Fungi are often overlooked as a major threat to human health, overshadowed by the more dramatic impacts of bacterial and viral infections. Yet these eukaryotic microbes pose a serious and growing danger, responsible for millions of deaths worldwide each year. And their success is largely due to their ability to manipulate the very immune defenses we rely on to keep us healthy.

Invasive fungal infections have become a global crisis, driven by the increasing number of immunocompromised individuals, the lack of rapid diagnostics, limited antifungal drugs, and the emergence of drug-resistant strains – a problem exacerbated by the COVID-19 pandemic. The World Health Organization has designated several fungal pathogens as top priorities, including Aspergillus fumigatus, Candida albicans, Candida auris, and Cryptococcus neoformans. These opportunistic fungi can cause life-threatening diseases, especially in patients with weakened immune systems.

What makes these fungi so dangerous is their remarkable ability to evade and exploit the human immune response. A key weapon in their arsenal is their capacity to manipulate the process of phagocytosis – the fundamental mechanism by which immune cells engulf and destroy foreign invaders. By subverting this crucial defense, fungal pathogens can not only survive inside host cells, but even use them as a means of spreading throughout the body.

The Phagocytic Crucible

Phagocytosis is the cellular process by which specialized immune cells, known as phagocytes, recognize, engulf, and destroy large particles like bacteria and fungal spores. This is a vital component of the innate immune system, providing a first line of defense against pathogens.

The phagocytic process begins when receptors on the surface of phagocytes, such as macrophages and neutrophils, detect molecular patterns associated with microbes. This triggers the rearrangement of the cell’s cytoskeleton, forming a “phagocytic cup” that envelops the target particle. The particle is then internalized within a membrane-bound compartment called a phagosome.

From here, the phagosome undergoes a maturation process, fusing with lysosomes to create a highly acidic and oxidizing environment known as the phagolysosome. This hostile milieu is designed to destroy the trapped pathogen through a barrage of hydrolytic enzymes and reactive oxygen species.

For many microbes, the phagolysosome represents the end of the line. But fungal pathogens have evolved sophisticated strategies to evade this fate. “All the major fungal pathogens we’re concerned about have developed ways to manipulate the phagosome, either by blocking its maturation or by escaping it altogether,” explains Lei-Jie Jia, a microbiologist at the Leibniz Institute for Natural Product Research and Infection Biology.

Masking their Identity

One key tactic employed by fungal invaders is to conceal their identity from the immune system. Many pathogenic fungi have evolved elaborate cell wall structures that shield their telltale molecular signatures, known as pathogen-associated molecular patterns (PAMPs), from recognition by phagocyte receptors.

For example, the dormant spores of A. fumigatus are coated with a hydrophobic “rodlet” layer and a pigment called DHN-melanin, which mask the underlying β-glucan molecules that would normally be detected by immune cells. Similarly, the opportunistic yeast Candida albicans obscures its β-glucan with a mannan-rich outer layer. And the deadly Cryptococcus neoformans hides behind a polysaccharide capsule.

“These protective cell wall structures not only help the fungi withstand environmental stresses, but also allow them to evade recognition by the host’s immune defenses,” says Jia.

In addition to masking PAMPs, some fungi actively disarm the host’s complement system – a key component of innate immunity that tags pathogens for phagocytic destruction. Secreted fungal proteins can bind to and degrade complement proteins like C3 and C4, preventing opsonization and phagocytosis.

Manipulating the Phagosome

Even when phagocytes do succeed in engulfing fungal invaders, the pathogens have evolved ingenious ways to subvert the phagosome’s killing mechanisms. A common tactic is to interfere with the phagosome maturation process, preventing it from developing into a fully acidified and oxidized phagolysosome.

For A. fumigatus, the key player is a surface protein called HscA. This molecule anchors a host protein complex called annexin A2-p11 (A2t) to the phagosomal membrane, blocking the recruitment of the small GTPase Rab7. Rab7 is a critical regulator of phagosome maturation, so its exclusion keeps the compartment in an immature state, allowing the fungal spores to germinate and proliferate.

Meanwhile, Cryptococcus neoformans uses its polysaccharide capsule and the enzyme urease to buffer the phagosomal pH, preventing acidification. And Candida albicans can induce the translocation of a host protein called STING to the phagosome, which may also disrupt maturation.

“By manipulating the phagosome in these ways, the fungi are able to create a relatively benign environment where they can survive and even replicate,” says Jia.

Escaping the Phagosome

But some fungal pathogens go a step further, actively escaping the phagosome altogether. This can happen through lytic mechanisms that induce host cell death, or through non-lytic “vomocytosis” where the pathogen-containing phagosome is expelled from the cell intact.

For example, the hyphae of A. fumigatus and C. albicans can physically rupture the phagosomal and cellular membranes, allowing the fungi to break free. Cryptococcus neoformans, on the other hand, has been observed triggering a form of programmed cell death called pyroptosis in macrophages, which leads to the release of the fungal cells.

Remarkably, Cryptococcus and other fungi can even hijack the host cell’s recycling pathways to exit the phagosome without killing the cell. The pathogen-containing phagosome is redirected to the cell surface and extruded, allowing the fungus to spread to new host cells.

“This non-lytic expulsion is a really clever strategy,” says Jia. “It enables the fungi to escape the phagosome without damaging the host cell, which could alert the immune system. They can just quietly slip away to infect other cells.”

Exploiting Nutritional Immunity

But the fungal onslaught doesn’t end at the phagosome. Even after breaching this initial barrier, pathogens must still contend with the host’s “nutritional immunity” – the sequestration of essential nutrients like iron, zinc, and copper that the microbes need to survive and proliferate.

Phagocytes like macrophages have evolved sophisticated mechanisms to starve invading pathogens of these vital minerals. For example, they can pump copper ions into the phagosome to toxic levels, or export essential trace metals like iron and zinc, depriving the fungi of these cofactors.

However, fungal pathogens have counter-strategies to overcome nutritional immunity. Many secrete specialized molecules that scavenge for scarce metals, or express high-affinity transporters to acquire them from the host. Cryptococcus neoformans, for instance, can bind to and degrade the host’s copper-binding proteins, allowing it to thrive even in copper-rich phagosomes.

“Nutritional immunity is a really important part of the host’s defense, but fungi have evolved ways to circumvent it,” explains Jia. “They’re able to maintain metal homeostasis even in the face of the host’s attempts to starve them.”

Harnessing the Phagosome

Given the fungi’s remarkable ability to manipulate the phagosome, researchers are exploring ways to harness this organelle as a target for antifungal therapies. The idea is to either deliver antimicrobial drugs directly to the phagosome, or to enhance the phagosome’s own killing capacity.

One promising approach is the use of nanoparticle-based drug delivery systems. By encapsulating antifungal compounds within liposomes or biodegradable polymers, scientists can potentially guide these nano-carriers straight to the phagosome through phagocytic uptake. Decorating the nanoparticles with ligands that bind to phagocyte receptors can further improve their targeting.

“The key is to get the drugs into the phagosome, where the fungi are hiding,” says Jia. “That way, you can achieve high local concentrations of the antifungal without subjecting the whole body to toxic side effects.”

Researchers are also investigating ways to boost the phagosome’s microbicidal activity, either by promoting its maturation or by enhancing the production of fungicidal molecules like reactive oxygen species. Treatments with cytokines like interferon-γ and granulocyte-macrophage colony-stimulating factor have shown promise in enhancing the phagosomal killing capacity of phagocytes.

“If we can find ways to retain the pathogens within the phagosome and ramp up its killing power, we may be able to more effectively eliminate these fungal invaders,” says Jia.

A Multifaceted Threat

The ability of fungal pathogens to manipulate phagocytosis is just one facet of their sophisticated arsenal. These eukaryotic microbes have evolved a diverse array of strategies to evade the host immune response, from masking their identity to exploiting nutritional vulnerabilities.

“Fungi are remarkably adept at navigating the complexities of the human immune system,” says Jia. “They’ve had millions of years to fine-tune these evasion mechanisms through their interactions with environmental predators like amoebas. So in many ways, they’re better equipped to handle our defenses than we are to handle them.”

This underscores the challenge of combating invasive fungal infections. With limited treatment options and the rise of drug resistance, new approaches are desperately needed. Targeting the phagosome represents a promising avenue, but it’s just one piece of a much larger puzzle.

“Fungi are a formidable foe,” Jia concludes. “If we want to turn the tide against these deadly invaders, we’ll need to deploy a multifaceted strategy that harnesses our deepening understanding of host-pathogen interactions. Only then can we truly outwit the fungal menace.”