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.

  • ★ The most complete critique of Bm1–Bm5 in the wiki, from the lab proposing the alternative. Sanz2019 judges the scheme “less categorical than the IgD/CD27 approach” when applied to peripheral blood, and lists four specific failures: it fails to separate transitional (IgD⁺CD38ʰⁱ) from pre-GC (Bm2′) cells; it coalesces different types of memory B cells; it does not separate resting naive (Bm1) from IgD⁺ unswitched memory; and it does not distinguish conventional CD27⁺ memory from IgD⁻CD27⁻ DN cells — which themselves contain a heterogeneous mix of atypical/tissue-based/exhausted memory and activated extrafollicular PB precursors. The review also notes the scheme was designed for tonsil, with the help of CD10, CD44, CD77 and CD23, not for blood (see Sanz2019 - Consistent Classification of Human B Cell Populations, review — no original data). For this wiki the last failure is decisive: the Bm scheme cannot see the DN compartment at all.

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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