2. Role of Neutrophils in COVID-19
Neutrophils are the immune system’s first responders having crucial functions in immunity and repair. Upon activation, they produce pro-inflammatory cytokines (including IL-6, TNF-α and IL-1β), generate reactive oxygen species (ROS), release hematopoietic serine proteases (neutrophil elastase, proteinase 3, and cathepsin G), microparticles and neutrophil extracellular traps (NETs). Neutrophils possess anti-microbial properties capable of not only killing both Gram-positive and Gram-negative bacteria, but they can also act as a double edged sword mediating tissue injury and perpetuating the inflammatory response.(Kolaczkowska and Kubes 2013) NETs, ROS and serine proteases can all independently, or collectively, upregulate thromboinflammatory processes.
In the context of COVID-19, neutrophilia signifies worse outcomes, with autopsy studies showing neutrophil infiltration in pulmonary capillaries, acute capillaries with fibrin deposition, extravasation of neutrophils in the alveolar space and neutrophil mucositis of the trachea.(Yao et al. 2020; Gu et al. 2005) Bost et al., revealed at least ten different neutrophil states present in blood and broncho-alveolar lavage of COVID-19 patients, with resting state phenotype mainly associated with mild patients and activated or immature phenotype associated with severe patients.(Bost et al. 2021) Their results suggested that COVID-19 is associated with a state of ‘immune silence’ (demonstrated by loss of neutrophil and monocyte immunosuppression and the replacement of lung memory CD8+ T cells by naïve T cells), correlating with severe clinical manifestation and outcome.(Bost et al. 2021) Diao et al., showed T-cells are dysfunctional with increased expression of exhaustion molecules related to heightened systemic inflammation, including IL-6 levels.(Diao et al. 2020) We have previously shown that IL-6 plays a major role in T-cell dependent Ang-II thromboinflammatory responses and in the activation and aggregation of platelets.(Senchenkova, Russell, et al. 2019) These findings support the hypothesis that drug discovery programmes based on T-cell dependent IL-6 signalling pathways may afford protection against thromboinflammation in COVID-19.
As observed with other pathological conditions (e.g. sickle cell disease),(Ansari et al. 2021) NET levels are increased in COVID-19 patients, with sera from COVID-19 patients being shown to trigger NET release from control neutrophils, and containing increased MPO-DNA complexes and citrullinated histone H3 levels which correlated with disease severity.(Veras et al. 2020; Zuo et al. 2020) These results suggest NETs may contribute to COVID-19 pathology and NET biomarkers may help to predict clinical worsening and VTE.
Neutrophil degranulation and NET formation exerts various intracellular danger-associated molecular patterns (DAMPs) activating pattern recognition receptors (PRRs) on nearby immune and non-immune cells releasing pro-inflammatory mediators.(Tomar et al. 2020) DAMPs activate properdin, factor B and C3, all components of the alternative pathway necessary to induce the complement cascade. Reports have shown increased activation of the complement system in severe COVID-19 patients.(Holter et al. 2020) Lung biopsies from COVID-19 patients have shown deposits of terminal complement components C5b-9, C4d, and mannose binding lectin (MBL)-associated serine protease (MASP)2, consistent with sustained, systemic activation of the complement pathways.(Magro et al. 2020) These findings highlight the therapeutic strategy of complement targeted therapies for COVID-19 mediated thrombosis.
The initial neutrophil response also leads to interactions with platelets via a variety of different mechanisms including Mac-1 (CD11b/CD18)/Glycoprotein Ib (CD42) and P-Selectin/P-selectin glycoprotein ligand-1 (PSGL-1), formation of fibrin cross-links (via Mac-1/fibrin interaction) and induction of extrinsic TF/Factor IIa pathway, generating thrombin.(De Meyer et al. 2016) TF-enriched NETs and a high neutrophil count are associated with increased disease severity and poor prognosis in COVID-19,(Skendros et al. 2020) amplifying the need for increased research regarding platelet-neutrophil interactions in thrombogenesis.