The purpose of this clinical research study is to learn if abatacept treatment of patients with active lupus nephritis who are also taking mycophenolate mofetil (MMF) and steroids as part of this study will control the nephritis despite a protocol-defined steroid taper; the endpoint is a "complete renal response", a composite including normalization of renal function (or stable normal function if function was normal at study entry) plus disappearance of protein and cells/casts from the urinary sediment. The safety of this treatment will also be studied

Inclusion Criteria:

• SLE as defined by meeting at least 4 of the 11 classification criteria of the American College of Rheumatology for the classification of Systemic Lupus Erythematosus, either sequentially or coincident. The 4 criteria need not be present at study entry
• Renal Biopsy within 12 months of randomization (Day 1) indicating active proliferative lupus glomerulonephritis ISN/RPS 2003 classification Class III or IV [excluding Class III (C), IV-S (C) and IV-g (C)] or WHO 1982 Classification Class III or IV (excluding Class IIIc, IVd).
• Active renal disease at the screening visit, as defined by: urinary protein/creatinine ratio ≥0.5 AND an active urinary sediment as defined by at least one of the following 3 criteria: i) >5 RBC/hpf OR ii) >5 WBC/hpf (with no evidence of a urinary tract infection) OR iii) cylindruria AND
• A Stable serum creatine ≤3 mg/dL

Exclusion Criteria:

• Subjects with a rise in serum creatine of ≥1 mg/dL within 1 month prior to the screening visit
• Subjects with drug-induced SLE, as opposed to idiopathic SLE
• Subjects with severe, unstable and/or progressive CNS lupus
• Subjects with autoimmune disease other than SLE as their main diagnosis (e.g.; RA, MS)
• Subjects who have received treatment with cyclophosphamide within 3 months of randomization (Day 1).
• Subjects who have received treatment with rituximab < 6 months prior to the screening visit