Ingestion of Bacteria by Antibody‐Coated Ehrlich Ascites Tumor Cells Mediated by Protein A

Endocytosis has been investigated with phagocytic cells such as macrophages, monocytes or polymorphonuclear leukocytes both in vivo and in vitro. The presence of opsonin is one of the prerequisites for phagocytosis in such cells Endocytoses of hormones and enzymes mediated by various specific receptors have also been investigated Here we report that Ehrlich ascites tumor cells (E-cells) sensitized with rabbit anti-E-cell antibody efficiently phagocytized Staphylococcus aureus organisms by way of the affinity between protein A molecules on staphylococcal cells and immunoglobulin molecules on tumor cells. Antibody molecules on the surface of tumor cells served as receptors in the process of ingestion of protein A-associated particulate substances. This study presents the first evidence for phagocytosis of ligand-associated particles with an artificially-attached receptor by cells which are usually considered not to be phagocytic. E-cells were harvested and sensitized with rabbit anti-E-cell antibody as described previously (19). After sensitization, tumor cells were washed three times with Eagle's Minimum Essential Medium (MEM). The antibody-sensitized tumor cells were incubated with formalinized S. aureus Cowan I with protein A at a dose of 108 colony forming units (CFU) per 106 tumor cells in MEM at 37 C in a 5% CO2 atmosphere. After 4 hr-incubation, the tumor cells were harvested by centrifugation (5 min at 1,200 rpm). The pellet was resuspended in MEM, and freshly prepared fixative of 1% glutaraldehyde (pH 7.4, 340 OsM) was added. Then the cells were centrifuged and the fixative was replaced with the same fresh fixative. After 1 hr the cells were washed, postfixed for 2 hr with 1% osmium tetroxide in phosphate buffer, dehydrated, and embedded in Epon 812. Thin sections were stained with uranyl acetate and lead citrate, and examined with a Hitachi H-500 electron microscope. As shown in Fig. No staphylococcal adsorption or ingestion by tumor cells was observed in experiments by the combined use of protein A-deficient mutant organisms with antibody-sensitized tumor cells or staphylococcal parental organisms with unsensitized tumor cells.