Exocytosis and macrophage-mediated antibody-dependent cellular cytotoxicity: a cautionary note

The appearance of soluble radioisotope from macrophages (M phi) that previously had ingested 51Cr-labelled antibody-coated sheep erythrocytes (51Cr-EA) was assessed to determine if exocytosis contributed to the pool of radioactivity used to measure antibody-dependent cellular cytotoxicity (ADCC). M phi preloaded for 60 min with 51Cr-EA lost the majority of the ingested 51Cr over a subsequent 3 hr incubation period. The exocytotic rate was independent of the number of 51Cr-EA ingested, but was temperature dependent with no appreciable release of 51Cr observed at 4 degrees. The exocytotic process was unaffected by sodium azide (10 mM) but was markedly enhanced in the presence of iodoacetate (50 mM), cytochalasin B (20 micrograms/ml) or lidocaine (0.5 mM). The results show that M phi rapidly digest engulfed target cells and that exocytosis can contribute sufficient soluble radioactivity to the extracellular pool to lead to misinterpretation of putative ADCC activity.