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.

Key Inclusion Criteria:

• Age: 12 to 70 years old.
• Diagnosis: Must have active B-cell mediated autoimmune disease (SLE, AAV, IIM, or SSc) confirmed by standard criteria.
• Disease Severity: Moderate to severe, requiring at least two prior treatments that were ineffective.
• Health Status: Adequate organ function to tolerate treatment.
• Consent: Able to provide informed consent or assent/obtain parental consent and comply with study procedures.

Key Exclusion Criteria:

• Pregnancy/Breastfeeding: Women must not be pregnant or nursing.
• Severe Organ Dysfunction: Significant heart, lung, liver, or kidney impairment.
• Active Infections: No recent or ongoing serious infections.
• Recent Cancer or Prior Cell Therapy: No active/recent malignancies, prior CAR T-cell therapy, or organ transplant.
• Allergies: No known allergies to study treatments.
• Weight Restriction: Must weigh at least 50 kg (110 lbs).