Immune-deficient Drosophila melanogaster: a model for the innate immune response to human fungal pathogens

Recent Advances in the Use of Drosophila melanogaster as a Model to Study Immunopathogenesis of Medically Important Filamentous Fungi

Airborne opportunistic fungi, including Aspergillus and other less common saprophytic molds, have recently emerged as important causes of mortality in immunocompromised individuals. Understanding the molecular mechanisms of host-fungal interplay in robust experimental pathosystems is becoming a research priority for development of novel therapeutics to combat these devastating infections. Over the past decade, invertebrate hosts with evolutionarily conserved innate immune signaling pathways and powerful genetics, such as Drosophila melanogaster, have been employed as a means to overcome logistic restrains associated with the use mammalian models of fungal infections. Recent studies in Drosophila models of filamentous fungi demonstrated that several genes implicated in fungal virulence in mammals also play a similarly important pathogenic role in fruit flies, and important host-related aspects in fungal pathogenesis are evolutionarily conserved. In view of recent advances in Drosophila genetics, fruit flies will become an invaluable surrogate model to study immunopathogenesis of fungal diseases.

Of model hosts and man: using Caenorhabditis elegans, Drosophila melanogaster and Galleria mellonella as model hosts for infectious disease research

The use of invertebrate model hosts has increased in popularity due to numerous advantages of invertebrates over mammalian models, including ethical, logistical and budgetary features. This review provides an introduction to three model hosts, the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster and the larvae of Galleria mellonella, the greater wax moth. It highlights principal experimental advantages of each model, for C. elegans the ability to run high-throughput assays, for D. melanogaster the evolutionarily conserved innate immune response, and for G. mellonella the ability to conduct experiments at 37°C and easily inoculate a precise quantity of pathogen. It additionally discusses recent research that has been conducted with each host to identify pathogen virulence factors, study the immune response, and evaluate potential antimicrobial compounds, focusing principally on fungal pathogens.

Drosophila melanogaster as a model organism for invasive aspergillosis

Mammalian hosts have traditionally been considered the "gold standard" models for studying pathogenesis and antifungal drug activity in invasive aspergillosis (IA). Nevertheless, logistical, economical, and ethical constraints make these host systems difficult to use for high-throughput screening of putative Aspergillus virulence factors and novel antifungal compounds. Here, we present Drosophila melanogaster, a heterologous non-vertebrate host with conserved innate immunity and genetic tractability, as an alternative, easy-to-use, and inexpensive pathosystem for studying Aspergillus pathogenesis and antifungal activity. We describe three different infection protocols (i.e., injection, rolling, ingestion) that introduce Aspergillus conidia at different anatomical sites of Toll-deficient Drosophila flies. These reproducible assays can be used to (1) determine the virulence of various Aspergillus strains and to (2) assess the anti-Aspergillus activity of orally absorbed antifungal agents in vivo. These methods can also be adapted to study pathogenesis and antifungal drug activity against other medically important human fungal pathogens.

Hosting infection: experimental models to assay Candida virulence

Although normally commensals in humans, Candida albicans, Candida tropicalis, Candida parapsilosis, Candida glabrata, and Candida krusei are capable of causing opportunistic infections in individuals with altered physiological and/or immunological responses. These fungal species are linked with a variety of infections, including oral, vaginal, gastrointestinal, and systemic infections, with C. albicans the major cause of infection. To assess the ability of different Candida species and strains to cause infection and disease requires the use of experimental infection models. This paper discusses the mucosal and systemic models of infection available to assay Candida virulence and gives examples of some of the knowledge that has been gained to date from these models.

Wild-type Drosophila melanogaster as a model host to analyze nitrogen source dependent virulence of Candida albicans

The fungal pathogen Candida albicans is a common cause of opportunistic infections in humans. We report that wild-type Drosophila melanogaster (OrR) flies are susceptible to virulent C. albicans infections and have established experimental conditions that enable OrR flies to serve as model hosts for studying C. albicans virulence. After injection into the thorax, wild-type C. albicans cells disseminate and invade tissues throughout the fly, leading to lethality. Similar to results obtained monitoring systemic infections in mice, well-characterized cph1Δ efg1Δ and csh3Δ fungal mutants exhibit attenuated virulence in flies. Using the OrR fly host model, we assessed the virulence of C. albicans strains individually lacking functional components of the SPS sensing pathway. In response to extracellular amino acids, the plasma membrane localized SPS-sensor (Ssy1, Ptr3, and Ssy5) activates two transcription factors (Stp1 and Stp2) to differentially control two distinct modes of nitrogen acquisition (host protein catabolism and amino acid uptake, respectively). Our results indicate that a functional SPS-sensor and Stp1 controlled genes required for host protein catabolism and utilization, including the major secreted aspartyl protease SAP2, are required to establish virulent infections. By contrast, Stp2, which activates genes required for amino acid uptake, is dispensable for virulence. These results indicate that nutrient availability within infected hosts directly influences C. albicans virulence.

Drosophila and Galleria insect model hosts: new tools for the study of fungal virulence, pharmacology and immunology

Over recent years we have witnessed the emergence of several non-vertebrate mini-hosts as alternative pathosystems for the study of fungal disease. These heterologous organisms have unique advantages, as they are economical, ethically expedient, and facile to use. Hence, they are amenable to high-throughput screening studies of fungal genomes for identification of novel virulence genes and of chemical libraries for discovery of new antifungal compounds. In addition, because they have evolutionarily conserved immunity they offer the opportunity to better understand innate immune responses against medically important fungi. In this review, we discuss how the insects Drosophila melanogaster and Galleria mellonella can be employed for the study of various facets of host-fungal interactions as complementary hosts to conventional vertebrate animal models.

Pathogen and host factors are needed to provoke a systemic host response to gastrointestinal infection of Drosophila larvae by Candida albicans

Candida albicans systemic dissemination in immunocompromised patients is thought to develop from initial gastrointestinal (GI) colonisation. It is unclear what components of the innate immune system are necessary for preventing C. albicans dissemination from the GI tract, but studies in mice have indicated that both neutropenia and GI mucosal damage are crucial for allowing widespread invasive C. albicans disease. Mouse models, however, provide limited applicability to genome-wide screens for pathogen or host factors - factors that might influence systemic dissemination following GI colonisation. For this reason we developed a Drosophila model to study intestinal infection by Candida. We found that commensal flora aided host survival following GI infection. Candida provoked extensive JNK-mediated death of gut cells and induced antimicrobial peptide expression in the fat body. From the side of the host, nitric oxide and blood cells influenced systemic antimicrobial responses. The secretion of SAP4 and SAP6 (secreted aspartyl proteases) from Candida was also essential for activating systemic Toll-dependent immunity.

Physical assessment of toxicology at nanoscale: nano dose-metrics and toxicity factor

In this work, we propose a systematic and reproducible evaluation of nanoparticles (NPs) toxicology in living systems, based on a physical assessment and quantification of the toxic effects of NPs by the experimental determination of the key parameter affecting the toxicity outcome (i.e., the number of NPs) and of the NPs "toxicity factor". Such a strategy was applied to a well determined scenario, i.e., the ingestion of citrate-capped gold NPs (AuNPs) of different sizes by the model system Drosophila melanogaster. Using these AuNPs as a reference toxicity standard, we were able to define different regions in the multiparametric space of toxicity, enabling the classification of the toxic levels of other nanomaterials, such as quantum dots and pegylated AuNPs. This approach may pave the way to a systematic classification of nanomaterials, leading to important developments in risk assessment and regulatory approval, as well as in a wide range of nanomedicine applications.

Pathogenesis of Candida albicans infections in the alternative chorio-allantoic membrane chicken embryo model resembles systemic murine infections

Alternative models of microbial infections are increasingly used to screen virulence determinants of pathogens. In this study, we investigated the pathogenesis of Candida albicans and C. glabrata infections in chicken embryos infected via the chorio-allantoic membrane (CAM) and analyzed the virulence of deletion mutants. The developing immune system of the host significantly influenced susceptibility: With increasing age, embryos became more resistant and mounted a more balanced immune response, characterized by lower induction of proinflammatory cytokines and increased transcription of regulatory cytokines, suggesting that immunopathology contributes to pathogenesis. While many aspects of the chicken embryo response resembled murine infections, we also observed significant differences: In contrast to systemic infections in mice, IL-10 had a beneficial effect in chicken embryos. IL-22 and IL-17A were only upregulated after the peak mortality in the chicken embryo model occurred; thus, the role of the Th17 response in this model remains unclear. Abscess formation occurs frequently in murine models, whereas the avian response was dominated by granuloma formation. Pathogenicity of the majority of 15 tested C. albicans deletion strains was comparable to the virulence in mouse models and reduced virulence was associated with significantly lower transcription of proinflammatory cytokines. However, fungal burden did not correlate with virulence and for few mutants like bcr1Δ and tec1Δ different outcomes in survival compared to murine infections were observed. C. albicans strains locked in the yeast stage disseminated significantly more often from the CAM into the embryo, supporting the hypothesis that the yeast morphology is responsible for dissemination in systemic infections. These data suggest that the pathogenesis of C. albicans infections in the chicken embryo model resembles systemic murine infections but also differs in some aspects. Despite its limitations, it presents a useful alternative tool to pre-screen C. albicans strains to select strains for subsequent testing in murine models.

Wild-type Drosophila melanogaster as an alternative model system for investigating the pathogenicity of Candida albicans

Candida spp. are opportunistic pathogens in humans, and their systemic infections display upwards of 30% mortality in immunocompromised patients. Current mammalian model systems have certain disadvantages in that obtaining results is time consuming owing to the relatively long life spans and these results have low statistical resolution because sample sizes are usually small. We have therefore evaluated the potential of Drosophila melanogaster as an additional model system with which to dissect the host-pathogen interactions that occur during Candida albicans systemic infection. To do this, we monitored the survival of wild-type flies infected with various C. albicans clinical isolates that were previously ranked for murine virulence. From our lifetime data we computed two metrics of virulence for each isolate. These correlated significantly with murine survival, and were also used to group the isolates, and this grouping made relevant predictions regarding their murine virulence. Notably, differences in virulence were not predictably resolvable using immune-deficient spz^−/− flies, suggesting that Toll signalling might actually be required to predictably differentiate virulence. Our analysis reveals wild-type D. melanogaster as a sensitive and relevant model system; one that offers immense genetic tractability (having an extensive RNA interference library that enables tissue-specific gene silencing), and that is easy to manipulate and culture. Undoubtedly, it will prove to be a valuable addition to the model systems currently used to study C. albicans infection.

The growing promise of Toll-deficient Drosophila melanogaster as a model for studying Aspergillus pathogenesis and treatment

Despite considerable progress over recent years, the prognosis of invasive aspergillosis (IA) remains unfavorable, reflecting an incomplete understanding of Aspergillus pathogenesis and suboptimal antifungal efficacy in vivo. Mammalian host systems including rodents and rabbits are important tools in elucidating antifungal drug activity and the immunopathogenesis of IA. Nonetheless, they are hampered by limitations that impose a "bottleneck" in mass screening of novel antifungal compounds and putative Aspergillus virulence factors including their cost, labor intensity and ethical constraints. Drosophila melanogaster is an invertebrate host with a long tract record of genetic studies and a simple, yet highly conserved innate immune system. Herein, we describe our experience using this fly model as a facile, non-laborious, inexpensive pathosystem for high-throughput screening of novel antifungal compounds and putative Aspergillus mutants, and studying antifungal innate immunity. We present three infection protocols (i.e., injection, rolling, ingestion) that introduce Aspergillus either directly into the hemolymph or at different epithelial surfaces of Toll-deficient Drosophila flies. As a proof of principle, we demonstrate attenuated virulence of known hypovirulent Aspergillus strains and protection of Aspergillus-infected flies given oral Aspergillus-active agents such is voriconazole. These protocols can be adapted for similar studies of other fungal pathogens. Crossing and generation of Toll-deficient Drosophila flies takes 3 weeks; Aspergillus conidial preparation takes 3 days; fly inoculation depending on the infection assay takes 1 to 6-8 hours; and assessment of fly survival, Aspergillus strain virulence, Drosophila innate host parameters and/or drug activity takes 4-8 days.

Porphyromonas gingivalis virulence in a Drosophila melanogaster model

Porphyromonas gingivalis has been implicated in the etiology of adult periodontitis. In this study, we examined the viability of Drosophila melanogaster as a new model for examining P. gingivalis-host interactions. P. gingivalis (W83) infection of Drosophila resulted in a systemic infection that killed in a dose-dependent manner. Differences in the virulence of several clinically prevalent P. gingivalis strains were observed in the Drosophila killing model, and the results correlated well with studies in mammalian infection models and human epidemiologic studies. P. gingivalis pathobiology in Drosophila did not result from uncontrolled growth of the bacterium in the Drosophila hemolymph (blood) or overt damage to Drosophila tissues. P. gingivalis killing of Drosophila was multifactorial, involving several bacterial factors that are also involved in virulence in mammals. The results from this study suggest that many aspects of P. gingivalis pathogenesis in mammals are conserved in Drosophila, and thus the Drosophila killing model should be useful for characterizing P. gingivalis-host interactions and, potentially, polymicrobe-host interactions.

Zebrafish as a model host for Candida albicans infection

In this work, the zebrafish model organism was developed to obtain a minivertebrate host system for a Candida albicans infection study. We demonstrated that C. albicans can colonize and invade zebrafish at multiple anatomical sites and kill the fish in a dose-dependent manner. Inside zebrafish, we monitored the progression of the C. albicans yeast-to-hypha transition by tracking morphogenesis, and we monitored the corresponding gene expression of the pathogen and the early host immune response. We performed a zebrafish survival assay with different C. albicans strains (SC5314, ATCC 10231, an hgc1 mutant, and a cph1/efg1 double mutant) to determine each strain's virulence, and the results were similar to findings reported in previous mouse model studies. Finally, using zebrafish embryos, we monitored C. albicans infection and visualized the interaction between pathogen and host myelomonocytic cells in vivo. Taken together, the results of this work demonstrate that zebrafish can be a useful host model to study C. albicans pathogenesis, and they highlight the advantages of using the zebrafish model in future invasive fungal research.

Toll-like receptors involved in the pathogenesis of experimental Candida albicans keratitis

Purpose. To investigate the expression and function of toll-like receptors (TLRs) during experimental keratomycosis. Methods. Scarified corneas of BALB/c mice were topically inoculated with Candida albicans and compared with control corneas by a murine gene microarray and immunostaining. Real-time reverse transcription polymerase chain reaction (RT-PCR) determined relative TLR gene expression in murine and human donor corneas. The scarified corneas of TLR2(-/-) mice, TLR4(-/-) mice, and C57BL/6J control mice were also inoculated with C. albicans, to determine relative severity, fungal load, and cytokine transcript levels. Results. TLR1, -2, -4, -6, and -13 were significantly upregulated (5- to 10-fold; P < 0.01) by microarray, and TLR1, -2, -4, and -13 were significantly increased (4- to 11-fold; P < 0.05) by real-time RT-PCR in BALB/c murine corneas. Similarly, TLR2, -6, and -13 were significantly upregulated (5- to 16-fold; P < or = 0.001) by real-time RT-PCR in C57BL/6J murine corneas the day after inoculation, and TLR2 and -13 remained significantly (P < 0.05) increased after 1 week. TLR2 transcript was also upregulated twofold (P = 0.04) in C. albicans-inoculated explanted human corneas. Although murine keratitis severity scores were similar, significantly more fungi were recovered from TLR2(-/-) mouse corneas (P = 0.04) than from TLR4(-/-) mouse corneas (P = 0.9). Tumor necrosis factor-alpha, interleukin 23, chemokine C-C ligands 3 and 4, and dectin-1 were significantly (P < 0.05) downregulated in C. albicans-infected corneas of TLR2(-/-) mice. Conclusions. TLR2 signals proinflammatory cytokines that control fungal growth during C. albicans keratitis. TLR13 may have an additional role in the innate immune response of murine corneal candidiasis.

Candida albicans Cas5, a regulator of cell wall integrity, is required for virulence in murine and toll mutant fly models

Candida albicans is the most common human fungal pathogen, yet the pathogenesis of C. albicans infection remains incompletely understood. We hypothesized that C. albicans has developed evolutionarily conserved mechanisms to invade disparate hosts and tested whether Toll mutant flies could serve as a model host for high-throughput screening of C. albicans virulence genes. We screened 34 C. albicans mutants defective in putative transcription factor genes (see http://www.tigr.org/tigr-scripts/e2k1/qzhao/page.cgi?num=1 ) by means of a previously established model of invasive candidiasis in Toll mutant flies. C. albicans mutants that displayed attenuated virulence in flies were subsequently tested for virulence in a mouse model of hematogenous candidiasis. Of the 34 C. albicans mutants tested, only the prototrophic cas5Delta/Delta mutant (strain VIC1186) exhibited attenuated virulence in Toll mutant flies that was restored in the complemented strain (VIC1190). Similarly, BALB/c mice infected intravenously with the cas5Delta/Delta mutant had significantly better survival and a lower fungal burden in kidneys and spleen than did those infected with the isogenic wild-type strain DAY185. CAS5 encodes a key transcriptional regulator of genes involved in cell wall integrity and lacks an orthologue in Saccharomyces cerevisiae. Our findings support the notion that Drosophila melanogaster is a promising model for large-scale studies of genes involved in the pathogenesis of C. albicans infection in mammals.

Drosophila melanogaster as a model host for studying Pseudomonas aeruginosa infection

Conservation of host signaling pathways and tissue physiology between Drosophila melanogaster and mammals allows for the modeling of human host-pathogen interactions in Drosophila. Here we present the use of genetically tractable Drosophila models of bacterial pathogenesis to study infection with the human opportunistic pathogen Pseudomonas aeruginosa. We describe and compare two protocols commonly used to infect Drosophila with P. aeruginosa: needle-pricking and injector-pumping. Each model has relevance for examining host components and bacterial factors in host defense and virulence. Fly survival and bacterial proliferation within host flies can be assessed as a measure of host susceptibility and pathogen virulence potential. The profiles of host responses toward P. aeruginosa virulent and non-virulent strains can be determined, enabling the identification of interaction-specific genes that could potentially favor or limit the initiation and progression of infection. Both of the protocols presented herein may be adapted for the inoculation and study of other microbial pathogens. P. aeruginosa cell preparation requires 24 h, fly inoculation 1 h, and fly survival and bacterial proliferation 1-4 d.

Phagocytosis of Candida albicans by RNAi-treated Drosophila S2 cells

Phagocytosis is a highly conserved aspect of innate immunity. Drosophila melanogaster has an innate immune system with many similarities to that of mammals and has been used to successfully model many aspects of innate immunity. The recent availability of Ribo Nucleic Acid interference (RNAi) libraries for Drosophila has made it possible to efficiently screen for genes important in aspects of innate immunity. We have screened an RNAi library representing 7216 fly genes conserved among metazoans to identify proteins required for the phagocytosis of the human fungal pathogen Candida albicans.

Pathogenesis of listeria-infected Drosophila wntD mutants is associated with elevated levels of the novel immunity gene edin

Drosophila melanogaster mount an effective innate immune response against invading microorganisms, but can eventually succumb to persistent pathogenic infections. Understanding of this pathogenesis is limited, but it appears that host factors, induced by microbes, can have a direct cost to the host organism. Mutations in wntD cause susceptibility to Listeria monocytogenes infection, apparently through the derepression of Toll-Dorsal target genes, some of which are deleterious to survival. Here, we use gene expression profiling to identify genes that may mediate the observed susceptibility of wntD mutants to lethal infection. These genes include the TNF family member eiger and the novel immunity gene edin (elevated during infection; synonym CG32185), both of which are more strongly induced by infection of wntD mutants compared to controls. edin is also expressed more highly during infection of wild-type flies with wild-type Salmonella typhimurium than with a less pathogenic mutant strain, and its expression is regulated in part by the Imd pathway. Furthermore, overexpression of edin can induce age-dependent lethality, while loss of function in edin renders flies more susceptible to Listeria infection. These results are consistent with a model in which the regulation of host factors, including edin, must be tightly controlled to avoid the detrimental consequences of having too much or too little activity.

Like any organism, fruit flies respond to invading microorganisms by mounting an immune defense. Many aspects of the immune defense in fruit flies are similar to the inflammatory response in mammals, including the harmful effects of a sustained response against persistent pathogenic infections. We found in the past that mutations in the gene wntD cause flies to succumb more easily to Listeria monocytogenes infections, apparently by losing an element of control over the inflammatory response. How does the wntD gene work? In this paper, we have identified genes that may mediate the susceptibility of wntD mutants to lethal infection. These genes include eiger, a homolog of the mammalian TNF gene, and a previously uncharacterized gene called edin (elevated during infection). Edin is expressed excessively in wntD mutant flies, and its expression also correlates with the level of pathogenesis induced by two different strains of Salmonella typhimurium. In its own right, overexpression of the edin gene can induce lethality, while losing edin function renders flies more susceptible to Listeria infection. Our results support a model in which the regulation of host factors, including edin, must be tightly controlled to avoid the detrimental consequences of having too much or too little activity.

Analysis of PRA1 and its relationship to Candida albicans- macrophage interactions

Phagocytosis of Candida albicans by either primary bone marrow-derived mouse macrophages or RAW 264.7 cells upregulated transcription of PRA1, which encodes a cell wall/membrane-associated antigen previously described as a fibrinogen binding protein. However, a pra1 null mutant was still able to bind fibrinogen, showing that Pra1p is not uniquely required for fibrinogen binding. As well, Pra1 tagged with green fluorescent protein did not colocalize with AlexaFluor 546-labeled human fibrinogen, and while PRA1 expression was inhibited when Candida was grown in fetal bovine serum-containing medium, Candida binding to fibrinogen was activated by these conditions. Therefore, it appears that Pra1p can play at most a minor role in fibrinogen binding to C. albicans. PRA1 gene expression is induced in vitro by alkaline pH, and therefore its activation in phagosomes suggested that phagosome maturation was suppressed by the presence of Candida cells. LysoTracker red-labeled organelles failed to fuse with phagosomes containing live Candida, while phagosomes containing dead Candida underwent a normal phagosome-to-phagolysosome maturation. Immunofluorescence staining with the early/recycling endosomal marker transferrin receptor (CD71) suggested that live Candida may escape macrophage destruction through the inhibition of phagolysosomal maturation.

Tetracycline alters drug susceptibility in Candida albicans and other pathogenic fungi

The tetracycline (TET) promoter has been used in several systems as an inducible regulator of gene expression. In control analyses, the standard Candida albicans laboratory strain SC5314 was found to have altered susceptibility to a variety of antifungal drugs in the presence of relatively high concentrations (50-200 microg ml(-1)) of TET. Altered susceptibility was most notable with exposure to amphotericin B (AMB), with a 32-fold increase in susceptibility, and terbinafine (TRB), with a 32-fold decrease in susceptibility. The TET/AMB synergy was observed in several clinical isolates of C. albicans and in the distantly related species Aspergillus fumigatus and Cryptococcus neoformans. The TET/AMB synergy is not related to efflux pump activity, as determined by FACS analyses and by analysis of a strain containing efflux pump deletions. Gene expression analyses by luciferase and by quantitative real-time reverse transcriptase PCR failed to identify significant alterations in expression of any genes associated with resistance. C. albicans grown with TET for 48 h does show a reduction in total cellular ergosterol. Analysis of growth curves suggests that the TET effect is associated with lack of a diauxic shift, which is related to a loss of mitochondrial function. MitoTracker fluorescent dye was used to demonstrate that TET has a direct effect on mitochondrial function. These results demonstrate the need for careful analysis of TET effects when using a TET-inducible promoter, especially in studies that involve antifungal drugs. This study defines some limits to the use of the TET-inducible promoter, and identifies effects on cells that are the result of TET exposure alone, not the result of expression of a targeted gene.

Innate immunity in Drosophila: Pathogens and pathways

Following in the footsteps of traditional developmental genetics, research over the last 15 years has shown that innate immunity against bacteria and fungi is governed largely by two NF-kappaB signal transduction pathways, Toll and IMD. Antiviral immunity appears to stem from RNA interference, whereas resistance against parasitoids is conferred by Toll signaling. The identification of these post-transcriptional regulatory mechanisms and the annotation of most Drosophila immunity genes have derived from functional genomic studies using "model" pathogens, intact animals and cell lines. The D. melanogaster host has thus provided the core information that can be used to study responses to natural microbial and metazoan pathogens as they become identified, as well as to test ideas of selection and evolutionary change. These analyses are of general importance to understanding mechanisms of other insect host-pathogen interactions and determinants of variation in host resistance.

Drosophila innate immunity and response to fungal infections

The fruit fly Drosophila melanogaster is an important model for the analysis of the interaction between host immune systems and fungal pathogens. Recent experiments have extended our understanding of the Toll-based signalling pathway critical to response to fungal infections, and identified new elements involved in cellular and humoral-based defences. The fly immune system shows remarkable sophistication in its ability to discriminate among pathogens, and the powerful genetics available to researchers studying the adult fly response, and the ability to manipulate cultured phagocytic cell lines with RNAi, are allowing researchers to dissect the molecular details of the process.

Dissecting innate immunity by germline mutagenesis

The innate arm of our immune system is the first line of defence against infections. In addition, it is believed to drive adaptive immune responses, which help fight pathogens and provide long-term memory. As such, the innate immune system is instrumental for protection against pathogens that would otherwise destroy their host. Although our understanding of the innate immune components involved in pathogen sensing and fighting is improving, it is still limited. This is particularly exemplified by increased documentation of innate immune deficiencies in humans that often result in high and recurrent susceptibility to infections or even death, without the genetic cause being evident. To provide further insight into the mechanisms by which pathogen sensing and eradication occur, several strategies can be used. The current review focuses on the forward genetic approaches that have been used to dissect innate immunity in the fruit fly and the mouse. For both animal models, forward genetics has been instrumental in the deciphering of innate immunity and has greatly improved our understanding of how we respond to invading pathogens.

Animal models: an important tool in mycology

Animal models of fungal infections are, and will remain, a key tool in the advancement of the medical mycology. Many different types of animal models of fungal infection have been developed, with murine models the most frequently used, for studies of pathogenesis, virulence, immunology, diagnosis, and therapy. The ability to control numerous variables in performing the model allows us to mimic human disease states and quantitatively monitor the course of the disease. However, no single model can answer all questions and different animal species or different routes of infection can show somewhat different results. Thus, the choice of which animal model to use must be made carefully, addressing issues of the type of human disease to mimic, the parameters to follow and collection of the appropriate data to answer those questions being asked. This review addresses a variety of uses for animal models in medical mycology. It focuses on the most clinically important diseases affecting humans and cites various examples of the different types of studies that have been performed. Overall, animal models of fungal infection will continue to be valuable tools in addressing questions concerning fungal infections and contribute to our deeper understanding of how these infections occur, progress and can be controlled and eliminated.

Exploiting amoeboid and non-vertebrate animal model systems to study the virulence of human pathogenic fungi

Experiments with insects, protozoa, nematodes, and slime molds have recently come to the forefront in the study of host–fungal interactions. Many of the virulence factors required for pathogenicity in mammals are also important for fungal survival during interactions with non-vertebrate hosts, suggesting that fungal virulence may have evolved, and been maintained, as a countermeasure to environmental predation by amoebae and nematodes and other small non-vertebrates that feed on microorganisms. Host innate immune responses are also broadly conserved across many phyla. The study of the interaction between invertebrate model hosts and pathogenic fungi therefore provides insights into the mechanisms underlying pathogen virulence and host immunity, and complements the use of mammalian models by enabling whole-animal high throughput infection assays. This review aims to assist researchers in identifying appropriate invertebrate systems for the study of particular aspects of fungal pathogenesis.

The host defense of Drosophila melanogaster

To combat infection, the fruit fly Drosophila melanogaster relies on multiple innate defense reactions, many of which are shared with higher organisms. These reactions include the use of physical barriers together with local and systemic immune responses. First, epithelia, such as those beneath the cuticle, in the alimentary tract, and in tracheae, act both as a physical barrier and local defense against pathogens by producing antimicrobial peptides and reactive oxygen species. Second, specialized hemocytes participate in phagocytosis and encapsulation of foreign intruders in the hemolymph. Finally, the fat body, a functional equivalent of the mammalian liver, produces humoral response molecules including antimicrobial peptides. Here we review our current knowledge of the molecular mechanisms underlying Drosophila defense reactions together with strategies evolved by pathogens to evade them.

Drosophila melanogaster Thor and response to Candida albicans infection

We used Drosophila melanogaster macrophage-like Schneider 2 (S2) cells as a model to study cell-mediated innate immunity against infection by the opportunistic fungal pathogen Candida albicans. Transcriptional profiling of S2 cells coincubated with C. albicans cells revealed up-regulation of several genes. One of the most highly up-regulated genes during this interaction is the D. melanogaster translational regulator 4E-BP encoded by the Thor gene. Analysis of Drosophila 4E-BP(null) mutant survival upon infection with C. albicans showed that 4E-BP plays an important role in host defense, suggesting a role for translational control in the D. melanogaster response to C. albicans infection.

Dual detection of fungal infections in Drosophila via recognition of glucans and sensing of virulence factors

The Drosophila immune system discriminates between various types of infections and activates appropriate signal transduction pathways to combat the invading microorganisms. The Toll pathway is required for the host response against fungal and most Gram-positive bacterial infections. The sensing of Gram-positive bacteria is mediated by the pattern recognition receptors PGRP-SA and GNBP1 that cooperate to detect the presence of infections in the host. Here, we report that GNBP3 is a pattern recognition receptor that is required for the detection of fungal cell wall components. Strikingly, we find that there is a second, parallel pathway acting jointly with GNBP3. The Drosophila Persephone protease activates the Toll pathway when proteolytically matured by the secreted fungal virulence factor PR1. Thus, the detection of fungal infections in Drosophila relies both on the recognition of invariant microbial patterns and on monitoring the effects of virulence factors on the host.

Role of mini-host models in the study of medically important fungi

Mini-host models have emerged as simple experimental systems to study the pathogenesis and host innate immune responses in fungal invaders and also to test drug efficacy against these organisms. A growing number of medically important fungi, including Aspergillus spp, Candida spp, Cryptococcus spp, and species in the class Zygomycetes, have been shown to infect and kill invertebrates such as roundworms, fruit flies, and wax moths. These studies have shown that several genes implicated in the virulence of fungi in mammalian models also have a similarly important pathogenic role in mini-host organisms. These mini-host models provide a unique opportunity of simultaneously exploring the molecular mechanisms of fungal pathogenicity and candidate agents with antifungal activity. Furthermore, the fact that some of these mini-hosts have well-defined genetics and conserved innate immunity offers the advantage of a comprehensive analysis of the molecular aspects of host immune response. We examine the relevance, advantages, and pitfalls of experimental systems of fungal infections in various mini-hosts and compare them with what is known in experimental systems in mammalian animal models.

Using non-mammalian hosts to study fungal virulence and host defense

Non-mammalian hosts have been used to study host-fungal interactions. Hosts such as Drosophila melanogaster, Caenorhabditis elegans, Acathamoeba castellanii, Dictyostelium discoideum, and Galleria mellonella have provided means to examine the physical barriers, cellular mechanisms and molecular elements of the host response. The Drosophila host-response to fungi is mediated through the Toll pathway, whereas in C. elegans the host-response is TIR-1-dependent. Virulence traits that are involved in mammalian infection are important for the interaction of fungi with these hosts. Screening of fungal virulence traits using mutagenized fungi to determine changes in fungal infectivity of non-mammalian hosts has been used to identify novel virulence proteins used to infect C. elegans such as Kin1 (a serine/threonine protein kinase) and Rom2 (a Rho1 guanyl-nucleotide exchange factor) from Cryptococcus neoformans. These heterologous non-mammalian hosts highlight the similarities and differences between different hosts in fungal pathogenesis and they complement studies in mammalian systems and those using other genetic approaches.

The pursuit of cryptococcal pathogenesis: heterologous hosts and the study of cryptococcal host–pathogen interactions

Analysis of the molecular mechanisms by which a pathogen interacts with the human host is most commonly performed using a mammalian model of infection. However, several virulence-related genes previously shown to be involved in mammalian infection with Cryptococcus neoformans have also been shown to play a role in the interaction of these pathogens with invertebrates, such as Acanthamoeba castellanii, Caenorhabditis elegans, Dictyostelium discoideum, Drosophila melanogaster and Galleria mellonella. The study of host-pathogen interactions using these model hosts has allowed rapid screening of mutant libraries and can be used for the study of evolutionarily preserved aspects of microbial virulence and host response.

Identification of Drosophila gene products required for phagocytosis of Candida albicans

Phagocytosis is a highly conserved aspect of innate immunity. We used Drosophila melanogaster S2 cells as a model system to study the phagocytosis of Candida albicans, the major fungal pathogen of humans, by screening an RNAi library representing 7,216 fly genes conserved among metazoans. After rescreening the initial genes identified and eliminating certain classes of housekeeping genes, we identified 184 genes required for efficient phagocytosis of C. albicans. Diverse biological processes are represented, with actin cytoskeleton regulation, vesicle transport, signaling, and transcriptional regulation being prominent. Secondary screens using Escherichia coli and latex beads revealed several genes specific for C. albicans phagocytosis. Characterization of one of those gene products, Macroglobulin complement related (Mcr), shows that it is secreted, that it binds specifically to the surface of C. albicans, and that it promotes its subsequent phagocytosis. Mcr is closely related to the four Drosophila thioester proteins (Teps), and we show that TepII is required for efficient phagocytosis of E. coli (but not C. albicans or Staphylococcus aureus) and that TepIII is required for the efficient phagocytosis of S. aureus (but not C. albicans or E. coli). Thus, this family of fly proteins distinguishes different pathogens for subsequent phagocytosis.

Mcr and the closely related Drosophila Tep proteins (proteins similar to mammalian secreted immune complement) bind to the surface of invading microbes and are required to promote the phagocytosis of specific pathogens.

Paralysis and killing of Caenorhabditis elegans by enteropathogenic Escherichia coli requires the bacterial tryptophanase gene

Pathogenic Escherichia coli, including enteropathogenic E. coli (EPEC), enterohaemorrhagic E. coli (EHEC), enteroinvasive E. coli (EIEC) and enterotoxigenic E. coli (ETEC) are major causes of food and water-borne disease. We have developed a genetically tractable model of pathogenic E. coli virulence based on our observation that these bacteria paralyse and kill the nematode Caenorhabditis elegans. Paralysis and killing of C. elegans by EPEC did not require direct contact, suggesting that a secreted toxin mediates the effect. Virulence against C. elegans required tryptophan and bacterial tryptophanase, the enzyme catalysing the production of indole and other molecules from tryptophan. Thus, lack of tryptophan in growth media or deletion of tryptophanase gene failed to paralyse or kill C. elegans. While known tryptophan metabolites failed to complement an EPEC tryptophanase mutant when presented extracellularly, complementation was achieved with the enzyme itself expressed either within the pathogen or within a cocultured K12 strains. Thus, an unknown metabolite of tryptophanase, derived from EPEC or from commensal non-pathogenic strains, appears to directly or indirectly regulate toxin production within EPEC. EPEC strains containing mutations in the locus of enterocyte effacement (LEE), a pathogenicity island required for virulence in humans, also displayed attenuated capacity to paralyse and kill nematodes. Furthermore, tryptophanase activity was required for full activation of the LEE1 promoter, and for efficient formation of actin-filled membranous protrusions (attaching and effacing lesions) that form on the surface of mammalian epithelial cells following attachment and which depends on LEE genes. Finally, several C. elegans genes, including hif-1 and egl-9, rendered C. elegans less susceptible to EPEC when mutated, suggesting their involvement in mediating toxin effects. Other genes including sek-1, mek-1, mev-1, pgp-1,3 and vhl-1, rendered C. elegans more susceptible to EPEC effects when mutated, suggesting their involvement in protecting the worms. Moreover we have found that C. elegans genes controlling lifespan (daf-2, age-1 and daf-16), also mediate susceptibility to EPEC. Together, these data suggest that this C. elegans/EPEC system will be valuable in elucidating novel factors relevant to human disease that regulate virulence in the pathogen or susceptibility to infection in the host.

Antifungal coating by biofunctionalized polyelectrolyte multilayered films

The surface of medical devices is a common site of bacterial and fungal adhesion, first step to the constitution of a resistant biofilm leading frequently to chronic infections. In order to prevent such complications, several physical and chemical modifications of the device surface have been proposed. Here, we experiment a new type of topical antifungal coating using the layer-by-layer technique. The nanometric multilayer film obtained by this technique is functionalized by the insertion of a chromogranin A-derived antifungal peptide (CGA 47-66, chromofungin). We show that the embedded peptide keeps its antifungal activity by interacting with the fungal membrane and penetrating into the cell. In vitro studies demonstrate that such an antifungal coating is able to inhibit the growth of yeast Candida albicans by 65% and completely stop the proliferation of filamentous fungus Neurospora crassa. The cytotoxicity of such a coating was also assessed by growing human gingival fibroblasts at its surface. Finally, the antifungal coating of poly(methylmethacrylate), a widely used material for biomedical devices, is successfully tested in an in vivo oral candidiasis rat model. Taken together, these results assessed the functionalized multilayer films containing a new potent antifungal non-toxic peptide, as a novel and promising technique for local antifungal protection.

Fruit flies as a minihost model for studying drug activity and virulence in Aspergillus

Invasive aspergillosis (IA) is a significant cause of morbidity and mortality in profoundly immunosuppressed patients. The mediocre efficacy of antifungals for IA in clinical practice and an incomplete understanding of the pathogenesis of IA contribute to its overall poor prognosis. Although logistically difficult for large scale use, conventional animal models of IA provide valuable information regarding both antifungal drug efficacy and Aspergillus mutant virulence. However, in the era of introduction of molecular biology techniques for studying Aspergillus and increasing antifungal options, the existing in vivo models of IA might be well complemented by nonvertebrate minihost models such as the Drosophila melanogaster (fruit fly). Drosophila may offer the distinct advantage of performing fast, inexpensive high-throughput screening of compounds for anti-Aspergillus activity and putative Aspergillus mutants for their role in Aspergillus virulence.

Toll-like receptors as key mediators in innate antifungal immunity

The Toll protein of Drosophila is a transmembrane receptor involved in dorsoventral polarization during embryonic development and recognition of infection. In mammals, Toll-like receptors (TLRs) constitute a novel protein family involved in innate immunity and respond to a wide spectrum of microorganisms, including fungi, bacteria, viruses, and protozoa. Specific agonists for nine of the ten members of the human TLR family have been described to date. TLRs as well as the TLR-associated adaptor molecule MyD88 have been implicated in the recognition of the fungal pathogens Candida albicans, Aspergillus fumigatus, Cryptococcus neoformans and Pneumocystis carinii. Moreover, several pathogen associated molecular patterns (PAMPs) located in the cell wall or cell surface of fungi have been identified as potential ligands. Yeast zymosan activates TLR2/ TLR6 heterodimers, whereas Saccharomyces cerevisiae- and C. albicans-derived mannan seems to be detected by TLR4. Phospholipomannan, present in the cell surface of C. albicans has been shown to be recognized by TLR2, while TLR4 mainly interacts with glucuronoxylomannan, the major capsular polysaccharide of C. neoformans. MyD88 has been implicated in TLR signalling of linear (1 --> 3)-beta-D-glucan, and of beta-glucan from P. carinii. These data point towards the ability of the innate immune system to utilize TLRs that are specific to different types and components of pathogenic fungi. Recent evidence further suggests that TLRs cooperate with other immune receptors involved in fungal recognition and that the selective induction of adaptor proteins finally leads to distinct signalling events upon fungal challenge.

Immune recognition of fungal β-glucans

The recognition of conserved microbial structures is a key aspect of metazoan immunity, and beta-glucans are emerging as a major target for the recognition of fungal pathogens. A number of receptors for these carbohydrates have been identified, which upon recognition, trigger a variety of immune responses. In contrast to many other systems, there is little apparent conservation in these mechanisms between vertebrates and invertebrates. In this review, we will highlight all the known receptors for beta-glucans and will discuss the various immune responses they can initiate, with reference to fungal infection, in both vertebrates and invertebrates.

Recognition of fungal pathogens by Toll-like receptors

Toll-like receptors (TLRs) have been identified as a major class of pattern-recognition receptors. Recognition of pathogen-associated molecular patterns by TLRs, either alone or in heterodimerization with other TLR or non-TLR receptors, induces signals responsible for the activation of the innate immune response. Recent studies have demonstrated a crucial involvement of TLRs in the recognition of fungal pathogens such as Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans. Through the study of fungal infection in knock-out mice deficient in either TLRs or TLR-associated adaptor molecules, it became apparent that specific TLRs such as TLR2 and TLR4 play differential roles in the activation of the various arms of the innate immune response. Recent data also suggest that TLRs offer escape mechanisms to certain pathogenic microorganisms, especially through TLR2-driven induction of anti-inflammatory cytokines. These new data have substantially increased our knowledge of the recognition of fungal pathogens, and the study of TLRs remains one of the most active areas of research in the field of fungal infections.