Aggregation of complement receptors on human neutrophils in the absence of ligand.

Abstract

Abstract. C3bi receptors (CR3) on human polymorphonuclear leukocytes (PMN) bind ligand-coated particles and promote their ingestion. The binding activity of CR3 is not constitutive but is transiently enabled by phorbol esters (Wright, S. D., and B. D. Meyer, 1986, J. Immunol. 136:1759-1764. Our observations indicate that the capacity of CR3 to bind ligand is tightly correlated with the degree of ligand-independent aggregation of the receptor in the plane of the membrane. Fixed PMN were labeled with anti-CR3 monoclonal antibodies and streptavidin colloidal gold before viewing in the electron microscope either en face or in thin section. On unstimulated PMN, gold particles marking CR3 were dispersed randomly. Stimulation of PMN for 25 min with phorbol myristate acetate (PMA) dramatically enhances binding of C3bi-coated particles, and the CR3 on such stimulated cells was observed in clusters containing more than six gold particles. CR3 was not aggregated over coated pits. After 50 min in PMA, the binding activity of CR3 falls, and the distribution of CR3 was again observed to be disperse. If a hydrophilic phorbol ester was washed away after a 20-min stimulation, binding activity remains elevated for at least 50 min, and CR3 remained aggregated. Thus, clustering of CR3 was temporally correlated with its ability to bind ligand and initiate phagocytosis. Unlike CR3, Fc receptors and HLA did not exhibit changes in their aggregation state in response to PMA. Treating PMN with formyl-methionyl-leucyl-phenylalanine, which enhances expression of CR3 but not its function, did not lead to aggregation of CR3. These observations suggest that a clustered configuration is a precondition necessary for binding ligand and signaling phagocytosis.

Coxiella burnetii Avoids Macrophage Phagocytosis by Interfering with Spatial Distribution of Complement Receptor 3

“…Thus, pseudopod formation and CR3 activity are distinct events, which is in agreement with other reports in which pseudopodal extensions and particle internalization are uncoupled (34). In neutrophils, a chemoattractant such as fMLP induces cell polarization without CR3 aggregation and phagocytosis increase, whereas phorbol esters induce both CR3 aggregation and phagocytosis without cell deformations (9). The mechanisms driving the distribution and the activation of CR3 in cells stimulated by C. burnetii or RANTES differently involved src kinases.…”

“…Although the mechanism of CR3 activation is incompletely understood, the capacity of CR3 to bind ligands efficiently is closely related to its aggregation state. For instance, CR3 clustering elicited by phorbol esters is correlated to increased binding and phagocytosis (9). The switch from an inactive to an active form is associated with an alteration in the mechanism of interaction of CR3 with the cytoskeleton (10).…”

Immune Complex-induced Integrin Activation and L-plastin Phosphorylation Require Protein Kinase A

“…There is evidence for activation-induced increases in integrin affinity (5-7), integrin clustering (8), and integrin diffusion (9). At a molecular level, phosphoinositidyl 3-kinase (PI 3-kinase) 1 (10), protein kinase C (11,12), and the Ca 2ϩ -dependent protease calpain (13) all can have a role in leukocyte integrin activation.…”

Lymphocyte adhesion mediated by integrins

“…Clustering of CR3 receptors has been reported to be temporally correlated with its ability to bind their ligand (Detmers et al, 1987). Moreover, bind ing of Ca2+ to LFA-1 is associated with clustering of receptors on the cell membrane, which facilitates LFA-1 activation by increasing the avidity of LFA-l-ligand interaction (van Kooyk, unpublished).…”