Conclusion
Our results suggest a novel role for sFasL signalling in the TNF-α-induced RCD programme in neutrophils during COVID-19 and a potential therapeutic target to curb inflammation and thus influence disease severity and outcome.
Methods
Regulated cell death phenotype of neutrophils isolated from critically ill COVID-19 patients or healthy donors and stimulated with COVID-19 or healthy plasma ex vivo was assessed by flow cytometry, time-lapse microscopy and cytokine multiplex analysis. Immunohistochemistry of COVID-19 patients and control biopsies were performed to assess the in situ neutrophil RCD phenotype. Plasma cytokine levels of COVID-19 patients and healthy donors were measured by multiplex analysis. Clinical parameters were correlated to cytokine levels of COVID-19 patients.
Results
COVID-19 plasma induced a necroptosis-sensitive neutrophil phenotype, characterised by cell lysis, elevated release of damage-associated molecular patterns (DAMPs), increased receptor-interacting serine/threonine-protein kinase (RIPK) 1 levels and mixed lineage kinase domain-like pseudokinase (MLKL) involvement. The occurrence of neutrophil necroptosis MLKL axis was further confirmed in COVID-19 thrombus and lung biopsies. Necroptosis was induced by the tumor necrosis factor receptor 1 (TNFRI)/TNF-α axis. Moreover, reduction of soluble Fas ligand (sFasL) levels in COVID-19 patients and hence decreased signalling to Fas directly increased RIPK1 levels, exacerbated TNF-driven necroptosis and correlated with disease severity, which was abolished in patients treated with glucocorticoids.