Neutrophils

Abstract

Production of neutrophils occupies almost two third of the activity in bone marrow. Production and release are governed by the level of granulocyte‐colony‐stimulating factor and by chemokines that provide either retention signals or release signals. Circulating neutrophils are directed to sites of infection by endothelial cells in postcapillary venules at sites of infection. The endothelial cells capture by‐passing neutrophils and guide the neutrophils across the endothelial cell layer. The neutrophils then migrate towards the invading microbes guided by signals produced by the microbes and sensed by the neutophils. Killing of microbes takes place largely in a phagocytic vacuole created by uptake of the microbes and fusion of granules with the membrane surrounding the microbes taken up. Here, killing is effectuated by granule proteins and by bactericidal reactive oxygen metabolites created by the neutrophil respiratory burst activity. Thereafter, the neutrophil may undergo apoptosis and become phagocytosed by macrophages or undergo NETosis with extrusion of nets of DNA that may limit bacterial spread.

Key Concepts

  • Production of neutrophils is quantitatively the most important activity in the bone marrow.
  • Endothelial cells that are activated by local infection/inflammation capture by‐passing neutrophils and guide them to the site of infection.
  • Bacteria are taken up by neutrophils into a phagocytic vacuole created by the plasma membrane which wraps around the ingested microbes and fuses with granules.
  • Killing is effectuated by a combination of exposure of microorganisms to highly reactive oxygen species generated by neutrophils in the phagocytic vacuole and by bactericidal proteins stored in granules and liberated to the phagocytic vacuole.
  • Death of neutrophils by apoptosis regulates the production of neutrophils by a feedback loop that tunes production of G‐CSF according to the rate of apoptosis.
  • Death by NETosis involves extrusion of DNA associated with antimicrobial peptides. This may provide defence against spread of microorganisms.

Keywords: chemotaxis; phagocytosis; opsonisation; degranulation; diapedesis; azurophil granules; specific granules; gelatinase granules; secretory vesicles; NADPH oxidase; superoxide; hydrogen peroxide; myeloperoxidase; Fc receptors; complement receptors; integrins; selectins; chemokines; sevenspan receptors; NETs

Figure 1. Neutrophil lifespan and stages of maturation, showing the flux through each compartment and the time in each compartment. The area of each section indicates the number of cells in each compartment. The stepwise increase through the first three compartments represents serial divisions of the cells in these compartments. In the promyelocyte stage, the azurophil granules (red dots) are formed, whereas the specific granules (purple organelles) are formed in the myelocytic stage. Note that no divisions occur after the myelocyte stage. (Reproduced from Bainton DF (1980) © Elsevier).
Figure 2. Neutrophil activation through G protein‐coupled receptors of the sevenspan supergene family. Most chemotaxin receptors belong to this gene family, being characterised by seven transmembrane‐spanning domains. The intracellular loops and C‐terminus define the association with different G proteins and, as a consequence, part of the secondary signalling cascades that may become activated upon ligand binding. The activation of cytoplasmic enzymes, such as phospholipases C, D and A2, and different protein kinases (PK), each with their own substrates and products, leads to several of the effector functions of neutrophils. PC, phosphatidylcholine; PIP2, phosphoinositol‐4,5‐bisphosphate.
Figure 3. Neutrophil influx into tissue. Initial interactions among selectin members and their respective carbohydrate receptor structures (sialyl Lewis‐X determinants) causes ‘rolling’ of the neutrophils (PMN) over the blood vessel wall (1). E‐selectin and ICAM‐1 are increased in expression on endothelial cells in infectious areas. E‐selectin binds to sialyl Lewis‐X on the neutrophils and ICAM‐1 to the β2‐integrin CR3 on the neutrophils. This last process causes a tight binding to and spreading of the neutrophils on the vessel wall (2). Platelet‐activating factor (PAF) and interleukin 8 (IL‐8), produced by endothelial cells (EC) in infectious areas, then induce the neutrophils to squeeze between the endothelial cells into the tissues. Neutrophils in the middle of the bloodstream continue to circulate (3).
Figure 4. Schematic representation of the three selectin members: L‐selectin (CD62L) on leucocytes, E‐selectin (CD62E) on activated endothelium and P‐selectin on activated endothelium and platelets. The number of sequence consensus repeats (SCR) differs among the selectin members. The EGF‐like domain represents the ligand‐binding site. This domain is called lectin‐like because of its ability to recognise a particular carbohydrate determinant, that is, sialyl Lewis‐X (sLex). EGF, epidermal growth factor.
Figure 5. The integrin supergene family. One β chain may associate with any one of various α chains. The β1‐integrins recognise extracellular matrix components. Only α4β1 (VLA‐4) is also able to bind to a cellular ligand, VCAM‐1, predominantly expressed on endothelial cells. Neutrophils predominantly express the β2‐integrins, CD11b/CD18 (CR3) in particular, at very high levels. This receptor is used as adhesion receptor through binding to ICAM‐1 and several different extracellular matrix (ECM) proteins, as well as an opsonin receptor for inactivated C3b (C3bi) fragments deposited on opsonised microorganisms.
Figure 6. Recognition, uptake and killing of microorganisms by neutrophils. Opsonised microorganisms bind with Fc regions of IgG antibodies to Fcγ receptors, and with C3b/C3bi fragments to complement receptors CR1 and CR3 on the surface of the neutrophils. As a result, the microorganisms are engulfed by the neutrophils and taken up into an intracellular phagosome. Neutrophil granules fuse with the phagosome membrane and deposit their contents into the phagosome. A membrane‐bound oxidase is activated and starts to generate superoxide (O2), also into the phagosome. The superoxide is spontaneously converted into hydrogen peroxide (H2O2), which reacts with myeloperoxide (MPO) released from the granules to yield additional toxic oxygen compounds. BPI, bactericidal permeability‐increasing protein.
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Borregaard, Niels(Sep 2015) Neutrophils. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001219.pub2]