Serum soluble interleukin-2 receptor is a useful biomarker for disease activity but not for differential diagnosis in IgG4-related disease and primary Sjögren's syndrome adults from a defined population

Objective. To identify biomarkers for disease activity in IgG4-related disease (IgG4-RD) and primary Sjögren's syndrome (pSS). Methods. Forty-three consecutive treatment- naïve patients with IgG4-RD, 62 patients with pSS, and 5 patients with sicca syndrome were enrolled. IgG4-RD and pSS disease activity was assessed based on the IgG4-RD responder index (IgG4-RD RI) and EULAR Sjögren's Syndrome Disease Activity Index (ESSDAI), respectively. The associations of biomarkers with disease activity were examined. Results. Comparison of the three diseases identified the serum levels of IgG, IgG4, IgG4/IgG ratio, IgE, and soluble interleukin-2 receptor (sIL-2R) for IgG4-RD and the serum levels of IgM and sIL-2R and lymphocyte counts for pSS as potential biomarkers of disease activity. Among these, serum sIL-2R levels correlate with baseline IgG4-RD RI scores and the number of affected organs in IgG4-RD (p=0.74, p<0.0001 and p=0.75, p<0.0001, respectively). Serum sIL-2R levels also correlate with ESSDAI scores and the number of affected organs in pSS (P=0.67, p<0.0001 and p=0.41, p<0.0001, respectively). Receiver operating characteristic curve analysis suggested serum sIL-2R levels as an efficient biomarker to distinguish the presence of extra-dacryosialadenitis involvements in IgG4-RD with a cut-off value of 424 U/mL (AUC=0.93, p<0.0001), and in pSS with 452 U/mL (AUC=0.89, p<0.0001). Serum sIL-2R levels decreased significantly after treatment in patients with IgG4-RD and pSS. Conclusion. Serum sIL-2R levels are a potentially valuable biomarker for evaluating disease activity and treatment response in IgG4-RD and pSS.