Anti-CD20 Therapy in B-Cell Disorders

CD20 Function

The function of CD20 on normal and malignant B cells is not yet fully understood. There is some evidence that CD20 is intimately linked to the B-cell receptor (BCR) signalling pathway, as well as to the microenvironmental interactions. The few mutations in MS4A1 that cause a complete absence of the cell surface protein CD20 result in decreased numbers of memory B cells in vivo, decreased immunoglobulin IgG levels, and an otherwise clinically impaired response to T-independent antigens (for example, pneumococcal polysaccharides) with a normal response to T-dependent antigens (for example, tetanus toxoid). CD20 can also be found in "lipid raft" microdomains and directly co-localize with the BCR. After BCR activation, CD20 is highly phosphorylated, and displays calcium channel-like properties, suggesting a role in calcium influx and downstream signaling. siRNA-mediated downregulation of CD20 on human B-cell lines strongly inhibits BCR-mediated calcium influx, and direct anti-CD20 cross-linking is also able to induce calcium signalling and downstream gene expression responses that resemble those upon BCR cross-linking. CD20 is upregulated on cells that exit lymph nodes and enter the peripheral blood of CLL patients through the CXCR4-SDF1 pathway, and the microenvironment-associated factors IL-4, TNFα and IFNα can also induce CD20 expression in vitro. This might at least partly explain the suboptimal activity of the BTK-inhibitor ibrutinib combined with rituximab (anti-CD20): ibrutinib blocks both BCR-signalling and the CXCR4-pathway, causing downregulation of CD20 and thereby attenuating its presence as a target for antibody action. In contrast, obinutuzumab requires lower CD20 levels and shows better combination efficacy. Additionally, CD20-deficient mice exhibit a 20–30% reduction in surface IgM levels and impaired BCR/CD19-dependent calcium mobilization, which some studies attribute to impaired calcium transport rather than storage release issues. Silencing CD20 in malignant B cells affects the phosphorylation of BCR-associated kinases (LYN, SYK) and downstream proteins (GAB1, ERK), suggesting that CD20 may participate in both "initiating and sustaining" BCR signaling or act as a "basal regulator."

Anti‑CD20 Monoclonal Antibodies

CD20 has been proposed to work as an ion channel, or to indirectly modulate calcium release triggered by the B-cell antigen receptor. Depletion of B cells with anti-CD20 monoclonal antibodies affects not only circulating B cells, but also a fraction of CD3⁺, CD4⁺, and CD8⁺ T cells that express CD20. The B-cell depletion results from a combination of different mechanisms, including apoptosis, complement-dependent cytotoxicity and ADCC. Stem cells, pro-B cells, plasmablasts and antibody-secreting plasma cells are not targeted, as they do not express CD20. Four anti-CD20 monoclonal antibodies are currently approved for the treatment of multiple sclerosis: rituximab, ocrelizumab, ofatumumab and ublituximab. The four drugs have different molecular structure, epitope targeted, and pharmacologic properties, including route of administration and dosing regimen, as well as mechanism of B-cell depletion, patterns of B-cell repopulation, risk profile and immunogenicity. Their capacity to mediate CDC and ADCC after binding also differs.

Table 1. Pre-treatment testing and safety monitoring for anti-CD20 monoclonal antibody therapy

(Source: Carlson AK, et al. 2024)

B cell depletion achieved by administration of mAbs can lead to infusion-related reactions (IRRs) and injection-related reactions. A number of mechanisms have been proposed, of which complement-dependent cytotoxicity is a relatively potent effector. Compared with rituximab and ofatumumab, ocrelizumab and ublituximab have a lower incidence of infusion or injection-related reactions likely due to their weak CDC activity. B cells have a protective role in infection and tumor surveillance, and this may occur by both antibody-dependent and antibody-independent pathways. Since plasma cells do not express CD20, long-term administration of anti-CD20 can lead to hypogammaglobulinemia. In this scenario, several management approaches are possible, including dose reduction or prolongation of mAb dosing interval, intravenous or subcutaneous supplementation of immunoglobulin, or discontinuation and switch to another DMT.

Anti-CD20 therapies have also been shown to affect T cell populations. It was found that anti-CD20 therapy not only directly depletes CD20⁺ T cells, but can also indirectly modulate other T cell populations by disrupting B:T cell interactions. In hematological studies, the number of CD20⁺ T cells was observed to rapidly decline in MS patients following treatment with ocrelizumab. CD3⁺CD20⁺ T cells were demonstrated to be nearly completely eradicated by 12 weeks of treatment with rituximab. In a different study, ofatumumab was also demonstrated to have the ability to modulate regulatory T cells with resultant decreases in non-suppressive regulatory T cells, increases in naive regulatory T cells and a reduced ratio of circulating T follicular helper cells to circulating T follicular regulatory cells. Other studies have also shown similar changes to T cell populations following ocrelizumab treatment. While the clinical relevance of CD20⁺ T cell depletion remains to be determined, it has been proposed as one possible mechanism of action of anti-CD20 monoclonal antibody therapies.