This is a phase 1 study of SYNCAR-001 + STK-009 in patients with severe, refractory systemic autoimmune rheumatic disease.

SYNCAR-001 + STK-009 is a 2-component human orthogonal (ho) IL-2 receptor-ligand cell therapy consisting of (1) SYNCAR-001, a CD19-directed chimeric antigen receptor T cell (CAR-T) co-expressing an engineered IL-2 beta receptor (hoRb); and (2) STK-009, an engineered pegylated IL-2 cytokine (hoIL-2) selective for hoRb. This study is being conducted to evaluate the safety and efficacy of a single dose of SYNCAR-001 followed by multiple subcutaneously administered doses of STK-009. No conditioning chemotherapy (lymphodepletion) will be administered. The study will follow a 3+3 design during dose escalation followed by dose expansion at the recommended phase 2 dose (RP2D).

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.