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Objective: The aim of this study is to report the safety and efficacy of CD19-targeting chimeric antigen receptor (CAR) T cells in a child with refractory juvenile dermatomyositis (JDM)., Methods: We describe a 12-year-old White male with severe, chronically active JDM refractory to multiple immunosuppressive treatment lines, including B cell depletion with rituximab. The patient received a single infusion of fresh, autologous, second-generation anti-CD19 CAR T cell product (lentiviral vector) manufactured on the Prodigy device (1 × 10 6 CAR T cells/kg), after lymphodepletion with cyclophosphamide (1,000 mg/m 2 over two days) and fludarabine (90 mg/m 2 over three days). Immunosuppressive and glucocorticoid treatment were withdrawn before leukapheresis and CAR T cell infusion., Results: Before anti-CD19 CAR T cell therapy, the patient had persistent severe skin and muscular disease activity. CAR T cells expanded significantly (peak at day 7, 32.69 cells/μL). Complete B cell depletion was documented on day 5 in the blood and at week 2 in the bone marrow. The patient presented with fever as part of mild cytokine release syndrome (grade 1), transient anemia (grade 2), and neutropenia (grade 4). Neither infection nor neurotoxicity were observed. Laboratory tests, magnetic resonance imaging, Physician Global Assessment of disease activity, Childhood Myositis Assessment Scale, and Cutaneous Assessment Tool for myositis were performed at baseline and follow-up to assess treatment response, showing remarkable progressive improvement that persists over time, even after B cell recovery., Conclusion: This patient with JDM with severe chronic disease, refractory to multiple treatments, achieved sustained B cell depletion and ongoing immunosuppressive drug-free clinical and radiologic improvement eight months after a single infusion of anti-CD19 CAR T cells., (© 2024 American College of Rheumatology.)