CR3-dependent phagocytosis by murine macrophages: different cytokines regulate ingestion of a defined CR3 ligand and complement-opsonized Cryptococcus neoformans.

Phagocytosis is a fundamental process in innate resistance to infection. We have used the pathogenic yeast Cryptococcus neoformans to study the interaction of this encapsulated organism with murine macrophages in vitro. In the absence of exogenous opsonins the encapsulated yeast is almost totally resistant to ingestion by murine macrophages. Owing to its ability to activate the alternative complement pathway, the anti-phagocytic properties of the polysaccharide capsule can be partially overcome following opsonization in vitro with non-immune mouse serum and subsequent phagocytosis via complement receptors. Here, we demonstrate the importance of the complement receptor type 3 (CR3) in in vitro phagocytosis of the yeast and in in vivo resistance to infection. In vitro, 70% of a population of resident murine macrophages are able to ingest C. neoformans and then only inefficiently (1-2 organisms per cell). Previously we have shown that tumour necrosis factor-alpha (TNF-alpha) and granulocyte-macrophage colony-stimulating factor (GM-CSF) efficiently enhance ingestion of serum-opsonized encapsulated C. neoformans, and we now show that the cytokines convert a population of resident macrophages to a state where all the cells are competent for ingestion of large numbers of yeasts (6-8 per cell). We also show that these cytokines have a direct effect on CR3, as enhanced levels of complement-opsonized sheep red blood cells (EIgMC) bind to macrophages activated in this way. However, cytokines that have previously been shown to enhance phagocytosis of EIgMC have no effect on ingestion of encapsulated C. neoformans. These results demonstrate that the cytokines regulating CR3-dependent ingestion of C. neoformans are different to those regulating ingestion of EIgMC and reinforce the importance of studying pathogens rather than inert ligands in understanding the regulation of phagocytosis.