Inclusion Criteria:  A diagnosis of SLE for at least 6 months prior to the Screening visit Meet the 2019 European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) criteria for SLE at Screening  Have moderate to severe disease activity despite stable standard-of-care medication defined as:  At screening: hSLEDAI score ≥6 (≥4 points of which must come from non-serological finding), OR at least one BILAG A or one B score; At randomization: ≥4-point drop in hSLEDAI, OR one BILAG letter grade improvement in at least one A or B score present at Screening, and investigator or central adjudication committee (CAC) rating of definite improvement or major or complete improvement  Able and willing to stop all lupus treatments, except antimalarials, corticosteroids (prednisone equivalent ≤ 10 mg), and NSAIDs  Exclusion Criteria:  Active lupus nephritis or active central nervous system manifestations of SLE Other inflammatory or autoimmune diseases that, in the opinion of the Investigator or CAC, may confound efficacy evaluations Common variable immunodeficiency syndrome or any other clinically significant immunodeficiency Known COVID-19 infection in the 4 weeks before Screening or positive SARS-CoV-2 RNA test Received a live attenuated vaccine within 2 months of Screening, received a non-live or mRNA vaccine within 2 weeks of Screening, or expecting to receive any vaccine during the study period Any recent infection requiring antibiotics within two weeks of Screening or any recent infection requiring IV antibiotics or hospitalization within 1 month of Screening Any condition for which, in the opinion of the Investigator or CAC, participation in the study would not be in the best interest of the patient (e.g., compromise their well-being) or that could prevent, limit, or confound the protocol-specified assessments Participated in any interventional clinical trial within 42 days prior to Screening or within five half-lives of the investigational product, whichever is longer Received rituximab or equivalent treatment that depletes B cells within 6 months of Screening unless return of B cells to pre-treatment value or normal range can be demonstrated. Received tumor necrosis factor inhibitors, interleukin antagonists, or other biologics, including belimumab, within 42 days or five half-lives of the agent, whichever is longer. Received IV immunoglobulin (IVIG) or IV cyclophosphamide within 2 months, prednisone ≥ 100 mg/day for more than 30 days within 2 months, or plasmapheresis within two months of the Screening visit.