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Neutrophil extracellular trap (NET) formation can generate short-term functional anucleate cytoplasts and can trigger loss of cell viability. We examined the role of non-apoptotic cell death signaling in NET formation by studying necroptotic human and mouse neutrophils. Necroptotic cell death signaling was activated by caspase-8 inhibition and pharmacological targeting of inhibitor of apoptosis proteins (IAPs). The specificity of cell death was confirmed using neutrophils from mice deficient in receptor-interacting protein kinase-3 (Ripk3), Ripk1 kinase activity (Ripk1D138N/D138N), caspase-8, or the plasma membrane-disrupting effector protein mixed lineage kinase domain-like (Mlkl). NETs were investigated with a combination of imaging and quantitative flow cytometry, immunogold electron microscopy, immunofluorescence microscopy, and ex vivo microbicidal assays to demonstrate functionality of NETs. In response to necroptotic stimuli, Mlkl translocates to the plasma membrane in neutrophils, and is required for downstream NADPH oxidase-independent reactive oxygen species production, loss of cytoplasmic granules, breakdown of the nuclear membrane, chromatin decondensation, histone hypercitrullination, and extrusion of bacteriostatic NETs. Neutrophils expressing a kinase-dead form of Ripk1 (Ripk1D138N/D138N) were unable to generate Ripk3/Mlkl-dependent NETs, or to undergo necroptosis or caspase-8-dependent apoptosis. NET formation that is triggered by necroptotic stimuli is dependent on TNF production, can be differentially modulated by the actions of G-CSF and IFNγ, and occurs concomitantly with the production, processing and release of IL-1α/β. Human necroptotic neutrophils also release NETs that kill S.aureus. Necroptotic NETs contain components of canonical NETs including dsDNA, hypercitrullinated histones, and neutrophil elastase, but also non-canonical components including Mlkl and membranes. Peptidylarginine deiminase 4 (Padi4) is required for extrusion of necroptotic NETs but not for chromatin decondensation. Padi4-deficient neutrophils are hypersensitive to necroptotic stimuli despite normal levels of phosphorylated Mlkl, indicating that Padi4 acts downstream of Mlkl activation. The failure of Padi4-deficient neutrophils to generate necroptotic NETs in the presence of membrane-associated Mlkl, suggests that the removal of Mlkl membrane-disrupting complexes by NETs can facilitate membrane repair and control the kinetics of neutrophil necroptosis. This work defines a distinct cell death signaling network downstream of Mlkl that promotes Padi4-dependent necroptotic NET release. Disclosures No relevant conflicts of interest to declare.

Neutrophilic dermatoses are a group of inflammatory skin disorders characterized by sterile infiltrates of neutrophils. These syndromes include pyoderma gangrenosum (PG) and Sweet's syndrome (SS), and they are associated with an increased risk of inflammatory bowel disease, rheumatoid arthritis, and hematologic malignancy, particularly monocytic or myelomonocytic leukemia (AML). IL-1 was first proposed in 1987 as a factor in SS, and the presence of "fragmented neutrophil nuclei" was hypothesized to contribute to disease. IL-1 has subsequently been reported at high levels in lesions of SS, and IL-1 neutralizing therapies have met with success in some SS patients. PG is also reported to be responsive to IL-1 neutralizing therapy, including PG patients with psoriatic arthritis. Splicing variants and promoter region deletions of protein tyrosine phosphatase-6 (PTPN6, or Src homology region 2 domain-containing phosphatase-1, SHP1) are a feature of SS and PG, consistent with the findings that mutations in Ptpn6 drive spontaneous IL-1R-dependent skin inflammation in mice. Mice lacking Ptpn6 develop a cutaneous inflammatory disease that is dependent on the IL-1 receptor (IL-1R), G-CSF, and neutrophils. We hypothesized that production of IL-1 by dying neutrophils drives skin inflammation in the setting of Ptpn6 deficiency. To investigate the mechanisms controlling pathogenic IL-1 release in mice lacking Ptpn6 specifically in neutrophils (Ptpn6∆PMN), we looked directly at cytokine production from neutrophils undergoing inflammatory forms of cell death. We found that stimuli engaging Ripk3/Mlkl-dependent necroptotic forms of cell death result in transcription, processing, and release of bioactive IL-1α and IL-1β from neutrophils. Production of IL-1α and IL-1β were increased in Ptpn6∆PMN neutrophils treated to undergo necroptosis. Ptpn6∆PMN neutrophils displayed increased rates of spontaneous cell death in the presence of G-CSF or IFN-γ alone, and were hypersensitive to necroptotic stimuli. These cell death abnormalities were absent in Ptpn6∆PMN Casp8∆PMN Mlkl-/- neutrophils, demonstrating that Ptpn6 regulates both apoptosis and necroptosis to prevent IL-1 release. We next examined the contribution of apoptosis and necroptosis in the development of spontaneous cutaneous inflammatory disease in Ptpn6∆PMN mice. Loss of either the Caspase-8-dependent apoptotic pathway or the Ripk3/Mlkl-dependent necroptotic pathway was not sufficient to prevent inflammation in mice. However, combined deletion of Caspase-8 and Ripk3/Mlkl protected Ptpn6∆PMN mice (Ptpn6∆PMN Casp8∆PMN Ripk3-/- and Ptpn6∆PMN Casp8∆PMN Mlkl-/- mice). Ripk1 is known to act as a physiological negative regulator of both Caspase-8-dependent apoptosis and Ripk3/Mlkl-dependent necroptosis. We found that the absence of Ripk1 in neutrophils in Ptpn6∆PMN Ripk1∆PMN mice resulted in heightened sensitivity of neutrophils to cell death and accelerated onset of cutaneous inflammatory disease. These results reveal Ripk1 as a critical physiological negative regulator of neutrophil inflammatory cell death and IL-1 production, and cutaneous inflammation. Together, these data emphasize dual functions for Ptpn6 in negative regulation of IL-1α/β transcription, and to prevent Caspase-8- and Ripk3/Mlkl-dependent cell death and concomitant IL-1α/β processing and release. These findings implicate neutrophils as the dominant producers of IL-1 in neutrophilic dermatoses, and identify novel therapeutic targets that could be exploited to control inflammatory forms of cell death in these skin disorders. Disclosures No relevant conflicts of interest to declare.