LAUSR

OBJECTIVE To investigate pathogenic mechanisms underlying juvenile dermatomyositis (JDM), we defined the effect of type I interferons (IFN I), interferon-α (IFNα) and interferon-β (IFNβ), on pediatric skeletal muscle function and expression of myositis-related proteins using an in vitro engineered human skeletal muscle model (myobundle). METHODS Primary myoblasts were isolated from three healthy pediatric donors and used to create myobundles that mimic functioning skeletal muscle in structural architecture and physiologic function. Myobundles were exposed to 0, 5, 10, or 20 ng/ml IFNα or IFNβ for 7 days and then functionally tested under electrical stimulation and analyzed immunohistochemically for structural and myositis-related proteins. Additionally, IFNβ-exposed myobundles were treated with Janus kinase inhibitors (JAKi) tofacitinib and baricitinib. These myobundles were also analyzed for contractile force and immunohistochemistry. RESULTS IFNβ, but not IFNα, was associated with decreased contractile tetanus force and slowed twitch kinetics. These effects were reversed by tofacitinib and baricitinib. IFN I paradoxically reduced myobundle fatigue, which did not reverse after JAKi. Additionally, IFN I correlated with MHC I upregulation, which normalized after JAKi treatment, but expression of myositis-specific autoantigens Mi-2, MDA5, and the ER stress marker GRP78 were variable and donor-specific after IFN I exposure. CONCLUSION IFNα and IFNβ have distinct effects on pediatric skeletal muscle, and these effects can partially be reversed by JAKi treatment. This is the first study illustrating effective use of a three-dimensional human skeletal muscle model to investigate JDM pathogenesis and test novel therapeutics.