Inflammasome activation in neutrophils of patients with severe COVID-19

Abstract

Infection by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) engages the inflammasome in monocytes and macrophages and leads to the cytokine storm in COVID-19. Neutrophils, the most abundant leukocytes, release neutrophil extracellular traps (NETs), which have been implicated in the pathogenesis of COVID-19. Our recent study shows that activation of the NLRP3 inflammasome is important for NET release in sterile inflammation. However, the role of neutrophil inflammasome formation in human disease is unknown. We hypothesized that SARS-COV-2 infection may induce inflammasome activation in neutrophils. We also aimed to assess the localization of inflammasome formation, (i.e. ASC speck assembly), and timing relative to NETosis in stimulated neutrophils by real time video microscopy. Neutrophils isolated from severe COVID-19 patients demonstrated that approximately 2% of neutrophils in both the peripheral blood and tracheal aspirates presented ASC speck. ASC speck was observed in neutrophils with an intact poly-lobulated nucleus, suggesting early formation during neutrophil activation. Additionally, 40% of nuclei were positive for citrullinated histone H3, and there was a significant correlation between speck formation and nuclear histone citrullination. Time-lapse microscopy in LPS-stimulated neutrophils from fluorescent ASC reporter mice showed that ASC speck formed transiently and at the microtubule organizing center, long before NET release. Our study shows that ASC speck is present in neutrophils from COVID-19 patients with respiratory failure and that it forms early in NETosis. Our findings suggest that inhibition of neutrophil inflammasomes may be beneficial in COVID-19.

Neutrophils in COVID-19: Not Innocent Bystanders

“…Active NLRP3 inflammasome is present in peripheral blood mononuclear cells (PBMCs) and post-mortem tissues of COVID-19 patients, and high expression of its derived products such as Casp1p20 and IL-18 were seen to correlate with disease severity and poor clinical outcome ( 47 ). NLRP3 inflammasome activation has also been described in neutrophils of severe COVID-19 patients ( 48 ). Aymonnier et al.…”

Specific NLRP3 Inflammasome Assembling and Regulation in Neutrophils: Relevance in Inflammatory and Infectious Diseases

“…Recently, the priming has been shown to be dispensable for NLRP3 inflammasome activation in human neutrophils as for human monocyte [ 120 , 121 ]. This new observation highlights the difference observed between the human and mouse models, and the importance of studying both models to decipher the underlying molecular mechanism.…”

“…This new observation highlights the difference observed between the human and mouse models, and the importance of studying both models to decipher the underlying molecular mechanism. However, the mechanism driving this alternative inflammasome activation is not known yet, which needs further exploration [ 120 ].…”

“…Moreover, as circulating neutrophils may differ from their extravasated counterparts, the role of inflammasome in airway neutrophils during COVID-19 remains to be clarified. However, circulating and airway neutrophils from severe COVID-19 patients were recently shown to have already ASC speck formed and release active IL-1β and IL-18 cytokines with positive citrullinated Histone 3 nuclei, suggesting a link between inflammasome and NETosis [ 120 ].…”

Caspase-1-driven neutrophil pyroptosis promotes an incomplete NETosis upon Pseudomonas aeruginosa infection

“…An intriguing feature of neutrophils is that, despite GSDMD activation, they resist canonical inflammasome-induced Caspase-1-dependent cell pyroptosis upon Salmonella Typhimurium and Burkholderia thaïlandensis -activated NLRC4 inflammasome or upon Nigericin/ATP-mediated NLRP3 inflammasome activation [3,5,27,28]. However, recent studies indirectly challenged canonical pyroptosis impairment in neutrophils by showing that sterile activators of the NLRP3 inflammasome also contribute to canonical inflammasome-dependent neutrophil death and subsequent NETosis [29,30]. If it does exist specific microbial species that can promote neutrophil canonical pyroptosis has remained an open question so far.…”