Dynamics of extracellular release of Aspergillus fumigatus DNAand galactomannan during growth in blood and serum

Aspergillus fumigatus is the major cause of invasive aspergillosis (IA),a disease associated with high rates of morbidity and mortality in patientsundergoing treatment for haematological malignancies. This study investigated A. fumigatus growth in vitro and in a murine model of IA inorder to provide insights into the dynamics of extracellular DNA and galactomannan (GM)release and their relevance to early diagnosis of IA. Following inoculationof whole blood with 20 A. fumigatus conidia ml−1,DNA that corresponded to the inoculum could be detected by PCR but GM wasnot detected in plasma separated from the blood sample, indicating that thefungus did not grow in whole blood. The quantities of DNA detected by PCR,and GM, were proportional to the amount of fungal biomass present in vitro. Fungal DNA could be detected in the sera of mice experimentally infectedwith A. fumigatus with maximum detection in cyclophosphamide-treatedmice.

Chronic granulomatous disease

Chronic granulomatous disease is an inherited disorder of the NADPH oxidase characterized by severe bacterial and fungal infections and disordered inflammation. We propose that NADPH oxidase has a key role in regulating acute neutrophilic and T cell responses, which in turn restrains fungal growth and calibrates the inflammatory response to minimize injury and allergy. In this model, superoxide-induced activation of indoleamine 2,3-dioxygenase (IDO) is a central mechanism by which the optimal balance of antifungal host defense and immune tolerance occurs. This model is based on studies in mice and requires correlation in humans.

Polymorphisms in toll-like receptor genes and susceptibility to pulmonary aspergillosis

Toll-like receptors (TLRs) are important components of innate immunity. We investigated the association between polymorphisms in the TLR2, TLR4, and TLR9 genes and susceptibility to noninvasive forms of pulmonary aspergillosis. A significant association was observed between allele G on Asp299Gly (TLR4) and chronic cavitary pulmonary aspergillosis (odds ratio [OR], 3.46; P =.003). Susceptibility to allergic bronchopulmonary aspergillosis was associated with allele C on T-1237C (TLR9) (OR, 2.49; P =. 043). No particular polymorphism was associated with severe asthma with fungal sensitization. These findings reinforce the importance of innate immunity in the pathogenesis of different forms of aspergillosis.

The contribution of PARs to inflammation and immunity to fungi

During inflammation, host- and microbial-derived proteases trigger the activation of protease-activated receptors (PARs), a family of G-protein-coupled receptors. We report here that activation of Toll-like receptors (TLRs) by fungi unmasks an essential and divergent role for PAR(1) and PAR(2) in downstream signaling and inflammation. TLRs activated PARs and triggered distinct signal transduction pathways involved in inflammation and immunity to Candida albicans and Aspergillus fumigatus. Inflammation was promoted by PAR(1) and PAR(2) activation in response to Candida and by PAR(2) inhibition in response to Aspergillus. This occurred by TLR regulation of PAR signaling, with TLR2 promoting PAR(1) activity, and TLR4 suppressing PAR(2) activity. Thus, tissue injury and pathogens induce signals that are integrated at the level of distinct TLR/PAR-dependent pathways, the exploitation or subversion of which contributes to divergence in microbial promotion of inflammatory response.

Induction and protection of autoimmune rheumatic diseases. The role of infections

It is thought that in genetically predisposed individuals, autoimmune diseases can be promoted and/or exacerbated by viruses, bacteria, or parasitic infectious agents. Pathogens can activate innate immune response interacting with Toll-like receptors that recognize pathogen-associated molecules. As a consequence of infections, a prolonged inflammatory response may occur leading to chronic inflammation with activation of adaptive immune response. In addition, the defective clearance of apoptotic infected cells, which progress- es to secondary necrosis, can foster the autoimmune reactions. Although numerous data from humans and/or animal models support the hypothesis of a direct contribution of pathogens to the induction of the disease, some infectious agents are able to prevent autoimmune disorders. In this review, data on the innate and adaptive immune response induced by pathogens are summarized, focusing on the possible protective or non-protective role of infections in the development of autoimmune diseases.

Immune regulation and tolerance to fungi in the lungs and skin

The balance of pro- and anti-inflammatory signaling is a prerequisite for successful host/fungal interactions and requires the coordinate actions of both innate and adaptive immune systems. Although inflammation is an essential component of the protective response to fungi, its dysregulation may significantly worsen fungal diseases and limit protective antifungal immune responses. The newly described Th17 develop - mental pathway may play an inflammatory role previously attributed to uncontrolled Th1 responses and serve to accommodate the seemingly paradoxical association of chronic inflammatory responses with fungal persistence in the face of an ongoing inflammation. In this scenario, unrestricted fungal growth could result from the activation of not only pathogenic Th17 cells, but also Th2 cells whose activation is strictly dependent on fungal burden. The capacity of regulatory T cells (Tregs) to inhibit aspects of innate and adaptive antifungal immunity is required for protective tolerance to fungi. Indoleamine 2,3-dioxygenase (IDO) and tryptophan catabolites contribute to such a homeostatic condition by providing the host with immune defense mechanisms adequate for protection, without necessarily eliminating fungal pathogens - which would impair immune memory - or causing an unacceptable level of tissue damage. IDO and tryptophan metabolites may prove to be potent regulators capable of taming overzealous or heightened inflammatory host responses.

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.

Impaired antifungal effector activity but not inflammatory cell recruitment in interleukin-6-deficient mice with invasive pulmonary aspergillosis

A murine model of infection, in which immunocompetent or immunosuppressed interleukin-6-deficient (IL-6(-/-)) mice were infected intranasally with Aspergillus fumigatus conidia and were monitored for parameters of fungal colonization and innate and adaptive immunity, was used to assess the role of IL-6 in invasive pulmonary aspergillosis (IPA). The results indicate that IL-6(-/-) mice were more susceptible than wild-type mice to IPA. Susceptibility was associated with increased inflammatory pathology, decreased antifungal effector functions of phagocytes, and impaired development of protective type 1 responses. Exposure to exogenous IL-6 restored antifungal effector activity.

Host immune reactivity and antifungal chemotherapy: the power of being together

Fungal infections, particularly those in immunocompromised hosts, have been a major therapeutic challenge over recent decades. The difficulty in diagnosing invasive infections, together with the emergence of unusual opportunistic pathogens and a pathogen shift in the spectrum of the causative organisms, have greatly hampered the effectiveness of antifungal therapy. Knowledge of the immunoregulation of fungal infections may provide new insights for therapeutic interventions. The appreciation of the importance of T helper cells and cytokines in the overall coordination of the effector immune response to fungi has offered novel opportunities to manipulate these processes and to alter the outcome of fungal infections while improving the therapeutic efficacy of antifungal drugs. Cytokine and cytokine antagonists, alone or in combination with antifungals, have the potential to overcome the specific defects of host immune reactivity predisposing to fungal infections. The current challenge lies in translating much of the information obtained in experimental and preclinical studies into therapeutic strategies providing prospects for the ultimate treatment of fungal infections.

Knockout mice as experimental models of virulence

Infection models with animals whose immune systems have been selectively altered by neutralization of endogenous cytokines or by deletion of a gene have provided a valuable means to study the function of cells or cytokines in the context of complex multidimensional interactions. In particular, knockout mice have allowed a deeper insight into the in vivo performance of antifungal innate and acquired immunity, whose interplay is considered fundamental in the general defense against infections. It is conceivable that such an integrated view of effector and regulatory immune mechanisms operating in opportunistic fungal infections would facilitate the search for cells, cytokines and molecular pathways that are essential to control fungal infectivity or oppose fungus-associated immunopathology.

Postgrafting administration of granulocyte colony-stimulating factor impairs functional immune recovery in recipients of human leukocyte antigen haplotype-mismatched hematopoietic transplants

In human leukocyte antigen haplotype-mismatched transplantation, extensive T-cell depletion prevents graft-versus-host disease (GVHD) but delays immune recovery. Granulocyte colony-stimulating factor (G-CSF) is given to donors to mobilize stem cells and to recipients to ensure engraftment. Studies have shown that G-CSF promotes T-helper (Th)-2 immune deviation which, unlike Th1 responses, does not protect against intracellular pathogens and fungi. The effect of administration of G-CSF to recipients of mismatched hematopoietic transplants with respect to transplantation outcome and functional immune recovery was investigated. In 43 patients with acute leukemia who received G-CSF after transplantation, the engraftment rate was 95%. However, the patients had a long-lasting type 2 immune reactivity, ie, Th2-inducing dendritic cells not producing interleukin 12 (IL-12) and high frequencies of IL-4- and IL-10-producing CD4(+) cells not expressing the IL-12 receptor beta(2) chain. Similar immune reactivity patterns were observed on exposure of donor cells to G-CSF. Elimination of postgrafting administration of G-CSF in a subsequent series of 36 patients with acute leukemia, while not adversely affecting engraftment rate (93%), resulted in the anticipated appearance of IL-12-producing dendritic cells (1-3 months after transplantation versus > 12 months in transplant recipients given G-CSF), of CD4(+) cells of a mixed Th0/Th1 phenotype, and of antifungal T-cell reactivity in vitro. Moreover, CD4(+) cell counts increased in significantly less time. Finally, elimination of G-CSF-mediated immune suppression did not significantly increase the incidence of GVHD (< 15%). Thus, this study found that administration of G-CSF to recipients of T-cell-depleted hematopoietic transplants was associated with abnormal antigen-presenting cell functions and T-cell reactivity. Elimination of postgrafting administration of G-CSF prevented immune dysregulation and accelerated functional immune recovery.

Defective antifungal T-helper 1 (TH1) immunity in a murine model of allogeneic T-cell-depleted bone marrow transplantation and its restoration by treatment with TH2 cytokine antagonists

Patients undergoing full haplotype-mismatched hematopoietic transplantations may experience severe intractable invasive fungal infections. To verify whether an imbalanced production of T-helper 1 (TH1) and TH2 cytokines may be responsible for susceptibility to fungal infections, C3H/HeJ (H-2(k)) recipient mice were lethally irradiated, received transplantations with T-cell-depleted allogeneic bone marrow (BM) cells from mice of H-2(d) haplotype, and were infected with Candida albicans. At different time-points after transplantation, mice were assessed for pattern of TH cytokine production and susceptibility to infection. The results show that a long-term, donor-type chimerism was achieved as early as 2 weeks after BM transplantation (BMT), at the time when high-level production of TH2 cytokines (interleukin-4 [IL-4] and IL-10) and impaired production of TH1 cytokines (interferon-gamma [IFN-gamma] and IL-12] were observed. At this time, mice were highly susceptible to both disseminated and mucosal infections, as indicated by decreased survival, uncontrolled fungal growth, and failure to develop protective TH1 immunity. However, a predominant production of TH1 cytokines was observed by week 5 after BMT, at the time when mice developed donor-type protective TH1 responses and were resistant to infections. Therapeutic ablation of IL-4 or IL-10 greatly increased resistance to candidiasis. These results indicate that a dysregulated production of TH cytokines occurs in mice undergoing T-cell-depleted allogeneic BMT. The transient predominant production of TH2 cytokines over that of IL-12 impaired the ability of mice to develop antifungal TH1 resistance, an activity that could be efficiently restored upon treatment with TH2 cytokine antagonists.

Cytokines in candidiasis and aspergillosis

Both innate and T helper (Th) immunity play a central role in fungal infections. A bi-directional influence exists between the two compartments of the immune system, mainly occurring through cytokine production. On the one hand, protective Th1 or nonprotective Th2 cells mediate resistance or susceptibility to disseminated and localized fungal infections by secreting cytokines with activating or deactivating signals for effector phagocytic cells. On the other hand, cells of the innate immune system regulate the development of antifungal T helper responses by producing directive cytokines, such as interleukin (IL)-12 and IL-10. In experimental models of Candida albicans and Aspergillus fumigatus infections, the administration or neutralization of selective cytokines and the use of cytokine-deficient mice have revealed the existence of a hierarchical pattern of cytokine mediated regulation of antifungal Th cell development and effector function. A finely regulated balance of directive cytokines, rather than the relative absence of opposing cytokines, appears to be required for optimal development and maintenance of protective Th1 reactivity to fungi. Thus, it is conceivable that some cytokines may have beneficial or deleterious effects on infection, depending on the dose and timing of endogenous production or exogenous administration. A better understanding of the different, sometimes unexpected, roles of cytokines is required for their use in prophylaxis and therapy of fungal infections, either alone or in combination with antifungal agents.

Interleukin 18 restores defective Th1 immunity to Candida albicans in caspase 1-deficient mice

Caspase 1, formerly designated interleukin 1beta (IL-1beta)-converting enzyme, processes pro-IL-1beta and pro-IL-18 to yield active cytokines that play a pivotal role in inflammation and cell activation. We show here the effect of caspase 1 deficiency on the inflammatory and adaptive immune responses to the fungus Candida albicans. Caspase 1 deficiency did not affect susceptibility to primary systemic infection with the fungus, as revealed by survival and fungal growth. However, Th1-mediated resistance to reinfection was greatly impaired in caspase 1-deficient mice, and this correlated with low-level production of IL-12 and gamma interferon. Early in infection, production of these cytokines and that of tumor necrosis factor alpha, IL-6, and, interestingly, IL-1beta occurred normally in caspase 1-deficient mice, while that of IL-18 was severely impaired. Exogenous administration of IL-18, more than IL-12, restored the Th1-mediated resistance to the infection. We conclude that, while caspase 1 is not indispensable for release of mature IL-1beta in candidiasis, the caspase 1-dependent production of IL-18 may represent an important and novel pathway for the expression of sustained Th1 reactivity to the fungus.

Innate and adaptive immunity in Candida albicans infections and saprophytism

Underlying acquired immunity to the fungus Candida albicans is usually present in adult immunocompetent individuals and is presumed to prevent mucosal colonization progressing to symptomatic infection. Exploration of immunological events leading to Candida resistance or susceptibility has indicated the central role of the innate and adaptive immune systems, the relative contribution of which may vary depending on the site of the primary infection. Nevertheless, acquired resistance to infection results from the development of Th1 responses. Cytokines produced by Thl cells activate phagocytic cells to a candidacidal state. In contrast, cytokines produced by Th2 cells inhibit Th1 development and deactivate phagocytic effector cells. Because reciprocal influences have been recognized between innate and adaptive Th immunity, it appears that an integrated immune response determines the life-long commensalism of the fungus at the mucosal level, as well as the transition from mucosal saprophyte to pathogen.