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NETosis is a special form of programmed cell death in neutrophils, which is characterized by the extrusion of DNA, histones, and antimicrobial proteins in a web-like structure known as neutrophil extracellular traps (NETs)

increased generation of reactive oxygen species (ROS) is a crucial intracellular process that causes NETosis

Another indirect route of SARS-CoV-2-induced NET production is platelet activation

When NETs are activated in the circulation, they can also induce hypercoagulability and thrombosis

In COVID-19, major NET protein cargos of NETs (i.e., NE, MPO, and histones) are significantly elevated.

SARS-CoV-2 can also infect host cells through noncanonical receptors such as C-type lectin receptors

Immunopathological manifestations, including cytokine storms and impaired adaptive immunity, are the primary drivers behind COVID-19, with neutrophil infiltration being suggested as a significant cause

NETosis and NETs are increasingly recognized as causes of vascular injury

SARS-CoV-2 and its components (e.g., spike proteins and viral RNA) attach to platelets and increase their activation and aggregation in COVID-19, resulting in vascular injury and thrombosis, both of which are linked to NET formation

NET formation may be caused by activated platelets rather than SARS-CoV-2 itself

NETosis, leading to aberrant immunity such as cytokine storms, autoimmune disorders, and immunosuppression.

early bacterial coinfections were more prevalent in COVID-19 patients than those infected with other viruses

NETosis and NETs may also have a role in the development of post COVID-19 syndromes, including lung fibrosis, neurological disorders, tumor growth, and worsening of concomitant disease

NETs and other by-products of NETosis have been shown to act as direct inflammation amplifiers. Hyperinflammation

“cytokine storm”

SARS-CoV-2 drives NETosis and NET formation to allow for the release of free DNA and by-products (e.g., elastases and histones). This may trigger surrounding macrophages and endothelial cells to secrete excessive proinflammatory cytokines and chemokines, which, in turn, enhance NET formation and form a positive feedback of cytokine storms in COVID-19

NET release enables self-antigen exposure and autoantibody production, thereby increasing the autoinflammatory response

patients with COVID-19 who have higher anti-NET antibodies are more likely to be detected with positive autoantibodies [e.g., antinuclear antibodies (ANA) and anti-neutrophil cytoplasmic antibodies (ANCA)]

COVID-19 NETs may act as potential inducers for autoimmune responses

have weakened adaptive immunity as well as a high level of inflammation

tumor-associated NETosis and NETs promote an immunosuppressive environment in which anti-tumor immunity is compromised

NETs have also been shown to enhance macrophage pyroptosis in sepsis

facilitating an immunosuppressive microenvironment

persistent immunosuppression may result in bacterial co-infection or secondary infection

can enhance this process by interacting with neutrophils through toll-like receptor 4 (TLR4), platelet factor 4 (PF4), and extracellular vesicle-dependent processes

NET-induced immunosuppression in COVID-19 in the context of co-existing bacterial infection

Following initial onset of COVID-19, an estimated 50% or more of COVID-19 survivors may develop multi-organ problems (e.g., pulmonary dysfunction and neurologic impairment) or have worsening concomitant chronic illness

NETs in the bronchoalveolar lavage fluid of severe COVID-19 patients cause EMT in lung epithelial cells

decreased E-cadherin (an epithelial marker) expression

COVID-19 also has a long-term influence on tumor progression

Patients with tumors have been shown to be more vulnerable to SARS-CoV-2 infection and subsequent development of severe COVID-19

patients who have recovered from COVID-19 may have an increased risk of developing cancer or of cancer progression and metastasis

awaken cancer cells

NETs have been shown to change the tumor microenvironment

enhance tumor progression and metastasis

vitamin C has been tested in phase 2 clinical trials aimed at reducing COVID-19-associated mortality by reducing excessive activation of the inflammatory response

vitamin C is an antioxidant that significantly attenuates PMA-induced NETosis in healthy neutrophils by scavenging ROS

vitamin C may also inhibit NETosis and NET production in COVID-19

Metformin

Vitamin C

NK cells are large granular lymphocytes that comprise approximately 10 % of circulating lymphocytes

all human NK cells express the CD56 antigen

treatment with various concentrations of IL‐2 in vivo may induce distinct functions within the NK cell compartment and, therefore, may have profound effects on NK cell‐mediated cytotoxicity

CD56bright

CD56dim

We show here that ADCC conducted by NK cells in vitro is enhanced by IL‐2 activation and is critically dependent on interactions between FcγRIII on NK cells and Herceptin‐coated tumor targets

administration of low‐dose IL‐2 to patients results in the marked expansion of a CD56+ population of immune effectors with the ability to lyse antibody‐coated cancer targets

NK cells represented only 7 % of lymphocytes prior to therapy but comprised over 50 % of the population after 10 weeks of low‐dose IL‐2

These data suggest that the enhanced ADCC seen following the expansion of NK cells with low‐dose IL‐2 is likely due to an increase in the overall number of NK cells

co‐administration of IL‐2 with rhu4D5 mAb will enhance activation of NK cell effector functions

Stimulation of NK cells with IL‐2 resulted in a significant increase in the lysis of rhu4D5‐coated targets

We have shown that costimulation with IL‐2 plus rhu4D5 results in significant production of IFN‐γ by NK cells with concomitant up‐regulation of cell‐surface activation and adhesion molecules

It has been previously demonstrated that continuous low‐dose IL‐2 can expand a CD56+ lymphocyte population, and we have now shown that this cell population is a potent mediator of ADCC against rhu4D5 mAb‐coated Her2 / neu+ targets

These results suggest that administration of low‐dose IL‐2 can be used to expand NK cell numbers, while higher doses may be used to enhance their cytolytic capacity in the setting of mAb therapy

we have demonstrated that NK cell lysis of Her2 / neu+ breast cancer cell lines in the presence of rhu4D5 mAb is markedly enhanced following stimulation with IL‐2

we have presented evidence that administration of low‐dose IL‐2 in vivo results in the expansion of a potent NK cell effector population

Our experiments suggest that NK cells costimulated with IL‐2 and immobilized IgG can secrete potent immunomodulatory cytokines which may serve to potentiate the anti‐tumor immune response.