Neuromyelitis optica spectrum disorder after treatment with pembrolizumab.

While immune checkpoint inhibitors (ICIs) have contributed to the development of therapeutic treatments for previously incurable advanced malignancies, they may induce immune-related adverse events (irAEs) in many organs including the CNS [1]. Because immune checkpoint molecules are predominantly expressed on T cells, irAEs are largely not B cell-mediated. Here, we report a patient who was treated with pembrolizumab (a PD-1 monoclonal antibody) for lung adenocarcinoma with brain metastasis, and who developed anti-aquaporin-4 antibody (AQP4-Ab) positive neuromyelitis optica spectrum disorder (NMOSD). We hypothesized that PD-1 immune checkpoint blockage might induce a B cell-mediated immune response in CNS resulting in this complication, which was further supported by the observation of a transient increase in plasmablasts in their CSF.