Immunity to fungal infections Flashcards
What re the four main phyla of fungi
Ascomycota – MAIN ONE- yeast, sac fungi- include aspergillus
Basidiomycota- mushrooms
Chytridiomycota- water based- don’t infect humans- probably the first to branch
Zygomycota- bread moulds
ascomycota and basidiomycota more closely related to each other than to the other two phyla
Summarise aspergillus
Ascomycota
Spores- which can be inhaled into the lungs or form an aspergilloma (ball of fungus) in the liver- with associated haemorrhages due to invasion
Summarise crypto coccus neoformans
Exist as yeast inside host at body temperature- hyphae outside the host
Basidiomycota
Capsule surrounded by adhesive fimbrae
Can cause meningitis- especially in HIV patients
MRI scans showing multiple cryptococcomas in the brain.
Summarise Candida albicans
Most common fungal infection
Yeast- but can turn into hyphae- yeast germ tubes
Cause mucosal infections- but can cause more systemic infections (i.e enter the blood) under the right immunocompromised infections
Can lodge in the retinal microvasculature- Candida endophthalmitis
Give examples of how the morphogenesis of fungi allows them to cause infection as a host
Candida albicans exist as single spores but they can become hyphae, which allows tissue invasion
Cryptococcus forms a capsule to evade phagocytosis
Aspergillus sp. are inhaled as conidia and invade tissues as hyphae
Outline the cellular immunity to fungi
§ Fungi are opsonised by pentraxin-3 and mannose-binding lectin (MBL)- at the mucosal surface
§ Cells involved:
o Phagocytes (neutrophils and macrophages) – are the first line of defence.
o NK cells – provide early INF-gamma- importance for preventing
o Dendritic cells – influence T-cell differentiation.
o Th1 and Th17 cells- important for mucosal immunity
Describe the different cytokines involved in fungal immunity
IL-10, 12, 18
IL-12- differentiation of T cells to TH1- leading to release of IFN-gamma and TNF-a- leading to innate immune system functions such as phagocytosis and neutrophil degranulation
IL-4- differentiation of T cells to TH2 (and inhibition of TH1)- leading to release of IL-4,5 which inhibits actions of the innate immune system.
Describe the important PRRs for. fungal immunity
TLRs
C-type lectins
TLRs- activate NFkB
C-type lectins (carbohydrate receptors)- activate inflammatory pathways - also main phagocytic receptors for fungi
Scavenger receptors
Damage receptors- recognise sterile information secondary to infection (DAMPSs)
Describe some PAMPs associated with fungi
RNA, DNA O-linked mannan N-linked mannan Chitin Alpha and beta glucan B-mannosides C3B or C3D coated B-glucan BAD1 Fungal proteases
Describe some DAMPs
Host proteases
S100B
Describe some PRRs which belong to the C-type lectin family
DECTIN 1 and 2
Mannose receptor
Galectin 3
Name a common disease that is caused by a variety of clinical syndromes
Chronic Mucocutaneous Candidiasis Apparent form birth Trush like infection- quite hypertrophic Sometimes infection of nails e.g. vulvovaginitis & onychomycosis.
Describe the relationship between human lectin 1 mutation and mucocutaneous fungal infection
o This leads to impaired macrophage IL-6 production and binding in response to fungal infections (e.g candida)
o Also leads to increased susceptibility to invasive aspergillosis in stem cell transplants.
Describe Human CARD9 deficiencies
CARD9- adapter molecule involved in intracellular C-type lectin receptor signalling
§ CARD-9 deficiency leads to chronic mucocutaneous candidiasis.
o CARD-9 is required for:
§ TNFa production in response to b-glucan stimulation.
§ T-cell Th17 differentiation (production of IFN-y and IL-17)
What can increase the risk of invasive aspergillosis after transplantation
§ TLR4 polymorphisms (loss of function) lead to an increased risk of Invasive Aspergillosis (IA) in transplantation (i.e. haematopoietic stem cell transplants).- also intersects with CMV which destroys the immune system
Heterozygotes for Dectin 1 mutation also associated with increased risk
Plasminogen Allelles Influence Susceptibility to Aspergillosis in Stem Cell Transplantation- Plasminogen Directly Binds Aspergillus fumigatus Conidia- TLR9 is actively recruited to Aspergillus fumigatus phagolysosomes
Summarise the major SNPs associated with susceptibility to invasive fungal infections and disease
Increased Susceptibility to Invasive Aspergillosis
•CXCL10 •IL1-R •IL23R •MASP2 •MBL2 •PLG •TLR2 •TLR4 •TLR6 •TLR9 •TNFR1 •TNFR2
Summarise why immune status leads to increased risk of fungal infection
Human Dectin 1 Deficiency: major fungal pattern recognition receptor, so failure of innate immunity - leading to chronic fungal infections
CARD9: downstream of Dectin 1 - adaptor molecule in signalling processes - so macrophages cannot mount an immune response, leading to chronic mucocutaneous candidiasis
Toll like receptors: loss of function or mutations in TLRs leads to increased risk of aspergillosis
Describe the potential role of fungal immunogenetics in the future
Better understanding of fungal immunogenetics will enable the development of personalised medicine
Can determine who would be at risk from invasive aspergillosis after a transplant
Which cell is most important in defence against fungal infection
Neutrophils
Macrophage-depleted mice are not susceptible to Aspergillus fumigatus infection
However neutropenic mice are suceptible to Aspergillus fumigatus infection- uninhibited growth
Describe the neutrophils in host fungal defence
o Neutrophil are very important in fungal defence:
§ Neutrophil NETS – neutrophils throw out chromatin “nets” to capture pathogens.
· These chromatin molecules outside the nucleus act as “danger signals” and recruit’s effector cells to the area as well.
Granulate- release DNA which form NETS- good for protection against hyphae as hyphae cannot be phagocytosed
Describe how fungal morphogenesis governs the dendritic cell modulation of adaptive immunity.
Hyphal forms (multi-cellular) = Th2 response- MHC II, co-stimulation, IL-4,10
Conidium (unicellular) = Th1 response -MHC II, co-stimulaiton, IL-12
TH1- IFN-y
TH-2 - IL-4 - morel likely to get allergic responses
o Fungal morphogenesis – fungi can transition between yeast, candida and hyphae forms (multicellular) and this can drive a modulation of Dendritic cell response and can be bad for the immune response (as it gets confused).
What does mucosal immunity govern
Mucosal immunity governs fungal tolerance and resistance
Depending on the cytokine released and the Th cells recruited and activated
What cytokine therapy has been shown to enhance clearance of invasive fungal infection?
IFN-y
Via activation of innate immune cell function and phagocytosis
Describe the principles of adoptive immunotherapy for fungal infection
If a patient is receiving a stem cell transplant, you can generate anti-fungal T cells, which can be cultured in large numbers and administered to the patient
Use CAR-T cells- stimulated with fungal antigen
Put back into patient after the transplant- associated with reduced antibody load
Give an example of gene therapy for chronic granulomatous disorder
Restoration of gp91 function
This is involved in the generation of NADPH oxidase – this generates reactive oxygen species, which is required to kill microbes
NOTE: another type of gene therapy = restoration of neutrophil NET formation
Describe the different cells important in fungal immunity
Both macrophages and neutrophils contribute to fungal immunity
However for Aspergillus neutrophils are of primary importance
Dendritic cells modulate adaptive immune responses
Adaptive T cell interferon-gamma responses augment host immunity to fungi
Interferon-gamma or adoptive T cell therapy have emerging utility for the treatment of fungal infections
Gene therapy for primary immunodeficiencies
Describe some common spores found in the air
Alternaria alternara, cladosporium herbarum - wet moulds in shower- can induce asthma
Aspergillus niger- otitis externa- ear infection- can also sensitise the lungs- link to asthma
Aspergillus fumigatus
Spores 3uM across (i.e. easily are drawn into the lungs)- can penetrate Lower in the lungs
Penicillum- similar to Aspergillus
Describe The damage response framework of microbial pathogenesis
Immune system too weak- may get disease and death
Normal immune system- commensalism and colonisation
Too strong- TH2 allergic response- allergy and potentially death
Describe the Taxonomic distribution of characterised allergens
Fungi- 16%
Describe the Prevalence of mould allergy induced by different fungal species in respective populations
Alternaria Aspergillus Candida Cladosporium Penicilium Tichophyton
Describe the important fungal reactions in fungal allergy
§ Important fungal reactions include type 1, 3, 4 hypersensitivity reactions.
o T1 – IgE-driven, involves histamine and leukotrienes, in minutes.
o T2 – IgG-, IgM-driven, involves complement, in 1-24 hours.
o T3 – IgG-, IgM-driven, involves complement, in 1-24 hours.
o T4 – T-cell-driven, involves lymphokines, in 2-3 days.
Describe the fungal infections related to type 1 hypersensitivity
Cellular response : SM constriction, eosinophil infiltration
Allergic rhinitis, allergic asthma, ABPA, ABPM
Describe the fungal infections related to type 3 and 4 hypersensitivity
Type 3: Infiltration and activation of granulocytes- hypersensitivity pneumonitis, aspergilloma
Type 4: T-cell and macrophage activation, ABPA, hypersensitivity pneumonitis.
much of these type 4 and type 3- not allergires- more Th1 granulomatous condition - without fungal infection- but get fibrotic lung disease driven by neutrophils
Why do fungal allergens cause a lot of damage in the lungs
Cross-rectivity with other allergens and host-proteins- so get auto-immune response against these too.
Summarise fungal allergens
Many fungal spores are inhaled daily
Host response may be normal, ineffective, or exaggerated
This leads to either allergic or invasive fungal disease Aspergillus is primary driver
Other fungi may also contribute
Type 1/111/IV hypersensitivty reactions
Describe the pathophysiology of allergic bronchopulmonary aspergillosis (ABPA)
Aberrant dendritic cell response
Th2 activation- class-switching of B cells to IG-E via IL-4
Leads to mast cell degranulation (leading to bronchoconstriction) and eosinophil activation
Describe the criteria for ABPA
• • • • • • • • • Predisposing conditions Asthma or cystic fibrosis
Obligatory criteria
Total baseline serum IgE >1000 IU/ml
Positive immediate hypersensitivity skin test or Aspergillus-specific IgE
Supportive criteria (more than 2 present)
Eosinophilia >500cells/ul
Serum precipitating or IgG antibodies to Aspergillus fumigatus Consistent radiographic abnormalities
Describe the radiological features of ABPA
Dilated bronchi with thick walls Ring or linear opacities Upper or central region predeliction Proximal bronchiectasis (fibrosis and widening of the airways) Lobar collapse (distal to mucus plug) due to mucous impaction Fibrotic scarring
Describe the hyper dense mucus sign on a CT of a patient with ABPA
The hyper dense mucus sign
Arrows highlight high attenuation mucoid impaction within abnormally dilated bronchi.
Summarise the management of ABPA
Corticosteroids
Itraconazole for steroid sparing effect
Benefit of itraconazole past 16 weeks unclear
Reduction in circulating IgE, steroid dependency and improved PFT
Itraconazole indicated if not responding to steroids or steroid- dependent
Role of inhaled steroids and other antifungals less clear Recombinant IgE monoclonal antibodies (omalizumab) may be useful- prevent mast cell and neutrophil degranulation
Describe Aspergillus rhino sinusitis
May be allergic or invasive
Increasingly common
Association with atopy and nasal polyposis
Raised total IgE, skin test and Aspergillus-specific RAST IgE (or sometimes IgG)
May also be caused by Bipolaris or Curvularia
Obliterated sinuses with hypo attenuated mucosa and enhancing material on imaging
Describe the treatment of fungal sinusitis
Oral corticosteroids
Surgical removal of obstructing nasal tissue
Systemic anti-fungal treatment has not been shown to be effective
Topical therapies being investigated
Describe severe asthma and fungal sensitisation
Severe asthma
Positive immediate skin test or in vitro specific IgE to >1 filamentous fungus
Exclusion of allergic bronchopulmonary aspergillosis
Controversial diagnosis due to concerns about generalised sensitization
Use of antifungals unclear
Describe the skin-prick test for asthma
Aspergillus fumigatus Alternaria Candida Cladosporium Penicilliuum Negative control Histamine House dust mite Cat Dog Grass
Describe hypersensitivity pneumonitis
Also known as extrinsic allergic alveolitis
Allergic response requiring long-term allergen exposure As a consequence often occupational
Cell-mediated delayed sensitivity reaction Allergen-specific precipitins usually present
Summarise the testing for fungal allergies
A variety of pulmonary allergies to fungi exist
Allergic bronchopulmonary aspergillosis is best recognised
Severe asthma with fungal sensitisation is controversial
Evidence for fungal sensitisation in hypersensitivity pneumonitis
Diagnosis driven by skin test, IgE and IgM in clinical relevant populations
What are the key characteristics of fungal pathogens
Fungal pathogens are largely opportunists, only causing infections and disease when host defences are breached. Patients with cancer are at particular risk because often they are immunocompromised, either because of their underlying malignancy and/or the treatment for their disease. AIDS patients and transplant recipients are also at risk. Pneumocystis pneumonia (PCP) caused by Pneumocystis jirovecii is associated with AIDS. At highest risk are patients with prolonged and profound neutropenia (low neutrophil count) after treatment with highly cytotoxic chemotherapy for haematological malignancies and recipients of haematopoietic stem cell transplantation (HSCT).
How does the immune response to fungi vary
The immune response to fungi varies with respect to the species encountered and anatomical site of infection. The fungal morphotype may also be an important determinant of the host response: whereas yeasts and spores are often effectively phagocytosed, the larger size of hyphae precludes effective ingestion.
Several shared defence mechanisms are operative in response to a range of fungi. Neutrophils, macrophages and monocytes are fundamentally important antifungal effector cells: phagocytes are a critical first line of defence. Fungal cell walls are fundamentally different in their structure from human plasma membranes, and are sensed by pattern recognition receptors of innate immunity
Describe the role of dendritic cells in fungal immunity
Dendritic cells have an instrumental role in linking innate and adaptive responses to a range of pathogenic fungi, including Aspergillus fumigatus, Cryptococcus neoformans and Candida albicans. The signals transmitted by dendritic cells activated by exposure to fungi vary depending on the encountered fungus and its morphotype, helping to shape the appropriate adaptive immune response. Th1 type CD4+ T cell responses predominate in protective responses to various fungal infections, with interferon-γ known to be a particularly important cytokine in this type of response.
Describe the resistance of fungi to complement-mediate lysis
Toll like receptors (TLRs) are also involved in sensing other fungal components. Phagocytes already residing in the target organs at the time of infection attempt to kill or damage fungi. Additional effector cells, including neutrophils and monocytes, are recruited to sites of infection by the action of inflammatory signals such as cytokines, chemokines and complement components. Fungi are potent activators of the complement system, resulting in opsonisation due to deposition of C3b on the fungal surface and recruitment of inflammatory cells as a result of C3a and C5a generation. However, fungi are resistant to complement-mediated lysis, presumably due to their thick cell wall.
Describe the complement pathways that fungi can activate
Fungi can activate the classical, alternative, and lectin complement pathways. Normal human serum contains antibodies to fungal cell wall components, particularly mannans, that can initiate classical pathway activation upon binding. Such antibodies may also directly opsonise fungi for recognition by phagocytic Fc receptors (FcRs). Activation of the lectin pathway occurs when recognition of exposed mannans by mannose-binding lectin (MBL) triggers MBL-associated serine proteases. In most models, complement deficiency makes mice more susceptible to experimental mycoses
Summarise chronic granulomatous disease
Patients with the rare genetic condition chronic granulomatous disease (CGD) have defects in the enzyme NADPH oxidase, and their neutrophils are unable to kill ingested pathogens. These patients are susceptible to invasive fungal infections, particularly with Aspergillus species, confirming the crucial role of neutrophils in defence against fungal infections.
Describe some PAMPs and their associated PRRs
1,3 β-glucans Dectin-1 CR3 (CD11c/CD18) CD5 CD36 SCARF1
Mannans Mannose receptor (CD206)
DC-SIGN (CD209)
Langerin (CD207)
Dectin-2
Chitin Mannose receptor (CD206)
What is the difference between hyphae and yeast
Yeast- unicellular
Hyphae- multi-cellular
Dimorphism important in immune response generated
Candida albicans- parent yeast cell, which can develop into budding yeast, peudohypa or germ tube- which can then develop into hyphae or pseudohypa
Apsergiullus also invade as hyphae
What is the difference between conidia and spores
Conidia are type of asexual spores (nonmotile) in fungi while a spore is a reproductive structure of fungi and some other organisms, containing one or more cells.
