A Phase 1 Study of SYNCAR-001 + STK-009 Without Conditioning Chemotherapy (Lymphodepletion) in Subjects With Severe, Refractory Systemic Autoimmune Rheumatic Disease

General Inclusion Criteria: Age ≥18 years at screening. SLE Inclusion Criteria: 1. Clinical diagnosis of SLE according to the 2019 European League Rheumatism EULAR/ACR classification criteria. 2. Subject must be positive for at least one of the following at screening: Anti-dsDNA (above the upper limit of normal \[ULN\]); or anti-Sm (above the ULN); or anti-Chromatin (above the ULN). 3. Subjects with active, severe, non-renal SLE or subjects with active proliferative LN SSc Inclusion Criteria: 1. Classified as SSc according to the ACR/EULAR classification criteria. 2. Diffuse cutaneous SSc (dcSSc) or SSc-associated ILD (SSc-ILD; significant or progressive). General Exclusion Criteria: 1. History of or active central nervous system manifestations of autoimmune disease. 2. Prior treatment with anti-CD19 adoptive T cell therapy, or any prior gene therapy product (e.g., CAR T cell therapy). SLE Exclusion Criteria: 1. Rapidly progressive glomerulonephritis. 2. End stage renal failure requiring dialysis or most recent renal biopsy with purely chronic lesions (Class III\[C\], IV-S\[C\], or IV-G\[C\]) if isolated renal disease. SSc Exclusion Criteria: 1. FVC \<50% of predicted or DLCO \<40% of predicted. 2. Pulmonary arterial hypertension (PAH) requiring PAH-specific treatment. Other protocol-defined criteria apply.