An Extension Study to Learn More About the Long-Term Safety of Litifilimab (BIIB059) Injections and Whether They Can Improve Symptoms of Adult Participants Who Have 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 will 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.

This is an extension study for all participants who completed study 230LE303 (NCT04895241) and 230LE304 (NCT04961567) (parent phase 3 studies) through Week 52 and did not discontinue litifilimab or placebo. Eligible participants from parent phase 3 studies will be followed for up to 180 weeks.

The primary objective of this study is to evaluate the long-term safety and tolerability of litifilimab in participants with active systemic lupus erythematosus (SLE).

The secondary objectives of this study are to evaluate the long-term effect of litifilimab on disease activity in participants with SLE, to evaluate the long-term effect of litifilimab in participants with SLE in maintaining low disease activity, to evaluate the effect of litifilimab in participants with active SLE in preventing irreversible organ damage, to assess long-term use of oral corticosteroid (OCS) with participants receiving litifilimab treatment, to assess the impact of litifilimab on participant-reported Health-Related Quality-of-Life Questionnaire (HRQoL), symptoms, and impacts of SLE, to evaluate long-term effect of litifilimab on laboratory parameters, and to evaluate immunogenicity of litifilimab.

Eligibility Criteria:
Key Inclusion Criteria:

* Participants who completed 1 of the 52-week of the double-blind placebo-controlled, parent Phase 3 studies (230LE303 (NCT04895241) and 230LE304 (NCT04961567)) on study treatments with either litifilimab or placebo to Week 48 and attended the last study assessment visit at Week 52

Key Exclusion Criteria:

* Early parent Phase 3 studies treatment terminators (participants who discontinued study treatment before Week 52)
* Early parent Phase 3 studies terminators (participants who withdrew from study participation and did not complete the 52-week treatment period)
* Participants who developed moderate-to-severe worsening of organ-specific lupus manifestations that would require a change in antimalarials and/or immunosuppressive therapy (initiation of new treatment or increase in dose above the allowed maximum dose)
* Use of other investigational drugs or off-label drugs used to treat SLE, cutaneous lupus, or lupus nephritis during the parent Phase 3 studies

NOTE: Other inclusion/exclusion criteria may apply.