see also:

• full blood examination (FBE)
• neutropenia
• neutrophilia
• innate immunity
• lymphocytes

• neutrophils are the dominant phagocytic granulocyte, representing up to 60% of all circulating leukocytes and play a key role in the acute stress response and innate immunity and have roles in combating infections from bacteria in particular, but also some viruses
• excessive activation may result in risk of serious thrombotic events via their formation of NETs (see at bottom)
• acute activation results in neutrophilia and if there are large numbers of immature forms released into the circulation, these are recorded as band forms on the full blood examination (FBE) or designated as “left shift” which often correlates with significant bacterial infection states

• neutrophils are in circulation until they encounter signals that initiate their trans-endothelial migration from the bloodstream to the interstitium
  • L-selectins and PSGL-1 mediate the rolling process along the endothelial layer
  • endothelial E-selectin engages the neutrophil PSGL-1
  • signaling through PSGL-1 by activated endothelium initiates β2 integrin extension, which slows down rolling, aided by the formation of membrane slings and tethers, resulting in selectin/integrin-mediated adhesion and arrest.
  • key chemoattractant receptors are CXCR1, as well as formyl peptide receptors 1 and 2 (FPR1, FPR2) and leukotriene B4 receptor BLT1
  • NB. recruitment to lung and liver seems to be selectin independent
• degranulation with release of
  • reactive oxygen species (ROS)
  • cytotoxic and proteolytics (e.g. MPO, cathelicidins and in human, α-defensins)
  • surface proteins and cytokines - can also regulate migration, transmigration (metalloproteinases), phagocytosis, and NET formation
  • can damage the target cell in a contact dependent manner, in which granules form pores by fusing with the membrane of the target or by exocytosis into the intracellular space
• phagocytosis of pathogens in response to pathogen or damage associated molecular patterns (PAMPs or DAMPs)
• neutrophil extracellular traps (NET) formation
• promote the immune response by:
  • releasing granules
  • producing cytokines
  • mediating the recruitment of other immune cells to sites of infection
• in an exacerbated immune response, neutrophils can cause considerable damage to the host such as extensive cell death, necrosis, vascular leakage, thrombus formation, and antibody-mediated auto-immune responses
• prevent excessive weight loss when the body is exposed to physiological stressors, such as low temperature
  • β3-adrenergic activation of adipocytes triggers a rapid influx of neutrophils into visceral white adipose tissue where they release signals that slow fat breakdown. Neutrophil recruitment requires the stimulation of both lipolysis and p38 MAPK in adipocytes, and is mediated by the secretion of leukotriene B4. Recruited neutrophils undergo activation in situ, and locally secrete IL-1β, which suppresses lipolysis and limits excessive energy loss. ¹⁾

• neutrophils in the periphery are comparatively short-lived compared to other immune cells, 6–24 h in circulation and possibly up to 7 days in tissue during an inflammatory response
• multipotent progenitors GMP
  • ⇒ unipotent neutrophil precursors in the bone marrow
    • ⇒ promyelocyte
      • ⇒ myelocyte
        • ⇒ meta-myelocyte
          • ⇒ banded neutrophil released into blood or tissues and express high levels of L-selectin (CD62L)
            • ⇒ mature neutrophil
              • ⇒ end of life or de-activated neutrophils express CXCR4 (C-X-C chemokine receptor type 4) which marks them for destruction in the spleen

• include immature neutrophils, resting neutrophils, primed neutrophils, and activated neutrophils.

• may arise from:
  • the activation and degranulation of mature NDNs
  • the release of immature neutrophils from the bone marrow
• LDNs are particularly prone to the spontaneous formation of NETs, primarily contributing to the lung injury associated with COVID-19, including vascular obstruction
• functional sub-classes:
  • immunosuppressive LDNs (granulocyte myeloid-derived suppressor cells or G-MDSCs)
    • Covid-19 associated LDNs
      • these may contribute to impaired lymphocyte responses during acute COVID-19 infection
      • 4 subsets representing different stages of maturation in the development of granulocytes, including promyelocytes, myelocytes, bands and mature granulocytes:
        • CD33⁺⁺CD16^-CD11b^-
        • CD33⁺CD16^-CD11b⁺
        • CD33^lowCD16⁺CD11b⁺
        • CD33^-CD16⁺CD11b^-/low
  • pro-inflammatory LDNs (low-density granulocytes or LDGs)

• neutrophils can release web-like structures called NETs, which capture and immobilize pathogenic microorganisms and produce elevated concentrations of myeloperoxidase (MPO) and defensins to resist exogenous infections
• NETs contain a combination of cell-free DNA, MPO, defensins, citrullinated histones and neutrophil granular proteins
• in addition to their role in fighting bacteria, their role in combating viral infections has been observed in various diseases, including human respiratory syncytial virus / human orthopneumovirus (HRSV), dengue fever, influenza, HIV / AIDS and COVID-19 coronavirus (2019-nCoV / SARS-CoV-2)
• neutrophil receptor activation (e.g. via TLRs) stimulates downstream events of the MEK/ERK pathway such as ROS production, neutrophil elastase release and citrullination of histone H3 that result in NET formation
• elevated levels of neutrophil activation and the formation of NETs can be associated with thrombosis as with COVID-19 coronavirus (2019-nCoV / SARS-CoV-2) ²⁾
• NETs can increase platelet activation as well as local thrombin formation via the excess extracellular histones associated with NETs having prothrombotic activity by inhibiting thrombin-dependent protein C activation
• NETs have been closely implicated in thrombotic events such as deep vein thrombosis, myocardial infarction, and thrombotic microangiopathy
• the extent of platelet-neutrophil aggregates are linked to disease severity and hypercoagulability and these aggregates express high TF levels, which is the main trigger of intravascular coagulation and thrombosis

• these include IL-1β, IL-1R, IL-6, and IL-17

• corticosteroids
  • but does not affect neutrophil priming and NET formation
• Janus kinase inhibitors
  • can modulate the production of cytokines by neutrophils through Janus kinases, have emerged as potential agents for COVID-19 treatment
• colchicine:
  • inhibits neutrophil recruitment, activation, cytokine production, inflammation and thrombosis
• Cytokine inhibitors
  • IL-6 receptor antagonists
  • IL-17 inhibitors
  • anakinra, an IL-1β and IL-1R receptor antagonist
• NET reduction agents
  • exogenous recombinant human deoxyribonuclease (DNase) can disrupt NETs’ structure and may compensate for the impaired degradation of NETs due to reduced DNase levels and activity, enhancing NETs clearance
    • but has limited impact on the pro-inflammatory components of NETs, such as histones and elastase
  • disulfiram
    • can inhibit NETs formation and downregulate innate immunity and complement/coagulation pathway