Microscopy analysis of zymosan phagocytosis by untreated and IL-4–treated ThioMΦs. ThioMΦs were plated on bacteriologic plastic for TEM or on coverslips for scanning electron microscopy in the same well. Cells were treated for 48 hours with IL-4 and challenged with zymosan (20 particles/MΦ) for 30 minutes at 37°C. Scanning electron microscopy showed that zymosan particles were no longer seen on the cell surface of untreated ThioMΦs, because all had been internalized by the cells (TEM data). In IL-4–treated ThioMΦs, many incomplete phagocytic cups were observed on the surface of MΦs, showing the arrest of the phagocytic cup closure (white arrows) induced by IL-4 treatment. Moreover, TEM showed that few zymosan particles were internalized in IL-4–treated ThioMΦs compared with untreated MΦs.