Abstract
Cryptococcus neoformans is a human opportunistic fungal pathogen responsible for ∼1/3 of HIV/AIDS deaths worldwide. This budding yeast expresses a polysaccharide capsule necessary for virulence. Capsule production inhibits phagocytosis by macrophages. Here we describe results that link copper homeostasis to capsule production and the inhibition of phagocytosis. Specifically, using Agrobacterium-mediated insertional mutagenesis, we identified an insertion in the promoter region of the putative copper transporter-encoding gene CTR2 that results in reduced expression of CTR2 and increased phagocytosis by murine RAW264.7 macrophages. The mutant also displayed sensitivity to copper starvation and defects in polysaccharide capsule production and melanization. These defects were all reversed by genetic correction of the promoter insertion by homologous targeting. Several melanization-defective mutants identified previously, those in the RIM20, RIM101, and VPS25 genes, also display sensitivity to copper starvation, reduced capsule production and increased phagocytosis. Together these results indicate a previously undescribed link between copper homeostasis to polysaccharide capsule production and phagocytosis inhibition in Cryptococcus neoformans.
Highlights
The fungus Cryptococcus neoformans is one of the leading causes of morbidity and mortality in immunocompromised patients, including organ transplant recipients on immunosuppressive therapy and AIDS patients
We identified a gene with homology to known copper transporters, which we have termed CTR2, and determined that CTR2 falls into a class of genes that when mutated all show an increased sensitivity to copper starvation, reduced melanization and capsule formation, and increased uptake by macrophages
This is in stark contrast to the rapid uptake by macrophages of other unopsonized yeast such as S. cerevisiae and Candida albicans, suggesting that C. neoformans possesses mechanisms for evading phagocytic cells
Summary
The fungus Cryptococcus neoformans is one of the leading causes of morbidity and mortality in immunocompromised patients, including organ transplant recipients on immunosuppressive therapy and AIDS patients. It is estimated that C. neoformans is responsible for 13–44% of the over 3 million AIDS-related deaths worldwide [1]. Cryptococcosis is typically associated with immunodeficient individuals, a recent outbreak in the Pacific Northwest among immunocompetent individuals has stressed the importance of understanding the complex interactions of this fungal pathogen with the host immune system. Cryptococcus neoformans is thought to be predominantly acquired through inhalation of spores or yeast into the lungs. Alveolar macrophages are believed to be one of the first lines of defense against cryptococcosis. Experimental evidence indicates that macrophages play an important role in host defense against cryptococcosis, especially early in infection [2,3]
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