Evaluate the Safety and Tolerability of CE211NS21 in Patients With AQP4-IgG-positive NMOSD Relapse

Inclusion Criteria: 1. Male or female adults aged 19 years or older and 65 years or younger (by full age) at the time of obtaining informed consent. 2. Patients who have a bone marrow donor who is 19 years or older and 70 years or younger (by full age) among their blood relatives. 3. Patients diagnosed with anti-aquaporin-4-immunoglobulin G (AQP4-IgG) positive neuromyelitis optica spectrum disorder based on serum testing at the time of screening. 4. Patients who have experienced a severe relapse† (first attack or acute relapse) within 28 days prior to screening and meet the following criteria. However, in patients with both transverse myelitis and optic neuritis, the criteria for the site of relapse are applied. \<Subgroup of Transverse Myelitis\> Expanded disability status scale (EDSS) scale of 4.0 or higher and 8.5 or lower. \<Subgroup of Optic Neuritis\> Best corrected visual acuity (BCVA) of 20/200 or lower in one or both eyes. †Relapse: New onset of neurological symptoms related to neuromyelitis optica or worsening of pre-existing neurological symptoms, objective changes on neurological examination (clinical findings or MRI findings) persisting for 24 hours or more, or the new onset of neurological symptoms or worsening of neurological symptoms requiring treatment. 5. Patients who can come to outpatient visits alone or with caregiver assistance 6. Women of childbearing potential who have not undergone sterilization must agree to use appropriate contraception\* until 12 months after the administration of the investigational product, and must provide evidence of not being in a childbearing state at the time of screening by meeting one of the following criteria: 7. For male patients who have not undergone a vasectomy: The patient must agree to use a barrier method of contraception (e.g., condoms) and ensure that he and his partner use appropriate contraception\* until 12 months after the administration of the investigational product, and must agree to refrain from donating sperm. 8. Patients who have been fully informed about this clinical trial, have voluntarily agreed to participate, and have given written consent to comply with the clinical trial's requirements. Exclusion Criteria: 1. Patients with any of the following cardiovascular diseases at the time of screening: * Myocardial infarction, unstable arrhythmia, and/or unstable angina within 6 months. * QTc interval ≥ 450 milliseconds or clinically significant changes on the electrocardiogram. * Congestive heart failure of New York Heart Association (NYHA) Class II or higher. * Stroke or transient ischemic attack (TIA) within 6 months. 2. Patients with a history of any other malignancy within 5 years prior to screening. 3. Patients who are HIV positive. 4. Patients deemed unsuitable for participation in this clinical trial by the investigator based on active hepatitis (HBV, HCV) test results. 5. Patients with acute or severe infections. 6. Patients for whom lumber puncture is contraindicated. 7. Patients with a history of major neurological diseases other than the target disease (including a history of polio). 8. Patients suspected of having demyelinating diseases other than the target disease or progressive multifocal leukoencephalopathy (PML). 9. Patients with a history of active infection within 4 weeks prior to screening (patients with conditions such as onychomycosis or dental caries may be allowed to participate in this clinical trial if deemed suitable by the investigator). 10. Patients who have undergone major surgery requiring general anesthesia within 4 weeks prior to screening. 11. Patients who are legally incapacitated, have active psychiatric disorders, or have severe neurological or psychiatric problems that could affect the conduct of the clinical trial. 12. Patients who have previously received other cell therapies. 13. Patients with hypersensitivity to bovine proteins, penicillin, or streptomycin antibiotics. 14. Patients with a history of hypersensitivity to any components of the investigational product. \<Related to contraindicated concomitant medications\> 15. Patients who require high-dose steroids exceeding 0.5 mg/kg/day or pulse steroid therapy at the baseline\*. 16. Patients who have received immunosuppressants or immunomodulators other than concomitantly allowable immunosuppressive therapy (IST), or any other investigational products within 12 weeks prior to screening (or within 5 times the medication's half-life). \<Related to laboratory tests\> 17. Patients whose laboratory test results at screening fall into any of the following categories: * ANC \<1,500/mm³ * Platelet count \<100,000/mm³ * Hemoglobin \<9.0 g/dL (Patients may be eligible if hemoglobin level recovers to 9.0 g/dL or higher. However, transfusions within 7 days prior to screening to meet this criterion are not allowed.) * Serum creatinine \>2.0xUNL * Total bilirubin \>2.0xUNL * AST, ALT \>3.0xUNL \<Others\> 18. Pregnant or breastfeeding women, or those with a positive pregnancy test at screening. 19. Patients who have been enrolled in another clinical trial within 12 weeks prior to screening (however, participation is allowed for those enrolled in non-interventional observational studies without the administration of investigational products). 20. Patients deemed unsuitable for participation in this clinical trial by the investigator for any other reasons.