EFB Dengue Literature Review

TLR7

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TLR7

Overview

TLR7 (Toll-like Receptor 7) is an endosomal pattern recognition receptor that senses single-stranded RNA (ssRNA). In B cells, TLR7 signalling triggers activation, proliferation, and — in the presence of appropriate cytokines — plasma cell differentiation. TLR7 is of central pathogenic significance in SLE, where endogenous RNA (from immune complexes containing Sm/RNP autoantigens) provides chronic TLR7 stimulation. In acute viral infections including dengue, exogenous viral ssRNA is the physiological TLR7 ligand.

Key Points from Literature

  • DN2/aNAV hyper-responsiveness to TLR7: R848 (TLR7/8 agonist) strongly induces CD25 expression and pERK/pMAPKp38 phosphorylation in DN2 and aNAV cells but not in SWM, DN1, or NAV. TLR7 also upregulates HLA-DR and CD86 in DN2 cells while downregulating inhibitory receptors CD72 and CD32b (see Jenks2018 - DN2 B Cells and EF Pathway in SLE, phospho-flow n=5–10).
  • TLR7 as required signal for EF differentiation: In vitro, TLR7 (R848) is essential for rNAV → aNAV → DN2 → PC differentiation. Removing R848 results in >95% cell death by day 7 and drastically reduced PC frequencies. Specific TLR7 inhibition with ODN 20959 produces the same effect (see Jenks2018 - DN2 B Cells and EF Pathway in SLE).
  • TLR7 overexpression in SLE B cells: SLE B cell transcriptomes are enriched for viral RNA sensors including TLR7 and IFIH1, plus the downstream kinase TBK1. This constitutes an overexpression of the pathway that drives DN2 activation (see Jenks2018 - DN2 B Cells and EF Pathway in SLE, RNA-seq).
  • TLR7 + IL-21 + IFN-γ = minimal signal for DN2 → PC: DN2 cells differentiate into PC through signal 3 alone (TLR7 + IL-21 + IFN-γ) without BCR stimulation or CD40L, and without extensive cell division (see Jenks2018 - DN2 B Cells and EF Pathway in SLE).
  • TLR7 gain-of-function mutations cause human SLE: Monogenic TLR7 GoF mutations result in human SLE with elevated ABC/DN2 cells. In mice, the orthologous TLR7 mutation induces lupus in a B cell-intrinsic, GC-independent fashion — ABC expansion and pathology proceed without GC involvement (see Sanz2025 - Human Atypical B Cells Overview, review citing Brown et al. 2022).
  • TLR7 plays an obligatory role in DN2 differentiation: TLR7 is required for both the initial activation of naive B cells into DN2 and for DN2 differentiation into ASC (see Sanz2025 - Human Atypical B Cells Overview, review citing Zumaquero et al. 2019).
  • Enhanced TLR7 activity through indirect mutations: Mutations enhancing endosomal TLR activity drive ABC/DN2 expansion and autoimmunity: NOX2 deficiency (NCF1/NCF2 loss-of-function) fails to terminate endosomal TLR signalling; UNC93B1 instability increases TLR7 activity. These defects break tolerance in a B cell-intrinsic fashion (see Sanz2025 - Human Atypical B Cells Overview, review).
  • SLE is enriched in all TLR7 pathway components: High TLR7 copy numbers and activity, elevated IFN-γ and IL-21 serum levels, high abundance of TLR7 ligands (including XIST lncRNA), and defective X chromosome inactivation all converge to amplify TLR7 signalling in SLE B cells (see Sanz2025 - Human Atypical B Cells Overview, review).
  • CD21lo cells have increased TLR7 sensitivity: CD21lo B cells show increased TLR7 sensitivity, which could contribute to their recruitment into the ABC compartment and SLE expansion (see Sanz2025 - Human Atypical B Cells Overview, review citing Zhu et al. 2024).
  • TLR7 inhibition as therapeutic strategy: Small molecule TLR7/8 inhibition (ENPATORAN) showed promising results in the phase II Willow study in cutaneous lupus. Targeted TLR7 inhibitor delivery via nanoparticles is also being explored (see Sanz2025 - Human Atypical B Cells Overview, review).
  • Dengue relevance: Dengue virus is an ssRNA flavivirus; during viraemia, TLR7 ligands are physiologically abundant. Whether the TLR7-driven EF differentiation programme operates during acute dengue — potentially driving the massive plasmablast expansion at days 7–10 — is a key open question.
  • Proposed mechanism for T-independent CSR in dengue via TLR7: GodoyLozano2016 explicitly proposes that endosomal DENV recognition by TLR7 provides a synergic signal with the BCR for T-independent class switch recombination, producing IgG-switched but poorly mutated antibodies. In mice, TLR7 and TLR9 synergise with BCR signalling to promote AID expression (required for both CSR and SHM); T-independent IgG responses against Polyomavirus require MyD88 (the canonical TLR adaptor). This provides a mechanistic link between the abundant TLR7 ligand (DENV ssRNA during viraemia) and the paradoxically low-SHM IgG observed in acute dengue — CSR is induced via TLR7+BCR but without the iterative SHM cycling of GC reactions (see GodoyLozano2016 - Lower IgG SHM Rates in Acute Dengue, proposed model, citing Pone et al. 2012 and Raval et al. 2013).

Contradictions & Debates

None documented in current wiki sources.

TRAF5, DN2 B Cell, Activated Naive B Cell, Plasmablast, Extrafollicular Response, ZEB2

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