CR3-DEPENDENT PHAGOCYTOSIS BY MURINE MACROPHAGES - DIFFERENT CYTOKINES REGULATE INGESTION OF A DEFINED CR3 LIGAND AND COMPLEMENT-OPSONIZED CRYPTOCOCCUS-NEOFORMANS
Ce. Cross et al., CR3-DEPENDENT PHAGOCYTOSIS BY MURINE MACROPHAGES - DIFFERENT CYTOKINES REGULATE INGESTION OF A DEFINED CR3 LIGAND AND COMPLEMENT-OPSONIZED CRYPTOCOCCUS-NEOFORMANS, Immunology, 91(2), 1997, pp. 289-296
Phagocytosis is a fundamental process in innate resistance to infectio
n. 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 ant
i-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 demonst
rate the importance of the complement receptor type 3 (CR3) in ill vit
ro phagocytosis of the yeast and in in vivo resistance to infection. I
n vitro, 70% of a population of resident murine macrophages are able t
o 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. neofo
rmans, and we now show that the cytokines convert a population of resi
dent macrophages to a state where all the cells are competent for inge
stion of large numbers of yeasts (6-8 per cell). We also show that the
se cytokines have a direct effect on CR3, as enhanced levels of comple
ment-opsonized sheep red blood cells (EIgMC) bind to macrophages activ
ated in this way. However, cytokines that have previously been shown t
o enhance phagocytosis of EIgMC have no effect on ingestion of encapsu
lated C. neoformans. These results demonstrate that the cytokines regu
lating CR3-dependent ingestion of C. neoformans are different to those
regulating ingestion of EIgMC and reinforce the importance of studyin
g pathogens rather than inert ligands in understanding the regulation
of phagocytosis.