1.5.2 Leukocyte mobility and chemotaxis

The emigration of circulating leukocytes from the blood into inflamed tissues have been refined into a ''three step'' process comprising: (a) rolling of leukocytes along the vasculature (mediated through transient interactions between selectin proteins and their carbohydrate ligands), followed by (b) activation of both neutrophils and endothelial cells and a high affinity interaction between integrins and glycoproteins of immunoglobulin superfamily, leading ultimaely to (c) extravasation (crawling along the endothelium, diapedesis, and migration into tissue) in response to a chemoattractant gradient (Figure 1.6).

 
Figure 1.6:   Mechanisms of leukocyte adherence to and transmigration across the endothelium (arranged according to G>P>Downey, 1994); PECAM: platelet endothelial cell adhesion molecule

Rolling leukocytes are generally defined as white cells that move through microvessels at a rate that is lower than that of red blood cells. In 30m diameter postcapillary venules, the red blood cell velocity is usually 1-3mm/s, whereas leukocytes roll at velocities ranging between 5 and 300m/s, with the most frequently observed rolling velocities lying between 20 and 60m/s. Rolling leukocytes are not always commited to either firmly adhering to the vessel wall or rolling along the entire vessel length; rolling leukocytes frequently detach and return to the mainstream of flowing blood. Leukocyte rolling is likely to occur also under normal physiological conditions in all tissues (gastrointestinal mucosa, skin, lung) that are continually exposed to extermal inflammatory stimuli that are physical and/or chemical in nature.

In inflamed tissue, leukocyte rolling frequently (but not always) leads to a stationary state in which the leukocyte remains firmly attacked to the endothelial cell surface, without rotation motion. This strong (high-affinity) adhesive interaction is often referred to as leukocyte sticking, firm adhesion, or adherence, terms that denote the absence of movement of the leukocyte along the length of the venule.

In the initial phase of an acute inflammatory response, circulating leukocytes are activated by exposure to inflammatory mediators including complement fragments (C5a), cytokines such as IL-1, IL-8 and TNF- and lipopolysaccharide or classical chemoattractants such as formylated methionine -- leucine - phenylalanine (FMLP) leading to their microvascular sequestration due in part to decreased deformability (i.e. increased cell stiffness) and in part to increased adhesiveness of the circulating leukocytes. Endothelial cells are similary activated, leading to enhanced expression of several adhesion molecules. Platelet activating factor (PAF) produced by endothelial cells may act on nearby neutrophils to potentiate their adhesion to the endothelium.

Transmigration of neutrophils across the endothelial barrier involves interaction between leukocyte integrins and endothelial ICAM, and between glycosylated aminoglycans on the neutrophil plasma membrane and PECAM-1, which is localized in the intercellular junctions of endothelial cells. In extravascular locations, interaction between extracellular matrix proteins and adhesion molecules, possibly by activation of cytosolic tyrosine kinases, facilitate the release of large quantities of toxic oxygen radicals and proteolytic enzymes.

Chemokines due to their selective chemoattractant activities for different types of leukocytes (Table 1.11) play an important role in the process of transmigration. The model of chemokine involvement in leukocyte trafficking might be summarized as follows:

(a)

a chemokine, sequestered in solid phase on the endothelial cell surface, is presented as a signal to trap a specific type of leukocyte as the cell is undergoing selectin-mediated rolling along the endothelium;

(b)

the leukocyte is selectively activated by the chemokine so that the cell stops rolling and become firmly adhered;

(c)

the adhered leukocyte rather ''crawls'' than swims along the chemotactic gradient formed by the chemokines on the endothelium;

(d)

the leukocyte undergoes diapedesis and migrates into the tissue space, while still responding to a chemotactic gradient.

In general, cell mobility represents the integration of many processes including adhesion (integrin-dependent), lamellar protrusion (actin-dependent), deadhesion (integrin-dependent), and contraction (actin and possibly myosin-dependent). Moving neutrophils assume a polarized morphology with an anterior lamellipodium extended in the direction of movement, a cell body that is elongated parallel to the axis of lamellar protrusion, and a knob-like tail or ''uropod''.

The importance of leukocyte adhesion molecules may be documented by the existence of leukocyte adhesion deficiency (LAD), a congenital disorder manifest as LAD-1 and LAD-2 syndromes. Leukocytes of patients with LAD-1 syndrome lack integrin expression. It occurs in two main forms: one with severe and the other with moderate clinical manifestations. In severe form, both - and -chains in the molecule of the LFA-1 subfamily are completely lacking. The severe deficiency affecting both boys and girls, is manifested by severe, life-threatening infections with high mortality (patients seldom survive beyond two years of age). The moderate deficiency is accompanied by partially integrin expression; patients express 2.5 to 6.0% of normal LFA-1, CR3 and CR4 levels and usually have only recurrent skin infections. Leukocytes from patients with LAD-2 syndrome failed to express sLe(CD15s) and therefore they are not able to bind to E-selectin and P-selectin. Consistent with the proposed role of selectins, there was a marked reduction in the rolling of leukocytes from these patients. This clearly indicates a requirement for the carbohydrate ligands recognized by the selectins. The LAD-2 patients, are suffering from recurrent bacterial infections but they can survive into childhood, with short stature and mental retardation due to disorder of fucose metabolism since fucose is an important component of sLe.