The immune function of granulocytes, monocytes, lymphocytes, and other specialized cells depends upon intercellular adhesion. In many cases the molecules mediating leukocyte cell adhesion belong to the Leu-CAM superfamily of adhesive molecules. To elucidate the events of homotypic aggregation in a quantitative fashion, we have examined the aggregation of neutrophils stimulated with formyl peptides, where aggregate formation is a transient reversible cell function. We have mathematically modeled the kinetics of aggregation using a linear model based on particle geometry and rates of aggregate formation and breakup. The time course was modeled as a three-phase process, each phase with distinct rate constants. Aggregate formation was measured on the flow cytometer; singlets and larger particles were distinguished using the intravital stain LDS-751. Aggregation proceeded rapidly after stimulation with formyl peptide (CHO-nle-leu-phe-nle-tyr-lys). The first phase lasted 30-60 s; this was modeled with the largest aggregation rate and smallest rate of disaggregation. Aggregate formation plateaued during the second phase which lasted up to 2.5 min. This phase was modeled with an aggregation rate nearly an order of magnitude less than that of the initial fast phase, whereas the disaggregation rate for this phase did not change significantly. A third phase where disaggregation predominated, lasted the remaining 2-3 min and was modeled with a four to fivefold increase of the disaggregation rate. The mechanism of cell-cell adhesion in the plateau phase was probed with the monoclonal antibody IB4 to the CD18 subunit of the adhesive receptor CR3. Based on these studies it appears that new aggregates do not form to a large degree after the first phase of aggregate formation is complete. However, new adhesive contact sites may form within the contact region of these adherent cells to keep the aggregates together.

Stimulation of human neutrophils with the chemotactic N-formyl peptide causes production of oxygen radicals and conversion of monomeric actin (G-actin) to polymeric actin (F-actin). The effects of the binary botulinum C2 toxin on the amount of F-actin and on neutrophil cell responses were studied. Two different methods for analyzing the actin response were used in formyl peptide-stimulated cells: staining of F-actin with rhodamine-phalloidin and a transient right angle light scatter. Preincubation of neutrophils with 400 ng/ml component I and 1,600 ng/ml component II of botulinum C2 toxin for 30 min almost completely inhibited the formyl peptide-stimulated polymerization of G-actin and at the same time decreased the amount of F-actin in unstimulated neutrophils by an average of approximately 30%. Botulinum C2 toxin preincubation for 60 min destroyed approximately 75% of the F-actin in unstimulated neutrophils. Right angle light scatter analysis showed that control neutrophils exhibited the transient response characteristic of actin polymerization; however, after botulinum C2 toxin treatment, degranulation was detected. Single components of the binary botulinum C2 toxin were without effect on the actin polymerization response. Fluorescence flow cytometry and fluorospectrometric binding studies showed little alteration in N-formyl peptide binding or dissociation dynamics in the toxin-treated cells. However, endocytosis of the fluorescent N-formyl peptide ligand-receptor complex was slower but still possible in degranulating neutrophils treated with botulinum C2 toxin for 60 min. The half-time of endocytosis, estimated from initial rates, was 4 and 8 min in control and botulinum C2 toxin-treated neutrophils, respectively.

The response of human neutrophils to N-formyl peptides were studied under conditions where ligand binding was controlled by infusing a cell suspension with the peptide over a time period comparable to the normal half-time for binding. Receptor occupancy was measured in real time with a fluorescently labeled peptide using flow cytometry. This binding was approximated by a simple reversible model using typical on (7 X 10(8) M- min-1) and off (0.35/min) rate constants and the infusion rates (0.02-0.2 nM/min). Under conditions of stimulus infusion intracellular calcium elevation, superoxide generation, and right angle light scatter and F-actin formation were measured. As the infusion rate was decreased into the range of 10 pM/min, lowering the rate of increase of receptor occupancy to approximately 0.5% per min, the calcium and right angle light scatter responses elongated in time and decreased in magnitude. Superoxide generation decreased below infusion rates of approximately 100 pM/min (occupancy increasing at a rate in the range of 5% per min). This behavior could contribute to differences between chemotactic responses, which appear to require low rates of receptor occupancy over long periods, and bactericidal or inflammatory responses (free radical generation and degranulation), which require bursts of occupancy of several percent of the receptors.

When exposed to the N-formylated chemoattractant peptides, neutrophils undergo a transient ruffling followed by a polarization that involves a redistribution of F-actin (Fechheimer, M., and S. H. Zigmond, 1983, Cell Motil., 3:349-361). The cells also undergo a biphasic right angle light scatter response whose first phase is maximal 10-15 s after exposure to the stimulus, and whose second phase is longer in duration and maximal only after 1 min or more (Yuli, I., and R. Snyderman, 1984, J. Clin. Invest. 73:1408-1417). We now report that the first phase is accompanied by a transient polymerization of actin (monitored by cytometric analysis of phallacidin staining according to the method of Howard, T. H., and W. H. Meyer, 1984, J. Cell Biol., 98:1265-1271) and the second phase is accompanied by a more sustained polymerization of actin. Based on correlated measurements of ligand binding (Sklar, L. A., D. A. Finney, Z. G. Oades, A. J. Jesaitis, R. G. Painter, and C. G. Cochrane, 1984, J. Biol. Chem., 259:5661-5669) and intracellular Ca++ elevation (under conditions where we use the fluorescent Ca++ chelator Quin 2 to modulate intracellular Ca++ levels), we conclude that this first phase requires less than 100 receptors/cell (out of 50,000) and does not require the release of intracellular stores of Ca++. In contrast, the sustained polymerization requires both the occupancy of thousands of receptors (an estimated 10% of the receptors per minute) and may be somewhat sensitive to the availability of intracellular Ca++. When ligand binding is interrupted, F-actin rapidly depolymerizes with a half-time of no greater than approximately 15 s, and the transient light scatter response decays toward its initial value in parallel. Partial disaggregation of the cells follows the recovery of these responses. Based on these observations, we suggest that transient actin polymerization and transient cell ruffling give rise to transient aggregation as long as degranulation is limited.

When human granulocytes were exposed to 50 nM N-formyl-Met-Leu-[3H]Phe at 37 degrees C they rapidly formed ligand-receptor complexes that dissociated 50-100 times more slowly than those on cells initially exposed to the peptide at 4 degrees C. These complexes of apparent higher affinity were stable after detergent solubilization of the cells with Triton X-100. The complexes co-isolated with the detergent insoluble cytoskeletal residues and were free of the cytosolic and Golgi markers, lactate dehydrogenase and galactosyl transferase, respectively. After 5 s of exposure to f-Met-Leu-Phe, 2,000-3,000 molecules of ligand per cell were trapped in such complexes. Continued exposure resulted in capture of a maximum of 14,000 molecules per cell by 5 min. Exposure at 15 degrees C, a temperature at which endocytosis of the receptor is prevented, resulted in complex formation at a linear rate for at least 20 min to levels twice those measured at 37 degrees C. At 4 degrees C, complex formation was approximately 10% of the maximum amount formed at 37 degrees C. Pulse-chase experiments revealed that the complex was in transient association with the cytoskeleton with a half life ranging between 30 s to 4 min depending on the length of the original incubation. Electron microscopic autoradiography indicated that after 1 min of incubation at 37 degrees C, the majority of the specific autoradiographic grains were localized to the outer circumference of the cellular cytoskeleton. After 4 min of incubation, the grains were less frequent at the cytoskeleton periphery but still threefold enriched over a random cellular distribution. We conclude that a metabolically controlled modulation of the state of the N-formyl chemotactic peptide receptor occurs in the plasma membrane which may be the result of transient association of ligand-receptor complex and the cell cytoskeleton.