How metals fuel fungal virulence, yet promote anti-fungal immunity

Abstract

Invasive fungal infections represent a significant global health problem, and present several clinical challenges, including limited treatment options, increasing rates of antifungal drug resistance and compounding comorbidities in affected patients. Metals, such as copper, iron and zinc, are critical for various biological and cellular processes across phyla. In mammals, these metals are important determinants of immune responses, but pathogenic microbes, including fungi, also require access to these metals to fuel their own growth and drive expression of major virulence traits. Therefore, host immune cells have developed strategies to either restrict access to metals to induce starvation of invading pathogens or deploy toxic concentrations within phagosomes to cause metal poisoning. In this Review, we describe the mechanisms regulating fungal scavenging and detoxification of copper, iron and zinc and the importance of these mechanisms for virulence and infection. We also outline how these metals are involved in host immune responses and the consequences of metal deficiencies or overloads on how the host controls invasive fungal infections.

Keywords: Copper; Fungal immunology; Iron; Macrophage; Nutritional immunity.

Fig. 1.

Fungal metal-sensing systems and regulatory mechanisms. (A) An overview of proteins and transcription factors used for responses to high and low environmental copper (Cu) in Cryptococcus neoformans. C. neoformans is unusual in that the same transcription factor, CnCuf1, regulates responses to both copper starvation (via upregulation of the copper transporters CnCtr1 and CnCtr4) and copper toxicity (via upregulation of the metallothioneins CnCmt1 and CnCmt2). CnCtr2 has more recently been linked to capsule formation to shield C. neoformans from innate immune recognition. (B) An overview of iron (Fe)-scavenging systems in C. neoformans under conditions of low iron availability, as would be found in the host. When free iron is limited, the transcription factor CnCir1 upregulates the iron permease CnCft1 and the ferroxidase CnCfo1, which form a complex to mediate iron import into the cell. CnCir1 also upregulates cell wall mannoprotein, CnCig1, to acquire iron from host haem. Furthermore, uptake of iron via endocytosis is mediated by the cytoplasmic protein CnVps23 and siderophores can be secreted to sequester extracellular iron. (C) Overview of zinc (Zn) uptake and storage systems in Candida albicans. When zinc is readily available, C. albicans stores zinc within intracellular zincosomes, the formation of which is controlled by CaZrc1. When zinc is limited in C. albicans, the transcription factor Zap1 upregulates the expression of CaPra1 to acquire this metal extracellularly and of the Zip and ZnT zinc transporter protein family.

Fig. 2.

Metal-dependent mechanisms of intracellular and extracellular killing by immune cells. (A-C) Inflammatory cytokines, such as IFNγ and GM-CSF can initiate mechanisms that restrict metals from intracellular fungi and phagosomes. (A) During bacterial infection, IFNγ has been shown to drive the pumping of copper (Cu) into phagosomes to cause toxicity, which occurs via the upregulation of the mammalian copper importers CTR1 and ATP7A. High copper levels also contribute to the production of reactive oxygen species (ROS) via NADPH oxidase, which further enhances microbial killing. Conversely, IFNγ limits copper from intracellular fungi via an unknown pathway, causing fungi to upregulate genes involved with copper starvation (e.g. HcCtr3 in Histoplasma capsulatum). IFN-γR, IFNγ receptor. (B) Iron (Fe) in macrophages has been shown to be limited from intracellular pathogens following stimulation with IFNγ and GM-CSF. Chloroquine is a drug that limits intracellular fungal growth, in part by preventing the release of iron from transferrin. Extrusion of iron from the phagosome into the cytoplasm via NRAMP1 helps support inflammatory responses, including the expression of iNOS, leading to the production of nitric oxide (NO), and pro-inflammatory cytokines such as IL-6 and TNFα via NFκB. (C) GM-CSF activation in macrophages upregulates the zinc (Zn) exporters SLC30A4 and SLC30A7, which remove zinc from the phagosomes to starve the residing fungi and move it into the cytoplasm, where it is subsequently chelated by metallothioneins Mt1 and Mt2. Zinc limitation in the phagosome and cytoplasm supports ROS production to support fungal killing. (D) Neutrophils may also participate in limiting copper, iron and zinc from larger, extracellular fungi via calprotectin and lipocalin, a key component of neutrophil extracellular traps (NETs).