Regulation of cell death and innate immune signaling in mouse neutrophils
Creators: name: Krebs, Philippe
name: Wicki, Simone
Publisher: name: (:null)
Year: 2016
Subjects: 610 Medicine &amp; health

description: Neutrophils are the most abundant leukocytes in human blood and are the first cells to be recruited to sites of infection. Recent evidence has emerged that neutrophils do not only play an essential role in the fight against intruding bacteria and fungi but also against viruses. Neutrophil lifespan must be tightly controlled in order to resolve inflammation and to avoid collateral tissue damage. Prolonged survival of activated neutrophils is implicated in numerous diseases such as rheumatoid arthritis and inflammatory bowel disease. Neutrophil survival is limited by activation of apoptosis induced either through the intrinsic (mitochondrial) pathway or through the death receptor (DR) pathway, including the DRs tumor necrosis factor receptor 1 (TNFR1) and FAS. In our studies we investigated the regulation of Toll like receptor 4 (TLR4), TNFR1 and FAS signaling by X-linked inhibitor of apoptosis protein (XIAP) and BH3interacting domain death agonist (BID) in mouse neutrophils. Whereas the role of XIAP in dendritic cells (DCs) and macrophages has been intensively studied, little is known on how this protein regulates cell death and innate immune signaling in neutrophils. In accordance with earlier reports, lipopolysaccharide (LPS) induced hypersecretion of IL-1β was observed also in Xiap-/- neutrophils, a process dependent on the NLRP3 inflammasome, caspase-1/-11 and likely also on caspase8. While LPS prolonged survival of wild-type (WT) as well as in Xiap-/- neutrophils, combined inhibition of XIAP, cIAP1 and cIAP2 using small molecule IAP antagonists (so-called Smac mimetics) sensitized neutrophils to caspase-dependent apoptosis initiated by RIPK1-induced TNFα production. Of note, inhibition of caspases caused a switch to RIPK3-dependent necroptosis. Accordingly, we investigated the cell Simone Katharina Wicki, Institute of Pharmacology, University of Bern CH 13 death induced by high concentrations of TNFα alone in neutrophils. Unlike in most other cell types, TNFα treatment alone elicited caspase-dependent apoptosis in mouse neutrophils, which could be abrogated in WT neutrophils by caspase inhibition, while loss of XIAP caused a switch to RIPK3-dependent necroptosis. In contrast to TNFR1 signaling, we could show that FAS-induced neutrophil killing is enhanced by cleavage of BID and crosstalk to intrinsic pathway indicating that neutrophils belong to type II cells. Even though FASL triggered clear caspasedependent apoptosis, inhibition of caspases failed to block cell death, unless RIPK1 was additionally inhibited. These data suggest that in response to FASL mouse neutrophils readily switch to necroptosis when caspases are blocked. The pathophysiological relevance of delayed neutrophil death in response to FASL upon loss of BID was investigated using the DSS-induced model of acute colitis. Based on significant differences in the weight loss in the course of the experiment, Bid-/- mice indeed developed a more severe colitis accompanied with impaired resolution of inflammation compared to WT controls. Our data indicate that neutrophils do not always respond to danger signals and death ligands in a similar fashion to DCs or macrophages. We provide evidence that XIAP antagonizes the shift from TNFα-induced apoptosis to necroptosis in mouse neutrophils. Moreover our data implicate that neutrophils may contribute to the hyperinflammation and progression of clinical manifestations observed in patients suffering from X-linked lymphoproliferative syndrome 2 (XLP-2). In terms of FAS signaling, inhibition of caspases alone caused a switch to necroptosis in mouse neutrophils. Even though the pathophysiological relevance of these findings are currently not clear, they are supportive of a backup cell death Simone Katharina Wicki, Institute of Pharmacology, University of Bern CH 14 mechanism upon encountering of pathogens (e.g. certain viruses) expressing caspase inhibitors.