EFB Dengue Literature Review

Bm Classification

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Bm Classification

Overview

The Bm (mature B cell) classification is an IgD/CD38-based flow cytometry gating framework that assigns peripheral blood B cells to developmental stages from naive through germinal centre to memory and plasma cell. Originally described by Pascual et al. (1994, J Exp Med) for tonsillar B cell subpopulations and extended to peripheral blood by Bohnhorst et al. (2001, J Immunol), it provides a CD27-independent orthogonal view of B cell differentiation complementary to the standard IgD/CD27 four-quadrant scheme.

The Bm framework became widely used in B cell immunology — particularly in SLE and infection studies — because it resolves the pre-germinal centre (Bm2ʹ) population and distinguishes transitional from naive cells, which the IgD/CD27 scheme conflates in the IgD⁺CD27⁻ naive gate.

Key Points from Literature

  • Bm1 (mature naive): IgD⁺, CD38⁻/^low, CD27⁻; resting naive B cells. In practice, Bm1 and Bm2 are often pooled as the “mature naive” gate (see Wei2007 - DN Memory B Cells in SLE; Anolik2004 - Rituximab and B Cell Abnormalities in SLE).
  • Bm2 (activated naive): IgD⁺, CD38^low, CD27⁻; includes CD23⁺ cells; partially overlaps with Bm1 depending on CD38 threshold.
  • Bm2ʹ (pre-GC / GC founder): IgD⁺, CD38^high, CD19⁺, CD20⁺, CD10⁺; these circulating pre-GC cells are expanded in SLE and were reported expanded in children with SLE (Arce et al. 2001, cited in Anolik2004 - Rituximab and B Cell Abnormalities in SLE). In healthy donors, Bm2ʹ cells represent a minor fraction (~4.7 ± 2.8%; see Anolik2004 - Rituximab and B Cell Abnormalities in SLE, n=5 controls).
  • Bm3/4 (GC B cells): IgD⁻, CD38^high, CD19⁺, CD20⁺; these are the dark-zone (Bm3) and light-zone (Bm4) GC B cells; present at very low frequency in peripheral blood under non-inflammatory conditions.
  • Early Bm5: IgD⁻, CD38^dull; newly emigrated memory cells; enriched in SLE relative to healthy donors (see Wei2007 - DN Memory B Cells in SLE).
  • Bm5 (resting memory): IgD⁻, CD38⁻/^low, CD27⁺ (majority); the predominant peripheral blood memory B cell gate; includes both IgG⁺ and IgA⁺ switched memory cells. A CD27⁻ (DN) fraction within Bm5 is IgD⁻CD38⁻CD27⁻ — these are the double-negative memory B cells (see Double-Negative B Cell; Wei2007 - DN Memory B Cells in SLE).
  • Plasma cells/plasmablasts (above Bm5): IgD⁻, CD38^very high (highest expression), CD19^low, CD20⁻; distinct from Bm3/4 by CD20 absence and CD19 downregulation (see Plasmablast; Anolik2004 - Rituximab and B Cell Abnormalities in SLE). In SLE this gate reaches 18.5 ± 17.9% of CD19⁺ B cells vs. 0.24 ± 0.23% in healthy controls (P=0.001; see Anolik2004 - Rituximab and B Cell Abnormalities in SLE, n=15 SLE, n=5 controls).
  • Transitional B cells: CD38^high, CD24^high, IgM^high; they overlap with Bm2ʹ in CD38 expression but are distinguishable by CD24 and IgM co-expression.

Practical Notes on Gating

  • The Bm2ʹ (pre-GC) and Bm3/4 (GC) gates are both CD38^high and IgD⁻/^low; the key distinguishing marker is IgD (retained in Bm2ʹ, lost in Bm3/4) and CD20 (expressed on both; absent on plasmablasts).
  • CD10 adds resolution: Bm2ʹ pre-GC cells are CD10⁺; mature memory Bm5 and DN cells are CD10⁻. This helps exclude transitional/pre-GC contaminants from the memory gate (see Conventional Flow Cytometry).
  • DN B cells (IgD⁻CD27⁻) fall within the Bm5 gate on an IgD/CD38 plot: they are IgD⁻ and CD38^low/⁻, indistinguishable from CD27⁺ Bm5 memory cells without adding CD27 to the panel.
  • The Bm framework was originally described for tonsil tissue; peripheral blood Bm subsets differ in frequency distribution and include far fewer Bm3/4 GC cells under steady-state conditions.

Contradictions & Debates

  • The Bm1/Bm2 boundary is instrument- and threshold-dependent; different groups set CD38 positivity at different points, leading to inconsistent reporting of the “activated naive” fraction.
  • Bm2ʹ pre-GC cells are expanded in some SLE patients but also in healthy individuals after vaccination or infection, making the clinical specificity of this finding context-dependent.
  • Whether the Bm framework or the IgD/CD27 four-quadrant scheme better captures biologically meaningful B cell subsets is debated; most rigorous studies use both axes simultaneously (adding CD27 to the IgD/CD38 plot) to resolve the ambiguity.

Conventional Flow Cytometry, Double-Negative B Cell, Plasmablast, Memory B Cell, CD38, IgD, CD27, Germinal Center

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