Sanchez 2025 — FM-IgG sensitisation runs through MRGPRX2/Mrgprb2 on mast cells

One-paragraph summary

This paper supplies what was structurally missing from the Goebel 2021 passive-transfer paradigm: a specific cellular and receptor mechanism for how FM patient IgG produces pain hypersensitivity in mice. Sanchez and Dong, in direct collaboration with Goebel, show that FM-IgG binds to mast cells via the MRGPRX2 receptor (human) / Mrgprb2 (mouse ortholog), triggering MC recruitment and IL-6 secretion. Critically, the IgG-induced mechanical and cold hypersensitivity is abolished by either (a) genetic deletion of Mrgprb2 or (b) ablation of Mrgprb2-expressing mast cells. Human translation: stimulating the human LAD2 mast cell line with FM-IgG elicited MRGPRX2-dependent IL-6 production. The finding unifies the H1 (autoantibody-mediated) and H2 (mast cell) axes of the project's mechanism map into a single chain: FM-IgG → MRGPRX2-bearing mast cells → IL-6 + MC mediators → sensory neuron sensitization. It also reframes the Goebel 2021 paradigm: the previously-emphasized antigen-specific binding to satellite glia (Seefried 2025, Hanani 2026) is not the only — and may not be the dominant — effector pathway. MRGPRX2 is a promiscuous receptor that binds cationic ligands non-canonically; the mechanism may not require a specific antigen.

Claims as triples

• IgG_fm — causes → mast_cell_activation [evidence: passive transfer + LAD2 stimulation; confidence: emerging]
• IgG_fm — binds → MRGPRX2 [evidence: LAD2 cells + Mrgprb2-deletion mice; confidence: emerging — NEW MECHANISM]
• MRGPRX2 — present_in → mast_cell [evidence: established receptor biology + this paper's confirmation; confidence: established]
• mast_cell — responds_to → IgG_fm [evidence: IL-6 secretion in LAD2; confidence: emerging]
• Mrgprb2_deletion — contradicts → IgG_fm_passive_transfer_hypersensitivity [evidence: KO mice protected; confidence: emerging — CAUSAL]
• mast_cell_ablation — contradicts → IgG_fm_passive_transfer_hypersensitivity [evidence: cell-depleted mice protected; confidence: emerging — CAUSAL]
• MRGPRX2_antagonism — predicted_intervention_for → fm_autoimmune [confidence: inferred — direct therapeutic implication]

Methods note

FM patient serum-IgG was purified and injected into mice across two experimental arms: 1. Wild-type vs Mrgprb2-knockout C57BL/6 mice — measured mechanical (von Frey) and cold (acetone) hypersensitivity post-transfer. Hypersensitivity developed in WT but not in Mrgprb2-KO mice. 2. Mast-cell-ablation mice (Mrgprb2-Cre × inducible DTR) — same protocol; ablation prevented hypersensitivity in WT genetic background, confirming that MCs (not just the receptor) are the effector cell.

In parallel, human LAD2 cells (a human mast cell line that expresses MRGPRX2) were stimulated with FM-IgG; IL-6 secretion was measured by ELISA and shown to be MRGPRX2-dependent (presumably via siRNA knockdown or pharmacological antagonist; full text needed to confirm method).

Authorship is significant: Andreas Goebel (the originator of the FM passive-transfer paradigm via Goebel 2021 and reviewer-author of Goebel 2025) is a co-author, signaling that this mechanism integrates with rather than supersedes the existing Goebel framework. Xinzhong Dong (Johns Hopkins HHMI investigator) is the senior MRGPR-receptor researcher whose lab originally characterized the MRGPRX2/Mrgprb2 receptor pair as the non-IgE MC-activating receptor for cationic ligands.

Limitations

• Preprint, not yet peer-reviewed. Posted to bioRxiv 2025-05-16. Methods and figures should be re-verified at publication. ACR conference abstract #494 (the queue's unknown-g-494 stub) is the preliminary version.
• The conference abstract framing was "PASC" (post-acute sequelae of SARS-CoV-2); the full preprint reframes to FM syndrome. The PASC framing of the abstract may indicate the lab is pursuing parallel work in PASC IgG via the same mechanism — worth tracking.
• The mechanism is established in mouse passive transfer; human-tissue confirmation is limited to the LAD2 cell-line experiment. Direct demonstration of MRGPRX2-mediated MC activation in human FM skin (or other tissue) would be the next required experiment.
• The relationship between this MRGPRX2-dependent mechanism and the satellite-glia-targeting antigen-specific mechanism (Seefried 2025's 9 binding clusters, Hanani 2026's multi-ganglia SGC activation) is not yet resolved. The paper does not rule out that both operate; it shows that MRGPRX2/MC is sufficient for the passive-transfer pain phenotype but does not exclude an independent SGC-binding contribution.
• Sample size of FM patient sera (the IgG pool) is not reported in the search summary; full text would clarify.

Open questions raised

• Q24 partial closure: Are the anti-SGC IgG-positive and MC-active FM patient subsets the same patients, distinct, or overlapping? This paper's mechanism suggests they may be the same patients viewed through different assays: anti-SGC IgG is the antigen-specific surrogate; MRGPRX2-mediated MC activation is the functional readout of the same circulating IgG pool. Co-measurement in a single cohort (anti-SGC ELISA + LAD2 IgG-activation assay) would directly test.
• Q-NEW (Q75): Does anti-SGC IgG titer correlate with MRGPRX2-activating capacity in the same patient serum? If yes, the two assays are measuring related quantities and either can serve as a stratifier. If not, they are independent biomarker axes and the H1 chain bifurcates downstream of "FM-IgG production."
• Q-NEW (Q76): Do existing MRGPRX2 antagonists (in clinical development for chronic urticaria, e.g., barzolvolimab and similar) attenuate FM symptoms in anti-SGC IgG-positive patients? This is a direct cure-path arm test using a clinical-stage compound class.
• Q-NEW (Q77): Is the MRGPRX2 mechanism FM-specific, or does it also operate for the Marie 2026 long-COVID IgG passive transfer paradigm? If yes, MRGPRX2 antagonism becomes a cross-condition cure-path arm for autoantibody-mediated post-infectious chronic pain.
• Does the lipedema-comorbid FM subset (35-40% prevalence per Amato 2026) have higher MRGPRX2-mediated IgG activation than non-lipedema FM? Predicted by H2-MC framework.

Triangulation notes

• Single most important mechanism paper for the project so far in 2026. Unifies H1 and H2 axes at the receptor level.
• Reframes Goebel 2021/2025 paradigm: the passive-transfer pain phenotype is now mechanistically explained as IgG → MRGPRX2 → MC → IL-6 → sensitization. The earlier assumption that IgG → SGC binding was the operative pathway is not refuted but is no longer required as the sole explanation.
• Reframes Seefried 2025 / Hanani 2026 / Krock 2023: anti-SGC IgG ELISA remains a valid clinical assay (it correlates with severity per Krock 2023), but the mechanism it indexes is now plausibly both antigen-specific glial-targeting and receptor-promiscuous MC activation. The Hanani 2026 multi-ganglia SGC activation may be downstream of MC mediators (released cytokines, tryptase, histamine acting on nearby SGCs) rather than direct IgG-on-SGC binding — testable distinction.
• Integrates with Lyons 2019 HαT/TPSAB1: HαT individuals have elevated baseline tryptase and increased MC count + MRGPRX2 expression per Simeone 2026. This paper predicts HαT-positive FM patients should have stronger IgG-passive-transfer response — i.e., HαT is a heritable amplifier of the FM-IgG-MRGPRX2 mechanism. Directly testable.
• Integrates with Christoforou 2026 high-dose cromolyn: if MRGPRX2 is the relevant MC receptor in FM, then cromolyn (a non-selective MC stabilizer) and selective MRGPRX2 antagonists should be ranked by FM-IgG-displacement capacity in vitro before clinical trial.
• Extends to PASC via Marie 2026 (also ingested this batch): Marie 2026 shows LC-IgG → DRG → pain in mice, mirroring Goebel paradigm. Whether the LC-IgG mechanism also operates through MRGPRX2 is the key question; Sanchez+Dong's ACR conference framing (which explicitly used PASC framing) suggests the lab is pursuing this.

Bridges

• B1 (Long COVID ↔ FM via autoantibody-mediated SFN) — major strengthening. The Marie 2026 LC-IgG passive transfer paper is structurally identical to the Sanchez 2025 FM-IgG paradigm; if MRGPRX2 mediates both, the bridge moves from bridging toward emerging.
• B3 (CRPS ↔ FM via pathogenic IgG) — strengthening. Goebel's earlier CRPS IgG passive-transfer work is the template; MRGPRX2 mechanism likely operates there too. Direct test possible.
• B-NEW: Chronic spontaneous urticaria ↔ FM via MRGPRX2 — propose new bridge. Urticaria research has driven MRGPRX2 antagonist development. If the receptor mechanism is shared, urticaria's clinical-stage compounds (e.g., barzolvolimab, CDX-0159) are candidate FM-autoimmune-subtype therapeutics. Adds to cure-path arm class.

Cure-path implications

This is the most direct mechanism-to-intervention paper in the project's evidence base for the H1×H2 unified subtype:

1. MRGPRX2 antagonists (clinical-stage) — barzolvolimab (Celldex) and CDX-0159 are in Phase 2/3 for chronic spontaneous urticaria; could be tested in anti-SGC-IgG-positive FM patients with no IND barrier. Stratified pilot trial design is straightforward. 2. Mast cell stabilizers — cromolyn, ketotifen, etc. revisited under the MRGPRX2 framing. Christoforou 2026's high-dose cromolyn pilot is consistent with this mechanism; ketotifen's negative Ang 2014 result may reflect insufficient receptor coverage rather than wrong mechanism. 3. FcRn blockade (efgartigimod, rozanolixizumab) — reduces circulating IgG; operates upstream of the MRGPRX2-binding step. Q48 of the open-questions file is now structurally re-validated. 4. Daratumumab (plasma-cell depletion) — operates further upstream still. The Fluge/Mella 2025 ME/CFS pilot is the existing anchor.

The cure-path-arm class for H1×H2 subtype now has FOUR distinct intervention levels: plasma cells (daratumumab) → circulating IgG (FcRn blockers, plasmapheresis) → receptor (MRGPRX2 antagonists) → MC stabilization (cromolyn). This is a more complete therapeutic hierarchy than for any other FM subtype.