Poly(I.C)-induced interferons enhance susceptibility of SCID mice to systemic candidiasis

Type I Interferons Ameliorate Zinc Intoxication of Candida glabrata by Macrophages and Promote Fungal Immune Evasion

Host and fungal pathogens compete for metal ion acquisition during infectious processes, but molecular mechanisms remain largely unknown. Here, we show that type I interferons (IFNs-I) dysregulate zinc homeostasis in macrophages, which employ metallothionein-mediated zinc intoxication of pathogens as fungicidal response. However, Candida glabrata can escape immune surveillance by sequestering zinc into vacuoles. Interestingly, zinc-loading is inhibited by IFNs-I, because a Janus kinase 1 (JAK1)-dependent suppression of zinc homeostasis affects zinc distribution in macrophages as well as generation of reactive oxygen species (ROS). In addition, systemic fungal infections elicit IFN-I responses that suppress splenic zinc homeostasis, thereby altering macrophage zinc pools that otherwise exert fungicidal actions. Thus, IFN-I signaling inadvertently increases fungal fitness both in vitro and in vivo during fungal infections. Our data reveal an as yet unrecognized role for zinc intoxication in antifungal immunity and suggest that interfering with host zinc homeostasis may offer therapeutic options to treat invasive fungal infections.

Interfering with immunity: detrimental role of type I IFNs during infection

Type I IFNs are known to inhibit viral replication and mediate protection against viral infection. However, recent studies revealed that these cytokines play a broader and more fundamental role in host responses to infections beyond their well-established antiviral function. Type I IFN induction, often associated with microbial evasion mechanisms unique to virulent microorganisms, is now shown to increase host susceptibility to a diverse range of pathogens, including some viruses. This article presents an overview of the role of type I IFNs in infections with bacterial, fungal, parasitic, and viral pathogens and discusses the key mechanisms mediating the regulatory function of type I IFNs in pathogen clearance and tissue inflammation.

Role of type I interferons in inflammasome activation, cell death, and disease during microbial infection

Interferons (IFNs) were discovered over a half-century ago as antiviral factors. The role of type I IFNs has been studied in the pathogenesis of both acute and chronic microbial infections. Deregulated type I IFN production results in a damaging cascade of cell death, inflammation, and immunological host responses that can lead to tissue injury and disease progression. Here, we summarize the role of type I IFNs in the regulation of cell death and disease during different microbial infections, ranging from viruses and bacteria to fungal pathogens. Understanding the specific mechanisms driving type I IFN-mediated cell death and disease could aid in the development of targeted therapies.

Type I IFN-mediated regulation of IL-1 production in inflammatory disorders

Although contributing to inflammatory responses and to the development of certain autoimmune pathologies, type I interferons (IFNs) are used for the treatment of viral, malignant, and even inflammatory diseases. Interleukin-1 (IL-1) is a strongly pyrogenic cytokine and its importance in the development of several inflammatory diseases is clearly established. While the therapeutic use of IL-1 blocking agents is particularly successful in the treatment of innate-driven inflammatory disorders, IFN treatment has mostly been appreciated in the management of multiple sclerosis. Interestingly, type I IFNs exert multifaceted immunomodulatory effects, including the reduction of IL-1 production, an outcome that could contribute to its efficacy in the treatment of inflammatory diseases. In this review, we summarize the current knowledge on IL-1 and IFN effects in different inflammatory disorders, the influence of IFNs on IL-1 production, and discuss possible therapeutic avenues based on these observations.

Type I interferon inhibits interleukin-1 production and inflammasome activation

Type I interferon (IFN) is a common therapy for autoimmune and inflammatory disorders, yet the mechanisms of action are largely unknown. Here we showed that type I IFN inhibited interleukin-1 (IL-1) production through two distinct mechanisms. Type I IFN signaling, via the STAT1 transcription factor, repressed the activity of the NLRP1 and NLRP3 inflammasomes, thereby suppressing caspase-1-dependent IL-1β maturation. In addition, type I IFN induced IL-10 in a STAT1-dependent manner; autocrine IL-10 then signaled via STAT3 to reduce the abundance of pro-IL-1α and pro-IL-1β. In vivo, poly(I:C)-induced type I IFN diminished IL-1β production in response to alum and Candida albicans, thus increasing susceptibility to this fungal pathogen. Importantly, monocytes from multiple sclerosis patients undergoing IFN-β treatment produced substantially less IL-1β than monocytes derived from healthy donors. Our findings may thus explain the effectiveness of type I IFN in the treatment of inflammatory diseases but also the observed "weakening" of the immune system after viral infection.

IFN-alpha/beta signaling is required for polarization of cytokine responses toward a protective type 1 pattern during experimental cryptococcosis

The antiviral activities of type I IFNs have long been established. However, comparatively little is known of their role in defenses against nonviral pathogens. We examined here the effects of type I IFNs on host resistance against the model pathogenic yeast Cryptococcus neoformans. After intratracheal or i.v. challenge with this fungus, most mice lacking either the IFN-alpha/beta receptor (IFN-alpha/betaR) or IFN-beta died from unrestrained pneumonia and encephalitis, while all wild-type controls survived. The pulmonary immune response of IFN-alpha/betaR-/- mice was characterized by increased expression of IL-4, IL-13, and IL-10, decreased expression of TNF-alpha, IFN-gamma, inducible NO synthetase, and CXCL10, and similar levels of IL-12 mRNA, compared with wild-type controls. Histopathological analysis showed eosinophilic infiltrates in the lungs of IFN-alpha/betaR-/- mice, although this change was less extensive than that observed in similarly infected IFN-gammaR-deficient animals. Type I IFN responses could not be detected in the lung after intratracheal challenge. However, small, but statistically significant, elevations in IFN-beta levels were measured in the supernatants of bone marrow-derived macrophages or dendritic cells infected with C. neoformans. Our data demonstrate that type I IFN signaling is required for polarization of cytokine responses toward a protective type I pattern during cryptococcal infection.

Effector function of leucocytes from susceptible and resistant mice against distinct isolates of Candida albicans

Neutrophils and macrophages were generated in vitro from mice that display either high or low tissue susceptibilities to Candida albicans infection and their ability to phagocytose and kill three isolates of the yeast with different virulence characteristics was evaluated. In the absence of opsonization, phagocytosis by BALB/c and CBA/CaH neutrophils was comparable, but the killing was very poor. Opsonization with normal serum slightly decreased phagocytosis, but it had markedly different effects on killing, either enhancing or inhibiting candidacidal activity, depending on the combination of yeast isolate and mouse strain. In contrast, BALB/c macrophages showed high levels of phagocytosis and killing of both unopsonized yeasts and opsonized yeasts; whereas killing of unopsonized yeasts by CBA/CaH macrophages was poor, it was markedly enhanced by opsonization.

The role of type I interferons in non-viral infections

For a long time, the family of type I interferons (IFN-alpha/beta) has received little attention outside the fields of virology and tumor immunology. In recent years, IFN-alpha/beta regained the interest of immunologists, due to the phenotypic and functional characterization of IFN-alpha/beta-producing cells, the definition of novel immunomodulatory functions and signaling pathways of IFN-alpha/beta, and the observation that IFN-alpha/beta not only exerts antiviral effects but is also relevant for the pathogenesis or control of certain bacterial and protozoan infections. This review summarizes the current knowledge on the production and function of IFN-alpha/beta during non-viral infections in vitro and in vivo.

Innate versus adaptive immunity in Candida albicans infection

Candida albicans is a common opportunistic pathogen, causing both superficial and systemic infection. Clinical observations indicate that mucocutaneous infections are commonly associated with defective cell-mediated immune responses, whereas systemic infection is more frequently seen in patients with deficiencies in neutrophil number or function. Analysis of mechanisms of host resistance against gastrointestinal and oral infection in mouse models has demonstrated an absolute dependence on CD4(+) T cells, although clearance also involves phagocytic cells. Both IL-12 and TNF-alpha appear to be important mediators, but mouse strain-dependent variations in susceptibility to infection may be related to T-cell enhancement of production of phagocytic cells by the bone marrow. In murine systemic infection, the role of innate and adaptive responses is less well defined. Studies in immunodeficient and T-cell-depleted mice suggest that clearance of the yeast may be predominantly a function of the innate response, whereas the adaptive response may either limit tissue damage or have the potential to cause immunopathology, depending on the host genetic context in which the infection takes place.

T cells augment monocyte and neutrophil function in host resistance against oropharyngeal candidiasis

The purpose of this study was to identify the cell populations involved in recovery from oral infections with Candida albicans. Monoclonal antibodies specific for CD4+ cells, CD8+ cells, and polymorphonuclear leukocytes were used to deplete BALB/c and CBA/CaH mice of the relevant cell populations in systemic circulation. Monocytes were inactivated with the cytotoxic chemical carrageenan. Mice were infected with 10(8) C. albicans yeast cells and monitored for 21 days. Systemic depletion of CD4+ and CD8+ T lymphocytes alone did not increase the severity of oral infection compared to that of controls. Oral colonization persisted in animals treated with head and neck irradiation and depleted of CD4+ T cells, whereas infections in animals that received head and neck irradiation alone or irradiation and anti-CD8 antibody cleared the infection in a comparable fashion. The depletion of polymorphonuclear cells and the cytotoxic inactivation of mononuclear phagocytes significantly increased the severity of oral infection in both BALB/c and CBA/CaH mice. High levels of interleukin 12 (IL-12) and gamma interferon (IFN-gamma) were produced by lymphocytes from the draining lymph nodes of recovering animals, whereas IL-6, tumor necrosis factor alpha, and IFN-gamma were detected in the oral mucosae of both naïve and infected mice. The results indicate that recovery from oropharyngeal candidiasis in this model is dependent on CD4+-T-cell augmentation of monocyte and neutrophil functions exerted by Th1-type cytokines such as IL-12 and IFN-gamma.

Oroesophageal candidiasis is lethal for transgenic mice with combined natural killer and T-cell defects

Germfree transgenic epsilon 26 (Tgepsilon26) mice, which express the full-length human CD3epsilon gene, have combined defects in natural killer (NK) cells and T cells were found to be extremely susceptible to oroesophageal (palate, tongue, esophagus) and gastric (cardia-antrum section) candidiasis. The gnotobiotic Tgepsilon26 mice die, apparently from severe oroesophageal candidiasis, within 2-4 weeks after their alimentary tracts are colonized with Candida albicans. The Tgepsilon26 mice manifest resistance to acute systemic candidiasis (intravenous injection) and to systemic candidiasis of endogenous origin for the first 2 weeks after their alimentary tracts are colonized with C. albicans. Granulocyte depletion data suggest that granulocytes, in the absence of functional NK cells and T cells, can protect Tgepsilon26 mice from acute systemic candidiasis and from systemic candidiasis of endogenous origin, for at least 14 days after alimentary tract colonization. Granulocytes and macrophages, in the absence of NK cells and T cells, are unable to protect Tgepsilon26 mice from lethal oroesophageal candidiasis and systemic candidiasis of endogenous origin which was evident in moribund Tgepsilon26 mice 2-4 weeks after colonization. Thus, non-T cells (i.e., NK cells) and T cells play important roles in resistance to oroesophageal and systemic (acute and of endogenous origin) candidiasis.

Intestinal lesions associated with disseminated candidiasis in an experimental animal model

In human patients, disseminated candidiasis, a life-threatening disease for immunocompromised patients, is often associated with intestinal lesions. In this study, we demonstrate that immunosuppressed gnotobiotic (IGB) piglets orally inoculated with wild-type Candida albicans developed extensive intestinal lesions and disseminated infection. Severe ulceration of the ileal mucosa was observed overlying regions of colonization and necrosis of the gut-associated lymphoid tissue. Despite the high susceptibility of IGB piglets to many microbial pathogens, an avirulent mutant strain of C. albicans failed to produce intestinal lesions and exhibited poor dissemination, demonstrating that these effects required virulent organisms. It is likely that in IGB piglets, as in human patients, intestinal lesions provide the mechanism for escape of C. albicans from the gastrointestinal tract. Multinucleated giant cells containing fungal organisms were observed within lymph nodes and lymphatic vessels, and as with other pathogens, such cells could provide a mechanism for dissemination of C. albicans.

Cytokine profiles in parotid saliva from HIV-1-infected individuals: changes associated with opportunistic infections in the oral cavity

The purpose of this study was to quantitate levels of cytokines in parotid saliva of subjects infected with human immunodeficiency virus-1 (HIV-1) and to determine if the cytokine profiles differ in subjects with an oral opportunistic infection, i.e., candidiasis or oral hairy leukoplakia. Parotid saliva samples were obtained from HIV-infected individuals with or without candidiasis or oral hairy leukoplakia and from healthy controls and were assessed by ELISA for levels of interleukin (IL)-1, IL-2, IL-4, IL-5, IL-10, transforming growth factor-beta, tumor necrosis factor-alpha and interferon (IFN)-gamma. Saliva from HIV-infected subjects with oral candidiasis had significantly higher levels of IFN-gamma than that seen in HIV-infected individuals with no oral disease and significantly higher levels of IL-2, IL-5 and IFN-gamma than saliva of healthy controls. No significant difference was seen in cytokine levels in saliva from HIV-infected subjects with no oral infections and healthy controls. The HIV-infected subjects with oral hairy leukoplakia displayed significantly higher levels of both IL-1 alpha and IFN-gamma compared with the HIV and no oral disease group and a higher level of IFN-gamma than seen in saliva from the healthy control group. In comparing cytokine levels from both HIV and oral disease groups, significant differences were detected in levels of IL-5 and IL-10. These results indicate that the profile of salivary cytokines is altered as a result of the oral opportunistic infection candidiasis or oral hairy leukoplakia and also by concurrent HIV infection.

Macrophages in resistance to candidiasis

Candida albicans, an increasingly common opportunistic pathogenic fungus, frequently causes disease in immunodeficient but not immunocompetent hosts. Clarifying the role of the phagocytic cells that participate in resistance to candidiasis not only is basic to understanding how the host copes with this dimorphic pathogen but also will expedite the development of innovative prophylactic and therapeutic approaches for treating the multiple clinical presentations that candidiasis encompasses. In this review, we present evidence that a diverse population of mononuclear phagocytes, in different states of activation and differentiation and from a variety of host species, can phagocytize C. albicans blastoconidia via an array of opsonic and nonopsonic mechanisms and can kill C. albicans blastoconidia and hyphae by means of oxygen-dependent and -independent mechanisms. Reactive nitrogen intermediates should now be added to the well-established candidacidal reactive oxygen intermediates of macrophages. Furthermore, what were thought to be two independent pathways, i.e., nitric oxide and superoxide anion, have now been shown to combine to form a potent macrophage candidacidal molecule, peroxynitrite. In contrast to monocytes and neutrophils, which are important in resistance to early stages of C. albicans infections, more differentiated macrophages activated by cytokines such as gamma interferon participate in the acquired resistance of hosts with C. albicans-specific, cell-mediated immunity. Evidence presented in this review demonstrates that mononuclear phagocytes, in some instances in the absence of other professional phagocytes such as neutrophils, play an import role in resistance to systemic and mucosal candidiasis.

Production and function of cytokines in natural and acquired immunity to Candida albicans infection

Host resistance against infections caused by the yeast Candida albicans is mediated predominantly by polymorphonuclear leukocytes and macrophages. Antigens of Candida stimulate lymphocyte proliferation and cytokine synthesis, and in both humans and mice, these cytokines enhance the candidacidal functions of the phagocytic cells. In systemic candidiasis in mice, cytokine production has been found to be a function of the CD4+ T helper (Th) cells. The Th1 subset of these cells, characterized by the production of gamma interferon and interleukin-2, is associated with macrophage activation and enhanced resistance against reinfection, whereas the Th2 subset, which produces interleukins-4, -6, and -10, is linked to the development of chronic disease. However, other models have generated divergent data. Mucosal infection generally elicits Th1-type cytokine responses and protection from systemic challenge, and identification of cytokine mRNA present in infected tissues of mice that develop mild or severe lesions does not show pure Th1- or Th2-type responses. Furthermore, antigens of C. albicans, mannan in particular, can induce suppressor cells that modulate both specific and nonspecific cellular and humoral immune responses, and there is an emerging body of evidence that molecular mimicry may affect the efficiency of anti-Candida responses within defined genetic contexts.

Gamma delta T cell-induced nitric oxide production enhances resistance to mucosal candidiasis

Despite the prevalence of gamma delta T cells in mucosae that are typically colonized by Candida albicans, little is known of the possible role of these cells in resistance to candidiasis. A sharp increase in the number of gamma delta T cells and macrophages following intraperitoneal inoculation of mice with C. albicans led us to examine the role of these cells in the immune response to C. albicans. We show that the gamma delta T cells enhance macrophage nitric oxide (NO) production and anti-candida activity, in vitro. We also propose that the gamma delta T cells regulate macrophage function during candidiasis in vivo as well, because depletion of these cells abrogated inducible NO synthase expression in mucosae and enhanced murine susceptibility to candidiasis.

Consequences of secondary or co-infections for immunity

While remarkable progress has been made using genetically altered mice to understand the importance of different cytokines in protecting against experimental infections or co-infections, an examination of the opportunistic infections that occur during HIV infection of humans does not yet show a clear picture of cytokine imbalance. Opportunistic infections appear to result from impairments in cells mediating innate resistance, such as natural killer cells, macrophages, and neutrophils. Some of these defects may not be corrected even if CD4+ T cells were suddenly restored to normal. The lessons from immunodeficient and gene knockout mice now need to be put to the test in the clinic.

A T cell-independent protective host response against Cryptococcus neoformans expressed at the primary site of infection in the lung

T cell-independent host resistance expressed against a primary lung infection with Cryptococcus neoformans was investigated. Following intratracheal inoculation of the yeast, BALB/cBy scid/scid mice or CD4+ plus CD8+ T cell-depleted BALB/cBy mice developed a primary lung infection that remained stable for several weeks before progressing and disseminating to kill the host. By contrast, normal BALB/cBy hosts resolved the infection after 4 to 8 weeks. Thy+ CD4- CD8- cells were found to accumulate in the pulmonary alveoli of infected scid/scid or normal mice. Depletion of these cells caused the infection to progress more rapidly and resulted 4 weeks later in a 30- to 70-fold increase in yeast numbers in the lungs and dissemination to extrapulmonary sites. Cytofluorometric studies revealed that the Thy+ CD4- CD8- cells responsible were negative for the CD3 T cell marker. A small percentage of these Thy+ CD3- cells expressed asialo-Gm1, but treatment with asialo-Gm1 antibody did not have the same infection-enhancing effect as Thy-1 monoclonal antibody treatment. Further experiments revealed that Thy-1 monoclonal antibody treatment had no effect on the establishment of infectious foci in the brain or liver following intravenous inoculation of the yeast. The data point to the existence of an early resistance mechanism for which Thy+ CD3- CD4- CD8- cells are essential. This mechanism of host defense, while insufficient for complete protection, may be capable of delaying the development of cryptococcal meningoencephalitis by restricting the growth of the yeast at primary sites of infection in the lungs, even in immunodeficient mice.

Natural killer cells do not play a dominant role in CD4+ subset differentiation in Candida albicans-infected mice

The effects of in vivo administration of monoclonal antibodies against NK-1.1-bearing cells on the early production of gamma interferon (IFN-gamma) in vitro and development of Th1-associated immunity were studied in mice infected with a live vaccine strain of Candida albicans. At 1 and 4 days postinfection, natural killer (NK) cell-enriched fractions from the spleens of antibody-treated mice displayed a dramatic reduction in 5E6+ lymphocytes and negligible anti-YAC-1 cytotoxic activity in vitro. Nevertheless, the frequency of IFN-gamma-producing cells in those fractions was reduced by less than half, on average, by anti-NK-1.1 treatment in vivo. In addition, the antibody-treated and infected mice demonstrated unchanged T helper cell responses, as measured by yeast-specific footpad reactions, resistance to reinfection, occurrence of antibodies of different isotypes, and production in vitro of interleukin-2 (IL-2), IFN-gamma, IL-4, and IL-10 by CD4+ cells. Therefore, although NK cells may contribute to early IFN-gamma production in Candida-vaccinated mice, these cells apparently do not play a dominant role in the qualitative development of yeast-specific T helper responses.

Enhancement of susceptibility of CB-17 mice to systemic candidiasis by poly(I . C)-induced interferon

Poly(I . C) enhanced the susceptibility of CB-17 (BALB/c) mice to acute systemic candidiasis. Poly(I . C), supernatants from poly(I . C)-treated macrophages, or alpha and beta interferons suppressed macrophage candidacidal activity in vitro. Thus, poly(I . C)-induced interferons may enhance the susceptibility of CB-17 mice to candidiasis by suppressing macrophage candidacidal activity in an autocrine fashion.

Effect of abrogation of natural killer cell activity on the course of candidiasis induced by intraperitoneal administration and gastrointestinal candidiasis in mice with severe combined immunodeficiency

Candida albicans CFU per gram of tissue recovered from livers, spleens, and kidneys of 12 severe combined immunodeficiency (scid) and 12 BALB/c mice 5 days after intraperitoneal (i.p.) administration of 10(7) C. albicans cells were not significantly different. Nine scid mice given normal rabbit serum (NRS) as a control and eight scid mice given anti-asialo-GM1 (alpha-ASGM1) had C. albicans CFU per gram recovered from livers and spleens 1 week after i.p. administration of C. albicans that were not significantly different, despite virtual elimination of natural killer (NK) cell activity in mice treated with alpha-ASGM1. At 2 weeks after i.p. administration, despite significantly increased NK cell activity in eight infected NRS-treated scid mice and virtual elimination of NK cell activity by alpha-ASGM1 treatment of eight scid mice, C. albicans CFU per gram recovered from livers and kidneys were not significantly different. At 2 weeks after intragastric administration of 2 x 10(6) C. albicans cells, eight NRS- and eight alpha-ASGM1-treated scid mice had identical proportions colonized with C. albicans and similar C. albicans CFU per gram recovered from feces. There was no evidence of hematogenous dissemination in either group. Similar results were seen 1 week after intragastric administration of 10(7) C. albicans cells. We conclude that NK cell activity is increased by i.p. administration of C. albicans in scid mice, but nontheless, abrogation of NK cell activity is not associated with enhanced susceptibility to candidiasis induced by i.p. administration and also is not associated with enhanced susceptibility to gastrointestinal colonization or hematogenous dissemination after intragastric administration of C. albicans.