The interaction of phagocytes and the large-sized parasite Cryptococcus neoformans: Cytochemical and ultrastructural study

The yeast Cryptococcus neoformans may develop under certain conditions a large polysaccharide capsule 50-100 μM in diameter and therefore cannot be phagocytosed by either polymorphonuclear cells (PMN's) or mononuclear phagocytes (MN's). The cellular defense mechanism - in various animals - against the yeast is composed by formation of ringlike structure of PMN's or MN's cells which surround the C. neoformans. Ring structures develop either in vivo or in vitro in tissue culture; destruction of the yeast occurs within 36-72 hours. Several hydrolases, such as acid phosphatase, β-glucuronidase and non-specific esterase were found to be released from the phagocytic cells into the enclosed yeast. Considerable reduction of NBT used as a marker for oxidative activity was observed in MN rings at contact regions of the MN cells and the yeast. Electron microscopic studies indicate that the phagocytic cells in the ring structure have many pseudopodes penetrating into the polysaccharide capsule of the yeast. Disintegration of the capsule was observed as well as phagocytosis of its material. A possible analogy between normal phagocytosis of small-sized bodies and the ring structure obtained when large bodies are involved is discussed.