This study will evaluate the safety and efficacy of MIL62 in patients with Neuromyelitis Optica Spectrum Disorder.

Inclusion Criteria:

1. Patients who meet the diagnostic criteria for NMOSD established by the International Panel for NMO Diagnosis (IPND) in 2015 and are seropositive for AQP4-IgG.
2. Male or female patients aged 18 to 70 years, inclusive of the endpoints..
3. Expanded Disability Status Scale(EDSS) score ≤ 7.
4. Phase Ib: At least 1 attack of NMOSD requiring rescue treatment within 2 years prior to screening. Phase III: At least 1 attack of NMOSD requiring rescue treatment within 1 year prior to screening; or at least 2 attacks of NMOSD requiring rescue treatment within 2 years prior to screening, including the first attack.
5. Glucocorticoid treatment prior to screening is allowed, and within 14 days before the first administration, the dose should be ≤20 mg/day of prednisone or its equivalent dose of glucocorticoids.
6. Patients who had an attack of the disease before screening must have stable or improved attack symptoms for at least 4 weeks prior to the first administration.
7. Voluntarily sign the informed consent form.

Exclusion Criteria:

1. Subjects who have received Rituximab, Inebilizumab, Ozanimod, Telitacicept, or any B-cell depleting drugs within 6 months prior to the screening period are allowed to enroll if their CD19 or CD20 positive B-cell counts are above the lower limit of normal; or if their CD4 positive T-lymphocyte counts are < 200 cells/μL.
2. Having used Tocilizumab, Satralizumab, Eculizumab, Efgartigimod, or other non-B-cell depleting biological agents with therapeutic effects on NMOSD, or mitoxantrone, or alkylating agents such as cyclophosphamide within 3 months prior to the first administration.
3. Phase Ib: Subjects who have used immunosuppressants such as azathioprine, mycophenolate mofetil, tacrolimus, cyclosporine, methotrexate, and cyclophosphamide before the first administration are eligible for enrollment, provided that the interval since discontinuing the drugs exceeds 5 times their half-life. Phase III: Subjects who have used immunosuppressants other than glucocorticoids within 1 month prior to the first administration, including but not limited to azathioprine, mycophenolate mofetil, tacrolimus, cyclosporine, and methotrexate, are excluded (exclusion is not required if the continuous use duration is ≤7 days).
4. Within 28 days prior to the first administration, plasma exchange (PE), moderate blood transfusion, or immunomodulatory drugs such as interferon β, interferon γ, or intravenous immunoglobulin (IVIG) have been used.
5. Live vaccines or attenuated vaccines were administered within 28 days prior to the first dose.