gms | German Medical Science

Introduction: The pathogenesis of necrotizing granulomatous inflammation, one of the key features of granulomatosis with polyangiitis (GPA), is still unclear. Necrotic cell death leads to the release of necrosis-specific endogenous danger molecules called alarmins. We hypothesize a pivotal role of the alarmins high-mobility group box protein 1 (HMGB1) and IL-33 for the initiation of inflammatory and autoimmune responses in GPA.

Methods: Tissue expression and cellular localization of HMGB1 and IL-33 as well as their corresponding receptors RAGE (receptor for advanced glycation end products) and ST2 (suppression of tumorigenicity 2) were analyzed by immunostaining in nasal tissues in GPA and chronic rhinosinusitis (CRS). Serum concentrations of soluble RAGE (sRAGE) and soluble ST2 (sST2), which act as decoy receptors for HMGB1 and IL-33, were measured in comparison to controls by ELISA. Cytokine response of monocytes challenged with necrotic supernatant of allogenic polymorphonuclear neutrophils (PMN) from GPA in comparison to controls was measured by cytokine arrays. Proteolytic cleavage of IL-33 by PMN was analyzed by western blot.

Results: Nasal tissues in GPA showed predominant cytoplasmic HMGB1 localization in contrast to mainly nuclear localization in CRS (p < 0.01), demonstrating active HMGB1 signaling. In addition, substantial RAGE expression particularly associated with macrophages (CD68⁺ cells) was observed in the necrotizing granulomatous inflammation compared to CRS. Necrosis-derived HMGB1 from PMN induced an increased cytokine release (e.g. IL-16, CXCL10, MIF) by monocytes from GPA patients compared to healthy controls (HC) in vitro, indicating attraction of cells involved in granuloma formation. In addition, elevated IL-33 expression as well as numbers of ST2⁺ cells in close proximity to lymphoid structures and necrosis were observed in granulomatous inflammation. Different cleavage fragments of IL-33 generated by PMN of GPA indicate an increased pro-inflammatory response compared to HC. However, neither sRAGE nor sST2 were elevated in sera from GPA compared to controls, suggesting a lack of negative feedback in GPA in both signaling pathways.

Conclusion: The IL-33/ST2 as well as the HMGB1/RAGE signaling pathways are involved in the induction and maintenance of granulomatous inflammation by driving local auto-amplificatory loops in GPA.