Background: Chimeric antigen receptor (CAR) T cells are highly effective in treating haematological malignancies, but associated toxicities and the need for lymphodepletion limit their use in people with autoimmune disease. To explore the use of CAR T cells for the treatment of people with autoimmune disease, and to improve their safety, we engineered them with RNA (rCAR-T)-rather than the conventional DNA approach-to target B-cell maturation antigen (BCMA) expressed on plasma cells. To test the suitability of our approach, we used rCAR-T to treat individuals with myasthenia gravis, a prototypical autoantibody disease mediated partly by pathogenic plasma cells., Methods: MG-001 was a prospective, multicentre, open-label, phase 1b/2a study of Descartes-08, an autologous anti-BCMA rCAR-T therapy, in adults (ie, aged ≥18 years) with generalised myasthenia gravis and a Myasthenia Gravis Activities of Daily Living (MG-ADL) score of 6 or higher. The study was done at eight sites (ie, academic medical centres or community neurology clinics) in the USA. Lymphodepletion chemotherapy was not used. In part 1 (phase 1b), participants with Myasthenia Gravis Foundation of America (MGFA) disease class III-IV generalised myasthenia gravis received three ascending doses of Descartes-08 to determine a maximum tolerated dose. In part 2 (phase 2a), participants with generalised myasthenia gravis with MGFA disease class II-IV received six doses at the maximum tolerated dose in an outpatient setting. The primary objective was to establish safety and tolerability of Descartes-08; secondary objectives were to assess myasthenia gravis disease severity and biomarkers in participants who received Descartes-08. This trial is registered with clinicaltrials.gov, NCT04146051., Findings: We recruited 16 individuals for screening between Jan 7, 2020 and Aug 3, 2022. 14 participants were enrolled (n=3 in part 1, n=11 in part 2). Ten participants were women and four were men. Two individuals did not qualify due to low baseline MG-ADL score (n=1) or lack of generalised disease (n=1). Median follow-up in part 2 was 5 months (range 3-9 months). There was no dose-limiting toxicity, cytokine release syndrome, or neurotoxicity. Common adverse events were headache (six of 14 participants), nausea (five of 14), vomiting (three of 14), and fever (four of 14), which resolved within 24 h of infusion. Fevers were not associated with increased markers of cytokine release syndrome (IL-6, IL-2, and TNF). Mean improvements from baseline to week 12 were -6 (95% CI -9 to -3) for MG-ADL score, -7 (-11 to -3) for Quantitative Myasthenia Gravis score, -14 (-19 to -9) for Myasthenia Gravis Composite score, and -9 (-15 to -3) for Myasthenia Gravis Quality of Life 15-revised score., Interpretation: In this first study of an rCAR-T therapy in individuals with an autoimmune disease, Descartes-08 appeared to be safe and was well tolerated. Descartes-08 infusions were followed by clinically meaningful decreases on myasthenia gravis severity scales at up to 9 months of follow-up. rCAR-T therapy warrants further investigation as a potential new treatment approach for individuals with myasthenia gravis and other autoimmune diseases., Funding: Cartesian Therapeutics and National Institute of Neurological Disorders and Stroke of the National Institutes of Health., Competing Interests: Declaration of interests VG has received honoraria as a consultant or advisory board member from Alexion Phermaceuticals, argenx, Immunovant, and Amylyx Pharmaceuticals and is employed by Biohaven Pharmaceuticals. MB received trial support and consulting fees from Alexion, Cartesian, Horizon, Immunovant, Sanofi, Takeda, UCB, and Ra Pharma. MK, MSS, and MVK are employees of and have ownership interest in Cartesian Therapeutics. MDM and CMJ are employees of Cartesian Therapeutics. CMJ is appointed as an employee of the University of Maryland and VA Maryland Health Care System. The views in this paper do not reflect the views of the state of Maryland or the US Government. GS has received research support from Cartesian Therapeutic, Immunovant, and argenx, paid to his institution; received consulting fees from UCB Pharma and Immunovant; received honoraria from argenx and Alexion; received travel support from argenx and Immunovant; and holds unpaid positions at the Myasthenia Gravis Foundation of California and the American Association of Neuromuscular and Electrodiagnostic Medicine. MHF has received honoraria as a consultant or advisory board member from argenx. TV is the University of South Florida site principal investigator for myasthenia gravis clinical trials sponsored by Alexion (now part of AstraZeneca), argenx, (now part of UCB), Horizon (now part of Viela Bio), Janssen (now part of Momenta), Regeneron, and Cartesian Therapeutics, and receives speaking or consulting honoraria relating to myasthenia gravis from Alexion, argenx, and UCB. JFH has received research support (paid to his institution) from Alexion Pharmaceuticals, argenx, Cartesian Therapeutics, the US Centers for Disease Control and Prevention, the Myasthenia Gravis Foundation of America, the Muscular Dystrophy Association, the National Institutes of Health (including the National Institute of Neurological Disorders and Stroke and the National Institute of Arthritis and Musculoskeletal and Skin Diseases), the Patient-Centered Outcomes Research Institute, Ra Pharmaceuticals (now UCB), and Takeda Pharmaceuticals; honoraria from Alexion Pharmaceuticals, argenx, Immunovant, NMD Pharma, Novartis Pharmaceuticals, Ra Pharmaceuticals (now UCB), Regeneron Pharmaceuticals, Sanofi US, and Viela Bio (now Horizon Therapeutics); he has also received non-financial support from Alexion Pharmaceuticals, argenx, Ra Pharmaceuticals (now UCB), and Toleranzia. TM has served in an advisory capacity for Alexion, Amicus, AnnJi, argenx, Arvinas, Ask Bio, Audentes, AvroBio, Horizon Therapeutics, Immunovant, Maze Therapeutics, Momenta (now Janssen), Sanofi-Genzyme, Spark Therapeutics, UCB, and Zogenix; serves on the speaker's bureau for argenx and Sanofi-Genzyme; serves on the medical advisory board for the Myositis Association, Neuromuscular Disease Foundation, Myasthenia Gravis Foundation of California, and Myasthenia Gravis Foundation of America; receives research funding from the Myositis Association, the Muscular Dystrophy Association, the National Institutes for Health, Alexion, Amicus, AnnJi, argenx, Audentes (now part of Astellas) Gene Therapy, Bristol-Myers-Squib, Cartesian Therapeutics, Grifols, ML-Bio, Momenta, Ra Pharmaceuticals, Sanofi-Genzyme, Spark Therapeutics, and Valerion; and serves on the data safety monitoring board for Acceleron, Avexis, Sarepta, and the National Institutes for Health. All other authors declare no competing interests., (Copyright © 2023 Elsevier Ltd. All rights reserved.)