EFB Dengue Literature Review

FCRL5

6 min read

FCRL5

Overview

FCRL5 (Fc Receptor-Like 5; also FcRH5) is a member of the Fc receptor-like family. Unlike FcRH4 (FCRL4), which mediates hypo-responsiveness in HIV exhausted memory cells, FCRL5 is induced by BCR signalling and can drive naive B cell proliferation. FCRL5 is expressed on DN2 and aNAV B cells in SLE and on CD21⁻ atypical memory cells in malaria, making it a cross-disease marker of extrafollicular/atypical B cell identity.

Key Points from Literature

  • Expressed on DN2 and aNAV: FCRL5 surface expression (MFI) is elevated on DN2 and aNAV cells relative to SWM, rNAV, and DN1 in both SLE and HCD. Validated at both RNA and protein level (see Jenks2018 - DN2 B Cells and EF Pathway in SLE).

  • FCRL4⁻/FCRL5⁺ pattern distinguishes SLE DN2 from HIV exhausted memory: SLE DN2 cells are FCRL4⁻ FCRL5⁺; HIV CD21⁻ DN cells are FCRL4⁺. A similar FCRL4⁻/FCRL5⁺ pattern has been reported in malaria atypical memory cells (citing Portugal et al. 2015). Despite FCRL5 expression, SLE DN2 cells retain intact proximal BCR signalling (BLNK phosphorylation after anti-IgG stimulation) — functionally distinct from HIV exhausted cells (see Jenks2018 - DN2 B Cells and EF Pathway in SLE).

  • In vitro upregulation: FCRL5 is upregulated on in vitro-generated DN2 cells (from rNAV + TLR7/IFN-γ/IL-21 cultures), confirming it is acquired during EF differentiation (see Jenks2018 - DN2 B Cells and EF Pathway in SLE).

  • FcRL5 as therapeutic target: FCRL5 is one of two preferred surface targets (alongside SLAMF7) for selective ABC/DN2 depletion within the B cell compartment — sparing resting naive and conventional memory cells (see Sanz2025 - Human Atypical B Cells Overview, review).

  • T-bet⁺/FcRL5⁺ defines memory ABC within CD27⁺ compartment: In influenza vaccination responses, T-bet⁺/FcRL5⁺ CD27⁺ memory ABC cells are poised for ASC differentiation and correlate with long-lived antibody responses, while T-bet⁻/FcRL5⁻ canonical memory cells have stem-like central memory properties (see Sanz2025 - Human Atypical B Cells Overview, review citing Nellore et al. 2023).

  • T-bet or FcRL5 can substitute for CD11c in gating: These markers identify similar populations when used vs. CXCR5 in the DN gate, providing flexibility for panel design (see Sanz2025 - Human Atypical B Cells Overview, review, Figure 2).

  • FCRL5 is a defining transcriptomic marker of the alternative B cell lineage: scRNA-seq placed FCRL5 among the core genes defining the alternative lineage (alongside TBX21 and ITGAX/CD11c). FCRL5 expression was consistent across atBC1, atBC2, atBC3, and MBC1 clusters, confirming it as a lineage-level marker rather than an activation-state marker (see Sutton2021 - Alternative Lineage B Cells in Vaccination and Infection, n=4, 10x Chromium scRNA-seq).

  • ★ FCRL5 is an INHIBITORY receptor whose engagement may explain why ABCs looked incapable of plasma-cell differentiation. Early in vitro work reported limited PC-differentiation capacity for atypical B cells (citing Portugal 2015 eLife and Sullivan 2015 PLoS Pathog, malaria-associated human atypical MBCs — note the latter is the paper that delineated FCRL5 on these cells). Later work attributes this substantially to stimulation mode: soluble anti-Ig fails to drive PC differentiation whereas membrane-bound anti-Ig succeeds, because inhibitory receptors — FcγRIIB, and per the review FCRL5 — must be physically excluded from the B cell immune synapse for BCR signalling to proceed, which only membrane-associated antigen achieves (see Glaros2025 - Multilayered Identity of B Cell Memory, review, no original data, citing Ambegaonkar 2020 Sci Adv).

    Verification note (2026-08-16). The Ambegaonkar primary abstract was independently checked during ingest because the review’s paraphrase and the paper’s title appear to point in opposite directions. The direction is confirmed: atypical MBCs “are unable to respond to soluble antigens” but “robustly respond to antigens that associate with cell surfaces, such as antigens in immune complexes” — the title’s “restricts responses to membrane-associated antigens” means responses are restricted to that antigen form. The abstract names FcγRIIB as the receptor characterized; FCRL5’s role is the review’s attribution.

  • ⚠ Assay-design consequence for dengue work. If FCRL5⁺/FcγRIIB^hi atypical cells cannot respond to soluble antigen, in vitro restimulation assays using soluble DENV E or NS1 would systematically under-report the functional capacity of precisely the population this wiki tracks. Membrane-associated or immune-complexed antigen presentation would be the informative comparator. See DN2 B Cell and In Vitro B Cell Stimulation.

  • A constraint on the cells→autoantibody bridge. The same verified abstract notes these cells are “unable to respond to fully soluble antigens, such as self-antigens.” A DN2/ABC cell bearing an autoreactive BCR would therefore be poorly activated by soluble self-antigen, implying that any atypical-cell route to autoantibody must run through membrane-associated or immune-complexed self-antigen. Relevant to the bridge-wiki/ thesis; see Atypical B Cell.

  • ★ FcRL5 is the SLE pole of the reciprocal FcRL4/FcRL5 split — the review’s central evidence that “atypical B cell” names more than one cell. Across DN B cells from RA, SLE, scleroderma and acute and chronic HIV, FcRL5 is high in SLE but not in HIV, exactly mirroring FcRL4, which is high in HIV (~21.1% of DN2-gated cells) and near-absent in SLE (~0.74%). Table 1 gives DN2 as FcRL5⁺ and DN1 as FcRL5⁻, and FcRL5⁺ is part of the phenotype induced in vitro by TLR7 + IFN + IL-21 (see Sanz2019 - Consistent Classification of Human B Cell Populations, review — no original data, figure adapted from Jenks2018 - DN2 B Cells and EF Pathway in SLE). See Atypical B Cell, Double-Negative B Cell.

  • FcRL5 is co-induced with CD11c and T-bet by the DN2-generating cocktail. SLE patient naive B cells stimulated with the full cocktail were 85% FcRL5⁺ at day 6 alongside 95% CD11c⁺; in patient blood, DN2 cells uniformly expressed high FcRL5 and high CD19 while DN1 cells did not (see Zumaquero2019 - IFN-gamma Programs T-bet-hi B Cells for ASC Differentiation, human, n=20 HD + n=40 SLE + in vitro reconstruction). FcRL5 therefore behaves as part of an inducible IFN-γ-driven module rather than an independent lineage marker — relevant to the Sanz2019 - Consistent Classification of Human B Cell Populations observation that FcRL5 and FcRL4 are reciprocally expressed between SLE and HIV DN cells.

  • Fcrl5 belongs to the three-gene signature separating T-bet⁺CD11c⁺ B cells from GC B cells in mouse. Sorted-population RNA-seq at day 12 found Itgax, Tbx21 and Fcrl5 lower in GC B cells, the reciprocal of the GC genes Bcl6, S1pr2 and Aicda being downregulated in the T-bet⁺ population (see Song2022 - Tfh Outside Germinal Centers Drive T-bet CD11c B Cells, mouse, LCMV-Armstrong, sorted RNA-seq, 3 sorts pooling 4–6 spleens). FCRL5 therefore tracks the GC-independent fate in an acute resolving viral infection, not only in the chronic-infection and autoimmune settings this page’s other sources cover.

Contradictions & Debates

None documented in current wiki sources.

DN2 B Cell, FcRH4, Activated Naive B Cell, Double-Negative B Cell, SLAMF7, T-bet, Extrafollicular Response, Atypical B Cell, Age-Associated B Cell, Plasmablast

Sources

Graph View

Backlinks