Fungi, dendritic cells and receptors: a host perspective of fungal virulence

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.

Protective tolerance to fungi: the role of IL-10 and tryptophan catabolism

The pro-inflammatory-anti-inflammatory signaling balance is required for successful host-fungus interactions. The occurrence and interference of regulatory T (T-reg) cells in fungal infections offers a valuable framework for events that occur at the host-fungus interface. Control of both the class and magnitude of the immune response might confer an evolutionary advantage whereby the host effectively fights infection but limits collateral immunopathology. Alternatively, fungus-induced immunosuppression could be a powerful immunoevasion strategy for the invading pathogen. Discovery of T-reg cells as a source of IL-10 might contribute to a better understanding of the opposing functions and variable levels of production of this prototypic immunoregulatory cytokine in fungal infections and inflammatory diseases. T-reg cells, however, might also represent an additional mechanistic level at which host responses are subverted by fungi.

Immunity to Aspergillus fumigatus: the basis for immunotherapy and vaccination

Efficient responses to fungi require different mechanisms of immunity. Dendritic cells (DCs) are uniquely able to decode the fungus-associated information and translate it into qualitatively different T helper (Th) immune responses. Murine and human DCs phagocytose conidia and hyphae of Aspergillus fumigatus through distinct recognition receptors. The engagement of distinct receptors translates into disparate downstream signaling events, ultimately affecting cytokine production and co-stimulation. Adoptive transfer of different types of DCs activates protective and non-protective Th cells as well as regulatory T cells, ultimately affecting the outcome of the infection in mice with invasive aspergillosis. The infusion of fungus-pulsed or RNA-transfected DCs also accelerates recovery of functional antifungal Th 1 responses in mice with allogeneic hematopoietic stem cell transplantation. Patients receiving T cell-depleted allogeneic hematopoietic stem cell transplantation are unable to develop antigen-specific T cell responses soon after transplant due to defective DC functions. Our results suggest that the adoptive transfer of DCs may restore immunocompetence in hematopoietic stem cell transplantation by contributing to the educational program of T cells. Thus, the remarkable furictional plasticity of DCs can be exploited for the deliberate targeting of cells and pathways of cell-mediated immunity in response to the fungus.

Human cytomegalovirus downregulates complement receptors (CR3, CR4) and decreases phagocytosis by macrophages

Human cytomegalovirus (HCMV) infection is associated with an increased susceptibility to opportunistic infections. Although the subversion of adaptive immune responses has been extensively studied, the consequences of HCMV infection on natural immune responses are not well documented. A striking selective downmodulation of CD11b/CD18 (CR3) or CD11c/CD18 (CR4) was found upon HCMV infection, on two models, the monocytic THP-1 cell line and monocyte- derived macrophages. HCMV-infected macrophages have an altered adhesion/phagocytic capacity to Candida albicans, a pathogen responsible for some opportunistic infections in immunocompromised patients. These results suggest a new mechanism implicated in the augmentation of opportunistic infections in HCMV patients.

Cytokines and fungal infections

The very poor outcome of invasive fungal infections (IFI) in patients with haematological malignancies or recipients of haematopoietic stem cell transplantation is largely attributed to their compromised host defence mechanisms. The restoration or augmentation of immune responses in these patients is now considered as one of the cornerstones of effective antifungal therapy. Major advances in the field of experimental immunology have provided insight on the important regulatory role of cytokines in both innate and adaptive immunity to fungal pathogens. Preclinical studies have convincingly demonstrated that immunomodulation with cytokines can enhance the antifungal activity of neutrophils and monocytes/macrophages as well as upregulate protective T-helper type 1 adaptive immune responses. Evidence on the clinical use of cytokines in immunocompromised hosts with IFI is, however, still scant and inconclusive. The present review summarizes experimental and clinical data on the role of cytokines in the immune response to fungal pathogens and on their potential use for prevention or treatment of fungal infections. Implications for future research are also briefly discussed.

Genes and molecules involved in Aspergillus fumigatus virulence

Aspergillus fumigatus causes a wide range of diseases that include mycotoxicosis, allergic reactions and systemic diseases (invasive aspergillosis) with high mortality rates. Pathogenicity depends on immune status of patients and fungal strain. There is no unique essential virulence factor for development of this fungus in the patient and its virulence appears to be under polygenetic control. The group of molecules and genes associated with the virulence of this fungus includes many cell wall components, such as beta-(1-3)-glucan, galactomannan, galactomannanproteins (Afmp1 and Afmp2), and the chitin synthetases (Chs; chsE and chsG), as well as others. Some genes and molecules have been implicated in evasion from the immune response, such as the rodlets layer (rodA/hyp1 gene) and the conidial melanin-DHN (pksP/alb1 gene). The detoxifying systems for Reactive Oxygen Species (ROS) by catalases (Cat1p and Cat2p) and superoxide dismutases (MnSOD and Cu, ZnSOD), had also been pointed out as essential for virulence. In addition, this fungus produces toxins (14 kDa diffusible substance from conidia, fumigaclavin C, aurasperon C, gliotoxin, helvolic acid, fumagilin, Asp-hemolysin, and ribotoxin Asp fI/mitogilin F/restrictocin), allergens (Asp f1 to Asp f23), and enzymatic proteins as alkaline serin proteases (Alp and Alp2), metalloproteases (Mep), aspartic proteases (Pep and Pep2), dipeptidyl-peptidases (DppIV and DppV), phospholipase C and phospholipase B (Plb1 and Plb2). These toxic substances and enzymes seems to be additive and/or synergistic, decreasing the survival rates of the infected animals due to their direct action on cells or supporting microbial invasion during infection. Adaptation ability to different trophic situations is an essential attribute of most pathogens. To maintain its virulence attributes A. fumigatus requires iron obtaining by hydroxamate type siderophores (ornitin monooxigenase/SidA), phosphorous obtaining (fos1, fos2, and fos3), signal transductional falls that regulate morphogenesis and/or usage of nutrients as nitrogen (rasA, rasB, rhbA), mitogen activated kinases (sakA codified MAP-kinase), AMPc-Pka signal transductional route, as well as others. In addition, they seem to be essential in this field the amino acid biosynthesis (cpcA and homoaconitase/lysF), the activation and expression of some genes at 37 degrees C (Hsp1/Asp f12, cgrA), some molecules and genes that maintain cellular viability (smcA, Prp8, anexins), etc. Conversely, knowledge about relationship between pathogen and immune response of the host has been improved, opening new research possibilities. The involvement of non-professional cells (endothelial, and tracheal and alveolar epithelial cells) and professional cells (natural killer or NK, and dendritic cells) in infection has been also observed. Pathogen Associated Molecular Patterns (PAMP) and Patterns Recognizing Receptors (PRR; as Toll like receptors TLR-2 and TLR-4) could influence inflammatory response and dominant cytokine profile, and consequently Th response to infec tion. Superficial components of fungus and host cell surface receptors driving these phenomena are still unknown, although some molecules already associated with its virulence could also be involved. Sequencing of A. fumigatus genome and study of gene expression during their infective process by using DNA microarray and biochips, promises to improve the knowledge of virulence of this fungus.

Toll-like receptor-2 is essential in murine defenses against Candida albicans infections

In this work, we studied the role of toll-like receptor-2 (TLR2) in murine defenses against Candida albicans. TLR2-deficient mice experimentally infected intraperitoneally (i.p.) or intravenously (i.v.) in vivo had very significant impaired survival compared with that of control mice. In vitro production of TNF-alpha and macrophage inhibitory protein-2 (MIP-2) by macrophages from TLR2-/- mice in response to yeasts and hyphae of C. albicans were significantly lower (80% and 40%, respectively; P <0.05) than production by macrophages from wild-type mice. This impaired production of TNF-alpha and MIP-2 probably contributed to the 41% decreased recruitment of neutrophils to the peritoneal cavity of i.p. infected TLR2-/- mice. In contrast, in vitro phagocytosis of yeasts and production of reactive oxygen intermediates (ROI) were not affected in macrophages from TLR2-/- animals. Our data indicate that TLR2 plays a major role in the response of macrophages to C. albicans, triggering cytokine and chemokine expression, and it is essential for in vivo protection against infection.

Prospects for dendritic cell vaccination against fungal infections in hematopoietic transplantation

Dendritic cells (DCs) are uniquely able to initiate and control the immune response to fungi. DCs function at three levels in the manipulation of the immune response to these pathogens. First, they mount an immediate or innate response to them, for example, by producing inflammatory mediators upon capture and phagocytosis; second, through these preceding innate functions, they decode the fungus-associated information and translate it in qualitatively different Th responses, and third, they are key in containing and dampening inflammatory responses by tolerization through the induction of regulatory T cells (Treg). DCs sense fungi in a morphotype-specific manner, through the engagement of distinct recognition receptors ultimately affecting cytokine production and costimulation. Both myeloid and plasmacytoid murine and human DCs phagocytose fungi and undergo functional maturation in response to them. However, their activation program for cytokine production was different, being IL-12 mainly produced by myeloid DCs and IL-12, IL-10 and IFN-alpha mainly produced by plasmacytoid DCs. This resulted in a distinct ability for T cell priming, being Th1, Th2, and Treg differently activated by the different DC subsets. The ability of fungus-pulsed DCs to prime for Th1 and Th2 cell activation upon adoptive transfer in vivo correlated with the occurrence of resistance and susceptibility to the infections, respectively. Antifungal protective immunity was also induced upon adoptive transfer of DCs transfected with fungal RNA. The efficacy was restricted to DCs transfected with RNA from yeasts or conidia but not with RNA from fungal hyphae. The effect was fungus-specific, as no cross-protection was observed upon adoptive transfer of DCs pulsed with either fungal species. The infusion of fungus-pulsed or RNA-transfected DCs accelerated the recovery of functional antifungal Th1 responses in mice with allogeneic hematopoietic stem cell transplantation (HSCT) and affected the outcome of the infections. As the ability of phagocytose fungi was defective in peripheral DCs from patients with HSCT, soon after the transplant, our findings suggest that the adoptive transfer of DCs may restore immunocompetence in HSCT by contributing to the educational program of T cells. Thus, the remarkable functional plasticity of DCs in response to fungi can be exploited for the deliberate targeting of cells and pathways of cell-mediated immunity in response to fungal vaccines.

Dectin-1 mediates macrophage recognition of Candida albicans yeast but not filaments

The ability of Candida albicans to rapidly and reversibly switch between yeast and filamentous morphologies is crucial to pathogenicity, and it is thought that the filamentous morphology provides some advantage during interaction with the mammalian immune system. Dectin-1 is a receptor that binds beta-glucans and is important for macrophage phagocytosis of fungi. The receptor also collaborates with Toll-like receptors for inflammatory activation of phagocytes by fungi. We show that yeast cell wall beta-glucan is largely shielded from Dectin-1 by outer wall components. However, the normal mechanisms of yeast budding and cell separation create permanent scars which expose sufficient beta-glucan to trigger antimicrobial responses through Dectin-1, including phagocytosis and activation of reactive oxygen production. During filamentous growth, no cell separation or subsequent beta-glucan exposure occurs, and the pathogen fails to activate Dectin-1. The data demonstrate a mechanism by which C. albicans shape alone directly contributes to the method by which phagocytes recognize the fungus.

A Crucial Role for Tryptophan Catabolism at the Host/Candida albicansInterface

By mediating tryptophan catabolism, the enzyme indoleamine 2,3-dioxygenase (IDO) has a complex role in immunoregulation in infection, pregnancy, autoimmunity, transplantation, and neoplasia. We hypothesized that IDO might affect the outcome of the infection in mice infected with Candida albicans by virtue of its potent regulatory effects on inflammatory and T cell responses. IDO expression was examined in mice challenged with the fungus along with the consequences of its blockade by in vivo treatment with an enzyme inhibitor. We found that IDO activity was induced at sites of infection as well as in dendritic cells and effector neutrophils via IFN-gamma- and CTLA-4-dependent mechanisms. IDO inhibition greatly exacerbated infection and associated inflammatory pathology as a result of deregulated innate and adaptive/regulatory immune responses. However, a role for tryptophan catabolism was also demonstrated in a fungus-autonomous fashion; its blockade in vitro promoted yeast-to-hyphal transition. These results provide novel mechanistic insights into complex events that, occurring at the fungus/pathogen interface, relate to the dynamics of host adaptation to the fungus. The production of IFN-gamma may be squarely placed at this interface, where IDO activation probably exerts a fine control over fungal morphology as well as inflammatory and adaptive antifungal responses.

Differences in expression of cell surface co-stimulatory molecules, Toll-like receptor genes and secretion of IL-12 by bone marrow-derived dendritic cells from susceptible and resistant mouse strains in response to Coccidioides posadasii

Coccidioides posadasii is a soil fungus that causes coccidioidomycosis or Valley Fever in the endemic regions of the southwestern US and Central America. Persons with decreased T cells reactivity and immune deficiency are at increased risk of developing severe disseminated infection. Among different mouse strains, DBA/2 mice are relatively resistant to C. posadasii whereas BALB/c mice are highly susceptible, and this discrepancy has been attributed to the difference in the development and expression of their Th1 cellular response. Dendritic cells (DC) are the most potent antigen-presenting cells that are activated after taking up pathogens or pathogens-derived antigens and regulate the immune response in the host, including Th1 cellular response. However, the DC responses against C. posadasii are not characterized. In the present study, we cultured bone-marrow derived DC (BMDC) from BALB/c and DBA/2 mice and infected with C. posadasii arthroconidia. The activation of BMDC was characterized by studying expression of cell surface co-stimulatory molecules (CD11c, MHC class II, CD40, CD80, and CD86), expression of genes encoding Toll-like receptors and release of IL-12. We found that the BMDC from DBA/2 mice showed significant upregulation of Toll-like receptor-2 and 4 genes expression, secretion of IL-12 (p<0.05) and modest increase in T cell co-stimulatory molecules as compared to BMDC from BALB/c mice. The data suggest that the differences in the activation status of DC in DBA/2 and BALB/c mice may be responsible for the discrepancy in their susceptibility to C. posadasii.

A vaccine against coccidioidomycosis is justified and attainable

Coccidioides is a fungal pathogen of humans which can cause a life-threatening respiratory disease in immunocompetent individuals. Recurrent epidemics of coccidioidal infections in Southwestern United States has raised the specter of awareness of this soil-borne microbe, particularly among residents of Arizona and Southern California, and has galvanized research efforts to develop a human vaccine against coccidioidomycosis. In this review, we discuss the rationale for such a vaccine, examine the features of host innate and acquired immune response to Coccidioides infection, describe strategies used to identify and evaluate vaccine candidates, and provide an update on progress toward development of a vaccine against this endemic pathogen.

Candida albicans cell wall glycans, host receptors and responses: elements for a decisive crosstalk

Candida albicans has adapted to live on the mucosal surfaces of animals. The human species has accepted it. By contrast to numerous other commensals, C. albicans has a prominent ability to invade virtually all tissues of a host presenting with natural or acquired defects in homeostasis. C. albicans uses considerable energy to synthesize glycans, which are present either as polymers or as glyconjugates. These glycan molecules play a prominent role in the biology of C. albicans by controlling the structure and plasticity of the cell wall, and are also involved in yeast-host interactions. These glycans are recognized as 'non-self' by host innate and adaptative immunity. The signal they induce in the host depends on the 'glycan code', which is determined by the nature of the sugar, the anomer type of linkage and branching, and the length of the oligosaccharide chains. However, this model is not static because the nature of the C. albicans molecule carrying such glycan codes and their expression at the cell wall surface also determines the host response, and, in turn, the regulation of cell wall glycan arrangement dynamics in C. albicans depends on host stimuli. Candida glycans therefore play an important role in the continuous interchange that regulates the balance between saprophytism and parasitism, and resistance and infection. A goal of current research concerning the virulence attributes of C. albicans will be to determine to what extent this species is able to regulate its glycan code as a response to the host.

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

We explored the host-pathogen interactions of the human opportunistic fungus Candida albicans using Drosophila melanogaster. We established that a Drosophila strain devoid of functional Toll receptor is highly susceptible to the human pathogen C. albicans. Using this sensitive strain, we have been able to show that a set of specific C. albicans mutants of different virulence in mammalian infection models are also impaired in virulence in Drosophila and remarkably display the same rank order of virulence. This immunodeficient insect model also revealed virulence properties undetected in an immunocompetent murine model of infection. The genetic systems available in both host and pathogen will enable the identification of host-specific components and C. albicans genes involved in the host-fungal interplay.

Dendritic cell-based vaccination against opportunistic fungi

Efficient responses to the different forms of fungi require different mechanisms of immunity. Dendritic cells (DCs) are uniquely able to decode the fungus-associated information and translate it in qualitatively different T helper (Th) immune responses, in vitro and in vivo. DCs sense fungi in a morphotype-specific manner, through the engagement of distinct recognition receptors ultimately affecting cytokine production and costimulation. Adoptive transfer of different types of DCs activates protective and non-protective Th cells as well as regulatory T cells and affects the outcome of the infections. DCs transfected with fungal RNA also restore antifungal resistance in hematopoietic transplantation. Thus, the remarkable functional plasticity of DCs in response to fungi can be exploited for the deliberate targeting of cells and pathways of cell-mediated immunity in response to fungal vaccines.

Thymosin alpha 1 activates dendritic cells for antifungal Th1 resistance through toll-like receptor signaling

Dendritic cells (DCs) show a remarkable functional plasticity in the recognition of Aspergillus fumigatus and orchestrate the antifungal immune resistance in the lungs. Here, we show that thymosin alpha 1, a naturally occurring thymic peptide, induces functional maturation and interleukin-12 production by fungus-pulsed DCs through the p38 mitogen-activated protein kinase/nuclear factor (NF)-kappaB-dependent pathway. This occurs by signaling through the myeloid differentiation factor 88-dependent pathway, involving distinct Toll-like receptors. In vivo, the synthetic peptide activates T-helper (Th) cell 1-dependent antifungal immunity, accelerates myeloid cell recovery, and protects highly susceptible mice that received hematopoietic transplants from aspergillosis. By revealing the unexpected activity of an old molecule, our finding provides the rationale for its therapeutic utility and qualify the synthetic peptide as a candidate adjuvant promoting the coordinated activation of the innate and adaptive Th immunity to the fungus.

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.

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.

Parenchymal organ, and not splenic, immunity correlates with host survival during disseminated candidiasis

We examined the relationship between host survival and renal and splenic immune responses in a murine model of hematogenously disseminated candidiasis. Male BALB/c mice were infected via tail vein injection with wild-type C. albicans or with an isogenic, Deltaefg1/Deltaefg1 hypha-deficient mutant. Host survival, organ fungal burden, intracellular cytokine content of splenic and kidney lymphocytes, and whole-organ cytokine profiles were determined. Wild-type C. albicans induced type 2 splenocyte responses with both nonfatal and fatal inocula. In the kidney, conversely, wild-type inocula causing no or low mortality induced type 1 responses and 100% fatal inocula induced type 2 or interleukin-10 (IL-10)-dominant responses. Hypha-deficient mutant C. albicans caused no or low mortality while inducing type 1 responses in both the spleen and kidney. To our knowledge, this is the first demonstration that host survival during systemic infection correlates with the type of immune response engendered in a nonlymphoid, parenchymal organ and not with the response in the spleen. Furthermore, the results provide in vivo confirmation that hyphal formation by C. albicans induces type 2 or IL-10-dominant host responses in tissues.

Release of arachidonic acid by stimulation of opsonic receptors in human monocytes: the FcgammaR and the complement receptor 3 pathways

The role of the opsonic receptors FcgammaR and CR3 on the release of arachidonic acid (AA) by human monocytes was studied using IgG-ovalbumin (OVA) equivalence immune complexes (IC), anti-OVA IgG bound to OVA-coupled latex beads, and C3bi-bound IC. Release of AA was produced by IC and latex-OVA beads bound to IgG, whereas binding of C3bi to IC inhibited the ability of IC to release AA. In contrast, coating of zymosan particles with C3bi enhanced AA release as compared with that produced by non-coated particles. Masking of C3bi on C3bi-bound IC by incubation with anti-C3 IgG resulted in the recovery of their ability to release AA, thereby suggesting that binding of C3b by IC reduces their flogogenic effects, whereas opsonization of microbial walls by complement may enhance their proinflammatory potential. The binding/uptake of opsonized zymosan particles was inhibited by anti-CR3 Ab and C3bi-bound IC, but not by beta-glucan, mannan, and anti-Toll-like receptor 2 Ab. These findings show that cooperative engagement of CR3 on both the lectin-like site involved in beta-glucan binding and the I-domain involved in C3bi binding, as it can be observed in the innate immune response, produces AA release, whereas the unique interaction of C3bi-bound IC with the I-domain of CR3, as it may occur in the adaptive immune response, diverts the IC lattice from a productive interaction with FcgammaR linked to AA release.

The interaction of human dendritic cells with yeast and germ-tube forms ofCandida albicansleads to efficient fungal processing, dendritic cell maturation, and acquisition of a Th1 response-promoting function

T helper cell type 1 (Th1) cell-mediated immunity plays a critical role in protection against the opportunistic pathogen Candida albicans. Virulence of the fungus is closely associated with its ability to form germ-tubes (GT), the early phase of the dimorphic transition from the commensal yeast (Y) to the more invasive hyphal (H) form. In this study, we examined the functional outcome of the interaction of Y or GT forms with human dendritic cells (DCs), professional antigen-presenting cells, which are pivotal for initiation and modulation of T cell responses. DCs phagocytosed and killed Y and GT cells with a comparable efficiency, becoming able to trigger strong proliferative responses by Candida-specific, autologous T cell clones. Both fungal forms induced DC maturation, as indicated by up-regulation of CD83, CD80, CD86, CD40, and major histocompatibility complex classes I and II surface antigens. Chemokine receptors were also modulated in Candida-DCs, which showed increased CCR7/CXCR4 and decreased CCR5 expression. Y- and GT-activated DCs differed in the pattern of cytokine expression. In particular, GT cells, in common with fully differentiated H cells, induced significantly more elevated levels of interleukin (IL)-10 than Y cells. Nevertheless, Y-, GT-, or H-pulsed DCs secreted comparable amounts of IL-12p70. In addition, irrespective of the fungal form triggering DC activation, Candida-DCs acquired the ability to prime naive T lymphocytes with a defined Th1 phenotype. Overall, our findings highlight the induction of substantially similar functional patterns in human DCs encountering the different forms of growth of C. albicans, both seemingly activating the Th1-type immunity which is characteristic of the healthy human subjects, naturally immunized and protected against the fungus.

The damage-response framework of microbial pathogenesis

The late twentieth century witnessed the emergence of numerous infectious diseases that are caused by microorganisms that rarely cause disease in normal, healthy immunocompetent hosts. The emergence of these diseases shows that the existing concepts of pathogenicity and virulence do not take into account the fact that both the microorganism and the host contribute to microbial pathogenesis. To address this impediment to studies of host-microorganism interactions, we propose a new theoretical approach to understanding microbial pathogenesis, known as the 'damage-response' framework.

Adaptation of Candida albicans to the host environment: the role of morphogenesis in virulence and survival in mammalian hosts

Although morphological flexibility could be a key contributor to fungal virulence, no molecular data has unambiguously established fungal morphogenesis as a virulence factor for Candida albicans, nor can specific forms of Candida be regarded as absolutely indicative of saprophytism or infection at a given site on the host. The fitness of the fungus in vivo probably reflects its adaptation to the variety of microenvironments in which this opportunist must survive.