Systemic lupus erythematosus (SLE) is an autoimmune disease with chronic or episodic inflammation in several organ systems, related to the presence of circulating and tissue‐deposited immune complexes (ICs) that stimulate leukocytes through Fcγ receptors (FcγR) with subsequent inflammation. Treatment with endoglycosidase S (EndoS), an IgG glycan–hydrolyzing bacterial enzyme from Streptococcus pyogenes, has shown beneficial effects in several experimental animal models of chronic inflammatory disease. This study was undertaken to investigate whether EndoS affects the proinflammatory properties of ICs and has the potential to be developed as a therapy for SLE.

ICs purified from SLE patients or RNA‐containing ICs formed in vitro were treated with EndoS and used in several assays reflecting different important features of SLE pathogenesis, such as phagocytosis by polymorphonuclear cells (PMNs) and plasmacytoid dendritic cells (PDCs), complement activation, and interferon‐α (IFNα) production by PDCs.

EndoS treatment abolished all proinflammatory properties of the ICs investigated. This included FcγR‐mediated phagocytosis by PDCs (P = 0.001) and subsequent production of IFNα (P = 0.002), IC‐induced classical pathway of complement activation (P = 0.008), chemotaxis, and oxidative burst activity of PMNs (P = 0.002). EndoS treatment also had a direct effect on the molecular structure of ICs, causing decreased IC size and glycosylation.

Our findings indicate that EndoS treatment has prominent effects on several pathogenetically important IC‐mediated events, and suggest that EndoS has the potential to be developed as a novel therapy for SLE.