Damage to Candida albicans hyphae and pseudohyphae by the myeloperoxidase system and oxidative products of neutrophil metabolism in vitro

Neutrophil Oxidative Burst Profile Is Related to a Satisfactory Response to Itraconazole and Clinical Cure in Feline Sporotrichosis

Despite the central role of cats in the transmission and amplification of Sporothrix, studies regarding immune response in feline sporotrichosis are scarce. In cats with sporotrichosis, neutrophil-rich lesions are usually associated to good general condition and lower fungal burden. However, the role of neutrophils in anti-Sporothrix immunity has been little explored in cats. Thus, the aim of this study was to evaluate the neutrophil oxidative burst in the blood of cats with sporotrichosis. Cats with sporotrichosis included in the study were treated with itraconazole (ITZ) alone or combined with potassium iodide (KI). The neutrophil oxidative burst was evaluated through a flow-cytometry-based assay using dihydrorhodamine 123 (background) and stimulation with Zymosan and heat-killed Sporothrix yeasts. The cure rate was 50.0% in cats under treatment with ITZ monotherapy and 90.9% in cats treated with ITZ + KI (p = 0.014), endorsing the combination therapy as an excellent alternative for the treatment of feline sporotrichosis. Higher percentages of Sporothrix-stimulated neutrophils were associated with good general condition (p = 0.003). Higher percentages of Sporothrix- (p = 0.05) and Zymosan-activated (p = 0.014) neutrophils before and early in the treatment were related to clinical cure in ITZ-treated cats. The correlation between oxidative burst and successful use of KI could not be properly assessed given the low number of failures (n = 2) in this treatment group. Nasal mucosa involvement, typically linked to treatment failure, was related to lower percentages of activated neutrophils in the background at the treatment outcome (p = 0.02). Our results suggest a beneficial role of neutrophils in feline sporotrichosis and a positive correlation between neutrophil activation and the cure process in ITZ-treated cats.

Human neutrophils ≠ murine neutrophils: Does it matter?

Human and murine neutrophils differ with respect to representation in blood, receptors, nuclear morphology, signaling pathways, granule proteins, NADPH oxidase regulation, magnitude of oxidant and hypochlorous acid production, and their repertoire of secreted molecules. These differences often matter and can undermine extrapolations from murine studies to clinical care, as illustrated by several failed therapeutic interventions based on mouse models. Likewise, coevolution of host and pathogen undercuts fidelity of murine models of neutrophil-predominant human infections. However, murine systems that accurately model the human condition can yield insights into human biology difficult to obtain otherwise. The challenge for investigators who employ murine systems is to distinguish models from pretenders and to know when the mouse provides biologically accurate insights. Testing with human neutrophils observations made in murine systems would provide a safeguard but is not always possible. At a minimum, studies that use exclusively murine neutrophils should have accurate titles supported by data and restrict conclusions to murine neutrophils and not encompass all neutrophils. For now, the integration of evidence from studies of neutrophil biology performed using valid murine models coupled with testing in vitro of human neutrophils combines the best of both approaches to elucidate the mysteries of human neutrophil biology.

Regulation of pseurotin A biosynthesis by GliZ and zinc in Aspergillus fumigatus

Recently, we reported that zinc regulates gliotoxin biosynthesis via ZafA, which is a zinc-responsive transcriptional activator. From an HPLC analysis of culture media of Aspergillus fumigatus, we found a trend of decreasing gliotoxin production but increasing pseurotin A and fumagillin production in proportion to the zinc concentration. The expression of the genes involved in pseurotin A biosynthesis was upregulated under high zinc concentrations. Furthermore, upregulated expression of pseurotin A biosynthetic genes and higher production of pseurotin A were observed in the zafA deletion strain. Interestingly, the deletion of gliZ, a transcriptional activator of gliotoxin biosynthesis genes, resulted in upregulated expression of pseurotin A biosynthetic genes and increased production of pseurotin A. We detected upregulation of fumR expression in the gliZ and zafA deletion mutants. The overexpression of gliZ observed in the zafA deletion mutant resulted in the failure of the mutant to increase pseurotin A production, which is a phenotype of the zafA deletion mutant. These results suggest that ZafA sequentially regulates pseurotin A biosynthesis through GliZ. Finally, we found through a murine virulence test that the gliZ and fumR double-deletion mutants showed a delayed death rate compared with the single-deletion mutants of either gliZ or fumR. Taken together, these results suggested that the biosynthesis of gliotoxin and pseurotin A are regulated in opposite ways by zinc utilization and that each secondary metabolite is synthesized when the synthesis of another secondary metabolite fails to protect it against the defense system of the host.

Dual identities for various alcohols in two different yeasts

Most of the yeast bypasses the developmental stage from simple unicellular yeast to elongated structure like hyphae. Regulation of this transition is governed by various quorum sensing and signalling molecules produced under different conditions of growth, that differ significantly, both physiologically and chemically. The evidence of fungal quorum sensing was uncovered ten years ago after the discovery of farnesol as first eukaryotic quorum sensing molecules in Candida albicans. In addition to farnesol, tyrosol was identified as second quorum sensing molecules in C. albicans controlling physiological activities. After the discovery of farnesol and tyrosol, regulation of morphogenesis through the production of chemical signalling molecules such as isoamyl alcohol, 2-phenylethyl alcohol, 1-dodecanol, E-nerolidol, etc. is reported in C. albicans. Some of the evidence suggests that the budding yeast Saccharomyces cerevisiae exhibits this type of regulation and the signals are regulated by aromatic alcohols which are the end product of amino acid metabolism. The effects of these molecules on morphogenesis are not similar in both yeasts, making comparisons hard. It is hypothesized that these signals works in microorganisms to derive a competitive advantage. Here, we present an example for utilization of competitive strategy by C. albicans and S. cerevisiae over other microorganisms.

Mechanism of Microbicidal Action of E-101 Solution, a Myeloperoxidase-Mediated Antimicrobial, and Its Oxidative Products

E-101 solution is a first-in-class myeloperoxidase-mediated antimicrobial developed for topical application. It is composed of porcine myeloperoxidase (pMPO), glucose oxidase (GO), glucose, sodium chloride, and specific amino acids in an aqueous solution. Once activated, the reactive species hydrogen peroxide (H₂O₂), hypochlorous acid, and singlet oxygen are generated. We evaluated the treatment effects of E-101 solution and its oxidative products on ultrastructure changes and microbicidal activity against methicillin-resistant Staphylococcus aureus (MRSA) and Escherichia coli Time-kill and transmission electron microscopy studies were also performed using formulations with pMPO or GO omitted. The glutathione membrane protection assay was used to study the neutralization of reactive oxygen species. The potency of E-101 solution was also measured in the presence of serum and whole blood by MIC and minimal bactericidal concentration (MBC) determinations. E-101 solution demonstrated rapid bactericidal activity and ultracellular changes in MRSA and E. coli cells. When pMPO was omitted, high levels of H₂O₂ generated from GO and glucose demonstrated slow microbicidal activity with minimal cellular damage. When GO was omitted from the formulation, no antimicrobial activity or cellular damage was observed. Protection from exposure to E-101 solution reactive oxygen species in the glutathione protection assay was competitive and temporary. E-101 solution maintained its antimicrobial activity in the presence of inhibitory substances, such as serum and whole blood. E-101 solution is a potent myeloperoxidase enzyme system with multiple oxidative mechanisms of action. Our findings suggest that the primary site where E-101 solution exerts microbicidal action is the cell membrane, by inactivation of essential cell membrane components.

Size matters - how the immune system deals with fungal hyphae

Fungal hyphae constitute a special challenge for the immune system, since they are too large to be phagocytosed. This review summarizes our current knowledge on those immune cells that are able to attack and eliminate hyphae and we discuss the different means that are employed by these cells in order to kill hyphae.

Characterization of myeloid-specific peroxidase, keratin 8, and dual specificity phosphatase 1 as innate immune genes involved in the resistance of crucian carp (Carassius auratus gibelio) to Cyprinid herpesvirus 2 infection

Myeloid-specific peroxidase (MPO), keratin 8 (KRT-8), and dual specificity phosphatase 1 (DUSP-1) are believed to play essential roles in innate immunity. Through suppression subtractive hybridization (SSH) analysis, we previously identified MPO, KRT-8, and DUSP-1 as the three genes that were the most significantly upregulated in crucian carp (Carassius auratus gibelio) that survived Cyprinid herpesvirus 2 (CyHV-2) infection. Here, we have further characterized these three genes and their response to pathogen challenge. The open reading frames (ORF) of MPO, KRT-8, and DUSP-1 were cloned by RACE technique and sequenced. The full-length cDNAs of the three genes contained ORFs of 2289, 1575 and 1083 bp respectively. The polypeptides from each ORF were projected to contain 762 (MPO), 524 (KRT-8), and 360 (DUSP-1) amino acids. Phylogenetic analysis showed that the three genes were most closely related to zebrafish. We found that MPO, KRT-8, and DUSP-1 were expressed at low levels in all of the tissues examined in healthy crucian carp. Quantitative real-time RT-PCR analysis indicated that MPO, KRT-8, and DUSP-1 mRNA expression was significantly upregulated within 72 h of CyHV-2 infection compared to mock infected controls. Maximum expression of MPO was detected at 24 hpi (2.71-fold, P < 0.05). While, 12 hpi (3.80-fold, P < 0.01) and 6 hpi (8.70-fold, P < 0.01) were the highest expression time points for KRT-8 and DUSP-1, respectively. In contrast, after Aeromonas hydrophila challenge, the transcripts of these three genes remained unchanged or slightly down-regulated. For the fish survived from viral infection, expression levels of MPO and KRT-8 were 2.72 fold and 2.47 fold higher than those of fish died from acute infection, and similar level of DUSP-1 was observed in samples of survived fish. These data suggested MPO, KRT-8 and DUSP-1 might be involved in the antiviral, but not antibacterial innate immune response in crucian carp. These findings also support the use of MPO and KRT-8 as immunological markers for a response to viral infection in crucian carp.

Candida albicans escapes from mouse neutrophils

Candida albicans, the most commonly isolated human fungal pathogen, is able to grow as budding yeasts or filamentous forms, such as hyphae. The ability to switch morphology has been attributed a crucial role for the pathogenesis of C. albicans. To mimic disseminated candidiasis in humans, the mouse is the most widely used model organism. Neutrophils are essential immune cells to prevent opportunistic mycoses. To explore potential differences between the rodent infection model and the human host, we compared the interactions of C. albicans with neutrophil granulocytes from mice and humans. We revealed that murine neutrophils exhibited a significantly lower ability to kill C. albicans than their human counterparts. Strikingly, C. albicans yeast cells formed germ tubes upon internalization by murine neutrophils, eventually rupturing the neutrophil membrane and thereby, killing the phagocyte. On the contrary, growth and subsequent escape of C. albicans are blocked inside human neutrophils. According to our findings, this blockage in human neutrophils might be a result of higher levels of MPO activity and the presence of α-defensins. We therefore outline differences in antifungal immune defense between humans and mouse strains, which facilitates a more accurate interpretation of in vivo results.

Nox2 modification of LDL is essential for optimal apolipoprotein B-mediated control of agr type III Staphylococcus aureus quorum-sensing

Staphylococcus aureus contains an autoinducing quorum-sensing system encoded within the agr operon that coordinates expression of virulence genes required for invasive infection. Allelic variation within agr has generated four agr specific groups, agr I-IV, each of which secretes a distinct autoinducing peptide pheromone (AIP1-4) that drives agr signaling. Because agr signaling mediates a phenotypic change in this pathogen from an adherent colonizing phenotype to one associated with considerable tissue injury and invasiveness, we postulated that a significant contribution to host defense against tissue damaging and invasive infections could be provided by innate immune mechanisms that antagonize agr signaling. We determined whether two host defense factors that inhibit AIP1-induced agrI signaling, Nox2 and apolipoprotein B (apoB), also contribute to innate control of AIP3-induced agrIII signaling. We hypothesized that apoB and Nox2 would function differently against AIP3, which differs from AIP1 in amino acid sequence and length. Here we show that unlike AIP1, AIP3 is resistant to direct oxidant inactivation by Nox2 characteristic ROS. Rather, the contribution of Nox2 to defense against agrIII signaling is through oxidation of LDL. ApoB in the context of oxLDL, and not LDL, provides optimal host defense against S. aureus agrIII infection by binding the secreted signaling peptide, AIP3, and preventing expression of the agr-driven virulence factors which mediate invasive infection. ApoB within the context of oxLDL also binds AIP 1-4 and oxLDL antagonizes agr signaling by all four agr alleles. Our results suggest that Nox2-mediated oxidation of LDL facilitates a conformational change in apoB to one sufficient for binding and sequestration of all four AIPs, demonstrating the interdependence of apoB and Nox2 in host defense against agr signaling. These data reveal a novel role for oxLDL in host defense against S. aureus quorum-sensing signaling.

Metabolomics reveals differential levels of oral metabolites in HIV-infected patients: toward novel diagnostic targets

The objective of the current study was to characterize the profile of oral metabolites in HIV-infected patients using metabolomics. Oral wash samples were collected from 12 HIV-infected and 12 healthy individuals (matched for age, sex, and ethnicity), processed, and analyzed by metabolomics. We detected 198 identifiable and 85 nonidentifiable metabolites; 27 identifiable metabolites were differentially present (12 increased, 15 decreased) in HIV-infected patients. Elevated metabolites included p-cresol sulfate, nucleotides (e.g., allantoin), and amino acids (e.g., phenylalanine, tryptophan), whereas decreased oral metabolites included fucose, fumarate, and N-acetylglucosamine. Pathway network analysis revealed the largest multinode network in healthy versus HIV-infected patients to involve carbohydrate biosynthesis and degradation. HIV-infected patients on antiretroviral therapy (ART) showed the largest number (12) of statistically significant metabolite correlation differences compared with healthy controls. Interestingly, the oral phenlyalanine:tyrosine ratio increased in ART-naive HIV-infected patients (mean ± SEM = 2.58 ± 0.87) compared with healthy individuals (1.33 ± 0.10, p = 0.062) or ART-experienced patients (1.78 ± 0.30, p = 0.441). This is the first study to reveal differential levels of oral metabolites in HIV-infected patients compared withj healthy volunteers, and that oral phenlyalanine:tyrosine ratio may be a useful marker for noninvasive monitoring of the immune status during HIV infection.

Neutrophil extracellular traps contain calprotectin, a cytosolic protein complex involved in host defense against Candida albicans

Neutrophils are the first line of defense at the site of an infection. They encounter and kill microbes intracellularly upon phagocytosis or extracellularly by degranulation of antimicrobial proteins and the release of Neutrophil Extracellular Traps (NETs). NETs were shown to ensnare and kill microbes. However, their complete protein composition and the antimicrobial mechanism are not well understood. Using a proteomic approach, we identified 24 NET-associated proteins. Quantitative analysis of these proteins and high resolution electron microscopy showed that NETs consist of modified nucleosomes and a stringent selection of other proteins. In contrast to previous results, we found several NET proteins that are cytoplasmic in unstimulated neutrophils. We demonstrated that of those proteins, the antimicrobial heterodimer calprotectin is released in NETs as the major antifungal component. Absence of calprotectin in NETs resulted in complete loss of antifungal activity in vitro. Analysis of three different Candida albicans in vivo infection models indicated that NET formation is a hitherto unrecognized route of calprotectin release. By comparing wild-type and calprotectin-deficient animals we found that calprotectin is crucial for the clearance of infection. Taken together, the present investigations confirmed the antifungal activity of calprotectin in vitro and, moreover, demonstrated that it contributes to effective host defense against C. albicans in vivo. We showed for the first time that a proportion of calprotectin is bound to NETs in vitro and in vivo.

How human neutrophils kill and degrade microbes: an integrated view

Neutrophils constitute the dominant cell in the circulation that mediates the earliest innate immune human responses to infection. The morbidity and mortality from infection rise dramatically in patients with quantitative or qualitative neutrophil defects, providing clinical confirmation of the important role of normal neutrophils for human health. Neutrophil-dependent anti-microbial activity against ingested microbes represents the collaboration of multiple agents, including those prefabricated during granulocyte development in the bone marrow and those generated de novo following neutrophil activation. Furthermore, neutrophils cooperate with extracellular agents as well as other immune cells to optimally kill and degrade invading microbes. This brief review focuses attention on two examples of the integrated nature of neutrophil-mediated anti-microbial action within the phagosome. The importance and complexity of myeloperoxidase-mediated events illustrate a collaboration of anti-microbial responses that are endogenous to the neutrophil, whereas the synergy between the phagocyte NADPH (nicotinamide adenine dinucleotide phosphate) oxidase and plasma-derived group IIA phospholipase A(2) exemplifies the collective effects of the neutrophil with an exogenous factor to achieve degradation of ingested staphylococci.

Integrin engagement mediates the human polymorphonuclear leukocyte response to a fungal pathogen-associated molecular pattern

Extravasation of leukocytes from peripheral blood is required for an effective inflammatory response at sites of tissue infection. Integrins help mediate extravasation and navigate the leukocyte to the infectious source. A novel role for integrins in regulating the effector response to a cell wall component of fungal pathogens is the subject of the current study. Although phagocytosis is useful for clearance of unicellular fungi, the immune response against large, noningestible hyphae is not well-understood. Fungal beta-glucan, a pathogen-associated molecular pattern, activates production of superoxide anion in leukocytes without the need for phagocytosis. To model polymorphonuclear leukocyte (PMN) recognition of fungi under conditions in which phagocytosis cannot occur, beta-glucan was covalently immobilized onto tissue culture plastic. Plasma membrane-associated respiratory burst was measured by reduction of ferricytochrome C. Results show that the human PMN oxidative burst response to immobilized beta-glucan is suppressed by addition of beta(1) integrin ligands to the beta-glucan matrix. Suppression was dose dependent and steric hindrance was ruled out. beta(1) integrin ligands did not affect respiratory burst to ingestible beta-glucan-containing particles, phorbol esters or live yeast hyphae. Furthermore, in the absence of matrix, Ab activation of VLA3 or VLA5, but not other beta(1) integrins, also prevented beta-glucan-induced respiratory burst. beta(1)-induced suppression was blocked and burst response restored by treating neutrophils with either the cell-binding fragment of soluble human Fn, cyclic RGD peptide, or Ab specific to VLA3 or VLA5. Together these findings extend the functional role of beta(1) integrins to include modulating PMN respiratory burst to a pathogen-associated molecular pattern.

The early transcriptional response of human granulocytes to infection with Candida albicans is not essential for killing but reflects cellular communications

Candida albicans is a polymorphic opportunistic fungus that can cause life-threatening systemic infections following hematogenous dissemination in patients susceptible to nosocomial infection. Neutrophils form part of the innate immune response, which is the first line of defense against microbes and is particularly important in C. albicans infections. To compare the transcriptional response of leukocytes exposed to C. albicans, we investigated the expression of key cytokine genes in polymorphonuclear and mononuclear leukocytes after incubation with C. albicans for 1 h. Isolated mononuclear cells expressed high levels of genes encoding proinflammatory signaling molecules, whereas neutrophils exhibited much lower levels, similar to those observed in whole blood. The global transcriptional profile of neutrophils was examined by using an immunology-biased human microarray to determine whether different morphological forms or the viability of C. albicans altered the transcriptome. Hyphal cells appeared to have the broadest effect, although the most strongly induced genes were regulated independently of morphology or viability. These genes were involved in proinflammatory cell-cell signaling, cell signal transduction, and cell growth. Generally, genes encoding known components of neutrophil granules showed no upregulation at this time point; however, lactoferrin, a well-known candidacidal peptide, was secreted by neutrophils. Addition to inhibitors of RNA or protein de novo synthesis did not influence the killing activity within 30 min. These results support the general notion that neutrophils do not require gene transcription to mount an immediate and direct attack against microbes. However, neutrophils exposed to C. albicans express genes involved in communication with other immune cells.

Contribution of the myeloperoxidase-dependent oxidative system to host defence against Cryptococcus neoformans

The in vivo contribution of reactive oxygen species produced by neutrophils against Cryptococcus infection is not widely recognized. Myeloperoxidase (MPO) is a neutrophil-specific enzyme that catalyses the production of hypohalous acids such as HOCl from H2O2. This study investigated the role of MPO in immunological defence against Cryptococcus neoformans in an MPO-deficient (MPO-/-) mouse model. The survival of MPO-/- mice infected either intranasally or intravenously with C. neoformans was lower than that of identically challenged wild-type mice. The MPO-/- mice that received intranasal injection of C. neoformans had significantly larger lung fungal burdens than wild-type mice. On day 7, MPO-/- mice had a significantly higher lung concentration of interleukin (IL)-4 and lower concentrations of IL-2, IL-12p70 and interferon (IFN)-gamma than wild-type mice, suggesting a weak Th1 response in the MPO-/- mice to C. neoformans. Pathologically, the MPO-/- mice with intranasal infection showed more severe pneumonia than wild-type mice, which was associated with an increase in the levels of IL-1alpha/beta in the lungs. In addition, in MPO-/- mice, the pulmonary infection disseminated to the brain with occasional meningitis. The keratinocyte-derived cytokine (KC) level in the brain of infected MPO-/- mice was higher than that of control mice. Both intranasal and intravenous infections resulted in a higher number of fungi in the spleen of MPO-/- mice compared to wild-type, suggesting decreased resistance to C. neoformans not only in the lungs but also in the spleen in the absence of MPO. Taken together, these data suggest a major role of MPO in the response to cryptococcal infection.

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.

Polymorphonuclear neutrophils deliver activation signals and antigenic molecules to dendritic cells: a new link between leukocytes upstream of T lymphocytes

Polymorphonuclear neutrophils (PMNs) are rapidly recruited to tissues upon injury or infection. There, they can encounter local and/or recruited immature dendritic cells (iDCs), a colocalization that could promote at least transient interactions and mutually influence the two leukocyte populations. Using human live blood PMNs and monocyte-derived iDCs, we examined if these leukocytes actually interacted and whether this influenced DC function. Indeed, coculture with live but not apoptotic PMNs led to up-regulation of membrane CD40, CD86, and human leukocyte antigen (HLA)-DR on DCs. Whereas CD40 up-regulation was dependent on soluble factors released by PMNs, as determined in cultures conducted in different chambers, cell contact was necessary for CD86 and HLA-DR up-regulation, a process that was inhibited by anti-CD18 antibodies, indicating that CD18 ligation was required. We also found that via a cell contact-dependent mechanism, DCs acquired Candida albicans-derived antigens from live as well as from apoptotic PMNs and could thus elicit antigen-specific T lymphocyte responses. Altogether, our data demonstrate the occurrence of cross-talk between human PMNs and DCs and provide new insights into the immune processes occurring upstream of the interactions between DCs and T lymphocytes.

The concomitant expression and availability of conventional and alternative, cyanide-insensitive, respiratory pathways in Candida albicans

The opportunistic oral pathogen Candida albicans expresses a cyanide-insensitive alternative oxidase (AOX) upon exposure to respiratory inhibitors that act downstream from coenzyme Q, and upon ageing of cells. To investigate whether the conventional pathway is retained when the alternative pathway is induced, cells were grown in the presence of sodium cyanide, a reversible inhibitor of cytochrome oxidase. AOX expression was monitored by Western blotting and the presence of cytochromes associated with complexes III and IV of the conventional pathway was monitored by recording spectra between 500 and 650 nm at 77K. The activities of complexes III and IV were determined in polarographic and enzyme-kinetic experiments using specific respiratory substrates and inhibitors. Results indicated that complexes III and IV are constitutively expressed and are functional in cells expressing AOX. Furthermore, the enzymatic activities of complexes III and IV were similar in mitochondrial preparations from cells grown with or without cyanide. We next investigated whether both pathways are simultaneously available for electron transfer from the Q pool to molecular oxygen. Respiration was virtually completely inhibited by the combination of cyanide and salicyl hydroxamic acid (SHAM) or antimycin A and SHAM, but only partly inhibited by either of these inhibitors alone. This indicates that electrons can in principle flow either through the conventional or the alternative respiratory pathway. The availability of two electron pathways in C. albicans and the potential use of either pathway endows this pleomorphic fungus with another level at which it can rapidly adjust to altered environmental conditions.

The immune response to fungal infections

During the past two decades, invasive fungal infections have emerged as a major threat to immunocompromised hosts. Patients with neoplastic diseases are at significant risk for such infections as a result of their underlying illness and its therapy. Aspergillus, Candida, Cryptococcus and emerging pathogens, such as the zygomycetes, dark walled fungi, Trichosporon and Fusarium, are largely opportunists, causing infection when host defences are breached. The immune response varies with respect to the fungal species and morphotype encountered. The risk for particular infections differs, depending upon which aspect of immunity is impaired. This article reviews the current understanding of the role and relative importance of innate and adaptive immunity to common and emerging fungal pathogens. An understanding of the host response to these organisms is important in decisions regarding use of currently available antifungal therapies and in the design of new therapeutic modalities.

Enhanced killing of Candida albicans by human macrophages adherent to type 1 collagen matrices via induction of phagolysosomal fusion

Candida albicans, a component of the normal flora of the alimentary tract and mucocutaneous membranes, is the leading cause of invasive fungal disease in premature infants, diabetics, and surgical patients and of oropharyngeal disease in AIDS patients. As little is known about the regulation of monocyte/macrophage anti-Candida activity, we sought to determine if fungicidal activity might be regulated by extracellular matrix proteins to which monocytes/macrophages are adherent in vivo. Compared to monocyte/macrophages that adhered to plastic, human monocytes and monocyte-derived macrophages that adhered to type 1 collagen matrices, but not to fibronectin, vitronectin, or laminin, demonstrated a significant increase in candidacidal activity. The enhancement of monocyte fungicidal activity was maintained over a 4-h period, whereas macrophage fungicidal activity was maximum at 1 h. Although adherence of monocytes and macrophages to collagen matrices concomitantly enhanced the production of superoxide anion, only the fungicidal activity of collagen-adherent monocytes was partially blocked by superoxide dismutase and catalase. Remarkably, we found that only 10% of the phagosomes in C. albicans-infected macrophages that adhered to plastic fused with lysosomes. In contrast, 80% of yeast-containing phagosomes of collagen-adherent macrophages fused with lysosomes. These data suggest that nonoxidative mechanisms are critical for human macrophage anti-Candida activity and that C. albicans pathogenicity is mediated, in part, by its ability to inhibit phagolysosomal fusion in macrophages.

Granulocytes govern the transcriptional response, morphology and proliferation of Candida albicans in human blood

Survival in blood and escape from blood vessels into tissues are essential steps for the yeast Candida albicans to cause systemic infections. To elucidate the influence of blood components on fungal growth, morphology and transcript profile during bloodstream infections, we exposed C. albicans to blood, blood fractions enriched in erythrocytes, polymorphonuclear or mononuclear leukocytes, blood depleted of neutrophils and plasma. C. albicans cells exposed to erythrocytes, mononuclear cells, plasma or blood lacking neutrophils were physiologically active and rapidly switched to filamentous growth. In contrast, the presence of neutrophils arrested C. albicans growth, enhanced the fungal response to overcome nitrogen and carbohydrate starvation, and induced the expression of a large number of genes involved in the oxidative stress response. In particular, SOD5, encoding a glycosylphosphatidylinositol (GPI)-anchored superoxide dismutase localized on the cell surface of C. albicans, was strongly expressed in yeast cells that were associated with neutrophils. Mutants lacking key genes involved in oxidative stress, morphology or virulence had significantly reduced survival rates in blood and the neutrophil fraction, but remained viable for at least 1 h of incubation when exposed to erythrocytes, mononuclear cells, plasma or blood lacking neutrophils. These data suggest that C. albicans genes expressed in blood were predominantly induced in response to neutrophils, and that neutrophils play a key role during C. albicans bloodstream infections. However, C. albicans is equipped with several genes and transcriptional programmes, which may help the fungus to counteract the attack of neutrophils, to escape from the bloodstream and to cause systemic infections.

Influence of endogenous pro-inflammatory cytokines on neutrophil-mediated damage ofCandida albicanspseudohyphae, quantified in a modified tetrazolium dye assay

For quantitative assessment of polymorphonuclear granulocyte (PMN)-mediated pseudohyphal damage, an improved tetrazolium (2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide; XTT) dye assay was developed. The modified assay proved to be a reliable indicator of viable pseudohyphal inoculum sizes. In addition, the influence of various endogenous pro-inflammatory cytokines on the capacity of PMN to damage Candida albicans pseudohyphae was investigated. PMN obtained from mice in which the genes encoding for tumor necrosis factor-alpha/lymphotoxin-alpha (TNF/LT), interferon-gamma (IFNgamma), interleukin (IL)-1alpha, or IL-1beta were disrupted, showed a significantly reduced pseudohyphal damage capacity in comparison with control PMN. The reduction amounted 25% for TNF-/- LT-/-, 11% for IFNgamma-/-, 21% for IL-1alpha-/-, and 34% for IL-1alpha-/-beta-/- PMN. In contrast, deficiency of IL-12 or IL-18 did not result in a diminished capacity to damage pseudohyphae and the capacity of PMN to damage Candida pseudohyphae was even slightly increased by 10% in IL-18-/- mice. These data suggest that endogenous pro-inflammatory cytokines are able to modulate antihyphal activity of PMN, the main effector cells against disseminated candidiasis by virtue of their capacity to kill both Candida blastoconidia and pseudohyphae.

Gene expression in HL60 granulocytoids and human polymorphonuclear leukocytes exposed to Candida albicans

Candida albicans is an opportunistic human pathogen causing both superficial and disseminated diseases. It is a dimorphic fungus, switching between yeast and hyphal forms, depending on cues from its microenvironment. Hyphae play an important role in the pathogenesis of candidiasis. The host's response to Candida infection is multifaceted and includes the participation of granulocytes as key effector cells. The aim of this investigation was to study host gene expression during granulocyte-Candida interaction. Effector cells were generated by the granulocytic differentiation of HL60 cells. The resulting cell population was shown to be morphologically and functionally equivalent to granulocytes and is therefore referred to as HL60 granulocytoids for the purposes of this study. Gene expression profiles were determined 1 h after hosts were infected with C. albicans. Three Candida-granulocytoid ratios were chosen to reflect different degrees of HL60 granulocytoid inhibition of C. albicans. The data demonstrate that at the high pathogen-host ratio, C. albicans modulated the HL60 granulocytoid's response by downregulating the expression of known antimicrobial genes. In addition, looking at the expression of a large number of genes, not all of which have necessarily been implicated in candidastatic or candidacidal mechanisms, it has been possible to describe the physiological response of the HL60 granulocytoid to an infectious challenge with C. albicans. Finally, some of the observed changes in HL60 granulocytoid gene expression were investigated in freshly isolated human polymorphonuclear leukocytes infected with C. albicans. Similar changes were seen in these primary human cells, lending support to the validity of this model.

The interaction of human dendritic cells with yeast and germ-tube forms ofCandida albicansleads to efficient fungal processing, dendritic cell maturation, and acquisition of a Th1 response-promoting function

T helper cell type 1 (Th1) cell-mediated immunity plays a critical role in protection against the opportunistic pathogen Candida albicans. Virulence of the fungus is closely associated with its ability to form germ-tubes (GT), the early phase of the dimorphic transition from the commensal yeast (Y) to the more invasive hyphal (H) form. In this study, we examined the functional outcome of the interaction of Y or GT forms with human dendritic cells (DCs), professional antigen-presenting cells, which are pivotal for initiation and modulation of T cell responses. DCs phagocytosed and killed Y and GT cells with a comparable efficiency, becoming able to trigger strong proliferative responses by Candida-specific, autologous T cell clones. Both fungal forms induced DC maturation, as indicated by up-regulation of CD83, CD80, CD86, CD40, and major histocompatibility complex classes I and II surface antigens. Chemokine receptors were also modulated in Candida-DCs, which showed increased CCR7/CXCR4 and decreased CCR5 expression. Y- and GT-activated DCs differed in the pattern of cytokine expression. In particular, GT cells, in common with fully differentiated H cells, induced significantly more elevated levels of interleukin (IL)-10 than Y cells. Nevertheless, Y-, GT-, or H-pulsed DCs secreted comparable amounts of IL-12p70. In addition, irrespective of the fungal form triggering DC activation, Candida-DCs acquired the ability to prime naive T lymphocytes with a defined Th1 phenotype. Overall, our findings highlight the induction of substantially similar functional patterns in human DCs encountering the different forms of growth of C. albicans, both seemingly activating the Th1-type immunity which is characteristic of the healthy human subjects, naturally immunized and protected against the fungus.

Stage-specific gene expression of Candida albicans in human blood

The pathogenic fungus Candida albicans commonly causes mucosal surface infections. In immunocompromised patients, C. albicans may penetrate into deeper tissue, enter the bloodstream and disseminate within the host causing life-threatening systemic infections. In order to elucidate how C. albicans responds to the challenge of a blood environment, we analysed the transcription profile of C. albicans cells exposed to human blood using genomic arrays and a cDNA subtraction protocol. By combining data obtained with these two methods, we were able to identify unique sets of different fungal genes specifically expressed at different stages of this model that mimics bloodstream infections. By removing host cells and incubation in plasma, we were also able to identify several genes in which the expression level was significantly influenced by the presence of these cells. Differentially expressed genes included those that are involved in the general stress response, antioxidative response, glyoxylate cycle as well as putative virulence attributes. These data point to possible mechanisms by which C. albicans ensures survival in the hostile environment of the blood and how the fungus may escape the bloodstream as an essential step in its systemic dissemination.

Impaired phagocyte respiratory burst responses to opportunistic fungal pathogens in transplant recipients: in vitro effect of r-metHuG-CSF (Filgrastim)

Phagocyte respiratory burst capacity in response to pathogenic fungi and the in vitro effects of granulocyte colony-stimulating factor (G-CSF) were examined in 15 normal volunteers and 39 transplant recipients (4 autologous and 4 allogeneic bone marrow, 3 heart, 10 liver, 8 lung, and 10 kidney). Chemiluminescence was measured for reaction mixtures of diluted whole blood, opsonized fungi, and luminol, with and without in vitro incubation with r-metHuG-CSF (Filgrastim). Transplant patients exhibited significantly impaired respiratory burst responses to conidia and yeast compared with controls, but this was reversed with Filgrastim. Responses to hyphae were low for both groups, and G-CSF had little or no effect. There was excellent correlation between responses to fungi and responses to opsonized zymosan. In vitro respiratory burst capacity is impaired in transplant recipients. This may predict susceptibility to invasive fungal infections. G-CSF can reverse impaired phagocyte function and is of potential benefit in the prevention and/or treatment of fungal infection in transplant patients.

Candida albicans is phagocytosed, killed, and processed for antigen presentation by human dendritic cells

Candida albicans is a component of the normal flora of the alimentary tract and also is found on the mucocutaneous membranes of the healthy host. Candida is the leading cause of invasive fungal disease in premature infants, diabetics, and surgical patients, and of oropharyngeal disease in AIDS patients. As the induction of cell-mediated immunity to Candida is of critical importance in host defense, we sought to determine whether human dendritic cells (DC) could phagocytose and degrade Candida and subsequently present Candida antigens to T cells. Immature DC obtained by culture of human monocytes in the presence of granulocyte-macrophage colony-stimulating factor and interleukin-4 phagocytosed unopsonized Candida in a time-dependent manner, and phagocytosis was not enhanced by opsonization of Candida in serum. Like macrophages (Mphi), DC recognized Candida by the mannose-fucose receptor. Upon ingestion, DC killed Candida as efficiently as human Mphi, and fungicidal activity was not enhanced by the presence of fresh serum. Although phagocytosis of Candida by DC stimulated the production of superoxide anion, inhibitors of the respiratory burst (or NO production) did not inhibit killing of Candida, even when phagocytosis was blocked by preincubation of DC with cytochalasin D. Further, although apparently only modest phagolysosomal fusion occurred upon DC phagocytosis of Candida, killing of Candida under anaerobic conditions was almost equivalent to killing under aerobic conditions. Finally, DC stimulated Candida-specific lymphocyte proliferation in a concentration-dependent manner after phagocytosis of both viable and heat-killed Candida cells. These data suggest that, in vivo, such interactions between DC and C. albicans may facilitate the induction of cell-mediated immunity.

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.

Caspase-3 Inhibition Partially Protects Oxidant Production in Apoptotic Human Neutrophils

Apoptotic PMN lose functional activity, which emphasizes the tissue injury limiting potential of PMN apoptosis. Caspase-3 activation is the first step in the execution phase of apoptosis. We hypothesized that PMN functional activity, as evidenced by oxidant production, can be restored in apoptotic PMN by inhibition of caspase-3.

METHODS

To accelerate PMN apoptosis, PMN were UV-irradiated for 15 min as previously described. PMN were pretreated with the caspase-3 inhibitor DEVD-fmk (100 microM) for 30 min prior to UV. PMN apoptosis was quantitated by flow cytometry with CD16 staining. Oxidant production in response to 10 microM PMA was quantitated fluorometrically using the method of Hyslop and Sklar. Caspase-3 activity was quantitated fluorometrically using a commercially available assay.

RESULTS

UV-treated PMN demonstrated a 3-fold increase in caspase-3 activity. This was associated with a significant increase in apoptotic PMN and a 10-fold decrease in oxidant production compared to control PMN. DEVD-fmk blocked increases in caspase-3 activity and significantly reduced PMN apoptosis. Oxidant production was increased 5-fold compared to UV-treated PMN but was still significantly less than control PMN.

CONCLUSIONS

In UV-accelerated PMN apoptosis, inhibition of caspase-3 activity partially protects oxidant production in apoptotic PMN. This suggests that signaling events in the initiation phase of PMN apoptosis, which are proximal to caspase-3 activation, may in part be responsible for loss of oxidant production in apoptotic PMN independent of caspase-3 activity.

Antioxidant systems in the pathogenic fungi of man and their role in virulence

In the last two decades, a variety of fungal antioxidants have attracted considerable interest, largely arising from their hypothetical role as virulence determinants. Melanin is a potent free radical scavenger and in Cryptococcus neoformans, there is now good evidence that the production of melanin is a significant virulence determinant. There is also recent evidence linking melanin biosynthesis to the virulence of Aspergillus fumigatus conidia. Superoxide dismutases are important housekeeping antioxidants and have an additional hypothetical role in virulence; however, although these enzymes have been biochemically characterized from Aspergillus and Cryptococcus, there is as yet no firm evidence that these enzymes are involved in pathogenicity. Catalase production may play some role in the virulence of Candida albicans but this enzyme has not been shown, as yet, to influence the virulence of A. fumigatus. There are some data supporting an antioxidant function for the acyclic hexitol mannitol in C. neoformans, but further investigations are required in this area. Research into the putative antioxidant activities of a range of other fungal enzymes, such as acid phosphatases, remains limited at this time.

Severe impairment in early host defense against Candida albicans in mice deficient in myeloperoxidase

Myeloperoxidase (MPO) catalyzes the reaction of hydrogen peroxide with chloride ion to produce hypochlorous acid (HOCl), which is used for microbial killing by phagocytic cells. Despite the important role of MPO in host defense, however, MPO deficiency is relatively common in humans, and most of these individuals are in good health. To define the in vivo role of MPO, we have generated by gene targeting mice having no MPO activity in their neutrophils and monocytes. The mice without MPO developed normally, were fertile, and showed normal clearance of intraperitoneal Staphylococcus aureus. However, they showed increased susceptibility to pneumonia and death following intratracheal infection with Candida albicans. Furthermore, the lack of MPO significantly enhanced the dissemination of intraperitoneally injected C. albicans into various organs during the first 7 days. Thus, MPO is important for early host defense against fungal infection, and the inability to generate HOCl cannot be compensated for by other oxygen-dependent systems in vivo in mice. The mutant mice serve as a model for studying pulmonary and systemic candidiasis.

Cloning and sequencing of a Candida albicans catalase gene and effects of disruption of this gene

Catalase plays a key role as an antioxidant, protecting aerobic organisms from the toxic effects of hydrogen peroxide, and in some cases has been postulated to be a virulence factor. To help elucidate the function of catalase in Candida albicans, a single C. albicans-derived catalase gene, designated CAT1, was isolated and cloned. Degenerate PCR primers based on highly conserved areas of other fungal catalase genes were used to amplify a 411-bp product from genomic DNA of C. albicans ATCC 10261. By using this product as a probe, catalase clones were isolated from genomic libraries of C. albicans. Nucleotide sequence analysis revealed an open reading frame encoding a protein of 487 amino acid residues. Construction of a CAT1-deficient mutant was achieved by using the Ura-blaster technique for sequential disruption of multiple alleles by integrative transformation using URA3 as a selectable marker. Resulting mutants exhibited normal morphology and comparable growth rates of both yeast and mycelial forms. Enzymatic analysis revealed an abundance of catalase in the wild-type strain but decreasing catalase activity in heterozygous mutants and no detectable catalase in a homozygous null mutant. In vitro assays showed the mutant strains to be more sensitive to damage by both neutrophils and concentrations of exogenous peroxide that were sublethal for the parental strain. Compared to the parental strain, the homozygous null mutant strain was far less virulent for mice in an intravenous infection model of disseminated candidiasis. Definitive linkage of CAT1 with virulence would require restoration of activity by reintroduction of the gene into mutants. However, initial results in mice, taken together with the enhanced susceptibility of catalase-deficient hyphae to damage by human neutrophils, suggest that catalase may enhance the pathogenicity of C. albicans.

Intact autocrine activation and cytokine production by PMNs from injured adults with elevated Candida antigen titres

Injured patients with Candida antigen titres have increased mortality due to sepsis. Polymorphonuclear leucocytes (PMNs) from injured patients with elevated Candida antigen titres demonstrate impaired function against Candida albicans growth when compared with PMNs from injury matched controls. To determine if PMN dysfunction is global, PMNs from patients with positive Candida antigen titres were evaluated for their ability to activate the anticandidal function of normal PMNs (autocrine activation) and to produce tumour necrosis factor (TNF) and interleukin 8 (IL8), known activators of PMN anticandidal function, this study demonstrates that the PMN dysfunction is not global, as PMN cytokine production and autocrine activation remain intact.

Lipopolysaccharide protects polymorphonuclear leukocytes from apoptosis via tyrosine phosphorylation-dependent signal transduction pathways

BACKGROUND

The present study was undertaken to determine if tyrosine phosphorylation signal transduction pathways, which are known to be activated in polymorphonuclear leukocytes (PMN) by lipopolysaccharide (LPS), play a role in priming of PMN oxidative burst and protection of PMN from apoptosis by LPS, and to determine if an interface between these two signaling pathways exists.

METHODS

PMN were combined with or without 10-fold serial dilutions (0.1 ng-1 microgram/ml) of LPS and incubated at 37 degrees C/5% CO2. After 24 h PMN apoptosis was assessed using fluorescence microscopy and DNA agarose gel electrophoresis. Additional PMN were pretreated with the tyrosine kinase inhibitors genistein and herbamycin A before addition of LPS. Tyrosine phosphorylation was detected by immunoblotting. Oxidant production was quantitated by following the oxidation of a chromophore to its fluorescent product.

RESULTS

LPS delayed the onset of apoptosis and prolonged the survival of PMN in a dose-dependent fashion. Both tyrosine kinase inhibitors blocked the protective effect of LPS on PMN apoptosis; however, only genistein blocked the priming effect of LPS on PMN oxidative burst.

CONCLUSIONS

Tyrosine phosphorylation signal transduction pathways are central to protection of PMN from apoptosis by LPS. Although tyrosine phosphorylation pathways also play a role in priming of the oxidative burst in PMN, our data suggest that there is not an interface between these important signaling pathways.

Redundant contribution of myeloperoxidase-dependent systems to neutrophil-mediated killing of Escherichia coli

Neutrophil microbicidal activity is a consequence of overlapping antimicrobial systems that vary in prominence according to the conditions of the neutrophil-microbe interaction, the nature of the microbe, and its metabolic state. In this study, normal, myeloperoxidase-deficient, and respiratory burst-deficient (chronic granulomatous disease [CGD]) neutrophils killed Escherichia coli with equivalent, high efficiencies. Killing by CGD and myeloperoxidase-deficient neutrophils was not augmented by supplements, such as exogenous H2O2 and myeloperoxidase, directed at ameliorating their metabolic defects, suggesting that nonoxidative microbicidal systems were sufficient for a full microbicidal effect. Neutrophils with an intact myeloperoxidase antimicrobial system (normal or appropriately supplemented deficient cells) were capable of rapidly suppressing E. coli DNA synthesis, while unsupplemented CGD or myeloperoxidase-deficient cells were far less effective, indicating that the myeloperoxidase system was active in normal neutrophils. The degree of DNA synthesis inhibition by myeloperoxidase-sufficient neutrophils could account, in a cell-free system, for most of the observed microbicidal activity. While the myeloperoxidase system was active and probably bactericidal, it was not rate limiting for microbicidal activity and appears to have been redundant with other microbicidal systems in the cell. Rapid and extensive inhibition of bacterial DNA synthesis appears to be an indicator of myeloperoxidase activity in neutrophils.

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.

Scavenging of hypochlorous acid by carvedilol and ebselen in vitro

1. The antihypertensive drug carvedilol and the antiinflammatory selenoorganic compound ebselen were tested for their ability to react with the reactive oxygen species hypochlorous acid (HOCl) in vitro. 2. Carvedilol scavenges HOCl at a rate sufficient to protect a model molecule catalase against inactivation by HOCl. 3. Ebselen was resistant to HOCl when its glutathione-peroxidase mimetic property was compared with that of glutathione peroxidase.

The involvement of nitric oxide in the anti-Candida albicans activity of rat neutrophils

Rat peritoneal neutrophils (PMN) spontaneously release nitric oxide (NO) when incubated in vitro. Addition of the NO synthase inhibitor L-monomethylarginine (L-NMMA) to the PMN reduces NO production and impairs the killing of the yeast Candida albicans, both effects being reversed by L-arginine. These data strongly suggest that oxidative metabolism of L-arginine by PMN is involved in the candidacidal activity of these cells. Rat blood PMN, which do not produce significant amounts of NO, exhibit a reduced killing capacity compared with peritoneal cells, except when they are obtained from lipopolysaccharide (LPS)-treated rats. In this case they produce measurable amounts of nitrite and express high fungicidal activity in vitro. Confirming the candidacidal activity of NO, the exposure of the C. albicans cultures to different concentrations of NO donors leads to a reduction in their survival. The candidacidal activity related to the NO pathway in rat PMN is phagocytosis dependent, since the activity can be inhibited by cytochalasin B. However, the oxidative products of oxygen released by rat PMN do not seem to be involved in their candidacidal activity, as incubation of the cells with phorbol myristate acetate (PMA) increases release of superoxide anion but does not affect the pattern of killing. Our results suggest that NO could be an important candidacidal pathway in rat neutrophils.

Fungal infection in surgical patients

Invasive fungal infections have become a major source of morbidity and mortality in the modern surgical intensive care unit. Patients at risk for invasion and dissemination are common, and are not as ill as thought previously. Severity of illness (APACHE II score > 10, ventilator use for >48 hours), antibiotics, central venous lines, total parenteral nutrition, burns, and immunosuppression are the most common risk factors. Recognition of these risk factors should arouse a high index of suspicion for the diagnosis of invasion or dissemination. Unfortunately, laboratory tests alone lack sensitivity and specificity. Therefore, the diagnosis of invasion and dissemination in the majority of cases requires the acquisition and proper interpretation of clinical evidence. Once the diagnosis is made, early systemic treatment is warranted. Reported toxicity and efficacy supports the use of fluconazole for most patients with invasive fungal infections. However, for the most critically ill patients amphotericin B remains the treatment of choice.

Synergy of fluconazole with macrophages for antifungal activity against Candida albicans

The possible between macrophages and fluconazole for antifungal activity against different isolates of C. albicans was studied. The susceptibility of C. albicans isolates to fluconazole (FCZ), when incubated in RPMI-1640 with 10% fetal bovine serum (FBS) and 10% fresh mouse serum (test medium, TM) was determined by using a quantitative culture methodology, Multiplication of isolate Sh27 was strongly inhibited by FCZ, even at 1.0 microgram/ml. However, FCZ even at 100 microgram/ml was not fungicidal. Resident murine peritoneal macrophages (MP) incubated for 48 h in RPMI-1640 + 10% FBS (tissue culture medium, TCM), then challenged with Sh27 in TM for 24 h, were fungistatic (20 +/ 9%, n = 4). Cultured macrophages synergized with FCZ (10 micrograms/ml) for fungicidal activity when co-cultured with sh27 in TM for 24 h (46 +/ 8%) and for 48 h (74 +/ 5%), n = 3. Macrophages and FCZ (10 micrograms/ml) could not synergize for significant killing of a less FCZ-sensitive C. albicans isolate 94-164. Multiplication of a FCZ-resistant isolate (94-20) was not inhibited by FCZ at 10 micrograms/ml TM; however, macrophages and FCZ (10 micrograms/ml) could synergize for fungistatic (64%), but not fungicidal, activity.

Comparative analysis of phagocytosis of fungal cells by insect hemocytes versus horse neutrophils

In this study, the phagocytic ability of Spodoptera exigua hemocytes was compared to horse neutrophils. In vitro assays showed that the insect granulocytes and horse neutrophils actively phagocytose FITC-labeled Paecilomyces farinosus blastospores opsonized with S. exigua hemolymph lectin or horse serum, respectively. Killing of fungal cells by the neutrophils and hemocytes was analyzed under in vitro conditions. Neutrophils reduced the growth of P. farinosus up to 65% whereas no fungicidal activity was observed with hemocyte monolayers. The production of oxygen metabolites by both phagocytic cells incubated with various elicitors (fungal cells, bacteria, phorbol myristate acetate) was examined using luminol-enhanced chemiluminescence. Phagocytosis of opsonized microbes by horse neutrophils resulted in marked increase of chemiluminescence activity whereas no chemiluminescence was detected in similarly challenged phagocytic insect hemocytes. Electron microscopy was used to examine phagocytic events and confirmed that insect phagocytes were unable to kill tested microbes.

Myeloperoxidase deficiency: an epidemiological study and flow-cytometric detection of other granular enzymes in myeloperoxidase-deficient subjects

MPO deficiency, as first studied in the 1960s, has been recorded with increasing frequency, following the introduction of the automated cytochemical count into clinical routine. However, with regard to the diseases correlated to MPO deficiency, no exact data on the frequency of co-existence have been recorded. Moreover, the question remains whether or not a further deficiency of other granular enzymes co-exists, especially with regard to acquired MPO deficiency. In order to answer these questions, an epidemiological study of more than 70,000 unselected patients was performed; the resulting prevalence of MPO deficiency was 0.15%. Within this patient group the intercellular content of elastase-like protease (ELP) and lactoferrin was measured semiquantitatively in a flow cytometer by means of indirect immunofluorescence staining. The frequency of coinciding diseases did not differ from the frequency of diseases in the hospital patients in general. The flow-cytometric studies revealed a normal content of ELP and lactoferrin in one group and a reduced content in another, suggesting the inherited form in the former and acquired MPO deficiencies in the latter group and thus indicating that differing mechanisms characterize the two forms of MPO deficiency. Nevertheless, we do not suggest distinguishing between acquired and inherited deficiencies solely with this technique. Instead, molecular-biologic and/or genetic methods should be referred to.

Inactivation of Escherichia coli penicillin-binding proteins by human neutrophils

Neutrophils use a variety of microbicidal mechanisms in their role as one of the primary arms of the human host defense system. We have previously observed that a cell-free system containing myeloperoxidase (MPO), one of the major components of the neutrophil's oxidative antimicrobial systems, inactivated microbial penicillin-binding proteins (PBPs), which mediate the formation of the peptidoglycan layer of eubacterial cell walls. This is a potentially important mechanism of MPO-mediated bacterial toxicity. Since numerous other microbicidal systems, both oxidative and nonoxidative, are used by whole neutrophils, we investigated the effect of intact neutrophils on Escherichia coli PBPs. Penicillin binding activity was progressively reduced by neutrophil exposure for all PBPs. Loss of penicillin binding activity correlated well with loss of microbial viability for almost all PBPs. Azide-treated neutrophils, MPO-deficient neutrophils, and chronic granulomatous disease neutrophils inactivated E. coli PBPs in a manner similar to that of normal neutrophils, suggesting that MPO-independent, and even oxygen-independent, microbicidal systems are also involved in inactivation of PBPs. PBP inactivation, an antimicrobial strategy used by beta-lactam-producing molds (and now by physicians), may be an important microbicidal mechanism used by human neutrophils.