A Study To Evaluate The Efficacy, Safety, Pharmacokinetics, And Pharmacodynamics Of Satralizumab In Patients With Anti-N-Methyl-D-Aspartic Acid Receptor (NMDAR) Or Anti-Leucine-Rich Glioma-Inactivated 1 (LGI1) Encephalitis

Inclusion Criteria:

Reasonable exclusion of tumor or malignancy before baseline visit (randomization)
Onset of autoimmune encephalitis (AIE) symptoms <=9 months before randomization
Meet the definition of "New Onset" or "Incomplete Responder" AIE
For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use adequate contraception during the treatment period and for at least 3 months after the final dose of satralizumab or placebo
For participants enrolled in the extended China enrollment phase at National Medical Products Administration (NMPA)-recognized sites: participants who are current residents of mainland China, Hong Kong, or Taiwan, and of Chinese ancestry

N-methyl-D-aspartic acid receptor (NMDAR) AIE Cohort

Age >=12 years
Diagnosis of probable or definite NMDAR encephalitis

Leucine-rich glioma-inactivated 1 (LGI1) AIE Cohort

Age >=18 years
Diagnosis of LGI1 encephalitis

Exclusion Criteria:

Any untreated teratoma or thymoma at baseline visit (randomization)
History of carcinoma or malignancy, unless deemed cured by adequate treatment with no evidence of recurrence for >=5 years before screening
For patients with NMDAR AIE, history of negative anti-NMDAR antibody in cerebrospinal fluid (CSF) using a cell-based assay within 9 months of symptom onset
Historically known positivity to an intracellular antigen with high cancer association or GAD-65
Historically known positivity to any cell surface neuronal antibodies other than NMDAR and LGI1
Confirmed paraneoplastic encephalitis
Confirmed central or peripheral nervous system demyelinating disease
Alternative causes of associated symptoms
History of herpes simplex virus encephalitis in the previous 24 weeks
Any previous/concurrent treatment with IL-6 inhibitory therapy (e.g., tocilizumab), alemtuzumab, total body irradiation, or bone marrow transplantation
Any previous treatment with anti-CD19 antibody, complement inhibitors, neonatal Fc receptor antagonists, anti-B-lymphocyte stimulator monoclonal antibody
Any previous treatment with T-cell depleting therapies, cladribine, or mitoxantrone
Treatment with oral cyclophosphamide within 1 year prior to baseline Treatment with any investigational drug (including bortezomib) within 24 weeks prior to screening
Concurrent use of more than one IST as background therapy
Contraindication to all of the following rescue treatments: rituximab, IVIG, high-dose corticosteroids, or intravenous (IV) cyclophosphamide
Any surgical procedure, except laparoscopic surgery or minor surgeries within 4 weeks prior to baseline, excluding surgery for thymoma or teratoma removal
Planned surgical procedure during the study
Evidence of progressive multifocal leukoencephalopathy
Evidence of serious uncontrolled concomitant diseases that may preclude patient participation
Congenital or acquired immunodeficiency, including HIV infection
Active or presence of recurrent bacterial, viral, fungal, mycobacterial infection, or other infection
Infection requiring hospitalization or treatment with IV anti-infective agents within 4 weeks prior to baseline visit
Positive hepatitis B (HBV) and hepatitis C (HCV) test at screening
Evidence of latent or active tuberculosis (TB)
History of drug or alcohol abuse within 1 year prior to baseline
History of diverticulitis or concurrent severe gastrointestinal (GI) disorders that, in the investigator's opinion, may lead to increased risk of complications such as GI perforation
Receipt of live or live-attenuated vaccine within 6 weeks prior to baseline visit
History of blood donation (1 unit or more), plasma donation or platelet donation within 90 days prior to screening
History of severe allergic reaction to a biologic agent
Active suicidal ideation within 6 months prior to screening, or history of suicide attempt within 3 years prior to screening
Any serious medical condition or abnormality in clinical laboratory tests that, in the investigator's judgment, precludes safe participation in and completion of the study
Pregnant or breastfeeding, or intending to become pregnant during the study or within 3 months after the final dose of study drug
Laboratory abnormalities at Screening