Studio per valutare l'efficacia, la sicurezza, la farmacocinetica (PK) e la farmacodinamica (PD) di satralizumab in partecipanti con encefalite da anticorpi anti-recettore N-metil-D-aspartato (NMDAR) o anti-glioma-inattivato ricco di leucina 1 (LGI1)

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 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 China's sites: participants who are current residents of mainland China, Hong Kong, or Taiwan, and of 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 with no evidence of recurrence for ≥ 5 years before screening
* For participants 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 glutamate decarboxylase 65 (GAD-65)
* Historically known positivity to any cell surface neuronal antibodies other than NMDAR and LGI1, in the absence of NMDAR and LGI1 antibody positivity
* Confirmed paraneoplastic encephalitis
* Confirmed central nervous system (CNS) or peripheral nervous system (PNS) 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 marrow transplantation
* Any previous treatment with anti-cluster of differentiation 19 antibody (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 immunosuppressive therapy (IST) as background therapy
* Contraindication to all of the following rescue treatments: rituximab, intravenous immunoglobulin (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
* 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 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
* History of suicide attempt within 3 years prior to screening except if this is clearly associated with and occurs during the acute phase of LGI-1 or NMDAR encephalitis
* 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