The role of phagocytic cells in resistance to disseminated candidiasis in granulocytopenic mice

The Role of B-Cells and Antibodies against Candida Vaccine Antigens in Invasive Candidiasis

Systemic candidiasis is an invasive fungal infection caused by members of the genus Candida. The recent emergence of antifungal drug resistance and increased incidences of infections caused by non-albicans Candida species merit the need for developing immune therapies against Candida infections. Although the role of cellular immune responses in anti-Candida immunity is well established, less is known about the role of humoral immunity against systemic candidiasis. This review summarizes currently available information on humoral immune responses induced by several promising Candida vaccine candidates, which have been identified in the past few decades. The protective antibody and B-cell responses generated by polysaccharide antigens such as mannan, β-glucan, and laminarin, as well as protein antigens like agglutinin-like sequence gene (Als3), secreted aspartyl proteinase (Sap2), heat shock protein (Hsp90), hyphally-regulated protein (Hyr1), hyphal wall protein (Hwp1), enolase (Eno), phospholipase (PLB), pyruvate kinase (Pk), fructose bisphosphate aldolase (Fba1), superoxide dismutase gene (Sod5) and malate dehydrogenase (Mdh1), are outlined. As per studies reviewed, antibodies induced in response to leading Candida vaccine candidates contribute to protection against systemic candidiasis by utilizing a variety of mechanisms such as opsonization, complement fixation, neutralization, biofilm inhibition, direct candidacidal activity, etc. The contributions of B-cells in controlling fungal infections are also discussed. Promising results using anti-Candida monoclonal antibodies for passive antibody therapy reinforces the need for developing antibody-based therapeutics including anti-idiotypic antibodies, single-chain variable fragments, peptide mimotopes, and antibody-derived peptides. Future research involving combinatorial immunotherapies using humanized monoclonal antibodies along with antifungal drugs/cytokines may prove beneficial for treating invasive fungal infections.

Agranulocytosis leads to intestinal Echinococcus multilocularis oncosphere invasion and hepatic metacestode development in naturally resistant Wistar rats

Susceptibility to Echinococcus multilocularis infection considerably varies among intermediate (mostly rodents) and dead-end host species (e.g. humans and pig), in particular regarding intestinal oncosphere invasion and subsequent hepatic metacestode development. Wistar rats are highly resistant to infection and subsequent diseases upon oral inoculation with E. multilocularis eggs, however, after immunosuppressive treatment with dexamethasone, rats become susceptible. To address the role of the cellular innate immunity, Wistar rats were individually or combined depleted of natural killer (NK) cells, macrophages (MΦ) and granulocytes (polymorphonuclear cells, PMN) prior to E. multilocularis egg inoculation. Although NK cell and MΦ depletion did not alter the resistance status of rats, the majority of PMN-depleted animals developed liver metacestodes within 10 weeks, indicating that PMN are key players in preventing oncosphere migration and/or development in Wistar rats. In vitro studies indicated that resistance is not caused by neutrophil reactive oxygen species or NETosis. Also, light microscopical examinations of the small intestine showed that oral inoculation of E. multilocularis eggs does not elicit a mucosal neutrophil response, suggesting that the interaction of oncospheres and neutrophils may occur after the former have entered the peripheral blood. We suggest to consider granulocytes as mediators of resistance in more resistant species, such as humans.

Mucosal IgA Prevents Commensal Candida albicans Dysbiosis in the Oral Cavity

The fungus Candida albicans colonizes the oral mucosal surface of 30–70% of healthy individuals. Due to local or systemic immunosuppression, this commensal fungus is able to proliferate resulting in oral disease, called oropharyngeal candidiasis (OPC). However, in healthy individuals C. albicans causes no harm. Unlike humans mice do not host C. albicans in their mycobiome. Thus, oral fungal challenge generates an acute immune response in a naive host. Therefore, we utilized C. albicans clinical isolates which are able to persist in the oral cavity without causing disease to analyze adaptive responses to oral fungal commensalism. We performed RNA sequencing to determine the transcriptional host response landscape during C. albicans colonization. Pathway analysis revealed an upregulation of adaptive host responses due to C. albicans oral persistence, including the upregulation of the immune network for IgA production. Fungal colonization increased cross-specific IgA levels in the saliva and the tongue, and IgA⁺ cells migrated to foci of fungal colonization. Binding of IgA prevented fungal epithelial adhesion and invasion resulting in a dampened proinflammatory epithelial response. Besides CD19⁺ CD138⁻ B cells, plasmablasts, and plasma cells were enriched in the tongue of mice colonized with C. albicans suggesting a potential role of B lymphocytes during oral fungal colonization. B cell deficiency increased the oral fungal load without causing severe OPC. Thus, in the oral cavity B lymphocytes contribute to control commensal C. albicans carriage by secreting IgA at foci of colonization thereby preventing fungal dysbiosis.

Active immunizations with peptide-DC vaccines and passive transfer with antibodies protect neutropenic mice against disseminated candidiasis

We previously report that peptide-pulsed dendritic cell (DC) vaccination, which targeting two peptides (Fba and Met6) expressed on the cell surface of Candida albicans, can induce high degree of protection against disseminated candidiasis in immunocompetent mice. Passive transfer of immune sera from the peptide immunized mice or peptide-related monoclonal antibodies demonstrated that protection was medicated by peptide-specific antibodies. In this study the efficacy of active and passive immunization against disseminated candidiasis was tested in mice with cyclophosphamide-induced neutropenia. Peptide-DC vaccines were given to mice prior to induction of neutropenia. We show active immunization with either Fba or Met6 peptide-DC vaccine significantly improved the survival and reduced the fungal burden of disseminated candidiasis in those immunocompromised mice. Importantly, we show that administration of two protective monoclonal antibodies also protect neutropenic mice against the disease, implying possibility of developing a successful passive immunotherapy strategy to treat the disease and protect against disseminated candidiasis. The results of this study are crucial as they address the fundamental questions as to whether the synthetic peptide vaccine induced immunity protects the host during a neutropenic episode. We anticipate that this peptide-vaccine study will serve as the foundation of future investigations into new peptide vaccines comprised of cell surface peptides from other medically important Candida species, as well as other fungi.

The Candida albicans ATO Gene Family Promotes Neutralization of the Macrophage Phagolysosome

Candida albicans is an opportunistic human fungal pathogen that causes a variety of diseases, ranging from superficial mucosal to life-threatening systemic infections, the latter particularly in patients with defects in innate immune function. C. albicans cells phagocytosed by macrophages undergo a dramatic change in their metabolism in which amino acids are a key nutrient. We have shown that amino acid catabolism allows the cell to neutralize the phagolysosome and initiate hyphal growth. We show here that members of the 10-gene ATO family, which are induced by phagocytosis or the presence of amino acids in an Stp2-dependent manner and encode putative acetate or ammonia transporters, are important effectors of this pH change in vitro and in macrophages. When grown with amino acids as the sole carbon source, the deletion of ATO5 or the expression of a dominant-negative ATO1(G53D) allele results in a delay in alkalinization, a defect in hyphal formation, and a reduction in the amount of ammonia released from the cell. These strains also form fewer hyphae after phagocytosis, have a reduced ability to escape macrophages, and reside in more acidic phagolysosomal compartments than wild-type cells. Furthermore, overexpression of many of the 10 ATO genes accelerates ammonia release, and an ato5Δ ATO1(G53D) double mutant strain has additive alkalinization and ammonia release defects. Taken together, these results indicate that the Ato protein family is a key mediator of the metabolic changes that allow C. albicans to overcome the macrophage innate immunity barrier.

Current progress in the biology of members of the Sporothrix schenckii complex following the genomic era

Sporotrichosis has been attributed for more than a century to one single etiological agent, Sporothrix schencki. Only eight years ago, it was described that, in fact, the disease is caused by several pathogenic cryptic species. The present review will focus on recent advances to understand the biology and virulence of epidemiologically relevant pathogenic species of the S. schenckii complex. The main subjects covered are the new clinical and epidemiological aspects including diagnostic and therapeutic challenges, the development of molecular tools, the genome database and the perspectives for study of virulence of emerging Sporothrix species.

Adaptive immune responses to Candida albicans infection

Fungal infections are becoming increasingly prevalent in the human population and contribute to morbidity and mortality in healthy and immunocompromised individuals respectively. Candida albicans is the most commonly encountered fungal pathogen of humans, and is frequently found on the mucosal surfaces of the body. Host defense against C. albicans is dependent upon a finely tuned implementation of innate and adaptive immune responses, enabling the host to neutralise the invading fungus. Central to this protection are the adaptive Th1 and Th17 cellular responses, which are considered paramount to successful immune defense against C. albicans infections, and enable tissue homeostasis to be maintained in the presence of colonising fungi. This review will highlight the recent advances in our understanding of adaptive immunity to Candida albicans infections.

Mincle polarizes human monocyte and neutrophil responses to Candida albicans

The distribution and function of the C-type lectin Mincle has not previously been investigated in human cells, although mouse models have demonstrated a non-redundant role for Mincle in the host response to fungal infections. This study identified an unusual pattern of reciprocal expression of Mincle on peripheral blood monocytes or neutrophils isolated from the same donor. Expression on monocytes was inversely correlated with phagocytosis and yeast killing, but was necessary for the induction of inflammatory cytokines in response to ex vivo Candida challenge. In contrast, Mincle expression on neutrophils was associated with phagocytic and candidacidal potential of those cells. Candida challenge upregulated Mincle expression but only in Mincle+ cells. These data highlight species-specific differences between the regulation of Mincle expression in mouse and man. Reciprocal expression of Mincle modified the candidacidal potential of monocytes or neutrophils, suggesting it may also polarize the type of host response to fungal infection.

Phagocytosis and intracellular killing of Candida albicans by murine polymorphonuclear neutrophils

Polymorphonuclear neutrophils (PMNs) are important phagocytes in the control of Candida infections. The phagocytic contribution of PMNs to host defence can by assessed by various methods, such as microbiological assays. However, assessment and definition of intracellular killing capacity can be a source of considerable confusion. A comparison of the growth of Candida in the presence of PMN with the growth of Candida in phagocyte-free suspensions may lead to an overestimation of killing capacity because PMNs can use both intracellular and extracellular killing mechanisms. Here, we describe the use of an adherent monolayer of exudate peritoneal PMNs that is used to differentiate between the process of phagocytosis and intracellular killing.

Macrophage-mediated responses to Candida albicans in mice expressing the human immunodeficiency virus type 1 transgene

The critical impairments of innate and adaptive immunity that cause susceptibility to mucosal candidiasis in human immunodeficiency virus (HIV) infection have not been fully determined. We therefore conducted an analysis of macrophage-mediated responses to Candida albicans in transgenic (Tg) mice expressing Nef, Env, and Rev of HIV type 1 (HIV-1) in CD4(+) T cells, dendritic cells, and macrophages and developing an AIDS-like disease (CD4C/HIV(MutA) Tg mice). Macrophages were successfully recruited to the oral and gastric mucosae of these Tg mice in response to chronic carriage of C. albicans and displayed polarization toward an alternatively activated phenotype. Functionally, peritoneal macrophages from uninfected Tg mice exhibited increased phagocytosis of C. albicans and enhanced production of interleukin 6 and monocyte chemoattractant protein 1, demonstrating that the HIV-1 transgene independently activates selected macrophage functions. Production of H(2)O(2) by macrophages from Tg mice primed with gamma interferon and treated with phorbol 12-myristate 13-acetate or C. albicans was moderately reduced, but expression of the HIV-1 transgene did not alter production of nitric oxide or reduce killing of C. albicans. A knockout of the inducible nitric oxide synthase (NOS2) gene in these Tg mice did not augment oral or gastrointestinal burdens during chronic carriage of C. albicans or cause systemic dissemination, likely due to a redundancy provided by partially preserved production of H(2)O(2) and oxygen-independent candidacidal mechanisms. Thus, the macrophage response to C. albicans is largely preserved in these Tg mice, and no functional macrophage defect appears to primarily determine the susceptibility to mucosal candidiasis.

Understanding the multiple functions of Gr-1(+) cell subpopulations during microbial infection

The murine cell surface determinant Gr-1 is expressed at high level on neutrophils. Depletion of polymorphonuclear leukocytes with anti-Gr-1(+) monoclonal antibody results in increased susceptibility and dysregulated immunity to many microbial pathogens, a finding widely interpreted to indicate the importance of neutrophils during infection. Yet, in recent years it has become clear that additional cell types express the Gr-1 determinant, including dendritic cell and monocyte subpopulations. In this review, we evaluate current knowledge on the functional aspects of Gr-1(+) cell populations. We focus on infection with the opportunistic protozoan Toxoplasma gondii, a case where host survival depends on an intact Gr-1(+) cell population.

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.

Host defence against disseminated Candida albicans infection and implications for antifungal immunotherapy

The different manifestations of Candida albicans infection are dictated by an underlying defect in the immune response of the host. Protective immunity to disseminated candidiasis, the manifestation of C. albicans infection discussed in this review, has traditionally been ascribed to innate immunity with emphasis on the role of granulocytes. Lately, however, immunological studies have learned that host defence against disseminated candidiasis is based on a complex interplay between innate and cell-mediated immunity. Despite the availability of new antifungal agents, mortality associated with disseminated C. albicans infection remains high. Immunotherapy that augments host defence is an important strategic option in the battle against disseminated candidiasis. Here, the authors review the chronological events in the pathogenesis of disseminated candidiasis that aid in predicting the impact of existing immunotherapy and the development of future immunomodulating strategies.

CD8+ T cells but not polymorphonuclear leukocytes are required to limit chronic oral carriage of Candida albicans in transgenic mice expressing human immunodeficiency virus type 1

Candida albicans causes oropharyngeal candidiasis (OPC) but rarely disseminates to deep organs in human immunodeficiency virus (HIV) infection. Here, we used a model of OPC in CD4C/HIV(Mut) transgenic (Tg) mice to investigate the role of polymorphonuclear leukocytes (PMNs) and CD8+ T cells in limiting candidiasis to the mucosa. Numbers of circulating PMNs and their oxidative burst were both augmented in CD4C/HIV(MutA) Tg mice expressing rev, env, and nef of HIV type 1 (HIV-1), while phagocytosis and killing of C. albicans were largely unimpaired compared to those in non-Tg mice. Depletion of PMNs in these Tg mice did not alter oral or gastrointestinal burdens of C. albicans or cause systemic dissemination. However, oral burdens of C. albicans were increased in CD4C/HIV(MutG) Tg mice expressing only the nef gene of HIV-1 and bred on a CD8 gene-deficient background (CD8-/-), compared to control or heterozygous CD8+/- CD4C/HIV(MutG) Tg mice. Thus, CD8+ T cells contribute to the host defense against oral candidiasis in vivo, specifically in the context of nef expression in a subset of immune cells.

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.

Animal models in the analysis of Candida host-pathogen interactions

An increasingly diverse array of clinically relevant animal models of candidiasis have been established that mimic both the immune perturbations of the host and tissue-specific features of candidiasis in humans. Cause-and-effect analysis of Candida host-pathogen interactions using these animal models has made a quantum leap forward in the genomic era, with the concurrent construction of C. albicans mutants with targeted mutations of putative virulence factors, the application of microarrays and other emerging technologies to comprehensively assess C. albicans gene expression in vivo, and construction of transgenic and knockout mice to simulate specific host immunodeficiencies. The opportunity to combine these powerful tools will yield an unprecedented wealth of new information on the molecular and cellular pathogenesis of candidiasis.

Immunopathogenesis of oropharyngeal candidiasis in human immunodeficiency virus infection

Oropharyngeal and esophageal candidiases remain significant causes of morbidity in human immunodeficiency virus (HIV)-infected patients, despite the dramatic ability of antiretroviral therapy to reconstitute immunity. Notable advances have been achieved in understanding, at the molecular level, the relationships between the progression of HIV infection, the acquisition, maintenance, and clonality of oral candidal populations, and the emergence of antifungal resistance. However, the critical immunological defects which are responsible for the onset and maintenance of mucosal candidiasis in patients with HIV infection have not been elucidated. The devastating impact of HIV infection on mucosal Langerhans' cell and CD4(+) cell populations is most probably central to the pathogenesis of mucosal candidiasis in HIV-infected patients. However, these defects may be partly compensated by preserved host defense mechanisms (calprotectin, keratinocytes, CD8(+) T cells, and phagocytes) which, individually or together, may limit Candida albicans proliferation to the superficial mucosa. The availability of CD4C/HIV transgenic mice expressing HIV-1 in immune cells has provided the opportunity to devise a novel model of mucosal candidiasis that closely mimics the clinical and pathological features of candidal infection in human HIV infection. These transgenic mice allow, for the first time, a precise cause-and-effect analysis of the immunopathogenesis of mucosal candidiasis in HIV infection under controlled conditions in a small laboratory animal.

Hematologic and immunophenotypic factors associated with development of Rhodococcus equi pneumonia of foals at equine breeding farms with endemic infection

Rhodococcus equi causes severe pyogranulomatous pneumonia in foals and in immunocompromised people. In mice, both CD4+ and CD8+ T lymphocytes contribute to host defense against R. equi, but CD4+ T lymphocytes are required for pulmonary clearance of the bacteria. In this prospective study of 208 foals at two equine breeding farms with endemic R. equi infections, we collected peripheral blood samples at 2 and 4 weeks of age and at the time of diagnosis of R. equi pneumonia. Samples were analyzed for concentrations of total and differential leukocytes, EqCD4+ and EqCD8+ T lymphocytes, and B lymphocytes. Thirty (14.4%) foals developed R. equi pneumonia. At the 2nd week of life, affected foals had significantly lower concentrations of white blood cells (WBC) and segmented neutrophils, significantly lower proportions of EqCD4+ T lymphocytes, and significantly higher proportions of EqCD8+ T lymphocytes. The EqCD4:EqCD8 ratio was significantly lower for affected foals. At the 4th week of life, affected foals had significantly lower concentrations of segmented neutrophils and EqCD4+ T lymphocytes than did unaffected foals. The ratio of EqCD4:EqCD8 was significantly lower for affected foals. Two- and 4-week-old foals with ratios of EqCD4:EqCD8<3 were significantly more likely to develop R. equi pneumonia. There was a significant farm effect which diluted our statistical power to detect differences; however; after adjusting for the farm effect, 2-week-old foals with ratios of EqCD4:EqCD8<3 remained significantly more likely to develop R. equi pneumonia. There were no significant differences in immunophenotypic variables between affected foals (at the time of diagnosis) and age-matched control foals. These data suggest that there are hematologic and immunophenotypic differences between affected and unaffected foals during the first 2-4 weeks of life, prior to onset of clinical signs of R. equi pneumonia. These differences may represent important immunologic mechanisms associated with increased susceptibility of individual foals to infection with R. equi. Because there was considerable overlap between values for affected and unaffected foals, we cannot yet recommend immunophenotyping of foals at endemically-infected farms as a clinically useful screening tool to identify foals at increased risk of developing R. equi pneumonia.

The Pathophysiology and Treatment of Candida Sepsis

Sepsis can occur during disseminated candidiasis, but its pathogenesis differs from that caused by typical prokaryotic pathogens. Complex interactions between defects in host defense and "relative" virulence factors expressed by Candida lead to dissemination of the saprophyte to parenchymal organs, and subsequently to onset of multiorgan failure. This review focuses first on the pathophysiology of Candida sepsis, detailing current understanding of host-pathogen interactions. We then consider the choice of antifungal and supportive treatments.

Phenotypic and functional changes on phagocytic cells recruited at the site of Candida albicans infection after chronic varied stress exposure

The transition of Candida albicans from commensalism to pathogenicity is associated with the immune status of the host; resistance to fungus involves macrophages (Mphi) and polymorphonuclear neutrophils (PMN), which act as effector cells. T-cell function is also involved. Previously, we found that in Wistar rats exposed to chronic varied stress (CVS) immediately after C. albicans infection (Ca-S group) some functions of phagocytic cells, such as killer activity and NO production, were strongly modified compared with unstressed, infected animals (Ca group). We examined the phenotypic and functional changes of these effector cells recruited at the site of C. albicans infection. The recruitment of peritoneal cells (PC) was markedly reduced in Ca-S animals and the arrival of Mphi and PMN was selectively diminished after CVS exposure. The integrin CD11b/CD18, implicated in migration and C. albicans phagocytosis, was downregulated in Mphi of Ca-S animals. The activation markers CD54 and MHC-II were upregulated in Mphi after fungal contact. The expression of CD54 was only changed in Ca-S rats. Finally, TNF-alpha production was reduced in PC of Ca-S animals, suggesting an impairment of functional activity. Taken together, the phenotypic and functional changes detected in effector cells may account for the decreased resistance to candidiasis seen in conjunction with CVS. The changes seen also expand our knowledge of the role of Mphi in the control of C. albicans dissemination.

Phagocytosis and intracellular killing of Candida albicans blastoconidia by neutrophils and macrophages: a comparison of different microbiological test systems

Polymorphonuclear neutrophils (PMN) and mononuclear phagocytes represent an important first line and effector function in the control of Candida infections. Their relative contribution to host defence is frequently assessed by means of microbiological assays. However, reported results are divergent and might well be associated with study design-related issues. In the present study, we compared frequently used microbiological candidacidal assays, with the purpose of determining the most adequate method for assessment of phagocytosis and intracellular killing. We concluded that microbiological assays using yeast-phagocyte suspensions are inappropriate for the assessment of intracellular killing of Candida blastoconidia by murine macrophages, due to adherence or clumping of cells. In contrast, an adherent monolayer of phagocytes can be applied as a single microbiological assay to independently study the process of phagocytosis and intracellular killing, by exudate peritoneal macrophages as well as exudate peritoneal PMN.

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.

Candida albicans and Candida krusei differentially induce human blood mononuclear cell interleukin-12 and gamma interferon production

Protection against Candida infection involves both innate and acquired immune responses, and cytokines produced by monocytes during the innate response may modify the acquired immune response by T cells. We hypothesized that Candida species which differ in pathogenicity can differentially induce production of immunoregulatory cytokines by human monocytes, which in turn modify T cells for immune responses to Candida. To test this hypothesis, we examined the effects of Candida albicans and Candida krusei on immunoregulatory cytokine production by human monocytes and gamma interferon (IFN-gamma) production by peripheral blood mononuclear cells (PBMC). Purified monocytes were incubated with live or heat-killed strains of C. albicans and C. krusei at the optimal Candida/monocyte ratio of 0.5. Cytokines in the supernatants were measured by enzyme-linked immunosorbent assay. Our data demonstrated that live C. albicans and C. krusei significantly induced interleukin-10 (IL-10), monocyte chemotactic factor 1, IL-1beta, and tumor necrosis factor alpha production by monocytes relative to unstimulated monocytes. In contrast, unlike C. krusei, pathogenic live strains of C. albicans induced no or only a minimal level of IL-12. The expression of IL-12 p40 mRNA levels by reverse transcription-PCR corroborated the IL-12 protein (p70) findings. In human PBMC, human blood monocytes were the major source of both IL-10 and IL-12 production in response to C. albicans and C. krusei. Upon activation of T cells in the presence of Candida-modified monocytes and antigen-presenting cells, IL-12 production by PBMC treated with Candida organisms correlated strongly with the level of IFN-gamma production by T cells. These results indicate that the virulence of C. albicans may be related to its ability to induce the monocytic type II cytokine IL-10, with a selective inhibition of IL-12 production, which may be responsible for the observed lack of T-cell IFN-gamma and may restrain an effective type I immune response to Candida.

Disparate requirement for T cells in resistance to mucosal and acute systemic candidiasis

Although highly susceptible to orogastric candidiasis, T-cell receptor delta- and alpha-chain knockout mice, deficient in gammadelta and alphabeta T cells, respectively, were found to be resistant to disseminated candidiasis of endogenous origin and to acute systemic candidiasis (resulting from intravenous injection).

Effects of macrophage colony-stimulating factor (M-CSF) on anti-fungal activity of mononuclear phagocytes against Trichosporon asahii

Trichosporon asahii is an emerging opportunistic pathogen in immunocompromised patients. Little is known about the mechanisms of host defence against T. asahii. We investigated the fungicidal activity of human peripheral blood monocytes and murine peritoneal macrophages against T. asahii isolates, and the effects of M-CSF on the anti-fungal activity of mononuclear phagocytes. We also established a neutropenic mouse model of disseminated trichosporonosis with T. asahii. M-CSF enhanced the phagocytic fungicidal activity of mononuclear cells, and infected mice treated with human M-CSF at 10 x 106 U/kg showed a significant improvement in survival rate, with fewer fungal colony counts in the lung compared with control mice. Mice treated with human M-CSF showed higher concentrations of tumour necrosis factor-alpha (TNF-alpha) in the lung and plasma compared with control mice. The survival rate was significantly reduced in mice treated with anti-mouse TNF-alpha. Our results showed that M-CSF enhanced the fungicidal activity of mononuclear phagocytes partly by production of TNF-alpha, and suggest that the administration of M-CSF to patients with disseminated trichosporonosis may be a useful adjunct to conventional anti-microbial therapy and prophylaxis.

Characterisation of Candida albicans infections of haematogenous and mucosal origin in mice lacking the interferon gamma receptor protein

Mice harbouring a null deletion mutation in the IFNgamma receptor gene were used to study the role of IFNgamma responsiveness during experimental systemic candidiasis of mucosal or haematogenous origin. After intravenous (i.v.) or intranasal (i.n.) challenge with Candida albicans the progression of infection and concomitant cellular and antibody anti-C. albicans immune responses were analysed. During the week following i.v. challenge, the rate of C. albicans multiplication in kidneys, liver and spleen was faster in IFNgammaR (-/-) than IFNgammaR (+/+) mice. As a result, IFNgammaR (-/-) mice perished earlier than IFNgammaR (+/+) mice when challenged with equal numbers of live yeast cells. However, the overall susceptibility of the two mouse strains, in terms of survival against different C. albicans challenge doses over a 60-day period, was similar. No differences were found in the cellular anti-C. albicans response generated by i.v. challenge in both mouse strains. In contrast the kinetics and strength of the serum anti-C. albicans antibody responses were markedly different. Significantly stronger, predominantly IgG2a antibody responses accompanied the eventual control of C. albicans infection in IFNgammaR (-/-) mice. Following intranasal challenge, there was no difference in the rate of C. albicans clearance from the lungs of IFNgammaR (-/-) and IFNgammaR (+/+) mice. However, 48 h after challenge, large, conspicuous abscesses appeared in the lungs, liver, kidneys and spleen of IFNgammaR (-/-) mice. These abscesses were characterised by the presence of C. albicans and abundant neutrophilic infiltrates, but very few macrophages. No such abscesses developed in i.n. challenged IFNgammaR (+/+) mice. In both mouse strains, i.n. challenge induced strong systemic anti-C. albicans cellular responses, but relatively low titre systemic antibody responses. Mucosal anti-C. albicans antibody responses were detected in IFNgammaR (+/+), but not IFNgammaR (-/-) mice. Splenic adherent macrophages obtained from IFNgammaR (-/-) mice exhibited a significantly lower candidacidal activity than those of IFNgammaR (+/+) mice, and as expected, were not responsive to IFNgamma. In summary, these data suggest that IFNgamma has a role in limiting C. albicans multiplication during the early stages of infection, as well as in preventing the development of C. albicans-associated abscesses. Activation of macrophages by IFNgamma might be pivotal in mediating this role.

Neutrophils play a protective nonphagocytic role in systemic Mycobacterium tuberculosis infection of mice

Evidence showing that neutrophils play a protective role in the host response to infection by different intracellular parasites has been published in the past few years. We assessed this issue with regard to the infection of mice with Mycobacterium tuberculosis. We found a chronic recruitment of neutrophils to the infection foci, namely, to the peritoneal cavity after intraperitoneal infection and to the spleen and liver after intravenous inoculation of the mycobacteria. However, bacilli were never found associated with the recruited neutrophils but rather were found inside macrophages. The intravenous administration of the antineutrophil monoclonal antibody RB6-8C5 during the first week of infection led to selective and severe neutropenia associated with an enhancement of bacillary growth in the target organs of the mice infected by the intravenous route. The neutropenia-associated exacerbation of infection was most important in the liver, where a bacterial load 10-fold higher than that in nonneutropenic mice was found; the exacerbation in the liver occurred both during and after the neutropenic period. Early in infection by M. tuberculosis, neutropenic mice expressed lower levels of mRNAs for gamma interferon and inducible nitric oxide synthase in the liver compared to nondepleted mice. These results point to a protective role of neutrophils in the host defense mechanisms against M. tuberculosis, which occurs early in the infection and is not associated with the phagocytic activity of neutrophils but may be of an immunomodulatory nature.

Evaluation of the intranasal challenge route in mice as a mucosal model for Candida albicans infection

The intranasal route was used to study Candida albicans infections in mice. Mice from two different inbred strains were challenged intranasally with C. albicans and the level of local and systemic colonization was monitored. DBA/2 mice were highly susceptible to challenge and viable C. albicans disseminated from the lungs to deeper tissues, including kidneys, liver and spleen within 48 h. In contrast, in BALB/c mice challenged in the same manner, C. albicans were retained within the lungs and cleared. Local and systemic anti-C. albicans immune responses were investigated. BALB/c mice exhibited higher titres of serum and mucosal anti-C. albicans IgA than DBA/2 mice. Splenocytes from BALB/c mice, but not from DBA/2 mice, produced detectable levels of interleukin-4 and -5 following stimulation with C. albicans antigens. Both DBA/2- and BALB/c-derived splenocytes produced interferon-gamma and interleukin-10 in response to similar stimulation. In conclusion, the intranasal route provided a simple, non-invasive murine model for investigating C. albicans infection through mucosal surfaces.

The Role of Recombinant Murine IL-12 and IFN-γ in the Pathogenesis of a Murine Systemic Candida albicans Infection

Studies on murine candidiasis suggest that resistance to disease is linked to a Th1 response and production of IFN-γ, while failure to elicit protection is associated with a Th2 response and production of IL-4 and IL-10. Experimental infection of C57BL/6 mice, IL-12 treatment of these mice, or both infection and IL-12 treatment resulted in a characteristic Th1 cytokine mRNA profile as measured by quantitative competitive PCR. Specifically, little or no IL-4 transcripts were detected, while IFN-γ message was elevated, particularly with IL-12 treatment. Despite its role in driving increased IFN-γ expression and production, IL-12 treatment, paradoxically, promoted disease progression in our model. Therefore, we examined the effect of IFN-γ neutralization on IL-12-induced susceptibility to infection. None of the systemically infected mice receiving IL-12 alone survived, while IL-12- and anti-IFN-γ-treated mice had a 70% survival rate, similar to that after infection alone. These results suggested that IFN-γ induced by IL-12 treatment contributed to lethality. However, in separate studies, IFN-γ knockout mice were more susceptible to infection than their wild-type counterparts, suggesting that IFN-γ is required for resistance. Nonetheless, infected IFN-γ knockout mice treated with recombinant murine IL-12 exhibited enhanced resistance, suggesting that the toxicities observed with IL-12 are directly attributable to IFN-γ and that an optimal immune response to Candida infections necessitates a finely tuned balance of IFN-γ production. Thus, we propose that although IFN-γ can drive resistance, the overproduction of IFN-γ during candidiasis, mediated by IL-12 administration, leads to enhanced susceptibility.

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.

Gamma interferon is not essential in host defense against disseminated candidiasis in mice

In vitro studies have suggested a role for interferon gamma (IFN-gamma) in host defense against disseminated candidiasis, but in vivo studies are inconclusive. We utilized homozygous IFN-gamma knockout (GKO) mice to determine if the cytokine is essential in host defense against this disease. Genotypes of mice were determined by PCR with specific primers for the normal or disrupted IFN-gamma gene. The GKO status of the mice was confirmed by an enzyme-linked immunosorbent assay, which showed no detectable IFN-gamma produced by their splenocytes stimulated by concanavalin A. To test the susceptibility of GKO mice to candidiasis, the animals were infected either intravenously (i.v.) or intragastrically (i.g.) with Candida albicans. GKO mice infected i.v. survived as long as wild-type (WT) mice and showed no difference in Candida CFU counts in liver, spleen, or kidneys compared to those for WT mice. When animals were given Candida i.g., at 3 h or at 10 or 21 days after infection, there was no dissemination of Candida to the lung, liver, spleen, or kidneys for either GKO or WT mice. There was no difference in Candida CFU counts recovered from the stomach or intestines between GKO and WT mice. Histological examination of the stomach cardial-atrium fold, where the fungus was located, showed that GKO mice did not have evidence of more tissue damage or fungal invasion than WT mice. Finally, the jejunum for both types of mice showed no evidence of tissue damage or fungal invasion. These studies indicate that IFN-gamma is not essential in host defense against C. albicans that originates from a mucosal site or that is given directly into the bloodstream in a mouse model.

Oral carriage of Candida albicans in murine AIDS

Oral candidiasis is a common fungal infection in patients infected with the human immunodeficiency virus (HIV). Although rare at the time of primary HIV infection, it is frequently found throughout the asymptomatic phase and is predictive of progressive immunodeficiency. However, the precise immune defect which results in outgrowth of commensal Candida albicans in HIV infection has not been identified. Mice infected with the Du5H(G6T2) mixture of mouse leukemia viruses develop a syndrome, designated murine AIDS (MAIDS), that has many of the immune abnormalities found in HIV infection. Retrovirus-infected C57BL/6 mice were examined for their ability to resist the development of oral candidiasis from the carrier state established after a self-limiting acute infection and to clear a subsequent secondary inoculum of oral C. albicans. Most of the mice orally colonized with C. albicans and then inoculated with the retrovirus mixture maintained a low-level oral carriage of C. albicans, while 30% of coinfected mice developed recurring 2- to 3-week episodes of acute Candida proliferation, separated by transient recoveries to the carrier state. The frequencies of CD4+ and CD8+ lymphocytes were, respectively, unchanged and significantly decreased (P < 0.05) in both cervical lymph nodes and spleens of coinfected mice compared to the corresponding frequencies in C. albicans-carrying, virus-free, age-matched control animals. Secretion of gamma interferon by concanavalin A (ConA)-stimulated spleen cells from Candida-carrying, retrovirus-infected mice was significantly decreased (P < 0.05) compared to that of C. albicans-carrying, retrovirus-free mice, in accordance with known abnormalities associated with MAIDS. However, production of this cytokine by ConA-stimulated or unstimulated cervical lymph node cells from coinfected mice was enhanced compared to that of virus-free animals colonized with C. albicans. Acquired resistance to reinfection with C. albicans was maintained in retrovirus-infected mice and was associated with a mucosal recruitment of CD8+ cells not observed in control mice. These results suggest that alterations in mucosal immunity which occur in MAIDS differ substantially from defects observed at other sites and that surrogate epithelial defense mechanisms may function locally to limit Candida proliferation.

Candida albicans stimulates cytokine production and leukocyte adhesion molecule expression by endothelial cells

Endothelial cells have the potential to influence significantly the host immune response to blood-borne microbial pathogens, such as Candida albicans. We investigated the ability (of this organism to stimulate endothelial cell responses relevant to host defense in vitro. Infection with C. albicans induced endothelial cells to express mRNAs encoding E-selectin, intercellular adhesion molecule 1, vascular cell adhesion molecule 1, interleukin 6, interleukin 8, monocyte chemoattractant protein 1, and inducible cyclooxygenase (cox2). All three leukocyte adhesion molecule proteins were expressed on the surfaces of the endothelial cells after 8 h of exposure to C. albicans. An increase in secretion of all three cytokines was found after 12 h of infection. Cytochalasin D inhibited accumulation of the endothelial cell cytokine and leukocyte adhesion molecule mRNAs in response to C. albicans, suggesting that endothelial cell phagocytosis of the organism is required to induce this response. Live Candida tropicalis, Candida glabrata, a nongerminating strain of C. albicans, and killed C. albicans did not stimulate the expression of any of the cytokine or leukocyte adhesion molecule mRNAs. These findings indicate that a factor associated with live, germinating C. albicans is required for induction of endothelial cell mRNA expression. Furthermore, since endothelial cells phagocytize killed C. albicans, phagocytosis is likely necessary but not sufficient for this organism to stimulate mRNA accumulation. In conclusion, the secretion of proinflammatory cytokines and expression of leukocyte adhesion molecules by endothelial cells in response to C. albicans could enhance the host defense against this organism by contributing to the recruitment of activated leukocytes to sites of intravascular infection.

Safety and efficacy of liposomal amphotericin B in allogeneic bone marrow transplant recipients

In a retrospective analysis, 79 allogeneic bone marrow recipients treated with AmBisome prophylactically or because of proven or suspected invasive fungal infection (IFI) were evaluated in 92 episodes. The median duration of treatment was 14 (range 1-112) days. The mean maximum dose given was 1.64 +/- 0.8 mg kg-1 day-1 and the mean total dose was 1.29 +/- 2.28 g. The overall incidence of reported adverse events was 194, of which none had a serious outcome. In six cases, the drug was withdrawn as a result of toxic or allergic reactions: dyspnoea and flush (3), urticaria (1), cholecystitis (1) and disorientation (one case, probably not related to AmBisome). No anaphylactoid reactions were seen. Laboratory findings, including low serum potassium (48% of the episodes), increased serum creatinine (38%) and increased serum sodium levels (7%), caused no major clinical problems. Thirteen cases of verified IFI were evaluated regarding the efficacy of AmBisome. Survival or cure of the mycotic infection occurred in 5/13 patients (38%). Two patients were treated with AmBisome (3.6 and 3.3 mg kg-1 day-1) because of verified IFI before BMT. One died of IFI. The other died of venoocclusive disease of the liver (VOD) without histological evidence of active IFI. We found a significant (P < 0.05) reduction in autopsy-proven IFI, 12/199 (6%) compared to the period when only conventional doses of amphotericin B were used, 26/227 (11%).

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.