The contribution of animal models of aspergillosis to understanding pathogenesis, therapy and virulence

Pathology and Epidemiology of Fungal Infections in Layer Chicken Flocks

Fungal infections have a key effect on the commercial poultry production and welfare. Infections caused by fungi and their food contaminants are zoonotic and influence food safety. Eggs and cooked meats remain major public health concerns. Therefore, this research is aimed at examining the pathology and understanding the epidemiology of fungal infection in layer chicken flocks. The study was carried out on twenty-layer flocks from Kafrelsheikh Governorate, Egypt, from January 2019 to December 2020. In total, 600 samples were collected from 100 healthy and diseased layer chickens from different organs (skin, liver, lung, kidney, spleen, and ovary). In this work, we present the clinical and pathological characteristics of some fungal pathogens (Aspergillus spp. and Fusarium spp.) in layer chicken flocks, as they are responsible for reducing the egg production. In total, 19 fungal strains were isolated from individual chickens, and these were analysed to determine the fungal species. The total proportion of fungal infections at the farm level was (3/20) 15%. The main clinical signs were emaciation and mortalities that reached $4.34\%\pm 0.84$ . We report the first isolation of Aspergillus piperis and Fusarium species from the ovary of poultry, which is the main reason for egg retention and multiple numerous nodules of occasional caseating centers in layer ovaries. The histopathological findings of Aspergillus infection are indicated by the presence of branched hyphae that tend to be numerous and progressive. Furthermore, we found spherules with multiple endospores of Fusarium spp. in the ovaries. Morphological and molecular identification and analysis were performed to confirm the etiological agents.

Current Models to Study the Sporothrix-Host Interaction

Sporotrichosis is a worldwide distributed subcutaneous mycosis that affects mammals, including human beings. The infection is caused by members of the Sporothrix pathogenic clade, which includes Sporothrix schenckii, Sporothrix brasiliensis, and Sporothrix globosa. The fungus can be acquired through traumatic inoculation of conidia growing in vegetal debris or by zoonotic transmission from sick animals. Although is not considered a life-threatening disease, it is an emergent health problem that affects mostly immunocompromised patients. The sporotrichosis causative agents differ in their virulence, host range, and sensitivity to antifungal drugs; therefore, it is relevant to understand the molecular bases of their pathogenesis, interaction with immune effectors, and mechanisms to acquired resistance to antifungal compounds. Murine models are considered the gold standard to address these questions; however, some alternative hosts offer numerous advantages over mammalian models, such as invertebrates like Galleria mellonella and Tenebrio molitor, or ex vivo models, which are useful tools to approach questions beyond virulence, without the ethical or budgetary features associated with the use of animal models. In this review, we analyze the different models currently used to study the host-Sporothrix interaction.

Interaction in vitro of pulmonary surfactant with antifungal agents used for treatment and prevention of invasive aspergillosis

BACKGROUND

Optimizing antifungal therapy is important to improve outcomes in severely immunocompromised patients.

OBJECTIVES

We analysed the in vitro interaction between pulmonary surfactant and antifungal agents used for management of invasive pulmonary aspergillosis.

METHODS

Amphotericin B formulations, mould-active triazoles and echinocandins were tested in vitro against 24 clinical isolates of different Aspergillus spp. with and without the addition of a commercial porcine surfactant (Curosurf®; Poractant alfa, Nycomed, Austria). The data are presented as MIC or minimum effective concentration (MEC) ranges, as MIC or MEC values that inhibited 90% of the isolates (MIC90 or MEC90) and as geometric mean (GM) MIC or MEC values.

RESULTS

For amphotericin B products, addition of surfactant to a final concentration of 10% led to a statistically significant reduction of the GM MIC for all Aspergillus isolates tested after 24 h (0.765 versus 0.552 mg/L; P < 0.05). For the mould-active triazoles, addition of 10% surfactant resulted in a significantly higher GM MIC at 48 h (0.625 versus 0.898 mg/L; P < 0.05). For the echinocandins, the addition of 10% surfactant led to a significantly higher GM MEC after both 24 h (0.409 versus 0.6532 mg/L; P < 0.01) and 48 h (0.527 versus 0.9378 mg/L; P < 0.01). There were no meaningful differences between individual members of the three existing classes of antifungal agents or between the different Aspergillus spp. tested.

CONCLUSIONS

Using EUCAST methodology, addition of porcine surfactant up to a concentration of 10% had a minor, and presumably non-relevant, impact on the in vitro activity of antifungal agents used in prophylaxis and treatment of invasive pulmonary aspergillosis.

A Murine Model for Chronic A. fumigatus Airway Infections

In addition to causing acute invasive infections in immunocompromised patients, the mold Aspergillus fumigatus causes chronic infections in patients with chronic pulmonary conditions such as cystic fibrosis. Here we describe a non-lethal model of chronic pulmonary aspergillosis in which immunocompetent mice are endotracheally infected with A. fumigatus conidia embedded in agar beads. This approach results in the establishment of hyphal infection within the airways of mice for up to a 28-day period and is amenable to the study of innate and adaptive antifungal responses, fungal mutant strains, and antifungal agents.

Animal models of invasive mycoses

Animal models of invasive fungal infections have been developed and are applied in a huge number of different animal species for a number of research purposes, for example, the study of pathogenesis, defense mechanisms, and therapeutic strategies. From the different models, which in most cases are based on the same fungal species and often the same strain, as in spontaneous human infections, fundamental results and knowledge of the diagnosis, progression, prophylaxis, and therapy have been achieved. However, in all models, one should be critical with respect to mimicking the disease entity of humans, which is often the focus of the research. In many of the models for instance, the time course is different to the one of humans, and in others, the propensity for localization and containment in specific organs does not parallel the situation in humans. Nevertheless, many animal models of invasive mycoses have proven valuable in a number of research areas. With regard to new generations of anti-mycotic drugs, the models play an essential role in demonstrating antifungal activity, as well as in demonstrating the absence of toxic side effects, a critical step which cannot be accomplished by in vitro studies.

Clinical and pathological features of aspergillosis due to Aspergillus fumigatus in broilers

BACKGROUND AND AIM

Aspergillus fumigatus is a ubiquitous pathogen causing aspergillosis in poultry. This research aimed to evaluate the clinical and pathological features of aspergillosis infection in broilers.

MATERIALS AND METHODS

A. fumigatus infection was induced experimentally by intra-air sac inoculation of a 1.7×108 spore suspension into broilers. Infected and non-infected birds were closely observed for the development of clinical signs of infection twice daily. Pathological samples were collected 5, 14, and 30 days post-infection (dpi) and examined by hematoxylin-eosin staining.

RESULTS

A total of 160 birds were included in this study. Clinical signs emerged at 3 dpi and became consistent at 5 dpi. A considerable decrease in severity and number of birds showing infection symptoms followed. The clinical signs of aspergillosis included anorexia (n=40; 50%), lethargy (n=32; 40%), dyspnea (n=38; 48%), and gasping (n=29; 36%). Macroscopic changes in the air sacs at 3 dpi included the development of minor lesions showing cloudiness, slight membrane thickening, and local exudates. Histopathological examination of the air sacs collected at 3 dpi indicated local inflammation surrounded by hyphae and spores. At 5 dpi, infected birds developed nodules, necrosis, and parenchymal consolidation of the lungs. Pulmonary changes, such as bronchopneumonia, spores, septate hyphae, and mild granulomatous inflammation, were also observed. At 14 dpi, multiple caseous nodules and plaques were found in the air sacs; plaque and necrosis in large areas of the lungs and severe multifocal granulomatous inflammation were noted.

CONCLUSION

The clinical symptoms of aspergillosis emerged at 3 dpi and gradually decreased beginning at 7 dpi. Similar pathological changes were observed in the air sacs and lungs. The results of this work provide additional information on the pathogenesis of aspergillosis.

Drosophila melanogaster as a model to study virulence and azole treatment of the emerging pathogen Candida auris

OBJECTIVES

Candida auris is an emerging, often MDR, yeast pathogen. Efficient animal models are needed to study its pathogenicity and treatment. Therefore, we developed a C. auris fruit fly infection model.

METHODS

TollI-RXA/Tollr632 female flies were infected with 10 different C. auris strains from the CDC Antimicrobial Resistance bank panel. We used three clinical Candida albicans strains as controls. For drug protection assays, fly survival was assessed along with measurement of fungal burden (cfu/g tissue) and histopathology in C. auris-infected flies fed with fluconazole- or posaconazole-containing food.

RESULTS

Despite slower in vitro growth, all 10 C. auris isolates caused significantly greater mortality than C. albicans in infected flies, with >80% of C. auris-infected flies dying by day 7 post-infection (versus 67% with C. albicans, P < 0.001-0.005). Comparison of C. auris isolates from different geographical clades revealed more rapid in vitro growth of South American isolates and greater virulence in infected flies, whereas the aggregative capacity of C. auris strains had minimal impact on their growth and pathogenicity. Survival protection and decreased fungal burden of fluconazole- or posaconazole-fed flies infected with two C. auris strains were in line with the isolates' disparate in vitro azole susceptibility. High reproducibility of survival curves for both non-treated and antifungal-treated infected flies was seen, with coefficients of variation of 0.00-0.31 for 7 day mortality.

CONCLUSIONS

Toll-deficient flies could provide a fast, reliable and inexpensive model to study pathogenesis and drug activity in C. auris candidiasis.

Preclinical Safety, Tolerability, Pharmacokinetics, Pharmacodynamics, and Antifungal Activity of Liposomal Amphotericin B

The improved safety profile and antifungal efficacy of liposomal amphotericin B (LAmB) compared to conventional amphotericin B deoxycholate (DAmB) is due to several factors including, its chemical composition, rigorous manufacturing standards, and ability to target and transit through the fungal cell wall. Numerous preclinical studies have shown that LAmB administered intravenously distributes to tissues frequently infected by fungi at levels above the minimum inhibitory concentration (MIC) for many fungi. These concentrations can be maintained from one day to a few weeks, depending upon the tissue. Tissue accumulation is dose-dependent with drug clearance occurring most rapidly from the brain and slowest from the liver and spleen. LAmB localizes in lung epithelial lining fluid, within liver and splenic macrophages and in kidney distal tubules. LAmB has been used successfully in therapeutic and prophylactic animal models to treat many different fungal pathogens, significantly increasing survival and reducing tissue fungal burden.

Acute lung injury and repair induced by single exposure of Aspergillus fumigatus in immunocompetent mice

Aim: Characterize the course of acute Aspergillus fumigatus lung infection in immunocompetent mice, investigating the immunological, pathological and tissue functional modifications. Materials & methods: C57BL/6 mice were intranasally infected with A. fumigatus conidia and euthanized to access inflammatory parameters. Results: Mice infected with A. fumigatus showed an inoculum-dependent lethality and body weight loss. An intense proinflammatory cytokine release, neutrophil infiltrate and pulmonary dysfunction was also observed in the early phase of infection. In the late phase of infection, proresolving mediators release, apoptosis and efferocytosis increased and lung tissue architecture is restored. Conclusion: Our study characterized an immunocompetent model of acute pulmonary Aspergillus infection in mice and opened an array of possibilities for investigations on interactions of A. fumigatus with host-immune system.

Comparison of intravitreal antifungal 100 μg voriconazole and 5 μg amphotericin B in experimental <em>Aspergillus flavus</em> endophthalmitis model in rabbits

BACKGROUND Fungal endophthalmitis is a sight-threatening disease associated with high morbidity and Aspergillus sp. is the most common causes. Voriconazole (VCZ) and Amphotericin B (AmB) are the most used antifungal drugs, while head-to-head comparison for in vivo intravitreal efficacy is still unknown. This study was aimed to compare the efficacy of both agents against Aspergillus flavus.  METHODS A randomized, masked, controlled-experimental study was conducted on 15 albino New Zealand white rabbits. Endophthalmitis was induced by intravitreal inoculation of Aspergillus flavus. Intravitreal injection was given 24 hours post-inoculation, the rabbits were divided into three groups; 100 μg/0.1 ml VCZ intravitreal injection, 5 μg/0.1 ml AmB, and control. Clinical evaluation of corneal opacity, aqueous cells and flare, and vitreous opacity using Yang’s method of quantification were performed at day 1, 3, 5, 7, and 10 after treatment. Mycology quantitative analysis and histopathological examination were performed at the final evaluation.  RESULTS Clinical evaluation showed improvement of inflammation in the VCZ and AmB treatment groups (Δ score −2.1 [2.8] and −1.0 [3.2]) compared with the control group (Δ score 0.8 [3.1]). Although the VCZ group demonstrated a better clinical response with less inflammation and relatively intact retina structures in the histopathology result. Number of fungal colony was significantly less in AmB group (CFU/0.1 ml, p < 0.05).  CONCLUSIONS Favorable clinical improvement was shown in VCZ group compared to AmB group. Intravitreal VCZ showed a better clinical response tendency for Aspergillus flavus-induced endophthalmitis in rabbits.

Detection of virulence factors and antifungal susceptibility of human and avian Aspergillusflavus isolates

Aspergillusflavus is the second leading cause of invasive and non-invasive aspergillosis. Secretion of hydrolytic enzymes is considered as a virulence factor in this species. Our work aimed to study in vitro production of some virulence factors, to evaluate the biofilm production against human and avian A. flavus isolates and to investigate the antifungal susceptibility agents. Hydrolytic enzymes, biofilm production and molecular typing were studied for 62 human and 36 avian A. flavus isolates by specific solid media and six microsatellite markers. The susceptibility to antifungal agents was evaluated for 37 human isolates. All human and avian A. flavus isolates showed positive activities of extracellular hydrolase: phospholipase, protease and hemolysin. A positive elastase activity was seen in 64.51% of human A. flavus isolates and 86.1% of avian A. flavus isolates. All A. flavus in these two populations formed biofilms. Statistical significant difference was observed for the mean phospholipase activities (P=0.025) and biofilm quantification (P=0.0001) between human and avian A. flavus isolates. The in vitro susceptibility results showed a resistance in 83.7%, 81.08% and 16.21% of A. flavus isolates respectively to amphotericin B, itraconazole and posaconazole. No association was noted between all virulence factors and the genotypes of human and avian isolates. Our study allowed us to show that human strains have a higher production of extracellular hydrolases and biofilm then avian strains. These virulence factors appear to act synergistically to contribute to the virulence of A. flavus strains. Moreover, significant correlation between virulence patterns and antifungal susceptibility profiles was observed.

Clinical Pharmacokinetics, Pharmacodynamics, Safety and Efficacy of Liposomal Amphotericin B

Since its introduction in the 1990s, liposomal amphotericin B (LAmB) continues to be an important agent for the treatment of invasive fungal diseases caused by a wide variety of yeasts and molds. This liposomal formulation was developed to improve the tolerability of intravenous amphotericin B, while optimizing its clinical efficacy. Since then, numerous clinical studies have been conducted, collecting a comprehensive body of evidence on its efficacy, safety, and tolerability in the preclinical and clinical setting. Nevertheless, insights into the pharmacokinetics and pharmacodynamics of LAmB continue to evolve and can be utilized to develop strategies that optimize efficacy while maintaining the compound's safety. In this article, we review the clinical pharmacokinetics, pharmacodynamics, safety, and efficacy of LAmB in a wide variety of patient populations and in different indications, and provide an assessment of areas with a need for further clinical research.

Comparative Assessment of Aspergillosis by Virtual Infection Modeling in Murine and Human Lung

Aspergillus fumigatus is a ubiquitous opportunistic fungal pathogen that can cause severe infections in immunocompromised patients. Conidia that reach the lower respiratory tract are confronted with alveolar macrophages, which are the resident phagocytic cells, constituting the first line of defense. If not efficiently removed in time, A. fumigatus conidia can germinate causing severe infections associated with high mortality rates. Mice are the most extensively used model organism in research on A. fumigatus infections. However, in addition to structural differences in the lung physiology of mice and the human host, applied infection doses in animal experiments are typically orders of magnitude larger compared to the daily inhalation doses of humans. The influence of these factors, which must be taken into account in a quantitative comparison and knowledge transfer from mice to humans, is difficult to measure since in vivo live cell imaging of the infection dynamics under physiological conditions is currently not possible. In the present study, we compare A. fumigatus infection in mice and humans by virtual infection modeling using a hybrid agent-based model that accounts for the respective lung physiology and the impact of a wide range of infection doses on the spatial infection dynamics. Our computer simulations enable comparative quantification of A. fumigatus infection clearance in the two hosts to elucidate (i) the complex interplay between alveolar morphometry and the fungal burden and (ii) the dynamics of infection clearance, which for realistic fungal burdens is found to be more efficiently realized in mice compared to humans.

The Genome of a Thermo Tolerant, Pathogenic Albino Aspergillus fumigatus

Biotechnologists are interested in thermo tolerant fungi to manufacture enzymes active and stable at high temperatures, because they provide improved catalytic efficiency, strengthen enzyme substrate interactions, accelerate substrate enzyme conversion rates, enhance mass transfer, lower substrate viscosity, lessen contamination risk and offer the potential for enzyme recycling. Members of the genus Aspergillus live a wide variety of lifestyles, some embrace GRAS status routinely employed in food processing while others such as Aspergillus fumigatus are human pathogens. A. fumigatus produces melanins, pyomelanin protects the fungus against reactive oxygen species and DHN melanin produced by the pksP gene cluster confers the gray-greenish color. pksP mutants are attenuated in virulence. Here we report on the genomic DNA sequence of a thermo tolerant albino Aspergillus isolated from rain forest composted floors. Unexpectedly, the nucleotide sequence was 95.7% identical to the reported by Aspergillus fumigatus Af293. Genome size and predicted gene models were also highly similar, however differences in DNA content and conservation were observed. The albino strain, classified as Aspergillus fumigatus var. niveus, had 160 gene models not present in A. fumigatus Af293 and A. fumigatus Af293 had 647 not found in the albino strain. Furthermore, the major pigment generating gene cluster pksP appeared to have undergone genomic rearrangements and a key tyrosinase present in many aspergilli was missing from the genome. Remarkably however, despite the lack of pigmentation A. fumigatus var. niveus killed neutropenic mice and survived macrophage engulfment at similar rates as A. fumigatus Af293.

RNA-Seq Profile Reveals Th-1 and Th-17-Type of Immune Responses in Mice Infected Systemically with Aspergillus fumigatus

With the increasing numbers of immunocompromised hosts, Aspergillus fumigatus emerges as a lethal opportunistic fungal pathogen. Understanding innate and acquired immunity responses of the host is important for a better therapeutic strategy to deal with aspergillosis patients. To determine the transcriptome in the kidneys in aspergillosis, we employed RNA-Seq to obtain single 76-base reads of whole-genome transcripts of murine kidneys on a temporal basis (days 0; uninfected, 1, 2, 3 and 8) during invasive aspergillosis. A total of 6284 transcripts were downregulated, and 5602 were upregulated compared to baseline expression. Gene ontology enrichment analysis identified genes involved in innate and adaptive immune response, as well as iron binding and homeostasis, among others. Our results showed activation of pathogen recognition receptors, e.g., β-defensins, C-type lectins (e.g., dectin-1), Toll-like receptors (TLR-2, TLR-3, TLR-8, TLR-9 and TLR-13), as well as Ptx-3 and C-reactive protein among the soluble receptors. Upregulated transcripts encoding various differentiating cytokines and effector proinflammatory cytokines, as well as those encoding for chemokines and chemokine receptors, revealed Th-1 and Th-17-type immune responses. These studies form a basic dataset for experimental prioritization, including other target organs, to determine the global response of the host against Aspergillus infection.

Molecular Insights Into Development and Virulence Determinants of Aspergilli: A Proteomic Perspective

Aspergillus species are the major cause of health concern worldwide in immunocompromised individuals. Opportunistic Aspergilli cause invasive to allergic aspergillosis, whereas non-infectious Aspergilli have contributed to understand the biology of eukaryotic organisms and serve as a model organism. Morphotypes of Aspergilli such as conidia or mycelia/hyphae helped them to survive in favorable or unfavorable environmental conditions. These morphotypes contribute to virulence, pathogenicity and invasion into hosts by excreting proteins, enzymes or toxins. Morphological transition of Aspergillus species has been a critical step to infect host or to colonize on food products. Thus, we reviewed proteins from Aspergilli to understand the biological processes, biochemical, and cellular pathways that are involved in transition and morphogenesis. We majorly analyzed proteomic studies on A. fumigatus, A. flavus, A. terreus, and A. niger to gain insight into mechanisms involved in the transition from conidia to mycelia along with the role of secondary metabolites. Proteome analysis of morphotypes of Aspergilli provided information on key biological pathways required to exit conidial dormancy, consortia of virulent factors and mycotoxins during the transition. The application of proteomic approaches has uncovered the biological processes during development as well as intermediates of secondary metabolite biosynthesis pathway. We listed key proteins/ enzymes or toxins at different morphological types of Aspergillus that could be applicable in discovery of novel therapeutic targets or metabolite based diagnostic markers.

Investigation of Aspergillus flavus in animal virulence

Aspergillus flavus is a common fungal pathogen of plants, animals and humans. Recently, many genes of A. flavus have been reported involving in regulation of pathogenesis in crops, but whether these genes are involved in animal virulence is still unknown. Here, we used a previous easy-to-use infection model for A. flavus based on mouse model by intravenous inoculation of A. flavus conidia. The outcome of infections in mice model showed that A. flavus NRRL3357 and laboratory strain CA14 PTS were both in dose dependent manner and highly reproducible. The progress of disease could be monitored by mice survival and histology analysis. Fungal burden analysis indicated it was gradually decreased within 7 days after infection. Moreover, aspergillosis caused by A. flavus significantly up-regulated gene expression levels of immune response mediators, including INF-γ, TNF-α, Dectin-1 and TLR2. Furthermore, the defined deletion A. flavus strains that previously displayed virulence in crop infection were also determined in this mouse model, and the results showed comparable degrees of infection in mice. Our results suggested that intravenous inoculation of conidia could be a suitable model for testing different A. flavus mutants in animal virulence. We hope to use this model to determine distinct A. flavus strains virulence in animals and study novel therapeutic methods to help control fungus diseases in the future.

Recognition of DHN-melanin by a C-type lectin receptor is required for immunity to Aspergillus

Our resistance to infection is critically dependent upon the ability of pattern recognition receptors to recognise microbial invasion and induce protective immune responses. One such family of receptors are the C-type lectins, which play central roles in antifungal immunity1. These receptors activate key effector mechanisms upon recognition of conserved fungal cell wall carbohydrates. However, several other immunologically active fungal ligands have been described, including melanin2,3, whose mechanisms of recognition remain largely undefined. Here we identify a C-type lectin receptor, Melanin sensing C-type Lectin receptor (MelLec), that plays an essential role in antifungal immunity through recognition of the naphthalene-diol unit of 1,8-dihydroxynaphthalene (DHN)-melanin. MelLec recognises melanin in conidial spores of Aspergillus fumigatus, as well as other DHN-melanised fungi and is ubiquitously expressed by CD31⁺ endothelial cells in mice. MelLec is also expressed by a sub-population of these cells in mice that co-express EpCAM and which were detected only in the lung and liver. In mouse models, MelLec was required for protection against disseminated infection with A. fumigatus. In humans, MelLec is also expressed by myeloid cells, and we identified a single nucleotide polymorphism of this receptor that negatively affected myeloid inflammatory responses and significantly increased susceptibility of stem-cell transplant recipients to disseminated Aspergillus infections. Thus MelLec is a receptor recognising an immunologically active component commonly found on fungi and plays an essential role in protective antifungal immunity in both mice and humans.

Rodent Models of Invasive Aspergillosis due to Aspergillus fumigatus: Still a Long Path toward Standardization

Invasive aspergillosis has been studied in laboratory by the means of plethora of distinct animal models. They were developed to address pathophysiology, therapy, diagnosis, or miscellaneous other concerns associated. However, there are great discrepancies regarding all the experimental variables of animal models, and a thorough focus on them is needed. This systematic review completed a comprehensive bibliographic analysis specifically-based on the technical features of rodent models infected with Aspergillus fumigatus. Out the 800 articles reviewed, it was shown that mice remained the preferred model (85.8% of the referenced reports), above rats (10.8%), and guinea pigs (3.8%). Three quarters of the models involved immunocompromised status, mainly by steroids (44.4%) and/or alkylating drugs (42.9%), but only 27.7% were reported to receive antibiotic prophylaxis to prevent from bacterial infection. Injection of spores (30.0%) and inhalation/deposition into respiratory airways (66.9%) were the most used routes for experimental inoculation. Overall, more than 230 distinct A. fumigatus strains were used in models. Of all the published studies, 18.4% did not mention usage of any diagnostic tool, like histopathology or mycological culture, to control correct implementation of the disease and to measure outcome. In light of these findings, a consensus discussion should be engaged to establish a minimum standardization, although this may not be consistently suitable for addressing all the specific aspects of invasive aspergillosis.

CD4+ T Cells Mediate Aspergillosis Vaccine Protection

Adaptive effector CD4⁺ T cells play essential roles in the defense against fungal infections, especially against invasive aspergillosis (IA). Such protective CD4⁺ T cells can be generated through immunization with specialized antifungal vaccines, as has been demonstrated for pulmonary Aspergillus fumigatus infections in mouse experiments. Adaptive transfer of fungal antigen-specific CD4⁺ T cells conferred protection onto non-immunized naive mice, an experimental approach that could potentially become a future treatment option for immunosuppressed IA patients, focusing on the ultimate goal to improve their otherwise dim chances for survival. Here, we describe the different techniques to analyze CD4⁺ T cell immune responses after immunization with a recombinant fungal protein. We present three major methods that are used to analyze the role of CD4⁺ T cells in protection against A. fumigatus challenge. They include (1) transplantation of CD4⁺ T cells from vaccinated mice into immunosuppressed naive mice, observing increasing protection of the cell recipients, (2) depletion of CD4⁺ T cells from vaccinated mice, which abolishes vaccine protection, and (3) T cell proliferation studies following stimulation with overlapping synthetic peptides or an intact protein vaccine. The latter can be used to validate immunization status and to identify protective T cell epitopes in vaccine antigens. In the methods detailed here, we used versions of the well-studied Asp f3 protein expressed in a bacterial host, either as the intact full length protein or its N-terminally truncated version, comprised of residues 15-168. However, these methods are generally applicable and can well be adapted to study other protein-based subunit vaccines.

Aspergillus in chronic lung disease: Modeling what goes on in the airways

Aspergillus species cause a range of respiratory diseases in humans. While immunocompromised patients are at risk for the development of invasive infection with these opportunistic molds, patients with underlying pulmonary disease can develop chronic airway infection with Aspergillus species. These conditions span a range of inflammatory and allergic diseases including Aspergillus bronchitis, allergic bronchopulmonary aspergillosis, and severe asthma with fungal sensitization. Animal models are invaluable tools for the study of the molecular mechanism underlying the colonization of airways by Aspergillus and the host response to these non-invasive infections. In this review we summarize the state-of-the-art with respect to the available animal models of noninvasive and allergic Aspergillus airway disease; the key findings of host-pathogen interaction studies using these models; and the limitations and future directions that should guide the development and use of models for the study of these important pulmonary conditions.

Interactions of Aspergillus fumigatus Conidia with Airway Epithelial Cells: A Critical Review

Aspergillus fumigatus is an environmental filamentous fungus that also acts as an opportunistic pathogen able to cause a variety of symptoms, from an allergic response to a life-threatening disseminated fungal infection. The infectious agents are inhaled conidia whose first point of contact is most likely to be an airway epithelial cell (AEC). The interaction between epithelial cells and conidia is multifaceted and complex, and has implications for later steps in pathogenesis. Increasing evidence has demonstrated a key role for the airway epithelium in the response to respiratory pathogens, particularly at early stages of infection; therefore, elucidating the early stages of interaction of conidia with AECs is essential to understand the establishment of infection in cohorts of at-risk patients. Here, we present a comprehensive review of the early interactions between A. fumigatus and AECs, including bronchial and alveolar epithelial cells. We describe mechanisms of adhesion, internalization of conidia by AECs, the immune response of AECs, as well as the role of fungal virulence factors, and patterns of fungal gene expression characteristic of early infection. A clear understanding of the mechanisms involved in the early establishment of infection by A. fumigatus could point to novel targets for therapy and prophylaxis.

RNAseq analysis of Aspergillus fumigatus in blood reveals a just wait and see resting stage behavior

Background

Invasive aspergillosis is started after germination of Aspergillus fumigatus conidia that are inhaled by susceptible individuals. Fungal hyphae can grow in the lung through the epithelial tissue and disseminate hematogenously to invade into other organs. Low fungaemia indicates that fungal elements do not reside in the bloodstream for long.

Results

We analyzed whether blood represents a hostile environment to which the physiology of A. fumigatus has to adapt. An in vitro model of A. fumigatus infection was established by incubating mycelium in blood. Our model allowed to discern the changes of the gene expression profile of A. fumigatus at various stages of the infection. The majority of described virulence factors that are connected to pulmonary infections appeared not to be activated during the blood phase. Three active processes were identified that presumably help the fungus to survive the blood environment in an advanced phase of the infection: iron homeostasis, secondary metabolism, and the formation of detoxifying enzymes.

Conclusions

We propose that A. fumigatus is hardly able to propagate in blood. After an early stage of sensing the environment, virtually all uptake mechanisms and energy-consuming metabolic pathways are shut-down. The fungus appears to adapt by trans-differentiation into a resting mycelial stage. This might reflect the harsh conditions in blood where A. fumigatus cannot take up sufficient nutrients to establish self-defense mechanisms combined with significant growth.

Electronic supplementary material

The online version of this article (doi10.1186/s12864-015-1853-1) contains supplementary material, which is available to authorized users.

Fungal model systems and the elucidation of pathogenicity determinants

Fungi have the capacity to cause devastating diseases of both plants and animals, causing significant harvest losses that threaten food security and human mycoses with high mortality rates. As a consequence, there is a critical need to promote development of new antifungal drugs, which requires a comprehensive molecular knowledge of fungal pathogenesis. In this review, we critically evaluate current knowledge of seven fungal organisms used as major research models for fungal pathogenesis. These include pathogens of both animals and plants; Ashbya gossypii, Aspergillus fumigatus, Candida albicans, Fusarium oxysporum, Magnaporthe oryzae, Ustilago maydis and Zymoseptoria tritici. We present key insights into the virulence mechanisms deployed by each species and a comparative overview of key insights obtained from genomic analysis. We then consider current trends and future challenges associated with the study of fungal pathogenicity.

Overview of vertebrate animal models of fungal infection

Fungi represent emerging infectious threats to human populations worldwide. Mice and other laboratory animals have proved invaluable in modeling clinical syndromes associated with superficial and life-threatening invasive mycoses. This review outlines salient features of common vertebrate animal model systems to study fungal pathogenesis, host antifungal immune responses, and antifungal compounds.

Proteomic demonstration of the recurrent presence of inter-alpha-inhibitor H4 heavy-chain during aspergillosis induced in an animal model

Invasive pulmonary aspergillosis remains a matter of great concern in oncology/haematology, intensive care units and organ transplantation departments. Despite the availability of various diagnostic tools with attractive features, new markers of infection are required for better medical care. We therefore looked for potential pulmonary biomarkers of aspergillosis, by carrying out two-dimensional (2D) gel electrophoresis comparing the proteomes of bronchial-alveolar lavage fluids (BALF) from infected rats and from control rats presenting non-specific inflammation, both immunocompromised. A bioinformatic analysis of the 2D-maps revealed significant differences in the abundance of 20 protein spots (ANOVA P-value<0.01; q-value<0.03; power>0.8). One of these proteins, identified by mass spectrometry, was considered of potential interest: inter-alpha-inhibitor H4 heavy-chain (ITIH4), characterised for the first time in this infectious context. Western blotting confirmed its overabundance in all infected BALF, particularly at early stages of murine aspergillosis. Further investigations were carried on rat serum, and confirmed that ITIH4 levels increased during experimental aspergillosis. Preliminary results in human samples strengthened this trend. To our knowledge, this is the first description of the involvement of ITIH4 in aspergillosis.

Aspergillus fumigatus from normal and condemned carcasses with airsacculitis in commercial poultry

Carcass inspection is important for the detection of certain diseases and for monitoring their prevalence in slaughterhouses. The objective of this study was to assess the occurrence of aspergillosis caused by Aspergillus fumigatus in commercial poultry, through mycological and histopathological diagnosis, and to verify the causal association between the aspergillosis diagnosis criteria and condemnation due to airsacculitis in broilers through a case-control study. The study was carried out with 380 samples. Lungs were collected from broilers that were condemned (95) or not condemned (285) due to airsacculitis directly from the slaughter line. Forty-six (12%) lung samples were positive for A. fumigatus in mycological culture. Among all samples, 177 (46.6%) presented histopathological alterations, with necrotic, fibrinous, heterophilic pneumonia; heterophilic pneumonia and lymphoid hyperplasia being the most frequent. Out of the 380 lungs analyzed, 65.2% (30) showed histopathological alterations and isolation of fungi. The statistical analysis (McNemar's chi-square test) indicated a significant association between the presence of histopathological lesions and the isolation of A. fumigatus. Mycological cultivation and histopathological diagnosis increase the probability of detecting pulmonary alterations in birds condemned by the Final Inspection System, which suggests that such diagnostic criteria can improve the assessment and condemnation of birds affected by airsacculitis.

Composite survival index to compare virulence changes in azole-resistant Aspergillus fumigatus clinical isolates

Understanding resistance to antifungal agents in Aspergillus fumigatus is of increasing importance for the treatment of invasive infections in immunocompromised patients. Although a number of molecular resistance mechanisms are described in detail, the potential accompanying virulence changes and impact on clinical outcome have had little attention. We developed a new measure of survival, the composite survival index (CSI) to use as a measure of the virulence properties of A. fumigatus. Using a novel mathematical model we found a strong correlation between the in vitro growth characteristics and virulence in vivo expressed as CSI. Our model elucidates how three critical parameters (the lag phase (τ), decay constant (λ), and growth rate (ν)) interact with each other resulting in a CSI that correlated with virulence. Hence, strains with a long lag phase and high decay constant were less virulent in a murine model of invasive aspergillosis, whereas high virulence for isolates with a high CSI was associated in vitro with rapid growth and short lag phases. Resistant isolates with cyp51A mutations, which account for the majority of azole resistant aspergillosis cases, did not show a lower virulence compared to azole-susceptible isolates. In contrast, the CSI index revealed that a non-cyp51A-mediated resistance mechanism was associated with a dramatic decrease in CSI. Because of its predictive value, the mathematical model developed may serve to explore strain characteristics in vitro to predict virulence in vivo and significantly reduce the number of experimental animals required in such studies. The proposed measure of survival, the CSI can be used more in a general form in survival studies to explore optimal treatment options.

Strain dependent variation of immune responses to A. fumigatus: definition of pathogenic species

For over a century microbiologists and immunologist have categorized microorganisms as pathogenic or non-pathogenic species or genera. This definition, clearly relevant at the strain and species level for most bacteria, where differences in virulence between strains of a particular species are well known, has never been probed at the strain level in fungal species. Here, we tested the immune reactivity and the pathogenic potential of a collection of strains from Aspergillus spp, a fungus that is generally considered pathogenic in immuno-compromised hosts. Our results show a wide strain-dependent variation of the immune response elicited indicating that different isolates possess diverse virulence and infectivity. Thus, the definition of markers of inflammation or pathogenicity cannot be generalized. The profound understanding of the molecular mechanisms subtending the different immune responses will result solely from the comparative study of strains with extremely diverse properties.

Experimental evidence that granulocyte transfusions are efficacious in treatment of neutropenic hosts with pulmonary aspergillosis

Although polymorphonuclear leukocytes (PMNs) are powerfully anti-Aspergillus, transfusion therapy remains controversial, with conflicting results, and experimental support has been lacking. We devised a pulmonary infection model in neutropenic BALB/c mice, used an antibacterial regimen to prevent confounding sepsis, and optimized PMN induction, purifications, and dose. Mice were given 150 mg/kg cyclophosphamide every 4 days and a gentamicin-vancomycin-clindamycin-imipenem regimen daily beginning 4 days before intranasal challenge with 5 × 10(5) Aspergillus conidia. This regimen produced leukopenia (~10% of normal white blood cell [WBC] count; ≤ 10% PMNs) for 10 days, without bacterial superinfection. PMN donors given 100 μg/kg recombinant murine granulocyte colony-stimulating factor (G-CSF) for 10 days yielded 11 × 10(7) to 13.6 × 10(7) WBC/ml (81 to 87% PMNs). Infected mice were given PMN transfusions intravenously. In 2 experiments with up to 70% mortality of neutropenic controls, transfusion of 10(7) PMNs 1 and 4 days after challenge had negligible effects on peripheral WBC counts but improved survival (P = 0.007, 0.02), decreased lung CFU (P = 0.03, 0.005), and cleared infection in 28 to 50% of survivors. Transfusion of 5 × 10(6) PMNs showed partial protection. Transfusions given every other day did not improve protection. Our present results provide an experimental basis for enthusiasm for PMN transfusions in the therapy of aspergillosis in humans.

Host immune defense against Aspergillus fumigatus: insight from experimental systemic (disseminated) infection

Model of systemic Aspergillus fumigatus infection induced by intravenous application of conidia is suitable for studying important aspects of invasive aspergillosis including relationship between infection and mortality, dissemination of infection and immune mechanisms involved in host resistance to this fungus. Use of this model allows the investigation of both innate and adaptive immune response characteristics in resistant/susceptible host, and investigating the contribution of genetic background and cytokine gene deficiency improves the knowledge of the diversity of mechanisms of immune response to Aspergillus infection. Studying of various aspects of systemic aspergillosis contributes to development of antifungal drugs.

Differential strain-related tissue immune response to sublethal systemic Aspergillus fumigatus infection in mice

Using a nonlethal systemic Aspergillus fumigatus infection, we have recently shown that similarly efficient elimination of fungus from spleens of prototypic Th1 (C57BL/6) and prototypic Th2 (BALB/c) mice is associated with differential immune responses. In light of these data and given the disseminated character of infection, the aim of the present study is to explore whether there are also strain-dependent differences in antifungal responses in peripheral tissues of infected mice. Although similar efficiency of conidia removal was noted in liver and kidneys of both strains, BALB/c mice seemed more prone to tissue injury. Compared with other nonlymphoid organs, lungs proved immunologically the most responsive in systemic aspergillosis. Lower numbers of neutrophils and macrophages in the lungs of infected BALB/c mice, delayed and lower (compared with C57BL/6 mice) expression of their oxidative activity, along with late IFN-γ and upregulated IL-4 production by lung cells might be responsible for slower elimination of A. fumigatus from the lungs of this mouse strain. The data obtained imply that lungs should be viewed as mandatory organ in evaluation of immune-mediated antifungal potential of drugs in models of systemic/disseminated infection and that strain differences noted in tissue responses should be taken into account in these settings.

Vaccination with mannan protects mice against systemic aspergillosis

Invasive aspergillosis is a major cause of mortality in immunocompromised patients and therapeutic options are often limited, thus a vaccine would be desirable. We presently studied acid-stable cell-wall mannan (α-1, 6-linked backbone highly branched with α-1, 2; α-1, 3; and β-1, 2-linked manno-oligomers) derived from C. albicans, with or without conjugation to bovine serum albumin (BSA), as a vaccine against systemic aspergillosis. Mice were vaccinated subcutaneously with mannan or mannan-BSA conjugate weekly 3 times, ending 2 weeks prior to infection with A. fumigatus conidia. Results showed that the protection induced by mannan is dose-dependent; 12 mg unconjugated mannan alone or > 0.3 mg mannan-BSA consistently enhanced survival (P < 0.05). Fungal burdens in brains and kidneys were reduced after > 0.3 mg of mannan-BSA (all P < 0.05). Mannan-induced protection was improved about 40-fold by conjugation of BSA to mannan. Mannan-BSA (500 kDa) was more protective than 40 kDa mannan-BSA. Mannan is a candidate for a cross-protective conjugate fungal vaccine.

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.

Embryonated chicken eggs as alternative infection model for pathogenic fungi

Embryonated eggs have been used as infection models for decades in virology and bacteriology. However, they can also be used as an attractive alternative infection model for studying fungal pathogenesis. Here, we discuss some general aspects which need to be considered when working with embryonated eggs as infection models. Furthermore, we provide detailed protocols and technical tips for infection of embryonated eggs with Aspergillus fumigatus and Candida albicans via the chorioallantois membrane, as well as sampling methods for downstream analyses.

A nebulized intra-tracheal rat model of invasive pulmonary aspergillosis

Animal models are particularly useful for the study of many infectious diseases, including those caused by fungi. Invasive pulmonary aspergillosis is most frequently studied in mouse models. We present here an animal model of this disease based on undernourished immunocompromised rats infected with Aspergillus fumigatus spores by intra-tracheal nebulisation.

Aspergillus fumigatus in Poultry

Aspergillus fumigatus remains a major respiratory pathogen in birds. In poultry, infection by A. fumigatus may induce significant economic losses particularly in turkey production. A. fumigatus develops and sporulates easily in poor quality bedding or contaminated feedstuffs in indoor farm environments. Inadequate ventilation and dusty conditions increase the risk of bird exposure to aerosolized spores. Acute cases are seen in young animals following inhalation of spores, causing high morbidity and mortality. The chronic form affects older birds and looks more sporadic. The respiratory tract is the primary site of A. fumigatus development leading to severe respiratory distress and associated granulomatous airsacculitis and pneumonia. Treatments for infected poultry are nonexistent; therefore, prevention is the only way to protect poultry. Development of avian models of aspergillosis may improve our understanding of its pathogenesis, which remains poorly understood.

Lessons from animal studies for the treatment of invasive human infections due to uncommon fungi

Clinical experience in the management of opportunistic infections, especially those caused by less common fungi, is, due to their rarity, very scarce; therefore, the most effective treatments remain unknown. The ever-increasing numbers of fungal infections due to opportunistic fungi have repeatedly proven the limitations of the antifungal armamentarium. Moreover, some of these fungi, such as Fusarium spp. or Scedosporium spp., are innately resistant to almost all the available antifungal drugs, which makes the development of new and effective therapies a high priority. Since it is difficult to conduct randomized clinical trials in these uncommon mycoses, the use of animal models is a good alternative for evaluating new therapies. This is an extensive review of the numerous studies that have used animal models for this purpose against a significant number of less common fungi. A table describing the different studies performed on the efficacy of the different drugs tested is included for each fungal species. In addition, there is a summary table showing the conclusions that can be derived from the analysis of the studies and listing the drugs that showed the best results. Considering the wide variability in the response to the antifungals that the different strains of a given species can show, the table highlights the drugs that showed positive results using at least two parameters for evaluating efficacy against at least two different strains without showing any negative results. These data can be very useful for guiding the treatment of rare infections when there is very little experience or when controversial results exist, or when treatment fails.

Real-time PCR and quantitative culture for monitoring of experimental Aspergillus fumigatus intracranial infection in neutropenic mice

The central nervous system (CNS) is the most common site of dissemination during Aspergillus infection. PCR has the potential to facilitate early diagnosis of CNS aspergillosis, which could assist in reducing disease mortality. In two experiments, neutropenic CD-1 male mice were infected intracranially with 5×10⁶ conidia of Aspergillus fumigatus. At time points up to 120 h after infection, mice were euthanized and samples of blood, brain, spinal cord and cerebrospinal fluid (CSF) were taken. The brain fungal burden was determined by quantitative culture, and fungal DNA was detected by quantitative PCR. Plating for A. fumigatus from the brain confirmed that all mice had burdens of log₁₀>3 from 4 to 120 h after infection. A. fumigatus DNA was detected in blood (88 %), brain (96 %), CSF (52 %) and spinal cord (92 %) samples. The brain and spinal cord contained the highest concentrations of fungal DNA. Adapting the extraction protocol to maximize yield from small sample volumes (10 µl CSF or 200 µl blood) allowed PCR detection of A. fumigatus in infected mice, suggesting the use of CSF and blood as diagnostic clinical samples for CNS aspergillosis.

CD4+ T cells mediate the protective effect of the recombinant Asp f3-based anti-aspergillosis vaccine

The mortality and morbidity caused by invasive aspergillosis present a major obstacle to the successful treatment of blood cancers with hematopoietic cell transplants. Patients who receive hematopoietic cell transplants are usually immunosuppressed for extended periods, and infection with the ubiquitous mold Aspergillus fumigatus is responsible for most cases of aspergillosis. Previously, we demonstrated that vaccination with recombinant forms of the A. fumigatus protein Asp f3 protected cortisone acetate-immunosuppressed mice from experimentally induced pulmonary aspergillosis. Here, we investigated the vaccine's protective mechanism and evaluated in particular the roles of antibodies and T cells. After vaccination, Asp f3-specific preinfection IgG titers did not significantly differ between surviving and nonsurviving mice, and passive transfer of anti-Asp f3 antibodies did not protect immunosuppressed recipients from aspergillosis. We experimentally confirmed Asp f3's predicted peroxisomal localization in A. fumigatus hyphae. We found that fungal Asp f3 is inaccessible to antibodies, unless both cell walls and membranes have been permeabilized. Antibody-induced depletion of CD4+ T cells reduced the survival of recombinant Asp f3 (rAsp f3)-vaccinated mice to nonimmune levels, and transplantation of purified CD4+ T cells from rAsp f3-vaccinated mice into nonimmunized recipients transferred antifungal protection. In addition, residues 60 to 79 and 75 to 94 of Asp f3 contain epitopes that induce proliferation of T cells from vaccinated survivors. Vaccine-primed CD4+ T cells are not expected to clear the fungal pathogen directly; however, they may locally activate immunosuppressed phagocytes that elicit the antifungal effect.