<h3>Objective:</h3> To explore immunological effects associated with ocrelizumab treatment in the blood of early, previously treatment-naive multiple sclerosis (MS) patients by multiparameter flow cytometry. <h3>Background:</h3> Dysregulation in immune regulatory networks is one hallmark of MS pathophysiology, particularly in early relapsing-remitting MS (RRMS). Considering the efficacy of B-cell-depleting therapies, it is of particular interest to explore the impact of ocrelizumab on peripheral immune signatures in a previously treatment-naive cohort. <h3>Design/Methods:</h3> Patients (treatment-naive, diagnosis of early-stage RRMS [age 20–56 years; disease duration ≤3 years]) received ocrelizumab 600 mg every 24 weeks. We performed comprehensive characterization of the quantitative changes of circulating T-cells, B-cells, NK-cells, myeloid cells, dendritic cells and their subsets and function in 65 patients by multicolor flow cytometry, measured relative to baseline (pre-infusion) at 24 and 48 weeks after first ocrelizumab infusion. Median cell counts (cells/μL) were evaluated using Kruskal-Wallis test followed by Dunn’s test with Bonferroni correction. <h3>Results:</h3> Analysis showed a significant (p<0.0001), stable and virtually complete depletion of all B-cell subsets including naive, memory, transitional and regulatory B-cells after 24 and 48 weeks. A transient increase (p<0.01) in T-cell subset (CD4/CD8 naive, memory, Th1, Th2, Th17, Tregs) counts was observed after 24 weeks that returned back to baseline levels after 48 weeks. This transient increase was accompanied by an increase in CD69 expression levels (CD4/CD8 T-cells) and HLA-DR+ CD8 T-cell counts at Week 24 compared with baseline, which decreased until 48 weeks. Notably, there was no reduction of CD4 and CD8 T-cells and their subsets up to 48 weeks. <h3>Conclusions:</h3> Ocrelizumab induced a significant depletion of all B-cell subsets in early-stage RRMS in ENSEMBLE. The changes induced on other lymphocyte subsets remained transient and there was no reduction in T-cell counts and the subsets analyzed as a consequence of B-cell depletion in the first year of treatment. <b>Disclosure:</b> Timo Wirth has nothing to disclose. Andreas Schulte-Mecklenbeck has nothing to disclose. Mrs. Steinberg has nothing to disclose. Dr. Raposo has received personal compensation for serving as an employee of F. Hoffman-La Roche. Dr. Raposo has received stock or an ownership interest from F.Hoffman-La Roche. The institution of Catharina Groß has received research support from Biogen. The institution of Catharina Groß has received research support from Roche. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Biogen. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Bristol Myers Squibb. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for EMD Serono. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Novartis. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Roche. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Sanofi. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Immunovant. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Johnson&Johnson / Janssen. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for UCB. Dr. Wiendl has received personal compensation in the range of $0-$499 for serving as a Consultant for Idorsia. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Actelion. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Argenx. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Gossamer Bio. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Immunic. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Merck. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for NexGen. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for PSI CRO. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Worldwide Clinical Trials. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for BD. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Lundbeck. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Merck Serono. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Novartis. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Janssen. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Alexion. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Argenx. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Bristol Myers Squibb. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Sandoz. The institution of Dr. Wiendl has received research support from Biogen. The institution of Dr. Wiendl has received research support from Roche. The institution of Dr. Wiendl has received research support from Deutsche Forschungsgemeinschaft. The institution of Dr. Wiendl has received research support from European Union. The institution of Dr. Wiendl has received research support from Alexion. The institution of Dr. Wiendl has received research support from Argenx. The institution of Dr. Wiendl has received research support from CSL Behring. The institution of Dr. Wiendl has received research support from Merck. The institution of Dr. Wiendl has received research support from UCB. Dr. Wiendl has received publishing royalties from a publication relating to health care. Dr. Wiendl has received publishing royalties from a publication relating to health care. Dr. Klotz has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Bayer. Dr. Klotz has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Biogen. Dr. Klotz has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Genzyme. Dr. Klotz has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Bristol Myers Squibb. Dr. Klotz has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Merck Serono. Dr. Klotz has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Novartis. Dr. Klotz has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Teva. Dr. Klotz has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Roche. Dr. Klotz has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Roche. Dr. Klotz has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Novartis. Dr. Klotz has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Alexion. Dr. Klotz has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Merck. Dr. Klotz has received personal compensation in the range of $5,000-$9,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Sanofi. Dr. Klotz has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Horizon. Dr. Klotz has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Viatris. Dr. Klotz has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for BMS. The institution of Dr. Klotz has received research support from Immunic AG. The institution of Dr. Klotz has received research support from Merck.