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.