EFB Dengue Literature Review

Extrafollicular Response

23 min read

Extrafollicular Response

Overview

The extrafollicular (EF) B cell response refers to antigen-driven B cell activation and differentiation that occurs outside of germinal centres — in the T cell zones and red pulp of the spleen, interfollicular areas of lymph nodes, and other extrafollicular anatomical niches. EF responses can produce short-lived plasmablasts, and in some contexts generate memory B cells with somatic hypermutation, without the full complement of GC-associated signals (particularly CD27 acquisition and high-rate SHM).

The EF response is increasingly recognised as a major branch of humoral immunity in acute viral infections, autoimmune diseases, and conditions of immune dysregulation. It is the central biological focus of this wiki, specifically as it pertains to dengue infection — a context where GC-independent B cell differentiation has been proposed but not yet comprehensively characterised.

Key Points from Literature

  • EF origin hypothesis for DN B cells: Wei et al. propose that IgD⁻CD27⁻ (double-negative) memory B cells in SLE develop via extrafollicular pathways, based on: (1) CD27 being normally acquired during GC reactions via CD40–CD154 interactions; (2) DN cells showing lower SHM rates (~3%) than CD27⁺ GC-derived memory cells (~5%); (3) murine evidence that SHM can occur in EF reactions (William et al. 2002, Science) (see Wei2007 - DN Memory B Cells in SLE, cross-sectional SLE cohort).

  • DC-mediated EF B cell activation: Qi et al. (2006, Science) showed that antigen-bearing dendritic cells can activate lymph node B cells in extrafollicular zones — cited in Wei2007 - DN Memory B Cells in SLE as mechanistic support for EF memory B cell generation.

  • CD40-independent class switching: Litinskiy et al. (2002, Nat Immunol) demonstrated that DCs can induce IgG/IgA class switching via BLyS and APRIL without CD40 signals — providing an EF route to isotype-switched antibodies (cited in Wei2007 - DN Memory B Cells in SLE).

  • EF responses in autoimmunity: The expansion of DN B cells in SLE, correlating with nephritis and autoreactive antibodies, suggests that pathological EF responses can generate autoreactive memory and potentially contribute to disease. This is in contrast to GC reactions, which in SLE appear dysregulated rather than absent (see Wei2007 - DN Memory B Cells in SLE).

  • DN expansion is driven by ongoing B cell dysregulation, not fixed programming: After rituximab-mediated B cell depletion and immune reconstitution, DN expansion resolved in effective depletors (P=0.05). This demonstrates that DN accumulation requires continuous input — likely from dysregulated naive/memory B cell precursors — rather than irreversible epigenetic programming of individual clones. In patients with incomplete depletion, resolution did not occur (see Anolik2004 - Rituximab and B Cell Abnormalities in SLE, phase I/II trial n=17).

  • Correlation between EF B cell expansion and disease severity: DN expansion correlates with VH4.34 autoreactive antibody titers (R²=0.8, P<0.05), stronger than the correlation for naive lymphopenia alone. This quantitative link supports the idea that EF-derived DN B cells are not bystander expansions but participate in the autoimmune process (see Anolik2004 - Rituximab and B Cell Abnormalities in SLE).

  • EF plasmablast dynamics: Circulating plasmablasts in SLE are predominantly short-lived (CD20⁻) cells replenished by CD20⁺ B cell precursors; their rapid decline after rituximab suggests they are products of ongoing EF differentiation rather than accumulated long-lived cells (see Anolik2004 - Rituximab and B Cell Abnormalities in SLE; see also Plasmablast).

  • Direct naive → EF → ASC pathway demonstrated in human SLE: A distinct subset of activated naive (acN) B cells (CD19^hi, IgD⁺, CD27⁻, MTG⁺, CD24⁻, CD21⁻, CD38^lo, CD23⁻) was identified as an important and persistent precursor of circulating ASCs during SLE flares. Up to 32.5% of acN sequences share clonal identity with co-circulating ASCs; 9 of 10 largest acN clones are direct ASC precursors. The average memory-to-naive ASC connectivity ratio is significantly lower in SLE than in vaccination responses, confirming that naive B cell recruitment — not memory recall — is a dominant feature of SLE flare ASC expansion (see Tipton2015 - ASC Diversity and Origin in SLE, NGS of sorted populations, n=5 SLE acute flare).

  • Germline-encoded autoreactivity without SHM proves EF differentiation: ASC clone 652-F6 had zero mutations in both VH and VL regions yet displayed strong reactivity to ANA, dsDNA, chromatin, and ribosomal P antigens. This is definitive proof that naive B cells can differentiate into autoreactive ASCs via EF pathways without any GC transit or SHM-driven affinity maturation (see Tipton2015 - ASC Diversity and Origin in SLE, single-cell monoclonal antibody analysis).

  • Low SHM as an EF biomarker in peripheral blood: ASCs in SLE flares show substantially lower VH mutation rates (avg 4.98%) than post-vaccination ASCs (avg 7.33%). Approximately 30–33% of SLE ASC sequences contain <3% VH mutation vs. ~10–12% post-vaccination. This <3% mutation fraction is the most quantitatively precise available benchmark for EF-derived plasmablasts in human peripheral blood (see Tipton2015 - ASC Diversity and Origin in SLE).

  • acN cell persistence sustains EF output over months: acN cells persist in circulation for months, continuously seeding new ASC progeny. Single-cell analysis confirmed clonal identity between acN cells and ASC clones detected by NGS 4 months earlier. Phylogenetic trees show unmutated acN precursors (0% SHM) co-existing with ASC progeny carrying up to 21.5% VH mutation, implying prolonged diversification from a long-lived EF-activated naive precursor (see Tipton2015 - ASC Diversity and Origin in SLE).

  • EF response in SLE produces polyclonal, bystander-activated ASCs alongside autoreactive expansions: ELISPOT showed that influenza- and tetanus-specific memory ASCs are spontaneously activated in SLE flares despite no recent immunization — evidence of broad bystander polyclonal activation of memory alongside selective clonal expansion of autoreactive naive cells (see Tipton2015 - ASC Diversity and Origin in SLE).

  • Complete human EF differentiation pathway mapped: rNAV → aNAV → DN2 → plasmablast. Jenks2018 integrates phenotypic, transcriptional (RNA-seq), epigenetic (ATAC-seq), functional (in vitro), and repertoire (BCR sequencing) evidence to define the full pathway. Key signals: TLR7 + IFN-γ + IL-21 drive each transition; CD40L and IL-4 inhibit it. The pathway operates without BCR stimulation or extensive cell division. Each step is confirmed by clonal connectivity (shared BCR sequences between aNAV, DN2, and PC in vivo) (see Jenks2018 - DN2 B Cells and EF Pathway in SLE, cross-sectional + in vitro + multi-omic).

  • DN2 cells are the EF pre-plasmablast: DN2 B cells (IgD⁻CD27⁻CXCR5⁻CD21⁻CD11c⁺CD19^hi) are poised PC precursors with high IRF4, BLIMP-1, and open PRDM1 chromatin by ATAC-seq. DN2 cultures produce IgG at higher per-cell levels than DN1 or SWM and generate ASC frequencies comparable to SWM by ELISPOT, despite requiring only TLR7 + IL-21 + IFN-γ (no BCR engagement). This establishes DN2 as the immediate precursor of EF-derived plasmablasts (see Jenks2018 - DN2 B Cells and EF Pathway in SLE).

  • TLR7 hyper-responsiveness is the mechanistic engine of EF activation: DN2 and aNAV cells show enhanced pERK and pMAPKp38 phosphorylation after R848 (TLR7 agonist) stimulation compared with rNAV, SWM, and DN1. This hyper-responsiveness is mechanistically explained by low expression of the negative TLR regulator TRAF5 and low TNFAIP3 (A20). Concurrently, CD40L stimulation fails to activate DN2 cells (no CD25 upregulation), consistent with a T cell-independent activation mode. This dual phenotype — TLR7-hyper-responsive, CD40L-unresponsive — is the functional signature of EF pathway cells (see Jenks2018 - DN2 B Cells and EF Pathway in SLE, phospho-flow + RNA-seq).

  • Transcriptional programme of EF effectors: DN2/aNAV cells express T-bet (TBX21) + ZEB2 (which cooperate to repress TCF7), high IRF4 + low IRF8 (promoting PC differentiation), and BLIMP-1 (PRDM1), while lacking the repressor cassette BACH2/FOXP1/FOXO1/BCOR that maintains the resting/memory state. This transcriptional programme is distinct from GC-derived SWM cells (which express TCF7/CXCR5/BACH2) and from DN1 cells (which share the SWM transcriptome) (see Jenks2018 - DN2 B Cells and EF Pathway in SLE, RNA-seq of sorted populations).

  • CD40L inhibits EF differentiation but not GC pathway: In vitro, CD40L stimulation inhibits rNAV differentiation into aNAV and DN2 but does not affect DN1 generation. This is the most direct evidence that GC (CD40-dependent) and EF (TLR7-dependent) pathways are not merely different stages of the same process but are antagonistically regulated (see Jenks2018 - DN2 B Cells and EF Pathway in SLE, in vitro differentiation).

  • Competing EF vs. GC endotypes predict clinical outcomes: SLE patients segregate into EF-dominant and memory/GC-dominant clusters by B cell profiling. ~75% of patients fall into EF or memory endotypes; higher severity and nephritis concentrate in the EF cluster. EF endotypes also predict reduced affinity maturation and neutralizing activity of SARS-CoV-2 mRNA vaccine responses in SLE patients (see Sanz2025 - Human Atypical B Cells Overview, review citing Jenks et al. 2021, Faliti et al. 2024).

  • EF responses associated with worse cancer outcomes: A large single-cell analysis of human tumors found EF responses associated with worse clinical outcomes and resistance to immunotherapy, promoted by an immunosuppressive T cell environment. This extends the EF endotype concept beyond autoimmunity and infection into cancer (see Sanz2025 - Human Atypical B Cells Overview, review citing Ma et al. 2024).

  • Self-limited EF autoreactivity is a normal feature of healthy immune responses: Healthy subjects generate naïve-derived DN2 cells producing dual-reactive (virus + self) antibodies with low or no SHM during primary SARS-CoV-2 infection. These autoreactive responses subside within months in healthy individuals — constituting self-limited EF autoreactivity. In genetically predisposed individuals (SLE), these responses may become perpetuated through elevated interferons, BAFF, IL-21, genetic susceptibility variants, and environmental factors (see Sanz2025 - Human Atypical B Cells Overview, review).

  • GC-independent autoimmunity confirmed by monogenic evidence: TLR7 gain-of-function mutations cause human SLE with expanded ABC/DN2 in a B cell-intrinsic, GC-independent fashion. In mice, the orthologous mutation induces lupus without GC involvement. Additional mutations enhancing TLR function (NOX2 loss-of-function, UNC93B1 instability) also drive ABC/DN2 expansion (see Sanz2025 - Human Atypical B Cells Overview, review citing Brown et al. 2022).

  • ZEB2 as primary driver, repressing GC entry: B cell-intrinsic Zeb2 is essential for the ABC transcriptional programme. Zeb2 represses Mef2b, a TF required for GC differentiation — providing a direct mechanistic link between EF differentiation and GC exclusion. Together with the GC-independent nature of autoimmunity in TLR7 gain-of-function mice, this establishes the molecular basis for EF/GC antagonism (see Sanz2025 - Human Atypical B Cells Overview, review citing Dai et al. 2024).

  • EF pathway in HIV: In HIV, AtB cells localise to extrafollicular areas of lymph nodes in the context of disorganised GCs, adding chronic HIV to the list of infections with dominant EF responses. A similar phenomenon occurs in lymph nodes and spleen of patients with lethal COVID-19 (see Sanz2025 - Human Atypical B Cells Overview, review).

  • First demonstration of EF pathway in acute human viral infection (COVID-19): Critically ill COVID-19 patients (CoV-A cluster) displayed hallmarks of EF B cell activation nearly indistinguishable from active SLE: expanded aN and DN2 cells, elevated DN2:DN1 ratios (P ≤ 0.0001 vs. HD), massive ASC expansion with CD138⁺ enrichment, contraction of unswitched memory cells, and a chemokine receptor shift from CXCR5 (follicular homing) to CXCR3 (IFN-γ-inflamed tissue homing). Hierarchical clustering of B cell population frequencies separated ICU from outpatients almost perfectly, driven by coordinated aN + DN2 + DN3 + ASC expansion (see Woodruff2020 - EF B Cell Responses in COVID-19, 24-marker spectral FCM, n=10 ICU-C, n=7 OUT-C, n=17 HD, n=7 SLE).

  • Central paradox: functional neutralizing antibodies + poor outcomes: CoV-A patients produced the highest anti-SARS-CoV-2 RBD antibodies (all isotypes) with confirmed in vitro neutralization by day 5 post-symptom onset — yet had the worst clinical outcomes (ICU, death). This establishes that EF-derived ASC responses can produce functional protective antibodies while simultaneously correlating with disease severity. The paradox challenges the assumption that antibody quantity/quality determines protection, and raises the possibility that the EF pathway itself — or its inflammatory accompaniments (IL-6, IP-10, CRP) — contributes to pathogenesis (see Woodruff2020 - EF B Cell Responses in COVID-19).

  • ASC repertoire in COVID-19 confirms EF origin: Single-cell V(D)J of ASCs from a CoV-A patient showed >50% germline (unmutated) VH clonotypes, balanced IgM/IgG1/IgA1 with ongoing class switching, and enriched autoreactive VH4-34 clones with preserved FR1 patches. This repertoire signature — newly recruited, germline-dominant, with defective tolerance — matches the EF ASC fingerprint established in SLE by Tipton2015 (see Woodruff2020 - EF B Cell Responses in COVID-19, 10x Chromium scV(D)J, 2,017 clonotypes).

  • DN2 expansion as a cellular correlate of inflammatory severity: DN2 frequency within the DN compartment correlated with CRP (r² = 0.39, P = 0.022), which in turn correlated with IL-6 and IP-10. Both IL-6 and IP-10 — previously identified as biomarkers of COVID-19 severity and the cytokine environment that drives EF responses in SLE — were elevated in the CoV-A cluster (see Woodruff2020 - EF B Cell Responses in COVID-19).

  • Validated by histopathological evidence of GC suppression: Kaneko et al. (2020) demonstrated loss of Bcl-6⁺ Tfh cells and germinal centers in spleens and lymph nodes of fatal COVID-19 cases. This supports that the peripheral EF dominance reflects actual GC failure — the EF pathway is not simply faster, it may be the only pathway available when GCs are suppressed (see Woodruff2020 - EF B Cell Responses in COVID-19, citing Kaneko et al. 2020, Cell).

  • EPIGENETIC HIERARCHY CONFIRMS EF DIFFERENTIATION PATHWAY: Integrated RRBS, ATAC-seq, and RNA-seq of 5 sorted B cell subsets from SLE patients and healthy controls established a linear differentiation trajectory: resting naive → T3 → activated naive → DN2/switched memory → ASC. DNA methylation phylogenetic analysis placed DN2 closest to ASC in both SLE and HC, confirming its position as the most terminally differentiated non-ASC subset. DN2 and aN cells were epigenetically closer to each other in SLE than in healthy controls, supporting accelerated differentiation through the aNAV→DN2 axis in SLE (see Scharer2019 - Epigenetic Programming in SLE B Cells, n=9 SLE + n=12 HC, African-American females).

  • SLE disease signature present in resting naive B cells — epigenetic priming of the EF pathway: 6,664 differentially methylated loci stratified all SLE vs. healthy samples. Resting naive B cells in SLE already showed 564 DMLs, 612 DEGs, and 402 DARs vs. HC, including upregulated NR4A1 (BCR-induced) and NR4A3 (TLR-induced) — indicating resting naive cells in SLE have received both BCR and TLR stimulation. This SLE signature propagated through all downstream subsets, demonstrating a transmissible epigenome that modulates the activation threshold for EF differentiation (see Scharer2019 - Epigenetic Programming in SLE B Cells).

  • Separate epigenetic programmes for EF vs. GC endpoints: DN2 (EF endpoint) accessible chromatin was enriched for T-BET, AP-1, and EGR motifs; isotype-switched memory (GC endpoint) was enriched for NF-κB, EBF, and OCT2 motifs. Despite similar DNA methylation states, these populations have fundamentally different chromatin architectures — confirming they represent distinct differentiation pathways, not different stages of the same pathway (see Scharer2019 - Epigenetic Programming in SLE B Cells).

  • T-BET programme is normal; AP-1/EGR amplification is disease-specific: T-BET motif enrichment in DN2 chromatin was shared by healthy controls and SLE — it is a normal feature of DN2 differentiation. AP-1 and EGR motif accessibility was amplified in SLE aN and DN2 cells beyond healthy levels, identifying AP-1/EGR as the disease-specific epigenetic layer that enhances EF differentiation in pathological immune environments (see Scharer2019 - Epigenetic Programming in SLE B Cells).

  • ATF3 as a key EF regulator in disease: ATF3 — induced by BCR/TLR stimulation and cellular stress — was the top SLE-specific transcription factor in the DN2 network. Its 98 target genes map to MTORC1, G2/M checkpoint, apoptosis, UPR, and TNF signalling pathways. ATF3 heterodimerises with Jun family members (all upregulated in SLE DN2) to function as an activator, potentially explaining the metabolic and proliferative programme of EF effector cells (see Scharer2019 - Epigenetic Programming in SLE B Cells).

  • DN2 cells resist apoptosis: GSEA showed all SLE B cell subsets except DN2 were enriched for G2/M checkpoint and apoptosis pathways. This unique negative enrichment in DN2 cells may explain their selective expansion in SLE — and potentially in any disease with EF activation (see Scharer2019 - Epigenetic Programming in SLE B Cells).

Dengue Context

  • FIRST DIRECT EVIDENCE OF EF B CELL ACTIVATION IN DENGUE (Ansari2025): In a prospective cohort of n=170 acute dengue adults, CD21⁻CD11c⁺ B cells within the IgD⁻CD27⁻ (DN) gate — phenotypically consistent with DN2 — are significantly expanded during acute infection. The T cell help arm is provided by Peripheral Helper T Cell (CXCR5⁻PD-1⁺), which constitute ~75% of activated CD4⁺ T cells and drive memory B cell→plasmablast differentiation via IL-21. This establishes the Tph→IL-21→memory B cell→plasmablast axis as the dominant B cell help pathway in acute dengue, operating extrafollicularly (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue, n=170 cohort + scRNA-seq + T-B coculture).

  • Neutralizing Ab paradox replicated in dengue: Anti-NS1 and anti-prM/M/E IgG are elevated in severe dengue, but FRNT₅₀ neutralizing titers do not differ between mild and severe groups. This mirrors the Woodruff2020 COVID-19 finding: EF-derived antibodies correlate with severity but not neutralization quality. In dengue, this has additional significance due to ADE — non-neutralizing cross-reactive IgG could directly enhance secondary infection (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue).

  • Concurrent EF + GC activity in dengue: Unlike the antagonistic EF/GC model from SLE (Jenks2018), acute dengue shows elevated CXCL13 (a Tfh/GC biomarker) alongside dominant Tph activation — suggesting EF and GC pathways operate simultaneously. This is consistent with the Sanz2025 endotype concept, where some patients may have mixed EF+GC responses (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue).

  • Memory B cells, not naive cells, are the dominant EF precursors in dengue: Tph cells preferentially drive class-switched memory B cell → plasmablast differentiation; naive B cells respond poorly. This contrasts with SLE, where naive (acN) cells are the dominant EF ASC precursors (Tipton2015). The difference likely reflects the memory recall component of dengue in an endemic setting — most patients have pre-existing cross-reactive memory from prior DENV exposure (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue, T-B coculture).

  • Tph severity association: CXCR5⁻PD-1⁺ Tph frequency is significantly higher in severe dengue. Combined with the severity-associated non-neutralizing IgG, this positions the Tph→EF pathway as a potential driver of immunopathology rather than protection (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue).

  • FOUNDATIONAL: Wrammert2012 establishes the magnitude benchmark for dengue plasmablasts — IgG-dominant, short-lived, consistent with EF output: The first systematic dengue plasmablast study (n=46, Bangkok) demonstrated >1,000-fold expansion peaking at day 6–7, averaging 47% of B cells, dominated by IgG (≥70% DENV-specific), with return to baseline by 1 month and no hypergammaglobulinemia. The transient, massive, IgG-dominant response with rapid contraction is phenotypically consistent with short-lived EF-derived plasmablasts. The lack of total IgG elevation despite >10⁵ ASCs/ml implies most cells die rather than home to long-lived niches — matching the SLE short-lived PB model from Anolik2004. This paper predates the EF framework but provides the quantitative target that GarciaBates2013 severity-stratified and Ansari2025 mechanistically attributed to Tph→IL-21 (see Wrammert2012 - Plasmablast Responses in Acute Dengue, n=46 cohort, 5-color FCM + ELISpot).

  • BCR repertoire data provide intermediate SHM — not clearly EF or GC: Parameswaran2013, the first BCR sequencing study in dengue, found that convergent CDR3-bearing B cells in acute dengue carry 4.4–6.9% V gene mutation. This is intermediate between the EF benchmark (<3% per Tipton2015) and full GC maturation (~7.3%). The convergent CDR3s are more prevalent in secondary dengue and derive from memory populations using multiple V gene families. Critically, these data come from unsorted PBMCs — the SHM distribution of sorted plasmablasts specifically remains unknown. The intermediate mutation rate could reflect: (a) a mixture of EF-derived low-SHM and GC-derived high-SHM cells, (b) memory cells that initially matured in GCs and are now being recalled through the EF pathway (Tph→memory B cell per Ansari2025), or (c) EF maturation with more extensive SHM than seen in SLE. Distinguishing these models requires sorted plasmablast BCR sequencing (see Parameswaran2013 - Convergent Antibody Signatures in Dengue, 454 pyrosequencing, n=60 dengue patients).

  • MOLECULAR EVIDENCE: Low SHM in acute dengue IgG plasmablasts supports GC-independent pathway (GodoyLozano2016): HTS of IgG VH cDNA from 19 acute dengue patients revealed globally lower SHM in the acute phase vs. post-convalescence (p<0.001), with SHM paradoxically lower in secondary than primary infections and in DWS+ than DWS−. Monte Carlo simulation confirmed the acute-phase signal derives predominantly from ASCs, not memory B cells. Biased IGHV segment usage (IGHV1-2, IGHV1-18, IGHV1-69 — all innate-like “natural antibody” segments) with polyclonal CDRH3 diversity supports germline-encoded recognition rather than affinity-matured recall. The authors propose a dual-pathway model: rapid GC-independent EF differentiation producing CSR-competent but SHM-deficient IgG, concurrent with the classical GC pathway. The proposed mechanism — TLR7-mediated endosomal DENV recognition synergising with BCR crosslinking to drive T-independent CSR without SHM — directly maps onto the Jenks2018 EF pathway (TLR7 + IFN-γ + IL-21) and provides the molecular evidence for the Ansari2025 Tph-driven EF model. The low SHM in secondary infections is consistent with original antigenic sin rapidly activating germline-coded cross-reactive B cells that bypass GC maturation (see GodoyLozano2016 - Lower IgG SHM Rates in Acute Dengue, n=19 acute + n=11 post-convalescent + n=10 controls, 454 pyrosequencing, 385,206 lineages).

  • HIGH SHM IN SORTED SECONDARY PBs COMPLICATES THE PURE EF MODEL: Priyamvada2016 found that sorted plasmablasts from secondary DHF carry mean 18.1 VH mutations (~6.5% nucleotide mutation) — comparable to influenza recall and significantly higher than the EF benchmark (<3% per Tipton2015). This high SHM in sorted PBs from secondary infections, combined with GodoyLozano2016’s globally low SHM from bulk IgG, suggests a dual-pathway model: (1) memory-derived PBs with high SHM (GC-matured during prior infection, now recalled through the Tph→memory B cell axis per Ansari2025); and (2) de novo EF-derived PBs with low SHM from naive recruitment. The relative contribution of each pathway likely varies with infection history — secondary infections are more memory-dominated, while primary infections may be more EF-dominated. This resolves the SHM paradox: the EF pathway is real (GodoyLozano2016 data) but operates alongside memory recall (Priyamvada2016 data), not instead of it. Evidence of Original Antigenic Sin (DENV1-biased neutralisation in DENV2 infection) further confirms that the high-SHM fraction is memory-derived, not de novo (see Priyamvada2016 - Cross-Reactive Memory Plasmablasts in Secondary Dengue, n=4 secondary DHF, 53 mAbs).

The earlier indirect dengue evidence remains relevant:

  • DENV-specific atypical MBCs (CD27⁻CD21⁻ = DN B cells) accumulate in 2° dengue durable to 18 months (Singh2026)
  • The question of whether these atypical MBCs are DN2 (EF-derived) or DN1 (GC-derived) is partially addressed by Ansari2025’s demonstration of CD21⁻CD11c⁺ EF B cells, but formal DN1/DN2/DN3 subdivision in dengue has not been performed

Contradictions & Debates

  • Whether EF-derived memory B cells (i.e., DN cells) are genuinely GC-independent or represent cells that initiated but aborted GC entry is debated. Wei et al. acknowledge both models: (a) independent EF lineage; (b) failed/abortive GC entrants. Jenks2018 substantially resolves this for DN2 cells specifically: they are EF-derived effectors (CD40L inhibits their generation; TLR7 drives it). The original EF-origin debate now applies primarily to whether some DN1 cells might also arise extrafollicularly, or whether DN1 is entirely GC-derived.

  • The lower SHM rate of DN vs. CD27⁺ memory cells is consistent with EF origin but is not conclusive — it could also reflect selection against high-affinity autoreactive clones within GCs rather than GC exclusion.

  • Whether the TLR7-driven EF pathway operates identically in acute viral infections (where TLR7 ligands are physiological viral ssRNA rather than pharmacological R848) remains unestablished. The SLE context may amplify TLR7 signalling through genetically elevated IRF5/IRF7 haplotypes that are not present in all individuals.

  • FIRST DENGUE DATA — Atypical MBCs and IgM+ MBCs as potential EF memory in dengue: In 2° dengue, DENV-specific atypical MBCs (CD27⁻CD21⁻ = DN B cells) are significantly expanded, durable to 18 months, and temporally correlated with later class-switched and activated MBC levels — suggesting they participate in the anti-DENV response rather than representing exhausted bystanders. DENV-specific IgM+ MBCs are the only subset significantly elevated at the acute 2° timepoint, consistent with rapid recall from an IgM+ memory compartment. The hypothetical working model positions atypical and IgM+ MBCs as the primary early responders in 2° immunity (vs. naive B cells and IgG+ MBCs in 1° immunity), raising the possibility that repeat DENV exposure selects for EF-like memory subsets rather than conventional GC-derived memory (see Singh2026 - DENV-Specific Memory B Cell Subsets, n=4/group longitudinal — small sample but internally consistent).

  • Caveat — EF vs GC origin unresolvable with this panel: Singh2026 lacks CXCR5, CD11c, and T-bet — the markers required to distinguish DN2 (EF effectors) from DN1 (GC-derived memory) within the CD27⁻CD21⁻ gate. The atypical MBC expansion in 2° dengue is therefore compatible with either EF or GC origin, or both (see Singh2026 - DENV-Specific Memory B Cell Subsets).

  • Clonal disconnect between dengue PBs and MBCs — implications for EF pathway: Appanna2016 showed plasmablasts and convalescent DENV-binding MBCs are clonally distinct (very few shared CDR3s, all IgM). E-specific IgG⁺ memory cells selectively feed the PB compartment (85% E-specific PB mAbs), while prM/complex-epitope MBCs follow a separate trajectory. VH4-34 and VH1-69 (autoantigen-binding VH families) were found specifically in PB-derived mAbs — paralleling the VH4-34 autoreactive clone enrichment in COVID-19 EF-derived ASCs (Woodruff2020) and SLE (Tipton2015). Whether the dengue PB wave’s VH4-34 enrichment represents transient EF autoreactivity (per the Sanz2025 self-limited model) is untested (see Appanna2016 - Plasmablasts as Subset of Memory B Cell Pool, n=12 dengue, mAb cloning + VH sequencing).

  • Plasmablast magnitude as a severity biomarker in dengue (GarciaBates2013): In the earliest severity-stratified dengue plasmablast study (n=84, Recife Brazil), plasmablasts averaged 46% of B cells in severe secondary dengue (peak 87%) vs. 5% in OFI — the highest reported for any human infection at that time. Plasmablast frequency increased with both secondary infection status and disease severity. >70% of IgG-secreting plasmablasts were DENV-specific, with 3-fold preference for the infecting serotype over heterotypic serotypes. Despite this massive virus-specific output, PRNT₅₀ neutralizing titers did not correlate with plasmablast frequency — the earliest independent confirmation of the neutralizing Ab paradox in dengue, predating the Ansari2025 FRNT₅₀ data by 12 years (see GarciaBates2013 - Plasmablast Response and Dengue Severity, n=84 cohort, ELISpot + PRNT).

  • B cell activation-induced death parallels T cell pathology: GarciaBates2013 reports that B cells in severe dengue undergo Ki-67⁺ proliferation, CD69⁺/CD95⁺ activation, and active caspase-3⁺ apoptosis (up to 60% of B cells in secondary DFC), with positive Ki-67/caspase-3 correlation. This mirrors the well-described T cell activation/apoptosis in dengue and suggests that both arms of adaptive immunity undergo activation-induced cell death during severe infection. B cell numbers are maintained despite apoptosis (leukopenia is granulocyte-driven), implying high turnover consistent with an EF differentiation wave producing and consuming cells simultaneously (see GarciaBates2013 - Plasmablast Response and Dengue Severity).

Double-Negative B Cell, DN2 B Cell, Activated Naive B Cell, Germinal Center, Memory B Cell, Plasmablast, Somatic Hypermutation, Class Switch Recombination, T-bet, TLR7, TRAF5, IRF4, BLIMP-1, BACH2, ATF3, EGR, PD-1, Peripheral Helper T Cell, IL-21, CD40L, FRNT, Original Antigenic Sin, Antibody-Dependent Enhancement

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