Directed exocytosis of secretory granules containing apolipoprotein E to the adherent surface and basal vacuoles of macrophages spreading on immobile immune complexes.

As macrophages spread on an immune complex-coated surface, large, clear basal vacuoles and numerous coated vesicles and tubules form rapidly and are occasionally contiguous with the adherent surface, creating a microcompartment between the immune complex-coated surface and the cell membrane. The present study explored the nature of this basal compartment by examining the distribution of a major secretory product of macrophages, apolipoprotein E (ApoE), and of a lysosomal enzyme, acid phosphatase, by enzyme and immunocytochemistry. Upon contact of the macrophages with the immune complexes, intracellular stores of ApoE were secreted rapidly in the first 10 to 20 minutes to the area of ligand-receptor interaction. ApoE filled the large basal vacuoles and was also found in invaginations on the adherent surface that were sealed to the influx of proteins and peptides from the bulk medium. In contrast, the contents of lysosomes were not redistributed to the basal compartment. By 2 hours most of the ApoE had appeared in the bulk medium, suggesting that the protein could move out of the basal compartment. These data suggest that specific ligand-Fc receptor interactions serve to target secretion by macrophages to selective focal areas of contact, and that there are also mechanisms for retrieval of material from these sites.