Association of Serum Calprotectin (S100A8/A9) Level With Disease Relapse in Proteinase 3–Antineutrophil Cytoplasmic Antibody–Associated Vasculitis

Objective S100A8/A9 (calprotectin) has shown promise as a biomarker for predicting relapse in antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). This study was undertaken to investigate serum S100A8/A9 level as a biomarker for predicting future relapse in a large cohort of patients with severe AAV.Methods Serum levels of S100A8/A9 were measured at baseline and months 1, 2, and 6 following treatment initiation in 144 patients in the Rituximab in ANCA-Associated Vasculitis trial (cyclophosphamide/azathioprine versus rituximab [RTX] for induction of remission) in whom complete remission was attained.Results Patients were divided into 4 groups: proteinase 3 (PR3)-ANCA with relapse (n=37), PR3-ANCA without relapse (n=56), myeloperoxidase (MPO)-ANCA with relapse (n=6), and MPO-ANCA without relapse (n=45). Serum S100A8/A9 level decreased in all groups during the first 6 months of treatment. The percentage reduction from baseline to month 2 was significantly different between patients who experienced a relapse and those who did not in the PR3-ANCA group (P=0.046). A significantly higher risk of relapse was associated with an increase in S100A8/A9 level between baseline and month 2 (P=0.0043) and baseline and month 6 (P=0.0029). Subgroup analysis demonstrated that patients treated with RTX who had increased levels of S100A8/A9 were at greatest risk of future relapse (P=0.028).Conclusion An increase in serum S100A8/A9 level by month 2 or 6 compared to baseline identifies a subgroup of PR3-ANCA patients treated with RTX who are at higher risk of relapse by 18 months. Since RTX is increasingly used for remission induction in PR3-ANCA-positive patients experiencing a relapse, S100A8/A9 level may assist in identifying those patients requiring more intensive or prolonged treatment.