Involvement of toll-like receptor 2 in experimental invasive pulmonary aspergillosis

Importance of TLR2 in early innate immune response to acute pulmonary infection with Porphyromonas gingivalis in mice

The periodontal pathogen Porphyromonas gingivalis is implicated in certain systemic diseases including atherosclerosis and aspiration pneumonia. This organism induces innate responses predominantly through TLR2, which also mediates its ability to induce experimental periodontitis and accelerate atherosclerosis. Using a validated mouse model of intratracheal challenge, we investigated the role of TLR2 in the control of P. gingivalis acute pulmonary infection. TLR2-deficient mice elicited reduced proinflammatory or antimicrobial responses (KC, MIP-1alpha, TNF-alpha, IL-6, IL-12p70, and NO) in the lung and exhibited impaired clearance of P. gingivalis compared with normal controls. However, the influx of polymorphonuclear leukocytes into the lung and the numbers of resident alveolar macrophages (AM) were comparable between the two groups. TLR2 signaling was important for in vitro killing of P. gingivalis by polymorphonuclear leukocytes or AM and, moreover, the AM bactericidal activity required NO production. Strikingly, AM were more potent than peritoneal or splenic macrophages in P. gingivalis killing, attributed to diminished AM expression of complement receptor-3 (CR3), which is exploited by P. gingivalis to promote its survival. The selective expression of CR3 by tissue macrophages and the requirement of TLR2 inside-out signaling for CR3 exploitation by P. gingivalis suggest that the role of TLR2 in host protection may be contextual. Thus, although TLR2 may mediate destructive effects, as seen in models of experimental periodontitis and atherosclerosis, we have now shown that the same receptor confers protection against P. gingivalis in acute lung infection.

Aspergillus fumigatus-induced interleukin-8 synthesis by respiratory epithelial cells is controlled by the phosphatidylinositol 3-kinase, p38 MAPK, and ERK1/2 pathways and not by the toll-like receptor-MyD88 pathway

Previous studies have established that phagocytes are key cells of the pulmonary innate immune defense against A. fumigatus, an opportunistic fungus responsible of invasive pulmonary aspergillosis. Macrophages detect A. fumigatus via Toll-like receptors 2 and 4 (TLR2 and -4) and respond by the MyD88-NF-kappaB-dependent synthesis of inflammatory mediators. In the present study, we demonstrate that respiratory epithelial cells also sense A. fumigatus and participate in the host defense. Thus, the interaction of respiratory epithelial cells with germinating but not resting conidia of A. fumigatus results in interleukin (IL)-8 synthesis that is controlled by phosphatidylinositol 3-kinase, p38 MAPK, and ERK1/2. Using MyD88-dominant negative transfected cells, we also show that IL-8 production is not dependent on the TLR-MyD88 pathway, although the MyD88 pathway is activated by A. fumigatus and leads to NF-kappaB activation. Thus, our results provide evidence for the existence of two independent signaling pathways activated in respiratory epithelial cells by A. fumigatus, one that is MyD88-dependent and another that is My88-independent and involved in IL-8 synthesis.

Immunomodulatory effects of voriconazole on monocytes challenged with Aspergillus fumigatus: differential role of Toll-like receptors

Voriconazole (VRC) has activity against Aspergillus fumigatus, the most frequent cause of invasive aspergillosis in immunocompromised patients. The combination of VRC and A. fumigatus hyphae induced a more pronounced profile of expression of genes encoding inflammatory molecules in human monocytes than Aspergillus alone did. Herein, we provide further evidence of the potential mechanism underlying this immunomodulatory effect of VRC on human monocytes in response to A. fumigatus hyphae. A significant additive antifungal effect was shown when VRC was combined with monocytes against A. fumigatus hyphae. Both A. fumigatus hyphae and VRC induced pronounced profiles of mRNA and protein expression of Toll-like receptor 2 (TLR2) as well as tumor necrosis factor alpha (TNF-alpha) in THP-1 monocytic cells compared to untreated cells. The VRC-induced increase was greater than that induced by hyphae. The combination of VRC and hyphae increased mRNA and protein expression of TLR2 and TNF-alpha to even higher levels than did either VRC or hyphae alone. In contrast, TLR4 expression, both at the mRNA and protein levels, was not increased by either VRC or hyphae or their combination. In addition, significantly more NF-kappaB was translocated to the nuclei of THP-1 cells treated with VRC than untreated cells. While VRC induced more NF-kappaB than hyphae did, treatment with the combination of the two factors induced the greatest NF-kappaB expression. The pronounced profile of TLR2 signaling, TNF-alpha expression, and NF-kappaB activation in the presence of VRC suggests an immunomodulatory effect leading to a more efficient response to A. fumigatus.

Histoplasma capsulatum yeast phase-specific protein Yps3p induces Toll-like receptor 2 signaling

Histoplasma capsulatum is a common cause of fungal infection in certain geographic areas, and although most infections are asymptomatic, it is capable of causing histoplasmosis, a disseminated, life-threatening disease, especially in immunocompromised individuals. A deeper understanding of this host-pathogen interaction is needed to develop novel therapeutic strategies to counter lethal infection. Although several lines of evidence suggest that this fungus is neurotropic in HIV patients, little is known about the immunobiology of Histoplasma infection in the central nervous system [CNS]. The goal of the present study was to understand the innate neuroimmune mechanisms that recognize H. capsulatum during the initial stages of infection. Using a 293T stable cell line expressing murine Toll-like receptor 2 [TLR2], we show here that TLR2 recognizes H. capsulatum cell wall protein Yps3p and induces the activation of NF-κB. In further experiments, we tested the ability of Yps3p to induce signaling from TLR2 in primary microglial cells, the resident brain macrophages of the CNS. Our data show that H. capsulatum Yps3p induced TLR2 signaling in wild-type microglia, but not in microglia isolated from TLR2 KO mice, confirming that Yps3p is a ligand for TLR2. Furthermore, Yps3p-induced TLR2 signaling was suppressed by vaccinia virus-encoded TLR inhibitors. This is the first demonstration of a fungal protein serving as a TLR ligand and mediating signaling in primary brain cells.

Dysregulation of toll-like receptor-2 (TLR-2)-induced effects in monocytes from patients with atopic dermatitis: impact of the TLR-2 R753Q polymorphism

BACKGROUND

Atopic dermatitis (AD) is often complicated by an enhanced susceptibility to bacterial skin infections, especially with Staphylococcus aureus. Toll-like receptors (TLR), especially TLR-2 recognizes cell wall components of S. aureus, e.g. lipoteichoic acid (LTA) and peptidoglycan (PGN). A heterozygous TLR-2 R753Q polymorphism occurs in a frequency of 11.5% in adult AD patients and has been shown to be associated with a severe phenotype of AD.

METHODS

The aim of this study was to investigate the impact of TLR-2 agonists (LTA, PGN and Pam3Cys) on cytokine production in human monocytes from AD patients with the TLR-2 R753Q polymorphism compared with that of AD patients with 'wild type' TLR-2 and control individuals to elucidate the functional role of the TLR-2 R753Q polymorphism.

RESULTS

Monocytes from AD patients with the TLR-2 R753Q mutation produced significantly more IL-6 and IL-12 compared with that of AD patients with nonmutated TLR-2 upon stimulation with TLR-2 agonists.

CONCLUSION

We show for the first time functional differences in TLR-2 responsiveness of monocytes from AD patients with the TLR-2 R753Q mutation compared with wild type AD patients in a ligand-dependent manner.

CLINICAL IMPLICATION

Our data support the emerging concept that AD patients have a dysbalance in innate and acquired immunity. TLR-2 may be essential in the pathogenesis and maintenance of AD and may be involved in the enhanced susceptibility to skin infections with S. aureus and in a higher inflammatory response in patients with AD carrying the TLR-2 polymorphism.

Stage-specific innate immune recognition of Aspergillus fumigatus and modulation by echinocandin drugs

The pulmonary innate immune system clears inhaled Aspergillus fumigatus conidia (spores) from terminal airways. Failure to control conidial germination in immune compromised hosts can result in hyphal tissue invasion and fatal disease. Insight into the molecular recognition of A. fumigatus by host leukocytes indicates that the innate immune system exploits obligate changes in fungal cell wall composition that occur at the first stage of germination, conidial swelling. Germinating spores activate at least two host signal transduction pathways. Surface exposure of fungal beta-(1,3) glucan, a polysaccharide constituent of the fungal cell wall, triggers dectin-1 signaling by host phagocytes. Spore germination leads to the induction of Toll-like receptor (TLR) signaling as well. This stage-specific recognition mechanism focuses host antifungal responses on cells with the potential for tissue invasion and may serve to limit potentially deleterious effects of inflammation in space and time. Fungal beta-(1,3) glucan not only activates host innate immune responses but also represents the target of echinocandin drugs. The activity of echinocandin drugs has largely been understood on the basis of pharmacologic growth inhibition of yeast and moulds, resulting in lysis of yeast cells and stunting of dysmorphic hyphae. The recognition that fungal beta-1,3 glucan activates dectin-1 signaling suggests that echinocandin drugs may exert immune modulatory effects by altering innate immune responses to drug-treated fungal cells, a view supported by recent data from studies on C. albicans, A. fumigatus, and non-Aspergillus moulds.

Fungal Recognition by TLR2 and Dectin-1

The innate immune system utilizes multiple receptors to recognize fungal pathogens, and the net inflammatory response is controlled by interactions between these receptors. Many fungi are recognized, at least in part, by Toll-like receptor 2 (TLR2) and Dectin-1. Examination of the roles these receptors play together and on their own is a useful model for understanding the interplay between innate immune receptors. This review focuses on the role(s) of TLR2 and Dectin-1 in triggering inflammatory responses, transcription factor activation, phagocytosis, and reactive oxygen production in response to fungi.

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.

Primary involvement of pharynx and peyer's patch in inhalational and intestinal anthrax

Bacillus anthracis causes three forms of anthrax: inhalational, gastrointestinal, and cutaneous. Anthrax is characterized by both toxemia, which is caused by secretion of immunomodulating toxins (lethal toxin and edema toxin), and septicemia, which is associated with bacterial encapsulation. Here we report that, contrary to the current view of B. anthracis pathogenesis, B. anthracis spores germinate and establish infections at the initial site of inoculation in both inhalational and cutaneous infections without needing to be transported to draining lymph nodes, and that inhaled spores establish initial infection in nasal-associated lymphoid tissues. Furthermore, we found that Peyer's patches in the mouse intestine are the primary site of bacterial growth after intragastric inoculation, thus establishing an animal model of gastrointestinal anthrax. All routes of infection progressed to the draining lymph nodes, spleen, lungs, and ultimately the blood. These discoveries were made possible through the development of a novel dynamic mouse model of B. anthracis infection using bioluminescent non-toxinogenic capsulated bacteria that can be visualized within the mouse in real-time, and demonstrate the value of in vivo imaging in the analysis of B. anthracis infection. Our data imply that previously unrecognized portals of bacterial entry demand more intensive investigation, and will significantly transform the current perception of inhalational, gastrointestinal, and cutaneous B. anthracis pathogenesis.

Anthrax is caused by Bacillus anthracis, a bacterial pathogen that forms spores, dormant bacteria that are highly resistant to destruction. Infections initiate from the introduction of spores into airways or damaged skin, or from the consumption of contaminated food. Within the host, spores germinate, then bacteria secrete toxins that cripple the immune response and sheath themselves in a capsule that prevents them from being phagocytosed. We strove to determine in real space and time where and when spores introduced by these three routes of infection germinate and how bacteria subsequently disseminate in a mouse model. This was achieved through the development of light-emitting B. anthracis that could be tracked inside a living mouse. Contrary to current models, our studies indicated that spores germinated in situ in the skin, the intestines, and the nasal passages without needing to be transported to lymph nodes. Furthermore, bacteria disseminate from initial sites of infection in a similar fashion, first to the draining lymph nodes, then the spleen, and finally the lungs and blood. These findings imply that spore interactions with local sites of entry are critical in the development of systemic disease and that disruption of these interactions may offer new methods of anthrax prevention.

Role of Toll-like receptors in lung innate defense against invasive aspergillosis. Distinct impact in immunocompetent and immunocompromized hosts

Toll-like receptors are key to pathogen recognition by a host and to the subsequent triggering of an innate immune response. Experimental and clinical evidence shows that defects in Toll-like receptors or in signaling pathways downstream from these receptors render hosts susceptible to various types of infection, including aspergillosis. Patients receiving an immunosuppressive regimen, including corticosteroid therapy or cytotoxic chemotherapy, are also susceptible to infections. Aspergillus fumigatus is an opportunistic pathogen that infects the lungs of immunosuppressed hosts. Here, we review the evidence that experimental inactivation of various Toll-like receptors and of their signaling pathways may worsen cases of invasive pulmonary aspergillosis. Moreover, the literature clearly indicates that the type of immunosuppression is very important, as it influences whether or not Toll-like receptors contribute to infection. The involvement of Toll-like receptors, based on the immunological status of the patient, should be considered if an immunosuppressive treatment must be administered.

Fungal infections in bone marrow transplant recipients

Invasive fungal infections (IFIs) can cause significant morbidity and mortality in patients after haematopoietic stem cell transplantation. The two most notorious pathogenic fungal species in this group of patients are Candida and Aspergillus. Risk factors for IFIs include: prolonged neutropaenia; fungal overgrowth and conditioning regiment-related mucositis; graft versus host disease; and steroid therapy. Clinical manifestations can be protean, and radiological changes are frequently nonspecific. Diagnostic methods include culture- and nonculture-based techniques. Some experts recommend IFI prophylaxis in the high-risk groups, such as patients with severe graft versus host disease who require prolonged immunosuppressive therapy or patients with a previous history of aspergillosis. Treatment options include therapy with azoles, including the newer agent voriconazole, amphotericin and caspofungin.

Murine splenocytes produce inflammatory cytokines in a MyD88-dependent response to Bacillus anthracis spores

Bacillus anthracis is a sporulating Gram-positive bacterium that causes the disease anthrax. The highly stable spore is the infectious form of the bacterium that first interacts with the prospective host, and thus the interaction between the host and spore is vital to the development of disease. We focused our study on the response of murine splenocytes to the B. anthracis spore by using paraformaldehyde-inactivated spores (FIS), a treatment that prevents germination and production of products associated with vegetative bacilli. We found that murine splenocytes produce IL-12 and IFN-gamma in response to FIS. The IL-12 was secreted by CD11b cells, which functioned to induce the production of IFN-gamma by CD49b (DX5) NK cells. The production of these cytokines by splenocytes was not dependent on TLR2, TLR4, TLR9, Nod1, or Nod2; however, it was dependent on the signalling adapter protein MyD88. Unlike splenocytes, Nod1- and Nod2-transfected HEK cells were activated by FIS. Both IL-12 and IFN-gamma secretion were inhibited by treatment with B. anthracis lethal toxin. These observations suggest that the innate immune system recognizes spores with a MyD88-dependent receptor (or receptors) and responds by secreting inflammatory cytokines, which may ultimately aid in resisting infection.

Antifungal immunity and adjuvant cytokine immune enhancement in cancer patients with invasive fungal infections

Invasive fungal infections are common in severely immunosuppressed patients with cancer and in recipients of haematopoietic transplants. Response to antifungal therapy alone is often inadequate. Pro-inflammatory cytokines are critical for promoting innate and adaptive cellular antifungal immune responses. Recombinant cytokines, including granulocyte-macrophage-colony stimulating factor and interferon-gamma, have been studied as adjuvant therapies for severely immunosuppressed cancer patients with difficult-to-treat invasive mycoses. The limited clinical experience to date shows a possible benefit of these cytokines, and further controlled clinical trials are needed to validate their routine use in cancer patients and stem-cell transplant recipients with invasive fungal infections who are likely to have a poor response to antifungal drug therapy.

Pneumocystis-mediated IL-8 release by macrophages requires coexpression of mannose receptors and TLR2

Interaction with the unique fungus Pneumocystis (Pc) promotes IL-8 release by human alveolar macrophages (AM), although the receptor(s) mediating IL-8 release have not been identified. TLR2 recognizes fungal components and mediates release of host defense cytokines and chemokines, although whether TLR2 mediates signaling in response to Pc is not known. In the current study, Pc induced IL-8 release by human AM, and AM pretreatment with anti-TLR2 neutralizing antibody reduced IL-8 release. However, in nonphagocytic human embryonic kidney (HEK)293 cells transfected with human TLR2 cDNA, incubation with Pc did not induce IL-8 release, whereas these same cells released IL-8 in response to the TLR2 agonist lipoteichoic acid. Targeted gene silencing of AM mannose receptors (MR; phagocytic receptors for Pc) using small interfering RNA also reduced Pc-mediated IL-8 release in human AM. However, HEK293 cells transfected with human MR cDNA alone did not release IL-8 in response to Pc. In contrast, HEK293 cells cotransfected with human TLR2 and human MR cDNA released IL-8 in response to Pc. In human AM, Pc promoted direct interaction of MR and TLR2, IL-8 release was reduced markedly upon simultaneous blocking of TLR2 and gene silencing of MR, and IL-8 release was dependent in part on transcription factor NF-kappaB and ERK1/2 and JNK MAPKs. These studies demonstrate that Pc-mediated IL-8 release by human AM requires the coexpression of MR and TLR2 and further supports the concept that combinatorial interactions of macrophage innate receptors provide specificity of host defense cell responses to infectious challenge.

Aspergillus fumigatus Induces Innate Immune Responses in Alveolar Macrophages through the MAPK Pathway Independently of TLR2 and TLR4

Aspergillus fumigatus causes invasive aspergillosis in immunosuppressed patients. In the immunocompetent host, inhaled conidia are cleared by alveolar macrophages. The signaling pathways of the alveolar macrophage involved in the clearance of A. fumigatus are poorly understood. Therefore, we investigated the role of TLRs in the immune response against A. fumigatus and their contribution to the signaling events triggered in murine alveolar macrophages upon infection with A. fumigatus conidia. Specifically, we examined the MAPKs and NF-kappaB activation and cytokine signaling. Our investigations revealed that immunocompetent TLR2, TLR4, and MyD88 knockout mice were not more susceptible to invasive aspergillosis as compared with wild-type mice and that the in vitro phosphorylation of the MAPKs ERK and p38 was not affected in TLR2, TLR4, or MyD88 knockout mice following stimulation with conidia. In vivo experiments suggest that ERK was an essential MAPK in the defense against A. fumigatus, whereas the activation of NF-kappaB appeared to play only a secondary role. In conclusion, our findings demonstrate that TLR2/4 recognition and MyD88 signaling are dispensable for the clearance of A. fumigatus under immunocompetent situations. Furthermore, our data stress the important role of ERK activation in innate immunity to A. fumigatus.

Phagocytosis of Aspergillus fumigatus conidia by murine macrophages involves recognition by the dectin-1 beta-glucan receptor and Toll-like receptor 2

Aspergillus fumigatus is a fungal pathogen causing severe infections in immunocompromised patients. For clearance of inhaled conidia, an efficient response of the innate immune system is required. Macrophages represent the first line of defence and ingest and kill conidia. C-type lectins represent a family of receptors, which recognize pathogen-specific carbohydrates. One of them is beta1-3 glucan, a major component of the fungal cell wall. Here we provide evidence that beta1-3 glucan plays an important role for the elimination of A. fumigatus conidia. Laminarin, a soluble beta1-3 glucan and antibodies to dectin-1, a well known beta1-3 glucan receptor, significantly inhibited conidial phagocytosis. On resting conidia low amounts of surface accessible beta1-3 glucan were detected, whereas high amounts were found on small spores that appear early during germination and infection as well as on resting conidia of a pksP mutant strain. Swollen conidia also display larger quantities of beta1-3 glucan, although in an irregular spotted pattern. Resting pksP mutant conidia and swollen wild-type conidia are phagocytosed with high efficiency thereby confirming the relevance of beta1-3 glucans for conidial phagocytosis. Additionally we found that TLR2 and the adaptor protein MyD88 are required for efficient conidial phagocytosis, suggesting a link between the TLR2-mediated recognition of A. fumigatus and the phagocytic response.

Strategies to enhance immune function in hematopoietic transplantation recipients who have fungal infections

The challenges in the treatment of systemic fungal infections after HSCT include: (1) changing epidemiology as less drug-susceptible saprophytic fungi are increasingly associated with human disease; (2) the difficulty of early and correct diagnosis, even with the new generation of enzymatic immunoassays; (3) the inability to reduce or eliminate predisposing factors, especially severe immune suppression in most transplant patients with these infections and (4) the uncertain role of antifungal drug combinations and risk of drug antagonism complicating effective empiric-pre-emptive therapy. Current, developing and future immune enhancement strategies including recombinant granulocyte- and granulocyte macrophage-colony stimulating factor (GM-CSF), interferon-gamma (IFN-gamma), adjuvant pro-inflammatory cytokine therapy during mobilized donor granulocyte transfusions, therapeutic potential of pentraxin, adaptive immune transfer and dendritic cell fungal vaccines. Improved understanding of the molecular pathogenesis of fungal infections and of the complexity of host antifungal immune responses has provided the critical information to readdress existing treatment paradigms and further evaluate the role of GM-CSF and IFN-gamma early in the course of therapy against life-threatening fungal infections in high-risk patients following stem cell transplantation.

An α-Glucan of Pseudallescheria boydii Is Involved in Fungal Phagocytosis and Toll-like Receptor Activation

The host response to fungi is in part dependent on activation of evolutionarily conserved receptors, including toll-like receptors and phagocytic receptors. However, the molecular nature of fungal ligands responsible for this activation is largely unknown. Herein, we describe the isolation and structural characterization of an alpha-glucan from Pseudallescheria boydii cell wall and evaluate its role in the induction of innate immune response. These analyses indicate that alpha-glucan of P. boydii is a glycogen-like polysaccharide consisting of linear 4-linked alpha-D-Glcp residues substituted at position 6 with alpha-D-Glcp branches. Soluble alpha-glucan, but not beta-glucan, led to a dose-dependent inhibition of conidia phagocytosis. Furthermore, a significant decrease in the phagocytic index occurred when alpha-glucan from conidial surface was removed by enzymatic treatment with alpha-amyloglucosidase, thus indicating an essential role of alpha-glucan in P. boydii internalization by macrophages. alpha-Glucan stimulates the secretion of inflammatory cytokines by macrophages and dendritic cells; again this effect is abolished by treatment with alpha-amyloglucosidase. Finally, alpha-glucan induces cytokine secretion by cells of the innate immune system in a mechanism involving toll-like receptor 2, CD14, and MyD88. These results might have relevance in the context of infections with P. boydii and other fungi, and alpha-glucan could be a target for intervention during fungal infections.

Immunity to fungi

Innate and adaptive immune responses target pathogenic fungi and provide defense against fungal infections. Recent studies demonstrate that specific host receptors recognize ligands that are unique to fungi and activate signaling cascades that lead to phagocytosis of fungi, generation of pro-inflammatory mediators, formation of reactive oxygen species, trafficking of inflammatory cells to sites of infection, and initiation of adaptive immune responses. Greater understanding of the molecular mechanisms that underlie antifungal defense has provided a framework for the investigation of protective vaccines and strategies for therapeutic adoptive cell transfer.

Signalling and oxidant adaptation in Candida albicans and Aspergillus fumigatus

Candida species and Aspergillus fumigatus were once thought to be relatively benign organisms. However, it is now known that this is not the case - Candida species rank among the top four causes of nosocomial infectious diseases in humans and A. fumigatus is the most deadly mould, often having a 90% mortality rate in immunocompromised transplant recipients. Adaptation to stress, including oxidative stress, is a necessary requisite for survival of these organisms during infection. Here, we describe the latest information on the signalling pathways and target proteins that contribute to oxidant adaptation in C. albicans and A. fumigatus, which has been obtained primarily through the analysis of mutants or inference from genome annotation.

Toll-like receptors: Recent advances, open questions and implications for aspergillosis control

Aspergillus fumigatus is a pathogenic mould that can cause severe and life-threatening infections in immunocompromised patients. Apart from novel and improved antifungals, additional strategies are required to protect patients at risk from developing invasive aspergillosis. Given the problems in diagnosis of this disease, important perspectives lie in attempts to elicit and strengthen a protective immunity. The innate immune system is the first line of defence against A. fumigatus. Phagocytes engulf and kill inhaled conidia, but also closely communicate with the adaptive immune system. Recognition of invading microbes is mediated by pattern recognition receptors (PRRs), and Toll-like receptors (TLR) 2 and TLR4 have been implicated in the immune response to A. fumigatus. The analysis of this process is hampered by the fact that A. fumigatus infections are inevitably coupled to germination resulting in the appearance of different fungal morphotypes, like conidia and hyphae. While conflicting data still exist on the relative importance of TLR2 and 4 in recognition of distinct A. fumigatus morphotypes, recent evidence suggests that certain TLR agonists can be used to divert the immune response towards an optimal fungicidal activity in the absence of detrimental inflammatory consequences.