EFB Dengue Literature Review

Phospho-Flow Cytometry

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Phospho-Flow Cytometry

Overview

Phospho-flow cytometry (phospho-flow) detects intracellular phosphorylation of signalling proteins in single cells by staining with phospho-specific antibodies after fixation and permeabilisation. It allows direct measurement of signalling pathway activation (e.g., ERK, p38 MAPK, STAT) in phenotypically defined cell subsets without the need for cell sorting. In B cell immunology, phospho-flow is used to compare TLR, BCR, and CD40 signalling responsiveness across B cell populations.

Key Points from Literature

  • TLR7 responsiveness of DN2 and aNAV cells: B cells stimulated with R848 (TLR7/8 agonist) and stained for phospho-ERK (pERK) and phospho-p38 MAPK (pMAPKp38). DN2 and aNAV cells showed significantly enhanced pERK and pMAPKp38 responses compared with rNAV, SWM, DN1, and NAV cells. This TLR7 hyper-responsiveness is the functional hallmark of EF pathway cells and is mechanistically explained by low expression of negative TLR regulators TRAF5 and TNFAIP3 (see Jenks2018 - DN2 B Cells and EF Pathway in SLE, phospho-flow on sorted/gated populations).

  • CD40L unresponsiveness of DN2: CD40L stimulation increased CD25 expression on rNAV and SWM cells but failed to activate DN2 cells. This result, assessed by surface marker upregulation rather than phospho-flow per se, complements the TLR7 phospho-flow data to establish the dual functional phenotype: TLR7-hyper-responsive + CD40L-unresponsive (see Jenks2018 - DN2 B Cells and EF Pathway in SLE).

  • ★ BCR signalling is intact across all three DN subsets — in health, in acute COVID-19, and after vaccination. DN1, DN2 and DN3 cells from healthy donors, mild and severe SARS-CoV-2 patients, and post-SARS-CoV-2-immunised individuals all maintained BCR signalling after IgG stimulation. DN2 showed the highest activation-marker expression (CD69, CD86) and the largest BCR signalling capacity of the three. This is the single strongest argument against reading DN cells as globally exhausted (see Beckers2023 - Origins and Functions of DN B Cells, review, citing Castleman 2022).

  • ⚠ Signalling readouts and activation-marker readouts disagree on the same cells. The same review reports a decreased percentage of CD86⁺ activated cells within DN B cells after CD40L or triple (CpG+anti-BCR+CD40L) stimulation relative to total B cells, in HD and MS patients — i.e. surface-activation assays call DN cells hyporesponsive while phospho-flow calls them signalling-competent. Methodological consequence: which assay is run determines the answer, so any claim about DN “responsiveness” must name its readout (see Beckers2023 - Origins and Functions of DN B Cells, review, own data; see In Vitro B Cell Stimulation, Double-Negative B Cell).

  • ★ Used to show that a cytokine changes a cell’s responsiveness rather than its resting signalling state — a design worth copying. Naive B cells were primed for 3 days under four conditions (anti-Ig + R848 alone, or plus IFN-γ, IL-2, or both), then washed and restimulated with IL-21 (10 ng/ml) for 20 min at 37°C, fixed/permeabilised with the BD Transcription Factor Phospho Buffer Set and stained for phospho-STAT3 (clone 13A3-1) (see Zumaquero2019 - IFN-gamma Programs T-bet-hi B Cells for ASC Differentiation, human, n=20 HD + n=40 SLE + in vitro reconstruction). The result depends entirely on the restimulation step: basal phospho-STAT3 at day 3 was similar and low across all four priming conditions, but after 20 min of IL-21 it was significantly higher in cells that had seen IFN-γ during priming. A resting-state phospho-flow readout would have found nothing. Pair with IL-21R — IL-21R protein was also 5.5–6-fold higher by day 6 in IFN-γ-primed cells.

Contradictions & Debates

  • Is the phospho-flow answer or the activation-marker answer the right one? Unresolved. Phospho-flow measures proximal signalling within minutes; CD86 upregulation measures a downstream transcriptional outcome hours later. A cell can signal normally and still fail to complete an activation programme, so the two are not strictly contradictory — but they cannot both be used to characterise DN cells in one sentence. See Beckers2023 - Origins and Functions of DN B Cells and Double-Negative B Cell Contradictions.

Conventional Flow Cytometry, DN2 B Cell, Activated Naive B Cell, TLR7, TRAF5, Extrafollicular Response

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