Neutrophils in COVID-19: Not Innocent Bystanders

Abstract

Unusually for a viral infection, the immunological phenotype of severe COVID-19 is characterised by a depleted lymphocyte and elevated neutrophil count, with the neutrophil-to-lymphocyte ratio correlating with disease severity. Neutrophils are the most abundant immune cell in the bloodstream and comprise different subpopulations with pleiotropic actions that are vital for host immunity. Unique neutrophil subpopulations vary in their capacity to mount antimicrobial responses, including NETosis (the generation of neutrophil extracellular traps), degranulation and de novo production of cytokines and chemokines. These processes play a role in antiviral immunity, but may also contribute to the local and systemic tissue damage seen in acute SARS-CoV-2 infection. Neutrophils also contribute to complications of COVID-19 such as thrombosis, acute respiratory distress syndrome and multisystem inflammatory disease in children. In this Progress review, we discuss the anti-viral and pathological roles of neutrophils in SARS-CoV-2 infection, and potential therapeutic strategies for COVID-19 that target neutrophil-mediated inflammatory responses.

Keywords: COVID-19; SARS-CoV-2; inflammation; innate immunity; neutrophil.

Figure 1

The neutrophil and clinical characteristics of COVID-19 patients. Activated neutrophils can produce cytokines such as IL-1β, IL-6, TNF-α, MIF, IL-12, TGF-β, IL-21, IL-23 and IL-27, contributing to a cytokine storm and the further development of ARDS and organ failure in COVID-19 patients. Pneumonia in COVID-19 patients is most likely to be caused by the production of proteases, cationic polypeptides, cytokines and ROS by neutrophils. Upon SARS-CoV-2 recognition by TLR7/8/9, protein arginase deiminase 4 (PAD4) is activated, which induces chromatin decondensation through histones citrullination and consequently NETs formation. Neutrophil nuclear membrane is disrupted by neutrophil elastase (NE) and gasdermin D which facilitates the formation of a pore in the neutrophil cell membrane and mediates release of the contents of NETs into the extracellular space. NETs induce macrophage activation and IL-1β production resulting in a positive loop with neutrophils and NETs formation. Macrophages also secrete CCR1, CCR2, IL-6 and TNF-α leading to further neutrophil recruitment. Extracellular histones presented in NETs causes cell cytotoxicity contributing with ARDS, sepsis and organ failure observed in COVID-19 patients. Extracellular DNA induces thick and viscous mucus production allowing bacteria colonization and respiratory failure. NETs also interact with fibrinogen, VWF and platelets causing thrombosis in several organs such as lung, kidney, liver and extremities. IL, interleukin; TNF, tumour necrosis factor; MIF, macrophage migration inhibitory factor; ARDS, acute respiratory distress syndrome; ROS, reactive oxygen species; PAD4, protein arginase deiminase 4; NETs, neutrophil extracellular trap; NE, neutrophil elastase; CCR, chemokine receptor; DNA, deoxyribonucleic acid; VWF, von Willebrand factor.

Figure 2

Neutrophil activation. ACE2 or L-SING receptors on neutrophils most likely recognise SARS-CoV2 via a spike (S) protein on its surface. Once the virus enters the cell, ssRNA viruses such as SARS-CoV-2 are recognised by TLR 7/8/9 which induce the activation of the MyD88 pathway. MyD88 activates TRAF3 and TRAF6 which result in the transcription of NF-κB and IRF7 associated genes. The activated NF-κB pathway leads to the transcriptional induction of proinflammatory cytokines, chemokines and additional inflammatory mediators in neutrophils. In addition, cytosolic viral RNA recruiting MDA5, RIF1 and PKR lead to the activation of TBK1 and the further activation of IRF3 resulting in the transcription of type I/II IFN genes. The positive stimulatory loop by type I IFN induces the production of more IFNs through the JAK/STAT pathway and the induction of Interferon Stimulated Genes (ISG). At the same time, SARS-CoV-2 possess ORF6, an accessory protein antagonist of IFNs by the inhibition of MDA5, TBK1, IRF3 and IRF9. ssRNA viruses also cause the recruitment of the NLRP3 inflammasome complex and the further activation of pro-caspase-1 resulting in the cleavage of pro-IL-1β and pro-IL-18 into the active forms. ACE2, angiotensin-Converting Enzyme 2; L-SING, L-Specific Intercellular adhesion molecule-3-Grabbing Non-integrin/CD209L; RNA, ribonucleic acid; ssRNA, Single-stranded RNA; TLR, toll-like receptor; MyD88, myeloid differentiation primary response 88; TRAF, tumor necrosis factor receptor (TNF-R)-associated factor; NF-κB, nuclear factor kappa B; IRF, Interferon Regulatory Factor; MDA, melanoma differentiation-associated protein; RIF, Replication Timing Regulatory Factor; PKR, protein kinase R; TBK, TANK Binding Kinase; IFN, interferon; ISG, Interferon Stimulated Genes; JAK-STAT, janus kinase; ORF, open Reading Frame; NLRP3, nod like receptor family, pyrin domain containing 3; IL, interleukin.