Sanz2019 - Consistent Classification of Human B Cell Populations

Tier B — review with zero original data. All figures are adapted from other publications, principally Jenks2018 - DN2 B Cells and EF Pathway in SLE. Its value is definitional and methodological, not evidentiary. Do not let it carry a mechanism claim alone.

Full citation: Sanz, I., Wei, C., Jenks, S. A., Cashman, K. S., Tipton, C., Woodruff, M. C., Hom, J., & Lee, F. E.-H. (2019). Challenges and opportunities for consistent classification of human B cell and plasma cell populations. Frontiers in Immunology, 10, 2458. https://doi.org/10.3389/fimmu.2019.02458

Raw file: [[raw/Sanz2019.pdf]]

Summary

The Sanz lab’s nomenclature paper. Its argument is that the human B cell field is generating irreconcilable results because populations are defined with too few, non-discriminating, and inconsistently applied surface markers — and because functional labels (“exhausted,” “anergic,” “regulatory”) get attached to populations that were never characterised well enough to earn them.

It proposes a 7-marker core panel and a canonical phenotype table covering every major human peripheral B cell population, including the DN compartment. For this wiki its most useful content is threefold: the recommended minimum marker set, the explicit phenotypic definitions of DN1 vs DN2 vs the FcRL4⁺ DN population, and a direct demonstration that the DN cells of SLE and of HIV are not the same cell — FcRL5 and FcRL4 are reciprocally expressed between them.

Study Design

• Type: Narrative review / methodological position paper. No original data.
• Sample size: Not applicable. Illustrative FACS plots are re-used from prior publications (chiefly Jenks et al. 2018 and Lee lab ASC work)
• Setting: Human peripheral blood, with reference to tonsil, spleen, bone marrow and non-malignant lymph node
• Population: Healthy donors, SLE, rheumatoid arthritis, scleroderma, acute and chronic HIV, post-vaccination

Key Findings

★ The recommended core panel

• Proper analysis of the major canonical human B cell subsets requires 7 markers plus dead-cell and doublet exclusion: (1) non-B lineage exclusion (CD3, CD14); (2) CD19; (3) IgD; (4) CD27; (5) CD38; (6) CD24; (7) CD21
• This set supports both widely used schemes simultaneously — IgD vs CD27 and IgD vs CD38 (Bm1–Bm5)
• The authors recommend initial categorisation on IgD, CD27 and CD38 together with CD24, and strongly recommend CD21 in the core set because it identifies activated cells within every parental population
• CD19 is preferred over CD20 when markers are limited, because CD19 intensity carries information about activation and helps distinguish short- from long-lived PC. CD20 adds discriminating power for ASC
• The Bm1–Bm5 scheme is judged less categorical than IgD/CD27 when applied to blood: it fails to separate transitional (IgD⁺CD38^hi^) from pre-GC (Bm2′), coalesces different memory types, does not separate resting naive (Bm1) from IgD⁺ unswitched memory, and 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 canonical DN definitions (Table 1)

• DN1: IgD⁻ CD27⁻ CD38⁺ CD24⁺ CD21⁺; FcRL4⁻ FcRL5⁻ CXCR5⁺; IgM/IgG/IgA⁺ → function given as “memory precursors”
• DN2: IgD⁻ CD27⁻ CD38⁻ CD24⁻ CD21⁻; T-bet⁺ CD11c⁺ FcRL5⁺ SLAMF7⁺ CXCR5⁻; IgM/IgG/IgA⁺ → function given as “extrafollicular ASC precursors”
• A third DN population: IgD⁻ CD27⁻ CD38⁻ CD24⁻ CD21⁻ FcRL4⁺ → “atypical/tissue-based memory” — phenotypically distinct from DN2 and listed separately
• Activated naive: IgD⁺ CD27⁻ CD38⁻ CD24⁻ CD21⁻; CD95⁺ CD23⁻ CD11c⁺ T-bet⁺ FcRL5⁺ SLAMF7⁺ CXCR5⁻ — described as “precursor of short-lived PB and GC reactions”
• Atypical tissue-based memory (CD27⁺): IgD⁻ CD27⁺ CD38^lo^ CD24^lo^ CD21⁻; FcRL4⁺ FcRL5⁺ → “mucosal surveillance; exhausted memory; BCR hypo-responsive memory”
• Note that activated naive and DN2 share essentially the whole activation signature (CD21⁻CD24⁻CD38⁻CD11c⁺T-bet⁺FcRL5⁺SLAMF7⁺CXCR5⁻) and differ by IgD alone

★ SLE DN cells and HIV DN cells are phenotypically opposite

• In DN B cells across RA, SLE, scleroderma, and acute and chronic HIV, FcRL4 and FcRL5 show a reciprocal pattern: FcRL4 is high in HIV DN2-gated cells (~21.1%) and near-absent in SLE (~0.74%), while FcRL5 is high in SLE but not HIV
• CD11c is highly expressed on SLE DN cells and is taken as indicative of DN2 as activated effector cells — explicitly contrasted with the HIV atypical memory B cells “which are thought to be anergic”
• The authors’ conclusion: this heterogeneity “is indicative of multiple DN B cell populations, and can only be resolved by further marker subgating on DN cells”

IFN-γ vs IL-4 reciprocal control (adapted from Jenks2018)

• Stimulating healthy-donor naive B cells with R848 + cytokines + IFN-γ, but not IL-4, produced both plasma cell differentiation and increased T-bet and CD11c with concomitant loss of CD21 and CD23
• The review states IL-4 and IFN-γ “exert similarly reciprocal regulation on the differentiation of T-bet⁺ B cells induced in SLE”
• Naive B cells from both healthy donors and SLE patients gained CD11c and lost CD21 under IFN-γ + R848 + cytokines
• CD21 loss occurred in all cultures and is read as marking activation state, recapitulating the in vivo DN2 / activated naive phenotype

★ Caveats the review raises about CD21^low^ gating

• CD21 downregulation marks activated memory cells in normal vaccination responses, HIV, malaria, and checkpoint-inhibitor-expanded memory — it is not disease-specific
• The same CD21^low^ feature is shared by CD11c⁺ activated naive, DN2, atypical/tissue-based memory, and T-bet⁺ B cells generally
• However, CD27⁻CD21^low^ cells have also been characterised as anergic in a fraction of RA and CVID patients. The review notes those studies gated on CD21⁻ without considering IgD, so would have included CD27⁻ switched cells with anti-IgM hyporesponsiveness
• Early transitional (T1) B cells can also be CD21^low^ and must be excluded using CD38, CD24 and/or CD10
• CD23 is mainly IL-4-induced after BCR or CD40 stimulation; CD23⁻ expansions in SLE may reflect activated naive cells rather than a distinct lineage

ASC subdivision

• CD138 vs CD38 on IgD⁻ cells resolves ASCs into five fractions spanning CD19⁺CD38^mid^CD138⁻ pre-PB through CD19⁻CD38^hi^CD138⁺ PC
• Pre-PB are enriched for BLIMP-1 while still expressing Pax5, signifying a transitional state toward ASC fate
• Circulating ASCs are largely Ki-67⁺; bone marrow ASCs show little Ki-67 — a newly-generated vs resident distinction
• Anergic naive cells (IgD⁺IgM^low/−^, “BND”) are ~<2% of blood B cells; IgM downregulation can also be caused by BCR activation, so IgM level alone should not be used to call anergy

Methods Used

Conventional Flow Cytometry, Bm Classification

Also discussed but not separately updated: Spectral Flow Cytometry, FACS Sorting, Compensation and FMO Controls — trimmed 2026-08-28. The review reports no methods of its own (every figure is adapted from prior work) and the PDF contains no mention of spectral cytometry, compensation or FMO controls at all; the single “flow sorted” reference sits in a figure legend adapted from Jenks2018 - DN2 B Cells and EF Pathway in SLE. The panel-design content it does carry is propagated to Conventional Flow Cytometry.

Entities Mentioned

CD19, CD20, CD21, CD24, CD27, CD38, CD10, CD23, CD11c, CD138, CD71, IgD, IgM, IgG, IgA, T-bet, FCRL5, FcRH4, SLAMF7, CXCR5, BLIMP-1, IFN-gamma, TLR7, DN2 B Cell, DN3 B Cell, Double-Negative B Cell, Activated Naive B Cell, Atypical B Cell, Plasmablast, Switched Memory B Cell, Early Memory B Cell, Tissue-Resident Memory B Cell

Concepts Addressed

Extrafollicular Response, Germinal Center, Memory B Cell, Class Switch Recombination, Follicular Exclusion

Relevance & Notes

Why it earns its place despite being Tier B. Sanz2025 - Human Atypical B Cells Overview is the more recent Sanz-lab overview and is already ingested, but Sanz2019 is the panel-design and nomenclature document, and that is a different job. Its Table 1 is the most complete canonical phenotype reference in the wiki, and it is directly actionable for DN2 Gating Strategy and DN2 Panel - Staining, Compensation, and Gating Protocol.

Direct bearing on the curator’s flow panel. The recommended 7-marker core (CD19, IgD, CD27, CD38, CD24, CD21 + dump) should be checked against the current 11-colour whole-blood panel. Two specific points to reconcile: CD24 is in the recommended core but is not obviously in the wiki’s documented panel, and the review’s warning that early transitional T1 cells are CD21^low^ means a CD21-anchored DN2 gate without CD24/CD38/CD10 exclusion can be contaminated from below. See DN2 Gating Strategy.

★ The most consequential finding for this wiki’s scope. The reciprocal FcRL4/FcRL5 pattern between HIV and SLE DN cells is direct phenotypic evidence that the “atypical B cell” label spans at least two different cells — an anergic FcRL4⁺ HIV/mucosal type and an activated FcRL5⁺CD11c⁺ SLE effector type. This matters because the wiki’s infectious-disease case has been partly built by transfer from autoimmunity, and this review says the transfer is not phenotypically free. It reinforces, from an independent angle, the “exhaustion framing was wrong for DN2” arc already recorded on DN2 B Cell and Atypical B Cell — while cautioning that the exhaustion framing may still be right for the HIV cell.

Where it agrees with the mechanism sources. The IFN-γ-not-IL-4 reciprocity restates Jenks2018 - DN2 B Cells and EF Pathway in SLE and is independently supported at the level of mechanism by Zumaquero2019 - IFN-gamma Programs T-bet-hi B Cells for ASC Differentiation (IFN-γ obligate for T-bet^hi^ pre-ASC formation) and, in murine ABCs, by Cancro2020 - Age-Associated B Cells’s IL-4 conditional antagonism.

A definitional point the wiki should adopt. Sanz2019 assigns DN1 the function “memory precursors” and DN2 “extrafollicular ASC precursors,” and lists a third, FcRL4⁺ DN population as atypical/tissue-based memory. The wiki’s DN3 B Cell page derives its DN3 concept from Lamprinou2026 - ABCs and DN B Cells’s DN1–DN4 taxonomy. These two schemes are not obviously the same partition and should not be silently merged — flagged as a Watch Item.

Limitations.

• Zero original data; every figure is adapted, mostly from a single prior paper by the same group.
• The classification is explicitly a proposal, and the review is candid that consensus does not exist.
• Table 1’s functional annotations (“memory precursors,” “extrafollicular ASC precursors”) are assignments of the kind the review’s own introduction warns against — function attributed on the basis of surface phenotype.
• Written in 2019, so it predates the COVID-19 EF literature, the CITE-seq/transcriptomic redefinitions in Sutton2021 - Alternative Lineage B Cells in Vaccination and Infection, and the DN3/DN4 expansions.

Questions Raised

• Does the wiki’s DN3 (from Lamprinou2026 - ABCs and DN B Cells) correspond to Sanz2019’s FcRL4⁺ DN row, or is it a different population? Both are CD21⁻ IgD⁻CD27⁻ and neither is DN2, but FcRL4 has never been measured in the DN3 papers.
• If activated naive and DN2 differ only by IgD across an otherwise identical activation signature, is IgD loss a discrete fate decision or simply a later timepoint on one trajectory?
• Should CD24 be added to the curator’s panel? Sanz2019 puts it in the recommended core; the wiki’s current gating does not obviously use it.
• Given that CD21^low^ marks activation in normal vaccination as well as in SLE and HIV, what is the healthy-donor baseline for a CD21-anchored DN2 gate in a dengue-endemic population with frequent prior infections?
• Is FcRL4 worth adding to the dengue panel to test whether dengue DN cells resemble the SLE (FcRL5⁺) or HIV (FcRL4⁺) pole?