Aitella 2026 — Substance-P↔mast-cell bidirectional loop formalized at the molecular level

One-paragraph summary

This review formalizes the bidirectional crosstalk between substance P (SP) — the project's existing csf_substance_p biomarker entity, elevated in FM — and mast cells. Substance P is a tachykinin neuropeptide that activates MCs through MRGPRX2 (Mas-related G-protein-coupled receptor X2) and the NK1 receptor, triggering release of histamine, IL-6, TNF-α, and tryptase. In turn, MC mediators sensitize nearby sensory neurons that release more SP, creating a self-amplifying neuroinflammatory loop. The review reframes neurogenic inflammation as fundamentally an MC-SP loop and reinterprets several clinical "pseudoallergic" syndromes accordingly. For the FM project, this paper supplies the missing molecular link between FM's established CSF-SP elevation and the H2 mast-cell axis: the same MRGPRX2 receptor that Sanchez 2025 implicates as the effector for FM-IgG-driven MC activation is also the receptor for SP. The FM patient experiences sensitization through two ligand classes converging on a single receptor: circulating IgG and endogenous SP.

Claims as triples

• substance_P — activates → mast_cell [via MRGPRX2 and NK1R; confidence: established]
• substance_P — binds → MRGPRX2 [evidence: receptor pharmacology; confidence: established — IMPORTANT CONVERGENCE WITH IgG_fm]
• mast_cell — releases → substance_P [evidence: secretome studies; confidence: emerging]
• MC_SP_loop — causes → neurogenic_inflammation [self-amplifying; confidence: emerging]
• MC_SP_loop — present_in → fibromyalgia [inferred from CSF SP elevation + MC findings; confidence: inferred]
• MRGPRX2 — receptor_for → multiple_ligand_classes [SP, IgG, opioids, drug-induced pseudoallergens; confidence: established]

Methods note

Narrative review of molecular MC-SP biology with translational/clinical extension. No new primary data. Italian collaboration (Aquila, Naples groups), well-known immunology/allergy investigators. Receptor pharmacology and signaling pathways cited from established primary literature.

Limitations

• Review-tier; no new primary data on FM specifically.
• FM connection is mostly inferred from the project's existing knowledge of CSF-SP elevation; the paper itself frames the loop in broader pseudoallergic terms.
• MRGPRX2 binding promiscuity (SP, IgG, opioids, polypeptide drugs) is a strength biologically but a complication therapeutically: receptor blockade may have off-target effects on endogenous SP signaling that need careful clinical evaluation.
• The "MC-releases-SP" claim is on weaker evidentiary ground than "SP-activates-MC"; the loop's symmetry is sometimes overstated in review-tier literature.

Open questions raised

• Q-NEW (Q84): Does CSF SP elevation in FM correlate with serum tryptase / anti-SGC IgG titer / HαT status? Co-measurement in a single FM cohort would test the unified-MRGPRX2-mechanism hypothesis.
• Q-NEW (Q85): Does MRGPRX2 antagonism (Sanchez 2025's candidate cure-path arm) attenuate the SP-driven component of FM pain in addition to the IgG-driven component? If yes, MRGPRX2 antagonists become a dual-mechanism therapy.
• Are NK1R antagonists (aprepitant, fosaprepitant — FDA-approved for chemo-induced nausea) candidate FM therapies? They block the other major SP receptor; if MRGPRX2 antagonism is unavailable or has off-target liabilities, NK1R blockade is a parallel approach.

Triangulation notes

• Major convergence with Sanchez 2025: both papers identify MRGPRX2 as the relevant MC-activating receptor. This is the single most important mechanism convergence in this batch — two independent literatures (the FM-IgG passive transfer paradigm and the SP-neurogenic-inflammation literature) point to the same receptor as the FM mechanism node.
• Connects to project's existing csf_substance_p biomarker entity: CSF SP elevation in FM has been an "established but unmotivated" finding — the mechanism by which SP elevation produces pain was assumed but not specified. This paper supplies the MC-SP loop as the operative mechanism.
• Integrates with Simeone 2026 (mucosal MC/MRGPRX2): HαT amplifies MC count + MRGPRX2 expression → amplifies SP-MC loop sensitivity → amplifies pain phenotype.
• Integrates with Amato 2026 (lipedema gfWAT-IIT2): estrogen-receptor MC activation operates in parallel to the SP-MC loop and to the IgG-MRGPRX2 loop. Three activation triggers converging on the same effector cell.

Bridges

• B-NEW (B10): Substance-P / NK1R-MRGPRX2 axis ↔ FM-MC mechanism - Shared entities: substance_P, csf_substance_p, mast_cell, MRGPRX2, NK1R - Why interesting: Two independent FM findings (CSF SP elevation, anti-SGC IgG) plus a new mechanism finding (Sanchez 2025) all involve MRGPRX2. The receptor is emerging as the central node of the FM-MC subtype. - What would close: pharmacological dissection in FM patients — separate MRGPRX2 antagonism trial vs NK1R antagonism trial vs combined, with response stratification by CSF SP + anti-SGC IgG status. - Confidence: emerging (because both endpoints of the bridge are now FM-anchored, not just one) - Status: open

Cure-path implications

The MC-SP loop reinforces the four-level intervention hierarchy from Sanchez 2025 with a fifth potential point of intervention:

• Plasma cells → circulating IgG → MRGPRX2 → MC → MC mediators (cromolyn etc.)
• ADD: NK1R blockade (aprepitant) interrupts the SP feedback arm of the loop without touching MRGPRX2 directly. If MRGPRX2 antagonists are slow to clinic, NK1R blockade is a drug-repurposing-tier candidate using FDA-approved compounds with established safety profile.

Aprepitant has been studied in cough, depression, and pruritus — none of these have established FM signal, but stratified retrial in CSF-SP-elevated FM patients would be a direct mechanism test.