Immune recognition of fungal β-glucans

Saccharomyces cerevisiae as a vaccine against coccidioidomycosis

Disseminated coccidioidomycosis is a life-threatening infection. In these studies, we examined protection against systemic murine coccidioidomycosis by vaccination with heat-killed Saccharomyces cerevisiae (HKY). CD-1 mice received HKY subcutaneously or by oral gavage with or without adjuvants once weekly beginning 3 or 4 weeks prior to infection; oral live Saccharomyces was also studied. All HKY sc regimens were equivalent, prolonging survival (P<or=0.005) and reducing fungal burden versus controls. Oral live Saccharomyces, but not HKY, prolonged survival (P=0.03), but did not reduce fungal burden. Survival of mice given HKY was equivalent to vaccination with formalin-killed spherules, but inferior in reduction of fungal burden. HKY was superior to a successful recombinant vaccine, PRA plus adjuvant. This novel heterologous protection afforded by HKY vaccination offers a new approach to a vaccine against coccidioidomycosis.

Evolutionarily conserved recognition and innate immunity to fungal pathogens by the scavenger receptors SCARF1 and CD36

Receptors involved in innate immunity to fungal pathogens have not been fully elucidated. We show that the Caenorhabditis elegans receptors CED-1 and C03F11.3, and their mammalian orthologues, the scavenger receptors SCARF1 and CD36, mediate host defense against two prototypic fungal pathogens, Cryptococcus neoformans and Candida albicans. CED-1 and C03F11.1 mediated antimicrobial peptide production and were necessary for nematode survival after C. neoformans infection. SCARF1 and CD36 mediated cytokine production and were required for macrophage binding to C. neoformans, and control of the infection in mice. Binding of these pathogens to SCARF1 and CD36 was β-glucan dependent. Thus, CED-1/SCARF1 and C03F11.3/CD36 are β-glucan binding receptors and define an evolutionarily conserved pathway for the innate sensing of fungal pathogens.

Beta-glucan, immune function, and upper respiratory tract infections in athletes

PURPOSE

This study investigated the effects of oat beta-glucan (BG) supplementation on chronic resting immunity, exercise-induced changes in immune function, and self-reported upper respiratory tract infection (URTI) incidence in human endurance athletes.

METHODS

Trained male cyclists were randomized to BG (N = 19) or placebo (P; N = 17) groups and under double-blind procedures received BG (5.6 g x d(-1)) or P beverage supplements for 2 wk before, during, and 1 d after a 3-d period in which subjects cycled for 3 h x d(-1) at approximately 57% maximal watts. URTI symptoms were monitored during BG supplementation and for 2 wk afterward. Blood samples were collected before and after 2 wk of supplementation (both samples, 8:00 a.m.), immediately after the 3-h exercise bout on day 3 (6:00 p.m.), and 14 h after exercise (8:00 a.m.) and were assayed for natural killer cell activity (NKCA), polymorphonuclear respiratory burst activity (PMN-RBA), phytohemagglutinin-stimulated lymphocyte proliferation (PHA-LP), plasma interleukin 6 (IL-6), IL-10, IL-1 receptor agonist (IL-1ra), and IL-8, and blood leukocyte IL-10, IL-8, and IL-1ra mRNA expression.

RESULTS

Chronic resting levels and exercise-induced changes in NKCA, PMN-RBA, PHA-LP, plasma cytokines, and blood leukocyte cytokine mRNA did not differ significantly between BG and P groups. URTI incidence during the 2-wk postexercise period did not differ significantly between groups.

CONCLUSIONS

An 18-d period of BG versus P ingestion did not alter chronic resting or exercise-induced changes in immune function or URTI incidence in cyclists during the 2-wk period after an intensified exercise.

Anti-beta-glucan antibodies in healthy human subjects

Previous data by our group demonstrated the antifungal efficacy of a vaccine consisting of laminarin (beta-(1,3)-glucan), conjugated with diphtheria toxoid, which generated protective anti-laminarin antibodies in mice. In this paper, we sought for the presence, isotype and subclass composition of natural anti-laminarin antibodies in an unselected population of human healthy subjects, in a comparison with antibodies directed against beta-(1,6)-glucan (pustulan) and branched beta-(1,3/1,6)-glucan (Pool 1) and mannan from Candida albicans. Almost all subjects showed detectable levels of anti-beta-glucan antibodies, with IgG largely prevailing on IgM, little, if any, IgA and no IgE. However, the titer of anti-beta-glucan antibodies was overall about 1log lower than that of anti-mannan antibodies of the corresponding isotype. In particular, the level of anti-laminarin IgG was the lowest one, its geometrical mean titer (95% confidence interval, CI) being 1838 (1245-2714) as compared to 8157 (6067-10,931) and 3940 (2911-5332) for pustulan and Pool 1 fungal glucan, respectively. Analysis of IgG subclass composition showed that IgG2 was the prevalent subclass against any antigen, and again the concentration of anti-laminarin IgG2 was the lowest one, its geometrical mean concentration being 0.13 (0.07-0.24)microg/ml as compared to anti-pustulan and anti-Pool 1 glucan and mannan IgG2 levels, which were 0.33 (0.2-0.5), 1.35 (0.9-2.0), and 36.1 (25.2-51.3)microg/ml, respectively. These data show that anti-laminarin antibodies are present at low levels in humans as compared to other anti-beta-glucan and, mostly, anti-mannan antibodies, and suggest that a protective antifungal vaccination in humans should attempt to tip the balance of antifungal antibodies in favour of the anti-laminarin ones.

The role of the cell wall in fungal pathogenesis

Fungal infections are a serious health problem. In recent years, basic research is focusing on the identification of fungal virulence factors as promising targets for the development of novel antifungals. The wall, as the most external cellular component, plays a crucial role in the interaction with host cells mediating processes such as adhesion or phagocytosis that are essential during infection. Specific components of the cell wall (called PAMPs) interact with specific receptors in the immune cell (called PRRs), triggering responses whose molecular mechanisms are being elucidated. We review here the main structural carbohydrate components of the fungal wall (glucan, mannan and chitin), how their biogenesis takes place in fungi and the specific receptors that they interact with. Different model fungal pathogens are chosen to illustrate the functional consequences of this interaction. Finally, the identification of the key components will have important consequences in the future and will allow better approaches to treat fungal infections.

Candida albicans cell surface superoxide dismutases degrade host-derived reactive oxygen species to escape innate immune surveillance

Mammalian innate immune cells produce reactive oxygen species (ROS) in the oxidative burst reaction to destroy invading microbial pathogens. Using quantitative real-time ROS assays, we show here that both yeast and filamentous forms of the opportunistic human fungal pathogen Candida albicans trigger ROS production in primary innate immune cells such as macrophages and dendritic cells. Through a reverse genetic approach, we demonstrate that coculture of macrophages or myeloid dendritic cells with C. albicans cells lacking the superoxide dismutase (SOD) Sod5 leads to massive extracellular ROS accumulation in vitro. ROS accumulation was further increased in coculture with fungal cells devoid of both Sod4 and Sod5. Survival experiments show that C. albicans mutants lacking Sod5 and Sod4 exhibit a severe loss of viability in the presence of macrophages in vitro. The reduced viability of sod5Δ/Δ and sod4Δ/Δsod5Δ/Δ mutants relative to wild type is not evident with macrophages from gp91phox^−/− mice defective in the oxidative burst activity, demonstrating a ROS-dependent killing activity of macrophages targeting fungal pathogens. These data show a physiological role for cell surface SODs in detoxifying ROS, and suggest a mechanism whereby C. albicans, and perhaps many other microbial pathogens, can evade host immune surveillance in vivo.

Caspofungin-mediated beta-glucan unmasking and enhancement of human polymorphonuclear neutrophil activity against Aspergillus and non-Aspergillus hyphae

BACKGROUND

We investigated whether caspofungin and other echinocandins have immune-enhancing properties that influence human polymorphonuclear neutrophil (PMN)-mediated mold hyphal damage.

MATERIALS AND METHODS

Using aniline blue staining, we compared patterns of beta-glucan exposure in Aspergillus fumigatus, Aspergillus terreus, Rhizopus oryzae, Fusarium solani, Fusarium oxysporum, Scedosporium prolificans, and Scedosporium apiospermum hyphae after caspofungin exposure. We also determined PMN-mediated hyphal damage occurring with or without preexposure to caspofungin or with preexposure to the combination of caspofungin and anti-beta-glucan monoclonal antibody, using 2,3-bis (2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino) carbonyl]-sH-tetrazolium hydroxide (XTT) assay.

RESULTS

Preincubation with caspofungin (32 microg/mL for R. oryzae; 0.0625 microg/mL for other isolates) increased exposure to beta-glucan. PMN-induced damage increased after caspofungin exposure and was further augmented by the addition of anti-beta-glucan antibody. Preincubation with micafungin or anidulafungin had similar effects on PMN-induced damage of A. fumigatus hyphae. Finally, preexposure of A. fumigatus, but not S. prolificans, to caspofungin induced expression of Dectin-1 by PMN.

CONCLUSIONS

The results of the present study suggest inducement of beta-glucan unmasking by echinocandins and enhancement of PMN activity against mold hyphae, thereby supporting the immunopharmacologic mode of action of echinocandins.

Heterologous expression of a position 2-substituted (1-->3)-beta-D-glucan in Lactococcus lactis

Exopolysaccharides play an important role in the rheology and texture of fermented foods, and among these beta-glucans have immunomodulating properties. We show that the overproduction of the Pediococcus parvulus GTF glycosyltransferase in an uncapsulated Lactococcus lactis strain results in synthesis and secretion (300 mg liter(-1)) of a position 2-substituted (1-->3)-beta-D-glucan that has potential use as a food additive.

Higher order structure of (1,3)-beta-D-glucans and its influence on their biological activities and complexation abilities

(1,3)-beta-D-Glucans form a group of biologically active biopolymers that exist in different structural organizations depending on the environmental conditions. The biological effect of (1,3)-beta-D-glucans is a core issue stimulating large research efforts of the molecular properties and their consequences for action as biological response modifiers. The fascination for these molecules increased further following the finding of their ability to form complexes of defined geometry with a number of structures, ranging from linear architectures as polymers or carbon nanotubes, to globular structures as gold particles or dye molecules. The fascinating information concerning the relationship between sample treatment history and molecular organization has not yet reached out to all the contributors within the field, resulting in unnecessary apparent inconsistencies in the literature. In addition to environmental conditions, the sample history is known to influence on the precise structural organization of these molecules. The present knowledge related to the structure of native as well as denatured, renatured and annealed (1,3)-beta-D-glucans is reviewed. The influence of their structural organization on the biological activity and complexation abilities is discussed, and some factors hindering progress in the understanding of their biological effects or complexation abilities are pointed out.

Evolution of polydom-like molecules: identification and characterization of cnidarian polydom (Cnpolydom) in the basal metazoan Hydractinia

End sequencing of random BAC clones from a Hydractinia symbiolongicarpus (Cnidaria: Hydrozoa) genomic library revealed a gene across a approximately 37.5kb region of the H. symbiolongicarpus genome sharing highest sequence identity and domain architecture to mammalian polydom that we in turn named cnidarian polydom (CnPolydom). Sharing all eight domain types characteristic of polydom and organized in a similar 5'-3' manner, CnPolydom was predicted to contain three additional domain types: PAN, FA58C, and CUB that are characteristic of CnPolydom. Expression analysis of CnPolydom from H. symbiolongicarpus (Hysy-CnPolydom) showed upregulation in response to bacterial and primarily fungal challenges, with transcripts produced specifically by a subset of interstitial stem cells (i-cells) and/or neural cells throughout the ectodermal tissue layer of feeding polyps (gastrozooids). This is the first description of a polydom-like molecule outside of Mammalia and provides evolutionary perspective on the ancestral structure and role of this pentraxin family clade.

The biological role of dectin-1 in immune response

The identification of antigen-recognizing receptors in the innate immune system significantly helped us to understand the initiation and regulation of the innate immune response. Dectin-1 was recently identified as the most important receptor for beta-glucan. Recognition of beta-glucan by dectin-1 triggers effective immune response, including phagocytosis and proinflammatory factor production, to eliminate infecting fungi, which especially benefits immunocompromised patients against opportunistic fungal infection. In addition, dectin-1 is involved in the adaptive immune response as well as autoimmune diseases and immune tolerance. In this paper, we will mainly focus on the characteristics of dectin-1 and its biological role in immune response.

Potentiation of Toll-like receptor-induced cytokine production by (1-->3)-beta-D-glucans: implications for the monocyte activation test

The monocyte activation test (MAT) has been introduced as an alternative for the detection of pyrogens in pharmaceuticals with the rabbit pyrogen test or the Limulus amebocyte lysate (LAL) test. The basis of the MAT is that pyrogens, via Toll-like receptors (TLRs) expressed on monocytes, stimulate cytokine production. Here, we report that, at concentrations that did not induce whole blood cytokine production when tested separately, (1-->3)-beta-D-glucans powerfully co-stimulated cytokine production (IL-6/IL-8) induced by ligands for TLR1/2, TLR2/6, TLR4, and TLR5. Experiments were performed to investigate the involvement of particular (1-->3)-beta-D-glucan receptors such as dectin-1. Spleen tyrosine kinase (Syk) inhibition attenuated the potentiating effects of (1-->3)-beta-D-glucans on TLR-induced cytokine production, suggesting that dectin-1 was involved. However, experiments with low molecular (1-->3)-beta-D-glucans such as laminarin argued against the involvement of dectin-1 in the co-stimulatory effects of (1-->3)-beta-D-glucans. Thus, although the receptors involved in the co-stimulatory actions of (1-->3)-beta-D-glucans on TLR-induced cytokine production are yet to be elucidated, it is clear that (1-->3)-beta-D-glucans may greatly affect MAT results and, when undetected in pharmaceuticals, may give rise to serious side-effects in patients co-exposed to other elicitors of innate immunity, such as during infections.

The lung responds to zymosan in a unique manner independent of toll-like receptors, complement, and dectin-1

In vitro studies indicate that the inflammatory response to zymosan, a fungal wall preparation, is dependent on Toll-like receptor (TLR) 2, and that this response is enhanced by the dectin-1 receptor. Complement may also play an important role in this inflammatory response. However, the relevance of these molecules within the in vivo pulmonary environment remains unknown. To examine pulmonary in vivo inflammatory responses of the lung to zymosan, zymosan was administered by intratracheal aerosolization to C57BL/6, TLR2- TLR4-, MyD88-, and complement-deficient mice. Outcomes included bronchoalveolar fluid cell counts. We next examined effects of dectin-1 inhibition on response to zymosan in alveolar macrophages in vitro and in lungs of C57BL/6, TLR2-, and complement-deficient mice. Finally, the effect of alveolar macrophage depletion on in vivo pulmonary responses was assessed. Marked zymosan-induced neutrophil responses were unaltered in TLR2-deficient mice despite a TLR2-dependent response seen with synthetic TLR2 agonists. TLR4, MyD88, and complement activation were not required for the inflammatory response to zymosan. Although dectin-1 receptor inhibition blocked the inflammatory response of alveolar macrophages to zymosan in vitro, in vivo pulmonary leukocyte recruitment was not altered even in the absence of TLR2 or complement. Depletion of alveolar macrophages did not affect the response to zymosan. Neither complement, macrophages, nor TLR2, TLR4, MyD88, and/or dectin-1 receptors were involved in the pulmonary in vivo inflammatory response to zymosan.

beta-Glucan of Candida albicans cell wall causes the subversion of human monocyte differentiation into dendritic cells

The functional consequences of treating human monocytes with purified and chemically characterized Candida albicans beta-glucan -- a major microbial pathogen associated molecular pattern -- on their differentiation into dendritic cells (DC) were investigated. We show here that beta-glucan-treated monocytes differentiated into mature DC (Glu-MoDC) with altered phenotype and functional behavior, similarly to DC derived from C. albicans germ-tubes-infected monocytes (Gt-MoDC). They failed to express CD1a and to up-regulate CD80 and DR molecules. Moreover, they produced IL-10 but not IL-12 and primed naive T cells without inducing their functional polarization into effector cells. Since C. albicans beta-glucan is a mixture of both beta-(1,3) and beta-(1,6) glucan, we investigated their relative contribution by the use of non-Candida beta-glucan structural analogs. We found that high molecular weight (MW) glucans beta-(1,6) pustulan and beta-(1,3) curdlan totally mimicked the effect of C. albicans beta-glucan, while the low MW beta-(1,3) glucan laminarin did not have any effect. Because beta-glucan is a common constituent of all fungi and is abundantly released in vivo during systemic fungal infection, this novel effect of beta-glucan has potential implications for host-parasite relationship in candidiasis and other mycoses. In particular, our data suggest that beta-glucan could bias noninfected, bystander monocytes, thus aggravating the general immunodeficiency, predisposing to systemic fungal infection.

Dietary supplementation with laminarin, a fermentable marine beta (1-3) glucan, protects against hepatotoxicity induced by LPS in rat by modulating immune response in the hepatic tissue

We tested the hypothesis that laminarin (LAM), a beta (1-3) polysaccharide extracted from brown algae, can modulate the response to a systemic inflammation. Male Wistar rats (n=7 per group) were fed a standard diet (control) or a diet supplemented with LAM for 25 days (5% during 4 days followed by 10% during 21 days). Thereafter, Escherichia coli lipopolysaccharides (LPS; 10 mg/kg i.p.) were injected and the animals were sacrificed 24 h after LPS challenge. The hypothermia, hyperglycemia and hypertriglyceridemia occurring early after LPS administration were less pronounced in LAM-treated rats than in controls. The increase in serum alanine aminotransferase (ALT), aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) activities - reflecting hepatic alterations - was lessened after LPS injection in LAM-treated rats compared to control rats. LAM treatment decreased serum monocytes number, nitrite (NO2) and tumor necrosis factor-alpha (TNF-alpha). LAM also modulated intra-hepatic immune cells: it lowered the occurrence of peroxidase-positive cells (corresponding to monocytes/neutrophils) and, in contrast, it increased the number of ED2-positive cells, corresponding to resident hepatic macrophages, i.e. Kupffer cells. In conclusion, the hepatoprotective effect of marine beta (1-3) glucan during endotoxic shock may be linked to its immunomodulatory properties. We propose that both lower recruitment of inflammatory cells inside the liver tissue and lower secretion of inflammatory mediators play a role in the tissue protective effect of LAM. These effects could be due to a direct effect of beta-glucan on immune cells, or to an indirect effect through their dietary fibre properties (fermentation in the gut).

Fecal water from ileostomic patients consuming oat beta-glucan enhances immune responses in enterocytes

Yeast, fungal, and dietary beta-glucans have immune-modulating effects in vitro and in vivo, as thought, mainly by affecting leukocytes; however, effects of oat beta-glucan on enterocytes have never been studied. As recognized, supplying oat beta-glucans as such to cells in culture directly is difficult because of solubility problems. Therefore, six ileostomic patients consumed, in random order, a control diet or an oat beta-glucan enriched diet (5 g) and from the collected ileostomic content, fecal water was prepared and added to two small intestinal cell lines (INT407, Caco-2) and two colon cell lines (HT29, T84) together with a cytokine cocktail (IL-1beta + INFgamma + TNFalpha). Several parameters reflecting immune-modulation were measured. As compared to placebo fecal water, beta-glucan enriched fecal water significantly increased IL-8 production in HT29 (5.0%; p = 0.046) and INT407 cells (22.0%; p = 0.028). Intercellular adhesion molecule (ICAM)-1 expression increased in T84 (11.0%; p = 0.028) and Caco-2 cells (20.4%; p = 0.075). These immune-stimulating effects were confirmed by enhancement of inflammatory expression profiles, as determined with an antibody array. Our findings show immune enhancement by fecal water from ileostomic patients consuming oat beta-glucan both in small intestinal and colon cell lines after stimulation, which is in agreement with documented effects in leukocytes. Whether these immune-stimulating effects on enterocytes contribute to the enhanced protection of the host against invading pathogens as observed both in animals and in humans, as well as the underlying mechanism, needs further evaluation.

Yeast glucan particles activate murine resident macrophages to secrete proinflammatory cytokines via MyD88- and Syk kinase-dependent pathways

The therapeutic benefits of fungal beta-glucans have been demonstrated as immuno-stimulating agents. In this study, we aimed to explore the mechanisms used by yeast beta-glucan-rich particles to activate murine resident macrophages for cytokine secretion. We demonstrated that resident macrophages were effectively activated by whole yeast beta-glucan particles (WGPs), such as with the upregulation of co-stimulatory molecules and the secretion of cytokines. The binding ability of WGPs and the levels of cytokine secretion in resident macrophages were significantly inhibited by soluble yeast beta-glucan but not by blockade of zymosan glucan receptor dectin-1. In addition, WGP-stimulated cytokine secretion was partially dependent on the MyD-88 pathway but was not significantly affected in CR3-deficient (CR3(-/-)) mice. Furthermore, we showed that Syk kinase was recruited upon WGP stimulation and was required for the production of cytokines. Taken together, these observations suggest that beta-glucan recognition is necessary but not sufficient to induce inflammatory response on resident macrophages. In addition, beta-glucan particles may use differential mechanisms for cytokine secretion in resident macrophages that may modulate both innate and adaptive immunity.

Oral and systemic administration of beta-glucan protects against lipopolysaccharide-induced shock and organ injury in rats

beta-Glucans are glucose polymers with a variety of stimulatory effects on the immune system. The objective of this study was to determine the effect of prophylactic oral administration of soluble Saccharomyces cerevisiae-derived beta-1,3/1,6-glucan (SBG) on the outcome of experimental endotoxaemia and shock-associated organ injury. Male Wistar rats were pretreated with SBG orally (SBGpo, 20 mg/kg/day) for 14 days, subcutaneously (SBGsc, 2 mg/kg/day) for 3 days, or vehicle (placebo). Rats were anaesthetized and subjected to endotoxaemia by intravenous infusion of Escherichia coli lipopolysaccharide (LPS) (6 mg/kg) or saline infusion (sham). We observed significant levels of plasma beta-glucan in the SBGpo group (P<0 x 5), although the SBGsc group had levels approximately 40-fold higher despite a 10-fold lower dose. SBG prophylaxis caused enhanced blood pressure recovery following LPS-induced blood pressure collapse. Oral treatment with SBG attenuated the LPS-induced rise in plasma creatinine levels (P<0 x 05), indicating protection against renal injury. SBG also attenuated the plasma levels of aspartate aminotransferase and alanine aminotransferase (SBGpo, P<0 x 01; SBGsc, P<0 x 01), indicating protection against LPS-induced hepatic injury. A moderate increase in baseline interleukin (IL)-1beta levels was observed in the SBGsc group (P< 0 x 05). In the LPS-challenged rats, plasma levels of proinflammatory cytokines was moderately reduced in both SBG-treated groups compared to placebo. SBG treatment, particularly oral administration, had a striking effect on the haemodynamics of LPS-treated rats, although only a minute fraction of the orally administered beta-glucan translocated to the circulation. Enhanced organ perfusion may thus be responsible for the attenuated levels of indicators of kidney and liver injury seen in SBG-treated rats.

Chronic inflammation: a failure of resolution?

Inflammation has evolved as a protective response to insult or injury, it's a primordial response that eliminates or neutralises foreign organisms or material, the resolution of inflammation encompasses the endogenous anti-inflammatory mechanisms that protect us against excessive tissue injury and promote the restoration of tissue structure and function. In fact, our well being and survival depends upon its efficiency and carefully-balanced control. In general, the innate inflammatory response initiates within minutes and, if all is well, resolves within hours. In contrast, chronic inflammation persists for weeks, months or even years. Here, we are going to discuss the key endogenous checkpoints necessary for mounting an effective yet limited inflammatory response and the crucial biochemical pathways necessary to prevent its persistence.

Medicinal importance of fungal beta-(1-->3), (1-->6)-glucans

Non-cellulosic beta-glucans are now recognized as potent immunological activators, and some are used clinically in China and Japan. These beta-glucans consist of a backbone of glucose residues linked by beta-(1-->3)-glycosidic bonds, often with attached side-chain glucose residues joined by beta-(1-->6) linkages. The frequency of branching varies. The literature suggests beta-glucans are effective in treating diseases like cancer, a range of microbial infections, hypercholesterolaemia, and diabetes. Their mechanisms of action involve them being recognized as non-self molecules, so the immune system is stimulated by their presence. Several receptors have been identified, which include: dectin-1, located on macrophages, which mediates beta-glucan activation of phagocytosis and production of cytokines, a response co-ordinated by the toll-like receptor-2. Activated complement receptors on natural killer cells, neutrophils, and lymphocytes, may also be associated with tumour cytotoxicity. Two other receptors, scavenger and lactosylceramide, bind beta-glucans and mediate a series of signal pathways leading to immunological activation. Structurally different beta-glucans appear to have different affinities toward these receptors and thus generate markedly different host responses. However, the published data are not always easy to interpret as many of the earlier studies used crude beta-glucan preparations with, for the most part, unknown chemical structures. Careful choice of beta-glucan products is essential if their benefits are to be optimized, and a better understanding of how beta-glucans bind to receptors should enable more efficient use of their biological activities.

Carboxymethyl β-glucan Binds to Corneal Epithelial Cells and Increases Cell Adhesion to Laminin and Resistance to Oxidative Stress

PURPOSE

Polysaccharides are frequently used as viscoelastic agents to improve pharmacokinetics of ophthalmic preparations. Recently, polysaccharides from yeast cell walls such as beta-glucans have emerged as bioactive molecules endowed with immunomodulatory and cytoprotective properties. In this study, we investigated the effects of carboxymethyl beta-glucan (CMG), a water-soluble derivative of yeast beta-glucan, on cultured rabbit corneal epithelial cells.

METHODS

We developed a fluorescein-labeled CMG to visualize its binding to corneal cells by means of digital microscopy and image deconvolution. The effects of CMG on adhesion and survival of corneal epithelial cells exposed to noxious stimuli were also studied.

RESULTS

CMG binds defined regions scattered throughout the body of corneal cells, suggesting binding specificity. Tridimensional reconstruction of fluorescence shows that binding is localized mainly at the plasma and nuclear membranes. Interestingly, CMG binding is highly represented at the level of focal adhesion of cells spreading onto laminin. Accordingly, CMG promotes adhesion of corneal epithelial cells to laminin without affecting their proliferation rate. CMG also protects cells from oxidative stress-dependent cell death, being ineffective in preventing ultraviolet B cytotoxicity.

CONCLUSIONS

Data show that CMG dynamically binds to corneal epithelial cells, promoting cell adhesion and resistance to oxidative stress.

Mechanism of macrophage activation by (1,4)-α-d-glucan isolated from Tinospora cordifolia

The signaling mechanism of the novel (1,4)-alpha-D-glucan (RR1) isolated from the medicinal plant Tinospora cordifolia was investigated in macrophages to evaluate its immunostimulating properties. When RAW264.7 macrophages were incubated with RR1 at 4 degrees C, the novel glucan inhibited the phagocytosis of unopsonized zymosan A bioparticles in a dose-dependent manner. RR1 also inhibited the binding and internalization of opsonized zymosan A bioparticles, although at a lower level than laminarin. Incubation of macrophages with anti-CD11b mAb followed by RR1 failed to show any inhibitory effect on RR1-induced TNF-alpha synthesis confirming that complement receptor 3 (CR3) is not involved in the opsonic binding and internalization of RR1 in macrophages unlike zymosan A. The anti-CD11b mAb has significant inhibitory effect on the zymosan A-induced tumor necrosis factor (TNF)-alpha synthesis. RR1 induced TNF-alpha synthesis in macrophages in a dose-dependent manner which can be completely inhibited by the NF-kappaB inhibitor caffeic acid phenethyl ester (CAPE) or curcumin. RR1 activated NF-kappaB in a time- and dose-dependent manner and this modulation of nuclear NF-kappaB activity is associated with the degradation of I-kappaB alpha thus facilitating the translocation of NF-kappaB into the nucleus. RR1-induced NF-kappaB activity peaks at 8 h of RR1 stimulation while I-kappaB alpha degradation occurred within 1 h of stimulation. RR1-induced NF-kappaB activation occurred through TLR6 signaling as evidenced by the synthesis of IL-8 in TLR6-transfected HEK293 cells. These results show that the novel (1,4)-alpha-D-glucan from Tinospora cordifolia activates the immune system through the activation of macrophages that occurs through TLR6 signaling, NF-kappaB translocation and cytokine production.

EST analysis of cDNA libraries from the entomopathogenic fungus Beauveria (Cordyceps) bassiana. II. Fungal cells sporulating on chitin and producing oosporein

In the accompanying paper [Cho, E.-M., Liu, L., Farmerie, W. & Keyhani, N. O. (2006). Microbiology 152, 2843-2854], the analysis of expressed sequence tag (EST) libraries derived from homogeneous single-cell populations of aerial conidia, in vitro blastospores and submerged conidia of the entomopathogenic fungus Beauveria (Cordyceps) bassiana has been reported. Here an extended EST analysis is presented of complex cell mixtures derived from fungal cells sporulating on chitin or grown under culture conditions inducing the production of the B. bassiana secondary metabolite, oosporein. Fungal material used for the construction of the libraries included germinating conidia and blastospores, mycelia, as well as cells in various developmental stages. Approximately 2,500 5' end sequences were determined from random sequencing of clones from each library, and were clustered into 277 contigs with 1,069 singlets, and 306 contigs with 1,064 singlets, for the chitin and oosporein libraries, respectively. Almost half (45-50 %) of the sequences in each library displayed either no significant similarity (e value >10(-4)) or similarity to hypothetical proteins found in the NCBI database. Approximately 20-25 % of the sequences in each library could be annotated by gene ontology terms. A comparative analysis between the two libraries, as well as the libraries in the accompanying paper, is presented. A set of 4,360 clustered and unique sequences was characterized. The data are indicative of a highly plastic gene expression repertoire being available to B. bassiana for growth during different environmental and developmental conditions, and provides a dataset for gene discovery and genome annotation.

A drug-sensitive genetic network masks fungi from the immune system

Fungal pathogens can be recognized by the immune system via their β-glucan, a potent proinflammatory molecule that is present at high levels but is predominantly buried beneath a mannoprotein coat and invisible to the host. To investigate the nature and significance of “masking” this molecule, we characterized the mechanism of masking and consequences of unmasking for immune recognition. We found that the underlying β-glucan in the cell wall of Candida albicans is unmasked by subinhibitory doses of the antifungal drug caspofungin, causing the exposed fungi to elicit a stronger immune response. Using a library of bakers' yeast (Saccharomyces cerevisiae) mutants, we uncovered a conserved genetic network that is required for concealing β-glucan from the immune system and limiting the host response. Perturbation of parts of this network in the pathogen C. albicans caused unmasking of its β-glucan, leading to increased β-glucan receptor-dependent elicitation of key proinflammatory cytokines from primary mouse macrophages. By creating an anti-inflammatory barrier to mask β-glucan, opportunistic fungi may promote commensal colonization and have an increased propensity for causing disease. Targeting the widely conserved gene network required for creating and maintaining this barrier may lead to novel broad-spectrum antimycotics.

Opportunistic fungal pathogens such as Candida albicans often cause fatal infections in patients with a compromised immune system. Unfortunately, current drugs often fail to halt fungal disease, are ineffective against drug-resistant strains, and have severe side effects. Despite the clear clinical significance of fungal infections, it is still not understood how fungi are recognized by the immune system. Candida has high levels of the structural molecule β-glucan in its cell wall, but the majority of its β-glucan is masked by a mannoprotein coat and is therefore invisible to the immune system. Masking of β-glucan may be a fungal virulence factor, because exposed β-glucan provokes a proinflammatory response that is important for mounting an effective immune response against the fungus and clearing the infection. By surveying the genome of the model fungus Saccharomyces cerevisiae (bakers' yeast), the authors discovered a genetic network required for masking β-glucan from the immune system. Mutation of genes in this network in C. albicans caused unmasking of β-glucan and an increased immune response to the fungus. The authors also found that sublethal doses of the antifungal drug caspofungin cause unmasking and lead to a greater immune response. Drugs targeting this fungally conserved masking network may provide new tools to fight fungal infections.

Natural and modified (1-->3)-beta-D-glucans in health promotion and disease alleviation

A number of polysaccharides with beta-glycosidic linkage are widespread in nature in a variety of sources. All have a common structure and the (1-->3)-beta-D-glucan backbone is essential. They have attracted attention over the years because of their bioactive and medicinal properties. In many cases their functional role is a mystery, in others it is well established. Because of their insoluble chemical nature, particulate (1-->3)-beta-D-glucans are not suitable for many medical applications. Various methods of changing or modifying the beta-D-glucan chemical structure and transforming it to a soluble form have been published. The beta-D-glucan bioactive properties can be affected positively or negatively by such modifications. This review examines beta-glucan sources in nature, health effects and structure-activity relationships. It presents the current state of beta-D-glucan solubilization methods and discusses their effectiveness and application possibilities for the future.

A novel glyco-conjugate vaccine against fungal pathogens

To generate a vaccine to protect against a variety of human pathogenic fungi, we conjugated laminarin (Lam), a well-characterized but poorly immunogenic beta-glucan preparation from the brown alga Laminaria digitata, with the diphtheria toxoid CRM197, a carrier protein used in some glyco-conjugate bacterial vaccines. This Lam-CRM conjugate proved to be immunogenic and protective as immunoprophylactic vaccine against both systemic and mucosal (vaginal) infections by Candida albicans. Protection probably was mediated by anti-beta-glucan antibodies as demonstrated by passive transfer of protection to naive mice by the whole immune serum, the immune vaginal fluid, and the affinity-purified anti-beta-glucan IgG fractions, as well as by administration of a beta-glucan-directed IgG2b mAb. Passive protection was prevented by adsorption of antibodies on Candida cells or beta-glucan particles before transfer. Anti-beta-glucan antibodies bound to C. albicans hyphae and inhibited their growth in vitro in the absence of immune-effector cells. Remarkably, Lam-CRM-vaccinated mice also were protected from a lethal challenge with conidia of Aspergillus fumigatus, and their serum also bound to and markedly inhibited the growth of A. fumigatus hyphae. Thus, this novel conjugate vaccine can efficiently immunize and protect against two major fungal pathogens by mechanisms that may include direct antifungal properties of anti-beta-glucan antibodies.

Immunomodulatory effects of beta-glucan on neutrophil function in fathead minnows (Pimephales promelas Rafinesque, 1820)

Stimulatory effects of yeast beta-1,3-1,6-glucans on neutrophils have long been recognized, but effects of glucans on degranulation of primary granules in fish neutrophils have not been previously reported. Neutrophil function was monitored during in vitro and in vivo application of glucans to non- (NS), acute- (AS) and chronically stressed (CS) fish. beta-Glucan proved to be a strong and quick (80%, 2 min) stimulant of degranulation. Dietary glucan increased degranulation in NS fish, and prevented a decrease in AS fish. Degranulation in CS fish returned to NS levels 3 days after the glucan diet was fed. Fathead minnows appear to be a useful model to investigate neutrophil degranulation in fish exposed to different environmental conditions and immunomodulators. Use of beta-glucans in fish diets prior to AS and during chronic stress can enhance neutrophil function, potentially increasing disease resistance and survival rates after transportation or exposure to poor water quality.