Aspergillus fumigatus evades immune recognition during germination through loss of toll-like receptor-4-mediated signal transduction

Interactions of Toll-like receptors with fungi

The role of Toll-like receptors (TLRs) in signaling immune responses to fungal pathogens is reviewed. TLR2 and TLR4, acting via the adapter protein MyD88, signal responses to Cryptococcus neoformans, Aspergillus fumigatus and Candida albicans in vitro, although the relative significance of these TLRs to host defenses in vivo is unclear.

Dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin mediates binding and internalization of Aspergillus fumigatus conidia by dendritic cells and macrophages

Aspergillus fumigatus is responsible for a large percentage of nosocomial opportunistic fungal infections in immunocompromised hosts, especially during cytotoxic chemotherapy and after bone marrow transplantation, and is currently a major direct cause of death in leukemia patients. Dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) is a type II C-type lectin that functions as an adhesion receptor and is used by viral and bacterial pathogens to gain access to human DC. We report that DC-SIGN specifically interacts with clinical isolates of A. fumigatus. DC-SIGN-dependent binding of A. fumigatus conidia can be demonstrated with stable transfectants and monocyte-derived DC and is inhibited by anti-DC-SIGN Abs. Binding and internalization of A. fumigatus conidia correlates with DC-SIGN cell surface expression levels and is abolished in the presence of A. fumigatus-derived cell wall galactomannans. The clinical relevance of this interaction is emphasized by the presence of DC-SIGN in lung DC and alveolar macrophages, and further illustrated by the DC-SIGN-dependent attachment of A. fumigatus conidia to the cell membrane of IL-4-treated monocyte-derived macrophages. Our results suggest the involvement of DC-SIGN in the initial stages of pulmonary infection as well as in fungal spreading during invasive aspergillosis.

Design of Efficacy Trials of Cytokines in Combination with Antifungal Drugs

Resolution of invasive fungal infections is often dependent on recovery from an immunocompromised state, which indicates that host defense mechanisms are extremely important in the clearance of fungal pathogens. Immunotherapy aimed at enhancement of host defense mechanisms may improve clinical outcome of invasive mycoses. The design of trials of immunotherapy against fungal pathogens requires profound knowledge of the host defense mechanisms that are involved in invasive fungal infections. Prospective phase II studies with recombinant granulocyte colony-stimulating factor and interferon-gamma have been done. Recombinant interferon-gamma is a candidate for phase III trials of adjunctive immunotherapy for cryptococcal meningitis, invasive aspergillosis, and candidemia, but the proper design of future trials will be crucial to establish whether immunotherapy is of clinical value in the treatment of invasive fungal infections.

Involvement of CD14, toll-like receptors 2 and 4, and MyD88 in the host response to the fungal pathogen Cryptococcus neoformans in vivo

The major capsular polysaccharide of Cryptococcus neoformans, glucuronoxylomannan (GXM), is recognized by Toll-like receptor 2 (TLR2), TLR4, and CD14. In these studies, mice deficient in CD14, TLR2, TLR4, and the TLR-associated adaptor protein, MyD88, were utilized to investigate the contribution of TLRs and CD14 to in vivo host defenses against C. neoformans. MyD88(-/-) mice had significantly reduced survival compared with wild-type C57BL/6 mice after intranasal (i.n.) and intravenous (i.v.) infection with live C. neoformans. CD14(-/-) mice had reduced survival when infected i.v., while TLR2(-/-) mice died significantly earlier after i.n. infection. Mortality was similar comparing TLR4 mutant C3H/HeJ mice and control C3H/HeOuJ mice following i.v. or i.n. challenge with C. neoformans. The course of pulmonary cryptococcosis was studied in more detail in the CD14(-/-), TLR2(-/-), and MyD88(-/-) mice. MyD88(-/-) mice infected i.n. had higher numbers of CFU in the lungs as well as higher GXM levels in the sera and lungs 7 days after infection than wild-type mice did. Surprisingly, there were no major differences in the levels of tumor necrosis factor alpha, interleukin-4 (IL-4), IL-10, IL-12p70, or gamma interferon in the lungs of C. neoformans-infected knockout mice compared with wild-type mice. Histopathologic analysis of the lungs on day 7 postinfection revealed minimal inflammation in all mouse groups. These studies demonstrate a major role for MyD88 and relatively minor roles for CD14 and TLR2 in the response to cryptococcal infection, with the decreased survival of MyD88(-/-) mice correlating with increased numbers of lung CFU and serum and lung GXM levels.

Recognition of fungal pathogens by Toll-like receptors

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

Toll-like receptors and microbes take aim at each other

Toll-like receptors (TLRs) are key components of effective innate immunity. As such, pathogenic microorganisms must evade recognition by TLRs, manipulate the consequences of TLR activation, or contend with the inflammatory consequences of TLR activation. Recent studies have improved our understanding of the various mechanisms by which pathogenic organisms manipulate the TLR recognition system.

Liver cells respond to Aspergillus fumigatus with an increase in C3 secretion and C3 gene expression as well as an expression increase in TLR2 and TLR4

Fungal infections by molds like Aspergillus fumigatus are an increasing health problem which can be fatal in immuno-compromised patients. In healthy individuals, these infections are easily eliminated by the innate and acquired immune system. Complement factor 3 (C3) has a key place within the complement cascade and C3 RNA expression can therefore be used to monitor an impending immune response. Employing a liver cell line (HepG2) as a model system, we have examined their responses to A. fumigatus or beta-glucan, a major component of the fungal wall. C3 RNA expression was increased after stimulation with both LPS and A. fumigatus as well as after incubation with beta-glucan, although with different kinetics. C3 protein release into the supernatant followed an inverse bell-shaped curve when cells were incubated with A. fumigatus or beta-glucan while during LPS stimulation, the release was more stable. HepG2 cells also express Toll-like receptors (TLRs) and both for TLR2 and TLR4, an expression increase was found. These data demonstrate that liver cells are able to react specifically to a fungal pathogen without the help of Kupffer cells.

Aspergillus fumigatus antigens activate innate immune cells via toll-like receptors 2 and 4

Invasive aspergillosis (IA) is a leading cause of mortality in haematological patients. Appropriate activation of the innate immune system is crucial for the successful clearance of IA. Therefore, we studied the Aspergillus fumigatus-mediated activation of human granulocytes and monocyte-derived immature dendritic cells (DCs), as well as murine bone marrow-derived DCs (BMDCs) from wild type, toll-like receptor (TLR)4-deficient, TLR2 knockout, and TLR2/TLR4 double deficient mice. Aspergillus fumigatus antigens induced the activation and maturation of immature DCs as characterized by CD83 expression, upregulation of major histocompatibility complex and co-stimulatory molecules. Moreover, fungal antigens enhanced the phagocytosis and production of interleukin (IL)-8 in granulocytes. The release of IL-12 by BMDCs in response to A. fumigatus antigens was dependent on the expression of TLR2, whereas the release of IL-6 was dependent on the expression of functional TLR4 molecules. The protein precipitate of A. fumigatus supernatant provided strong stimulation of DCs and granulocytes, indicating that a factor secreted by A. fumigatus might activate innate immune cells. In conclusion, A. fumigatus antigens induced the activation of DCs and granulocytes. Our results indicated that this activation was mediated via TLR2 and TLR4. Future studies are needed to assess the clinical impact of these findings in patients at high risk for IA.

The Contribution of the Toll-Like/IL-1 Receptor Superfamily to Innate and Adaptive Immunity to Fungal Pathogens In Vivo

In vitro studies have indicated the importance of Toll-like receptor (TLR) signaling in response to the fungal pathogens Candida albicans and Aspergillus fumigatus. However, the functional consequences of the complex interplay between fungal morphogenesis and TLR signaling in vivo remain largely undefined. In this study we evaluate the impact of the IL-1R/TLR/myeloid differentiation primary response gene 88 (MyD88)-dependent signaling pathway on the innate and adaptive Th immunities to C. albicans and A. fumigatus in vivo. It was found that 1) the MyD88-dependent pathway is required for resistance to both fungi; 2) the involvement of the MyD88 adapter may occur through signaling by distinct members of the IL-1R/TLR superfamily, including IL-1R, TLR2, TLR4, and TLR9, with the proportional role of the individual receptors varying depending on fungal species, fungal morphotypes, and route of infection; 3) individual TLRs and IL-1R activate specialized antifungal effector functions on neutrophils, which correlates with susceptibility to infection; and 4) MyD88-dependent signaling on dendritic cells is crucial for priming antifungal Th1 responses. Thus, the finding that the innate and adaptive immunities to C. albicans and A. fumigatus require the coordinated action of distinct members of the IL-1R/TLR superfamily acting through MyD88 makes TLR manipulation amenable to the induction of host resistance to fungi.

Toll-like receptors and the host defense against microbial pathogens: bringing specificity to the innate-immune system

Toll-like receptors (TLRs) have been identified as a major class of pattern-recognition receptors. Recognition of pathogen-associated molecular patterns (PAMPs) by TLRs, alone or in heterodimerization with other TLR or non-TLR receptors, induces signals responsible for the activation of genes important for an effective host defense, especially proinflammatory cytokines. Although a certain degree of redundancy exists between signals induced by the various TLRs, recent studies have identified intracellular pathways specific for individual TLRs. This leads to the release of cytokine profiles specific for particular PAMPs, and thus, TLRs confer a certain degree of specificity to the innate-immune response. In addition to the activation of the innate-immune response, TLR-mediated recognition represents a link between the innate- and acquired-immune systems, by inducing the maturation of dendritic cells and directing the T helper responses. Alternatively, recent data have also suggested TLR-mediated escape mechanisms used by certain pathogenic microorganisms, especially through TLR2 induction of anti-inflammatory cytokines. Finally, the crucial role of TLRs for the host defense against infections has been strengthened recently by the description of patients partially defective in the TLR-activation pathways.

Immunity to fungal infections

The topic of immunity to fungal infections is of interest to a wide range of disciplines, from microbiology to immunology. It is of particular interest in terms of therapy of HIV-infected individuals, and patients with cancer or individuals who have received transplants. Understanding the nature and function of the immune response to fungi is an exciting challenge that might set the stage for new approaches to the treatment of fungal diseases, from immunotherapy to vaccines. The past decade has witnessed the development of a wide range of new approaches to elucidate events that occur at the host-fungus interface.