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BATRACHOCHYTRIUM DENDROBATIDIS - Coggle Diagram
BATRACHOCHYTRIUM DENDROBATIDIS
infection of a host
consequences
key factors that determine outcome
infectious process
importance of steps
STEPS:
adherence
invasion
replication
host damage
immune evasion
1 more item...
swimming zoospore as the infectious stage
infectious occurs when zoospores land on skin or mouthparts and encyst (stop movement, settle, resorb the flagellum and form a cell wall). attach via adhesive molecules
moves from the surface of the skin to the
stratum granulosum
of the epidermis and matures in the
stratum corneum
by sending germination tubes through one or more cell layers injecting the content of the zoospore cyst into the cytoplasms of deeper cells of the host epidermis
enters healthy cells, grows in size and develops into an urn shaped zoosporangium (haploid reproductive structure) in which zoospores develop. intracellular location and process of injecting zoospore contents into deeper epidermal cells may permit Bd evasion of host immune surveillance
as infected skin cells move towards surface, zoosporangium matures, discharge papilla opens and mature zoospores swim out.
zoospores are motile for 24h at 23ºC before they encyst or die but survive more than 48h at 4-14–C
movement is directed towards nutritional cues such as glucose, lactose, cysteine and keratin (major component of amphibian skin cells
remains confined to mature skin or tadpoles mouth parts
https://www.jstor.org/stable/23016317?searchText=&searchUri=&ab_segments=&searchKey=&refreqid=fastly-default%3A62bafcc5d0a25b347fe11313a302ca2a&seq=2
impairs ion transport in skin
endocrine response that balances cutaneous ion transport elevates glucocorticoid or mineralocorticoid responses resulting in immune suppression
restricted to keratinised epidermis of amphibians (mouthparts of anuran tadpoles, skin of adult amphibians)
EARLY INFECTION
chemotaxis of infectious zoospores towards skin surface
encounter mucus and any associated constitutive defences of the skin
virulence
ecology
mechanisms of immune evasion
mechanism
contribution to infectious process
facilitation of lifecycle
releases factors that inhibit lymphocyte proliferation in vitro by induction of apoptosis (immunosuppressive factors)
immune system reorganisation at metamorphosis
amphibian immune system develops in two phases
rapid expansion during tadpoles life followed by significant loss of lymphocytes during climax of metamorphosis
provides a good opportunity for Bd to rapidly establish itself in epidermis
tadpole mouthparts harbour fungus during larval life
emerging zoospores may infect newly developing adult skin establishing overwhelming infections
second expansion of lymphocyte populations and development of adult like recognition repertoire following involution of immune system (1)
loss of lymphocytes at metamorphosis due to rapid increase in corticosteroid hormones that occurs during this period
stress that elevates corticosteroid hormones interfere with immune mechanisms of resistance to Bd
amphibian lymphocytes are inhibited with nano molar concentrations of corticosterone (glucocorticoid) and aldosterone (mineralocorticoid)
reversible
in vitro
and
in vivo
by corticosteroid receptor antagonist RU496
tadpoles also elevate corticosterone in response to food deprivation
other natural events that elevate corticosteroid hormones
metamorphosis
breeding activity of explosive-breeding species
decreased food resources (stress)
also inhibits renewal of amphibian skin peptides following norepinephrine induced depletion of the peptides
physiological responses of frogs suffering from advanced stages of chytridiomycosis may also alter corticosteroid hormone levels
decrease in lymphocyte numbers in weeks preceding death
suppression of lymphocyte response
soluble factors derived from cell wall actively interferes with adaptive immunity and prevents clearance of pathogen
inhibits proliferation of splenocytes
immunodeficiency thus occurs as insufficient T lymphocytes and B lymphocytes are present to respond to and eliminate Bd
some lymphocyte genes are down regulated in spleen of susceptible but not resistant/tolerant in vivo species
allows Bd to infect immunocompetent hosts
explains why species lacking innate mucosal immune defenses are so susceptible to chytridiomycosis
molecules that alter specific host mechanisms
contributions to overall infectious process
models that elucidate host-pathogen interactions
in vitro models
studies about antimicrobial peptides
species expressing a mix of skin peptides that potently inhits Bd growth in vitro are among common/resistant species
declining/endangered species express peptides with poor in vitro anti-Bd activity
studies about immune suppression
soluble factors released by Bd can inhibit proliferation of both T and B lymphocytes.
can induce apoptosis of lymphocyte populations
factors are released by either live or dead zoosporangia
iv immune experiments unfolding proliferation of splenic lymphocytes in culture
macrophage phagocytosis not similarly affected
apoptosis via TUNEL and caspase assays
programmed cell death positively associated with infection load and morbidity
apoptosis may be a pathogen virulence becanism
immune inhibition activity associated with Bd supernatants (measured delayed-type-hypersensitivity responses
Bd peoduces two metabolites capable of inhibiting lymphocyte proliferation and survival in vitro
methylthioadenosine
kynurenine
animal models
need for a susceptible model species to carry out efficient research
gene expression studies
susceptible individuals express a greater number and variety of dysregulated immune genes during late stage infections than more resistant individuals
susceptible individuals may mount massively dysregulated and non-protective immune responses
immunopathology is likely associated with DAMPs induced late in infection as the pathogen damages skin cells in order to release subsequent generations of zoospores
dysregulated response may rapidly disrupt cellular homeostatic mechanisms
significant depletion of "immune nutrient factor" alpha-ketoglutarate and associate metabolite glutamate in severely infected animals
metabolic dysregulation has carry on effects on numerous other aspects of cell homeostasis, particularly cellular energy metabolism (alpha-ketoglutarate is a key intermediate of the Krebs cycle)
immunopathology may not only cause their immune responses to be ineffective at eliminating pathogen, but may contribute to host morbiidity and mortality due to extensive disruption of cellular homeostasis and consumption of energy resources
studies suggest massive disruption of homeostatic mechanisms in susceptible individuals
epithelial stability
water
ion transport
musculoskeletal functions
host immune response to the disease
protective consequences
harmful consequences
combined effects of immune system remodelling and development of keratinized epidermis across the body during metamorphosis, may help to explain why newly metamorphosed anoraks are particularly vulnerable to chytridiomycosis
uncertainties
limited evidence for the adaptive immune response to Bd
little evidence for herd immunity
herd immunity threshold relies on infection transmission being density-dependent rather than frequency depended as is expected for amphibian breeding aggregations
threshold is unlikely to occur where force of infecition is unaffected by presence of resistant individuals
indirectly transmitted pathognes
those with multiple host species/life stages with differing tolerance and susceptibility to infection
temporal patterns of enzootic Bd infection appear regulated by season and temperature rather than field adaptive immunity
limited activation of protective immune response
decrease in total and IgY serum antibody responses in infection
L caerulea
vs uninfected frogs
via anti-sheep red blood cells [SBC] haemagglutination assay
circulating numbers of lymphocytes greatly reduced in infected frogs
histopath results show mild response to no lymphocyte response
conflicting reports on up regulation of MHC genes, B/T lymphocytes, immunoglobulins
some signals of down regulated T cells
complex set of interactions
temperature dependent
temporally dependent
inflammation
benefit to host
benefit to pathogen
reported in 40% of skin sites sampled in susceptible species
consist of infiltration of neutrophils, lymphocytes and macrophages
RESPONSES TO CHYTRID
IMMUNDE DEFENSES OF THE SKIN
secreted antimicrobial peptides
amphibian skin is rich in mucous and granular glands (poison/serous glands)
mucous glands produce material rich in heavily glycosylated muffins and mucopolysaccharides that help keep the skin moist
granular glands
epithelial cells fuse to form a syncytium and centre of gland is filled with granules packed with active peptides
surrounded by a layer of myoepithelial cells with sympathetic axons terminating between the cells
possess
alpha
-adrenoreceptors, and epinephrine or nor-epinephrine induce contraction and release of the contents
after discharge, gland regenerates and peptide contents are resotred
produce bioactive peptides including neuropeptides and antimicrobial peptides that are thought to play a role in defence against predators as well as microbes
both glands composed of a layer of epithelial cells surrounding a secretory compartment
active against gram positive and gram negative bacteria, fungi, protozoa and viruses
cationic, hydrophobic and have ability to form an amphipatic
alpha
-helix in a membrane-mimetic environment
structure bestows an ability to disturb biological membranes
not common among all families
inhibit growth and colonisation of Bd fungus
can be detected in skin secretions of tadpoles of some species
other species do not appear to express defensive antimicrobial peptides until granular glands mature at metamorphosis
immunoglobulins
anti fungal metabolites produces by symbiotic skin bacteria
skin of amphibians inhabited by a rich array of skin microbes
2,4-diacetylphloroglucinol
indole-3-carboxaldehyde
violacein
exerts synergistic effects with AMPs to inhibit Bd growth
mucus containing antibodies capable of binding to
B. dendrobatidis
adaptive lymphocyte-mediated immune response
should develop to clear the infection
development of an effective cell mediated defence is impaired
epidermis and dermis contain antigen-presenting cells such as lymphoid dendritic cells, macrophages, B-lymphocytes and T-lymphocytes
immunisation against Bd induced elevated pathogen-specific IgM and IgY serum antibodies
conventional Immunization protocols that induce elevated serum antibodies would not necessarily be protective against this pathogen which is confined to the skin
process inducing elevated mucosal antibody responses might be more successful
mucus secretions from exposed
X. laevis
contain significant amounts of IgM, IgY, IgX antibodies capable of binding to Bd
EARLY INFECTION
expected immune mechanisms upon initial exposure to Bd, assuming constitutive defences such as AMPs and bacteria are inssufficiant
pathogen recognition leads to infiltration of innate immune cells, including macrophages and granulocytes such as neturophils
ecdysis (skin sloughing)
functions as a constitutive innate immune defence mechanism by physically removing skin microbiota
occurs soon after dark every 3-4 days
increases with ambient temperature
abnormal amount with smaller shed pieces is a clinical sign of chytridiomycosis
result of sporangia initiating premature keratinisation and cell death in infected epidermal cells
in concert with hyperplasia and stimulated epidermal turnover
rate increases with infection but behaviour, duration and rhythmicity remains the same
diseased frogs typically do not eat sloughed skin as normal
varies on species intrinsic susceptibility
can result in clearance of infection
likely to be both beneficial and detrimental
removes symbiotic skin bacteria and disrupts skin homeostasis
most resistant species demonstrate increased sloughing rates at lower infectious burdens as an effective induced defence response
natural mucosal antibodies generated by innate-like B cells
may inhibit zoospoewaa
polyreactive against highly conserved pathogen epitopes
typically ended by germline genes
not much known about efficacy against Bd
lysozyme and other defensive enzymes
constitutively expressed antimicrobial enyzyme found in body fluids and mucosal linings
potent bactericidal activity
typically considered antibacterial but may also possess anti fungal properties
increase in expression of lysozyme genes in skin of infected frogs (Xenopus tropicalis)
antimicrobial peptides (AMPs)
-
KEY COMPONENT OF RESPONSE
typically small hydrophobic cationic peptides produced by serous glands that provide non-specific defence against pathogenic organisms
four main categories of defensive molecules
alkaloids
steroids
biogenic amines
other peptides and proteins
range and quality is remarkable
released at a low continuous rate at rest
mild activation of sympathetic nervous system sufficient to stimulate the contraction of glad-associated muscle fibres and release of larger quantities of AMPs to the skin surface
unlikely that zoospore invasion alone would be sufficient stimulus to produce this response
in vitro
assays found to inhibit growth of various pathogens
reliable predictors of natural resistance of amphibians to chytridiomycosis
efficacy against Bd varies substantially by species and other factors
intrinsic peptide efficacy against Bd demonstrated
in vitro
concentration, number and type of peptides produced
rate and location of release to the skin surface in response to microbial pathogens
presence of host- and Bd-secreted proteases that may degrade AMPs
depletion led to increased infection probability in resistant amphibian species
infected frogs demonstate decreased expression
skin microbiome may inhibit zoospore colonisation
bacteria species
Janithinobacterium lividum
has shown particular promise for anti-Bd properties of secreted metabolite violacein
INTERMEDIATE INFECTION
recognition of antigens by dendritic cells and then differentiate into antigen presenting cells and migrate to the spleen enabling antigen specific selection of lymphocytes
simultaneously membrane-bound immunoglobulin on naive B lymphocytes is exposed to extracellular Bd antigens (transported via the blood circulatory and lymphatic systems)
with assistance of T helper cells these B cells are activated to respond to infection
LATE INFECTION
involves lymphocyte clonal expansion
differentiation into plasma cells
activated T cells
cytotoxic
T helper cells
production of antibodies by plasma cells
RECOVERY
cohort of selected memory lymphocytes should remain which activate at re-exposure
pattern recognition receptors
detect common fungal structures and initiate inflammation
invading microbes prompt host recognition
antigens contain widely recognised structural moieties known as pathogen-associated molecular patterns (PAMPs) that are common among different groups of microorganisms
bing to host germqline-encoded pattern recognition receptors expressed within or on cells of the innate immune system and nonprofessional immune cells
little known about amphibian pattern recognition receptors
homologous genes to mammalian PRRs
AMPHIBIAN IMMUNE RESPONSES
concerning b cell response compared to mammals
affinity maturation of antibody is relatively poor, may be related to lack of germinal centers important for the selection of B cells expressing antigen receptors with higher affinity
debdritic cells perform additional duty of follicular dendritic cells (specialised cells critical for antigen specific B cell activation), further suggests a less powerful B cell response capacity in amphibians
differentiated B cells have phagocytic capabilities unlike mammalian lymphocytes
antibodies consist of IgM, IgX (mainly mucosal expression) and IgY (induced via T-cell dependent responses) (latter two functionally analogous to IgA and IgG of mammals. spleen contains two further isotopes (IgD - homologous to mammal and fish IgD; and IgF - no known mammalian homolog
CD8 T cell response is similarly MHC class I-restricted and critical for host resistance to viral infection
CD8 T cell expansion not as extensive as in mammals
prominent immune surveillance system based on a large family of MHC class II-like genes regulating the development and function of innate-like T cells critical for host antimicrobial defenses (recently unveiled in
Xenopus
spp.
similar systems likely to exist across all aquatic vertebrates
TADPOLES
competent but functionally less well developed than the immune system of post-metamorphic and adult amphibians
immune system undergoes substantial remodelling accompanies by immunosuppression during metamorphosis through until about 6 months post-metamorphosis
immunoglobulin repertoire typically smaller and less specific in tadpoles, the thymus involutes and is reformed during metamorphosis, expression of MHC class I and II molecules greatly expands
THE IMMUNOOCOMPETENT UNINFECTED STATE
epidermis
constitutes an immediate innate physical defence barrier against pathogen invasion
consists of cell layers that mature as they migrate toward the skin surface
layers
basal lamina
stratum germinative (roughly cuboidal or columnar-shaped proliferative cells)
stratum spinosum
stratum granulosum
stratum corneum - highly differentiated keratinised squamous epithelial cells found at surface of the skin
superficial cells joined by tight junctions which help maintain the skin barrier
intermittent sloughing may assist in the physical removal of skin microorganisms
layer of mucous produced by mucous glands sits on the surface
contains a number of defensive molecules
lysozyme
enzymes produced by phagocytes and keratinocytes
antimicrobial peptides secreted via serous glands
mucosal antibodies
commensal symbiotic bacterial communities together with their secreted antimicrobial compounds
a number of peripheral immune surveillance cells typically present
epifermal dendritic cells including putative Langerhans cells
Xenopus
Langerhans cells express high levels of MHC class II molecules
various dendritic cells likely to serve as efficient antigen presenting cells
dendritic epidermal T cells
gamma/delta T cells
deeper dermal layers
highly collagenous
deeper stratum compactum
tthicker and more superficial stratum spongiosum
compactum and spongiosum separated by the substantial amorpha granular calcified layer (some species only)
provides nutrition and sensory integration to the epidermis via a network of capillaries and nerves that course through dermis
contains serous (granular or poison) glands and mucous glands are also present within the dermis
serous glands often widely distributed throughout the skin
often widely distributed throughout the skin
able to discharge their contents in response to noxious stimuli
contains pigment bearing chromatophores
contains smooth muscle fibers
in uninfected state the repertoire of naive B and T lymphocytes lie mainly quiescent in spleen, blood, secondary lymphoid organs (liver, intestine).
in amphibians spleen functions as primary and major secondary lymphoid organisms
each possess a unique and specific antigen receptor combination
environments
mechanisms of sensing environments
mechanisms of response to environments
contribution to infectious process
lower temperatures
immune systems of amphibians undergoes involution during natural hibernation
antibody responses and skin graft rejection (classical T cell mediated response) are significantly impaired by lower temperatures
prevalence of infection increases in cooler seasons
Bd persists at 4-14ºC
sheds more zoospores and are active over a longer period of time
soil bacterial colonies with anti-Bd properties
stress
predisposes hosts to chytrid via corticosterone responses
environmental stressors
poor nutritional status
high densities
exposure to predator cues
elevations in corticosteroids
inhibit humoral response
reduces numbers and viability of circulating lymphocytes
predisposes to disease and is ALSO a result of infection
does not mediate sublethal effects (growth and mass)
variability of disease
development of disease depends on the species infected capability to produce defensive antimicrobial peptides
pattern of decline in genus
Atelopus
attributed to extreme temperature variations impairing immunity, shifting balance between immunity and disease
MHC variants
differential survival of amphibians infected maps to genetic variation in MHC class II peptide-binding region (PBR)
acquired resistance associated with particular amino acid residue positions situatied in exon 2 of the MHC class IIB gene encoding beta-1 segment of the antigen binding groove that presents epitopes to T ccells
stability of antigen-MHC complex depends mainly on deep pockets within the binding groove that interact directly with antigen residues
toxins
fungal toxin poisons hosts
factors that cause commensal to become pathogenic
transmission
diagnostics
sensitivity
specificity
public health impacts
why is it a problem
why is it STILL a problem
treatment
challenges
positives/benefits
immunisation
promising results with intraperitoneal injection of heat-killed Bd
high-titer pathogen-specific IgM and IgY serum antibody response 14 days post final immunization
Bd binding mucosal antibodies demostrated after repeated exposure to Bd
potential upregulated immunoglobulin genes suggest production of memory lymphocytes
not consistently effective across species
cycles of exposure to live zoospores followed by treatment appear to produce a more robust immune response than killed zoospores
host-pathogen interactions
chytridiomycosis causes a range of changes to host epidermis
hyperkeratosis
impedes respiration and water balance
hyperplasia
ulceration
erosions
necrosis
effectors associated with Bd such as CRN13 can induce necrosis
reduced gene expression levels of keratin and collagen
key components of structural skin integrity
severe infection clinical signs
lethargy
abnormal posture
anorexia
peripheral erythema
increased skin shedding
mortality over a period of 2-6 weeks post exposure
lower plasma concentrations of sodium and potassium electrolytes
induces asystolic cardiac arrest
decreased expression of ion channel genes
consistent with observation of electrolyte imbalance directly preceding death, attributable perhaps to physical disruption of the epidermis
increased expression of potassium/chloride genes in Bd susceptible species
tadpoles may exhibit blunting of mouthparts and sublethal effects on growth and development but infection not usually fatal until metamorphosis and widespread keratinisation
interferes with feeding
metabolism
phylogeny
Chytridiomycota
are a phylum of microscopic predominantly saprobic fungi with a biphasic life cycle consisting of motile flagellated zoospores and reproductive zoosporangium
BACKGROUND
causes skin disease chytridiomycosis
highly virulent vertebrate fungi
implicated in worldwide amphibian declines
causes inhibited electrolyte transport
reduction in plasma sodium and potassium concentrations
asystolic cardiac arrest