CD20 in Lymphoma: A Predictive Biomarker for Anti-CD20 Therapy

Introduction

Few molecules in diagnostic hematopathology have shaped clinical practice as profoundly as CD20. It serves a dual role: it is a lineage marker that helps identify B-cell lymphomas, and it is a predictive biomarker that informs eligibility for anti-CD20 monoclonal antibody therapy. This article explains what CD20 testing measures, how it is performed, how results are interpreted, and what those results mean for treatment eligibility — framed educationally for pathologists, oncologists, trainees, and informed patients.

What the Test Measures

CD20 is a cell-surface protein expressed by B lymphocytes across most of their maturation. Its expression begins at the pre-B/immature B-cell stage and is present on mature B cells, but it is characteristically lost as B cells terminally differentiate into plasma cells. This biology matters clinically for two reasons. First, the presence of CD20 confirms B-cell lineage in the workup of a lymphoid neoplasm. Second, because CD20 sits on the cell surface, it is accessible to therapeutic antibodies — making it an ideal drug target [2].

The therapeutic exploitation of CD20 began with rituximab, a monoclonal antibody that transformed the management of B-cell lymphomas and has been described as launching a genuine revolution in antibody-based cancer therapy [2]. Subsequent "successor" anti-CD20 antibodies have followed the same principle: engaging the CD20 antigen on malignant B cells to trigger their destruction.

How It Is Tested

CD20 expression is assessed by two complementary methods.

Immunohistochemistry (IHC) is applied to formalin-fixed, paraffin-embedded (FFPE) tissue — the standard specimen for tissue biopsies and excisions. IHC uses an anti-CD20 antibody to stain the tissue, producing a membranous staining pattern in positive cells that the pathologist evaluates in the context of tissue architecture. This allows CD20 status to be correlated directly with morphology.

Flow cytometry is performed on fresh specimens — peripheral blood, bone marrow aspirate, fluid, or disaggregated tissue. Flow provides quantitative, single-cell measurement of surface CD20 and is especially valuable for characterizing circulating or bone-marrow-based disease, and for resolving admixed populations.

Preanalytic constraints differ by method. IHC depends on adequate fixation of FFPE tissue; flow cytometry requires viable, fresh cells and must be performed promptly, since cell viability degrades with time. The choice of assay therefore depends on the specimen available and the clinical question.

Scoring for CD20 as a predictive marker is fundamentally categorical: expression is either present or lost. Unlike some biomarkers that rely on graded percentage thresholds, the clinically actionable distinction here is whether the malignant B cells display CD20 that a therapeutic antibody could engage.

What Each Result State Means

CD20-positive (CD20+): The malignant B cells express CD20. In the diagnostic setting, this supports B-cell lineage. In the predictive setting, it indicates the tumor bears the target of anti-CD20 monoclonal antibodies.

CD20-negative (CD20−): The malignant cells do not display detectable CD20. This result requires careful interpretation because it can arise from several distinct scenarios:

• Plasmacytic differentiation. Because CD20 is normally downregulated as B cells mature into plasma cells, neoplasms with plasmacytic phenotype may be genuinely CD20-negative as a matter of their underlying biology.
• Post-rituximab loss. CD20 expression can be lost after anti-CD20 treatment. A tumor that was CD20-positive at diagnosis may become CD20-negative on a subsequent biopsy following therapy — an important consideration when evaluating relapsed or refractory disease.

The distinction between a lymphoma that never expressed CD20 and one that has lost expression under therapeutic pressure is clinically meaningful, and it underscores why repeat biopsy and repeat biomarker assessment at relapse can be informative.

What It Determines for Treatment Eligibility

The central predictive function of CD20 is straightforward: CD20 is the target of anti-CD20 therapy (rituximab and its successors) [2]. A CD20-positive result indicates that the tumor expresses this target and therefore informs eligibility for the anti-CD20 monoclonal antibody drug class.

It is essential to frame this correctly. CD20 status is one input into a treatment decision — it establishes whether the therapeutic target is present, not whether any individual patient should receive a specific agent. Conversely, loss of CD20 expression has therapeutic implications, because a target that is absent cannot be engaged by an antibody directed against it. When CD20 is lost — whether through plasmacytic biology or after prior anti-CD20 exposure — the rationale for anti-CD20 therapy is correspondingly weakened. These are educational statements about drug-class eligibility, not individualized treatment recommendations.

Caveats and What Is Evolving

Several nuances deserve emphasis:

• CD20 loss is dynamic. Because expression can change over the course of disease and treatment, a single CD20 result is a snapshot. The evidence suggests that antigen loss after anti-CD20 exposure is a recognized mechanism relevant to relapsed disease.
• Assay concordance. IHC and flow cytometry measure the same antigen through different technical routes and with different specimen requirements. Apparent discordance can reflect preanalytic factors, differing sensitivities, or sampling of different disease compartments rather than true biological difference.
• Classification context. CD20 interpretation always occurs within the framework of modern lymphoma classification, which integrates morphology, immunophenotype, and molecular findings to arrive at a diagnosis [1]. CD20 is one marker within that integrated assessment, not a standalone determinant.
• Categorical, not graded. For CD20 as a predictive marker, the actionable question remains presence versus loss. This contrasts with biomarkers in other tumor types where contested intermediate cutoffs (for example, "low-expression" categories seen with certain solid-tumor markers) have generated ongoing debate. CD20's predictive use rests on the more binary presence-versus-absence framework, though careful interpretation of "negative" results — distinguishing true absence from technical false-negativity — remains a practical challenge.

Summary

CD20 is both a diagnostic lineage marker and a predictive biomarker. Assessed by IHC on FFPE tissue or by flow cytometry on fresh specimens and scored as present versus lost, it identifies whether a B-cell lymphoma bears the target of anti-CD20 monoclonal antibody therapy. A positive result informs eligibility for the anti-CD20 drug class; a negative result — whether from plasmacytic biology or post-treatment loss — carries therapeutic implications that must be interpreted within the full diagnostic context.

References

1. Alaggio R, Amador C, Anagnostopoulos I, et al. The 5th edition of the WHO Classification of Haematolymphoid Tumours: Lymphoid Neoplasms. Leukemia. 2022. doi:10.1038/s41375-022-01620-2.
2. Weiner GJ. Anti-CD20 monoclonal antibodies: reviewing a revolution. Seminars in Hematology. 2019. PMCID: PMC6343614.