Inclusion Criteria:

• Diagnosis of NMDAR encephalitis, defined by both (a) and (b):  
  • A subacute onset of change in mental status consistent with autoimmune encephalitis;
  • A positive cell-based assay for anti-NMDA receptor IgG antibody in the CSF confirmed in study-specified laboratories.  
• Age ≥ 18 years.
• Written informed consent and any locally required authorization (e.g., Health Insurance Portability and Accountability Act [HIPAA] in the United States of America (USA), European Union [EU] Data Privacy Directive in the EU) obtained from the participant/legal representative prior to performing any protocol-related procedures, including screening evaluations.  
• Females of childbearing potential who are sexually active with a nonsterilized male partner must agree to use a highly effective method of contraception beginning at screening or upon discharge from hospitalization/inpatient rehabilitation (for participants who were incapacitated at the time of screening), and to continue precautions for 6 months after the final dose of investigational product.  
• Nonsterilized males who are sexually active with a female partner of childbearing potential must agree to use a highly effective method of contraception at screening or upon discharge from hospitalization/inpatient rehabilitation (for participants who were incapacitated at the time of screening), and to continue precautions for 3 months after the final dose of investigational product. Male patients with female partners of childbearing potential must have that female partner use at least one form of highly effective contraception, starting at least one menstrual cycle before (the male patient's) first study drug administration and continuing until at least 3 months after their male partner's last dose of the study drug.  
• Willing to forego other immunomodulatory therapies (investigational or otherwise) for NMDAR encephalitis during the study.  
• Patient must have received at least 3 days of methylprednisolone 1000 mg IV or equivalent corticosteroid within 30 days prior to randomization (Day 1). In addition, patients must have received EITHER of the following treatments within 30 days before randomization.  
  • IVIg, at a minimum dose of 2 g/kg;
  • Plasma exchange or plasmapheresis, with a minimum of 5 treatments.
    • NOTE: These treatments may be provided during the screening period, but must be completed prior to randomization.  
• mRS of ≥ 3 at the screening visit, indicating at least moderate disability.
• Ability and willingness to attend study visits and complete the study  

Exclusion Criteria:  

• Any condition that, in the opinion of the investigator, would interfere with  the evaluation or administration of the investigational product,  interpretation of participant safety or study results, or would make  participation in the study an unacceptable risk. This specifically includes  recent history (last 5 years) of herpes simplex virus encephalitis or known  central nervous system demyelinating disease (e.g., multiple sclerosis).  
• Presence of an active or chronic infection that is serious in the opinion of  the investigator.  
• Concurrent/previous enrollment in another clinical study involving an  investigational treatment within 4 weeks or 5 published half-lives of the  investigational treatment, whichever is the longer, prior to randomization.  
• Lactating or pregnant females, or females who intend to become pregnant  anytime from study enrollment to 6 months following last dose of  investigational agent.  
• Known history of allergy or reaction to any component of the investigational  agent formulation or history of anaphylaxis following any biologic therapy.  
• At screening (one repeat test may be conducted to confirm results prior to  randomization within the same screening period), any of the following:
  • Aspartate transaminase (AST) > 2.5 × upper limit of normal (ULN);
  • Alanine transaminase (ALT) > 2.5 × upper limit of normal (ULN);
  • Total bilirubin > 1.5 × ULN (unless due to Gilbert's syndrome);
  • Platelet count < 75,000/μL (or < 75 × 10^9/L);
  • Hemoglobin < 8 g/dL (or < 80 g/L);
  • Total white blood count <2,500 cells/mm^3;
  • Total immunoglobulin < 600 mg/dL;
  • Absolute neutrophil count < 1200 cells/μL;
  • CD4 T lymphocyte count < 300 cells/µL.
• Receipt of the following at any time prior to randomization:  
  • Alemtuzumab;
  • Total lymphoid irradiation;
  • Bone marrow transplant;
  • T-cell vaccination therapy.
• Receipt of rituximab or any experimental B-cell depleting agent, unless the CD19 B-cell level has returned to above the lower limit of normal prior to randomization.
• Receipt of any of the following within 3 months prior to randomization:
  • Natalizumab (Tysabri®);
  • Cyclosporine;
  • Methotrexate;
  • Mitoxantrone;
  • Cyclophosphamide;
  • Azathioprine;
  • Mycophenolate mofetil.
• Severe drug allergic history or anaphylaxis to two or more food products or  medicines (including known sensitivity to acetaminophen/paracetamol,  diphenhydramine or equivalent antihistamine, and methylprednisolone or  equivalent glucocorticoid).  
• Known history of a primary immunodeficiency (congenital or acquired) or an  underlying condition such as human immunodeficiency virus (HIV) infection or  splenectomy that predisposes the participant to infection.  
• Confirmed positive test for hepatitis B serology (hepatitis B surface antigen and core antigen) and/or hepatitis C PCR positive at screening.  
• History of cancer, apart from ovarian or extra-ovarian teratoma (also known as a dermoid cyst) or germ cell tumor, or squamous cell carcinoma of the skin or basal cell carcinoma of the skin. Squamous cell and basal cell carcinomas should be treated with documented success of curative therapy > 3 months prior to randomization.
•  Any live or attenuated vaccine within 3 weeks prior to Day 1 (administration of killed vaccines is acceptable).
• Bacillus of Calmette and Guérin (BCG) vaccine within 1 year of enrollment.
• Recurrence of previously treated NMDAR encephalitis within the last 3 or 5 years, or suspicion of symptomatic untreated NMDAR encephalitis of greater than 3 months duration at the time of screening.

Eligibility last updated 10/4/21. Questions regarding updates should be directed to the study team contact.