THE MULTISTEP PROCESS OF HOMOTYPIC NEUTROPHIL AGGREGATION - A REVIEW OF THE MOLECULES AND EFFECTS OF HYDRODYNAMICS

Homotypic adhesion of neutrophils stimulated with chemoattractant is a nalogous to capture on vascular endothelium in that both processes are supported by L-selectin and beta(2)-integrin adhesion receptors, Unde r hydrodynamic shear, cell adhesion requires that receptors bind suffi cient ligand over the duration of intercellular contact to withstand t he hydrodynamic stresses. Using cone and plate viscometry to apply a u niform linear shear field to suspensions of neutrophils and flow cytom etry to quantitate the size distribution of aggregates formed over the time course of formyl peptide stimulation, we conducted a detailed ex amination of the affect of shear rate and shear stress on the kinetics of cell aggregation. The efficiency of aggregate formation was fit fr om a mathematical model based on Smoluchowski's two-body collision the ory. Over a range of venular shear rates (400-800 s(-1)), similar to 9 0% of the single cells are recruited into aggregates ranging from doub lets to groupings larger than sextuplets. Adhesion efficiency fit to t he kinetics of aggregation increased with shear rate from similar to 2 0% at 100 s(-1) to a maximum level of similar to 80% at 440 s(-1). Thi s increase to peak adhesion efficiency was dependent on L-selectin and beta(2)-integrin, and was resistant to shear stress up to similar to 7 dyn/cm(2). When L-selectin was blocked with antibody, beta(2)-integr in (CD11a,b) supported adhesion at low shear rates (< 400 s(-1)). Aggr egates formed over the rapid phase of aggregation remain intact and re sistant to shear up to 120 s. At the end of this plateau phase of stab ility, aggregates spontaneously dissociate back to singlets. The rate of cell disaggregation is linearly proportional to the applied shear r ate. The binding kinetics of selectin and integrin appear to be optimi zed to function within discrete ranges of shear rate and stress, provi ding an intrinsic mechanism for the transition from neutrophil tetheri ng to firm but reversible adhesion.