Abstract Text: Inflammatory myopathies are autoimmune muscle disorders with unmet therapeutic needs. Their pathophysiology has remained difficult to unravel due to the paucity of relevant experimental models. ICOS pathway invalidation in NOD mice results in a switch of autoimmunity from diabetes towards myositis, providing a valuable murine model. Indeed, we show that Icos-/- NOD mice exhibit reduced muscle strength and muscle histopathological features including macrophage and IFNg-secreting CD4 T cell infiltration. Muscle holoproteome and transcriptome analyses suggested metabolic alterations and mitochondrial defects while elevated reactive oxygen species confirmed the presence of muscle oxidative stress. To demonstrate that inflammatory infiltrates are the cause of metabolic dysfunction observed in myofibers, we performed Nanostring spatial transcriptome analysis. Through segmentation with desmin and CD45, we analyzed the whole transcriptomic profile of myofibers adjacent or not to large immune cell clusters, and of muscle-infiltrating immune cells themselves. We found a strong downregulation of the skeletal muscle contraction machinery in myofibers from diseased Icos-/- NOD mice. The expression of genes encoding proteins involved in key mitochondrial metabolic processes, mitochondrial dynamics and structure stability was also strongly reduced. For many of these genes, the decrease was exacerbated in myofibers directly adjacent to large immune clusters. Histoenzymology, ex vivo oxidative phosphorylation assessment and electron microscopy confirmed severe mitochondrial defects in myofibers. N-acetyl cysteine therapy significantly ameliorated mitochondrial alterations and clinical signs of myositis. Together, our data show that the pathophysiology of myositis involves severe mitochondrial defects in myofibers caused by tissue inflammation that can be attenuated by antioxidant therapy.
