Atacicept in Lupus Nephritis Patients Taking Stable Regimen of Mycophenolate Mofetil

Inclusion Criteria: * Male or female subjects, ≥ 18 years of age, who provide written informed consent * Subjects must have a diagnosis of SLE satisfying ≥ 4 of 11 ACR criteria, and must have had a renal biopsy during screening or within the previous 18 months demonstrating class III (A or A/C), IV (A or A/C), V, or concomitant III/V or IV/V LN as defined by the International Society of Nephrology/Renal Pathology Society (ISN/RPS). * Subjects must have a urine protein: creatinine ratio ≥ 2 mg/mg (≥ 226.2 mg/mmol), and either a positive test for antinuclear antibody (ANA) (HEp-2 ANA ≥ 1:80) and/or anti-double stranded deoxyribonucleic acid (dsDNA) (≥ 30 IU/mL) at screening. * Subjects must have started induction therapy for LN at least 5 months prior to Trial Day 1, be considered to have received continuous treatment for LN during the 5 months prior to Trial Day 1, and have received a stable dose of MMF ≥ 1 g/day, with or without corticosteroids, for at least 8 weeks prior to Trial Day 1. Exclusion Criteria: * Recent changes in immunosuppressant, ACD inhibitors for ARBs * Use of azathioprine, cyclosporine, tacrolimus, or cyclophosphamide or other biologics within 8 weeks prior to Trial Day 1. * Serum IgG \< 6 g/L * Estimated Glomerular Filtration Rate (GFR) ≤ 30 mL/min per 1.73 m2 * History of Demyelinating Disease * Significant Hematuria and/or Proteinuria due to a reason(s) other than LN. Evaluation should be done according to the local standard of care * Breast feed or pregnancy * Legal Incapacity or limited legal capacity