A Study to Evaluate the Efficacy, Safety, Pharmacokinetics (PK), and Pharmacodynamics (PD) of Satralizumab in Participants 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 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 fromheterosexual intercourse) or use adequate contraception during the treatment periodand for at least 3 months after the final dose of satralizumab or placebo

• For participants enrolled in the extended China enrollment phase at China's sites:participants who are current residents of mainland China, Hong Kong, or Taiwan, andof Chinese ancestry

NMDAR AIE Cohort:

• Age ≥ 12 years

• Diagnosis of probable or definite NMDAR encephalitis

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 withno evidence of recurrence for ≥ 5 years before screening

• For participants with NMDAR AIE, history of negative anti-NMDAR antibody incerebrospinal fluid (CSF) using a cell-based assay within 9 months of symptom onset

• Historically known positivity to an intracellular antigen with high cancerassociation or glutamate decarboxylase 65 (GAD-65)

• Historically known positivity to any cell surface neuronal antibodies other thanNMDAR and LGI1, in the absence of NMDAR and LGI1 antibody positivity

• 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 interleukin-6 (IL-6) inhibitory therapy (e.g., tocilizumab), alemtuzumab, total body irradiation, or bone marrowtransplantation

• Any previous treatment with anti-cluster of differentiation 19 antibody (CD19antibody), 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 priorto screening

• Concurrent use of more than one immunosuppressive therapy (IST) as backgroundtherapy

• Contraindication to all of the following rescue treatments: rituximab, intravenousimmunoglobulin (IVIG), high-dose corticosteroids, or intravenous (IV)cyclophosphamide

• Any surgical procedure, except laparoscopic surgery or minor surgeries within 4weeks 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

• Congenital or acquired immunodeficiency, including human immunodeficiency virus (HIV) infection

• Active or presence of recurrent bacterial, viral, fungal, mycobacterial infection,or other infection

• Infection requiring hospitalization or treatment with IV anti-infective agentswithin 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 asGI 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 donationwithin 90 days prior to screening

• History of severe allergic reaction to a biologic agent

• History of suicide attempt within 3 years prior to screening except if this isclearly associated with and occurs during the acute phase of LGI-1 or NMDARencephalitis

• Any serious medical condition or abnormality in clinical laboratory tests that, inthe investigator's judgment, precludes safe participation in and completion of thestudy

• Pregnant or breastfeeding, or intending to become pregnant during the study orwithin 3 months after the final dose of study drug