Inclusion Criteria - Diagnosed with Systemic Lupus Erythematosus (SLE) at least 24 weeks before the screening visit. - Meet the European Alliance of Associations for Rheumatology (EULAR)/American College of Rheumatology (ACR) 2019 classification criteria for SLE. - One of the following: positive antinuclear antibodies (ANA) ≥ 1:80 at screening OR positive anti dsDNA OR positive anti Smith (anti Sm) as determined by the central laboratory at screening. - Total Systemic Lupus Erythematosus Disease Activity Index-2K (SLEDAI-2K) score ≥ 6 points and clinical SLEDAI 2K score ≥ 4 points with joint involvement, and/or cutaneous vasculitis, and/or rash. - Lupus headache, alopecia, organic brain syndrome, and mucosal ulcers must be recorded on SLEDAI 2K, if indicated, but do not count toward the points required for screening at entry. - At least one SLE background therapy (immunosuppressant and/or antimalarial) is required for ≥ 12 weeks before the screening visit, must be at a stable dose for ≥ 8 weeks before the screening visit, and must remain stable until randomization and throughout study participation. - Oral corticosteroid (OCS; prednisone or equivalent) background therapy is permitted but not required. For participants taking OCS, the dose must be stable for ≥ 2 weeks before the screening visit, cannot exceed 30 mg/day at screening, and must remain stable until the Week 4 visit. Participants can be on an OCS as well as an antimalarial and/or an immunosuppressant. Exclusion Criteria - Diagnosis of drug-induced SLE rather than idiopathic SLE. - Other autoimmune diseases (eg, multiple sclerosis, psoriasis, inflammatory bowel disease, etc.) are excluded. Participants with type I autoimmune diabetes mellitus, thyroid autoimmune disease, Celiac disease, or secondary Sjögren's syndrome are not excluded. - SLE overlap syndromes including, but not limited to, rheumatoid arthritis, scleroderma, and mixed connective tissue disease are excluded. - Active or unstable lupus neuropsychiatric manifestations, including, but not limited to, any condition defined by BILAG A criteria. - Active, severe Class III, and IV, lupus nephritis that requires or may require treatment with cytotoxic agents or high-dose CS. - History of congenital or acquired immunodeficiency. - Known active infection, or any major episode of infection requiring hospitalization or treatment with parenteral (intramuscular or IV) antimicrobial agents (eg, antibiotics antiviral, antifungal, or antiparasitic agents) within 30 days of randomization, or treatment with oral antimicrobial agents within 2 weeks of randomization. - Currently on any therapy for chronic infection (eg, pneumocystis, herpes zoster, cytomegalovirus, invasive bacterial or fungal infections, or atypical mycobacteria). - Taking more than 1 immunosuppressant at screening. - Other protocol-defined Inclusion/Exclusion criteria apply.