Inflammatory cytokines as new potential targets for the treatment of genetic and non-genetic myopathies

Several diseases affecting skeletal muscles, such as Duchenne’s muscular dystrophy (DMD) and juvenile dermatomyositis (JDM), are characterized by a consistent and prolonged muscle inflammatory response favoring tissue damage and interfering with the regeneration processes. Even though these diseases have different pathogenic mechanisms, with DMD being a genetic disease caused by mutations in the dystrophin gene and JDM being an autoimmune disorder. Inflammatory mediators play a role in the pathogenesis of all these diseases, therefore further research to pinpoint the exact mechanisms by which inflammatory cytokines contribute to muscle damage in JDM and DMD is strategic to properly guide potential targeted therapeutic approaches. This would be important to identify new targets and develop targeted therapies. In the present study, we focused our attention on the role of two key pro-inflammatory cytokines, interleukin-6 (IL-6) and interferon-γ (IFNγ) in the pathogenesis of muscle damage in DMD and JDM. These cytokines regulate the local immune response but, at the same time, they can directly affect muscle satellite cell differentiation and influence muscle homeostasis. The high levels of these cytokines during chronic inflammation in muscle tissue could interfere with muscle regeneration and favor tissue damage. We have shown that the inflammatory cytokine IL-6 and its receptor IL-6 receptor (IL-6R) were overexpressed in muscle biopsies from patients with DMD. IL-6 and IL-6R were up-regulated in the muscle of mdx mice (the animal model of DMD) during the necrotic phase of the mdx muscle. In mdx muscles, we also found an up-regulation of ADAM17, a metalloproteinase involved in the shedding of membrane IL-6R and in the release of soluble IL-6R (sIL-6R). sIL-6R mediates a peculiar IL-6 signaling modality, the so-called trans-signaling, involved in pro-inflammatory processes, such as recruitment of mononuclear cells, inhibition of T-cell apoptosis and inhibition of T-reg differentiation. On the contrary, IL-6 classical-signaling, through the membrane IL-6R, mediates the anti-inflammatory and regenerative activities of IL-6, such as regeneration of intestinal epithelial cells, inhibition of epithelial apoptosis and the activation of the hepatic acute phase response. We therefore tested the effect of the inhibition of IL6 trans-signaling, using an interleukin-6 receptor (1F7) neutralizing antibody, on the dystrophic phenotype of mdx mice. Blockade of endogenous IL6 trans-signaling significantly reduced the myonecrosis of dystrophic muscle and improved the physical endurance of mice during treadmill exercise. To evaluate the possible role of IL6 transsignaling on muscle regeneration processes, the in vitro treatment of C2C12 myoblasts with recombinant IL-6 and sIL-6R decreased the expression of muscle differentiation markers at day 3 of differentiation. These results show a role of IL6 trans-signaling in myonecrosys, in loss of muscle 7 performance and in the muscle regenerative processes suggesting that this pathway is a potential therapeutic target in DMD patients. Performing a gene expression analysis on muscle biopsies of DMD and JDM patients, we found that IFNγ and IFNγ-inducible genes were strongly up-regulated in JDM muscle. We hypothesized a possible role of IFNγ in the pathogenesis of JDM. Expression levels of IFNγ, IFNγ-inducible genes (type II IFN score) also correlated with typical histopathological features of JDM muscle biopsies, such as infiltration of CD3+ and CD68+ cells and perifascicular atrophy. Moreover, muscle type II IFN score correlated with obal disease activity (PGA) observed at time of biopsy. Interestingly, JDM patients with high muscle type II IFN score reached clinically inactive disease significantly later than patients with low muscle type II IFN score. Our data provide an alternative pharmacological strategy based on IL-6 trans-signaling blockade for treatment of DMD and circumvent the major problems associated with conventional therapy. Moreover, the increased expression of IFNγ and IFNγ-inducible genes in muscle of JDM patients and their association with histological and clinical features further support the pathogenic role of IFNγ in JDM.