Elevated IgG4 serum levels among primary Sjögren's syndrome patients: do they unmask underlying IgG4-related disease?

OBJECTIVE: To determine IgG4 levels in a cohort of consecutive patients with primary Sjögren's syndrome (SS) and other autoimmune diseases and explore whether they associate with distinct clinical, serologic, and histopathologic features.
METHODS: Serum IgG4 levels were measured in 133 primary SS patients and 49 healthy donors (HDs). Seventy-four lupus and 54 rheumatoid arthritis (RA) patients served as disease controls. Immunohistochemical IgG4 analysis was performed in paraffin-embedded minor salivary gland (MSG) tissues.
RESULTS: Raised IgG4 serum levels (>135 mg/dl) were detected in 10 (7.5%) of 133 primary SS patients (high-IgG4 group), in 8 (10.8%) of 74 lupus patients, in 7 (12.9%) of 54 RA patients, and in 1 (2%) of 49 HDs. Compared to the normal-IgG4 (<135 mg/dl) primary SS group, high-IgG4 patients exhibited increased prevalence of IgG4-related features (autoimmune cholangitis, autoimmune pancreatitis, and interstitial nephritis), lower rates of antinuclear antibody positivity, and higher IgG2 and IgE levels. Positive staining for IgG4+ plasma cells with an IgG4:IgG ratio ≥40% was detected in 3 of 6 available MSG tissues in the high-IgG4 group. Criteria of possible or definite IgG4-related disease (IgG4-RD) were fulfilled by 10 (7.5%) of 133 of our primary SS cohort.
CONCLUSION: Raised IgG4 serum levels were detected in 7.5% of primary SS patients in association with IgG4RD-reminiscent clinical, serologic, and histopathologic features. Whether this high-IgG4 primary SS group represents a misclassified IgG4-RD group or a distinct primary SS subtype remains to be further explored in future studies.