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(a) structure and function

The selectin family consist of three m em brane glycoproteins, including E-, L- and P-selectins, th a t mediate leukocyte-endothelial cell interactions by binding carbohydrate ligands on opposing cells (McEver, 1991; Varki et al., 1992). The extracellular region of each of them contains an amino-terminal motif th a t is characteristic of Ca++ dependent or C- type lectins, followed by an epidermal growth factor (EGF)-like domain and a series of consensus repeats like those in complement-regulatory proteins (CRP) (Fig. 1.5). Although both lectin and EGFdomains are required to mediate neutrophil adhesion, the three dimensional structure of the ligand-binding region of human E-selectin revealed limited contact between the two domains (Graves et al, 1994). The transm em brane region of selectins is followed by a short cytoplasmic domain. The longest cytoplasmic tail is found in P-selectin (35 residues), one function of which is to direct P-selectin to the secretory granule membranes of endothelial cells and platelets (Disdier et al., 1992). When these cells are activated by thrombin or histamine, P-selectin is redistributed within m inutes to the cell surface as granule membranes fuse with the plasma membrane. No function has yet been shown for the cytoplasmic regions of L- and E- selectins. E-selectin is expressed by cytokine activated endothelium. It appears on the endothelial cell surface 2-4 hours following induction of its synthesis by inflammatory cytokines such as IL-1 or T N F-a. L-selectin is constitutively expressed on the surface of monocytes, neutrophils and a subsets of lymphocytes until they are activated (Table 1.2).

P- and E-selectin bind to myeloid cells, eosinophils (Bochner et al., 1991), a subset of natural killer cells (Lobb et al., 1991) and memory T lymphocytes (Picker et al., 1991; Moore et al., 1992). Thus both molecules may mediate leucocyte extravasation during acute, chronic and allergic

P-selectin L-selectin 1 2 COOH E-selectin COOH lectin domain EGF domain complement binding domains ? ? ? ? ? ? ? ? 6666 16666 COOH

Figure 1.5: Schem atic representation o f selectin m olecule

inflammatory responses. L-selectin binds to the HEV of peripheral lymph nodes (Imai et al., 1991) and to cultured endothelial cells activated hy cytokines (Spertini et al., 1991) (Table 1.2). Therefore, L-selectin has been implicated in both homing of lymphocytes to peripheral lymph nodes and the "rolling" of neutrophils on the endothelium near acute inflammatory sites (described further below).

T ab le 1.2: C h a ra c te ris tic s o f se lec tin s

N am e E x p re sse d by T a rg e t cells P-selectin rapidly-activated

platelets and endothelial cells neutophils, monocytes eosinophils, lymphocyte subsets E-selectin cytokine-activated endothelial cells neutrophils,monocytes eosinophils, lymphocyte subsets L-selectin monocytes, neutrophils,

lymphocyte subsets

activated endothelial cells of peripheral lymph nodes, high endothelial venule

All selectins appear to recognise a sialylated carbohydrate determ inants on their counter-receptors. The carbohydrate ligand for L- selectin is related to tetrasacharide sialyl Lewis^ (sLe®) and Lewis* (sLe*) and contains sialic acid and sulphate (Rosen, 1993). E- and P-selectin recognise carbohydrate structures th a t are distinct but are closely related to the sLe^ and sLe*. The carbohydrate ligands for L- and P-selectin are O-linked to specific mucin-like molecules. Thus L-selectin recognizes two mucins in high endothelial venules, glycosylation dependent cell adhesion

molecule 1 (GlyCAM-1), which is secreted (Rosen, 1993) and CD34, which is on the cell surface (Baumhueter et al., 1993), while the mucin-like P- selectin glycoprotein ligand (PSGL-1) is a disulphide-linked dimer of 120 kD (Sako et al., 1993).

(b) rolling and tight adhesion: the cooperative roles of selectins and integrins

The cu rren t model for leucocyte movement from th e blood circulation into tissues involves an early phase of rolling during which a brief and reversible attachm ent to the endothelium occurs, followed by a phase of tight adhesion and transendothelial migration. Selectins are involved in the earliest rolling interactions, while integrins are required for the tig h t adhesion (Lasky, 1993). The la tte r process needs an appropriate activation signal to be delivered to the leukocytes which will triger an increase in the affinity of leukocyte integrins for endothelial ligands.

The mechanism underlying the ability of selectins to support rolling is not very well understood, but an involvement of P-selectin has been demonstrated in vitro (Lawrence and Springer, 1991). Other studies have suggested a role for L-selectin, since antibodies against L-selectin and recombinant soluble forms of L-selectin can inhibit rolling in vivo (Ley et al., 1991; von Adrian et al., 1991). The recent studies of Mayadas et al. (1993) in P-selectin deficient mice showed approximately 2.5 times more circulating neutrophils than in wild type mice. These findings suggest th at defects in P-selectin expression lead to a deficiency in neutophil- endothelial cell adhesion, preventing neutrophils from m igrating efficiently into tissues. All these results support the view th a t P- and L- selectins act cooperatively a t the early stages of neutrophil rolling and inflammation, while E-selectin, which is maximally expressed 4 hours

after induction of an inflammatory response, may be the major mediator of leucocyte rolling during the later phases of inflammation (Lasky, 1993).