Background Granulomatosis with polyangiitis (GPA) is characterized by extravascular necrotizing granulomatous inflammation and systemic anti-neutrophil cytoplasmic autoantibody (ANCA) – associated vasculitis. The pathogenic mechanisms, especially of the granulomatous inflammation, are still not fully understood. Cell death and tissue damage could be a driving force for chronic inflammation in GPA.
Objectives To investigate the role of the damage-associated molecular patterns (DAMPs) IL-33 and high-mobility group box 1 (HMGB1) in the granulomatous inflammation of GPA.
Methods Tissue expression of HMGB1, IL-33 and their corresponding DAMP receptors “receptor for advanced glycation end products” (RAGE) and “suppression of tumorigenicity 2” (ST2), respectively, were analyzed in nasal GPA biopsies compared to chronic rhinosinusitis (CRS) by immunostaining. Circulating soluble RAGE (sRAGE) and soluble ST2 (sST2) were measured in GPA sera compared to CRS or healthy controls (HC) using ELISA. Expression of HMGB1 and the autoantigen proteinase 3 (PR3) in nasal GPA tissues and on isolated neutrophils was determined by immunofluorescence staining. Full-length IL-33 was cleaved in vitro by either activated, necrotic or NETotic neutrophils. The biological activity of these IL-33-cleavage fragments was tested by subsequent stimulation of human mast cells (HMC-1). The resulting activation of mitogen-activated protein kinases (MAPK) was determined by western blot.
Results Increased tissue expression of extranuclear HMGB1 (p<0.01) and strong upregulation of RAGE was observed in the necrotizing granulomatous inflammation compared to CRS. In addition, substantial numbers of IL-33 and ST2 expressing cells were found in close proximity to necrotic areas and lymphoid structures in the granulomatous inflammation compared to CRS. However, neither sRAGE nor sST2 were elevated in GPA sera compared to controls. In GPA tissue co-localization of HMGB1 and PR3 was detected on neutrophils. This co-localization could be induced on isolated neutrophils by apoptosis in vitro. Full-length IL-33 was processed by activated, necrotic or NETotic neutrophils into different cleavage products. IL-33 fragments produced by necrotic GPA neutrophils induced a significantly stronger MAPK activity in HMC-1 compared to those from HC.
Conclusions In this study, we demonstrate a direct link between dying neutrophils and necrosis-derived DAMPs HMGB1 and IL-33 in the granulomatous inflammation of GPA. Differential cleavage of IL-33 by dying neutrophils, IL-33-dependent MAPK activation and high tissue expression of RAGE and ST2 suggest a considerable role of these DAMPs in sustaining chronic inflammation in GPA. Moreover, HMGB1/PR3 co-localization on apoptotic neutrophils reveals a possible adjuvant function of HMGB1 for the autoimmune response against PR3.
Acknowledgement Supported by German Research Foundation grants CRU170 (AM, PL), EXC306 (PL) and RTG1727 (AM, PL).
Disclosure of Interest None declared