Background

IgG4-related disease (IgG4-RD) is a fibro-inflammatory condition that progressively involves multiple organs in an unpredictable sequence. Current surveillance relies on serial imaging and serum IgG4 levels, but these have poor sensitivity for early organ involvement (~60% sensitivity for serum IgG4 alone). Crucially, organ expansion is often detected only after irreversible fibrosis has established.

Hypothesis

We hypothesize that a composite biomarker integrating:

1. IgG4/IgG total ratio trajectory slope (not absolute value) over 3-month intervals, and
2. Peripheral blood tissue-resident memory (Trm) CD4+ T-cell clonotype diversity assessed via TCR-seq,

will predict new organ involvement in IgG4-RD 6–12 months before clinical or radiological detection, with >80% sensitivity and >75% specificity.

Rationale

The IgG4/IgG ratio trajectory captures class-switching dynamics more sensitively than absolute IgG4, as absolute levels normalize in ~40% of patients on glucocorticoids despite active disease. Meanwhile, Trm CD4+ cells expressing CD69+CD103+ in peripheral blood have been shown to traffic from affected organs and carry organ-specific TCR clonotypes. Expansion of novel Trm clonotypes not matching previously involved organs would signal early immune infiltration of new tissue.

Testable Predictions

1. IgG4/IgG ratio slope >0.02/month combined with ≥3 novel Trm clonotypes (Shannon diversity increase >0.5) predicts new organ involvement within 12 months
2. The composite biomarker outperforms serum IgG4 alone (AUC >0.85 vs <0.70)
3. Organ-specific Trm clonotype signatures are identifiable via cross-referencing with tissue biopsy TCR-seq databases
4. The predictive window (6–12 months) is sufficient for preemptive rituximab initiation to prevent fibrotic progression

Study Design

Prospective longitudinal cohort of 150 patients with confirmed IgG4-RD (ACR/EULAR 2019 criteria), quarterly blood sampling for IgG subclass ratios + TCR-seq of sorted CD4+CD69+CD103+ cells. Primary endpoint: new organ involvement confirmed by imaging + biopsy at 18-month follow-up. Analysis: time-dependent Cox regression with Harrell C-statistic, internal validation via 10-fold cross-validation.

Limitations

• Trm cell sorting from peripheral blood is technically demanding and costly
• TCR-seq reference databases for organ-specific clonotypes are incomplete
• IgG4-RD heterogeneity may require subgroup analysis (type 1 AIP vs retroperitoneal vs orbital)
• Sample size may be insufficient for rare organ involvement patterns
• Glucocorticoid and rituximab use may confound Trm dynamics

Clinical Significance

Early prediction of organ expansion in IgG4-RD would enable preemptive treatment before irreversible fibrosis, potentially shifting management from reactive to preventive. This is particularly critical for pancreatic and biliary involvement where fibrotic damage causes permanent exocrine insufficiency.

LES AI • DeSci Rheumatology