Idiopathic Inflammatory Myositis (IIM)

This is a phase 1 study designed to evaluate the safety, pharmacokinetics (PK), and anti-B-cell activity of FT819 following treatment with or without auxiliary medicinal product (AMP) in participants with moderate-to-severe active systemic lupus erythematosus (SLE) with or without nephritis, antineutrophilic cytoplasmic antibody (ANCA)-associated vasculitis (AAV), idiopathic inflammatory myositis (IIM), and systemic sclerosis (SSc). The study will consist of a dose-escalation stage, followed by an expansion stage to further evaluate the safety and activity of FT819.

The goal of this clinical trial is to learn about how an umbilical cord lining-derived stem cell (ULSC) product performs when treating Dermatomyositis/Polymyositis (DM/PM), also known as idiopathic inflammatory myopathy (IIM) in adults. It will assess safety and efficacy in relieving symptoms of DM/PM with ULSC administered in three intravenous (IV) doses of 150 million cells per dose. The main questions that this study plans to answer are: * Is ULSC as safe as placebo (a look-alike saline without cells) in repeated IV infusion? * Does ULSC improve symptoms of DM/PM after three doses? Researchers will compare ULSC to placebo and evaluate changes from baseline (before first dose) to after each dose and after all three doses are completed per treatment study period. * For participants undergoing steroid (e.g., prednisone) therapy for DM/PM, does ULSC allow their steroid dose to be reduced? Does ULSC reduce need for rescue therapy? Participants will have been diagnosed with either DM or PM: * Diagnosed according to the EULAR/ACR 2017 Classification Criteria for idiopathic inflammatory myositis (IIM), which includes DM and PM. * Positive for myositis-associated antibody or undergone evaluation to exclude mimics. Participants in this study will: * Participate for total of 25 months with 15 in-person clinic visits and 8 virtual visits on phone or video call. * Receive both ULSC and placebo for a total of 6 IV infusions (260 mL) 3 months apart. * Receive 3 doses of ULSC and 3 doses placebo in either of two sequences, as assigned: ULSC first and placebo second, or placebo first and ULSC second. * If undergoing steroid therapy, will have steroid dose taper prescribing lower doses starting two weeks after the second infusion. * Return for follow-up visits after each dose and up to 12 months after final dose. * Have follow-ups including self-reported questionnaires, physical exam, muscle strength and endurance tests, blood tests, pulmonary function tests, and other assessments.

This is an open-label, single-dose, prospective, monocenter, Phase I interventional study (not first-in-human) to assess the safety, tolerability, and preliminary efficacy of autologous anti-CD19 CAR-T cells as an advanced therapy medicinal product in adult patients; with severe and refractory systemic autoimmune rheumatic diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), Sjogren's disease (SjD), systemic sclerosis (SSc) and idiopathic inflammatory myositis (IIM).