This study looks at how safe and effective an injection called litifilimab (BIIB059) is for adults with systemic lupus erythematosus (SLE). SLE is a disease where the immune system attacks the body by mistake. Participants will continue their usual medicines like antimalarials, steroids, and immunosuppressants. The study will check if litifilimab helps reduce the disease activity and improve symptoms over 52 weeks. Researchers will use special tools to measure changes in symptoms, like the SLE Responder Index (SRI) and the Patient Global Assessment - Visual Analog Scale (PGA-VAS).

Participants will either get litifilimab or a placebo (a fake treatment with no real medicine) as injections every 4 weeks. The study is "double-blind," meaning neither the participants nor the researchers will know who is getting the real treatment. The study will have up to 22 visits and last up to 80 weeks for each participant, including a follow-up period for safety checks.

• Duration: Up to 80 weeks with 22 visits.
• Participants receive injections every 4 weeks.
• Double-blind: neither researchers nor participants know who gets the real drug.