The role of Toll-like receptors and C-type lectins for vaccination against Candida albicans

Advancing Vaccine Strategies against Candida Infections: Exploring New Frontiers

Candida albicans, along with several non-albicans Candida species, comprise a prominent fungal pathogen in humans, leading to candidiasis in various organs. The global impact of candidiasis in terms of disease burden, suffering, and fatalities is alarmingly high, making it a pressing global healthcare concern. Current treatment options rely on antifungal drugs such as azoles, polyenes, and echinocandins but are delimited due to the emergence of drug-resistant strains and associated adverse effects. The current review highlights the striking absence of a licensed antifungal vaccine for human use and the urgent need to shift our focus toward developing an anti-Candida vaccine. A number of factors affect the development of vaccines against fungal infections, including the host, intraspecies and interspecies antigenic variations, and hence, a lack of commercial interest. In addition, individuals with a high risk of fungal infection tend to be immunocompromised, so they are less likely to respond to inactivated or subunit whole organisms. Therefore, it is pertinent to discover newer and novel alternative strategies to develop safe and effective vaccines against fungal infections. This review article provides an overview of current vaccination strategies (live attenuated, whole-cell killed, subunit, conjugate, and oral vaccine), including their preclinical and clinical data on efficacy and safety. We also discuss the mechanisms of immune protection against candidiasis, including the role of innate and adaptive immunity and potential biomarkers of protection. Challenges, solutions, and future directions in vaccine development, namely, exploring novel adjuvants, harnessing the trained immunity, and utilizing immunoinformatics approaches for vaccine design and development, are also discussed. This review concludes with a summary of key findings, their implications for clinical practice and public health, and a call to action for continued investment in candidiasis vaccine research.

Microbial exopolysaccharides-β-glucans-as promising postbiotic candidates in vaccine adjuvants

The urgent task of creating new, enhanced adjuvants is closely related to our comprehension of their mechanisms of action. A few adjuvants have shown sufficient efficacy and low toxicity to be allowed for use in human vaccines, despite the fact that they have a long history and an important function. Adjuvants have long been used without a clear understanding of how precisely they augment the immune response. The rational production of stronger and safer adjuvants has been impeded by this lack of information, which necessitates more mechanistic research to support the development of vaccines. Carbohydrate structures-polygalactans, fructans, β-D-glucans, α-D-glucans, D-galactose, and D-glucose-are desirable candidates for the creation of vaccine adjuvants and immunomodulators because they serve important functions in nature and are often biocompatible, safe, and well tolerated. In this review, we have discussed recent advances in microbial-derived carbohydrate-based adjuvants, their immunostimulatory activity, and the implications of this for vaccine development, along with the critical view on the microbial sources, chemical composition, and biosynthetic pathways.

In vitro and in vivo efficacies of Dectin-1-Fc(IgG)(s) fusion proteins against invasive fungal infections

Fungal infections have increased in the last years, particularly associated to an increment in the number of immunocompromised individuals and the emergence of known or new resistant species, despite the difficulties in the often time-consuming diagnosis. The controversial efficacy of the currently available strategies for their clinical management, apart from their high toxicity and severe side effects, have renewed the interest in the research and development of new broad antifungal alternatives. These encompass vaccines and passive immunization strategies with monoclonal antibodies (mAbs), recognizing ubiquitous fungal targets, such as fungal cell wall β-1,3-glucan polysaccharides, which could be used in early therapeutic intervention without the need for the diagnosis at species-level. As additional alternatives, based on the Dectin-1 great affinity to β-1,3-glucan, our group developed broad antibody-like Dectin1-Fc(IgG)(s) from distinct subclasses (IgG2a and IgG2b) and compared their antifungal in vitro and passive immunizations in vivo performances. Dectin1-Fc(IgG2a) and Dectin1-Fc(IgG2b) demonstrated high affinity to laminarin and the fungal cell wall by ELISA, flow cytometry and microscopy. Both Dectin-1-Fc(IgG)(s) inhibited H. capsulatum and C. neoformans growth in a dose-dependent fashion. For C. albicans, such inhibitory effect was observed with concentrations as low as 0.098 and 0.049 µg/mL, respectively, which correlated with the impairment of the kinetics and lengths of germ tubes in comparison to controls. Previous opsonization with Dectin-1-Fc(IgG)(s) enhanced considerably the macrophage antifungal effector functions, increasing the fungi macrophages-interactions and significantly reducing the intraphagosome fungal survival, as lower CFUs were observed. The administration of both Dectin1-Fc(IgG)(s) reduced the fungal burden and mortality in murine histoplasmosis and candidiasis models, in accordance with previous evaluations in aspergillosis model. These results altogether strongly suggested that therapeutic interventions with Dectin-1-Fc(IgG)(s) fusion proteins could directly impact the innate immunity and disease outcome in favor of the host, by direct neutralization, opsonization, phagocytosis, and fungal elimination, providing interesting information on the potential of these new strategies for the control of invasive fungal infections.

The Role of HMGB1 in Invasive Candida albicans Infection

INTRODUCTION

High mobility group box 1 (HMGB1) is an important "late" inflammatory mediator in bacterial sepsis. Ethyl pyruvate (EP), an inhibitor of HMGB1, can prevent bacterial sepsis by decreasing HMGB1 levels. However, the role of HMGB1 in fungal sepsis is still unclear. Therefore, we investigated the role of HMGB1 and EP in invasive C. albicans infection.

METHODS

We measured serum HMGB1 levels in patients with sepsis with C. albicans infection and without fungal infection, and control subjects. We collected clinical indices to estimate correlations between HMGB1 levels and disease severity. Furthermore, we experimentally stimulated mice with C. albicans and C. albicans + EP. Then, we examined HMGB1 levels from serum and tissue, investigated serum levels of tumor necrosis factor α (TNF-α) and interleukin 6 (IL-6), determined pathological changes in tissues, and assessed mortality.

RESULTS

Serum HMGB1 levels in patients with severe sepsis with C. albicans infection were elevated. Increased HMGB1 levels were correlated with procalcitonin (PCT), C-reactive protein (CRP), 1,3-β-D-Glucan (BDG) and C. albicans sepsis severity. HMGB1 levels in serum and tissues were significantly increased within 7 days after mice were infected with C. albicans. The administration of EP inhibited HMGB1 levels, decreased tissue damage, increased survival rates and inhibited the release of TNF-α and IL-6.

CONCLUSIONS

HMGB1 levels were significantly increased in invasive C. albicans infections. EP prevented C. albicans lethality by decreasing HMGB1 expression and release. HMGB1 may provide an effective diagnostic and therapeutic target for invasive C. albicans infections.

Virulence Factors in Candida species

Fungal diseases are severe and have very high morbidity as well as up to 60% mortality for patients diagnosed with invasive fungal infection. In this review, in vitro and in vivo studies provided us with the insight into the role of Candida virulence factors that mediate their success as pathogens, such as: membrane and cell wall (CW) barriers, dimorphism, biofilm formation, signal transduction pathway, proteins related to stress tolerance, hydrolytic enzymes (e.g. proteases, lipases, haemolysins), and toxin production. The review characterized the virulence of clinically important C. albicans, C. parapsilosis, C. tropicalis, C. glabrata and C. krusei. Due to the white-opaque transition in the mating-type locus MTL-homozygous cells, C. albicans demonstrates an advantage over other less related species of Candida as a human commensal and pathogen. It was reviewed that Candida ergosterol biosynthesis genes play a role in cellular stress and are essential for Candida pathogenesis both in invasive and superficial infections. Hydrolases associated with CW are involved in the host-pathogen interactions. Adhesins are crucial in colonization and biofilm formation, an important virulence factor for candidiasis. Calcineurin is involved in membrane and CW stress as well as virulence. The hyphae-specific toxin, named candidalysin, invades mucosal cells facilitating fungal invasion into deeper tissues. Expression of this protein promotes resistance to neutrophil killing in candidiasis. The virulence factors provide immunostimulatory factors, activating dendric cells and promoting T cell infiltration and activation. Targeting virulence factors, can reduce the risk of resistance development in Candida infections.

Development of Fish Immunity and the Role of β-Glucan in Immune Responses

Administration of β-glucans through various routes, including immersion, dietary inclusion, or injection, have been found to stimulate various facets of immune responses, such as resistance to infections and resistance to environmental stress. β-Glucans used as an immunomodulatory food supplement have been found beneficial in eliciting immunity in commercial aquaculture. Despite extensive research involving more than 3000 published studies, knowledge of the receptors involved in recognition of β-glucans, their downstream signaling, and overall mechanisms of action is still lacking. The aim of this review is to summarize and discuss what is currently known about of the use of β-glucans in fish.

Barley beta-Glucan and Zymosan induce Dectin-1 and Toll-like receptor 2 co-localization and anti-leishmanial immune response in Leishmania donovani-infected BALB/c mice

Toll-like receptors (TLRs), TLR2 in particular, are shown to recognize various glycans and glycolipid ligands resulting in various immune effector functions. As barley β-glucan and zymosan are the glycans implicated in immunomodulation, we examined whether these ligands interact with Dectin-1, a lectin-type receptor for glycans, and TLR2 and induce immune responses that can be used against Leishmania infection in a susceptible host. The binding affinity of barley β-glucan and zymosan with Dectin-1 and TLR2 was studied in silico. Barley β-glucan- and zymosan-induced dectin-1 and TLR2 co-localization was studied by confocal microscopy and co-immunoprecipitation. These ligands-induced signalling and effector functions were assessed by Western blot analyses and various immunological assays. Finally, the anti-leishmanial potential of barley β-glucan and zymosan was tested in Leishmania donovani -infected macrophages and in L. donovani-infected BALB/c mice. Both barley β-glucan and zymosan interacted with TLR2 and dectin-1, but with a much stronger binding affinity for the latter, and therefore induced co-localization of these two receptors on BALB/c-derived macrophages. Both ligandsactivated MyD88- and Syk-mediated downstream pathways for heightened inflammatory responses in L. donovani-infected macrophages. These two ligands induced T cell-dependent host protection in L. donovani-infected BALB/c mice. These results establish a novel modus operandi of β-glucans through dectin-1 and TLR2 and suggest an immuno-modulatory potential against infectious diseases.

Upregulated miR-155 inhibits inflammatory response induced by C. albicans in human monocytes derived dendritic cells via targeting p65 and BCL-10

Background

Candida albicans (C. albicans) is one of the most common fungal pathogens causing superficial and systemic infections. The innate immune system is the first defense line against C. albicans infection. MiR-155, a multifunctional microRNA (miRNA), has been proved to be a crucial regulator in innate immune response against bacterial and virus. However, the biological function of miR-155 in innate immune response against C. albicans infection remains unknown.

Methods

The expression miR-155, as well as inflammatory factors [interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ)], in monocytes derived dendritic cells (DCs) during heat-killed C. albicans infection was detected by quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR). The biological functions of miR-155 were investigated with "gain- and loss-of-function" experiments. Potential targets of miR-155 were identified by bioinformatics analysis, luciferase assay and western blot. Small interfering RNA (siRNA) was used to validate the function of miR-155 target.

Results

C. albicans increased the expression of miR-155 and pro-inflammatory factors. MiR-155 induced by C. albicans was depended on Dectin-1-spleen tyrosine kinase (Syk)/Raf-1-MAPK signaling pathway. Furthermore, miR-155 suppressed the secretion of pro-inflammatory cytokines induced by C. albicans by targeting NF-κB p65 and B cell leukemia/lymphoma 10 (BCL-10).

Conclusions

In conclusion, up-regulated miR-155 acts as a negative feedback regulator in the innate immune response against C. albicans infection.

Fungal polysaccharides

Fungal bioactive polysaccharides are well known and have been widely used in Asia as a part of the traditional diet and medicine. In fact, some biopolymers (mainly β-glucans or glycoconjugate) have already made their way to the market as antitumor or immunostimulating drugs. In the last decades, the relationship between structure and activity of polysaccharides and their detailed mode of action have been the core of intense research to understand and utilize their medicinal properties. Most of the antitumor polysaccharides belong to conserved β-glucans, with a linear β-(1→3)-glucan backbone and attached β-(1→6) branch. Structurally different β-glucans appear to have different affinities toward their receptors and thus generate markedly different host responses. However, their antitumor activities are mainly influenced by molecular mass, degree of branching, conformation, and structure modification of the polysaccharides. β-Glucans act on several immune receptors including Dectin-1, complement receptor (CR3) and TLR-2/6, then trigger both innate and adaptive response and enhance opsonic and nonopsonic phagocytosis. Various receptor interactions explain the possible mode of actions of polysaccharides.

β-glucans as potential immunoadjuvants: A review on the adjuvanticity, structure-activity relationship and receptor recognition properties

β-glucans, a group of polysaccharides exist in many organism species such as mushrooms, yeasts, oats, barley, seaweed, but not mammalians, have a variety of biological activities and applications in drugs and other healthcare products. In recent years, β-glucans have been studied as adjuvants in anti-infection vaccines as well as immunomodulators in anti-cancer immunotherapy. β-glucans can regulate immune responses when administered alone and can connect innate and adaptive immunity to improve immunogenicity of vaccines. When β-glucans act as immunostimulants or adjuvants, a set of receptors have been revealed to recognize β-glucans, including dectin-1, complement receptor 3 (CR3), CD5, lactosylceramide, and so on. Therefore, this review is mainly focused on the application of β-glucans as immune adjuvants, the receptors of β-glucans, as well as their structure and activity relationship which will benefit future research of β-glucans.

Reduced PICD in Monocytes Mounts Altered Neonate Immune Response to Candida albicans

Background

Invasive fungal infections with Candida albicans (C. albicans) occur frequently in extremely low birthweight (ELBW) infants and are associated with poor outcome. Phagocytosis of C.albicans initializes apoptosis in monocytes (phagocytosis induced cell death, PICD). PICD is reduced in neonatal cord blood monocytes (CBMO).

Hypothesis

Phagocytosis of C. albicans causes PICD which differs between neonatal monocytes (CBMO) and adult peripheral blood monocytes (PBMO) due to lower stimulation of TLR-mediated immune responses.

Methods

The ability to phagocytose C. albicans, expression of TLRs, the induction of apoptosis (assessment of sub-G1 and nick-strand breaks) were analyzed by FACS. TLR signalling was induced by agonists such as lipopolysaccharide (LPS), Pam3Cys, FSL-1 and Zymosan and blocked (neutralizing TLR2 antibodies and MYD88 inhibitor).

Results

Phagocytic indices of PBMO and CBMO were similar. Following stimulation with agonists and C. albicans induced up-regulation of TLR2 and consecutive phosphorylation of MAP kinase P38 and expression of TNF-α, which were stronger on PBMO compared to CBMO (p < 0.005). Downstream, TLR2 signalling initiated caspase-3-dependent PICD which was found reduced in CBMO (p < 0.05 vs PBMO).

Conclusion

Our data suggest direct involvement of TLR2-signalling in C. albicans-induced PICD in monocytes and an alteration of this pathway in CBMO.

Molecular Mechanisms Associated With Particulate and Soluble Heteroglycan Mediated Immune Response

Immune responses are outcomes of complex molecular machinery which occur inside the cells. Unravelling the cellular mechanisms induced by immune stimulating molecules such as glycans and determining their structure-function relationship are therefore important factors to be assessed. With this viewpoint, the present study identifies the functional receptor binding unit of a well characterized heteroglycan and also delineates the cellular and molecular processes that are induced upon heteroglycan binding to specific cell surface receptors in immune cells. The heteroglycan was acid hydrolysed and it was revealed that 10-30 kDa fractions served as the functional receptor binding unit of the molecule. Increasing the size of 10-30 kDa heteroglycan showed prominent immune activity. The whole soluble heteroglycan was also conjugated with hyperbranched dendrimers so as to generate a particulate form of the molecule. Dectin-1 and TLR2 were identified as the major receptors in macrophages that bind to particulate as well as soluble form of the heteroglycan and subsequently caused downstream signaling molecules such as NF-κβ and MAPK to get activated. High levels of 1L-1β and IL-10 mRNA were observed in particulate heteroglycan treated macrophages, signifying that increasing the size and availability of the heteroglycan to its specific receptors is pertinent to its biological functioning. Upregulated expression of PKC and iNOS were also noted in particulate heteroglycan treated RAW 264.7 cells than the soluble forms. Taken together, our results indicate that biological functions of immunomodulatory heteroglycan are dependent on their size and molecular weight. J. Cell. Biochem. 117: 1580-1593, 2016. © 2015 Wiley Periodicals, Inc.

Immunological basis of anti-Candida vaccines focused on synthetically prepared cell wall mannan-derived manno-oligomers

The increasing incidence of diseases caused by Candida species and complications in individuals with impaired immunity require new strategies for candidiasis treatment and prevention. The available therapies are often of limited effectiveness in immunocompromised patients, resulting in treatment failures, chronic infections and high mortality rates. Research directed at identifying the composition of an effective vaccine is required. Mannan forms the outermost layer of the Candida cell wall and has an essential role in modulation of anti-Candida host immune responses. Therefore, Candida cell wall mannan and synthetically prepared manno-oligomer-based glycoconjugates are the foci of attention in vaccine candidate development. Almost all of the existing human vaccines mediate protection through neutralizing antibodies. Th1-based and/or Th17-based cellular immune responses, rather than antibody-mediated immunity, mediate protection against candidiasis. Findings of published studies indicate that analysis of cellular immune responses as well as antibody responses is necessary when assessing the immunomodulatory properties of manno-oligomer-based glycoconjugates that are potential anti-Candida vaccine candidates.

Co-immunization of cattle with a vaccine against babesiosis and Lactobacillus casei increases specific IgG1 levels to Babesia bovis and B. bigemina

The effect of Lactobacillus casei administered along with a live attenuated vaccine vs. bovine babesiosis (VAC) on induction of IgG1 and IgG2 antibodies to Babesia bovis and Babesia bigemina was assessed by the indirect fluorescent antibody test (IFAT) in bovines of an endemic babesiosis area before (day 0) and after vaccination (days 15 and 30). We previously reported that L. casei increases the efficiency of VAC under controlled conditions and under extreme conditions in the field; however, the levels of IgG1 and IgG2 antibodies to B. bovis and B. bigemina are not known in vaccinated animals. Twenty-one dairy cows were allocated into three groups (seven animals per group): unvaccinated, vaccinated with VAC and vaccinated simultaneously with VAC and L. casei (VAC-LC). All animals were kept in a babesiosis endemic area at Tlalixcoyan, Veracruz. At days 15 and 30 after vaccination, the average levels of IgG1 to B. bovis and to B. bigemina were significantly higher in VAC-LC group than levels observed in VAC and control groups (P<0.01). Levels of IgG2 were similar in VAC and VAC-LC groups but higher than in the control group (P<0.01). When tested in in vitro cultures of B. bovis, sera from VAC-LC group significantly inhibited parasite growth as compared with the sera of the other two groups. It is suggested that the efficiency improvement of VAC, in part, is due to the L. casei boost of IgG1 over IgG2 antibodies to B. bovis and B. bigemina when the bacteria is co-inoculated with this vaccine.

Invasive Candida infections and the harm from antibacterial drugs in critically ill patients: data from a randomized, controlled trial to determine the role of ciprofloxacin, piperacillin-tazobactam, meropenem, and cefuroxime

OBJECTIVE

Use of antibiotics in critically ill patients may increase the risk of invasive Candida infection. The objective of this study was to determine whether increased exposure to antibiotics is associated with increased prevalence of invasive Candida infection.

DESIGN

Substudy using data from a randomized controlled trial, the Procalcitonin And Survival Study 2006-2010.

SETTING

Nine multidisciplinary ICUs across Denmark.

PATIENTS

A total of 1,200 critically ill patients.

INTERVENTION

Patients were randomly allocated to either a "high exposure" antibiotic therapy (intervention arm, n = 604) or a "standard exposure" guided by current guidelines (n = 596).

MEASUREMENTS AND MAIN RESULTS

Seventy-four patients met the endpoint, "invasive Candida infection," 40 in the high exposure arm and 34 in standard exposure arm (relative risk = 1.2; 95% CI, 0.7-1.8; p = 0.52). Among medical patients in the high exposure arm, the use of ciprofloxacin and piperacillin/tazobactam was 51% and 75% higher than in the standard exposure arm; no difference in antibiotic exposure was observed between the randomized arms in surgical patients. Among medical intensive care patients, invasive Candida infection was more frequent in the high exposure arm (6.2%; 27/437) than in standard exposure arm (3.3%; 14/424) (hazard ratio = 1.9; 95% CI, 1.0-3.6; p = 0.05). Ciprofloxacin used at study entry independently predicted invasive Candida infection (adjusted hazard ratio = 2.1 [1.1-4.1]); the risk gradually increased with duration of ciprofloxacin therapy: six of 384 in patients not exposed (1.6%), eight of 212 (3.8%) when used for 1-2 days (hazard ratio = 2.5; 95% CI, 0.9-7.3), and 31 of 493 (6.3%) when used for 3 days (hazard ratio = 3.8; 95% CI, 1.6-9.3; p = 0.002). Patients with any ciprofloxacin-containing antibiotic regimen the first 3 days in the trial had a higher risk of invasive Candida infection than did patients on any antibiotic regimen not containing ciprofloxacin (unadjusted hazard ratio = 3.7; 95% CI, 1.6-8.7; p = 0.003; adjusted hazard ratio, 3.4; 95% CI, 1.4-8.0; p = 0.006).

CONCLUSIONS

High exposure to antibiotics is associated to increased risk of invasive Candida infection in medical intensive care patients. Patients with ciprofloxacin-containing regimens had higher risk of invasive Candida infection. Other antibiotics, such as meropenem, piperacillin/tazobactam, and cefuroxime, were not associated with such a risk.

Shared characteristics between Mycobacterium tuberculosis and fungi contribute to virulence

Mycobacterium tuberculosis, an etiologic agent of tuberculosis, exacts a heavy toll in terms of human morbidity and mortality. Although an ancient disease, new strains are emerging as human population density increases. The emergent virulent strains appear adept at steering the host immune response from a protective Th1 type response towards a Th2 bias, a feature shared with some pathogenic fungi. Other common characteristics include infection site, metabolic features, the composition and display of cell surface molecules, the range of innate immune receptors engaged during infection, and the ability to form granulomas. Literature from these two distinct fields of research are reviewed to propose that the emergent virulent strains of M. tuberculosis are in the process of convergent evolution with pathogenic fungi, and are increasing the prominence of conserved traits from environmental phylogenetic ancestors that facilitate their evasion of host defenses and dissemination.

Immunomodulation of Fungal β-Glucan in Host Defense Signaling by Dectin-1

During the course of evolution, animals encountered the harmful effects of fungi, which are strong pathogens. Therefore, they have developed powerful mechanisms to protect themselves against these fungal invaders. β-Glucans are glucose polymers of a linear β(1,3)-glucan backbone with β(1,6)-linked side chains. The immunostimulatory and antitumor activities of β-glucans have been reported; however, their mechanisms have only begun to be elucidated. Fungal and particulate β-glucans, despite their large size, can be taken up by the M cells of Peyer's patches, and interact with macrophages or dendritic cells (DCs) and activate systemic immune responses to overcome the fungal infection. The sampled β-glucans function as pathogen-associated molecular patterns (PAMPs) and are recognized by pattern recognition receptors (PRRs) on innate immune cells. Dectin-1 receptor systems have been incorporated as the PRRs of β-glucans in the innate immune cells of higher animal systems, which function on the front line against fungal infection, and have been exploited in cancer treatments to enhance systemic immune function. Dectin-1 on macrophages and DCs performs dual functions: internalization of β-glucan-containing particles and transmittance of its signals into the nucleus. This review will depict in detail how the physicochemical nature of β-glucan contributes to its immunostimulating effect in hosts and the potential uses of β-glucan by elucidating the dectin-1 signal transduction pathway. The elucidation of β-glucan and its signaling pathway will undoubtedly open a new research area on its potential therapeutic applications, including as immunostimulants for antifungal and anti-cancer regimens.

Mannan-binding lectin inhibits Candida albicans-induced cellular responses in PMA-activated THP-1 cells through Toll-like receptor 2 and Toll-like receptor 4

Background

Candida albicans (C. albicans), the most common human fungal pathogen, can cause fatal systemic infections under certain circumstances. Mannan-binding lectin (MBL),a member of the collectin family in the C-type lectin superfamily, is an important serum component associated with innate immunity. Toll-like receptors (TLRs) are expressed extensively, and have been shown to be involved in C. albicans-induced cellular responses. We first examined whether MBL modulated heat-killed (HK) C. albicans-induced cellular responses in phorbol 12-myristate 13-acetate (PMA)-activated human THP-1 macrophages. We then investigated the possible mechanisms of its inhibitory effect.

Methodology/Principal Finding

Enzyme-linked immunosorbent assay (ELISA) and reverse transcriptasepolymerase chain reaction (RT-PCR) analysis showed that MBL at higher concentrations (10–20 µg/ml) significantly attenuated C. albicans-induced chemokine (e.g., IL-8) and proinflammatory cytokine (e.g., TNF-α) production from PMA-activated THP-1 cells at both protein and mRNA levels. Electrophoretic mobility shift assay (EMSA) and Western blot (WB) analysis showed that MBL could inhibit C. albicans-induced nuclear factor-κB (NF-κB) DNA binding and its translocation in PMA-activated THP-1 cells. MBL could directly bind to PMA-activated THP-1 cells in the presence of Ca²⁺, and this binding decreased TLR2 and TLR4 expressions in C. albicans-induced THP-1 macrophages. Furthermore, the binding could be partially inhibited by both anti-TLR2 monoclonal antibody (clone TL2.1) and anti-TLR4 monoclonal antibody (clone HTA125). In addition, co-immunoprecipitation experiments and microtiter wells assay showed that MBL could directly bind to the recombinant soluble form of extracellular TLR2 domain (sTLR2) and sTLR4.

Conclusions/Significance

Our study demonstrates that MBL can affect proinflammatory cytokine and chemokine expressions by modifying C. albicans-/TLR-signaling pathways. This study supports an important role for MBL on the regulation of C. albicans-induced cellular responses.

Saccharomyces as a vaccine against systemic candidiasis

We have shown heat-killed Saccharomyces (HKY) is a protective vaccine against aspergillosis and coccidioidomycosis. To test the hypothesis that the efficacy of HKY- induced protection may be due to the cross-reactive antigens in the cell walls of the different fungi, we studied the effect of HKY against systemic candidiasis. Male CD-1 mice were given different regimens of HKY subcutaneously prior to intravenous challenge with Candida albicans. Compared to PBS controls, the administration of HKY (6 × 10(7)) 3, 4 or 6 times prolonged survival (all P < 0.05) and reduced fungal load in the kidney (all P < 0.05). An HKY dose of 1.2 × 10(8) given 4 times prolonged survival (P = 0.02), but showed dose-limiting toxicity. HKY given by an oral route, or by a subcutaneous route with alum as an adjuvant, did not improve survival. Overall, we found that HKY protects mice from infection by Candida albicans in a dose-and regimen-dependent manner. To understand the protection induced by HKY against different fungal species, additional studies of epitope mapping are warranted.

Recent progress in vaccines against fungal diseases

Diseases caused by fungi are increasingly impacting the health of the human population and now account for a large fraction of infectious disease complications in individuals with impaired immunity or breached tissue defenses. Antifungal therapy is often of limited effectiveness in these patients, resulting into treatment failures, chronic infections and unacceptable rates of mortality, morbidity and their associated costs. Consequently there is a real medical need for new treatments and preventive measures to combat fungal diseases and, toward this goal, safe and efficacious vaccines would constitute major progress. After decades of complacency and neglect of this critically important field of research, remarkable progress has been made in recent years. A number of highly immunogenic and protective vaccine formulations in preclinical setting have been developed, and at least two have undergone Phase 1 clinical trials as preventive and/or therapeutic tools against candidiasis.

Inherited variation in pattern recognition receptors and cancer: dangerous liaisons?

The group of pattern recognition receptors includes families of Toll-like receptors, NOD-like receptors, C-type lectin receptors, and RIG-I-like receptors. They are key sensors for a number of infectious agents, some of which are carcinogenic, and they launch an immune response against them. Inherited structural variation in genes encoding these receptors and proteins of their signaling pathways may affect their function, modulating cancer risk and features of cancer progression. Relevant malignancies, valuable gene polymorphisms, prime questions about future directions, and answers to these questions are analyzed in this review. It is possible to suggest that polymorphisms of genes encoding pattern recognition receptors and proteins of their signaling pathways may be associated with almost all cancer types, particularly with those in which carcinogenic infectious agents are responsible for the substantial share of cases (namely gastric cancer, colorectal cancer, liver cancer, cervical cancer, and nasopharyngeal carcinoma). The concept of selection of polymorphisms for further oncogenomic investigation, based on a combination of results from basic and epidemiological studies, is proposed.

Innate and adaptive antifungal immune responses: partners on an equal footing

Adaptive immunity has long been regarded as the major player in protection against most fungal infections. Mounting evidence suggest however, that both innate and adaptive responses intricately collaborate to produce effective antifungal protection. Dendritic cells (DCs) play an important role in initiating and orchestrating antifungal immunity; neutrophils, macrophages and other phagocytes also participate in recognising and eliminating fungal pathogens. Adaptive immunity provides a wide range of effector and regulatory responses against fungal infections. Th1 responses protect against most forms of mycoses but they associate with significant inflammation and limited pathogen persistence. By contrast, Th2 responses enhance persistence of and tolerance to fungal infections thus permitting the generation of long-lasting immunological memory. Although the role of Th17 cytokines in fungal immunity is not fully understood, they can enhance proinflammatory or anti-inflammatory responses or play a regulatory role in fungal immunity all depending on the pathogen, site/phase of infection and host immunostatus. T regulatory cells balance the activities of various Th cell subsets thereby permitting inflammation and protection on the one hand and allowing for tolerance and memory on the other. Here, recent developments in fungal immunity research are reviewed as means of tracing the emergence of a refined paradigm where innate and adaptive responses are viewed in the same light.

Impact of aging on dendritic cell functions in humans

Aging is a paradox of reduced immunity and chronic inflammation. Dendritic cells are central orchestrators of the immune response with a key role in the generation of immunity and maintenance of tolerance. The functions of DCs are compromised with age. There is no major effect on the numbers and phenotype of DC subsets in aged subjects; nevertheless, their capacity to phagocytose antigens and migrate is impaired with age. There is aberrant cytokine secretion by various DC subsets with CDCs secreting increased basal level of pro-inflammatory cytokines but the response on stimulation to foreign antigens is decreased. In contrast, the response to self-antigens is increased suggesting erosion of peripheral self tolerance. PDC subset also secretes reduced IFN-α in response to viruses. The capacity of DCs to prime T cell responses is also affected. Aging thus has a profound affect on DC functions. Present review summarizes the effect of advancing age on DC functions in humans in the context of both immunity and tolerance.

Immune escape of the human facultative pathogenic yeast Candida albicans: the many faces of the Candida Pra1 protein

Infectious diseases caused by human pathogenic fungi represent a major and global health problem. Based on the limited efficacy of existing drugs and the increasing resistance to antifungal compounds, new strategies are urgently needed to fight such fungal infections. The medically important pathogen Candida albicans can exist as an opportunistic yeast, but can also cause severe diseases, septicaemia, and death. In order to establish new strategies to fight Candida infections and to interfere with survival of the pathogen, it is highly relevant to understand the molecular and immunology interactions between the pathogen C. albicans and the human host. This immune cross talk has moved into the focus of infectious disease research. In this review, we summarize the diverse and multiple levels of the immune cross talk between the fungal pathogen C. albicans and the human host. In particular, we define how one single fungal protein Pra1 (pH-regulated antigen 1) interferes and controls host immune attack at multiple levels and thus contributes to fungal immune escape. Candida Pra1 represents a promising candidate for immune interference.

Host-pathogen interactions and virulence-associated genes during Candida albicans oral infections

Oral infections with Candida albicans are very common diseases in even only mildly immunocompromised patients. By using genome-wide microarrays, in vitro infection models and samples from patients with pseudomembranous candidiasis, several genes have been identified which encode known and unknown fungal factors associated with oral infection. The expression of selected genes has been investigated via qRT-PCR in both in vitro models and in vivo samples from patients. Several lines of evidence suggest that fungal morphology plays a key role in adhesion to and invasion into oral epithelial cells and mutants lacking regulators of hyphal formation are attenuated in their ability to invade and damage epithelial cells. Adhesion is mediated by hyphal-associated factors such as Hwp1 and the Als adhesin family. Hyphal formation facilitates epithelial invasion via two routes: active penetration and induced endocytosis. While induced endocytosis is predominantly mediated by the adhesin and invasin Als3, active penetration seems to be supported by hydrolase activity and mechanical pressure. Expression profiles reflect the morphological switch and an adaptive response to neutral pH, non-glucose carbon sources, and nitrosative stress.

Importance of innate mucosal immunity and the promises it holds

The body defense mechanism has evolved to protect animals from invading pathogenic microorganisms and cancer. It is able to generate a diverse variety of cells and molecules capable of specifically recognizing and eliminating a limitless variety of foreign invaders. These cells and molecules act together in a dynamic network and are known as the immune system. Innate mucosal immunity consists of various recognition receptor molecules, including toll-like receptors, NOD-like receptors, and RIG-I-like receptors. These recognition receptor molecules recognize various invading pathogens effectively, and generate an immune response to stop their entry and neutralize their adverse consequences, such as tissue damage. Furthermore, they regulate the adaptive response in cases of severe infection and also help generate a memory response. Most infections occur through the mucosa. It is important to understand the initial host defense response or innate immunity at the mucosal surface to control these infections and protect the system. The aim of this review is to discuss the effects and functions of various innate mucosal agents and their importance in understanding the physiological immune response, as well as their roles in developing new interventions.

Pathogen-associated molecular patterns on biomaterials: a paradigm for engineering new vaccines

Vaccine development has progressed significantly and has moved from whole microorganisms to subunit vaccines that contain only their antigenic proteins. Subunit vaccines are often less immunogenic than whole pathogens; therefore, adjuvants must amplify the immune response, ideally establishing both innate and adaptive immunity. Incorporation of antigens into biomaterials, such as liposomes and polymers, can achieve a desired vaccine response. The physical properties of these platforms can be easily manipulated, thus allowing for controlled delivery of immunostimulatory factors and presentation of pathogen-associated molecular patterns (PAMPs) that are targeted to specific immune cells. Targeting antigen to immune cells via PAMP-modified biomaterials is a new strategy to control the subsequent development of immunity and, in turn, effective vaccination. Here, we review the recent advances in both immunology and biomaterial engineering that have brought particulate-based vaccines to reality.

Applied biotechnology for production of immunoglobulin Y specific to hepatitis A virus

A new protocol for producing polyclonal antibody against hepatitis A virus (HAV) is described. Twenty hens were immunized three times with a commercial HAV vaccine and HAV from a cell culture with three types of adjuvants: CpG oligodeoxynucleotides (CpG-ODN), incomplete Freund's adjuvant and an alum adjuvant. In each of the last two booster inoculations, blood from the birds was collected and tested for HAV antibodies. Egg yolk was separated from egg white and immunoglobulin Y (IgY) antibody was then purified by polyethylene glycol 6000. The mean yield of total protein in yolk was 22.62 mg/mL. Specific activity of the antibody was tested using commercial ELISA, Western blotting, and in vitro neutralization assay demonstrating that anti-HAV IgY bound specifically. After the first immunization, birds immunized with HAV from cell culture plus incomplete Freund's adjuvant with/without CpG-ODN showed highest levels of anti-HAV IgY in serum (p<0.05). Viral combination with CpG-ODN resulted in early response of anti-HAV serum in hens, reflecting the amount of IgY transferred to the egg yolk (p<0.05). The results suggest that egg yolk may be a large scale source of specific antibodies against hepatitis A virus. Further applications of this method have yet to be tested.