Kinetics of β2-integrin and L-selectin bonding during neutrophil aggregation in shear flow

Activated neutrophils aggregate in a shear field via bonding of L-selectin to P-selectin glycoprotein ligand-l (PSGL-1) followed by a more stable bonding of LFA-1 (CD11a/CD18) to intercellular adhesion molecule 3 (ICAM-3) and Mac-1 (CD11b/CD18) to an unknown counter receptor. Assuming that the Mac-1 counter receptor is ICAM-3-like in strength and number, rate processes were deconvoluted from neutrophil homoaggregation data for shear rates (G) of 100-3000 s(-1) with a two-body hydrodynamic collision model (Tandon and Diamond, 1997. Biophys. J. 73:2819-2835). For integrin-mediated aggregation (characteristic bond strength of 5 mu dynes) in the absence of L-selectin contributions, an average forward rate of k(f) = 1.57 x 10(-12) cm(2)/s predicted the measured efficiencies for G = 100-800 s(-1). For a selectin bond formation rate constant equal to the integrin bond formation rate constant, the colloidal stability of unactivated neutrophils was satisfied for a reverse rate of the L-selectin-PGSL bond corresponding to an average bond half-life of 10 ms at a characteristic bond strength of 1 mu dyne. Colliding neutrophils initially bridged by at least one L-selectin-PSGL-1 bond were calculated to rotate from 8 to 50 times at G = 400 to 3000 s(-1), respectively, before obtaining mechanical stability in sheared fluid of either 0.75 or 1.75 cP viscosity. Thus for G > 400 s(-1), the interaction time needed for the rotating aggregates to become stable was relatively constant at 52.5 +/- 8.5 ms, largely independent of shear rate or shear stress. Aggregation data and the colloidal stability criterion can provide a consistent set of forward and reverse rate constants and characteristic bond strengths for a known time-dependent stoichiometry of receptors on cells interacting in a shear flow field.