H5b — Immune-Barrier-Calcium Intersection Chain (candidate, sibling to H5)

Status: project synthesis document, version 0.1, 2026-05-24 Drawn from: ontology entities staged 2026-05-24 (H5b module — immune_barrier_calcium_intersection_module, 4 new gene entities ORAI1/STIM1/OCLN/TJP1, plus shared MYO9B/CTLA4 from H5); bridges B-Gen-3 and B-Gen-4; open questions Q-IBC-1..9; the project's CA-I1 intersection in calcium-dysregulation-hypothesis.md; the H1×H2 unified mechanism in fm_h1_h2_interactive.html. Purpose: define H5b as a calcium-handling-amplifier-stratified version of the H1×H2 mechanism. H5b is anchored on the recognition that a six-gene set — ORAI1 + STIM1 + MYO9B + OCLN + TJP1 + CTLA4 — deliberately straddles three project domains (immune × barrier × calcium) and docks directly into the existing CA-I1 calcium intersection.

TL;DR

H5b says: within FM-IgG-positive patients, there exists a subgroup whose mast-cell degranulation responsiveness is genetically amplified at the calcium-handling layer (ORAI1/STIM1 risk-allele carriers → magnified SOCE → magnified MRGPRX2-driven degranulation per unit FM-IgG stimulus) AND whose autoreactive T-cell pool is similarly amplified at the same molecular substrate (ORAI1/STIM1 SOCE → NFAT activation → autoreactive B-cell help) AND whose gut barrier carries multi-component tight-junction vulnerability (MYO9B + OCLN + TJP1 variants) AND whose T-cell tolerance is impaired (CTLA4 variants). Five vulnerabilities, one coherent patient phenotype, and a clinical-stage drug at each layer.

Confidence at chain level: bridging. No FM-direct ORAI1/STIM1 evidence; the CA-I1 framing is project-internal in calcium-dysregulation-hypothesis.md but not yet anchored on a primary FM paper. The right substrate-level test is ORAI1/STIM1 genotyping in FM-IgG-positive vs FM-IgG-negative cohorts (Q-IBC-1). The most mechanistically-informative test is functional SOCE measurement on patient-derived MC or PBMCs (Q-IBC-2). The most concrete cure-path-arm test is a stratified CRAC channel inhibitor pilot (CM4620 / Auxora — Calcimedica Phase 2/3 pancreatitis program, no IND barrier, Q-IBC-3).

Sibling relationship to H5

H5 (autoimmune-genetic-risk chain — celiac+T1DM signature) and H5b (immune-barrier-calcium intersection chain) are sibling candidates per Ken's option-b decision 2026-05-24. They are not competing hypotheses; they are two different gene-set framings of how to stratify the H1 FM-IgG-positive subset.

Shared between H5 and H5b: CTLA4 (T-cell coinhibitory checkpoint), MYO9B (Rho-GAP barrier-integrity).

H5-specific (8 genes): HLA-DQ2/DQ8, PTPN22, IL2/IL21 locus, IL2RA, TNF, CLDN2, TGM2.

H5b-specific (4 genes): ORAI1, STIM1, OCLN, TJP1.

The H5-H5b direction-decision question is Q-IBC-8: do these two modules identify the same FM-IgG-positive patients, mostly the same patients, or orthogonal subgroups? Joint genotyping of all 13 distinct loci on a single 200-patient FM-IgG-positive cohort would resolve whether to consolidate into H5 v0.2 or keep as sibling candidates.

Why H5b is structurally different from H5

H5 supplies a celiac-spectrum autoimmunity genetic substrate upstream of the H1 chain. H5b supplies a calcium-handling amplifier genetic substrate at the existing H1×H2 effector layer. Two consequences:

1. H5b docks into the existing CA-I1 intersection. The calcium-dysregulation hypothesis v0.2 (calcium-dysregulation-hypothesis.md) defines CA-I1 as "Mast cell × STIM1/Orai1 SOCE downstream of MRGPRX2 / NK1R / ER-IP3R." H5b says: this calcium-handling layer is genetically modulable at the patient level, and FM-IgG-positive patients with SOCE-amplifying ORAI1/STIM1 variants have the H1×H2 mechanism turned up at the substrate level.

2. H5b is multi-effector-cell coherent at the molecular level. ORAI1/STIM1 → SOCE operates at both T cells (NFAT activation → autoreactive B-cell help → FM-IgG production) and mast cells (CA-I1 intersection — Sanchez 2025 mechanism amplification). One molecular system, two FM-relevant cell types, both already in the project's H1×H2 framework. The H5b prediction is therefore not "find a new chain" — it is "the same chain runs harder in carriers of these variants."

The differentiating implication: CRAC channel inhibition (CM4620 / Auxora) would interrupt both arms simultaneously. Where MRGPRX2 antagonism (barzolvolimab) blocks one specific receptor input to MC, CRAC blockade acts on the downstream calcium-handling layer regardless of receptor input. The two interventions are mechanistically additive in H5b-stratified FM-IgG-positive patients with active Sanchez 2025 mechanism.

The H5b chain (vertical, top-to-bottom)

``` LEVEL 1 — GENETIC SUBSTRATE (three-domain intersection) ├── IMMUNE: CTLA4 (T-cell coinhibitory checkpoint — shared with H5) ├── IMMUNE + CALCIUM: │ ORAI1 — CRAC channel pore-forming subunit │ STIM1 — ER calcium sensor gating ORAI1 │ (these two are THE SOCE molecular system; CA-I1 in calcium hypothesis) └── BARRIER: MYO9B — Rho-GAP regulator (shared with H5) OCLN — occludin (tight-junction integral protein) TJP1 — ZO-1 (tight-junction scaffold)

LEVEL 2 — TRIGGER INTERSECTION (same as H1 — dysbiosis, viral, stress, trauma)

LEVEL 3 — PROCESS LAYERS ├── T-cell hyperactivation │ ← ORAI1/STIM1 SOCE → NFAT activation │ ← CTLA4 tolerance failure ├── MC hyperresponsiveness │ ← ORAI1/STIM1 SOCE in MC (CA-I1 intersection) │ → magnified Sanchez 2025 MRGPRX2 response per unit FM-IgG └── Barrier failure ← MYO9B Rho-GAP variants ← OCLN integral-protein variants ← TJP1 scaffold variants

LEVEL 4 — DOWNSTREAM EFFECTORS (merges with H1) ├── Germinal-center hyperactivity (IL-21 — shared with H5) ├── Plasma cell expansion → FM-IgG production └── Bile acid translocation (Jakobsson 2026 — H1 anchor)

LEVEL 5 — H1×H2 CONVERGENCE (amplified at CA-I1) ├── FM-IgG → MRGPRX2 (Sanchez 2025) ├── ★ MC SOCE response amplified by ORAI1/STIM1 variants (H5b-specific) ├── MC degranulation → mediators (IL-6, histamine, tryptase, TNF-α, chymase) └── Satellite glia + sensory neuron sensitization (Hanani 2026)

LEVEL 6 — SYMPTOMS (same as H1) ├── Widespread pain └── Multimodal hypersensitivity ```

The architectural claim of H5b: levels 1–4 supply the genetic-substrate amplifier; level 5 is where the amplification expresses itself in the existing H1×H2 mechanism. Same downstream as H1/H5 — but with a calcium-handling-layer magnifier that is measurably present in patient cells (Q-IBC-2 functional SOCE assay).

The 6 genes — what each contributes

Immune + Calcium intersection (the H5b signature)

ORAI1 — Pore-forming subunit of the CRAC channel. Mediates SOCE at the plasma membrane downstream of STIM1-sensed ER calcium depletion. Critical for T-cell NFAT activation AND mast cell degranulation. Loss-of-function variants cause SCID with autoimmunity (illustrating the central role); H5b candidate variants are gain-of-function or amplification-risk variants that magnify SOCE. Pharmacologically tractable via CRAC channel inhibitors (CM4620 / Auxora — Calcimedica Phase 2/3 acute pancreatitis program). The single most differentiating gene in the H5b module because it ties the immune mechanism to the calcium mechanism via one molecular substrate.

STIM1 — ER membrane calcium sensor. Detects ER calcium depletion and oligomerizes at ER-PM junctions to activate ORAI1. The gating partner of ORAI1. Cell-type-specific knockouts in mice show STIM1 in T cells is required for autoimmunity development AND STIM1 in MC is required for IgE- and MRGPRX2-mediated degranulation — directly relevant to FM-IgG → MC mechanism (Sanchez 2025). H5b prediction: ORAI1 and STIM1 risk-allele carriers will show enrichment in the FM-IgG-positive subset because the substrate that amplifies MRGPRX2-driven degranulation is shared with the substrate that amplifies T-cell-driven autoantibody production.

Barrier (multi-component tight-junction vulnerability)

MYO9B (shared with H5) — Rho-GAP regulating epithelial barrier integrity upstream of the tight junction itself. Supplies the regulatory layer.

OCLN (occludin) — Tetraspan tight-junction integral membrane protein. Contributes to barrier sealing and cross-talk with the cytoskeleton (via ZO-1 / TJP1 scaffolding). Distinct from claudins. Expression dysregulation in IBD and celiac. H5b adds OCLN to the barrier vulnerability layer alongside CLDN2 (H5) — three different molecular layers of the tight junction now spanned by the combined H5 + H5b framing.

TJP1 (ZO-1) — Tight-junction scaffold protein anchoring claudins and occludin to actin. The structural backbone. ZO-1 redistribution is the standard in-vitro signature of tight-junction disruption — making TJP1/ZO-1 the natural cellular assay target for larazotide-like interventions.

Immune (shared with H5)

CTLA4 — T-cell coinhibitory checkpoint. Risk variants impair Treg function and let autoreactive T-cell activation slip through. Druggable via abatacept (FDA-approved RA). H5b adds: response should be magnified in patients who are ALSO ORAI1/STIM1 risk-allele carriers because both genes operate at the T-cell layer — abatacept restores the coinhibitory signal that the genetic risk attenuates, and the SOCE-amplifier variant adds to the activation pressure being restrained.

CA-I1 anchoring — the structural payoff

The crucial architectural feature of H5b is its dock into the existing CA-I1 intersection. From calcium-dysregulation-hypothesis.md v0.2:

> I-1: Mast cell | STIM1/Orai1 SOCE downstream of MRGPRX2 / NK1R / ER-IP3R | FM-IgG, substance P, estrogen | Sanchez 2025, Aitella 2026, Wang 2025 | MRGPRX2 antagonists (barzolvolimab); NK1R antagonists (aprepitant); CRAC blockers; MC stabilizers (cromolyn)

H5b adds the genetic-risk dimension to this intersection: the patient-level question of who carries the SOCE-amplifying variants and who therefore responds preferentially to CRAC blockade vs. MRGPRX2 antagonism. This converts CA-I1 from a population-level mechanism description into a patient-stratifiable cure-path arm.

The discriminating prediction: CRAC channel inhibitor pilot in H5b-stratified FM patients should respond at meaningfully higher rates than unselected FM-IgG-positive patients. A 30-50 patient stratified open-label study with primary endpoint FIQR reduction at 12 weeks would supply the CA-I1 closing evidence the calcium hypothesis currently lacks.

Intervention break-points

Six H5b cure-path-arm candidates, ordered from genetic stratification through three downstream therapy classes plus shared H5 candidates:

Note: the most discriminating cure-path-arm test is the CRAC blocker vs MRGPRX2 antagonist head-to-head in H5b-stratified patients. Both target MC degranulation but at different molecular levels — CRAC blockade at the calcium-handling substrate, MRGPRX2 antagonism at the receptor. If CRAC blockade outperforms MRGPRX2 antagonism in H5b carriers, that anchors H5b as a real distinct subset. If the two are equivalent, H5b adds patient stratification but not new intervention biology.

What promotes H5b from bridging to emerging

A single positive finding at any of the following gates promotes H5b:

1. ORAI1/STIM1 risk-allele enrichment in FM-IgG-positive vs FM-IgG-negative cohorts (Q-IBC-1) — promotes B-Gen-3. 2. Functional SOCE elevation on patient-derived MC or PBMCs in FM-IgG-positive vs FM-IgG-negative (Q-IBC-2) — promotes B-Gen-3, more informatively than Q-IBC-1 because it bypasses genotype heterogeneity. 3. Symptom-load reduction in CRAC channel inhibitor pilot in H5b-stratified FM patients (Q-IBC-3) — promotes H5b chain itself, AND supplies the FM-direct CA-I1 evidence the calcium hypothesis currently lacks (Q-IBC-9). 4. Joint H5 + H5b genotyping on a 200-patient FM-IgG-positive cohort (Q-IBC-8) — resolves direction-decision: consolidate or keep as siblings.

The lowest-cost gate is Q-IBC-1 (ORAI1/STIM1 genotyping ~$100/patient). The most mechanistically-informative gate is Q-IBC-2 (functional SOCE). The most cure-path-defining gate is Q-IBC-3 (CRAC blocker stratified pilot).

What would refute H5b

H5b would be refuted (not merely sharpened) by:

1. ORAI1/STIM1 risk-allele carriage frequency in FM-IgG-positive patients indistinguishable from the general population in a 200-patient or larger cohort. 2. Functional SOCE measurement in FM-IgG-positive patient cells indistinguishable from FM-IgG-negative patient cells AND from healthy controls. 3. Negative result on CRAC channel inhibitor pilot in well-stratified H5b-positive FM cohort with adequate power.

(1) is cheap (~$5-10K for 100 patients); (2) requires functional cell biology setup but is established methodology; (3) requires a stratified pilot trial. The framework is therefore disciplined — near-term-falsifiable at each layer.

Cross-references

• H1×H2 unified mechanism: fm_h1_h2_interactive.html — the convergent MC + MRGPRX2 effector node that H5b amplifies.
• Calcium-dysregulation hypothesis (CA-I1): calcium-dysregulation-hypothesis.md — the project framework H5b docks into.
• H5 (sibling): h5-autoimmune-genetic-risk-chain.md — the celiac+T1DM-signature framing.
• Bridges: B-Gen-3 (H5b ↔ CA-I1) + B-Gen-4 (H5b T-cell SOCE ↔ H1 autoantibody pathway) in synthesis/bridges.md.
• Open questions: Q-IBC-1..9 in synthesis/open-questions.md.

Position in the project's chain inventory

H5b is structurally distinct from H4 (different upstream mechanism), structurally distinct from H5 (different genetic-substrate framing), and structurally anchored to the H1×H2 mechanism (same downstream effector layer, amplified at the calcium-handling substrate).

Status

• Synthesis document: drafted 2026-05-24, version 0.1
• Ontology entities: staged at bridging tier (11 entities including module)
• Bridges: B-Gen-3, B-Gen-4 staged at bridging tier
• Open questions: Q-IBC-1..9 logged
• FM-direct evidence: none at any locus
• Direction-decision pending: alongside H5, awaiting joint H5/H5b cohort overlap analysis (Q-IBC-8)