Inclusion Criteria:

• Primary membranous nephropathy (MN), confirmed by kidney biopsy obtained in the past 36 months.
• Anti-PLA2R positive.
• Estimated Glomerular Filtration Rate (eGFR) ≥ 40 ml/min/1.73m^2 while on maximally tolerated renin-angiotensin system (RAS) blockade.
• Proteinuria: 
  • ≥ 4 and < 8 g/day that has been present for ≥ 3 months while on while on maximally tolerated RAS blockade; or
  • ≥ 8 g/day while on maximally tolerated RAS blockade. 
• Blood pressure while on maximally tolerated RAS blockade: 
  • Systolic blood pressure ≤ 140 mmHg; and
  • Diastolic blood pressure ≤ 90 mmHg.

Exclusion Criteria:

• Secondary cause of membranous nephropathy (MN) (e.g., systemic lupus erythematosus (SLE), drug, infection, malignancy) suggested by review of the subject's medical history and/or clinical presentation.
• Rituximab use within the previous 12 months.
• Rituximab use > 12 months ago:
  • With an undetectable CD19 B cell count; or
  • Did not result in a complete remission (CR) or partial remission (PR) with rituximab treatment alone (e.g., without other immunosuppressive or immunomodulatory therapy).
• Use of anti-B cell therapy other than rituximab within the previous 12 months (or 5 half-lives, whichever is greater).
• Cyclophosphamide use within the past 3 months.
• Use of other immunosuppressive medications, such as cyclosporine or tacrolimus, within the past 30 days.
• Use of corticosteroids within the past 30 days.
• Use of any biologic investigational agent, defined as any drug not approved for sale in the country it is used, in the previous 12 months.
• Use of any non-biologic investigational agent in the past 30 days (or 5 half-lives, which ever is greater).
• Unstable kidney function defined as > 15% decrease in the Estimated Glomerular Filtration Rate (eGFR) during the previous 3 months.
• Decrease in proteinuria by 50% or more during the previous 12 months.
• White blood cell (WBC) count < 3.0 x 10^3/µl.
• Absolute neutrophil count < 1.5 x 10^3/µl.
• Moderately severe anemia (hemoglobin < 9mg/dL).
• History of primary immunodeficiency.
• Serum immunoglobulin A (IgA) < 10 mg/dL.
• Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) = ≥ 2 times the upper limit of normal (ULN).
• Positive human immunodeficiency virus (HIV) serology.
• Positive hepatitis C virus (HCV) serology, unless treated with anti-viral therapy with achievement of a sustained virologic response (undetectable viral load 24 weeks after cessation of therapy).
• Evidence of current or prior infection with hepatitis B, as indicated by a positive HBsAg, positive HBcAb, or positive HBsAb serology without history of vaccination.
• Positive QuantiFERON - tuberculosis (TB) Gold test results.
  • Note: Tuberculin Purified Protein Derivative (PPD) test may be substituted for QuantiFERON - TB Gold test.
• History of malignant neoplasm within the last 5 years.
  • Exception: basal cell or squamous cell carcinoma of the skin treated with local resection only, or carcinoma in situ of the uterine cervix treated locally and with no evidence of metastatic disease for 3 years.
• Absence of individualized, age-appropriate cancer screening.
• Women of child-bearing potential who are pregnant, nursing, or unwilling to be sexually inactive or use FDA-approved contraception until study week 104.
• Acute or chronic infection, including:
  • current use of suppressive therapy for chronic infection, hospitalization for treatment of infection in the past 60 days; or
  • parenteral anti-microbial (including anti-bacterial, anti-viral, or anti-fungal agents) use in the past 60 days for infection.
• History of anaphylactic reaction to parenteral administration of contrast agents, human or murine proteins or monoclonal antibodies, including:
  • rituximab; or
  • belimumab.
• Evidence of serious suicide risk, including:
  • any history of suicidal behavior in the last 6 months;
  • any suicidal ideation in the last 2 months; or 
  • who, in the investigator's judgment, pose a significant suicide risk.
• Evidence of current drug or alcohol abuse or dependence, or a history of drug or alcohol abuse or dependence in the past 12 months.
• Vaccination with a live vaccine within the past 30 days.
• Other diseases or conditions which, in the opinion of the investigator, would put the subject at risk or confound the results of the study. 
• Inability to comply with study and follow-up procedures.