MICR5831 L26: Fungal Virulence 12/9/25

What are the main groups of human fungal pathogens?

1) Cutaneous Dermatophytes

2) Subcutaneous

3) Opportunistic

4) Systemic/True Pathogens

What type of human fungal pathogen is this?

-Generally retain similar morphology as the environmental forms

-Elicit lower immune responses

Cutaneous/Dermatophytes

What type of human fungal pathogen is this?

-Cause more tissue damage and immune reaction

Subcutaneous

What type of human fungal pathogen is this?

-Often exist as commensals but can cause systemic disease

Opportunistic

What type of human fungal pathogen is this?

-Often pulmonary and can spread systemically

Systemic/True Pathogens

What are some common human fungal pathogens?

-C. neoformans

-P. jirovecii

-C. albicans

-H. capsulatum

-A. fumigatus

-T. rubrum

Which fungi are most likely to have major adaptations that allow them to change, skin or body fungi?

Fungi that inhabit the body often

How does this fungus shapeshift inside the host?

-Cryptococcus neoformans

-Spore to budding capsule

-Variable cell size

How does this fungus shapeshift inside the host?

-Histoplasma capsulatum

-Conidia to budding

-Alpha glucan outer layer

How does this fungus shapeshift inside the host?

-Aspergillus fumigatus

Conidia to hyphae

How does this fungus shapeshift inside the host?

-Coccidioides immitis

Arthrospore to spherules with endospores

How does this fungus shapeshift inside the host?

-Candida albicans

Hyphae and pseudohyphae

How does this fungus shapeshift inside the host?

-Pneumocystis jirovecii

Asexual trophic form and sexual cysts containing spores

What are mechanisms that enhance fungi survival and damage host?

1) Adherence

2) Host cell invasion

3) Dissemination

4) Structural integrity, prevents cell damage

5) Resistance to complement, phagocytosis, adaptive immunity

6) Nutrition acquired in host

What part of fungi mediates interaction with the host?

Fungal cell wall

Which parts of the fungal cell wall are conserved structurally or heterogeneous?

Conserved: Inner layers

Heterogeneous: Outer layers

What are the components of the fungal cell membrane?

-Transmembrane proteins

-Ergosterols

What are the components of the fungal cell wall, from inside to outside?

1) Chitin

2) Glucan

3) Mannoprotein

What are the properties of these components of the fungal cell wall?

1) Chitin

2) Mannans

3) Molecules released

4) Dimorphism (hyphae -> yeast)

1) Hard to penetrate

2) Beta glucans are shielded from receptors

3) Degrades and inhibits phagocytosis/complement system

4) Dissemination, protect from immune response

What fungal cell wall component is this?

-One of commonest polysaccharides in nature

-Maintains strength/structure, adapts to stress

-1 to 20% of cell wall, synthesized from n-acetylglucosamine

-Not found in humans

Chitin

What fungal cell wall component is this?

-Highly variable structure, alpha/beta glucose moieties

-Activates PAMPS and immune response via Dectin receptor

-Facilitates binding to other cell wall components

-Most abundant cell wall polysaccharide

Glucan

What type of Glucan is most abundant in the fungal cell wall?

Beta 1,3 glucans

What is the main receptor for glucans that activate PAMPS and the host immune response?

Dectin

Which glucan is hydrophobic? Which glucan is hydrated?

Hydrophobic: Alpha glucan

Hydrated: Beta glucan

Describe the structure of Beta glucan

B-D-glucose is linked by glycosidic chains and branches

What fungal cell wall component is this?

-Linked O and N attached to glycoproteins

-Outermost layer of wall

-Protect glucans from immune surveillance

Mannans

What receptors are associated with Mannans?

DC-SIGN, Dectin-2, MBP, Mincle, SP-A, SP-D

What is the role of mannoproteins/mannans in yeast?

-Cover surface of cell

-Not covalently bonded to polysaccharide core of wall

What is the role of mannoproteins/mannans in hyphae?

Integral part of the cell wall

What fungal cell wall component is this?

-Polyketide synthase pathway produces it

-Crosslinked to polysaccharides

-Interacts with chitin, mannan and Mannoprotein

Melanin

What yeast species is Melanin linked to?

-Cryptococcus

-Candida albicans

What filamentous fungi species is Melanin linked to?

-Fonsecaea pedrosa

-Aspergillus

What thermally dimorphic species is Melanin linked to?

-Paracoccioides brasiliensis

-Coccidioides

-Histoplasma capsulatum

What pathway produces Melanin?

Polyketide synthase

True or False: Fungi without melanin show greater dissemination in hosts

False, melanin is linked with higher dissemination and is harder for the immune system to kill

What does Melanin do protect from environmental damage/the immune response?

-Porosity decreased

-Dissemination increased

-Alters cytokine response

-Decreases phagocytosis

-Reduces microbicidal/antifungal toxicity

True or False: Yeasts have more chitin in their cell walls than hyphal fungi

False, hyphal fungi have more chitin

Is this hyphal fungi or an encapsulated yeast?

Cell Wall Components:

-B-1,3-glucan, B-1,3-1,4-glucan, B-1-,6-glucan, a-1,3-glucan

-Galactomannan

-Melanin

-Rodlet

-Chitin

Hyphal Fungi

What is the influence of the hyphal/fungal cell wall on virulence and resistance?

-Does not activate host immune response (Th2, Th1)

-Hides fungi from immune response

Name an example of an encapsulated yeast

Cryptococcus

What is unusual about encapsulated yeasts such as Cryptococcus?

-Variable capsule as added protection

-Dynamic structure, responds to environment

Is this hyphal fungi or an encapsulated yeast?

-Capsule: GXM, GalXM

-Cell Wall: B-1,3-glucan, B-1,6-glucan, a-1,3-glucan

-Chitin

-Chitosan

-Melanin

Encapsulated yeast

What is the influence of the yeast capsule/cell wall on virulence and resistance?

-Hides fungi from immune response

-Does not activate Th1 or Th2 response

-Resistance to antifungals and stress

-Dissemination is increased

Name an example of a dimorphic fungi with hyphal, pseudohyphal and budding yeast forms

Candida

What sets Candida apart from other thermally dimorphic fungi?

Yeast and hyphae exist in human hosts

What are the differences between the Candida hyphae and yeast cell walls?

-Yeast has more Beta glucans

-Hyphae has more Mannoproteins

-Chitin is about the same in both

What is this?

-Ability to make 2 types of vegetative cells

-Senses/responds to environmental changes

-Disease in immunocompetent people

Thermal dimorphism

What are environmental changes that trigger thermally dimorphic fungi to change forms?

-Heat

-CO2, cysteine, estradiol

What fungi are thermally dimorphic?

-Ascomycota

-Basidiomycota

-Zygomycota

What yeast specific genes are responsible for subverting the immune response/causing disease in immunocompetent people?

BAD1 and CBP1

What happens after a thermally dimorphic yeast like T. marneffei in its saprophytic phase enters the host?

1) Conidia are released

2) Attachment to host lung tissues

3) Host phagocytosis into macrophages

4) Fungal dimorphic switching

How does resistance increase for T. marneffei after being phagocytosed in host macrophages?

-Phagosomal environment

-Oxidative and nitrosative stress

-Heat stress

-Glucose starvation

How does production/upregulation increase for T. marneffei after being phagocytosed in host macrophages?

-Melanin

-HSP

-Catalase-peroxidase

-SOD

-Iron acquistion

-Alternative carbon source

-EVs

How do adhesins contribute to fungal virulence?

1) Adhesins on surface wall adhere to host cell

2) Prevent expulsion, aid cell invasion

3) Covalently-bound to glucans/chitin

4) Specific adhesion molecules

5) Specific host cell receptors

6) Creates biofilms (Candida)

Why are most adhesins covalently-bound to B-1,3-glucan and chitin?

They are GPI-modified cell wall proteins

Which fungus has this adhesin?

-BAD1 protein

-Binds yeast to CD11b, CD18, and CD14 receptors on human macrophages

Blastomyces dermatidis

Which fungus has this adhesin?

-ALS (agglutinin-like sequence)

-Binds to laminin, collagen, fibronectin, endothelial and/or epithelial cells

Candida albicans

Which fungus has this adhesin?

-Eap1, a GPI-modified cell wall protein

-Binds to epithelial cells

Candida albicans

Which fungus has this adhesin?

-Int1, a RGD-protein

-Binds to epithelial cells

Candida albicans

Which fungus has this adhesin?

-Mannan core/oligomannosyl side chains from phosphomannoprotein

-Binds to marginal zone around macrophages

Candida albicans

Which fungus has this adhesin?

-Epa1, a GPI-modified cell wall protein

-Binding to epithelial cells

Candida glabrata

How do enzymes contribute to fungal virulence?

1) Degrades host tissues, disseminates pathogen

2) Impairs host immune responses

3) Nutrition/iron acquisition mechanism

4) Secreted into host tissues directly/via vesicle

What are some enzymes that are used by pathogenic fungi to degrade host tissues?

-Phospholipases

-Lipases

-Proteases (serine, aspartic and metalloproteases)

-DNAse

-Acid phosphatase

-Urease

What molecules released by the host immune response do pathogenic fungi need to protect themselves from?

ROS and RNS species

How can pathogenic fungi defend themselves against reactive oxygen radicals and reactive nitrogen species?

-Produce enzymes to degrade/resist host radicals

-Catalases, superoxide dismutase and HSP protect against ROS

-Zinc, copper, peroxidase and melanin resist oxidation

How do fungi use mycotoxins as virulence factors?

-Secondary metabolites act as mycotoxins

-Released in situ during infection or by contamination of foodstuff

-Ingested or inhaled

What pathogenic fungal toxin is this?

-Aspergillus

-Induces Apoptosis

-Reduces macrophage function, inhibits cytokine production

Gliotoxin

What pathogenic fungal toxin is this?

-Candida albicans

-Damages epithelial cells

-Destabilizes host cell membranes

Candidalysin

What pathogenic fungal toxin is this?

-Aspergillus

-Cytotoxic to cells

-Damages the liver

Aflatoxin

How do fungi acquire bio-available iron?

1) Reductive mechanisms

2) Siderophores

3) Heme assimilation

What fungal iron acquisition mechanism is this?

-Ferric reductases reduce ferric iron to soluble ferrous iron

-Multicopper ferroxidase re-oxidates it to ferric iron at cell surface

-Transport across the cell surface by a permease

Reductive mechanisms

What enzyme reduces ferric iron to soluble ferrous iron?

Ferric reductase

What enzyme re-oxidates soluble ferrous iron to ferric iron at the cell surface?

Multicopper ferroxidase

What enzyme transports ferric iron across the cell surface?

Permease

What fungal iron acquisition mechanism is this?

-Bind to iron via specific transporters

Siderophores/Non-reductive Mechanisms

What fungal iron acquisition mechanism is this?

-Most iron in host bound to hemoglobin

-Some fungi can remove iron from hemoglobin

Heme assimilation

What fungal virulence factor is this?

-Calcium-activated protein phosphatase

-Moderates stress responses in Cryptococcus and Candida

-Aids host cell adherence

Calcineurin

What is this?

-Calcineurin

-Calcium-activated protein phosphatase

-Moderates stress responses in Cryptococcus and Candida

-Aids host cell adherence

What are some stress responses does Calcineurin mediate in Cryptococcus and Candida?

-Inhibits nonspecific host effector mechanisms

-Protects against complement and ROS

What fungal virulence factor is this?

-Protects fungi from oxidative damage

-Contributes to brain pathology

-Importance in Cryptococcal CNS infections

Mannitol

What is this?

-Mannitol

-Protects fungi from oxidative damage

-Contributes to brain pathology

-Importance in Cryptococcal CNS infections

List out the 7 fungal virulence factors

1) Thermal dimorphism

2) Adhesins

3) Enzymes

4) Defense against ROS and RNS

5) Toxins

6) Iron Acquisition

7) Calcineurin and Mannitol

Which molecule is not a major component of fungal cell walls?

-Chitin

-Mannoprotein

-Mycolic acid

-Glycan

Mycolic acid

The fungal virulence factor Calcineurin can influence pathogenicity by?

-Moderating the stress responses to heat, cations and pH

-Allowing the yeast to develop a capsule

-Degrading lysosomal radicals

-Enhances acquisition of heme

Moderating the stress responses to heat, cations and pH