Biomolecular Recognition: The Current Challenge - Dynamic Force Spectroscopy and Biomolecular Recognition - page 6

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FIGURE 1.2 Leukocyte arrest on activated endothelium. Rapidly flowing leukocytes are first
tethered by receptors such as P-selectin, which appear on the membrane of activated endothe-
lial cells (1). Then, they begin rolling with a nearly 100-fold velocity decrease (2), which
allows them to detect activating mediators such as chemokines on endothelial surfaces. These
molecules activate leukocyte integrins, which results in firm cell adhesion (3). Then, cells
migrate toward endothelial cell junctions (4) and undergo impressive deformation that allows
them to traverse the endothelial junctions. Finally, after crossing the basal membrane, they
accede to inflamed tissues (5).

strong leukocyte attachment and arrest. Displacement toward interendothelial junc-
tions and migration to the peripheral tissues then follow. The progress of molecu-
lar biology and monoclonal antibody technology allowed identification of the adhe-
sion molecules involved in leukocyte/endothelial interaction during the 1980s, and
the overall mechanisms of rolling and firm adhesion were disclosed in 1991 [115]
[189]. Briefly, proper stimulation of endothelial cells was shown to generate rapid
expression of so-called selectin molecules on the membranes of endothelial cells.
Thus, P-selectin that is stored in specialized granules may be externalized within
minutes. P-selectin is a long (about 40 nm) molecule the distal extremity of which
bears a binding sites specific for characteristic structures bearing the sialyl

Lewis x
tetrasaccharide. This ligand is exposed on molecules borne by leukocyte membranes
such as PSGL-1 (a 40-nm carbohydrate-rich molecule). The jerky rolling motion
may be accounted for by a rapid formation and dissociation of P-selectin/PSGL-1
bonds as shown with model systems [24]. During the rolling phase, leukocyte adhe-
sion receptors belonging to the integrin family get activated by molecules linked
to the endothelial cell pericellular matrix. These integrins then strongly bind to
their ligand and induce a durable arrest. As an example, leukocyte integrin LFA-1
(which means lymphocyte function associated-1, also called CD11a/CD18) will bind
to ICAM-1 (intercellular cell adhesion molecule-I, CD54) on endothelial cell sur-
faces. Flow chambers (see below) were used to study leukocyte arrest on planar sur-
faces coated with endothelial cell monolayers or molecules. A question that rapidly
emerged consisted of understanding why P-selectin/PSGL-1 interaction resulted in
rolling, whereas integrin/ligand association could not occur in the absence of rolling,
even if leukocyte integrins were activated before the experiment. Since the affin-
ity of P-selectin/PSGL-1 and integrin/ligand interaction fell into the same range,
it was soon suggested that P-selectin/PSGL-1 interaction might display peculiar
physical properties, with high association and dissociation rates (allowing rapid
cell attachment and detachment) and high mechanical strength to resist hydrody-
namic forces (otherwise, these interactions would not generate any detectable cell
arrest).

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