Study of Inebilizumab in Pediatric Subjects With Neuromyelitis Optica Spectrum Disorder

Inclusion Criteria: * Male or female participants, minimum body weight of 15 kg, age 2 to \< 18 years at the time of screening. * Positive serum anti-AQP4-IgG result at screening and diagnosed with NMOSD according to the criteria of Wingerchuk et al, 2015. * Documented history of one or more NMOSD acute relapses within the last year, or 2 or more NMOSD acute relapses within 2 years prior to screening. Exclusion Criteria: * Any condition that, in the opinion of the Investigator, would interfere with the evaluation or administration of the Investigational Product or interpretation of participant safety or study results. * 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 Day 1. * Evidence of significant hepatic, renal, or metabolic dysfunction or significant hematological abnormality (one repeat test may be conducted to confirm results within the same screening period). * B-cell counts \< one-half of the lower limit of normal (LLN) for age according to the central laboratory. * Receipt of the following at any time prior to Day 1: 1. Alemtuzumab 2. Total lymphoid irradiation 3. Bone marrow transplant 4. T-cell vaccination therapy * Receipt of rituximab or any experimental B-cell depleting agent within 6 months prior to screening unless B-cell counts have returned to ≥ one-half the LLN. * Receipt of intravenous immunoglobulin (IVIG) within one month prior to Day 1. * Receipt of any of the following within 2 months prior to Day 1: 1. Cyclosporine 2. Methotrexate 3. Mitoxantrone 4. Cyclophosphamide 5. Tocilizumab 6. Satralizumab 7. Eculizumab * Receipt of natalizumab (Tysabri®) within 6 months prior to Day 1. * Severe drug allergic history or anaphylaxis to 2 or more food products or medicine (including known sensitivity to acetaminophen/paracetamol, diphenhydramine or equivalent antihistamine, and methylprednisolone or equivalent glucocorticoid). * Diagnosed with a concurrent autoimmune disease that is uncontrolled (unless approved by the medical monitor). * Recent receipt of live/attenuated vaccine or blood transfusion. Receipt of any of the following: 1. Any live or attenuated vaccine within 4 weeks prior to Day 1 (administration of killed vaccines and nucleoside-modified mRNA-based vaccines is acceptable; the Sponsor recommends that Investigators ensure all participants are up to date on required vaccinations prior to study entry). 2. Bacillus Calmette Guérin vaccine within one year of screening. 3. Blood transfusion within 4 weeks prior to screening or during screening. * Clinically significant serious active or chronic viral, bacterial, or fungal infection that requires treatment with anti-infectives, hospitalization, or, in the Investigator's opinion, represents an additional risk to the participant, within 2 months prior to Day 1. * Known history of congenital or acquired immunodeficiency (e.g., due to human immunodeficiency virus \[HIV\] infection, splenectomy, immunosuppression-related or idiopathic T-cell deficiencies) that predisposes the participant to infection. * Positive test for chronic hepatitis B infection at screening, defined as either: a. Positive hepatitis B surface antigen (HBsAg), or b. Positive hepatitis B core (HBc) antibody (anti-HBc) plus negative hepatitis B surface (HBs) antibody (anti-HBs). * Positive test for hepatitis C virus antibody. * Negative test for varicella zoster virus (VZV)-IgG. * History of cancer, apart from squamous cell or basal cell carcinoma of the skin treated with documented success of curative therapy \> 3 months prior to Day 1. * History of active or latent tuberculosis (TB), or a positive QuantiFERON®-TB Gold test at screening, unless treatment for TB was completed per local guidelines. Participants with latent TB or a positive QuantiFERON®-TB Gold test who are actively on anti-TB treatment can enroll if they have completed at least one month of anti-TB treatment and intend to complete the full course of anti-TB treatment. Participants with an indeterminate QuantiFERON®-TB Gold test result can enroll if a repeat QuantiFERON®-TB Gold test is negative or a tuberculin skin test is negative. * For participants who may undergo MRI scans: 1. Unable to undergo an MRI scan (e.g., hypersensitivity to Gd-containing MRI contrast agents, implanted pacemakers, defibrillators, or other metallic objects on or inside the body that limit performing MRI scans), or 2. Unable to tolerate or comply with the MRI procedure.