Fungal Pathogenesis

Fungi enter the body through five routes: inhalation of spores to the alveoli, colonisation of skin and mucosa followed by breach, traumatic implantation, translocation of gut commensals, and intravascular devices.

Growth at 37 degrees Celsius is the first hurdle. Mammalian body temperature excludes most of the fungal kingdom. Pathogenic fungi have adapted in different ways:

• Thermally dimorphic fungi convert from mould to yeast or spherule at body temperature.
• Aspergillus fumigatus grows up to about 55 degrees Celsius, and Candida auris grows at 42 degrees Celsius, which allows survival through fever.
• Heat shock protein 90 and the calcineurin pathway support growth at 37 degrees Celsius and during fever. Calcineurin is required for growth of Cryptococcus at 37 degrees Celsius, so calcineurin inhibitors such as tacrolimus have antifungal activity against Cryptococcus in vitro, although their immunosuppressive action dominates clinically.
• Dermatophytes grow best at the cooler temperature of the skin surface and are restricted to keratinised tissue.

Attachment precedes colonisation, invasion and biofilm formation.

• Candida albicans binds epithelial cells, endothelial cells and plastic through surface adhesins of the agglutinin like sequence (Als) family and Hwp1. Als3 binds host cadherins and also acts as a ferritin receptor.
• Aspergillus fumigatus conidia carry a hydrophobin rodlet layer that makes them airborne and masks recognition. Conidia bind laminin, fibrinogen and surfactant proteins A and D.
• Dermatophyte arthroconidia attach to corneocytes within hours and then germinate and penetrate the stratum corneum.
• Pneumocystis jirovecii attaches to type 1 pneumocytes through major surface glycoprotein, with host fibronectin and vitronectin as bridging molecules.
• Mucorales spore coat protein CotH3 binds the endothelial receptor glucose regulated protein 78, which is upregulated by high glucose and iron, in ketoacidosis.
• Cryptococcus capsule polysaccharide aids attachment to lung epithelium.
• Candida glabrata adheres through the Epa family of lectin like adhesins.
• Candida auris colonises skin through its own adhesins and forms aggregates that persist on skin and surfaces.

Many fungi change form on entering the host, and the form determines the pathogenic function.

• Candida albicans: yeast cells colonise and disseminate through the bloodstream. Hyphae invade tissue, express adhesins and proteases, and secrete candidalysin. Serum, 37 degrees Celsius, neutral or alkaline pH, carbon dioxide and nutrient limitation trigger the switch. Mutants locked in the yeast form or the hyphal form are attenuated in animal models, so both forms contribute.
• Thermally dimorphic fungi: conversion at 37 degrees Celsius produces the form that resists phagocytic killing. Coccidioides spherules are too large to be ingested.
• Cryptococcus: capsule enlargement and titan cell formation (50 to 100 micrometres, polyploid) in the lung resist phagocytosis, while small cells of 2 to 4 micrometres disseminate.
• Aspergillus: swelling of conidia and germ tube formation after inhalation produce hyphae, which are the invasive form.
• Malassezia: yeast to hyphal conversion in heat and humidity produces pityriasis versicolor.
• Moulds in blood and tissue: Aspergillus terreus, Fusarium, Scedosporium and Lomentospora prolificans produce adventitious conidia or yeast like forms in tissue, which allows these moulds to appear in blood cultures. Aspergillus fumigatus almost never does.

Candida albicans enters epithelium by two routes. In induced endocytosis, hyphal Als3 and Ssa1 bind epithelial cadherins and trigger uptake of the fungus. In active penetration, hyphal pressure together with secreted aspartyl proteases drives the fungus between and through cells. Hyphae secrete candidalysin, a 31 amino acid peptide cleaved from Ece1, which forms pores in epithelial membranes, triggers calcium influx and a danger response through the epidermal growth factor receptor and MAP kinase signalling, and recruits neutrophils.

Aspergillus fumigatus conidia that escape clearance germinate in the alveolus. Hyphae then traverse the epithelium and the endothelium and extend through tissue planes.

Dermatophytes secrete subtilisins and fungalysins that digest keratin, with sulphite release reducing keratin disulphide bonds. Invasion is restricted to dead keratin.

Mucorales invade tissue along nerves (perineural spread) as well as through vessels, which accounts for rapid extension from sinus to orbit and brain.

Gut translocation of Candida depends on loss of epithelial integrity through chemotherapy induced mucositis, ischaemia or surgery, and on loss of anaerobic bacterial competitors.

Angioinvasion is the defining pathogenic event of invasive mould infection in neutropenia and of mucormycosis in ketoacidosis.

• Aspergillus: in neutropenic patients hyphae penetrate and fill small and medium vessels. Thrombosis causes haemorrhagic infarcts. The imaging counterparts are the halo sign early in disease (haemorrhage around a nodule) and the air crescent sign later, with neutrophil recovery. In patients who are not neutropenic, for example on corticosteroids, injury arises more from neutrophil mediated necrosis and airway invasion, with less angioinvasion.
• Mucorales: CotH3 binding to glucose regulated protein 78 drives endothelial uptake and damage. Ketoacidosis raises glucose regulated protein 78 and releases iron from transferrin, so the host environment is tuned to favour invasion. Vessel invasion causes thrombosis, ischaemic necrosis and the black eschar of rhino orbital disease.
• Candida: haematogenous spread causes microabscesses through endothelial invasion and transmigration, with the kidney, brain, eye, liver and spleen as common targets.

• Masking of pathogen associated molecules: outer layers of mannan, alpha 1,3 glucan, capsule or hydrophobin cover immunostimulatory beta glucan.
• Size and form: titan cells and spherules cannot be phagocytosed.
• Interference with complement and antimicrobial peptides: surface proteins and secreted proteases degrade or sequester complement components.
• Antigenic variation: Pneumocystis varies its major surface glycoprotein through a large gene family expressed one at a time.
• Modulation of host signalling: shed polysaccharide and secreted effectors reduce cytokine production, impair leukocyte migration and promote anti inflammatory pathways (for example BAD1 of Blastomyces induces transforming growth factor beta).
• Resistance to oxidative killing: melanin, catalases and superoxide dismutases neutralise reactive oxygen species.
• Nutritional adaptation: high affinity iron and zinc acquisition systems overcome nutritional immunity.

The common targets of dissemination are the brain, eye, kidney, liver, spleen, skin and bone. Cryptococcal urease promotes entrapment of yeasts in brain microvasculature and crossing of the endothelium.

Fungal disease results from the interaction of microbial and host factors. Damage may follow too little response (uncontrolled fungal growth) or too much (immunopathology). Many fungal diseases are mixed.