<br>        Inclusion Criteria:<br><br>          1. Patients must be diagnosed as having either neuromyelitis optica (NMO) or NMO spectrum<br>             disorder (NMOSD), defined as the following:<br><br>               1. NMO as defined by Wingerchuk et al. 2006 criteria (requires all of the following<br>                  3 criteria: I. Optic neuritis, II. Acute myelitis, III. At least two of three<br>                  supportive criteria: Contiguous spinal cord lesion identified on a magnetic<br>                  resonance imaging (MRI) scan extending over 3 vertebral segments; Brain MRI not<br>                  meeting diagnostic criteria for multiple sclerosis (MS); NMO-IgG seropositive<br>                  status)<br><br>               2. NMOSD as defined by either of the following Wingerchuk 2007 criteria with<br>                  anti-AQP4 antibody (Ab) seropositive status at screening (i. Idiopathic single or<br>                  recurrent events of longitudinally extensive myelitis [=3 vertebral segment<br>                  spinal cord MRI lesion]; ii. Optic neuritis: recurrent or simultaneous<br>                  bilateral); For patients aged 12 to 17 years, a minimum of 4 patients should be<br>                  positive for anti-AQP4Ab status at screening<br><br>          2. Clinical evidence of at least 2 documented relapses (including first attack) in the<br>             last 2 years prior to screening, at least one of which has occurred in the 12 months<br>             prior to screening<br><br>          3. EDSS score from 0 to 6.5 inclusive at screening<br><br>          4. Age 12 to 74 years, inclusive at the time of informed consent<br><br>          5. One of the following baseline treatments must be at stable dose as a monotherapy for 8<br>             weeks prior to baseline: Azathioprine; Mycophenolate mofetil; Oral corticosteroids.<br>             For participants aged 12 to 17 years, either of the following baseline treatments for<br>             relapse prevention can be allowed: Azathioprine + oral corticosteroids; Mycophenolate<br>             mofetil + oral corticosteroids<br><br>          6. Ability and willingness to provide written informed consent and to comply with the<br>             requirements of the protocol<br><br>        For adolescents who may be enrolled after the end of the double-blind period, the inclusion<br>        criterion 2 is as follows (other criteria are same): Clinical evidence of at least 2<br>        documented relapses (including first attack) prior to screening.<br><br>        Exclusion Criteria:<br><br>        Exclusion criteria related to previous or concomitant therapy:<br><br>          1. Any previous treatment with IL-6 inhibitory therapy (e.g. tocilizumab), alemtuzumab,<br>             total body irradiation or bone marrow transplantation at any time<br><br>          2. Any previous treatment with anti-CD20, eculizumab, belimumab, interferon, natalizumab,<br>             glatiramer acetate, fingolimod, teriflunomide or dimethyl fumarate within 6 months<br>             prior to baseline<br><br>          3. Any previous treatment with anti-CD4, cladribine or mitoxantrone within 2 years prior<br>             to baseline<br><br>          4. Treatment with any investigational agent within 3 months prior to baseline<br><br>             Exclusions for general safety:<br><br>          5. Pregnancy or lactation<br><br>          6. For patients of reproductive potential, a positive result from a serum pregnancy test<br>             at screening, or not willing to use reliable means of contraception (physical barrier<br>             [patient or partner] in conjunction with a spermicidal product, contraceptive pill,<br>             patch, injectables, intrauterine device or intrauterine system) during the treatment<br>             period and for at least 3 months after the last dose of study drug<br><br>          7. Any surgical procedure (except for minor surgeries) within 4 weeks prior to baseline<br><br>          8. Evidence of other demyelinating disease or progressive multifocal leukoencephalopathy<br>             (PML)<br><br>          9. Evidence of serious uncontrolled concomitant diseases that may preclude patient<br>             participation, such as: other nervous system disease, cardiovascular disease,<br>             hematologic/hematopoiesis disease, respiratory disease, muscular disease, endocrine<br>             disease, renal/urologic disease, digestive system disease, congenital or acquired<br>             severe immunodeficiency<br><br>         10. Known active infection (excluding fungal infections of nail beds or caries dentium)<br>             within 4 weeks prior to baseline<br><br>         11. Evidence of chronic active hepatitis B or C<br><br>         12. History of drug or alcohol abuse within 1 year prior to baseline<br><br>         13. History of diverticulitis that, in the Investigator's opinion, may lead to increased<br>             risk of complications such as lower gastrointestinal perforation<br><br>         14. Evidence of active tuberculosis (TB; excluding patients receiving chemoprophylaxis for<br>             latent TB infection)<br><br>         15. Evidence of active interstitial lung disease<br><br>         16. Receipt of any live or live attenuated vaccine within 6 weeks prior to baseline<br><br>         17. History of malignancy within the last 5 years, including solid tumors, hematologic<br>             malignancies and in situ carcinoma (except basal cell and squamous cell carcinomas of<br>             the skin, or in situ carcinoma of the cervix uteri that have been completely excised<br>             and cured)<br><br>         18. History of severe allergic reaction to a biologic agent (e.g. shock, anaphylactic<br>             reactions)<br><br>         19. Active suicidal ideation within 6 months prior to screening, or history of suicide<br>             attempt within 3 years prior to screening<br><br>         20. Following laboratory abnormalities at screening*.<br><br>               1. White blood cells (WBC) <3.0 x10^3/microliter (µL)<br><br>               2. Absolute neutrophil count (ANC) <2.0 x10^3/µL<br><br>               3. Absolute lymphocyte count <0.5 x10^3/µL<br><br>               4. Platelet count <10 x 10^4/µL<br><br>               5. Aspartate aminotransferase (AST) or alanine aminotranferase (ALT) >1.5 times the<br>                  upper limit of normal (ULN) * If retest is conducted, the last value of retest<br>                  before randomization must meet study criteria.<br><br>        For adolescents who may be enrolled after the end of the double-blind period, the<br>        annotation in the exclusion criterion 20 is as follows (other criteria are same): * If<br>        retest is conducted, the last value of retest before baseline must meet study criteria<br>