Margherita Giannini

Letters to the Editor; Authors: M. Giannini, B. Geny, A. Meyer

Les myosites sont des maladies auto-immunes rares dont l’inflammation du muscle squelettique est le dénominateur commun de tous les sous-types. Les glucocorticoïdes (GC) sont la première ligne de traitement, cependant leur utilisation est empirique et ils induisent de nombreux effets indésirables. Par une approche translationnelle, nous avons identifié le mécanisme de la réponse thérapeutique aux GC ce qui permettra d’améliorer la prise en charge des patients atteints de myosites. Même quand les signes d’activité des myosites sont résolus, les patients présentent des dégâts musculaires qui sont difficiles à quantifier dans la pratique clinique courante. Nous avons démontré que la masse musculaire est un paramètre pertinent pour évaluer les dégâts musculaires chez ces patients. Lorsque la masse et la force sont mesurées en même temps, elles permettent d’identifier les patients souffrant de sarcopénie et myosite. Enfin, nous avons contribué à caractériser un nouveau sous-type de myosite, la scléromyosite.

Autoantibody internalization has been implicated in autoimmune disease pathogenesis, yet its mechanisms, and generality across different diseases, cell types, and affected tissues remain poorly defined. Using bulk RNA sequencing, we identified reproducible, autoantibody-specific transcriptomic signatures consistent with autoantigen dysfunction in muscle biopsies from patients with anti-Mi2 dermatomyositis and anti-PM/Scl scleromyositis across independent cohorts. Electroporation of purified patient IgG into primary cultures of healthy cells was sufficient to induce the corresponding transcriptomic programs in vitro . Direct immunofluorescence demonstrated immunoglobulin internalization into subcellular compartments matching the localization of the autoantigen in different affected tissues. Spatial transcriptomic analyses revealed that antibody-secreting cells translocated cytoplasmic material (i.e., immunoglobulin RNA) into adjacent affected cells expressing autoantibody-specific transcripts. The disease-specific transcripts were present not only in muscle fibers, but also in other cells, including macrophages, endothelial cells, and fibroblasts. Autoantibody-induced transcriptomic programs were associated with cell damage and autoantibody-specific reactive inflammatory programs, including activation of type I interferon and TGF-ß1 signaling in anti-Mi2 dermatomyositis and activation of type II interferon in anti-PM/Scl scleromyositis. Antibody internalization was also observed in different tissues from patients with other autoimmune diseases, including anti-U1RNP mixed connective tissue disease, anti-Ku overlap syndrome, and anti-Scl70 systemic sclerosis. Together, these findings establish autoantibody internalization as a shared pathogenic mechanism across diverse autoimmune diseases, providing a unifying framework for conditions driven by autoantibodies against intracellular antigens.

Malgré les traitements immunomodulateurs, une proportion importante de patients atteints de myopathies inflammatoires (MI), même lorsque l’activité de la maladie est faible, présentent un handicap persistant. Cette condition, appelée « dommage », est fréquente et corrélée à la mortalité. Notre étude a examiné l’association entre le déséquilibre de diverses myokines, mesurées à l’inclusion, et le « dommage total » d’étendue et celui de sévérité, évalués grâce aux scores de l’IMACS ( International Myositis Assessment and Clinical Studies ), et le « dommage global » établi par un médecin. À cette fin, ont été inclus 40 patients adultes atteints de MI, avec une durée de maladie de 12 mois et plus, une activité de la maladie faible depuis au moins 6 mois, et 30 volontaires sains (VS). Le taux sérique de FABP-3 ( fatty acid-binding protein 3 ) était 1,8 fois plus élevé chez les patients atteints de MI par rapport aux VS, et positivement corrélé aux scores de dommage total (étendue et sévérité), ainsi qu’au score de dommage global. Parmi les domaines de l’IMACS, les associations les plus fortes ont été observées pour les atteintes gastro-intestinales, cardiovasculaires et musculaires. En conclusion, un taux circulant élevé de FABP-3 est un biomarqueur candidat de dommages dans les MI. Compte tenu du rôle de FABP-3 dans le métabolisme énergétique et la trophicité musculaires, il pourrait également jouer un rôle dans la physiopathologie de cette condition. Ces résultats pourraient donc avoir un impact sur la quantification et le développement de traitements pharmacologiques pour les dommages associés aux MI.

Objectives: To evaluate whether the impairment of aquaporin-4 (AQP4), a water channel protein important for muscle function, may be associated with dysphagia in idiopathic inflammatory myopathies (IIMs). Swallowing muscle inflammation in IIM has been documented in case reports, but direct correlations with limb muscles impairment are lacking. Methods: AQP4 was immunolocalized in limb muscle biopsies. The percentage of AQP4 expressing myofibers was compared in IIM patients with and without dysphagia, and healthy controls, and correlated with muscular strength, creatine kinase level and myositis damage index. Additionally, myosin heavy chain (fast and neonatal), cluster of differentiation 68 (CD68) and matrix metalloproteinase-2 (MMP2) were immunolocalized with sarcolemma AQP4 to evaluate whether an increase in innate immunity and extracellular matrix degradation were reflected in the modified AQP4 expression. Results: AQP4 immunostaining was significantly decreased in myofibers of dysphagic patients compared with normophagic patients and controls. Moreover, although both IIM groups had CD68-positive cells surrounding AQP4-negative myofibers, only in dysphagic patients AQP4-negative myofibers surrounded by CD68-positive and MMP2-positive cells were significantly increased, suggesting that inflammation from blood vessels and inflammatory cells affects AQP4 immunolocalization. Conclusion: Myofiber hypotrophy appears to be due to inflammation from CD68-positive and MMP2-positive cells, whose activity disrupts the extracellular matrix and may detach AQP4 from the sarcolemma.

Introduction Inflammatory myopathies (IM) are a group of severe autoimmune diseases, sharing some similarities, whose cause is unknown and treatment is empirical. While C-protein-induced myositis (CIM), the most currently used mouse model of IM, has removed some roadblocks to understand and improve the treatment of IM, it has only been partially characterised and its generation limited by poor reproducibility. This study aimed at optimising the generation and the characterisation of CIM. Methods In silico analysis was run to identify the top three specific and immunogenic regions of C-protein. The cognate polypeptides were synthesised and used to immunise C57BL/6N mice. Grip strength, walking ability, serum creatine kinase levels and muscle pathology (histological and electron microscopic features) were assessed. Immune cell proportions and interferon signature in muscles were also determined. Results Among the three C-protein polypeptides with the highest immunogenic score, immunisation with the amino acids 965–991 induced the most severe phenotype (experimental myositis (EM)) characterised by 37% decrease in strength, 36% increase in hind base width, 45% increase in serum creatine-kinase level and 80% increase in histological inflammatory score. Optical and electron microscopy revealed mononuclear cell infiltrate, myofibre necrosis, atrophy, major histocompatibility complex-I expression as well as sarcolemmal, sarcomeric and mitochondrial abnormalities. Autoantibodies targeting C-protein, proinflammatory T-lymphocytes, macrophages, and type I and II interferon-stimulated transcripts were detected within the muscle of EM mice. Conclusion EM recapitulates the common hallmarks of IM. This costless, high throughput, reproducible and robust model, generated in the most commonly used background for genetically engineered mice, may foster preclinical research in IM.

Introduction: Inflammatory myopathies (IM) are a group of severe autoimmune diseases, sharing some similarities, whose cause is unknown and treatment is empirical. While C-protein-induced myositis (CIM), the most currently used model of IM, has removed some roadblock to understand and improve the treatment of IM, it has only been partially characterised and its generation limited by reproducibility issues. This study aimed at optimising the generation and the characterisation of CIM. Methods: In silico analysis was run to identify the top-3 specific and immunogenic regions of C-protein. The cognate polypeptides were synthetised and used to immunise C57BL/6N mice. Grip strength, walking ability, serum creatine-kinase levels and muscle pathology (histological and electron microscopic features) were assessed. Immune cell proportions and interferon signature in muscles were also determined. Results: Among the three C-protein polypeptides with the highest immunogenic score, amino acids 965-991 induced the most severe phenotype (i.e., 37% decrease in strength, 36% increase in hind base width, 45% increase in serum creatine-kinase level, 80% increase in histological inflammatory score) from day (D) 14 to at least D31 after immunisation [experimental myositis (EM)]. Optical and electron microscopy revealed mononuclear cell infiltrate, myofibre necrosis, atrophy, MHC-I expression as well as sarcolemmal, sarcomeric and mitochondrial abnormalities. Proinflammatory T-lymphocytes, macrophages, type-I and II interferon-stimulated transcripts were found within the muscle of EM mice. Conclusion: EM recapitulates the common hallmarks of IM. This costless, high throughput, reproducible and stable model, generated in the most commonly used background for genetically engineered mice, may foster pre-clinical research in IM. Key messages: What is already known on this topic: C-protein-induced myositis is currently the most used model of inflammatory myopathies but has been partially characterised and its generation is limited by reproducibility issues. What this study adds.: Immunisation against the polypeptide encompassing C-protein amino acids 965-991 induces a costless, high throughput, reproducible and stable model of myositis (experimental myositis) that recapitulates the common hallmarks of inflammatory myopathies. How this study might affect research, practice or policy: Experimental myositis, generated in the most used background for genetically engineered mice (C57BL/6N), might foster pre-clinical research in IM.