EFB Dengue Literature Review

FcRH4

7 min read

FcRH4

Overview

FcRH4 (Fc Receptor-Like 4; also FCRL4, IRTA1) is an ITIM-containing Fc receptor homolog expressed on a subset of human memory B cells. It can exert powerful inhibitory effects on BCR signalling. Critically, FcRH4 expression distinguishes tissue-resident (tonsillar/mucosal) CD27⁻ memory B cells from circulating peripheral blood CD27⁻ (double-negative) memory B cells — a distinction with practical implications for flow cytometry panel design.

FcRH4 is closely related to FcRH5 (FCRL5), which has emerged in more recent literature as a marker of atypical/T-bet⁺ B cells in acute infections including malaria, SARS-CoV-2, and potentially dengue.

Key Points from Literature

  • PBL DN B cells (IgD⁻CD27⁻) in both healthy subjects and SLE patients are uniformly FcRH4⁻; FcRH4 expression is absent from all peripheral blood B cell subsets (see Wei2007 - DN Memory B Cells in SLE, n=29 healthy + n=36 SLE cross-sectional).

  • In tonsil, FcRH4 is expressed in a fraction of CD27⁻ cells (whether IgD⁺ or isotype-switched), and in a subset of Bm5 CD27⁻ cells. Tonsillar DN B cells thus contain both FcRH4⁺ and FcRH4⁻ fractions (see Wei2007 - DN Memory B Cells in SLE).

  • This FcRH4 expression pattern was first described by Ehrhardt et al. (2005, J Exp Med) as marking a “distinctive tissue-based population of memory B cells” — cited in Wei2007 - DN Memory B Cells in SLE as the comparator population.

  • The absence of FcRH4 on PBL DN cells distinguishes Wei et al.’s circulating DN population from the Ehrhardt tissue-resident population; the recirculating FcRH4⁻ DN cells are proposed to be the biologically relevant population in peripheral blood studies (see Wei2007 - DN Memory B Cells in SLE).

  • FCRL4⁻ on SLE DN2 cells — a key distinguishing feature: DN2 B cells (IgD⁻CD27⁻CXCR5⁻CD21⁻CD11c⁺) in SLE are FCRL4⁻ but FCRL5⁺. This FCRL4⁻/FCRL5⁺ pattern sharply distinguishes DN2 cells from HIV-associated exhausted CD21⁻ memory B cells, which are FCRL4⁺. The FCRL4⁻ status of DN2 cells also indicates that they retain intact BCR signalling capacity (FCRL4 is an ITIM-bearing inhibitory receptor), consistent with their functional role as active pre-plasmablasts rather than anergic/exhausted cells (see Jenks2018 - DN2 B Cells and EF Pathway in SLE, flow cytometry on SLE + HCD cohorts).

  • Note on FcRH5/FCRL5: FcRH5 (a related family member) has emerged as a marker of atypical B cells in malaria and COVID-19 literature. The FCRL4⁻/FCRL5⁺ pattern in SLE DN2 cells is now the most precisely defined phenotype within this receptor family for extrafollicular effector B cells. See FCRL5 for full characterisation.

  • Historical origin of the “atypical memory” label: The initial proposal for a separate AtB memory compartment stemmed from the identification of blood CD27⁻ CD21lo B cells in HIV patients with phenotypic features similar to FcRH4⁺ tissue-based memory cells (Ehrhardt et al. 2005). The extension of the FcRH4⁺ tissue-based label to circulating CD27⁻ cells is now recognised as misleading, since circulating DN cells are uniformly FcRH4⁻ (see Sanz2025 - Human Atypical B Cells Overview, review).

  • FCRL4 appears in the review-level marker set for human atypical B cells. Human ABCs are described as typically lacking CD27 and CD21 while often expressing T-bet with CD11c, CXCR3, FCRL4, and FCRL5 (see Glaros2025 - Multilayered Identity of B Cell Memory, review, no original data). ⚠ This sits in tension with the wiki’s DN2 definition, which is explicitly FCRL4⁻ — the absence of FCRL4 is what distinguishes SLE DN2 cells from HIV exhausted memory and from tissue-based FCRL4⁺ memory (Jenks2018 - DN2 B Cells and EF Pathway in SLE; see DN2 B Cell). The most likely reading is that the review’s marker list is a union across the heterogeneous “ABC” umbrella — which includes tissue-associated FCRL4⁺ populations — rather than a conjunctive definition of any one cell. Treat FCRL4 as an umbrella-level marker, not a DN2 marker.

  • FCRL4 originally defined a tissue-based memory population. The review cites the founding description of FcRH4/FCRL4 as “an immunoregulatory molecule” defining a distinctive tissue-based population of memory B cells (citing Ehrhardt 2005 J Exp Med), which is consistent with its appearance in the atypical marker set via tissue-resident rather than circulating cells. See Tissue-Resident Memory B Cell.

  • Corroboration of the tonsil⁺/blood⁻ split, with a functional inference attached. A 2023 review restates that FcRH4⁺ DN cells predominate in tonsil while peripheral blood DN cells are FcRH4⁻ in both HD and SLE, and adds the authors’ speculation that this is why circulating DN cells are more responsive to activation and expansion than their tissue-resident counterparts — i.e. FcRH4 is offered as the brake that the blood population lacks (see Beckers2023 - Origins and Functions of DN B Cells, review, citing Wei 2007 / Ehrhardt 2003 — note this is the 2003 PNAS paper, not the 2005 J Exp Med paper cited elsewhere on this page).

  • ★ [2026-08-27] FcRL4⁺ cells are ~15–18% of blood DN B cells in HIV — but ~1% in healthy controls and in IgG4-related disease (n=5 per group, p<0.0001). FcRL4 was included in a 13-colour B cell panel specifically to distinguish the IgG4-RD DN compartment from the HIV DN compartment, and it did so cleanly (see Allard-Chamard2023 - DN3 B Cells Infiltrate Inflamed Tissues, n=5 per group).

  • ⚠ [2026-08-27] This qualifies, and partly conflicts with, the standing wiki claim that FcRH4 is absent from all peripheral blood B cell subsets. Wei2007 - DN Memory B Cells in SLE reported FcRH4 absent from every blood subset in healthy donors (n=29) and SLE (n=36). Allard-Chamard’s healthy and IgG4-RD values (~1%) are consistent with Wei; the discrepancy is confined to HIV, which Wei did not study. The reconciliation is therefore most likely disease context rather than measurement error — chronic viraemic HIV appears to be the setting in which blood DN cells acquire FcRL4. Practical consequence: FcRH4-negativity cannot be assumed for blood DN cells in a chronic viral infection, and FcRH4 is therefore a less clean tissue-residency discriminator than the wiki has treated it. Both n’s are small (n=5 per group here) (see Allard-Chamard2023 - DN3 B Cells Infiltrate Inflamed Tissues).

  • ★ FcRL4 is the HIV pole of the reciprocal split, and it defines a third DN population in Table 1. In DN cells across RA, SLE, scleroderma and acute/chronic HIV, FcRL4 is high in HIV DN2-gated cells (~21.1%) and near-absent in SLE (~0.74%), while FCRL5 runs the opposite way. Table 1 lists an IgD⁻CD27⁻CD38⁻CD24⁻CD21⁻FcRL4⁺ population, annotated “atypical/tissue-based memory”, separately from DN2 — phenotypically identical on the core markers and distinguished only by FcRL4. The review’s reading is that CD11c⁺ SLE DN2 are activated effectors while the FcRL4⁺ HIV cells “are thought to be anergic” (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). Open in this wiki: whether that FcRL4⁺ row is the same cell as DN3 B Cell — FcRL4 has never been measured in the DN3 papers.

Contradictions & Debates

  • ⚠ Internal tension within the same review: are blood DN cells FcRH-negative or FcRH-positive? Beckers2023 - Origins and Functions of DN B Cells states in §2 that DN and CD20^hi CD27⁻CD21^lo B cells express multiple inhibitory receptors including FcRH3-5 in HIV-infected, malaria-infected, anti-SARS-CoV-2-immunised, and young and aged healthy donors — while §5 of the same review restates that peripheral blood DN cells are FcRH4⁻ in HD and SLE. FcRH4 falls inside the FcRH3-5 range, so the two statements cannot both hold for healthy-donor blood as written. The most likely reconciliation is that “FcRH3-5” in §2 is carried over from source studies reporting FcRH3 and/or FcRH5 without FcRH4, or applies to the CD20^hi CD21^lo HIV tissue-like memory population rather than to DN cells. Until the primaries are traced, the wiki keeps the Wei2007 primary result — circulating DN cells are FcRH4⁻ — as the operative claim, and treats the inhibitory-receptor finding as applying to FcRH3/FcRH5 and to the exhausted CD21^lo compartment.

Double-Negative B Cell, DN2 B Cell, FCRL5, CD27, Memory B Cell, Extrafollicular Response, Atypical B Cell, Age-Associated B Cell, Tissue-Resident Memory B Cell

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