Use of DBA/2N mice in models of systemic candidiasis and pulmonary and systemic aspergillosis

Complement-pentraxins synergy: Navigating the immune battlefield and beyond

The complement is a crucial immune defense system that triggers rapid immune responses and offers efficient protection against foreign invaders and unwanted host elements, acting as a sentinel. Activation of the complement system occurs upon the recognition of pathogenic microorganisms or altered self-cells by pattern-recognition molecules (PRMs) such as C1q, collectins, ficolins, and pentraxins. Recent accumulating evidence shows that pentraxins establish a cooperative network with different classes of effector PRMs, resulting in synergistic effects in complement activation. This review describes the complex interaction of pentraxins with the complement system and the implications of this cooperative network for effective host defense during pathogen invasion.

Development of a murine model to study the cerebral pathogenesis of Aspergillus fumigatus

IMPORTANCE

Molds are environmental fungi that can cause disease in immunocompromised individuals. The most common pathogenic mold is Aspergillus fumigatus, which is typically inhaled into the lungs and causes invasive pulmonary disease. In a subset of these patients, this infection can spread from the lungs to other organs including the brain, resulting in cerebral aspergillosis. How A. fumigatus causes brain disease is not well understood and these infections are associated with extremely high mortality rates. Thus, we developed an animal model to study the pathogenesis of cerebral aspergillosis to better understand this disease and develop better treatments for these life-threatening infections.

Protective role of host complement system in Aspergillus fumigatus infection

Invasive aspergillosis (IA) is a life-threatening fungal infection for immunocompromised hosts. It is, therefore, necessary to understand the immune pathways that control this infection. Although the primary infection site is the lungs, aspergillosis can disseminate to other organs through unknown mechanisms. Herein we have examined the in vivo role of various complement pathways as well as the complement receptors C3aR and C5aR1 during experimental systemic infection by Aspergillus fumigatus, the main species responsible for IA. We show that C3 knockout (C3^-/-) mice are highly susceptible to systemic infection of A. fumigatus. Intriguingly, C4^-/- and factor B (FB)^-/- mice showed susceptibility similar to the wild-type mice, suggesting that either the complement pathways display functional redundancy during infection (i.e., one pathway compensates for the loss of the other), or complement is activated non-canonically by A. fumigatus protease. Our in vitro study substantiates the presence of C3 and C5 cleaving proteases in A. fumigatus. Examination of the importance of the terminal complement pathway employing C5^-/- and C5aR1^-/- mice reveals that it plays a vital role in the conidial clearance. This, in part, is due to the increased conidial uptake by phagocytes. Together, our data suggest that the complement deficiency enhances the susceptibility to systemic infection by A. fumigatus.

Soluble mediators in anti-fungal immunity

Although soluble mediators of our innate immune system have a substantial impact on invading microbes, their role against fungal pathogens has been underexplored. Constituting the humoral immunity, soluble mediators comprise the complement system, collectins, acute-phase proteins, antibodies and antimicrobial peptides. These components can prevent fungal infection either by directly interacting with invading microbes, leading to their aggregation (microbistatic), destruction (microbicidal) or linking them to cellular immunity. The composition of soluble-mediator varies with human body-fluids, resulting in different antifungal mechanisms. Moreover, cellular immune system deploys both oxidative and non-oxidative mechanisms to destroy extracellular or internalized fungal pathogens; however, cellular immune activation is mainly influenced as well as regulated by soluble mediators. This review outlines the antifungal mechanism employed, directly or indirectly, by soluble mediators, and in response, the evading strategies of the fungal pathogens.

The complement system in Aspergillus fumigatus infections and its crosstalk with pentraxins

Aspergillosis is a life-threatening infection mostly affecting immunocompromised individuals and primarily caused by the saprophytic fungus Aspergillus fumigatus. At the host-pathogen interface, both cellular and humoral components of the innate immune system are increasingly acknowledged as essential players in the recognition and disposal of this opportunistic mold. Fundamental hereof is the contribution of the complement system, which deploys all three activation pathways in the battle against A. fumigatus, and functionally cooperates with other soluble pattern recognition molecules, including pentraxins. In particular, preclinical and clinical observations point to the long pentraxin PTX3 as a nonredundant and complement-dependent effector with protective functions against A. fumigatus. Based on past and current literature, here we discuss how the complement participates in the immune response to this fungal pathogen, and illustrate its crosstalk with the pentraxins, with a focus on PTX3. Emphasis is placed on the molecular mechanisms underlying such processes, the genetic evidence from human epidemiology, and the translational potential of the currently available knowledge.

Enolase From Aspergillus fumigatus Is a Moonlighting Protein That Binds the Human Plasma Complement Proteins Factor H, FHL-1, C4BP, and Plasminogen

The opportunistic fungal pathogen Aspergillus fumigatus can cause severe infections, particularly in immunocompromised individuals. Upon infection, A. fumigatus faces the powerful and directly acting immune defense of the human host. The mechanisms on how A. fumigatus evades innate immune attack and complement are still poorly understood. Here, we identify A. fumigatus enolase, AfEno1, which was also characterized as fungal allergen, as a surface ligand for human plasma complement regulators. AfEno1 binds factor H, factor-H-like protein 1 (FHL-1), C4b binding protein (C4BP), and plasminogen. Factor H attaches to AfEno1 via two regions, via short conserved repeats (SCRs) 6-7 and 19-20, and FHL-1 contacts AfEno1 via SCRs 6-7. Both regulators when bound to AfEno1 retain cofactor activity and assist in C3b inactivation. Similarly, the classical pathway regulator C4BP binds to AfEno1 and bound to AfEno1; C4BP assists in C4b inactivation. Plasminogen which binds to AfEno1 via lysine residues is accessible for the tissue-type plasminogen activator (tPA), and active plasmin cleaves the chromogenic substrate S2251, degrades fibrinogen, and inactivates C3 and C3b. Plasmin attached to swollen A. fumigatus conidia damages human A549 lung epithelial cells, reduces the cellular metabolic activity, and induces cell retraction, which results in exposure of the extracellular matrix. Thus, A. fumigatus AfEno1 is a moonlighting protein and virulence factor which recruits several human regulators. The attached human regulators allow the fungal pathogen to control complement at the level of C3 and to damage endothelial cell layers and tissue components.

"Candida Albicans Interactions With The Host: Crossing The Intestinal Epithelial Barrier"

Formerly a commensal organism of the mucosal surfaces of most healthy individuals, Candida albicans is an opportunistic pathogen that causes infections ranging from superficial to the more life-threatening disseminated infections, especially in the ever-growing population of vulnerable patients in the hospital setting. In these situations, the fungus takes advantage of its host following a disturbance in the host defense system and/or the mucosal microbiota. Overwhelming evidence suggests that the gastrointestinal tract is the main source of disseminated C. albicans infections. Major risk factors for disseminated candidiasis include damage to the mucosal intestinal barrier, immune dysfunction, and dysbiosis of the resident microbiota. A better understanding of C. albicans' interaction with the intestinal epithelial barrier will be useful for designing future therapies to avoid systemic candidiasis. In this review, we provide an overview of the current knowledge regarding the mechanisms of pathogenicity that allow the fungus to reach and translocate the gut barrier.

Pentraxins in Complement Activation and Regulation

The complement is the first line of immune defense system involved in elimination of invading pathogens and dying host cells. Its activation is mainly triggered by immune complexes or pattern recognition molecules (PRMs) upon recognition against non-self or altered self-cells, such as C1q, collectins, ficolins, and properdin. Recent findings have interestingly shown that the pentraxins (C-reactive protein, CRP; serum-amyloid P component, SAP; long pentraxin 3, PTX3) are involved in complement activation and amplification via communication with complement initiation PRMs, but also complement regulation via recruitment of complement regulators, for instance C4b binding protein (C4BP) and factor H (fH). This review addresses the potential roles of the pentraxins in the complement system during infection and inflammation, and emphasizes the underlining implications of the pentraxins in the context of complement activation and regulation both under physiological and pathological conditions.

Aspergillus fumigatus conidial metalloprotease Mep1p cleaves host complement proteins

Innate immunity in animals including humans encompasses the complement system, which is considered an important host defense mechanism against Aspergillus fumigatus, one of the most ubiquitous opportunistic human fungal pathogens. Previously, it has been shown that the alkaline protease Alp1p secreted from A. fumigatus mycelia degrades the complement components C3, C4, and C5. However, it remains unclear how the fungal spores (i.e. conidia) defend themselves against the activities of the complement system immediately after inhalation into the lung. Here, we show that A. fumigatus conidia contain a metalloprotease Mep1p, which is released upon conidial contact with collagen and inactivates all three complement pathways. In particular, Mep1p efficiently inactivated the major complement components C3, C4, and C5 and their activation products (C3a, C4a, and C5a) as well as the pattern-recognition molecules MBL and ficolin-1, either by directly cleaving them or by cleaving them to a form that is further broken down by other proteases of the complement system. Moreover, incubation of Mep1p with human serum significantly inhibited the complement hemolytic activity and conidial opsonization by C3b and their subsequent phagocytosis by macrophages. Together, these results indicate that Mep1p associated with and released from A. fumigatus conidia likely facilitates early immune evasion by disarming the complement defense in the human host.

Aspf2 From Aspergillus fumigatus Recruits Human Immune Regulators for Immune Evasion and Cell Damage

The opportunistic fungal pathogen Aspergillus fumigatus can cause life-threatening infections, particularly in immunocompromised patients. Most pathogenic microbes control host innate immune responses at the earliest time, already before infiltrating host immune cells arrive at the site of infection. Here, we identify Aspf2 as the first A. fumigatus Factor H-binding protein. Aspf2 recruits several human plasma regulators, Factor H, factor-H-like protein 1 (FHL-1), FHR1, and plasminogen. Factor H contacts Aspf2 via two regions located in SCRs6–7 and SCR20. FHL-1 binds via SCRs6–7, and FHR1 via SCRs3–5. Factor H and FHL-1 attached to Aspf2-maintained cofactor activity and assisted in C3b inactivation. A Δaspf2 knockout strain was generated which bound Factor H with 28% and FHL-1 with 42% lower intensity. In agreement with less immune regulator acquisition, when challenged with complement-active normal human serum, Δaspf2 conidia had substantially more C3b (>57%) deposited on their surface. Consequently, Δaspf2 conidia were more efficiently phagocytosed (>20%) and killed (44%) by human neutrophils as wild-type conidia. Furthermore, Aspf2 recruited human plasminogen and, when activated by tissue-type plasminogen activator, newly generated plasmin cleaved the chromogenic substrate S2251 and degraded fibrinogen. Furthermore, plasmin attached to conidia damaged human lung epithelial cells, induced cell retraction, and caused matrix exposure. Thus, Aspf2 is a central immune evasion protein and plasminogen ligand of A. fumigatus. By blocking host innate immune attack and by disrupting human lung epithelial cell layers, Aspf2 assists in early steps of fungal infection and likely allows tissue penetration.

Animal Models of Aspergillosis

Aspergillosis is an airborne fungal disease caused by Aspergillus spp., a group of ubiquitous molds. This disease causes high morbidity and mortality in both humans and animals. The growing importance of this infection over recent decades has created a need for practical and reproducible models of aspergillosis. The use of laboratory animals provides a platform to understand fungal virulence and pathophysiology, assess diagnostic tools, and evaluate new antifungal drugs. In this review, we describe the fungus, various Aspergillus-related diseases in humans and animals and various experimental animal models. Overall, we highlight the advantages and limitations of the animal models, the experimental variables that can affect the course of the disease and the reproducibility of infection, and the critical need for standardization of the species, immunosuppressive drugs, route of infection, and diagnostic criteria to use.

Humoral immune responses to infection: common mechanisms and unique strategies to combat pathogen immune evasion tactics

Humoral immune responses are crucial for protection against invading pathogens and are the underlying mechanism of protection for most successful vaccines. Our understanding of how humoral immunity develops is largely based on animal models utilizing experimental immunization systems. While these studies have made enormous progress for the field and have defined many of the fundamental principles of B cell differentiation and function, we are only now beginning to appreciate the complexities of humoral immune responses induced by infection. Co-evolution of the adaptive immune system and the pathogenic world has created a diverse array of B cell responses to infections, with both shared and unique strategies. In this review, we consider the common mechanisms that regulate the development of humoral immune responses during infection and highlight recent findings demonstrating the evolution of unique strategies used by either host or pathogen for survival.

Infections associated with the use of eculizumab: recommendations for prevention and prophylaxis

PURPOSE OF REVIEW

Eculizumab inhibits complement effector functions and has significantly impacted the treatment of paroxysmal nocturnal hemoglobinuria and atypical hemolytic uremic syndrome. However, the risks of potentially life-threatening infections, notably with Neisseria spp. in addition to its cost, are major challenges in clinical practice. In this review, we characterize and summarize the infectious complications reported with the use of eculizumab in the context of its typical and expanding indications.

RECENT FINDINGS

Use of eculizumab is rapidly extending to the fields of transplantation and neurology. Eculizumab has been primarily associated with an increased risk of meningococcal infections. Immunization against its commonest serotypes (ABCWY) is now possible with the advent of the meningococcal B vaccine. A combined ABCWY vaccine is underway. Preventive strategies against breakthrough Neisseria infections should also include chemoprophylaxis. Less is known about the association of eculizumab with other infections as recently reported. Surrogate markers of complement blockade, notably CH50, and eculizumab efficacy may help in the risk assessment of infection.

SUMMARY

Eculizumab has opened new horizons in the treatment of complement-mediated disorders. Prophylactic and immunization strategies against the risk of Nesseria spp. infections are sound and feasible. The use of eculizumab is expanding beyond complement-mediated diseases to transplantation and neurological disorders. Further research is needed to better define and stratify the risk of infection and prevention strategies in patients with the latter indications.

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.

An overview of the synergy and crosstalk between pentraxins and collectins/ficolins: their functional relevance in complement activation

The complement system is an innate immune defense machinery comprising components that deploy rapid immune responses and provide efficient protection against foreign invaders and unwanted host elements. The complement system is activated upon recognition of pathogenic microorganisms or altered self-cells by exclusive pattern recognition molecules (PRMs), such as collectins, ficolins and pentraxins. Recent accumulating evidence shows that the different classes of effector PRMs build up a co-operative network and exert synergistic effects on complement activation. In this review, we describe our updated view of the crosstalk between previously unlinked PRMs in complement activation and the potential pathogenic effects during infection and inflammation.

Deciphering the Counterplay of Aspergillus fumigatus Infection and Host Inflammation by Evolutionary Games on Graphs

Microbial invaders are ubiquitously present and pose the constant risk of infections that are opposed by various defence mechanisms of the human immune system. A tight regulation of the immune response ensures clearance of microbial invaders and concomitantly limits host damage that is crucial for host viability. To investigate the counterplay of infection and inflammation, we simulated the invasion of the human-pathogenic fungus Aspergillus fumigatus in lung alveoli by evolutionary games on graphs. The layered structure of the innate immune system is represented by a sequence of games in the virtual model. We show that the inflammatory cascade of the immune response is essential for microbial clearance and that the inflammation level correlates with the infection-dose. At low infection-doses, corresponding to daily inhalation of conidia, the resident alveolar macrophages may be sufficient to clear infections, however, at higher infection-doses their primary task shifts towards recruitment of neutrophils to infection sites.

Review of current methods for characterizing virulence and pathogenicity potential of industrial Saccharomyces cerevisiae strains towards humans

Most industrial Saccharomyces cerevisiae strains used in food or biotechnology processes are benign. However, reports of S. cerevisiae infections have emerged and novel strains continue to be developed. In order to develop recommendations for the human health risk assessment of S. cerevisiae strains, we conducted a literature review of current methods used to characterize their pathogenic potential and evaluated their relevance towards risk assessment. These studies revealed that expression of virulence traits in S. cerevisiae is complex and depends on many factors. Given the opportunistic nature of this organism, an approach using multiple lines of evidence is likely necessary for the reasonable prediction of the pathogenic potential of a particular strain. Risk assessment of S. cerevisiae strains would benefit from more research towards the comparison of virulent and non-virulent strains in order to better understand those genotypic and phenotypic traits most likely to be associated with pathogenicity.

Antibodies generated against Streptococci protect in a mouse model of disseminated aspergillosis

Invasive aspergillosis (IA) resulting from infection by Aspergillus fumigatus is a leading cause of death in immunosuppressed populations. There are limited therapeutic options for this disease and currently no vaccine. There is evidence that some anti-A. fumigatus mAbs can provide protection against IA. However, vaccine development has been impeded by a paucity of immunological targets on this organism demonstrated to provide protective responses. Sialylated oligosaccharide epitopes found on a variety of pathogens, including fungi and group B streptococci (GBS), are thought to be major virulence factors of these organisms facilitating pathogen attachment to host cells and modulating complement activation and phagocytosis. Because some of these oligosaccharide structures are conserved across kingdoms, we screened a panel of mAbs raised against GBS serotypes for reactivity to A. fumigatus. This approach revealed that SMB19, a GBSIb type-specific mAb, reacts with A. fumigatus conidia and hyphae. The presence of this Ab in mice, as a result of passive or active immunization, or by enforced expression of the SMB19 H chain as a transgene, results in significant protection in both i.v. and airway-induced models of IA. This study demonstrates that some Abs generated against bacterial polysaccharides engage fungal pathogens and promote their clearance in vivo and thus provide rationale of alternative strategies for the development of vaccines or therapeutic mAbs against these organisms.

Effective Neutrophil Phagocytosis of Aspergillus fumigatus Is Mediated by Classical Pathway Complement Activation

Aspergillus fumigatus is an important airborne fungal pathogen and a major cause of invasive fungal infections. Susceptible individuals become infected via the inhalation of dormant conidia. If the immune system fails to clear these conidia, they will swell, germinate and grow into large hyphal structures. Neutrophils are essential effector cells for controlling A. fumigatus infection. In general, opsonization of microbial particles is crucial for efficient phagocytosis and killing by neutrophils. Although the antibodies present in human serum do bind to all fungal morphotypes, we observed no direct antibody-mediated phagocytosis of A. fumigatus. We show that opsonization, phagocytosis and killing by neutrophils of A. fumigatus is complement-dependent. Using human sera depleted of key complement components, we investigated the contribution of the different complement initiation pathways in complement activation on the fungal surface. We describe the classical complement pathway as the main initiator of complement activation on A. fumigatus swollen conidia and germ tubes. Antibodies play an important role in complement activation and efficient innate recognition, phagocytosis and killing of A. fumigatus by neutrophils.

Interference of Aspergillus fumigatus with the immune response

Aspergillus fumigatus is a saprotrophic filamentous fungus and also the most prevalent airborne fungal pathogen of humans. Depending on the host’s immune status, the variety of diseases caused by A. fumigatus ranges from allergies in immunocompetent hosts to life-threatening invasive infections in patients with impaired immunity. In contrast to the majority of other Aspergillus species, which are in most cases nonpathogenic, A. fumigatus features an armory of virulence determinants to establish an infection. For example, A. fumigatus is able to evade the human complement system by binding or degrading complement regulators. Furthermore, the fungus interferes with lung epithelial cells, alveolar macrophages, and neutrophil granulocytes to prevent killing by these immune cells. This chapter summarizes the different strategies of A. fumigatus to manipulate the immune response. We also discuss the potential impact of recent advances in immunoproteomics to improve diagnosis and therapy of an A. fumigatus infection.

Fungi in the cystic fibrosis lung: bystanders or pathogens?

Improvement to the life expectancy of people with cystic fibrosis (PWCF) brings about novel challenges including the need for evaluation of the role of fungi in the cystic fibrosis (CF) lung. To determine if such organisms represent bystanders or pathogens affecting clinical outcomes we review the existing knowledge from a clinical, biochemical, inflammatory and immunological perspective. The prevalence and importance of fungi in the CF airway has likely been underestimated with the most frequently isolated filamentous fungi being Aspergillus fumigatus and Scedosporium apiospermum and the major yeast Candida albicans. Developing non-culture based microbiological methods for fungal detection has improved both our classification and understanding of their clinical consequences including localized, allergic and systemic infections. Cross-kingdom interaction between bacteria and fungi are discussed as is the role of biofilms further affecting clinical outcome. A combination of host and pathogen-derived factors determines if a particular fungus represents a commensal, colonizer or pathogen in the setting of CF. The underlying immune state, disease severity and treatment burden represent key host variables whilst fungal type, form, chronicity and virulence including the ability to evade immune recognition determines the pathogenic potential of a specific fungus at a particular point in time. Further research in this emerging field is warranted to fully elucidate the spectrum of disease conferred by the presence of fungi in the CF airway and the indications for therapeutic interventions.

Aspergillus-associated airway disease, inflammation, and the innate immune response

Aspergillus moulds exist ubiquitously as spores that are inhaled in large numbers daily. Whilst most are removed by anatomical barriers, disease may occur in certain circumstances. Depending on the underlying state of the human immune system, clinical consequences can ensue ranging from an excessive immune response during allergic bronchopulmonary aspergillosis to the formation of an aspergilloma in the immunocompetent state. The severest infections occur in those who are immunocompromised where invasive pulmonary aspergillosis results in high mortality rates. The diagnosis of Aspergillus-associated pulmonary disease is based on clinical, radiological, and immunological testing. An understanding of the innate and inflammatory consequences of exposure to Aspergillus species is critical in accounting for disease manifestations and preventing sequelae. The major components of the innate immune system involved in recognition and removal of the fungus include phagocytosis, antimicrobial peptide production, and recognition by pattern recognition receptors. The cytokine response is also critical facilitating cell-to-cell communication and promoting the initiation, maintenance, and resolution of the host response. In the following review, we discuss the above areas with a focus on the innate and inflammatory response to airway Aspergillus exposure and how these responses may be modulated for therapeutic benefit.

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.

In vitro-in vivo correlation of voriconazole resistance due to G448S mutation (cyp51A gene) in Aspergillus fumigatus

Invasive pulmonary aspergillosis continues to be associated with a high mortality despite timely and appropriate therapy. Although host immunity plays a major role in poor clinical response, antifungal drug resistance cannot be ignored. Our studies were aimed 1) to study the mechanism of drug resistance in voriconazole strains of Aspergillus fumigatus, 2) to establish a causal relationship between cyp51A mutation and voriconazole resistance (VRC-R), and 3) to determine whether VRC-R due to cyp51A mutation correlated with in vivo resistance. A point mutation (G448S) involving cyp51A gene in VRC-R isolate was associated with resistance to VRC but not to posaconazole (POS); POS had superior activity to VRC in reducing lung fungal burden and mortality in mice infected with a VRC-R mutant of A. fumigatus. Our study demonstrated that azole resistance is based on specific site of cyp51A mutation and that in vitro VRC-R correlates with in vivo resistance.

Complement Attack against Aspergillus and Corresponding Evasion Mechanisms

Invasive aspergillosis shows a high mortality rate particularly in immunocompromised patients. Perpetually increasing numbers of affected patients highlight the importance of a clearer understanding of interactions between innate immunity and fungi. Innate immunity is considered to be the most significant host defence against invasive fungal infections. Complement represents a crucial part of this first line defence and comprises direct effects against invading pathogens as well as bridging functions to other parts of the immune network. However, despite the potency of complement to attack foreign pathogens, the prevalence of invasive fungal infections is increasing. Two possible reasons may explain that phenomenon: First, complement activation might be insufficient for an effective antifungal defence in risk patients (due to, e.g., low complement levels, poor recognition of fungal surface, or missing interplay with other immune elements in immunocompromised patients). On the other hand, fungi may have developed evasion strategies to avoid recognition and/or eradication by complement. In this review, we summarize the most important interactions between Aspergillus and the complement system. We describe the various ways of complement activation by Aspergillus and the antifungal effects of the system, and also show proven and probable mechanisms of Aspergillus for complement evasion.

Immunogenetics of invasive aspergillosis

Invasive aspergillosis is one of the most important infections in hematopoietic stem cell transplant recipients, with an incidence rate of 5-15% and an associated mortality of 30-60%. It remains unclear why certain patients develop invasive aspergillosis while others, undergoing identical transplant regimen and similar post transplant immunosuppression, do not. Over the last decade, pattern recognition receptors such as Toll-like receptors (TLRs) and the C-type lectin receptors (CLRs) have emerged as critical components of the innate immune system. By detecting specific molecular patterns from invading microbes and initiating inflammatory and subsequent adaptive immune responses, pattern recognition receptors are strategically located at the molecular interface of hosts and pathogens. Polymorphisms in pattern recognition receptors and downstream signaling molecules have been associated with increased or decreased susceptibility to infections, suggesting that their detection may have an increasing impact on the treatment and prevention of infectious diseases in the coming years. Infectious risk stratification may be particularly relevant for patients with hematologic malignancies, because of the high prevalence and severity of infections in this population. This review summarizes the innate immune mechanisms involved in Aspergillus fumigatus detection and the role of host genetic polymorphisms in susceptibility to invasive aspergillosis.

Complement C3 plays an essential role in the control of opportunistic fungal infections

The innate recognition of fungal pathogens is a crucial first step in the induction of protective antifungal immunity. Complement is thought to be one key component in this process, facilitating fungal recognition and inducing early inflammation. However, the roles of the individual complement components have not been examined extensively. Here we have used mice lacking C3 to examine its role in immunity to opportunistic fungal pathogens and show that this complement component is essential for resistance to infections with Candida albicans and Candida glabrata. We demonstrate that the absence of C3 impairs fungal clearance but does not affect inflammatory responses. We also show that the presence of C3 contributes to mortality in mice challenged with very high doses of Saccharomyces cerevisiae, although these effects were found to be mouse strain dependent.

Vaccine-induced cellular immune responses differ from innate responses in susceptible and resistant strains of mice infected with Coccidioides posadasii

Susceptibility to Coccidioides spp. varies widely in humans and other mammals and also among individuals within a species. Among strains of mice with various susceptibilities, immunohistopathology revealed that C57BL/6 mice were highly susceptible to the disease following intranasal infection, DBA/2n mice were intermediate, and Swiss-Webster mice were innately resistant. Resistant Swiss-Webster mice developed prominent perivascular/peribronchiolar lymphocytic cuffing and well-formed granulomas with few fungal elements and debris in the necrotic center, surrounded by a mantle of macrophages, lymphocytes, and fibrocytes. Susceptible C57BL/6 mice became moribund between 14 and 18 days postinfection, with overwhelming numbers of neutrophils and spherules and very few T cells, the drastic reduction of which was associated with failure and death, while intermediate DBA/2n mice controlled the fungal burden but demonstrated progressive lung inflammation with prominent suppuration, and they deteriorated clinically. Vaccinated C57BL/6 mice had an early and robust lymphocyte response, which included significantly higher Mac2(+), CD3(+), and CD4(+) cell scores on day 18 than those of innately resistant SW mice and DBA/2n mice; they also had prominent perivascular/peribronchiolar lymphocytic infiltrates not present in their unvaccinated counterparts, and they appeared to be resolving lesions by day 56 compared to the other two strains, based on significantly lower disease scores and observably smaller and fewer lesions with few spherules and neutrophils.

Complement induction and complement evasion in patients with cerebral aspergillosis

Cerebral aspergillosis is a mostly lethal infection of the central nervous system. Former results identified low cerebral complement levels as one cause for insufficient immune reaction. Therefore we studied cerebral complement expression after fungal invasion and investigated putative mechanisms of Aspergillus spp to cope with the complement-induced selection pressure. Brain tissue derived from patients with cerebral aspergillosis or non-infected individuals was analyzed immunohistochemically for complement synthesis. Correlations between expression levels were determined statistically. Increased complement synthesis, a prerequisite for strengthened antifungal potency, was visible in resident astrocytes, neurons, oligodendrocytes as well as in infiltrating macrophages after fungal infection. Surprisingly, microglia, although regarded as major immune cells, only marginally participated in synthesis of most complement proteins. Several evasion mechanisms were found that help the fungus to establish a cerebral infection even in the presence of complement: Fungal hyphae limit the surface deposition of C3 and thus interfere with complement-dependent phagocytosis. Furthermore, the "sealing off" in brain abscesses not only inhibits fungal spreading but also forms protection shields against complement attack. Complement indeed seems to represent an important selection pressure and evokes the development of fungal evasion mechanisms. Counteractions for these evasion processes might represent interesting therapeutic approaches.

Complement and fungal pathogens: an update

Fungal infections are a serious complication in immunocompromised patients such as human immunodeficiency virus-infected individuals, patients with organ transplantations or with haematological neoplasia. The lethality of opportunistic fungal infection is high despite a growing arsenal of antimycotic drugs, implying the urgent need for supportive immunological therapies to strengthen the current inefficient antimicrobial defences of the immunocompromised host. Therefore, increasing effort has been directed to investigating the interplay between fungi and the host immunity and thus to find starting points for additional therapeutic approaches. In this article, we review the actual state of the art concerning the role of complement in the pathogenesis of fungal infections. Important aspects include the activation of the complement system by the fungal pathogen, the efficiency of the complement-associated antimicrobial functions and the arsenal of immune evasion strategies applied by the fungi. The twin functions of complement as an interactive player of the innate immunity and at the same time as a modulator of the adaptive immunity make this defence weapon a particularly interesting therapeutic candidate to mobilise a more effective immune response and to strengthen in one fell swoop a broad spectrum of different immune reactions. However, we also mention the 'Yin-Yang' nature of the complement system in fungal infections, as growing evidence assigns to complement a contributory part in the pathogenesis of fungus-induced allergic manifestations.

Plasminogen alleles influence susceptibility to invasive aspergillosis

Invasive aspergillosis (IA) is a common and life-threatening infection in immunocompromised individuals. A number of environmental and epidemiologic risk factors for developing IA have been identified. However, genetic factors that affect risk for developing IA have not been clearly identified. We report that host genetic differences influence outcome following establishment of pulmonary aspergillosis in an exogenously immune suppressed mouse model. Computational haplotype-based genetic analysis indicated that genetic variation within the biologically plausible positional candidate gene plasminogen (Plg; Gene ID 18855) correlated with murine outcome. There was a single nonsynonymous coding change (Gly110Ser) where the minor allele was found in all of the susceptible strains, but not in the resistant strains. A nonsynonymous single nucleotide polymorphism (Asp472Asn) was also identified in the human homolog (PLG; Gene ID 5340). An association study within a cohort of 236 allogeneic hematopoietic stem cell transplant (HSCT) recipients revealed that alleles at this SNP significantly affected the risk of developing IA after HSCT. Furthermore, we demonstrated that plasminogen directly binds to Aspergillus fumigatus. We propose that genetic variation within the plasminogen pathway influences the pathogenesis of this invasive fungal infection.

Efficacy of SPK-843, a novel polyene antifungal, in comparison with amphotericin B, liposomal amphotericin B, and micafungin against murine pulmonary aspergillosis

SPK-843, a new polyene antifungal, exhibited dose-dependent efficacy on murine pulmonary aspergillosis models. SPK-843 doses of higher than 1.0 mg/kg of body weight exhibited no renal toxicities and a tendency toward better survival prolongation than the estimated maximum tolerated doses of amphotericin B (Fungizone) (1.0 mg/kg) and liposomal amphotericin B (AmBisome) (8.0 mg/kg).

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.

Role of nitrogen and carbon transport, regulation, and metabolism genes for Saccharomyces cerevisiae survival in vivo

Saccharomyces cerevisiae is both an emerging opportunistic pathogen and a close relative of pathogenic Candida species. To better understand the ecology of fungal infection, we investigated the importance of pathways involved in uptake, metabolism, and biosynthesis of nitrogen and carbon compounds for survival of a clinical S. cerevisiae strain in a murine host. Potential nitrogen sources in vivo include ammonium, urea, and amino acids, while potential carbon sources include glucose, lactate, pyruvate, and fatty acids. Using mutants unable to either transport or utilize these compounds, we demonstrated that no individual nitrogen source was essential, while glucose was the most significant primary carbon source for yeast survival in vivo. Hydrolysis of the storage carbohydrate glycogen made a slight contribution for in vivo survival compared with a substantial requirement for trehalose hydrolysis. The ability to sense and respond to low glucose concentrations was also important for survival. In contrast, there was little or no requirement in vivo in this assay for any of the nitrogen-sensing pathways, nitrogen catabolite repression, the ammonium- or amino acid-sensing pathways, or general control. By using auxotrophic mutants, we found that some nitrogenous compounds (polyamines, methionine, and lysine) can be acquired from the host, while others (threonine, aromatic amino acids, isoleucine, and valine) must be synthesized by the pathogen. Our studies provide insights into the yeast-host environment interaction and identify potential antifungal drug targets.

Efficacy of micafungin alone or in combination against experimental pulmonary aspergillosis

Mortality from invasive pulmonary aspergillosis approaches 80% with few useful therapeutic options available. In these studies, we examined the efficacy of micafungin (MICA) alone or in combination with other antifungals in a model of pulmonary aspergillosis in immunosuppressed DBA/2 mice infected intranasally with conidia of Aspergillus fumigatus 10AF. In the initial study, groups of mice were given saline, or 1, 3 or 10 mg kg(-1) of MICA b.i.d., s.c. All saline controls, and 90% of untreated mice succumbed to infection. The efficacy of MICA was difficult to assess because of an apparent toxicity at 10 mg kg(-1). MICA given at 1 mg/kg significantly prolonged survival over the saline controls (P = 0.008). MICA at 3 or 10 mg kg(-1) versus the saline controls approached significance. No treatment regimen differed in efficacy. The efficacy of combination therapy was assessed, with mice given either no treatment, MICA at 1 mg/kg/dose, 0.8 mg kg(-1) of intravenous amphotericin B (AMB), 100 mg kg(-1) of oral itraconazole (ICZ), or 100 mg/kg/dose of twice-daily subcutaneous nikkomycin Z (NIK). AMB alone and MICA + AMB or MICA +NIK significantly prolonged survival (P < 0.05 - 0.02) over that of the controls. ICZ alone, ICZ+MICA and NIK alone did not significantly prolong survival. MICA alone at 1 mg/kg approached significance in prolonging survival. The combination of MICA and ICZ appeared to be potentially antagonistic. Although AMB+MICA was efficacious, no synergistic activity was noted for any of the regimens. Overall, these results indicate that MICA has moderate activity against pulmonary aspergillosis and might be useful in combination with conventional AMB.

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

Animal models of aspergillosis have been used extensively to study various aspects of pathogenesis, innate and acquired host-response, disease transmission and therapy. Several different animal models of aspergillosis have been developed. Because aspergillosis is an important pulmonary disease in birds, avian models have been used successfully to study preventative vaccines. Studies done to emulate human disease have relied on models using common laboratory animal species. Guinea pig models have primarily been used in therapy studies of invasive pulmonary aspergillosis (IPA). Rabbits have been used to study IPA and systemic disease, as well as fungal keratitis. Rodent, particularly mouse, models of aspergillosis predominate as the choice for most investigators. The availability of genetically defined strains of mice, immunological reagents, cost and ease of handling are factors. Both normal and immunosuppressed animals are used routinely. These models have been used to determine efficacy of experimental therapeutics, comparative virulence of different isolates of Aspergillus, genes involved in virulence, and susceptibility to infection with Aspergillus. Mice with genetic immunological deficiency and cytokine gene-specific knockout mice facilitate studies of the roles cells, and cytokines and chemokines, play in host-resistance to Aspergillus. Overall, these models have been critical to the advancement of therapy, and our current understanding of pathogenesis and host-resistance.

Antifungal activity of the local complement system in cerebral aspergillosis

Dissemination of aspergillosis into the central nervous system is associated with nearly 100% mortality. To study the reasons for the antifungal immune failure we analyzed the efficacy of cerebral complement to combat the fungus Aspergillus. Incubation of Aspergillus in non-inflammatory cerebrospinal fluid (CSF) revealed that complement levels were sufficient to obtain a deposition on the surface, but opsonization was much weaker than in serum. Consequently complement deposition from normal CSF on fungal surface stimulated a very low phagocytic activity of microglia, granulocytes, monocytes and macrophages compared to stimulation by conidia opsonized in serum. Similarly, opsonization of Aspergillus by CSF was not sufficient to induce an oxidative burst in infiltrating granulocytes, whereas conidia opsonized in serum induced a clear respiratory signal. Thus, granulocytes were capable of considerably reducing the viability of serum-opsonized Aspergillus conidia, but not of conidia opsonized in CSF. The limited efficacy of antifungal attack by cerebral complement can be partly compensated by enhanced synthesis, leading to elevated complement concentrations in CSF derived from a patient with cerebral aspergillosis. This inflammatory CSF was able to induce (i) a higher complement deposition on the Aspergillus surface than non-inflammatory CSF, (ii) an accumulation of complement activation products and (iii) an increase in phagocytic and killing activity of infiltrating granulocytes. However, levels and efficacy of the serum-derived complement were not reached. These data indicate that low local complement synthesis and activation may represent a central reason for the insufficient antifungal defense in the brain and the high mortality rate of cerebral aspergillosis.

Temporal events in the intravenous challenge model for experimental Candida albicans infections in female mice

We characterized the intravenous (i.v.) challenge model for disseminated Candida albicans infection in female BALB/c and DBA/2 mice. Clearance of fungi from the bloodstream and appearance of fungi in tissues were measured at intervals after challenge with various doses of C. albicans. The wild-type isolate SC5314 and derived strains CAF2-1 and CAI-4 transformed with CIp10 were of equal virulence in the model. Variability in mouse survival times, kidney fungal burdens and cachexia was lowest when challenge inocula were within the range 10(4)-10(5) CFU g(-1) body weight in BALB/c mice, but brain fungal burdens and outcomes in DBA/2 mice were variable for all inocula tested. Critical times in the development of infections in optimally challenged BALB/c mice were at 5-10 h (bloodstream fully cleared of fungi), 24 h (start of exponential fungal growth in kidneys) and 48 h (50% of blood cultures become positive). Differential involvement of right and left kidneys occurred almost exclusively in mice challenged with <2 x 10(4) CFU g(-1). We conclude that the i.v. challenge model in female BALB/c mice is now sufficiently well characterized to permit more refined experimentation in future virulence studies with C. albicans mutants.

A Novel T Cell Receptor Transgenic Animal Model of Seborrheic Dermatitis-Like Skin Disease

We have characterized a novel animal model of the common inflammatory skin disease seborrheic dermatitis (SD) that involves the expression of the self-specific 2C transgenic T cell receptor on the DBA/2 genetic background. Opportunistic fungal pathogens are present in the primary histological lesions and severe disease can be mitigated by the administration of fluconazole, demonstrating a role for infection in disease pathogenesis. Spontaneous disease convalescence occurs at 70-90 d of age and is preceded by an expansion of CD4+ T cells that partially restores the T cell lymphopenia that occurs in these animals. The adoptive transfer of syngeneic CD4+ T cells into pre-diseased DBA/2 2C mice completely abrogates the development of cutaneous disease. The pattern of disease inheritance in DBA/2 backcrosses suggests that one, or a closely linked group of genes, may control disease penetrance. Bone marrow reconstitution experiments demonstrated that the DBA/2 susceptibility factor(s) governing disease penetrance is likely non-hematopoietic since bone marrow from disease-resistant 2C mice can adoptively transfer the full disease phenotype to non-transgenic DBA/2 animals. This model implicates fungal organisms and CD4+ T cell lymphopenia in the development of a SD-like condition and, as such, may mimic the development of SD in acquired immunodeficiency syndrome.

The role of complement in invasive fungal infections. Die Rolle des Komplements bei invasiven Pilzinfektionen

New therapeutic approaches enable organ transplantations and guarantee longer survival for AIDS patients or patients with haematological neoplasia. The price for these medical advances is immunosuppression and thus enhanced susceptibility to opportunistic fungal infections. As a consequence invasive fungal infections are on the march in modern medicine. Therapeutic limitations and difficulties strongly demand for a deeper understanding of the interaction between the various fungi and the hosts' innate and adaptive immune defence system. This understanding is the essential prerequisite for a potential therapeutic approach, which may support specifically the insufficient antifungal attack of the host. In the present article, we therefore review the current knowledge of the role of the complement system as a central part of innate immunity and as a fine tuner of adaptive immunity in the pathogenesis of invasive fungal infections, such as aspergillosis, candidosis, cryptococcosis, paracoccidioidomycosis, blastomycosis and histoplasmosis.

Genetic susceptibility to vaginal candidiasis

To enable future studies on host resistance factors and therapy, inbred and outbred mouse strains were tested for susceptibility to vaginal candidiasis. Groups of mice were given 0.5 mg estradiol 3 days before and 4 days after intravaginal challenge with a suspension of Candida albicans. On day 1 after challenge, a swab was used to quantitate infection in all groups and to assure equivalent infection levels. On day 6, this was repeated and the experiment was terminated. BALB/c, the reference strain in repeated experiments, was susceptible, showing persistent infection with levels of cfu at day 6 falling within a range between a twofold decrease and a fourfold increase in relation to day 1 levels. CD-1 outbred mice were markedly resistant, with day 6 cfu levels showing a 74- to 87-fold decrease with respect to day 1 levels, whereas other outbred strains (CF-1, SW, ICR) were susceptible. A BALB/c substrain (ByJ) was also susceptible. With exception of CBA/J, which showed modest resistance, all inbred strains were similarly susceptible, including DBA/2, AKR/J, C3H/HeN, A/J and C57BL/6. The differences between CD-1 and BALB/c mice were also seen with a second C. albicans isolate. Our results show susceptibility to vaginal candidiasis is independent of the major histocompatibility locus H2 haplotype and any effect ascribable to use of particular commercial mouse suppliers. Differences among mouse strains in susceptibility to C. albicans, as seen in previous studies involving nonvaginal challenge routes, are not reflected in this vaginal candidiasis model; in general, such resistance patterns appear specific to the route of challenge administration. The resistance seen in mouse strain CD-1 is of particular interest in that CD-1 is known to be resistant to endocrine disruption by estrogen. Our results suggest this estrogen insensitivity may have broad-ranging effects on processes other than gametogenesis, including vaginal susceptibility to candidiasis.

Modulation of C5aR expression on human neutrophils by encapsulated and acapsular Cryptococcus neoformans

Cryptococcus neoformans and cryptococcal surface polysaccharides influenced C5aR expression on human polymorphonuclear neutrophils (PMN). Encapsulated and acapsular strains produced dramatically different effects. Treatment of PMN with acapsular cryptococci up-regulated C5aR expression; whereas treatment with encapsulated cells suppressed C5aR expression. Glucuronoxylomannan (GXM), the principal constituent of the cryptococcal capsule, was responsible for such inhibition. Increased C5aR expression following treatment with acapsular cryptococci was accompanied by increased binding of C5a to PMN, increased superoxide production in response to stimulation with C5a, and an increased chemotactic response to C5a. Conversely, decreased C5aR expression following treatment with encapsulated cryptococci or acapsular cryptococci that had been pretreated with GXM was accompanied by decreased binding of C5a to PMN and a decreased chemotactic response to C5a. Our results raise the possibility that the down-regulation of C5aR expression by encapsulated cryptococci might alter PMN function at the site of cryptococcal infection.

Correlation of the frequency of petite formation by isolates of Saccharomyces cerevisiae with virulence

In previous studies on the colony phenotype switching of Saccharomyces cerevisiae, we observed that the least virulent isolates formed greater numbers of petite colonies when grown at body temperature, 37 degrees C. To determine if there is a link between virulence and petite formation, we examined the frequency of spontaneous petite formation for virulent clinical isolates (YJM128, YJM309), an intermediate virulent segregant of YJM128 (YJM145) and avirulent clinical (YJM308) and nonclinical S. cerevisiae (Y55, YJM237) after growth at 37 degrees C. The rank order of increasing frequency of petite formation was YJM128 = YJM145 < YJM309 < Y 55 < YJM308 = YJM237, which is similar to the rank-order of virulence in CD-1 mice. To assess the virulence of petites in vivo, two mouse models, CD-1 and DBA/ 2N, were infected i.v. with 10(7) cfu of either the parental grand or a spontaneously derived petite from one of four isolates previously classified with differing degrees of virulence: YJM128, YJM309, YJM145 and Y55. In both CD-1 and DBA/2N, the mean log10 cfu of grands recovered from the brain was significantly higher than that of the petites (P<0001). Overall, petites were significantly less virulent than the parental strains. However, death of some DBA/2N mice caused by YJM128 petite 1 showed that petites are not totally avirulent. To see if S. cerevisiae isolates form petite colonies in vivo, both mouse models were infected with parental grands of YJM128 and Y55. Recovered colonies were counted and confirmed as grand or petite, and the frequency of petite colonies in the brain, the target organ, correlated with the in vitro results. Overall, these studies show an inverse correlation between the frequency of petite-colony formation and the previously determined virulence of S. cerevisiae in CD-1 mice. Furthermore, petites were significantly less virulent than the parental grands, in most cases, and petites are spontaneously formed in vivo at a frequency inversely correlated to the virulence of the strain.

The effect of canine macrophages on the adherence and growth of Blastomyces dermatitidis yeast: evidence of a soluble factor that enhances the growth of B. dermatitidis yeast

Blastomycosis is a medically important systemic fungal infection of dogs and humans. Phagocytic cells are the first line of cellular defence against B. dermatitidis, and are a prominent feature in the lesions and exudate of canine blastomycosis. The adherence of B. dermatitidis yeast to canine phagocytes, and the effects of such adherence on the growth of B. dermatitidis yeast, has not been previously reported. The results of this study demonstrate that canine complement enhances the adherence of B. dermatitidis yeast to canine macrophages. Initiation of the canine complement cascade by B. dermatitidis yeast appeared to occur predominantly by the classical pathway. Adherence of B. dermatitidis yeast to canine macrophages enhanced the growth of the yeast. In the absence of macrophages, this effect could be duplicated by incubating yeast in conditioned medium from co-cultures of macrophages and yeast. This observation suggests that a soluble factor is involved in the growth enhancement of the yeast, These findings provide new insights into the adherence of B. dermatitidis yeast to canine macrophages, and how adherence influences the proliferation of B. dermatitidis yeast.

Aspergillus fumigatus and aspergillosis

Aspergillus fumigatus is one of the most ubiquitous of the airborne saprophytic fungi. Humans and animals constantly inhale numerous conidia of this fungus. The conidia are normally eliminated in the immunocompetent host by innate immune mechanisms, and aspergilloma and allergic bronchopulmonary aspergillosis, uncommon clinical syndromes, are the only infections observed in such hosts. Thus, A. fumigatus was considered for years to be a weak pathogen. With increases in the number of immunosuppressed patients, however, there has been a dramatic increase in severe and usually fatal invasive aspergillosis, now the most common mold infection worldwide. In this review, the focus is on the biology of A. fumigatus and the diseases it causes. Included are discussions of (i) genomic and molecular characterization of the organism, (ii) clinical and laboratory methods available for the diagnosis of aspergillosis in immunocompetent and immunocompromised hosts, (iii) identification of host and fungal factors that play a role in the establishment of the fungus in vivo, and (iv) problems associated with antifungal therapy.