Lecture 6: Fungal Infections

Question 1

Fungal Infections regularly seen in

Answer 1

HIV infected patients
Cancer patients undergoing chemotherapy
Transplant patients on immunosuppressive medication
Patients taking long term corticosteroids

Question 2

Facts about fungi
Examples
Functions

Answer 2

Eukaryotic micro organisms

Includes yeast, moulds, polypores, plant parasitic rusts and smuts

Important role as decomposers, pathogens (plants and animals), carbon cycling in soils and mediate mineral nutrition in plants

Question 3

Fungal diseases are called myscoses and are characterised based on:

Answer 3

Site of infection
Route of infection
Virulence - the severity or harmfulness

Question 4

Fungal Disease: Site of infection different mycoses

Answer 4

1.Superficial mycoses: Caused by fungi known as dermatophytes and are usually restricted to the non-living keratinized components of the skin, hair and nails

2.Subcutaneous mycoses: Driven by saprophytic fungi which cause chronic nodules or ulcers in subcutaneous tissues following trauma. Eg. Chromomycosis, sporotrichosis & mycetoma

3.Respiratory mycoses: Infections caused by soil sprophytes producing acute lung infections or granulomatous lesions eg. Coccidioidomycosis

4.Candidiasis: Caused by candida albicans (ubiquitous commensal) causes superficial (rarely systemic) infections of the skin and mucosal membranes

Question 5

Fungal Disease: route of infection types

Answer 5

Endogenous – presence of fungi in the gut/human microbiota

Exogenous – Inhalation of fungi from environment

Question 6

Fungal Disease: Virulence

Answer 6

Primary – pathogenic organism
Opportunistic – commensal organism

Question 7

predisopsing factors for specific fungal pathogens

Answer 7

1. Candida (mucosal)
   = impaired cellular immunity, neutropenia
2. Candida (disseminated)
   = impaired muscoa, neutropenia

3.Aspergillus
= Neutropenia, high dose corticosteroids

4.Cryptopcoccus
= impaired cellular immunity, corticosteriods

5. Zygomycetes
   = Neutropenia, corticosteroids, diabetic ketoacidosis
6. Fusarium
   = Neutropenia, corticosteriods, impaired skin
7. Scedosporium
   = neutropenia
8. Trichosporon
   = Neutropenia, impaired skin

Question 8

Candida
Pneumocystis
Histoplasma Capsulatum
Blastomyces Dermatitidis
Malassezia
Cryptococcus

Source and Diseases caused

Answer 8

Candida
Commensal on skin, GI, vagina

Vaginitis
Skin/nail infections
Oropharyngeal infections

Pneumocystis
Host specific

Pneumonia

Histoplasma Capsulatum
In soil, contaminated birds/bats, infected by inhalation of spores

Acute pneumonia
Self-limited flu like symptoms
Disseminated disease
chronic pulmonary disease

Blastomyces Dermatitidis
Soil, decaying wood. infected by inhalation of spores

Skin lesions/subcutaneous nodules, disseminated disease, acute/chronic pulmonary disease

Malassezia
Commensal of skin

Cutaneous infections
Allergic atopic eczema

Cryptococcus
Soil, pigeons, trees

Pneumonia
Meningitis

Question 9

virulence factors that allow fungal survival and persistence in the host (5)

Answer 9

1. complementary structures - adherance to host tissues and the extracellular matrix

2.production of phospholipases, proteases and elastases that cause tissue damage and impairment of host defences

3. switch to metabolic pathways that are required for intracellular survival
4. thermotolerance (the ability to grow at 37°C)

5.exist in different forms and to reversibly switch from one to the other during infection

Eg: dimorphic fungi, which transform from saprobic filamentous moulds to unicellular yeasts in the host

some species of Candida can grow in different forms, such as yeasts, blastospores, pseudohyphae and hyphae, depending on infection sites

Question 10

3 mechanisms of fungal immunity

Answer 10

Tolerance
protection
clearance

Question 11

The TLR-PAMP recognition system

Answer 11

Lecture Slide

Question 12

Draw the fungal wall
And functions of B-glycans, Chitin and Mannans

Answer 12

Lecture Slide

Β-glycans
β1,3 - scaffolding
β1,6 –branches

Chitin (provides structural stability)

Mannans – often linked to proteins

Question 13

The innate response to fungi serves two main purposes:

depends heavily on?

Answer 13

Surveillance and elimination

depends heavily on phagocytic cells
Especially neutrophils, macrophages & dendritic cells
Macrophages mediate the first steps of an effective antifungal host defense
Neutrophils essential in elimination

Question 14

Innate recognition of fungi: Dendritic Cells and macrophages

Answer 14

bind to components of fungal cell walls usingpattern recognition receptors(PRRs)
C-type lectin receptors(CLRs, e.g. Dectin-1)
Toll-like receptors(TLRs, e.g. TLR2)

Question 15

What happens when PRRS bind to PAMPS

Answer 15

when PRRs bind fungi they induce intracellular signalsor associated molecules (FcRγ).

These intracellular signals will in turn initiate:
1.phagocytosis
2.Initiation of killing mechanisms(e.g. production of reactive oxygen species)
3.helps drive thedevelopment of adaptive immunity

Question 16

Innate cellular components: Epithelial Cells, Macrophage, Neutrophil, dendritic and platelet roles

+ DIAGRAM

Answer 16

Fungi can invade tissue by inducing endocytosis or actively penetrating the epithelium. Epithelial cells can produce antimicrobial peptides with antifungal activities and proinflammatory cytokines that can recruit other immune cells, such as monocytes, which will also contribute to fungal clearance.

Tissue resident macrophages can phagocyte and kill fungal pathogens;
neutrophils can also produce ROS (reactive oxygen species) and release neutrophil extracellular traps (NETs) that capture fungi and contain antimicrobial proteins that inhibit their growth.

Dendritic cells can migrate to lymph nodes and activate specific T-cell responses depending on the microbial morphology and site of infection. Innate lymphoid cells can also produce proinflammatory cytokines (IL-22) that contribute to mucosal antifungal responses.

Finally, in the last steps of infection, fungi breach the endothelium allowing them access to the bloodstream where they can activate platelets to produce cytokines and molecules with antifungal activity (CCL5 and PF4).

Question 17

Adaptive Response cells and roles

Answer 17

Major Adaptive immune cell subset involved are CD4+ Th1 and Th17 cells

Th1 cells secrete IFN-y & TNF-a and are responsible for fungal clearance and inflammation

Th17 cells secrete IL-17a & IL-17F and are responsible for neutrophil recruitment and inflammation

Question 18

Innate and adaptive immunity to fungal infections diagram

Answer 18

lecture slide

Question 19

Functions of antibodies in response to fungi

Answer 19

1.opsonization of fungi
2. complement activation,
3.antibody-dependent cell toxicity
4.Immunomodulation
5.neutralisation of fungal PAMPS

Question 20

Role of TNF-α, IFN-γ, and IL-17A and IL-17F

Answer 20

TNF-α, IFN-γ, and IL-17A and IL-17F. These cytokines stimulate the antifungal effector functions of phagocytes, as well as the generation of optimal T-cell-dependent immunity.

IFN-γ and help for opsonizing antibodies

IL-17A to mobilize neutrophils and induce defensins may contribute to a prompt and efficient control of the infection at the different body site

Question 21

3 steps and description of fungal avoidance mechanisms of immune system response

Answer 21

1.Stealth— hide from detection

2.Control
-activate host immune inhibitory mechanisms
-guide immune responses toward types that are not especially effective against the microorganism

3.Attack— produce molecules that destroy host immune defences

Question 22

fungal avoidance mechanisms diagram and examples

Answer 22

Stealth:
Dimorphism
Biofilm
Capsule
Melanin

Control:
Complement inhibiton
Avoiding phagocytic process
Apoptosis induction
Manipulation of phagosome maturation

Attack:
Destruction via ROS and RNS

Question 23

Stealth:
Explain Dimorphism in more detail with specific example of fungi

Explain Biofilm in more detail with specific example of fungi

Answer 23

Dimorphism - Histoplasma capsulatum andBlastomyces dermatitidi
The cell wall is altered; the filamentous form contains both β- and α-glucans, but conversion to the yeast form is accompanied by increased production of α-(1,3)-glucan (much less immunogenic) and has been correlated to reduced virulence.

Biofilm - C. albicans
Biofilms are microbial communities that are attached to surfaces and held together by an extracellular matrix. Growth in a mass increases the resistance of the organisms to environmental stress, their resistance to antifungal activities and also effectively shields them from attack by phagocytes.

Question 24

Control:
Explain Complement inhibition in more detail with specific example of fungi

Explain Avoiding phagocytic process in more detail with specific example of fungi

Answer 24

Complement inhibition -
To avoid elimination by the complement pathway, it is known thatC.albicansandAspergillusspp. secrete proteins onto their surface that bind to regulatory complement proteins. When attached to the fungal surface, these proteins inhibit the complement cascade and thus allow the evasion of complement attack

Avoiding phagocytic process - C. albicansundergo efficient phagocytosis; however, they are also able to develop hyphae inside and outside the macrophage, multiplying intracellularly, destroying the phagocytic cell, and escaping ingestion. During this transition,C. albicansproduces hyphae within macrophages to kill the cell or outside the macrophage to avoid phagocytosis and limit the recruitment of additional macrophages.

Question 25

Attack:
Explain the purpose of RNS and ROS

Answer 25

If host defense mechanisms cannot be avoided completely or controlled sufficiently, the last resort for a pathogen is simply to survive or destroy the defences by the destruction of reactive oxygen species (ROS) and reactive nitrogen species (RNS). ROS and RNS have combined powerful antimicrobial effects and are sufficient to eliminate pathogens

Question 26

Draw a diagram and explain Cryptococcus neoformansinfection in humans AND the mechanisms used to avoid immune response

Answer 26

(1) Killing and clearance by macrophages after phagocytosis,

(2) The encapsulated fungi are able to escape from phagocytosis and further establish virtual infection after pulmonary colonization, from asymptomatic latency period to pneumonia,

(3) Part of the phagocytosed fungi could escape killing and further parasitize the host cells like a Trojan horse which transports the pathogenic fungi into the CNS via circulation. After crossing the BBB, Cn that accumulates in the CNS would result in lethal cryptococcal meningitis especially in susceptible hosts.

Lecture slide