The purpose of this study is to understand the effects of long-term treatment with inebilizumab on circulating levels of immunoglobulins, B-cell counts, and other safety measures, and to further monitor repletion of immunoglobins and B-cell counts in participants with NMOSD who discontinue treatment. The objectives include: 1. To establish the nadir in circulating immunoglobulins (Ig) during chronic treatment with inebilizumab and ascertain the time needed to ensure restoration of pre-treatment baseline serum levels of IgG and IgM after discontinuation of treatment 2. To characterize B-cell counts throughout treatment with inebilizumab and after discontinuation until repletion of Immunoglobulin (Ig levels) 3. To assess long-term safety of inebilizumab 4. To assess other long-term effects of inebilizumab