Interleukin-21 (IL-21) is a pleotropic cytokine that plays an important role in the regulation of both innate and adaptive immune responses. In the present study, we investigated whether serum IL-21 positivity is associated with disease activity in patients with all variants of antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). Sixty patients with AAV from a monocentric prospective cohort were enrolled from November 2016 to May 2018 in this study. On the day of visit, clinical manifestations including Birmingham vasculitis activity score (BVAS) were assessed, routine laboratory tests were performed, and blood samples were collected. Isolated sera were stored at − 80 °C on the same day to measure serum IL-21. The definition of generalised AAV set by the European Vasculitis Study group was adopted. In addition, serum IL-21 positivity was compared between patients with different autoimmune diseases and healthy controls. The mean age was 59.3 years, and 39 patients (65.0%) were women. Of patients, 23 (38.3%) had new-onset AAV, whereas 28 (46.7%) had generalised AAV. Serum IL-21 was detected in 16 patients (26.7%). Patients with serum IL-21 positivity exhibited a higher risk of having generalised AAV than those without (relative risk 5.250, p = 0.012). No difference in serum IL-21 positivity was observed among patients with AAV, rheumatoid arthritis, and systemic lupus erythematosus and healthy controls. Among patients with serum IL-21 positivity, serum IL-21 became negative at initial visit following the decrease in BVAS. Serum IL-21 positivity might be a useful biomarker to indicate the disease activity of AAV.
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