BackgroundOcrelizumab is an effective treatment for relapsing and primary-progressive multiple sclerosis (MS). However, the effect of ocrelizumab on leptomeningeal (LM) inflammation is unknown. ObjectiveTo investigate whether ocrelizumab reduces LM inflammation by reducing the exposure to Epstein-Barr virus (EBV)-infected B cells in relapsing-remitting (RR) MS. MethodsThis was a Phase IV, prospective, open-label, single-center, observational, longitudinal pilot study of RRMS patients who started treatment with ocrelizumab (NCT03025269). Clinical, MRI and EBV-antibodies outcomes at baseline, 12- and 24-month of the study were evaluated. The MRI outcomes included T2, T1 and T1-contrast enhancing (CE) lesion counts and volumes, LM CE count, and percentage brain volume changes. Results27 RRMS patients started ocrelizumab and 24 remained on the treatment for whole duration of the study. Most patients remained stable (74.1%) or improved (18.5%) in their disability status. At baseline, 42.3% of patients showed LM CE lesions. The majority of patients remained stable in their LM CE status over the follow-up (72.7%). A significant decrease in percentage volume loss of cortex (p=0.009), GM (p=0.01) and thalamus (p=0.038) was detected, while T1-LV increased (p=0.02). A significant decrease of EBNA-1 IgG (p=0.013) was evidenced. An infusion-related allergic reaction led to discontinuation of the medication in one patient at first dose. ConclusionsTreatment with ocrelizumab was safe and clinically effective. Brain volume loss and accumulation of T1-LV occurred. While ocrelizumab decreased humoral response to EBV possibly by reducing B cells, it did not reduce LM inflammation.