gms | German Medical Science

Background: Cell death and dysregulation of dead cell clearance play a crucial role in the initiation of chronic inflammatory diseases and autoimmunity. The cardinal pathological features of granulomatosis with polyangiitis are necrotizing granulomatous inflammation and necrotizing autoimmune vasculitis. Previously, we have reported on dysregulated apoptosis and efferocytosis predisposing to chronic inflammation in GPA. Here we provide evidence, that a recently discovered regulated form of inflammation-inducing necrosis named necroptosis is also involved in pathophysiology of GPA.

Methods: Polymorphonuclear leukocytes (PMN) and peripheral blood mononuclear cells (PBMC) were isolated from the blood of GPA patients and healthy controls (HC) (n = 5 each). mRNA as well as protein were extracted and gene expression and protein levels of the necroptosis markers receptor interacting serine/threonine kinase 1 (RIP1), RIP3, cellular inhibitor of apoptosis protein-1 (cIAP1), caspase 8 and mixed lineage kinase domain like pseudokinase (MLKL) were determined by qPCR and western blot, respectively. Tissue expression of phosphorylated MLKL (pMLKL) was investigated by immunohistochemistry of nasal biopsies from GPA patients and non-autoimmune inflamed tissue controls (n = 3 each).

Results: PBMC as well as PMN from GPA and HC express all of the investigated cell death/necroptosis markers on the mRNA and protein level. There was a slight upregulation of RIP1 and caspase 8 in PMN of GPA patients compared to HC on the mRNA and protein level suggestive of the formation of the ripoptosome. Expression of caspase 8 and RIP1 was significantly higher (p < 0.05) in PMN compared to PBMC in GPA. pMLKL expression was observed in inflamed tissue from nasal biopsies from GPA, especially in close proximity to the mucosal epithelium.

Conclusion: Our results indicate for the first time a possible involvement of necroptosis in GPA. Circulating PMN express major factors of the ripoptosome, a molecular death platform leading to either apoptosis or necroptosis, depending on the recruitment of other factors like RIP3. Identification of pMLKL within the inflamed tissue suggests ongoing necroptosis in GPA. Altogether, regulated necrosis could represent a potential therapeutic target in GPA.