Neutrophils are the most abundant of the circulating immune cells and are the first to be recruited to sites of inflammation. Neutrophils are a heterogeneous group of immune cells from which are derived extracellular traps (NETs), reactive oxygen species, cytokines, chemokines, immunomodulatory factors, and alarmins that regulate the recruitment and phenotypes of neutrophils, macrophages, dendritic cells, T cells, and B cells. In addition, cytokine-stimulated neutrophils can express class II major histocompatibility complex and the internal machinery necessary for successful antigen presentation to memory CD4+ T cells. This may be relevant in the context of vaccine memory. Neutrophils thus emerge as orchestrators of immune responses that play a key role in determining the outcome of infections, vaccine efficacy, and chronic diseases like autoimmunity and cancer. This review aims to provide a synthesis of current evidence as regards the role of these functions of neutrophils in homeostasis and disease.

How Neutrophils Shape the Immune Response: Reassessing Their Multifaceted Role in Health and Disease

During infections, neutrophil extracellular traps act like a meshwork of molecules that captures microbes. In contrast, during sterile inflammation the presence of NETs is usually associated with tissue damage and uncontrolled inflammation. In this context, DNA acts both as activator of NETs formation and immunogenic molecule fueling inflammation within the injured tissue microenvironment. Pattern recognition receptors that specifically bind to and get activated by DNA such as Toll-like receptor-9 (TLR9), cyclic GMP-AMP synthase (cGAS), Nod-like receptor protein 3 (NLRP3) and Absence in Melanoma-2 (AIM2) have been reported to play a role in NETs formation and detection. However, how these DNA sensors contribute to NETs-driven inflammation is not well understood. Whether these DNA sensors have unique roles or on the contrary they are mostly redundant is still elusive. In this review, we summarize the known contribution of the above DNA sensors to the formation and detection of NETs in the context of sterile inflammation. We also highlight scientific gaps needed to be addressed and propose future direction for therapeutic targets.

/pdf/who-and-how-dna-sensors-in-nets-driven-inflammation-1ylmcbs2.pdf

Who and how, DNA sensors in NETs-driven inflammation

Tissue‐infiltrating neutrophils (TINs) secrete various signaling molecules to establish paracrine communication within the inflammatory milieu. It is imperative to identify molecular mediators that control this secretory phenotype of TINs. The present study uncovers a secretory neutrophil subset that exhibits increased pro‐inflammatory cytokine production and enhanced migratory capacity which is highly related with periodontal pathogenesis. Further analysis identifies the OTU domain‐containing protein 1 (OTUD1) plays a regulatory role in this secretory neutrophil polarization. In human and mouse periodontitis, the waning of inflammation is correlated with OTUD1 upregulation, whereas severe periodontitis is induced when neutrophil‐intrinsic OTUD1 is depleted. Mechanistically, OTUD1 interacts with SEC23B, a component of the coat protein II complex (COPII). By removing the K63‐linked polyubiquitin chains on SEC23B Lysine 81, the deubiquitinase OTUD1 negatively regulates the COPII secretory machinery and limits protein ER‐to‐Golgi trafficking, thus restricting the surface expression of integrin‐regulated proteins, CD9 and CD47. Accordingly, blockade of protein transport by Brefeldin A (BFA) curbs recruitment of Otud1‐deficient TINs and attenuates inflammation‐induced alveolar bone destruction. The results thus identify OTUD1 signaling as a negative feedback loop that limits the polarization of neutrophils with secretory phenotype and highlight the potential application of BFA in the treatment of periodontal inflammation.

The Deubiquitinase OTUD1 Suppresses Secretory Neutrophil Polarization And Ameliorates Immunopathology of Periodontitis

Neutrophils play a critical role in the immune response to infection and tissue injury. However, recent studies have shown that neutrophils are a heterogeneous population with distinct subtypes that differ in their functional properties. Moreover, aging can alter neutrophil function and exacerbate immune dysregulation. In this review, we discuss the concept of neutrophil heterogeneity and how it may be affected by aging. We then examine the implications of neutrophil heterogeneity and aging for COVID-19 pathogenesis and wound healing. Specifically, we summarize the evidence for neutrophil involvement in COVID-19 and the potential mechanisms underlying neutrophil recruitment and activation in this disease. We also review the literature on the role of neutrophils in the wound healing process and how aging and neutrophil heterogeneity may impact wound healing outcomes. Finally, we discuss the potential for neutrophil-targeted therapies to improve clinical outcomes in COVID-19 and wound healing.

Neutrophil heterogeneity and aging: implications for COVID-19 and wound healing

<title>Abstract</title> BACKGROUND: The persistence of vasculo-thrombotic complications has been put forward as a possible contributing factor in the long COVID (LC) syndrome. OBJECTIVES: Given the recently reported separate demonstration of the association of LC with elevated levels of fibrin amyloid microclots (FAM) and with those neutrophil extracellular traps (NETs), markers that are linked to thromboinflammation, this study considers the association of FAM with NETs. RESULTS: The results show that NETs markers are quantitatively and structurally associated with the size and number of FAM in patients with LC. These markers showed a strong diagnostic performance, both independently and when combined. CONCLUSIONS: Our study revealed that NETs may be a component of circulating FAM, We suggest that higher NETs formation promotes the stabilization of FAM in the circulation, leading to deleterious effects which contribute causally to the LC syndrome. 

Circulating microclots are structurally associated with Neutrophil Extracellular Traps and their amounts are strongly elevated in long COVID patients

Both reviewers agree that this study may have important implications for DNase as a therapeutic agent for COVID-19. Reviewers point out the small sample size and short follow-up period as potential limitations. 

Review 2: "Targeting Neutrophils Extracellular Traps (NETs) Reduces Multiple Organ Injury in a COVID-19 Mouse Model"

Thrombosis remains a major problem in our society, manifesting across multiple demographic groups and with high associated morbidity and mortality. Thrombus development is the result of a complex mechanism in which multiple cell types and soluble factors play a crucial role. One cell that has gained the most attention in recent years is the neutrophil. This key member of the innate immune system can form neutrophil extracellular traps (NETs) in response to activating stimuli in circulation. NETs form a scaffold for thrombus formation, both initiating the process and stabilizing the final product. As the first responders of the host immune system, neutrophils have the flexibility to recognize a variety of molecules and can quickly interact with a range of different cell types. This trait makes them sensitive to exogenous stimuli. NET formation in response to pathogens is well established, leading to immune-mediated thrombus formation or immunothrombosis. NETs can also be formed during sterile inflammation through the activation of neutrophils by fellow immune cells including platelets, or activated endothelium. In chronic inflammatory settings, NETs can ultimately promote the development of tissue fibrosis, with organ failure as an end-stage outcome. In this review, we discuss the different pathways through which neutrophils can be activated toward NET formation and how these processes can result in a shared outcome: thrombus formation. Finally, we evaluate these different interactions and mechanisms for their potential as therapeutic targets, with neutrophil-targeted therapies providing a future approach to treating thrombosis. In contrast to current practices, such treatment could result in reduced pathogenic blood clot formation without increasing the risk of bleeding. 

The coming of age of neutrophil extracellular traps in thrombosis: Where are we now and where are we headed?

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Review 2: "Targeting Neutrophils Extracellular Traps (NETs) Reduces Multiple Organ Injury in a COVID-19 Mouse Model"

The coming of age of neutrophil extracellular traps in thrombosis: Where are we now and where are we headed?