Massó 2026 — Anti-GFAP antibodies elevated in FM; numerically increased serum GFAP

One-paragraph summary

Proof-of-concept case-control study (Martínez-Lavín group) measuring serum glial fibrillary acidic protein (GFAP) and anti-GFAP antibodies in 47 women with fibromyalgia vs 31 healthy controls, using pre-COVID-pandemic serum samples to avoid SARS-CoV-2 confounding. The hypothesis: dorsal root ganglion (DRG) satellite glial cells (SGCs) — the proposed target of the FM autoimmune axis — express GFAP as an early activation marker; environmental stressors are known to induce GFAP upregulation and structural modifications (including citrullination) that render the protein immunogenic. Anti-GFAP antibody optical density was significantly elevated in fibromyalgia patients (median 0.04 vs 0.02 HC; p=0.025), with 9 of 47 (19%) FM patients positive at the 99th-percentile control threshold. Serum GFAP itself was numerically higher in FM (244.0 vs 211.4 pg/mL) with borderline statistical significance (p=0.057). For the FM project, this paper identifies anti-GFAP IgG as a candidate component of the broader anti-SGC IgG fraction (Seefried 2025 identified 9 binding clusters; GFAP is a plausible target for one cluster). The 19% positive rate is notable: FM autoimmune subset is ~37% by Seefried 2025 broad assay; anti-GFAP may represent roughly half of the autoimmune subset. Critically for the retinal-biomarker priority, GFAP is the canonical marker of activated Müller cells (radial retinal glia) — meaning anti-GFAP antibodies in FM patient serum could plausibly cross-react with retinal Müller cells, providing a direct mechanistic linkage between the H1 autoimmune chain and retinal substrate alterations imageable on OCT. This was not tested in the paper but is a tractable extension predicted by the unified framework.

Claims as triples

• IgG_fm — binds → GFAP [evidence: anti-GFAP ELISA OD elevated in FM; confidence: emerging]
• anti_GFAP_antibody — present_in → fibromyalgia [19% positive at 99th-percentile threshold; confidence: emerging]
• anti_GFAP_antibody — predicts → fibromyalgia [diagnostic discrimination at the population level; confidence: emerging]
• GFAP — present_in → fibromyalgia [borderline elevation in serum; confidence: emerging]
• IgG_fm — binds → satellite_glia [via anti-GFAP mechanism; confidence: emerging — reinforces existing project triple]
• GFAP — present_in → muller_cell [established retinal-biology mechanism; inferred relevance to retinal substrate via cross-reactivity hypothesis; confidence: bridging]

Methods note

47 women with FM (ACR criteria; pre-pandemic samples) vs 31 healthy women. Two assays: (a) custom ELISA using recombinant human GFAP as antigen, measuring patient IgG binding optical density; (b) commercial human GFAP ELISA kit for serum GFAP quantification. Threshold for anti-GFAP-Ab positivity set at 99th-percentile of HC OD value (0.127). Statistical analysis: Mann-Whitney U tests for between-group comparisons.

Limitations

• Preprint, not yet peer-reviewed (Research Square).
• Female-only cohort — doesn't address whether the finding extends to male FM patients (rarer but present).
• Proof-of-concept sample size (n=47+31) — replication in larger cohorts essential before claiming established status.
• The 19% positive rate is at the 99th-percentile threshold; less stringent thresholds would give different rates. Threshold selection in autoimmune biomarker work is consequential.
• Does not test whether anti-GFAP antibodies cause SGC activation or pain in mice (no passive transfer; no functional readout) — the mechanistic claim is correlative.
• Does not test cross-reactivity with retinal Müller cells (a tractable extension predicted by the unified framework).

Open questions raised

• Are anti-GFAP-antibody-positive FM patients the same patients as the broader anti-SGC-IgG-positive cohort (Seefried 2025 ~37%)? (Direct overlap question — measures the antigen-specific fraction within the broader anti-SGC reactivity.)
• Do anti-GFAP antibodies from FM patients bind retinal Müller cells, and is Müller-cell activation imageable in vivo via OCT inner-retinal-layer changes? (Direct retinal-substrate extension predicted by the unified H1 framework. Tractable in stored FM serum + retinal-tissue immunohistochemistry.)
• Does the anti-GFAP-positive subset respond preferentially to plasma-cell-targeting therapy (daratumumab) or FcRn blockade (efgartigimod, rozanolixizumab)?
• Does the citrullinated form of GFAP have higher binding affinity than native GFAP for FM-patient IgG? (Tests the proposed environmental-stress → citrullination → immunogenicity chain.)

Triangulation notes

• Extends the anti-SGC IgG chain (Seefried 2025; Krock 2023; Hanani 2026; Sanchez 2025; Martínez-Lavín group's earlier work) at the antigen-specificity level. Identifies one specific candidate target (GFAP) within the broader anti-SGC reactivity that Seefried 2025 identified as 9 binding clusters. GFAP corresponds to a likely SGC-activation-marker-target cluster.
• Direct mechanistic linkage to the project's retinal-biomarker priority via Müller-cell GFAP expression. Müller cells are the retinal radial glia and the dominant GFAP-expressing cells in the retina. Anti-GFAP IgG in FM patient serum could in principle bind and activate Müller cells, producing inner-retinal-layer alterations imageable on OCT. This is the cleanest mechanistic candidate for an objective retinal biomarker of FM autoimmune-subtype activity.
• Aligns with I-2 calcium intersection (calcium-dysregulation-hypothesis.md v0.2): Müller cells are the retinal counterpart of DRG satellite glia in the I-2 (satellite-glia × Ca²⁺ waves) framework. GFAP-targeted autoantibody attack on Müller cells would extend the multi-ganglion SGC mechanism (Hanani 2026) to the retinal Müller cell substrate.
• Connects to Tian 2026 microglial-receptor-signaling axis: GFAP-positive reactive astrocytes share inflammatory-state signaling with reactive microglia in the gut-CNS-microglia chain. Reactive Müller cells in the retina would parallel this.

Bridges

• B-Re-5 (Anti-GFAP autoantibodies ↔ retinal Müller cell substrate, NEW 2026-05-23). One endpoint FM-anchored (this paper); other endpoint retinal-substrate-anchored (Müller cell GFAP expression, established retinal biology). Confidence: bridging — predicted but not tested in FM tissue. Closing: anti-GFAP IgG immunohistochemistry on retinal Müller cell preparations; OCT inner-retinal-layer assessment in anti-GFAP-positive FM patients.

Cure-path implications

The 19% anti-GFAP-positive subset is a candidate stratifier for the project's H1-cure-path program: these patients should respond preferentially to plasma-cell depletion (daratumumab), FcRn blockade (efgartigimod, rozanolixizumab), plasmapheresis + IVIG, or MRGPRX2 antagonism (barzolvolimab — via the shared MC effector cell from Sanchez 2025). The anti-GFAP ELISA Massó 2026 developed is a candidate stratifier assay alongside the broader Seefried-type anti-SGC ELISA, with the advantage of being antigen-specific.