| The first step in leukocyte accumulation at inflammatory sites in vivo is rolling on the vessel wall, usually mediated by selectins. The close proximity to the vessel wall allows the rolling leukocytes to survey endothelium for other inflammatory signals, and to engage further adhesion receptors that enable firm adhesion and diapedesis. |
Rolling interactions can be reconstituted in vitro, when purified selectin molecules are present on the wall of a parallel wall flow chamber. The cells roll with a characteristic jerky motion. Each jerk appears to reflect the dissociation of a cluster of bonds between the cell and the substrate. Rapid movement forward until the cell is held by another cluster of bonds, toward the rear of the cell, helps promote the formation of a new cluster of bonds under the center of the cell. Rolling requires a rapid rate of bond formation and dissociation, as the zone of adhesion is translated downstream. Visualization of the asymmetric nucleus of the neutrophil shows that cells truly roll.
Lawrence, M. B., and Springer, T. A. (1991). Leukocytes roll on a selectin at physiologic flow rates: distinction from and prerequisite for adhesion through integrins, Cell 65, 859-873. |
| All three steps can be reconstituted in vitro. Both P-selectin and ICAM-1 are incorporated on the wall of a flow chamber. Neutrophils are infused and begin rolling on the P-selectin. There is no interaction with ICAM-1, because the aMb2 and aLb2 integrins on the neutrophil have not been activated. Subsequently, a chemoattractant such as a bacterial N-formylated peptide (fMLP) is added to the perfusate. When it reaches the position of the rolling neutrophils in the flow chamber, it rapidly activates G protein-coupled receptor signaling pathways, which activate integrins, resulting in firm adhesion to the ICAM-1. The video segment is a time-lapse that is accelerated 6-fold.
Lawrence, M. B., and Springer, T. A. (1991). Leukocytes roll on a selectin at physiologic flow rates: distinction from and prerequisite for adhesion through integrins, Cell 65, 859-873.
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When shear is below the threshold, as at the beginning of this segment, cells are free in flow and thus move at the "hydrodynamic velocity". When the flow rate and hence the shear is increased, the neutrophils "catch" on the peripheral node addressin and roll, and paradoxically move more slowly despite the higher flow rate. Note that while rolling, the cells move in jerky steps, reflecting dissociation of receptor-ligand bonds. The rolling cells also exhibit sideways excursions, since bonds are not always exactly under the center of the cell. While free in flow, the cells move smoothly. The transition between rolling and freedom in flow can be demonstrated through many repeated cycles of variation in flow rate.
Finger, E. B., Puri, K. D., Alon, R., Lawrence, M. B., von Andrian, U. H., and Springer, T. A. (1996). Adhesion through L-selectin requires a threshold hydrodynamic shear, Nature 379, 266-269.
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