Residency · Residency · Rheumatology

IgG4-Related Disease

Introduction

IgG4-related disease is a fibroinflammatory condition of relatively recent recognition that can affect virtually any organ system. It is characterized by tumefactive (mass-forming) lesions, a dense lymphoplasmacytic infiltrate rich in IgG4-positive plasma cells, and a distinctive pattern of storiform fibrosis. The unifying diagnosis of IgG4-RD has brought together previously disparate entities that were once considered independent conditions, including autoimmune pancreatitis, Riedel thyroiditis, retroperitoneal fibrosis, Mikulicz disease, and sclerosing cholangitis. The mass-forming nature of IgG4-RD means it is frequently mistaken for malignancy, making accurate diagnosis essential. The typical patient is 60 to 70 years old at diagnosis, with a male-to-female ratio of approximately 2-3:1, although this ratio varies by the organ predominantly affected.

Pathogenesis

The pathogenesis of IgG4-RD has been substantially clarified in recent years. CD4-positive cytotoxic T lymphocytes have been identified as key pathogenic effector cells, producing IL-1beta, TGF-beta, and IFN-gamma, and playing a critical role in driving tissue fibrosis. T follicular helper cells drive B cell class switching to IgG4 and promote germinal center reactions. B cell activation leads to expansion of plasmablasts and plasma cells that produce large quantities of IgG4. Paradoxically, IgG4 itself is likely not pathogenic; it is an anti-inflammatory antibody that undergoes Fab-arm exchange, becoming bispecific and functionally monovalent, and thus unable to form immune complexes. The fibrosis that characterizes IgG4-RD is driven by TGF-beta, which activates fibroblasts to produce collagen in the distinctive storiform (cartwheel) pattern. Complement activation is rare in IgG4-RD because IgG4, unlike IgG1 and IgG3, does not efficiently activate the classical complement pathway.

Clinical Manifestations

Pancreaticobiliary (Most Common Index Presentation)

Type 1 autoimmune pancreatitis is the classic pancreaticobiliary manifestation of IgG4-RD. It presents with diffuse pancreatic enlargement producing the characteristic "sausage-shaped pancreas" on cross-sectional imaging. The typical clinical presentation is painless obstructive jaundice, which closely mimics pancreatic adenocarcinoma. Histologically, the pancreas shows lymphoplasmacytic infiltrate with fibrosis, and serum IgG4 levels are elevated. A defining feature is the dramatic response to glucocorticoid therapy, which serves as both a therapeutic and diagnostic criterion. IgG4-sclerosing cholangitis produces biliary strictures that mimic cholangiocarcinoma. The critical distinction from primary sclerosing cholangitis is that IgG4-sclerosing cholangitis responds to glucocorticoid therapy, whereas PSC does not.

Head and Neck

The orbit is the most common head and neck site affected by IgG4-RD, manifesting as lacrimal gland enlargement (dacryoadenitis), orbital pseudotumor, and proptosis. These findings can closely mimic orbital lymphoma, necessitating biopsy. Salivary gland involvement, most commonly the submandibular glands and less frequently the parotid glands, was formerly designated as "Mikulicz disease" or "Kuttner tumor" before being recognized as IgG4-RD. Thyroid involvement takes the form of Riedel thyroiditis, a fibrous thyroiditis producing a "woody hard" thyroid gland with associated hypothyroidism. Sinonasal involvement can produce destructive or mass-forming lesions that mimic granulomatosis with polyangiitis.

Retroperitoneal

Retroperitoneal fibrosis is an important manifestation of IgG4-RD, with approximately 50 to 60 percent of cases previously classified as "idiopathic" retroperitoneal fibrosis now recognized as IgG4-related. Fibrous tissue surrounds the aorta and ureters, potentially causing hydronephrosis through ureteral encasement. Cross-sectional imaging reveals a characteristic periaortic soft tissue cuff. Malignancy-associated retroperitoneal fibrosis, including lymphoma and carcinomatosis, must be excluded.

Aortic and Vascular

IgG4-related aortitis and periaortitis produce inflammatory aortic aneurysms with periaortic inflammation. Coronary arteritis is rare but can cause myocardial infarction, representing a potentially life-threatening vascular complication.

Renal

Tubulointerstitial nephritis is the most common renal manifestation of IgG4-RD, presenting as multiple hypodense cortical nodules on contrast-enhanced CT and showing a plasma cell-rich interstitial infiltrate with abundant IgG4-positive cells on biopsy. Membranous nephropathy is a less common renal manifestation, characterized by IgG4 deposits along the glomerular basement membrane.

Pulmonary

Pulmonary involvement in IgG4-RD is protean, presenting as nodules, ground-glass opacities, interstitial pneumonia, or pleural thickening and effusion. Multiple radiographic patterns have been described, including solid nodular, bronchovascular, and alveolar interstitial patterns. These diverse presentations can mimic malignancy, sarcoidosis, or lymphangitic carcinomatosis, frequently requiring biopsy for definitive diagnosis.

Other Organs

Additional organ involvement includes hypertrophic pachymeningitis with dural thickening, constrictive pericarditis, IgG4-prostatitis with elevated PSA mimicking prostate cancer, and subcutaneous nodules.

Multi-organ Involvement

More than 60 percent of patients with IgG4-RD have two or more organs involved, either synchronously or metachronously. Common organ combinations include concurrent pancreatic and biliary disease, combined lacrimal and salivary gland involvement, and coexisting retroperitoneal and aortic disease. New organ involvement can develop over years, making long-term monitoring essential for all patients.

<image>A multi-organ involvement diagram of IgG4-related disease. Show a central human body figure with arrows pointing to affected organs, each with a small illustration panel. (1) Orbits: Bilateral lacrimal gland enlargement with proptosis; CT showing enlarged lacrimal glands. (2) Salivary glands: Bilateral submandibular gland swelling. (3) Thyroid: Woody hard thyroid mass (Riedel thyroiditis). (4) Lungs: Multiple nodules and ground-glass opacities on CT. (5) Pancreas: "Sausage-shaped" diffusely enlarged pancreas on CT with delayed enhancement and capsule-like rim. (6) Bile ducts: Strictures mimicking cholangiocarcinoma on MRCP. (7) Retroperitoneum: Periaortic and periureteral fibrosis causing hydronephrosis on CT. (8) Kidneys: Multiple hypodense cortical nodules on contrast CT representing TIN. Include a central histopathology inset showing the three hallmark features: dense lymphoplasmacytic infiltrate, storiform (cartwheel) fibrosis, and obliterative phlebitis. Label all organs and findings.</image>

Diagnosis

2019 ACR/EULAR Classification Criteria

The 2019 ACR/EULAR classification criteria provide a structured approach to diagnosing IgG4-RD. Entry criteria require characteristic clinical or radiologic involvement of a typical organ combined with exclusion of mimicking conditions. Exclusion criteria mandate the absence of fever, highly positive ANA or anti-dsDNA, highly positive RF or anti-CCP, positive ANCA, eosinophilia, and specific features of other defined diseases. Weighted inclusion criteria are distributed across 8 domains encompassing histopathology (0-14 points), immunostaining (0-14 points), serology (4 points for IgG4 greater than twice the upper limit of normal), bilateral lacrimal, parotid, or submandibular gland involvement (6 points), chest findings (4 points), pancreaticobiliary disease (8-19 points), renal involvement (6-10 points), and retroperitoneal disease (8 points). A threshold of 36 or more points classifies a patient as having IgG4-RD, with a sensitivity of 82 percent and specificity of 97.8 percent.

Histopathology (Gold Standard)

Histopathologic examination remains the gold standard for diagnosing IgG4-RD. Three cardinal histopathologic features define the disease. First, a dense lymphoplasmacytic infiltrate composed of T cells, B cells, and abundant plasma cells. Second, storiform fibrosis, characterized by a distinctive cartwheel pattern of fibroblast and collagen arrangement. Third, obliterative phlebitis, in which venous inflammation leads to luminal obliteration. The IgG4-positive plasma cell count should exceed 10 per high-power field in biopsy specimens, with higher thresholds (greater than 30 per HPF) applied in certain organs. The IgG4-to-total-IgG ratio should exceed 40 percent on immunostaining. While all three cardinal features need not be present simultaneously, at least 2 of 3 are required for a definite histopathologic diagnosis.

Serology

Serum IgG4 levels are elevated above 135 mg/dL in approximately 60 to 70 percent of IgG4-RD patients. However, this biomarker is not specific, as elevated levels occur in approximately 5 percent of healthy individuals and in various other conditions including allergic disease, parasitic infections, pancreatic cancer, and eosinophilic granulomatosis with polyangiitis. Very high levels exceeding four times the upper limit of normal are more specific and tend to correlate with multi-organ disease. Conversely, serum IgG4 can be normal in 30 to 40 percent of biopsy-proven cases, particularly in single-organ disease. The prozone effect must be recognized: very high IgG4 concentrations may produce falsely normal results on nephelometric assays. Circulating plasmablasts (CD19+CD20-CD27+CD38+) represent a more sensitive and specific biomarker than serum IgG4 and are currently the best available tool for monitoring disease activity. Complement levels (C3/C4) may be low in IgG4-related tubulointerstitial nephritis, an unusual finding among IgG4-RD organ manifestations. Mild eosinophilia is observed in some patients, though marked eosinophilia should prompt consideration of EGPA or parasitic infection.

Imaging

CT and MRI demonstrate mass-forming or diffuse organ enlargement with a delayed enhancement pattern. PET-CT with 18F-FDG is valuable for identifying active disease sites, assessing the full extent of organ involvement, and monitoring treatment response. Magnetic resonance cholangiopancreatography is essential for evaluating biliary strictures. Renal CT reveals multiple round or wedge-shaped cortical lesions characteristic of tubulointerstitial nephritis.

Differential Diagnosis (Critical to Exclude)

The differential diagnosis of IgG4-RD is of paramount clinical importance and includes several conditions that must be rigorously excluded. Malignancy, including lymphoma (orbital, salivary, retroperitoneal), pancreatic cancer, and cholangiocarcinoma, is the most critical differential. Tissue biopsy should always be performed before assuming a diagnosis of IgG4-RD, as IgG4-positive cells can be present in both lymphoma and carcinoma. Sarcoidosis is distinguished by non-caseating granulomas, elevated ACE, and a different demographic profile with higher prevalence in African Americans. Granulomatosis with polyangiitis is characterized by ANCA positivity, necrotizing granulomatous inflammation, and destructive sinusitis. Sjogren syndrome is differentiated by the presence of anti-Ro/SSA and anti-La/SSB antibodies and focal lymphocytic sialadenitis on lip biopsy. Castleman disease is an IL-6-driven condition with hyaline-vascular or plasma cell histologic subtypes in which IgG4-positive cells can be present. Rosai-Dorfman disease is identified by the characteristic finding of emperipolesis and S100-positive histiocytes.

Management

Glucocorticoids (First-line)

Glucocorticoids remain the first-line treatment for IgG4-RD. The standard regimen is prednisone at 30 to 40 mg daily (or 0.6 mg/kg/day) for 2 to 4 weeks, followed by a gradual taper over 3 to 6 months. The initial response rate is approximately 90 percent, with rapid clinical improvement. However, the relapse rate is substantial, ranging from 25 to 50 percent during or after glucocorticoid tapering. Predictors of relapse include multi-organ disease, IgG4 levels exceeding four times the upper limit of normal, elevated circulating plasmablasts, and biliary or proximal biliary involvement.

Steroid-Sparing Agents

Conventional immunosuppressants are used as steroid-sparing agents, though evidence is limited compared to biologics. Options include azathioprine at 2 to 2.5 mg/kg/day, mycophenolate mofetil at 1.5 to 3 g/day, and methotrexate at 15 to 25 mg weekly. Despite their use, relapse rates remain high at approximately 30 to 50 percent.

Rituximab

Rituximab is the most effective steroid-sparing agent for IgG4-RD, working through depletion of B cells including IgG4-producing plasmablasts. Dosing follows either the rheumatologic protocol (1000 mg IV on day 1 and day 15) or the oncologic protocol (375 mg/m2 weekly for 4 doses). Response rates of 80 to 95 percent have been reported, with declining IgG4 levels and often undetectable plasmablasts after treatment. Maintenance therapy involves retreatment based on clinical relapse or rising plasmablast levels, typically every 6 to 12 months. Rituximab may be used as first-line therapy in organ-threatening disease, avoiding the need for prolonged glucocorticoid courses.

Emerging Therapies

Several novel therapeutic approaches are under investigation. Elotuzumab, an anti-SLAMF7 antibody, targets CD4-positive cytotoxic T lymphocytes, the key pathogenic effector cells. Xmab5871, an anti-CD19 antibody, targets plasmablasts. JAK inhibitors are being explored based on the T cell-driven pathology underlying IgG4-RD.

Monitoring

Disease monitoring in IgG4-RD requires a multimodal approach. Clinical response should be assessed through organ-specific evaluations including imaging and functional testing. Serum IgG4 trends, rather than absolute values, are most informative. Circulating plasmablasts remain the best biomarker for tracking disease activity and predicting relapse. PET-CT is useful for restaging, with decreasing FDG uptake indicating treatment response. Organ-specific monitoring should include serial assessment of renal function, pancreatic function, and hepatobiliary imaging as appropriate.

<image>A treatment algorithm for IgG4-related disease. Start with "Suspected IgG4-RD" → tissue biopsy (gold standard) → confirm histopathologic features (lymphoplasmacytic infiltrate, storiform fibrosis, obliterative phlebitis, IgG4+ plasma cells). If confirmed, assess urgency: acute organ-threatening disease (obstructive jaundice, ureteral obstruction, renal failure) → urgent intervention (biliary stent, ureteral stent) + GC 30-40 mg/day or rituximab. Non-urgent: GC 30-40 mg/day x 2-4 weeks → taper. Reassess at 3 months: if complete response and low relapse risk → taper off GC, monitor. If relapse or high-risk features (multi-organ, high IgG4, elevated plasmablasts) → rituximab 1000 mg x 2 + GC-sparing maintenance. If rituximab-refractory → consider emerging therapies. Show monitoring panel at bottom: serum IgG4, plasmablasts, organ-specific imaging, CRP, complement levels (if renal involvement).</image>

Key Clinical Pearls

• IgG4-RD frequently mimics malignancy; always biopsy before assuming IgG4-RD
• Serum IgG4 is elevated in only 60-70% of patients and is not specific; circulating plasmablasts are a better biomarker
• Type 1 autoimmune pancreatitis responds dramatically to glucocorticoids; lack of response should prompt reconsideration of diagnosis (especially pancreatic cancer)
• Rituximab is the most effective steroid-sparing agent and can be used first-line for organ-threatening disease
• Storiform fibrosis + obliterative phlebitis + lymphoplasmacytic infiltrate = histopathologic triad of IgG4-RD
• IgG4-related retroperitoneal fibrosis accounts for the majority of "idiopathic" retroperitoneal fibrosis

References

1. Wallace ZS, et al. The 2019 American College of Rheumatology/European League Against Rheumatism Classification Criteria for IgG4-Related Disease. Arthritis Rheumatol. 2020;72(1):7-19.
2. Stone JH, et al. IgG4-related disease. N Engl J Med. 2012;366(6):539-551.
3. Khosroshahi A, et al. International consensus guidance statement on the management and treatment of IgG4-related disease. Arthritis Rheumatol. 2015;67(7):1688-1699.
4. Mattoo H, et al. Clonal expansion of CD4+ cytotoxic T lymphocytes in patients with IgG4-related disease. J Allergy Clin Immunol. 2016;138(3):825-838.
5. Carruthers MN, et al. Rituximab for IgG4-related disease: a prospective, open-label trial. Ann Rheum Dis. 2015;74(6):1171-1177.

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