Exocytosis of zymosan particles by human phagocytes

The kinetics of human leucocyte phagocytosis and exocytosis of fluorescein-isothiocyanate (FITC)-labelled zymosan particles were studied by flow cytometry (FCM). The leucocytes rapidly associated with zymosan particles, and internalization was confirmed by a fluorescence quenching technique. For incubation periods longer than about 60 min, exocytosis of ingested particles was observed. All human phagocytes ejected zymosan particles. The rate of exocytosis was about 1 particle per phagocyte per h, and was independent of the number of internalized particles. Exocytosis was dependent on temperature and glucose, but did not require Ca++ and Mg++ ions. Phagocytosis and exocytosis occurred concurrently, and phagocytosis accelerated ejection of previously internalized particles. The exocytosed particles were partially degraded by the phagocytes, and rephagocytosis of ejected zymosan particles was slower than the uptake of control particles. Phagocytes undergoing exocytosis remained intact during the 210 min examined. The findings indicate a heretofore neglected vector in the interaction of prey and phagocytosing cells.