immunity, vol.25, issue.3, pp.383-392, 2006.
DOI : 10.1016/j.immuni.2006.08.010

I. Charo, S. Myers, A. Herman, C. Franci, A. Connolly et al., Molecular cloning and functional expression of two monocyte chemoattractant protein 1 receptors reveals alternative splicing of the carboxyl-terminal tails., Proceedings of the National Academy of Sciences, vol.91, issue.7, pp.2752-2756, 1994.
DOI : 10.1073/pnas.91.7.2752

B. De-paepe, D. Bleecker, and J. , ?-chemokine receptor expression in idiopathic inflammatory myopathies, Muscle & Nerve, vol.272, issue.5, pp.621-627, 2005.
DOI : 10.1002/mus.20294

A. Delogu, A. Schebesta, Q. Sun, K. Aschenbrenner, T. Perlot et al., Gene Repression by Pax5 in B Cells Is Essential for Blood Cell Homeostasis and Is Reversed in Plasma Cells, Immunity, vol.24, issue.3, pp.269-281, 2006.
DOI : 10.1016/j.immuni.2006.01.012

N. Rulli, M. Rolph, A. Srikiatkhachorn, S. Anantapreecha, A. Guglielmotti et al., Protection From Arthritis and Myositis in a Mouse Model of Acute Chikungunya Virus Disease by Bindarit, an Inhibitor of Monocyte Chemotactic Protein-1 Synthesis, Journal of Infectious Diseases, vol.204, issue.7, pp.1026-1030, 2011.
DOI : 10.1093/infdis/jir470

A. Tournadre, V. Lenief, A. Eljaafari, and P. Miossec, Immature muscle precursors are a source of interferon-?? in myositis: Role of Toll-like receptor 3 activation and contribution to HLA class I up-regulation, Arthritis & Rheumatism, vol.361, issue.2, pp.533-541, 2012.
DOI : 10.1002/art.33350

N. Okiyama, T. Sugihara, Y. Iwakura, H. Yokozeki, N. Miyasaka et al., Therapeutic effects of interleukin-6 blockade in a murine model of polymyositis that does not require interleukin-17A, Arthritis & Rheumatism, vol.35, issue.Pt 1, pp.2505-2512, 2009.
DOI : 10.1002/art.24689

M. Hofmann, S. Drury, C. Fu, W. Qu, A. Taguchi et al., RAGE Mediates a Novel Proinflammatory Axis, Cell, vol.97, issue.7, pp.889-901, 1999.
DOI : 10.1016/S0092-8674(00)80801-6