We have measured the kinetics of specific antibody-dependent binding (and phagocytosis) of haptenated lipid vesicles to RAW264 macrophages as a function of antibody density on the vesicle surface and lipid composition of the vesicles. Fluid vesicles (dimyristoylphosphatidylcholine at 37 "C) bind much more rapidly to macrophages than do solid vesicles (dipalmitoylphosphatidylcholine at 37 "C). Inclusion of cholesterol in the dipalmitoylphosphatidylcholine vesicle membrane results in a large enhancement in binding rate, whereas inclusion of cholesterol in the dimyristoylphosphatidylcholine membrane has a much smaller effect (37 "C). The rate of vesicle binding also depends on the antibody density on the vesicle surface. In the presence of cytochalasin B, vesicle phagocytosis is completely inhibited in the case of dimyristoylphosphatidylcholine vesicles and partially inhibited in the case of dipalmitoylphosphatidylcholine vesicles. An analysis of the vesicle to macrophage binding kinetics has beenx e interaction of a phagocytic cell with an antibody-coated target cell can be thought of as an example of cell-cell recognition and triggering. Antigens on the target cell are first "recognized" by specific antibodies; the Fc stems of the bound antibodies are "recognized" by the Fc receptors of the phagocytic cell, which in turn triggers phagocytosis or cytolysis of the target cell. In the present work we have simplified this "model" of cell-cell recognition even further by using haptenated lipid vesicles in place of the target cell, since the physical and chemical properties of such vesicles can be controlled with considerable precision. By varying these physical and chemical properties and by studying the phagocytic activity, one can hope to understand better the molecular events that are typically involved in cell-cell recognition. We have investigated the relation between molecular motion in the vesicle target membranes and the kinetics of vesicle binding and/or phagocytosis by RAW264 macrophages. We have also attempted to provide a quantitative analysis of the kinetic results. Other studies of specific antibody-dependent interactions between cells of the immune system and haptenated lipid membranes include work by Geiger & Schreiber (1 979), Henkart &Blumenthal (1975), andHafeman et al. (1979).The present study has been carried out in parallel with studies of the kinetics of specific antibody-dependent activation of the first component of complement by similar lipid target membranes. The results we have obtained for specific antibody-dependent binding of haptenated lipid target membranes are strikingly similar to those observed for the specific anti-
The specific antibody-dependent stimulation of the respiratory burst (cyanide-insensitive oxygen consumption, 1-C-glucose oxidation) of RAW264 macrophage cell line by haptenated lipid vesicles depends strongly on the physical properties of the lipid membrane, as well as the surface density of antibodies on the vesicles. Lipid membranes that are "solid" at 37 degrees C (dipalmitoylphosphatidylcholine, DPPC) are much more effective, per vesicle bound, than are "fluid" membranes (dimyristoylphosphatidylcholine, DMPC). Vesicle membranes that have both fluid and solid regions (DPPC containing < 20 mol % cholesterol) show both enhanced binding rates (due to the fluid regions) and enhanced respiratory rates (due to the solid regions). In contrast to these results, the specific antibody-dependent respiratory burst of neutrophils due to haptenated vesicles parallels the antibody-dependent vesicle binding and shows no significant difference between fluid and solid target membranes.
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