Antifungal type 1 responses are upregulated in IL-10-deficient mice

C57BL/6 mice are highly resistant to infections caused by Candida albicans and Aspergillus fumigatus. To elucidate the role of IL-10 produced by C57BL/6 mice during these infections, parameters of infection and immunity to it were evaluated in IL-10-deficient and wild-type mice with disseminated or gastrointestinal candidiasis or invasive pulmonary aspergillosis. Unlike parasitic protozoan infection, C. albicans or A. fumigatus infection did not induce significant acute toxicity in IL-10-deficient mice, who, instead, showed reduced fungal burden and fungal-associated inflammatory responses. The increased resistance to infections as compared to wild-type mice was associated with upregulation of innate and acquired antifungal Th1 responses, such as a dramatically higher production of IL-12, nitric oxide (NO) and TNF-alpha as well as IFN-gamma by CD4+ T cells. Pharmacological inhibition of NO production greatly reduced resistance to gastrointestinal candidiasis, thus pointing to the importance of IL-10-dependent NO regulation at mucosal sites in fungal infections. These results are reminiscent of those obtained in genetically susceptible mice, in which IL-10 administration increased, and IL-10 neutralization decreased, susceptibility to C. albicans and A. fumigatus infections. Collectively, these observations indicate that the absence of IL-10 augments innate and acquired antifungal immunity by upregulating type 1 cytokine responses. The resulting protective Th1 responses lead to a prompt reduction of fungal growth, thus preventing tissue destruction and lethal levels of proinflammatory cytokines.

Interleukin-4 causes susceptibility to invasive pulmonary aspergillosis through suppression of protective type I responses

Aspergillus fumigatus, an opportunistic fungal pathogen, causes multiple allergic and nonallergic airway diseases. Invasive pulmonary aspergillosis (IPA) is a nonallergic, life-threatening disease of immunocompromised patients. In a murine model of IPA, interleukin (IL)-4-deficient (IL-4-/-) BALB/c mice were used to examine the role of IL-4 in lung pathology and immune responses. IL-4-/- mice were more resistant than wild-type mice to infection caused by multiple intranasal injections of viable A. fumigatus conidia. Resistance was associated with decreased lung inflammatory pathology, impaired T helper (Th)-2 responses (including lung eosinophilia), and an IL-12-dependent Th1 response. In contrast, development of host-detrimental antifungal Th2 cells occurred in IL-12-/- and interferon-gamma-/- mice and in IL-4-/- mice when subjected to IL-12 neutralization. These results demonstrate that IL-4 renders mice susceptible to infection with A. fumigatus by inhibition of protective Th1 responses. IL-4 appears to have a distinct role in the pathogenesis of allergic and nonallergic lung diseases caused by the fungus.

Immunity to Candida albicans: Th1, Th2 cells and beyond

Resistance to Candida albicans infection in mice results from the development of T helper (Th) type 1 cell responses. Cytokines produced by Th1 cells activate macrophages and neutrophils to a candidacidal state. The development of Th2 responses underlines susceptibility to infection, because cytokines produced by Th2 cells inhibit Th1 development and deactivate phagocytic effector cells. With the recognition of the reciprocal influences between innate and adaptive Th immunity, it appears that the coordinated action of these two lines of immune defense is required to efficiently oppose the infectivity of the fungus and to determine its lifelong commensalism at the mucosal level.

Cytokines and mycoses

Cell-mediated immunity (CMI) has been shown, over many decades of clinical observation and bench research, to be central to the outcome of invasive fungal infections. In recent years, understanding the role of messenger molecules (cytokines), in coordinating and augmenting cellular immunity has been ascendant. These studies have made it possible to consider using cytokines, now available in abundant quantities via recombinant DNA technologies, to treat fungal infections. In this symposium, the most important fungal pathogens that cause infections in humans, particularly in immunocompromised patients, are considered, with emphasis on how recent experimental work may lead to a better understanding of the role of cytokines and their use in therapy.

Type 1 and type 2 cytokines: from basic science to fungal infections

At the present time, it is clear that Th1 responses afford protection against the fungi; however, the development, maintenance and function of the protective immune responses are complex mechanisms and are influenced by multiple factors. The route of infection has been shown to affect initial cytokine production and, consequently, the induction of protective Th1 responses. The ability of different isolates of the same fungal agent to induce and sustain a protective response has also been emphasized. Protective immune responses have been shown to vary in genetically different mouse strains after infection. In addition, these protective responses, such as cellular influx and cytokine production, also vary within the same animal depending on the tissue infected. The functional dominance of certain cytokines over others in influencing development and maintenance of protective responses has been discussed. Certain cytokines may act differently in hosts lacking important components of their innate or immune repertoire. It is evident from these presentations that a more comprehensive understanding of the protective mechanisms against different fungal agents is emerging. However, there is still much to learn before cytokine modulatory therapy can be used effectively without risk in the human host.

Antibody and/or cell-mediated immunity, protective mechanisms in fungal disease: an ongoing dilemma or an unnecessary dispute?

Historically there has been controversy on the relative importance of antibody- and cell-mediated immune responses in the protection against fungal pathogens. The controversy was fuelled by the difficulties encountered in obtaining consistent results with polyclonal antibody experiments and I inducing long-lasting immune protection by vaccinations with induce stron cell-mediated responses. Recent studies indicate that both antibody- and cell-mediated immune responses can contribute to host protection against Candida albicans and C. neoformans. At the present time the major issue is not the relative importance of antibody- and cell-mediated immune responses but rather, the mechanisms by which the two arms of the immune system function and cooperate.

Cytokine- and T helper-dependent lung mucosal immunity in mice with invasive pulmonary aspergillosis

The role of cytokine- and T helper (Th)-dependent lung mucosal antifungal immunity in murine invasive pulmonary aspergillosis (IPA) was investigated. Intact or leukopenic DBA/2 mice were resistant or highly susceptible, respectively, to infection caused by multiple intranasal injections of viable Aspergillus fumigatus conidia. Resistance was associated with unimpaired innate antifungal activity of pulmonary phagocytic cells, concomitant with high-level production of tumor necrosis factor (TNF)-alpha and interleukin (IL)-12 and the presence of interstitial lymphocytes producing interferon-gamma and IL-2. Conversely, production of TNF-alpha and IL-12 was down-regulated in highly susceptible mice, which also had defective innate antifungal immunity and high-level production of IL-4 and IL-10 by lung lymphocytes. Resistance was increased in susceptible mice upon local IL-4 or IL-10 neutralization or IL-12 administration. These results indicate that, similar to observations in mice with disseminated aspergillosis, innate and Th1-dependent immunity play an essential role in host defense against IPA.

IL-10 is required for development of protective Th1 responses in IL-12-deficient mice upon Candida albicans infection

IL-12 is both required and prognostic for Th1 development in mice with Candida albicans infection. To delineate further the physiologic role of IL-12 in antifungal immunity, mice deficient for this cytokine were assessed for susceptibility to C. albicans infections, and for parameters of innate and adaptive immunity. IL-12-deficient mice were highly susceptible to gastrointestinal infection or to reinfection and showed elevated production of Candida-specific IgE and IL-4 and defective production of IFN-gamma. The failure to mount protective Th1 responses occurred despite the presence of an unimpaired innate antifungal immune response, which correlated with unaltered IFN-gamma production, but defective production of, and responsiveness to, inhibitory IL-10. IL-10 or IL-12 neutralization increased the innate antifungal resistance in wild-type mice. However, in IL-12-deficient mice, treatment with exogenous IL-12 or IL-10 impaired IL-4 production and increased resistance to infection, through a negative effect on the CTLA-4/B7-2 costimulatory pathway. These results confirm the obligatory role of IL-12 in the induction of anticandidal Th1 responses, and indicate the existence of a positive regulatory loop between IL-12 and IL-10 that may adversely affect the innate antifungal response, but is required for optimal costimulation of IL-12-dependent CD4+ Th1 cells.

IFN-gamma is required for IL-12 responsiveness in mice with Candida albicans infection

To elucidate the role of IFN-gamma in antifungal CD4+ Th-dependent immunity, 129/Sv/Ev mice deficient for IFN-gamma receptor (IFN-gammaR(-/-)) were assessed for susceptibility to gastrointestinal or systemic Candida albicans infection and for parameters of innate and adaptive T helper immunity. IFN-gammaR(-/-) mice failed to mount protective Th1-mediated acquired immunity upon mucosal immunization or in response to a live vaccine strain of the yeast. The impaired Th1-mediated resistance correlated with defective IL-12 responsiveness, but not IL-12 production, and occurred in the presence of an increased innate antifungal resistance. The development of nonprotective Th2 responses was observed in IFN-gammaR(-/-) mice upon mucosal infection and subsequent reinfection. However, under experimental conditions of Th2 cell activation, the occurrence of Th2 cell responses was similar in IFN-gammaR(-/-) and in IFN-gammaR(+/+) mice. These results indicate the complex immunoregulatory role of IFN-gamma in the induction of mucosal and nonmucosal anticandidal Th cell responses; IFN-gamma is not essential for the occurrence of Th2 responses but is required for development of IL-12-dependent protective Th1-dependent immunity.

A 70-kilodalton recombinant heat shock protein of Candida albicans is highly immunogenic and enhances systemic murine candidiasis

The 70-kDa recombinant Candida albicans heat shock protein (CaHsp70) and its 21-kDa C-terminal and 28-kDa N-terminal fragments (CaHsp70-Cter and CaHsp70-Nter, respectively) were studied for their immunogenicity, including proinflammatory cytokine induction in vitro and in vivo, and protection in a murine model of hematogenous candidiasis. The whole protein and its two fragments were strong inducers of both antibody (Ab; immunoglobulin G1 [IgG1] and IgG2b were the prevalent isotypes) and cell-mediated immunity (CMI) responses in mice. CaHsp70 preparations were also recognized as CMI targets by peripheral blood mononuclear cells of healthy human subjects. Inoculation of CaHsp70 preparations into immunized mice induced rapid production of interleukin-6 (IL-6) and tumor necrosis factor alpha, peaking at 2 to 5 h and declining within 24 h. CaHsp70 and CaHsp70-Cter also induced gamma interferon (IFN-gamma), IL-12, and IL-10 but not IL-4 production by CD4+ lymphocytes cocultured with splenic accessory cells from nonimmunized mice. In particular, the production of IFN-gamma was equal if not superior to that induced in the same cells by whole, heat-inactivated fungal cells or the mitogenic lectin concanavalin A. In immunized mice, however, IL-4 but not IL-12 was produced in addition to IFN-gamma upon in vitro stimulation of CD4+ cells with CaHsp70 and CaHsp70-Cter. These animals showed a decreased median survival time compared to nonimmunized mice, and their mortality was strictly associated with organ invasion by fungal hyphae. Their enhanced susceptibility was attributable to the immunization state, as it did not occur in congenitally athymic nude mice, which were unable to raise either Ab or CMI responses to CaHsp70 preparations. Together, our data demonstrate the elevated immunogenicity of CaHsp70, with which, however, no protection against but rather some enhancement of Candida infection seemed to occur in the mouse model used.

Endogenous interleukin 4 is required for development of protective CD4+ T helper type 1 cell responses to Candida albicans

Interleukin (IL)-4-deficient mice were used to assess susceptibility to systemic or gastrointestinal Candida albicans infections, as well as parameters of innate and elicited T helper immunity. In the early stage of systemic infection with virulent C. albicans, an unopposed interferon (IFN)-gamma response renders IL-4-deficient mice more resistant than wild-type mice to infection. Yet, IL-4-deficient mice failed to efficiently control infection in the late stage and succumbed to it. Defective IFN-gamma and IL-12 production, but not IL-12 responsiveness, was observed in IL-4-deficient mice that failed to mount protective T helper type 1 cell (Th1)-mediated acquired immunity in response to a live vaccine strain of the yeast or upon mucosal immunization in vivo. In vitro, IL-4 primed neutrophils for cytokine release, including IL-12. However, late treatment with exogenous IL-4, while improving the outcome of infection, potentiated CD4(+) Th1 responses even in the absence of neutrophils. These findings indicate that endogenous IL-4 is required for the induction of CD4(+) Th1 protective antifungal responses, possibly through the combined activity on cells of the innate and adaptive immune systems.

Defective co-stimulation and impaired Th1 development in tumor necrosis factor/lymphotoxin-alpha double-deficient mice infected with Candida albicans

To define the immunological functions of tumor necrosis factor (TNF) in Candida albicans infection, TNF/lymphotoxin (LT)-alpha double-deficient mice were assessed for susceptibility to systemic or gastrointestinal infection and parameters of innate and adaptive Th immunity. When compared to wild-type mice, TNF/LT-alpha-deficient mice were more susceptible to either type of infection caused by virulent or low-virulence C. albicans cells. Susceptibility to infection correlated with impaired development of protective Th1 responses, in spite of the production of bioactive IL-12. The occurrence of predominant Th2 responses was associated with both impaired antifungal effector functions of neutrophils and a defective expression of co-stimulatory molecules on macrophages. All functions were improved upon administration of recombinant TNF-alpha, also resulting in increased resistance to infection. These findings indicate that the protective effect of TNF-alpha in candidiasis relies on the induction of antifungal Th1 responses, possibly occurring through stimulation of antifungal effector functions and co-stimulatory activities of phagocytic cells.

Initiation of T-helper cell immunity to Candida albicans by IL-12: the role of neutrophils

The Th1/Th2 paradigm of acquired immunity is proving essential for a better understanding of immunoregulation in candidiasis and perhaps other fungal infections, conditions that may be life-threatening in humans and difficult to control by chemotherapy alone, especially in neutropenic and severely immunocompromised patients. In its basic conception applied to Candida infection in mice, this paradigm calls for: (i) an association between Th1 responses and the onset/maintenance of phagocyte-dependent immunity, critical for opposing infectivity of the commensal, focusing an infection, and clearing the yeast from infected tissue; (ii) the ability of the yeast to activate Th2 response as an evasive strategy; (iii) the reciprocal regulation of Th1 and Th2 responses, resulting in a dynamic balance between these two types of reactivity. This balance, in concert with a variety of environmental factors, may regulate the status of the yeast as a commensal or pathogen in the mucosal tissue of colonized humans, but may also determine the outcome of deep-seated systemic infections once hematogenous dissemination of the yeast has occurred. An important corollary of this hypothesis may be the possible combined effects on Th immunity of Candida carriage/infection and various disease states. While immune deficiency or dysregulation, resulting in an altered cytokine balance as may occur in AIDS, can reasonably be expected to increase local infectivity of the yeast, it is even more intriguing that antifungal chemotherapy will resolve some of the unusual skin (atopic dermatitis-like) disorders frequently observed in this clinical setting, patients with AIDS. Besides, an immunopathologic role for Candida has been suggested for atopic dermatitis, atopy, and other conditions, overtly associated or not with Candida. Thus, the Th cell dichotomy to Candida may have important implications not only for regulation of the balance between commensalism and infection, but may also contribute to the onset or dominance of Th2 responses in other disease states. A similar example, although with different effects, may be provided by the temporary improvement seen in atopic dermatitis patients in concomitance with acute severe infections, an effect that has been proposed to result from transient down-regulation of the predominant Th2 cell reactivity. With a view to either controlling Candida infections or opposing Candida-related immunopathology, the promotion of yeast-specific Th1 responses appears to be a promising immunotherapeutic approach. This, in principle, could be achieved by subtraction of Th2 cytokines or by administration of Th1-promoting cytokines. However, our initial studies with exogenous IL-12 were unsuccessful, suggesting that the recombinant cytokine: (i) is unable to oppose Th2 differentiation driven in vivo by IL-4/IL-10; (ii) may induce endogenous IL-10 production as a regulatory response, and (iii) may potentate local inflammatory responses in gastrointestinal infection or even trigger IFN-gamma-dependent mechanisms of fungal septic shock.. More recent studies seem to provide encouraging results, at least under specific conditions of testing. In acute candidemia, neutrophils appear to be a major source of the directive cytokines, IL-12 and IL-10, thus contributing to the selection of Th1 and Th2 cell responses to LVS or virulent infection, respectively. Neutrophils may also be an important source of IL-10 released in response to challenge with exogenous IL-12. As a result, the Th1-promoting role of IL-12 may be largely unopposed (by IL-10 induction) in neutropenic mice, which would otherwise succumb to LVS challenge. These animals are, in fact, cured by replacement therapy with IL-12 and acquire durable, Th1-associated anticandidal protection. These findings may be very important for immunotherapy of fungal infections in humans. Neutropenic patients are those at the highest risk for developing systemic candidal infections. (ABSTRACT TRUNCATED)

Interleukin-12 in infectious diseases

Interleukin-12 (IL-12) is a potent immunoregulatory cytokine that is crucially involved in a wide range of infectious diseases. In several experimental models of bacterial, parasitic, viral, and fungal infection, endogenous IL-12 is required for early control of infection and for generation and perhaps maintenance of acquired protective immunity, directed by T helper type 1 (Th1) cells and mediated by phagocytes. Although the relative roles of IL-12 and gamma interferon in Th1-cell priming may be to a significant extent pathogen dependent, common to most infections is that IL-12 regulates the magnitude of the gamma interferon response at the initiation of infection, thus potentiating natural resistance, favoring Th1-cell development; and inhibiting Th2 responses. Treatment of animals with IL-12, either alone or as a vaccine adjuvant, has been shown to prevent disease by many of the same infectious agents, by stimulating innate resistance or promoting specific reactivity. Although IL-12 may enhance protective memory responses in vaccination or in combination with antimicrobial chemotherapy, it is yet unclear whether exogenous IL-12 can alter established responses in humans. Continued investigation into the possible application of IL-12 therapy to human infections is warranted by the role of the cytokine in inflammation, immunopathology, and autoimmunity.

The T cell response against fungal infections

A variety of pathological conditions, including impaired immune function, is believed to underlie host susceptibility to fungal infections and to determine both the severity and the characteristic of the associated pathology. Although the redundancy and the interdependence of antifungal responses may not favor the proper dissection and appreciation of individual effector mechanisms, the T helper type 1/type 2 paradigm of acquired immunity to fungi is proving essential for a better understanding of the host response from a regulatory perspective. The recent understanding of the importance of the different T helper cell subsets in fungal infections and the increasing appreciation of the reciprocal regulation between the innate, humoral, and adaptive immune systems in the development of optimal antimicrobial immunity have offered us new clues which may lead to an understanding of T cell dependent immunity to fungi.

Impaired generation of bone marrow B lymphocytes in mice deficient in C/EBPbeta

CAAT/enhancer binding proteins (C/EBP) are a family of transcription factors that mediates adipocyte differentiation and the regulation of genes expressed in immune responses and inflammation, such as interleukin-6 (IL-6), IL-8, and granulocyte colony-stimulating factor (G-CSF). We investigated the role of C/EBPbeta (NF-IL6) in the generation of bone marrow B lymphocytes by taking advantage of C/EBPbeta-/- mice. We found that the expansion of bone marrow (BM) B lymphocytes was impaired in long-term lymphoid cultures from C/EBPbeta-/- mice. Consistent with this finding, the number of BM B cells was decreased in C/EBPbeta-/- mice. Both the levels of IL-7 gene expression and bioactive IL-7 from BM stromal cells were decreased in C/EBPbeta-/- mice. Furthermore, the proliferative responsiveness of BM B-cell precursors to IL-7 was also reduced as compared to wild-type mice, indicating that C/EBPbeta is required for the generation of BM B cells induced by IL-7. Accordingly, IL-7 stimulates the C/EBPbeta DNA-binding activity of normal BM pre-B lymphocytes as well as of 70Z/3 pre-B cells. These results point to C/EBPbeta as a critical signaling molecule in BM B lymphopoiesis.

Induction of protective Th1 responses to Candida albicans by antifungal therapy alone or in combination with an interleukin-4 antagonist

Resistance or susceptibility to disseminated and mucosal Candida albicans infections in mice correlates with the development of protective or nonprotective T helper (Th) cell responses. To determine whether immunomodulatory activity on Th cell functions is an effect beyond that provided by antifungal therapy, mice with disseminated or gastrointestinal infection were treated with amphotericin B or fluconazole and assessed for mortality, fungus burden in the organs, and parameters of Th cell-dependent immunity. Both antimycotics produced protective CD4+ Th1 cell responses, as revealed by increased production of interleukin (IL)-12 and interferon-y, decreased production of IL-4, delayed-type hypersensitivity to fungal antigen, and the disappearance of antigen-specific IgE. Concomitant neutralization of endogenous IL-4 greatly increased the antifungal efficacy and the Th1-promoting activity of both agents. These results indicate that successful antifungal therapy alone or in combination with cytokine antagonists may rely on the induction of an appropriate Th antifungal cell response.

Neutrophil production of IL-12 and IL-10 in candidiasis and efficacy of IL-12 therapy in neutropenic mice

Neutrophil defects predispose to severe fungal infections, yet the immunomodulatory role of these cells is poorly defined. The contribution of neutrophils to the early cytokine balance governing Th1 and Th2 cell development was examined in mice with candidiasis. Neutrophils secreted IL-12 and IL-10, correlating with the respective development of self-limiting (Th1-associated) and progressive (Th2-associated) disease. Exogenous IL-12 was effective in protecting neutropenic hosts susceptible to infection. These results suggest that 1) neutrophils, via their ability to release cytokines, play an active role in determining the qualitative development of the T cell response, and 2) their early role in anticandidal immunity can be replaced by exogenous IL-12.