The purpose of this study is to determine the ability of XmAb5871 to maintain Systemic Lupus Erythematosus (SLE) disease activity improvement achieved by a brief course of disease-suppressing steroid therapy

Inclusion Criteria:

• Patients with a diagnosis of SLE as defined by the ACR criteria
• Patients have a history of a (+) ANA, (+) ENA or a (+) anti-dsDNA serology documented within one year prior to randomization
• Investigator has assessed the patient and in their judgment, the SLE disease activity is not organ threatening
• Both investigator and patient agree that it is acceptable to discontinue their current immunosuppressant SLE medications and receive a brief course of IM steroid therapy
• If patients are on oral steroids, they must be on the equivalent of ≤15 mg/day of prednisone to enter screening, and must be able to taper to ≤10 mg/day by randomization

Exclusion Criteria:

• History or evidence of a clinically unstable/uncontrolled disorder, condition or disease, other than SLE that, in the opinion of the investigator would pose a risk to patient safety or interfere with the study evaluation, procedures or completion
• Patients who have organ threatening manifestations of SLE including active Class 3 or 4 lupus nephritis requiring induction or maintenance therapy or any other disorder for which stopping SLE therapy is contraindicated
• Active CNS lupus such as seizures or psychosis that in the opinion of the investigator would preclude participation
• Unstable hemolytic anemia or thrombocytopenia
• Patient is pregnant or breast feeding, or planning to become pregnant while participating in the study
• Use of any biologic therapy (including belimumab) within 6 months of randomization, or prior exposure to a monoclonal antibody directed to CD20 (such as rituximab) within 12 months of randomization