Differences in the ingestion mechanisms of IgG and C3b particles in phagocytosis by neutrophils

Abstract

A prominent feature of the surface interaction between the phagocyte and particles coated with IgG and C3b is its remarkably discriminatory character. IgG primarily promotes ingestion whereas C3b primarily promotes attachment. However, in several systems the C3b molecule is reported to cause membrane pertubation as efficiently as the IgG molecule. In this study we have used yeast particles coated with specific IgG or C3b as preys in a newly developed phagocytic assay. The number of particles interacting with polymorphonuclear leucocytes (PMN) was correlated with effector responses monitored as percentage ingested particles, superoxide anion production, and sensitivity of the ingestion process to cytochalasin B. During phagocytosis only 60% of PMN-associated yeast-C3b particles were ingested compared with 95% of yeast IgG. In cytochalasin B treated PMN, where ingestion was virtually abolished, attached yeast IgG-induced superoxide anion production whereas the same number of attached yeast C3b did not. Attempts to induce superoxide anion production in the C3b system failed by increasing incubation time, number of added particles or by increasing the concentration of the opsonizing protein. A low concentration of cytochalasin B (1 μg/ml) decreased the IgG-dependent ingestion to 65% without affecting the C3b-dependent ingestion. The results indicate the existence of different ingestion processes in PMN—one more `basal' ingestion process represented by the C3b-promoted ingestion independent of membrane activation and another `active' ingestion process represented by the IgG-promoted ingestion dependent on membrane activation and an active response of the PMN.