Patient-derived myobundles as a platform for modeling muscle weakness in rare pediatric rheumatic diseases: a proof-of-concept study

Muscle weakness is prominent in several rare pediatric rheumatic diseases, including juvenile dermatomyositis (JDM) and immune-mediated necrotizing myopathy (IMNM), and is less well characterized in children with inflammatory arthritis (arthritis-related weakness, AR). Mechanisms underlying muscle dysfunction in these conditions remain poorly understood, in part due to the lack of physiologically relevant in vitro models. This proof-of-concept study evaluated the feasibility of generating three-dimensional human skeletal muscle constructs (myobundles) from the muscle of pediatric patients with JDM, IMNM, and AR. Myogenic progenitor cells were isolated from muscle tissue of pediatric patients with JDM, IMNM, and AR, as well as from a healthy pediatric control (one donor per condition, with myobundles as technical replicates). Myoblast purity and morphology were assessed by desmin and vimentin co-staining. Myobundle function was evaluated by measuring electrically evoked contractile force and twitch kinetics. Structural features were assessed by sarcomeric α-actinin (SAA) and vimentin immunofluorescence, with quantitative analysis of cross-sectional area, myofiber diameter, striated myofiber organization, nuclei density, and SAA- and vimentin-positive percent areas. Given the single donor per condition, data are presented descriptively without inferential statistics. Myoblast purity was high across all donors (83–96%). Myoblasts from the IMNM donor appeared elongated and spindle-shaped. Relative to myobundles from the healthy pediatric donor, myobundles from all three donors with weakness showed reduced contractility, altered striated myofiber organization, and decreased myofiber diameter. Myobundles from the JDM donor had a larger cross-sectional area relative to controls, and MHC class I and MxA were not detected at baseline yet showed robust induction following in vitro treatment with type I interferon (IFNβ). Myobundles from the AR donor exhibited faster twitch kinetics and greater fatigability than controls, although this donor had the highest clinical strength scores. Myobundles from the IMNM donor showed pronounced functional impairment, with markedly reduced force generation, slowed twitch kinetics, reduced SAA-positive area, and disrupted myofiber organization. Functional myobundles can be generated from limited muscle tissue available in rare pediatric rheumatic diseases, and donor-specific functional and structural differences are measurable in vitro. Confirmation that these represent reproducible disease-associated phenotypes will require additional donors within each condition.