Ocrelizumab-Induced Leukocyte Subpopulations Dynamics in Multiple Sclerosis

Background/Objectives: Multiple sclerosis (MS) is a chronic autoimmune disease of the CNS characterized by immune-mediated inflammation and neurodegeneration. Based on its immunoablative effect targeting the CD20-positive B lymphocytes, ocrelizumab has emerged as a valid therapeutic option for MS. Although the on-treatment dynamics of leukocyte subpopulations are crucial for optimizing therapeutic strategies, little data are available in this field. Here, we aim to investigate how pre- and post-infusion immune-cell subsets change and correlate with each other, and how CD19+ B cell repopulation evolves over time. Methods: In an observational, retrospective, and prospective real-world study, we studied 51 relapsing-remitting MS ocrelizumab-treated patients who underwent flow cytometric immunophenotyping before and after each infusion and at three-month intervals, up to the eighth (4 years), for a total of 408 cycles. Results: After an initial decrease, post-infusion lymphocyte count returned to normal, while total white blood cells showed a sawtooth pattern driven by post-infusion increases. Granulocytes and natural killer cell counts remained substantially stable, while monocytes showed a significant cyclic pattern: increasing post-infusion, decreasing at the intermediate time point, with a more pronounced reduction pre-infusion, and rising again after re-administration. CD3+ and CD4+ T cells exhibited the same cyclic sawtooth pattern observed for monocytes. The CD4+/CD8+ ratio progressively increased, reflecting a significant elevation of CD4+, CD3+ and, to a lesser extent, CD8+ T cells. Pre-infusion CD19+ B cell counts progressively decreased, approaching zero by the fourth treatment year. Pre-treatment CD3+, CD4+, CD8+, lymphocyte, monocyte, and granulocyte counts differed significantly from on-treatment values. Correlation analyses showed that the major relationships among leukocyte subsets remained preserved throughout treatment. Conclusions: This study details leukocyte subpopulation dynamics during ocrelizumab therapy, unveiling previously unrecognized immunomodulatory properties beyond B-cell depletion. Ocrelizumab predominantly affects adaptive immunity, with potential indirect effects on innate immune compartments, and is associated with changes across immune-cell populations. These findings could have implications for individualized treatment strategies, but their direct application in this field still requires confirmatory studies on immunophenotyping and peripheral B-cell repopulation dynamics.