Pathophysiology of IgG4-Related Disease
IgG4-related disease is fundamentally an immune-mediated fibroinflammatory disorder characterized by IgG4-positive lymphoplasmacytic infiltration of affected organs, driven by dysregulated T and B cell responses that culminate in progressive tissue fibrosis. 1
Core Pathological Features
The disease manifests through three cardinal histopathological hallmarks that define tissue involvement 1:
- IgG4-positive lymphoplasmacytic infiltrate (>10 IgG4+ plasma cells per high-power field with IgG4+/IgG+ ratio >40%)
- Storiform fibrosis (characteristic whorled pattern of collagen deposition)
- Obliterative phlebitis (inflammatory venous occlusion)
These pathological changes occur across virtually any organ system, though the pancreas (type 1 autoimmune pancreatitis), biliary tract (IgG4-related sclerosing cholangitis), salivary glands, kidneys, retroperitoneum, and lungs are most commonly affected 1.
Immunological Mechanisms
T Cell-Mediated Pathogenesis
The disease is orchestrated by specific T cell subsets that drive both inflammation and IgG4 class-switching 2, 3:
T follicular helper 2 cells (Tfh2) are the dominant drivers in most phenotypes, particularly Mikulicz disease with systemic involvement. These cells promote germinal center formation, plasmablast differentiation, and IgG4 class-switching through IL-4 and IL-13 secretion 2, 3
Tfh1 cells become important in autoimmune phenotypes where IgG1-type autoantibodies are pathogenic, inducing IgG1 class-switching via IFN-γ 2
CX3CR1+ cytotoxic T cells dominate in the retroperitoneal fibrosis/aortitis phenotype, representing a distinct immunological pathway 2
Regulatory T cells (Tregs) paradoxically contribute to IgG4 class-switching rather than suppressing inflammation 2
B Cell and Antibody Responses
B cell dysregulation is central to pathogenesis 3, 4:
- Expansion of circulating plasmablasts and IgG4+ plasma cells
- IgG4 class-switching driven by Th2 cytokines (IL-4, IL-13)
- Recent identification of disease-specific autoantigens, suggesting true autoimmune mechanisms 4
- Enhanced IgG4 production resulting from Th2-skewed immunity 5
The efficacy of B cell depletion therapy (rituximab) confirms the pathogenic role of B cells beyond simple antibody production 4, 6.
Cytokine Networks
Multiple cytokines perpetuate the inflammatory cascade 4:
- IL-5 drives eosinophil survival and tissue infiltration
- IL-4 and IL-13 promote Th2 responses and IgG4 class-switching
- TGF-β mediates progressive fibrosis
- Innate immunity activation via Toll-like receptors (TLRs) and M2 macrophage polarization 4
Clinical Phenotype-Specific Pathogenesis
IgG4-RD can be stratified into distinct clinical phenotypes with different dominant immune mechanisms 2:
- Retroperitoneal fibrosis/aortitis phenotype: CX3CR1+ CTLs predominate
- Mikulicz disease with systemic involvement: Tfh2 cells dominate
- Malignancy-associated phenotype: Increased CXCR5+ CD2-double negative T cells with decreased naive CD8+ T cells
- Allergy-associated phenotype: Different T cell profile compared to malignancy phenotype
- Autoimmune phenotype: Tfh1 involvement with IgG1 autoantibody production
Complement Activation
In patients with hypocomplementemia, complement pathway activation is mediated by IgG1 or IgG2 antibodies rather than IgG4, implicating Tfh1 cells in this disease subset 2. This represents a distinct pathogenic mechanism from typical IgG4-RD.
Genetic and Environmental Factors
While the etiology remains incompletely understood 7, 8:
- Genetic associations with HLA-DQ in MPO-ANCA-positive cases
- ANCA-negative disease linked to variants in mucosal response genes (GPA33, IL5)
- Environmental triggers remain speculative but may include allergens, infections, or other immune stimuli
Progressive Fibrosis
The ultimate pathological endpoint is organ fibrosis leading to functional impairment 2, 8. The transition from inflammation to fibrosis involves:
- Chronic lymphoplasmacytic infiltration
- M2 macrophage activation promoting fibroblast proliferation
- TGF-β-mediated collagen deposition
- Obliterative vascular changes compromising tissue perfusion
This fibrotic process, once established, may be irreversible even with immunosuppression, explaining why early diagnosis and treatment are critical to prevent permanent organ damage 1, 6.