CD20 B-cell Depletion

CD20 is a non-glycosylated phosphoprotein expressed on the surface of pre-B cells, mature B lymphocytes, and most B-cell malignancies, but absent from haematopoietic stem cells and plasma cells. Its precise physiological function remains incompletely understood, though it likely functions as a calcium channel and is involved in B-cell activation and proliferation. The restricted expression pattern — present on the target B-cell populations but absent from the precursors (protecting haematopoietic reconstitution) and terminally differentiated plasma cells (preserving existing antibody repertoire) — makes CD20 an ideal therapeutic target for both B-cell malignancies and autoimmune diseases driven by B-lymphocyte pathology.

Anti-CD20 monoclonal antibodies are classified as type I (rituximab, ofatumumab) or type II (obinutuzumab) based on their CD20 binding epitope and conformational changes induced upon binding. Type I antibodies translocate CD20 into lipid rafts, facilitating complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC). Type II antibodies such as obinutuzumab are glycoengineered to enhance FcgammaRIII binding and ADCC, and uniquely elicit direct non-apoptotic cell death, while showing reduced CDC. In CLL, obinutuzumab plus chlorambucil demonstrated superior PFS versus rituximab plus chlorambucil in the CLL11 trial, establishing a clinical correlate of these mechanistic differences.

In multiple sclerosis, CNS B-cell and plasmablast involvement in both relapsing and progressive disease has established CD20 depletion as a highly effective treatment strategy. Ocrelizumab (anti-CD20, IV, 6-monthly) demonstrated superiority over interferon-beta-1a in relapsing MS and became the first therapy approved for primary progressive MS in any regulatory jurisdiction. Ofatumumab (anti-CD20, SC, monthly) provides self-administration convenience with comparable efficacy. Ublituximab offers rapid B-cell depletion with a differentiated infusion schedule. Sustained B-cell depletion and the optimal timing of monitoring (B-cell repopulation, immunoglobulin levels) are important considerations for long-term management.