Long-term, Open-label, Safety Study of Inebilizumab in Neuromyelitis Optica Spectrum Disorder (NMOSD)

Inclusion Criteria: 1. Age 18 years or above, and able to provide 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 subject/legal representative prior to performing any protocol-related procedures, including screening evaluations. 2. Have completed at least 2 years in the open-label period of the N-MOmentum study or are newly initiating inebilizumab treatment at a participating site. 3. Females of childbearing potential who are sexually active with a nonsterilized male partner must use a highly effective method of contraception as outlined in the protocol (subjects in the Czech Republic only must use 1 additional method of contraception) from screening, and must agree to continue using such precautions for 6 months after treatment with inebilizumab; cessation of contraception after this point should be discussed with a responsible physician. 4. Nonsterilized males who are sexually active with a female partner of childbearing potential must use a male condom plus spermicide (subjects in the Czech Republic only must use 1 additional method of contraception) from Day 1 for 3 months after receipt of last treatment with inebilizumab. 5. Sterilized males, without the appropriate post-vasectomy documentation on the absence of sperm in the ejaculate, who are sexually active with a female partner of childbearing potential must use a condom and spermicide from Day 1 for 3 months after receipt of the last treatment with inebilizumab. Exclusion Criteria: 1. Have any condition that would place the participant at unacceptable risk of complications, interfere with evaluation of inebilizumab or confound the interpretation of participant safety or study results. 2. Received rituximab or any other B-cell depleting agent after exit from N-MOmentum study, or within the last 12 months prior to screening for non N-MOmentum participants. 3. Known history of allergy or reaction to any component of inebilizumab formulation or history of anaphylaxis following any biologic therapy 4. Have a severe clinically significant infection, including active chronic infection such as hepatitis B 5. Have active or untreated latent tuberculosis 6. Have a history of progressive multifocal leukoencephalopathy (PML) 7. Is severely immunocompromised state 8. Have active malignancies