Infections caused by fungal pathogens pose a serious and steadily increasing threat
to susceptible individuals worldwide
1
with a specific impact on treatment of immunocompromised patients. Fungi represent
a distinct kingdom among the Eukarya, making the design and validation of therapeutic
compounds that may counteract fungal infections a highly challenging and unfortunately
massively neglected field in the pharmaceutical pipeline.
2
Mechanisms and determinants of fungal pathogenesis are far from being understood comprehensively
but this is a pre-requisite to define what has been coined the human virulome of pathogenic
fungi.
3
As an infectious disease is commonly the outcome of a potential pathogen encountering
a susceptible host, both sides of this intertwined interplay need to be studied with
the aim to put fungal characteristics in the appropriate context of virulence.
4
Therefore, suitable models of infection are an essential requirement when testing
distinct features of the pathogen as virulence-determining factors or to validate
therapeutic interventions such as efficiencies of antimycotic substances. Small vertebrates
like mice, rats, rabbits, guinea pigs, or hamsters have been established extensively
for such purposes, based on their relative convenience with respect to handling and
manipulation. The in-depth knowledge of the murine immune system that is accompanied
by its highly advanced molecular biology and genetic accessibility has cemented the
prime role of mice as hosts in infection studies, serving as proxy for the susceptible
human host that is confronted with fungal pathogens. Yet, financial, infrastructural,
and especially ethical issues limit the implementation of mammalian infection models
to study the virulome of fungi. Apart from this, following the course of infection
in individual hosts to assess fungal burden, histopathology, or immune responses is
generally hampered by the application of invasive procedures that usually require
culling of the infected cohort animal-by-animal.
Addressing these aspects, Coste and co-workers describe in the recent issue of Virulence
their efforts to combine 2 of the most recent developments in fungal infection research,
that is the use of an alternative insect mini-host accompanied by the implementation
of a suitable reporter system based on bioluminescence that allows longitudinal studies
in single infected animals.
5
Driven by the need for alternatives replacing mammalian systems to study fungal infections,
established model organisms such as the zebrafish Danio rerio, the nematode Ceanorhabditis
elegans or the soil amoeba Dictyostelium discoideum and Acanthamoeba castellanii have
been validated in recent years to cover various characteristics of fungal virulence
and host response.
6-12
This has been complemented by making use of embryonated chicken eggs, which allowed
virulence studies addressing relevant aspects such as invasion, dissemination, or
immune reactions.
13
In promoting such initiatives, invertebrate insect hosts have a long-standing tradition
to elucidate mechanisms of immunity against fungal pathogens and virulence determinants,
ranging from seminal early studies in the fruit fly Drosophila melanogaster that opened
the field of receptor-mediated recognition of pathogen-associated molecular patterns,
the so-called PAMPs, to recent ones in locusts that appear suitable to mimic invasion
of the central nervous systems by pathogenic fungi.
14-16
Due their ease of housing and handling and thereby obviating administrative as well
as ethical concerns, insect larvae have emerged as most suitable for infection studies;
this is accompanied by several other benefits, such as relatively low purchase costs,
their survival at appropriate temperatures that support growth of fungal pathogens,
or the feasibility of precise and non-traumatic inoculation with defined infectious
doses due to their relatively large size.
For the opportunistic pathogen Candida albicans, the silkworm Bombyx mori or caterpillars
of the tobacco hornworm Manduca sexta had been established most recently as suitable
replacement hosts to monitor virulence traits, while larvae of the greater wax moth
Galleria mellonella have served as predominant mini-host for Candida infections since
the year 2000.
17-19
These larvae lack an adaptive immune response but hold various hemocytes that eliminate
invading pathogens via phagocytosis; moreover, humoral components of innate immunity,
like antimicrobial peptides, are mounted upon infection, with differences being evident
for the response against bacterial or fungal pathogens.
20
Wax moth larvae have served to address various aspects of the host-fungus interplay
in the context of pathogenesis, ranging from pattern recognition triggering nodulation
to filamentation and other traits of virulence and even the efficiency of antifungals.
21-23
A systematic analysis of C. albicans mutants impaired in the yeast-to-hyphal transition
in both the murine systemic infection model as well as the Galleria system revealed
a reliably good correlation,
22
and studies with the yeast pathogen Cryptococcus neoformans or the human-pathogenic
mold Aspergillus fumigatus have led to similar conclusions.
24,25
A recent study from the Coste group however also demonstrated significant discrepancies
between the 2 infection models.
26
Yet, G. mellonella larvae have earned their merits in studying fungal infections that
is only dampened by the evident lack of a reference genome sequence and missing tools
of molecular biology for genetic engineering.
When following the course of disease and/or therapeutic treatment in an appropriate
infection model system, crucial parameters such as symptomatic signs but also pathogen
distribution and load need to be monitored. In this respect, sensitive reporter systems
have been developed that are based on bioluminescence, i.e. photon emission conjunct
with substrate oxidation that is catalyzed by light-generating enzymes. These so-called
luciferases have evolved in prokaryotes as well as eukaryotes, and several enzymes
of either origin are nowadays established, differing with respect to physico-chemical
characteristics such as the nature of (co-)substrates, signal peak wavelength, or
emission kinetics.
27
Emitted photons are commonly detected by the use of highly sensitive charged coupled
device (CCD) cameras to yield a quantitative read-out of bioluminescence. Accordingly,
a longitudinal insight about pathogen distribution in the infected organism might
be gained without the need for terminal inspection, which evidently reduces the number
of hosts to be monitored. For fungal pathogens, several studies have demonstrated
the usefulness of but also the limitations of bioluminescence imaging in murine infection
systems.
28-35
A major restriction lies in the obligate need for external substrate application,
which is in contrast to bacterial pathogens that might be transformed with thoroughly
characterized and adapted bioluminescence operons.
36
Due to the orthogonality of such systems, these cannot be employed in fungal organisms,
while eukaryotic bioluminescent systems are only characterized with respect to the
luciferase activity but not substrate-generating pathways.
In the recent issue of Virulence, Eric Delarze and colleagues have described the fruitful
combination of both approaches, employing Galleria mellonella larvae as hosts for
infections by the human commensal Candida albicans and implementing bioluminescence
as reporter read-out to enhance studies on virulence traits as well as antifungal
treatment.
5
By transferring a validated and optimized expression module for surface display of
the luciferase enzyme from the copepod Gaussia princeps
29
to a wild-type isolate and congenic deletion strains, bioluminescent signals could
be quantified ex vivo from the pulp of infected and sacrificed larvae that correlated
to fungal burdens deduced from colony forming units to some degree. Most importantly,
the fungal infection could also be monitored over time in living wax moth larvae by
using a non-toxic and water-soluble formulation of the substrate coelenterazine, WCTZ.
This allowed in vivo kinetic studies of C. albicans infections in this alternative
host and represents a step forward in elucidating virulence characteristics of this
common yeast pathogen and in monitoring options for antifungal treatment. From this
study, further perspectives emerge but also shortcomings and limitations of the system
became evident: Obviously the described achievements pave the road for large scale
studies to yield significant insights at high reliability without the need for infecting
numerous cohorts of susceptible mice. This proof-of-concept study might also spark
additional initiatives with other fungal pathogens for which bioluminescence as well
as the wax moth larvae infection model had been successfully established. Yet, drawbacks
emerge from the apparent lack of a standardized set of congenic C. albicans strains
that carry the integrated reporter constructs at defined and identical numbers. Infecting
wax moth larvae can also not reflect the various facets of C. albicans pathogenicity
that range from superficial colonization to systemic dissemination. In this context,
however, the Gaussia-derived bioluminescence system had been characterized exhaustingly
in murine infection models to reveal shortcomings that may be associated with substrate
instability, emission characteristics, or tissue penetration issues.
29
Accordingly, the necessity of external substrate application in bioluminescence studies
on fungal pathogens is still an unresolved issue that needs to be addressed to significantly
further the field. The recent insights by Delarze et al., however, make a strong case
for developing bioluminescence imaging approaches in alternative mini-hosts with the
aim to study fungal virulence.