Siglec-9 Agonism Reduces ANCA-Mediated Neutrophil Responses In Vitro

Background: Sialic acid-binding Ig-like lectin 9 (Siglec-9) is constitutively expressed on neutrophils and monocytes. The expression and potential role of siglec-9 in ANCA-associated vasculitis (AAV) is yet to be determined. We aimed to examine the expression of siglec-9 in patients with AAV and explore the impact of siglec-9 agonism on ANCA-mediated neutrophil responses in vitro.

Methods: Leukocytes and serum were isolated from peripheral venous blood of AAV patients and siglec-9 expression was measured by flow cytometry and ELISA, respectively. Immunohistochemistry was performed on kidney biopsies of AAV patients with AAGN and stained for siglec-9 and leukocyte markers. Functional studies were done using healthy donor neutrophils in the presence of ANCA and siglec-9 mAb to investigate its role in apoptosis and ROS production.

Results: We found increased serum siglec-9 expression in active AAV compared to remission AAV and a positive correlation with disease activity. Neutrophils and intermediate (CD14+/CD16+) monocytes from PR3-ANCA patients displayed higher siglec-9 expression compared to MPO-ANCA patients. Siglec-9 expression in AAGN was restricted to areas of active inflammation. We observed increased siglec-9 shedding in neutrophils following ANCA stimulation. Siglec-9 agonism in these neutrophils was associated with increased apoptosis and reduced ROS production compared to isotype control and unstimulated neutrophils.

Conclusions: Our study suggests that siglec-9 is expression correlates with disease activity in AAV. Our functional studies support a potential role for siglec-9 in modulating ANCA-mediated neutrophil responses. Further evaluation is required to determine the relevance of these findings on neutrophil-endothelial interactions.