Leukocyte rolling and firm adhesion in the microvasculature : functional significance for leukocyte recruitment in inflammation

Abstract

Leukocyte recruitment to tissues is a key component in the inflammatory process.In order for circulating leukocytes to extravasate, they need to interact with vascularendothelium in several sequential steps. Slow rolling along the endothelial liningis the initial step in this sequence of events, followed by firm adhesion to endotheliumand subsequent transmigration across the vessel wall. These interactions are governedby adhesion molecules expressed on the surface of leukocytes and endothelial cells,i.e. Ieukocyte rolling is mediated predominantly by adhesion molecules of the selectinfamily whereas firm adhesion is mediated by integrin molecules. This intravital microscopicstudy aimed at investigating leukocyte/endothelium interactions in inflammation withfocus on the functional significance of leukocyte rolling for subsequent steps inthe leukocyte extravasation process.

The quantitative relationship between chemoattractant-induced leukocyte firm adhesionand the preceding rolling interaction was investigated in rat mesenteric venules.Systemic treatment with fucoidin, a sulfated polysaccharide which interferes withselectin-receptor function, resulted in a dose-dependent inhibition of leukocyterolling. Secondary to this effect, chemoattractant-induced firm leukocyte adhesionwas reduced in proportion to the decrease in leukocyte rolling. The data demonstratea close relationship between the extent of leukocyte rolling and the firm adhesiveresponse, and that the initial rolling interaction is a necessary precondition forsubsequent steps in the extravasation process. Further studies in the rabbit showedthat inhibition of leukocyte rolling with fucoidin profoundly reduced neutrophilinfiltration in inflammatory skin lesions, and leukocyte accumulation in the cerebrospinalfluid in an experimental meningitis model, thus illustrating a therapeutic potentialof inhibiting leukocyte rolling (e.g. with carbohydrate molecules) in inflammatorydisorders.

Heparin and related glycans may, similarly to fucoidin, interfere with leukocyte/endotheliuminteractions. It was demonstrated that heparin has inhibitory actions on both leukocyterolling and chemoattractant-induced firm adhesion, and that it may increase localblood flow. Attenuation of the firm adhesive response at low concentrations of heparincould be ascribed mainly to inhibition of selectin-mediated leukocyte rolling, whereasat high concentrations of heparin a direct effect on integrin-mediated firm adhesionwas indicated. Because of interference with several microvascular functions, strictdose-dependent responses to heparin treatment were not found, illustrating a complexinterplay between local blood flow, leukocyte rolling, and firm adhesion as determinantsof leukocyte recruitment to tissues in inflammation.

These interrelationships and the influence of microhemodynamics on leukocyte/endotheliuminteractions were characterized in more detail. The rolling leukocyte flux was directlyproportional to the total leukocyte flux, and to the venular blood flow. Consequently,the rolling/total leukocyte flux fraction did not vary with blood flow and showedno correlation to wall shear rate. Firm leukocyte adhesion evoked by chemoattractantstimulation was quantitatively related to the rolling leukocyte flux, and hence tothe venular blood flow, while there was no correlation to the wall shear rate. Thedata indicate that leukocyte adhesion to the venular endothelium, because of itsdependence on the preceding rolling interaction, is proportional to the microvesselblood flow, which emphasizes the importance of tissue hyperemia for leukocyte recruitmentin inflammation.

Analysis of the rolling characteristics of polymorphonuclear (PMN) and mononuclear(MN) leukocytes indicated that the great majority of PMN leukocytes passing in venulesof the rat mesentery rolled along the endothelial lining, whereas MN leukocytes wererolling only to a small extent under the same basal conditions. In cytokine-activatedtissue, on the other hand, MN leukocyte rolling was substantially increased. Differencesbetween the leukocyte subpopulations in their binding interaction with the endothelialselectins and a dependence of MN leukocyte rolling on a4 integrins were suggestedto govern the discrete rolling characteristics, and are likely to contribute to thetemporal selectivity in the recruitment of leukocyte subclasses to inflamed tissuesites.

Keywords: adhesion molecules, blood flow, chemoattractants, endothelium, flowcytometry, fucoidin, heparin, inflammation, integrins, intravital microscopy, leukocyteadhesion, leukocyte extravasation, leukocyte rolling, meningitis, mesentery, microcirculation,rabbit, rat, selectins

ISBN 91-628-2830-4