Abstract • 178

To learn more about the nature of the infectious mechanism, internalisation of C.albicans was studied in mouse macrophages (MØ) by confocal immunofluorescence and electron microscopy. Uptake of candida yeasts had characteristics of phagocytosis, required intact actin filaments and a microtubular network. Candida-phagosomes rapidly attracted lysosome-associated glycoprotein (Lamp)-rich vacuoles, indicative of fusion with late endosomes and lysosomes. Phagosomes also fused with early endosomes and became transferrin receptor positive. The yeasts developed germ tubes within phagolysosomes, distended their membranes and escaped, destroying the non-activated MØ. In contrast to yeasts, germ tubes penetrated intact MØ. even when phagocytosis was blocked and attracted Lamp-rich organelles. When endoplasmic reticulum Ca2+- ATP-ase was inhibited, germ tubes could penetrate but failed to recruit lysosomes. Inhibition of the lysosomal H+ -ATP-ase and associated lysosomal fusion reduced germ tube formation of candida within the phagolysosomes. These data suggest that rapid recruitment of late endocytic/lysosomal compartments by internalising C. albicans favours differentiation of a relatively invasive, filamentous form of this pathogen.