Background Immunoglobulin gamma-3 subclass, also called the immunoglobulin gamma 3 chain C (IGHG3), normally comprises 5%–8% of all IgG in humans. The salivary IGHG3 was increased in Korean patients with systemic lupus erythematosus (SLE). This study tried to assess the levels of IGHG3 in serum, saliva, and urine in patients with SLE.

Methods Serum, saliva, and urine samples were collected from 181 patients with SLE and 99 age- and sex-matched healthy controls (HCs) with clinical data. The levels of IGHG3 in each body fluid were measured and analyzed to assess their associations with clinical manifestations and disease activity of SLE.

Results Serum IGHG3 was 478.1 ± 160.9 and 364.4 ± 97.9 μg/mL (p<0.001) in patients with SLE and HCs. Salivary IGHG3 was 3,078.9 ± 2,473.8 and 1,413.6 ± 1,075.3 ng/mL (p<0.001), and urinary IGHG3 was 64.0 ± 74.5 and 27.1 ± 16.2 ng/mL (p<0.001) in patients with SLE and HCs, respectively. Area under the receiver operating characteristic curve for diagnosis of SLE was 0.737 for serum IGHG3 (95% confidence interval (CI), 0.673–0.8), 0.755 for salivary IGHG3 (95% CI, 0.694–0.815), and 0.733 for urinary IGHG3 (95% CI, 0.67–0.795). Serum IGHG3 was correlated with leukocyte count (r=-0.219, p=0.003), lymphocyte count (r=-0.221, p=0.03), anti-dsDNA antibody (r=0.22, p=0.003), C3 (r=-0.23, p=0.002), salivary IGHG3 was correlated with ESR (r=0.173, p=0.024), and urinary IGHG3 was correlated with hemoglobin (r=-0.183, p=0.021), erythrocyte sedimentation rate (r=0.204, p=0.01), anti-dsDNA antibody (r=0.262, p=0.001), C3 (r=-0.202, p=0.011), and SLE disease activity index (r=0.332, p<0.01). In addition, urinary IGHG3 was higher in patients with nephritis compared in those not (119.55 ± 110.0 vs. 49.8 ± 54.4, p<0.01).

Conclusions The levels of IGHG3 were significantly increased in serum, saliva and urine of patients with SLE, and urinary IGHG3 level was associated with disease activity and renal involvement of SLE.