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.

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.

Iron overload alters innate and T helper cell responses to Candida albicans in mice

The effect of iron overload on susceptibility of mice to Candida albicans infection and on the type of T helper (Th) immunity elicited was investigated. Iron overload greatly increased susceptibility to disseminated infection with low-virulence C. albicans cells of exogenous origin. The candidacidal activity and the ability to release nitric oxide and bioactive interleukin (IL)-12 were greatly impaired in neutrophils and macrophages from infected mice. CD4 T cells from spleens of iron-overloaded mice were found to produce high levels of IL-4 and IL-10 and low levels of interferon-gamma. Treatment of iron-overloaded mice with the iron chelator, deferoxamine, resulted in the cure of mice from infection, restored the antifungal effector and immunomodulatory functions of the phagocytic cells, and allowed the occurrence of CD4 Th1 protective antifungal responses. These data indicate that iron overload may negatively affect CD4 Th1 development in mice with candidiasis, a function efficiently restored by therapy with deferoxamine.

Early T cell unresponsiveness in mice with candidiasis and reversal by IL-2: effect on T helper cell development

To investigate the role and effect of IL-2 in the genesis of Th1 and Th2 responses to Candida albicans in vivo, we assessed the levels of IL-2 production and the Ag-specific proliferative response in mice with healing or nonhealing infection and the effects of IL-2 neutralization or administration on the course and outcome of infection and on the type of CD4+ Th immunity elicited. High levels of IL-2 production and Ag-specific proliferation in vitro correlated with disease progression in susceptible mice. In contrast, resolution of infection in resistant mice was accompanied by the induction of Ag-specific hyporesponsiveness and impaired IL-2 production. Progression of infection did not occur in susceptible mice treated with anti-IL-2 or anti-IL-2R mAbs; conversely, disease resolution was prevented in resistant mice treated with IL-2. CD4+ Th1 cell responses were present in BALB/c mice rendered resistant by IL-2 neutralization and CD4+ Th2 responses in mice rendered susceptible by IL-2 treatment. The presence of IL-2 restored Ag-specific responsiveness in vitro and correlated in vivo with the expansion of CD4+ MEL-149(low) cells capable of producing IL-2 and IL-4 both in vitro and in vivo as observed in adult thymectomized mice. These results indicate that production of IL-2 early in infection correlates with the induction of IL-4-producing CD4+ Th2 cells, while a transient loss of T cell responsiveness, such as IL-2 production, appears to be required for CD4+ Th1 occurrence in mice with candidiasis.

An immunoregulatory role for neutrophils in CD4+ T helper subset selection in mice with candidiasis

Granulocytes may serve immunoregulatory and effector roles in different limbs of the immune response to infection. Using live vaccine strain or virulent challenge in mucosal or systemic infection of mice with Candida albicans, we examined the effect of mAb-mediated depletion of neutrophils on the course of primary and secondary challenge and on development of CD4+ cell-dependent immunity. We obtained evidence of deleterious effects of neutrophil depletion occurring at the time of infection under all conditions of testing, both in naive and in previously immunized mice. In contrast, neutrophil depletion appeared to benefit the hosts late in the course of an overwhelming systemic infection. In an attempt to correlate neutrophil function with the nature of the T cell response, we tested the ability of neutrophils to produce cytokines associated with functionally distinct CD4+ Th cell responses to Candida. We found that neutrophils were endowed with the capacity to secrete IL-12 and IL-10 in vitro in response to the yeast. Neutrophil ablation early in the course of Th1-associated, self-limiting infection appeared to change the qualitative development of the T cell response, and rendered mice susceptible to infection. In addition to long recognized contributions to acute anti-candidal responses, these data suggest an important role for neutrophils both in initiation and in expression of Candida-specific immunity.

An immunoregulatory role for neutrophils in CD4+ T helper subset selection in mice with candidiasis

Granulocytes may serve immunoregulatory and effector roles in different limbs of the immune response to infection. Using live vaccine strain or virulent challenge in mucosal or systemic infection of mice with Candida albicans, we examined the effect of mAb-mediated depletion of neutrophils on the course of primary and secondary challenge and on development of CD4+ cell-dependent immunity. We obtained evidence of deleterious effects of neutrophil depletion occurring at the time of infection under all conditions of testing, both in naive and in previously immunized mice. In contrast, neutrophil depletion appeared to benefit the hosts late in the course of an overwhelming systemic infection. In an attempt to correlate neutrophil function with the nature of the T cell response, we tested the ability of neutrophils to produce cytokines associated with functionally distinct CD4+ Th cell responses to Candida. We found that neutrophils were endowed with the capacity to secrete IL-12 and IL-10 in vitro in response to the yeast. Neutrophil ablation early in the course of Th1-associated, self-limiting infection appeared to change the qualitative development of the T cell response, and rendered mice susceptible to infection. In addition to long recognized contributions to acute anti-candidal responses, these data suggest an important role for neutrophils both in initiation and in expression of Candida-specific immunity.

Early T cell unresponsiveness in mice with candidiasis and reversal by IL-2: effect on T helper cell development

To investigate the role and effect of IL-2 in the genesis of Th1 and Th2 responses to Candida albicans in vivo, we assessed the levels of IL-2 production and the Ag-specific proliferative response in mice with healing or nonhealing infection and the effects of IL-2 neutralization or administration on the course and outcome of infection and on the type of CD4+ Th immunity elicited. High levels of IL-2 production and Ag-specific proliferation in vitro correlated with disease progression in susceptible mice. In contrast, resolution of infection in resistant mice was accompanied by the induction of Ag-specific hyporesponsiveness and impaired IL-2 production. Progression of infection did not occur in susceptible mice treated with anti-IL-2 or anti-IL-2R mAbs; conversely, disease resolution was prevented in resistant mice treated with IL-2. CD4+ Th1 cell responses were present in BALB/c mice rendered resistant by IL-2 neutralization and CD4+ Th2 responses in mice rendered susceptible by IL-2 treatment. The presence of IL-2 restored Ag-specific responsiveness in vitro and correlated in vivo with the expansion of CD4+ MEL-149(low) cells capable of producing IL-2 and IL-4 both in vitro and in vivo as observed in adult thymectomized mice. These results indicate that production of IL-2 early in infection correlates with the induction of IL-4-producing CD4+ Th2 cells, while a transient loss of T cell responsiveness, such as IL-2 production, appears to be required for CD4+ Th1 occurrence in mice with candidiasis.

Th1 and Th2 cytokines in mice with invasive aspergillosis

With a murine model of invasive aspergillosis we investigated cytokine production by CD4+ T helper cells and the effects of cytokine administration or neutralization on the course and outcome of infection. Patterns of susceptibility and resistance to infection were obtained with different strains of mice injected with different inocula of Aspergillus fumigatus conidia. Mice surviving the primary infection also resisted a subsequent lethal infection that was associated with production of gamma interferon by CD4+ T splenocytes. Impaired neutrophil antifungal activity, observed in susceptible mice, was concomitant with a predominant production of interleukin-4 (IL-4) by CD4+ splenocytes. In these mice, exogenous administration of IL-12 failed to induce resistance to infection; in contrast, treatment with soluble IL-4 receptor cured more than 70% of the mice from primary infection and resulted in the onset of acquired resistance to a subsequent lethal infection. These findings indicate that in murine invasive aspergillosis, production of IL-4 by CD4+ T cells may be one major factor discriminating susceptibility and resistance to infection.

CD4+ T-helper-cell responses in mice with low-level Candida albicans infection

Resistance and susceptibility to Candida albicans infection have been shown to be dependent upon the activation of CD4+ T helper (Th) type 1 or Th2 cells, respectively. To study the type, kinetics, and cytokine dependency of CD4+ Th-cell responses in low-level C. albicans infection, susceptible mice were infected with sublethal doses of C. albicans and assessed for parameters of CD4+ Th-dependent immunity. Interleukin (IL)-12 and gamma interferon were always produced early in infection regardless of the pathogen load. In contrast, production of IL-4, and hence Th2-cell reactivity, was strictly dose dependent, being induced at the higher dose of the fungus. Production of IL-12 correlated with a successful control of infection in mice exposed to the lower doses of C. albicans but not with the development of acquired immunity. An antigenic stimulus appeared to be required for IL-12 to induce a protective anticandidal response. Cytokine depletion in vivo revealed that neutralization of IL-4 was protective early but not late in infection, suggesting a different role for IL-4 in the induction versus maintenance of an ongoing anticandidal Th response. Late in infection, an exacerbative effect was also observed upon IL-12 neutralization. These results indicate that the fungal burden and timing of cytokine appearance greatly influence CD4+ Th induction and effector functions in mice with candidiasis.

Impaired neutrophil response and CD4+ T helper cell 1 development in interleukin 6-deficient mice infected with Candida albicans

To define the role of interleukin (IL)6 in Candida albicans infection, IL-6 deficient mice were assessed for susceptibility to systemic or gastrointestinal infection, as well as for parameters of elicited T helper cell (Th) immunity. IL-6-deficient mice were more susceptible than wild-type mice to either type of infection caused by virulent C. albicans. In response to systemic challenge with a live vaccine strain of yeast, IL-6-deficient mice failed to mount Th1-associated protective immunity, but the resulting Th2-biased response could be redirected to the Th1 phenotype by IL-10 neutralization. Severe impairment of the macrophage and neutrophil response to infection was observed in IL-6-deficient mice, but administration of IL-6 would increase both neutrophil response and resistance to infection. IL-6 seems to oppose the Th2-promoting role of IL-10 in candidiasis, its early regulatory activity involving effects on neutrophil function.

TGF-beta is important in determining the in vivo patterns of susceptibility or resistance in mice infected with Candida albicans

Resistance and susceptibility of mice to systemic infection with the fungus Candida albicans are associated with the preferential expansion of Th1 and Th2 cells, respectively. In this study, endogenous production of TGF-beta was found to be increased soon after infection of healer mice with a live vaccine strain of the fungus, but down-regulated in nonhealer mice with virulent yeast challenge. Although not affecting the outcome of primary challenge, serologic ablation of TGF-beta in the former animals abrogated development of acquired resistance and resulted in impaired production of IL-12/IFN-gamma and higher expression of IL-4/IL-10 at the time of reinfection with virulent yeast. A CD4+ population expressing the memory phenotype, CD44highMEL-14low, which appeared to be expanded by yeast infection of nonhealer mice, was similarly increased in the healer mice by anti-TGF-beta treatment. In vitro rTGF-beta impaired the candidacidal function of IFN-gamma-activated macrophages. Yet in nonhealer mice infected with virulent C. albicans, administration of rTGF-beta delayed progression of the disease, which was concomitant with the detection of lower levels of IL-4. In addition to previous evidence for an obligatory role of IFN-gamma and IL-12 in Candida-driven Th1 cell differentiation in vivo, the present data establish TGF-beta as a third cytokine, the presence of which may be required for optimal Th1 development leading to long-lived anticandidal resistance.

TGF-beta is important in determining the in vivo patterns of susceptibility or resistance in mice infected with Candida albicans

Resistance and susceptibility of mice to systemic infection with the fungus Candida albicans are associated with the preferential expansion of Th1 and Th2 cells, respectively. In this study, endogenous production of TGF-beta was found to be increased soon after infection of healer mice with a live vaccine strain of the fungus, but down-regulated in nonhealer mice with virulent yeast challenge. Although not affecting the outcome of primary challenge, serologic ablation of TGF-beta in the former animals abrogated development of acquired resistance and resulted in impaired production of IL-12/IFN-gamma and higher expression of IL-4/IL-10 at the time of reinfection with virulent yeast. A CD4+ population expressing the memory phenotype, CD44highMEL-14low, which appeared to be expanded by yeast infection of nonhealer mice, was similarly increased in the healer mice by anti-TGF-beta treatment. In vitro rTGF-beta impaired the candidacidal function of IFN-gamma-activated macrophages. Yet in nonhealer mice infected with virulent C. albicans, administration of rTGF-beta delayed progression of the disease, which was concomitant with the detection of lower levels of IL-4. In addition to previous evidence for an obligatory role of IFN-gamma and IL-12 in Candida-driven Th1 cell differentiation in vivo, the present data establish TGF-beta as a third cytokine, the presence of which may be required for optimal Th1 development leading to long-lived anticandidal resistance.

Mechanisms of resistance to fungal infections

In the past year, significant advances have been made in our understanding of immune defenses to fungal infections, which may be instrumental in the development of rational approaches to immunodiagnosis and therapy of these infections. The highlights have been the result of the direct application of advances in molecular biology and basic immunology, particularly cytokine research, leading to improved definition of fungal antigens and increased understanding of the roles of functionally distinct T-cell subsets, the activity of which may be either host-protective or disease-promoting.

T helper cell dichotomy to Candida albicans: implications for pathology, therapy, and vaccine design

Acquired immunity to Candida albicans is believed to prevent mucosal colonization of adult immunocompetent individuals from progressing to symptomatic infection. Resistance to disease appears to correlate with the detection of delayed-type hypersensitivity responses in vivo and a T helper type 1 (Th1) cytokine secretion profile in vitro. Cellular immunodeficiency, particularly HIV infection, greatly increases the risk of mucosal infection, confirming that CD(4+)-cell-directed immunity is effective locally in controlling infectivity of the yeast. While Th1-type CD4+ cell activation resulting in phagocyte-dependent immunity clearly represents an important mechanism of anticandidal resistance, clinical observations suggest that Th2-type CD4+ cell reactivity may be triggered by Candida antigens in several disease states, including symptomatic infections and immunopathology. This may imply that a Th1-type pattern of reactivity characterizes the saprophytic yeast carriage and resistance to disease by healthy humans, whereas Th2-type responses would be mostly associated with pathology. Moreover, Candida-specific T helper responses, namely humoral and cell-mediated immunity, appear to be reciprocally regulated, as typically occurs in experimental models of parasitic and retroviral infection, where the Th1/Th2 paradigm of acquired immunity has been best characterized. Recent studies, besides providing direct evidence for the occurrence of cross-regulatory Th1 and Th2 responses in mice with candidiasis, emphasize the potential of cytokine/anticytokine therapy for recruiting Candida-specific responses toward protective, Th1-type CD4+ cell reactivity. At the same time, these studies call attention to the possible consequences of C. albicans infection for immunopathology, allergy, and coinfection.

A TH1-TH2-like switch in candidiasis: new perspectives for therapy

An imbalance in TH1-type and TH2-type responses may allow Candida albicans to modify the host response to favor its own persistence. This hypothesis has important consequences for allergy, autoimmunity and co-infection, and also highlights a potential role for cytokine and anti-cytokine therapy in Candida-related pathology.

Interleukin-4 and -10 exacerbate candidiasis in mice

Neutralization of endogenous interleukin (IL)-4 or IL-10 in mice with Candida albicans infection initiates or accelerates development of a T helper (Th)1-associated protective response. Here, we report the effect of IL-4 and IL-10 administration on the course of systemic or gastrointestinal (GI) candidiasis and on the development of Th immunity using yeast/host combinations that result either in Th1-associated self-limiting infection (healer mice) or in Th2-associated progressive disease (nonhealer mice). Treatment with IL-4 or IL-10 greatly exacerbated the course of systemic infection in nonhealer mice and rendered healer mice, inoculated with attenuated yeast cells, susceptible to infection. Under the latter conditions of yeast challenge and IL-4/IL-10 administration, the development of a fatal disease was associated with inhibition of IL-12 production and detection of progressive Th2 cell dominance. In contrast, in healer mice allowed to resolve their infections and to develop long-lived anti-candidal resistance, the expression of this acquired resistance was not impaired by IL-4 and/or IL-10, as shown by the outcome of reinfection with virulent yeast cells. In the GI model of infection, both IL-4 and IL-10 were found to exacerbate the course of infection and to induce the appearance of CD4+ T cells producing high levels of IL-4 and IL-10 in Peyer's patches. These findings demonstrate that exogenous IL-4 and IL-10 may greatly affect the development of Th responses to C. albicans in vivo, but do not modify the expression of established and predominant Th1 cell reactivity.

Lymphoproliferative disorder and imbalanced T-helper response in C/EBP beta-deficient mice

C/EBP beta is considered a key element of interleukin-6 (IL-6) signalling as well as an important transcriptional regulator of the IL-6 gene itself. We describe here how mice lacking C/EBP beta develop a pathology similar to mice overexpressing IL-6 and nearly identical to multicentric Castleman's disease in human patients, with marked splenomegaly, peripheral lymphadenopathy and enhanced haemopoiesis. Humoral, innate and cellular immunity are also profoundly distorted, as shown by the defective activation of splenic macrophages, the strong impairement of IL-12 production, the increased susceptibility to Candida albicans infection and the altered T-helper function. Our data show that C/EBP beta is crucial for the correct functional regulation and homeostatic control of haemopoietic and lymphoid compartments.

T helper cell type 1 (Th1)- and Th2-like responses are present in mice with gastric candidiasis but protective immunity is associated with Th1 development

The relative contributions of local T helper cell type 1 (Th1)- and Th2-like responses to the course of primary and secondary gastrointestinal (GI) candidiasis were examined in adult immunocompetent BALB/c mice. Both Th1 cytokines, such as interferon-gamma (IFN-gamma), and the Th2 cytokines, interleukin (IL)-4 and IL-5, were produced by CD4+ cells from Peyer's patches (PP) and mesenteric lymph nodes at a time when the fungus was cleared from the stomach and intestine. Augmentation of antigen-specific Th2-like responses by treatment with cholera toxin did not modify the course of disease. In contrast, treatment with soluble IL-4 receptor, which increased Th1 cells, was associated with enhanced yeast clearance. In addition, IFN-gamma but not IL-4 mRNA was present in PP and spleen CD4+ cells in mice resistant to subsequent GI inoculation. Activation of Th1- but not Th2-like responses may be responsible locally for controlling GI candidiasis and generating protective immunity.