Proof-of-concept study: longitudinal immune biomarker signatures in treatment-naïve IgG4-related disease.

Immunoglobulin G4-related disease (IgG4-RD) is a chronic, systemic, immune-mediated fibroinflammatory disorder characterized by heterogeneous organ involvement and variable clinical expression. Reliable biomarkers are needed to quantify inflammatory activity, distinguish active inflammation from irreversible damage, and monitor treatment response. While serum IgG4 is a recognized marker of disease burden in proliferative phenotypes, its diagnostic specificity is limited and its correlation with disease activity is inconsistent across the clinical spectrum, particularly in predominantly fibrotic or localized subsets. In this context, B-cell activation and cytokine-related circulating signatures may provide complementary, hypothesis-generating information on inflammatory activity. Prospective, longitudinal, single-centre study of 10 consecutively included (October 2022-December 2024) newly-diagnosed treatment-naïve patients with IgG4-RD, assessed at baseline, 6, and 12 months. Disease activity was evaluated using the IgG4-RD Responder Index (RI), clinician-assessed activity, and 18F-FDG PET/CT; cumulative injury was measured with the IgG4-RD Damage Index (DI); and laboratory assessments included eosinophil count, erythrocyte sedimentation rate, C-reactive protein, total IgG, IgG4, complement components 3 and 4 (C3 and C4), and IgE. Circulating interleukin (IL) and interferon (IFN) cytokines (IL-1β, IL-4, IL-5, IL-10, IL-13, IFN-γ, IL-21) and predefined cytokine ratios were quantified by multiplex assay, and B-cell subsets were analysed by flow cytometry. Longitudinal changes and immune signature-activity associations were assessed using univariate linear mixed-effect models with patient-level random intercepts. Patients were aged 75.7 years (median), 70% male, had predominantly retroperitoneal/aortic involvement, and 80% had baseline RI ≥ 4. RI decreased over follow-up, while DI remained low-to-moderate (= 2 at 6 and 12 months). IgG4 decreased by 12 months (mean - 35.8 mg/dL; p = 0.016) and also IL-4 and IL-21 over time. Higher RI was associated with increased IFN-γ (p = 0.0412), IL-10 (p = 0.0077), IL-13 (p = 0.0086), IL-5 (p = 0.0051), total IgG (p = 0.0010), and eosinophils (p = 0.0027), and inversely associated with the plasmablast/IgG4 ratio (p = 0.0264). In the exploratory PET/CT subgroup, PET-positive status was associated with higher total IgG (p = 0.0194), requiring cautious interpretation. DI was associated with higher CRP (p = 0.0455), and ESR tracked the IFN-γ/IL-5 ratio (p = 0.0093). Conventional inflammatory markers showed limited and selective associations, whereas selected cytokine-related measures showed nominal associations with clinical activity measures and appeared biologically plausible within predefined pathogenic axes. In treatment-naïve IgG4-RD, selected circulating cytokines and derived immune signatures showed preliminary associations with inflammatory activity measures and may provide information complementary to serum IgG4. These findings remain exploratory and require validation in larger, phenotypically diverse cohorts with appropriate control groups and correction strategies for multiple testing.