abstract
The morbidity, mortality and economic burden associated with fungal infections, together with the emergence
of fungal strains resistant to current antimicrobial agents, necessitate broadening our understanding
of fungal pathogenesis and discovering new agents to treat these infections. Using invertebrate hosts, especially
the nematode Caenorhabditis elegans and the model insects Drosophila melanogaster and Galleria
mellonella, could help achieve these goals. The evolutionary conservation of several aspects of the innate immune
response between invertebrates and mammals makes the use of these simple hosts an effective and
fast screening method for identifying fungal virulence factors and testing potential antifungal compounds.
The purpose of this review is to compare several model hosts that have been used in experimental mycology
to-date and to describe their different characteristics and contribution to the study of fungal virulence and
the detection of compounds with antifungal properties. This article is part of a Special Issue entitled: Animal
Models of Disease.
1. Introduction
In recent years, systemic fungal disease incidence has rapidly
increased, especially in immunocompromised patients [1]. This
presents a significant problem, as these diseases are associated with
high mortality (ranging from 40 to 67% for candidiasis and over 88%
for invasive aspergillosis) [2,3], and new fungal strains resistant to
multiple antifungal agents are being encountered in the ICU setting
[4]. These facts underscore the need to better understand the pathophysiology
of different fungal infections, which could help reveal
new targets of drug therapy and lead to the development of new antifungal
agents.
The murine model is one of the most commonly used models for
studying fungal infection because of the similarity of murine and
human physiology and immune systems.However, ethical and logistical
constraints associatedwith the use ofmice in such experiments slowthe
evolution of our understanding in the field of mycology, which must
progress more rapidly if we wish to hinder the ever-growing resistance
of fungal pathogens to antimicrobial compounds.
This emerging need for an easier and fastermethod of in vivo experiments
on fungal pathogenesis can be addressed by using invertebrate
model hosts. Many studies have shown that, although invertebrates
are separated by millions of years of evolution frommammals, many aspects
of the innate immune system are conserved between the species
[5]. Additionally, their low cost, simplicity of use and short life span
make invertebrates ideal candidates for large-scale studies. Finally,
there are no ethical constraints in the use of invertebrates, which further
facilitates their use for in vivo experimentation. The purpose of
this review is to elucidate the characteristics of different invertebrate
model hosts that have been used to study fungal diseases, as well as
the relative advantages and potential drawbacks associated with each.
2. Drosophila melanogaster
Since the groundbreaking discovery by Lemaitre et al. [6] that the
Toll pathway serves an essential role in Drosophila defense against
pathogenic fungi, accompanied by the widely reproduced figure of
dead Toll-deficient flies covered by Aspergillus fumigatus hyphae, the
fruit-fly has been used widely in experiments of fungal pathogenesis.
The sequencing of its whole genome [7], the creation of RNAi libraries
[8] that permit selective deactivation of specific genes, and the availability
of mutant strains facilitate studies of different aspects of its immune
response to fungal pathogens. For example, two main pathways of
microbial resistance in the fruit fly have been described. The Imd pathway
appears to be more important against Gram-negative bacteria,
while the immune reaction against Gram-positive bacteria and fungi
follows a different pathway, the Toll/Spätzle pathway [9]. Specifically,
as reviewed elsewhere [10,11], fungal invasion is recognized via pattern
recognition receptors,most importantly GNBP-3,which recognizes fungal
beta-glucans and triggers a reaction leading to the processing of a
small molecule, called Spätzle, which functions as the ligand of the
Toll receptor. The activation of the Toll pathway leads in turn, through
the recruitment of proteins like MyD88 and the transcription factor
NF-kB, to the release of molecules with potent antifungal activity called
drosomycin [12] and metchnikowin [13]. Furthermore, the Drosophila
immune system seems to be capable of recognizing elements of
fungal virulence and triggering the Toll pathway via another protease,
called Persephone, independent of the GNBP-3 mechanism [14]. This
microbial-specific response seems to apply to systemic infections, while
in epithelial infections the Imd pathway seems to coordinate Drosophila
defense [15–17]. Newer studies have also shown the importance of pathways
not related to either Toll or Imd in the immune response of the fruit
fly, like the JNK and the JAK/STAT pathways [18]. Further, Chen et al.
showed that the p38 pathway that is part of the Toll cascade inmammals
participates in the immune response of Drosophila independent of the
Toll pathway, by up-regulating the expression of heat-shock proteins
and suppressing JNK activity [19]. It is important to note, however, that
despite similarities in the innate immune response between the fruit
fly and humans, there are many differences that pose serious limitations
to themodel. First, Drosophila does not have an adaptive immune system
and lacks antibodies. Additionally, its innate immune system lacks natural
killer cells, dendritic cells and cytokines that play a crucial role in the
human immune response. Finally, the fruit fly cannot be easily used as a
model system for tissue-specific infections in humans.
Despite these technical limitations, Drosophila flies have been
used as an experimental model for fungal virulence factors and in
studies that aim to identify novel antifungal compounds. For these
purposes Toll-deficient flies are used, as wild-type flies are resistant
to most types of fungal infection. For example, studies have shown
that, as in mammals, hyphal formation is an important virulence
factor for Candida infection in flies [20], though the Cryptococcus
neoformans main virulence factor in mammals–capsule formation–
does not seem to play an equally important role in the Drosophila
model [21]. Lionakis et al. have developed a protocol for the testing
of several antifungal compounds in a Drosophila model of invasive aspergillosis
[22]. The administration of antifungal compounds in flies
can happen either orally or via direct injection of the compound,
however the latter requires special equipment and expertise [10].
Interestingly, a novel study by Glittenberg et al. provides insight
into how wild-type Drosophila strains can be used to study Candida
virulence, thus expanding the model and potentially making it more
relevant to mammals [23]. Different researchers have recently provided
evidence that, while wild-type Drosophila flies cannot be killed
by Candida glabrata, they also cannot completely eliminate the infection
as C. glabrata cells can remain viable inside the insect's phagocytes,
just as occurs in mammals [24]. Finally, a method that utilizes
Drosophila hemocyte-derived S2 cells has been used as an alternative
approach to study fungal virulence and host–pathogen interactions.
In one study, researchers showed that white Candida albicans cells
are more susceptible to phagocytosis by host hemocytes than opaque
C. albicans cells, thus suggesting that a change from white to opaque
cell type might help the fungus evade immune detection [25]. In another
study, investigators utilized Drosophila S2 cells and RNA interference
and found 57 host genes that play an important role in
C. neoformans infection including genes associated with autophagy
[26]. In both of these studies, results were validated by comparison
to results obtained using murine macrophages.
In summary, D. melanogaster is a promising model for the study of
host immune response, pathogen virulence factors, and efficacy of antimicrobial
compounds in fungal diseases. Its main limitations are that it
requires special lab equipment (e.g. a “fly room” and incubators) and
technical expertise in handling the flies [27].