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), 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.