A Multicenter, Randomized, Dose-Blind, Phase 3 Long-Term Extension Study to Evaluate Continuous Safety and Efficacy of Litifilimab (BIIB059) in Adult Participants With Active Systemic Lupus Erythematosus

In this study, researchers will learn more about a study drug called litifilimab (BIIB059) in participants with systemic lupus erythematosus (SLE). The study will focus on participants who have active disease and are already taking standard of care medications. These may include antimalarials, steroids, and immunosuppressants. This is an extension study of 230LE303 and 230LE304 (TOPAZ-1 and TOPAZ-2). It will enroll participants who completed the treatment periods of either one of the parent studies. The main objective of the study is to learn more about the long-term safety of litifilimab. The main question researchers want to answer is: \- How many participants have adverse events and serious adverse events? Researchers will also learn about the effect litifilimab has on controlling symptoms of SLE and lowering its activity. They will measure symptoms of SLE over time using a variety of scoring tools. These include the SLE Responder Index (SRI), the Systemic Lupus Erythematosus Disease Activity Index-2000 (SLEDAI-2K), and the British Isles Lupus Activity Group-2004 (BILAG-2004), among others. Researchers will also study how participants' immune systems respond to litifilimab. Additionally, they will measure the effect litifilimab and SLE have on the quality of life of participants using a group of questionnaires. The study will be done as follows: * The Week 52 visit of studies 230LE303 and 230LE304 will be Day 1 of this study. * Participants who were receiving either a high or low dose of litifilimab in the parent studies will continue receiving the same doses. * Participants who were receiving placebo in the parent studies will be randomized to receive either a high or low dose of litifilimab. * All participants will receive litifilimab as injections under the skin once every 4 weeks. The treatment period will last 156 weeks. Participants may continue to take their standard of care medications. * Neither the researchers nor the participants will know which doses of litifilimab the participants are receiving. * There will be a follow-up safety period that lasts up to 24 weeks. * In total, participants will have up to 47 study visits. The total study duration for participants will be up to 180 weeks. Optional Substudy: Some participants may be invited to join an optional substudy after being in the main study for at least 4 months. This substudy will test a new injector device for giving litifilimab. The injector device is an automatic device that delivers the full dose in one injection without needing to push a plunger. Researchers will compare the safety and tolerability of the injector device and how the body reacts to it to the current prefilled syringe method. The substudy will last 3 months and will include about 120 participants.