Trial: Observational post-hoc analysis of the phase 3 ORATORIO PPMS study assessing choroid plexus (CP) MRI changes with ocrelizumab vs placebo. Drug/intervention: Ocrelizumab—humanized IgG1 monoclonal antibody (immunotherapy) targeting CD20 on B cells, depletes circulating CD20+ B cells via antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity, and apoptosis, reduces B-cell antigen presentation, costimulation, and proinflammatory cytokines. Cells/pathways targeted: CD20+ B lymphocytes (pre-B to memory, not plasma cells), downstream T-cell activation, and CNS autoimmune inflammation at the CP/CSF barrier, with CP enhancement on T1/T2-FLAIR MRI as the readout.