EFB Dengue Literature Review

CD20

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CD20

Overview

CD20 (MS4A1) is a membrane-spanning calcium channel expressed on B lineage cells from the pre-B stage through mature B cells. Its expression is lost on plasma cells and plasmablasts, and is not present on early bone marrow B cell precursors. In B cell immunology, CD20 serves two roles: (1) as a flow cytometry marker that distinguishes mature B cells from plasma cells and plasmablasts, and (2) as the therapeutic target of rituximab (anti-CD20 monoclonal antibody), which depletes CD20⁺ B cells via ADCC, complement-dependent cytotoxicity, and apoptosis.

The absence of CD20 on plasmablasts and plasma cells has important implications: rituximab depletes the CD20⁺ B cell compartment but cannot directly eliminate ongoing antibody-secreting cells, leading to the dissociation between B cell depletion and serologic response observed in clinical trials.

Key Points from Literature

  • Plasmablast identification: Plasmablasts are CD20⁻, enabling their discrimination from CD20⁺ B cells. In flow cytometry, the combination CD38^high, CD19^low, CD20⁻ defines peripheral blood plasmablasts; CD38^high, CD19⁺, CD20⁺ defines the pre-GC (Bm2ʹ) population (see Anolik2004 - Rituximab and B Cell Abnormalities in SLE, n=15 SLE patients, n=5 controls).
  • Indirect plasmablast depletion after rituximab: Despite being CD20⁻, circulating plasmablasts declined rapidly in select patients after rituximab (e.g., patient 11: 40% → 14% at 2 months). This implies a short-lived plasmablast pool continuously replenished by CD20⁺ B cell precursors. Plasmablasts that persist after CD20⁺ precursor depletion likely represent a long-lived subpopulation or cells homing to survival niches (see Anolik2004 - Rituximab and B Cell Abnormalities in SLE).
  • CD20 downregulation upon B cell activation: B cell activation is associated with downregulation of surface CD20 via movement into lipid rafts and endocytosis — a potential mechanism for rituximab resistance in highly activated SLE B cells (Anolik et al. 2003, Eur J Immunol — cited but not yet ingested).
  • Residual B cells after effective depletion: After 95–99% B cell depletion, residual cells are predominantly switched memory with a CD20⁺ phenotype (CD20⁺, CD38^low, CD27⁺, IgD⁻). Their persistence may reflect trafficking from peripheral lymphoid tissue rather than incomplete depletion per se (see Anolik2004 - Rituximab and B Cell Abnormalities in SLE).

Contradictions & Debates

  • Pre-GC (Bm2ʹ) cells express CD20 and are therefore in principle susceptible to rituximab, yet patients with high pre-GC baseline frequencies were more likely to have incomplete depletion. Whether this reflects CD20 downregulation, intrinsic resistance, or ongoing antigen-driven GC reactions replenishing these cells faster than rituximab can deplete them is unresolved (see Anolik2004 - Rituximab and B Cell Abnormalities in SLE).

  • CD20⁻/low as original dengue plasmablast gate criterion: In the first systematic dengue plasmablast study, Wrammert2012 defined plasmablasts as CD19⁺CD3⁻CD20⁻/low CD27^high CD38^high. The CD20⁻/low criterion is the core plasmablast discriminator, consistent with the SLE data. This gating was performed on whole blood with BD Trucount for absolute enumeration (see Wrammert2012 - Plasmablast Responses in Acute Dengue, 5-color conventional FCM).

  • CD20⁻ criterion in dengue plasmablast gate: In the GarciaBates2013 gating scheme, CD20⁻ combined with CD38⁺ within the CD27⁺CD21⁻ fraction distinguishes plasmablasts from activated memory B cells (CD20⁺CD38⁻/lo). This confirms the CD20⁻ plasmablast phenotype in dengue, consistent with SLE data (see GarciaBates2013 - Plasmablast Response and Dengue Severity, LSRII FCM).

  • CD20⁻/⁺ as primary PB vs. MBC discriminator in dengue clonal analysis: Appanna2016 used CD20 as the key bifurcation marker: CD19⁺CD20⁻CD27^hiCD38^hi for plasmablasts (days 3–7) vs. CD19⁺CD20⁺CD27⁺ for memory B cells (days 16–166). The clonal disconnect between these two CD20-defined compartments (very few shared CDR3s, all IgM) demonstrates that CD20 loss marks a functionally and clonally distinct effector population during acute dengue (see Appanna2016 - Plasmablasts as Subset of Memory B Cell Pool, FACSAria, n=12 dengue).

Plasmablast, Double-Negative B Cell, CD19, CD38, Bm Classification, Germinal Center

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