Neutrophil movement

Egress of neutrophils from the bone marrow to the circulation, and diapedesis from the circulation to the tissues, involves squeezing of the neutrophils through the blood vessel wall, in between the endothelial cells and through the basement membrane. For this purpose, neutrophils create gaps between endothelial cells of about 1 jjuti, without disturbing the endothelial cell layer. Thanks to its lobulated nucleus the neutrophil is very flexible, which greatly facilitates its passage through these pores (a rounded neutrophil has a diameter of 12-15 pm). It is not known which humoral factors cause egress from the bone marrow in healthy individuals.

Adherence of neutrophils to endothelial cells in postcapillary venules is facilitated by passive move

Figure 1 Neutrophil life span and stages of maturation. The ordinate shows the flux through each compartment and the abscissa shows the time in each compartment. The area of each of the compartments gives the number of cells in each compartment. The stepwise increase in cell numbers through the first three compartments represents serial divisions of the cells in these compartments. Note that no divisions occur after the myelocyte stage. (Reproduced with permission from Bainton DF (1980) The cells of inflammation: a general view. In Weissmann G (ed) The Cell Biology of Inflammation, vol 2, pp 1-25. Amsterdam: Elsevier/North-Holland.)

Figure 1 Neutrophil life span and stages of maturation. The ordinate shows the flux through each compartment and the abscissa shows the time in each compartment. The area of each of the compartments gives the number of cells in each compartment. The stepwise increase in cell numbers through the first three compartments represents serial divisions of the cells in these compartments. Note that no divisions occur after the myelocyte stage. (Reproduced with permission from Bainton DF (1980) The cells of inflammation: a general view. In Weissmann G (ed) The Cell Biology of Inflammation, vol 2, pp 1-25. Amsterdam: Elsevier/North-Holland.)

ment of the neutrophils to the vessel wall, which causes reversible interaction with selectin molecules on the endothelial cells, influenced by the fluid flow (neutrophil rolling). This is followed by a more stable interaction with accessory molecules, such as ICAM-1 (neutrophil spreading), which leads to dia-pedesis and movement into the tissues. During inflammation these processes are strongly increased as a result of the formation of chemotactic agents in the inflamed area, such as the complement fragment C5a, the tripeptide formyl-Met-Leu-Phe (fMLP, interleukin 8 (IL-8) and leukotriene B4. These agents increase the surface expression of certain adhesion molecules on the neutrophils, such as the CD 18 inte-grin p2 family, and also activate these molecules for enhanced binding to endothelial cell structures, probably by changing their configuration. Moreover, these chemotactic agents also induce the neutrophils to move to areas of higher concentration of these agents. This is the process of chemotaxis, which directs the cells toward the infected or inflamed area. On the other hand, endotoxin, released by microorganisms, induces macrophages to generate IL-1

and tumor necrosis factor (TNF). These agents, including endotoxin, affect the endothelial cells by upregulating the adhesion proteins ICAM-1, VCAM-1 and E-selectin, thus promoting neutrophil adherence. Finally, endothelial cells also generate platelet-activating factor and IL-8 under these conditions, which promotes neutrophil migration.

The biological significance of adherence and migration is clearly demonstrated by the clinical symptoms of patients with leukocyte adhesion deficiency, i.e. severe, recurrent bacterial infections, impaired wound healing, persistent leukocytosis and severe deficiency in mounting an inflammatory response. Neutrophils from these patients lack all three CD18 adherence proteins, are strongly-depressed in their ability to adhere to endothelial cells or other neutrophils (aggregation) and are chemotactically inactive.

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