This is a randomized, double-blind, placebo-controlled phase 2 study to evaluate the efficacy, safety and tolerability of an autologous T-cells expressing a chimeric antigen receptor (CAR) directed to B-Cell maturation antigen (BCMA) in patients with autoantibody-mediated myositis, including antisynthetase syndrome (ASyS) and dermatomyositis (DM).

Inclusion Criteria:

• Confirmed diagnosis of one of the following:

Dermatomyositis (DM): Probability score ≥55% on the 2017 EULAR/ACR (European Alliance of Associations of Rheumatology/ American College of Rheumatology) criteria for classification of dermatomyositis (corresponding to diagnosis of 'probable or definite' DM). OR Antisynthetase Syndrome (ASyS): Diagnosis based on ACR/EULAR Classification Criteria (1)."

• Participants must have dermatomyositis or antisynthetase syndrome with muscle and/or skin involvement.
• Refractory or intolerance to standard therapy.
• Stable background immunosuppressive therapy for ≥8 weeks.
• Adequate hematologic, renal, hepatic, and pulmonary function (SpO₂ ≥92% on room air).
• Informed consent, compliance with visits, contraception, and vaccinations required.

Exclusion Criteria:

• Isolated interstitial lung disease (ILD) without muscle or skin involvement

  • Severe irreversible muscle damage or advanced weakness (e.g., wheelchair-bound).
  • Interstitial lung disease (ILD) requiring oxygen, severe pulmonary impairment (FVC ≤45%, DLCO ≤40%), or pulmonary hypertension.
  • Other inflammatory myopathies (PM, IMNM, IBM, cancer- or drug-induced myositis, overlap myositis except Sjögren's).
  • Other severe neuromuscular, cardiac, pulmonary, or systemic autoimmune diseases requiring immunosuppression.
  • Significant uncontrolled chronic illnesses or psychiatric conditions interfering with participation.
  • Pregnancy or lactation.
  • Recent use of prohibited immunosuppressants/biologics or investigational agents (per washout periods).
  • Live vaccination within 4 weeks.
  • History of primary immunodeficiency, organ or bone marrow transplant.
  • Active or uncontrolled infections: HBV, HCV, HIV, tuberculosis, or recurrent/severe infections.