Saccharomyces cerevisiae as a vaccine against coccidioidomycosis

Application of anti-fungal vaccines as a tool against emerging anti-fungal resistance

After viruses and bacteria, fungal infections remain a serious threat to the survival and well-being of society. The continuous emergence of resistance against commonly used anti-fungal drugs is a serious concern. The eukaryotic nature of fungal cells makes the identification of novel anti-fungal agents slow and difficult. Increasing global temperature and a humid environment conducive to fungal growth may lead to a fungal endemic or a pandemic. The continuous increase in the population of immunocompromised individuals and falling immunity forced pharmaceutical companies to look for alternative strategies for better managing the global fungal burden. Prevention of infectious diseases by vaccines can be the right choice. Recent success and safe application of mRNA-based vaccines can play a crucial role in our quest to overcome anti-fungal resistance. Expressing fungal cell surface proteins in human subjects using mRNA technology may be sufficient to raise immune response to protect against future fungal infection. The success of mRNA-based anti-fungal vaccines will heavily depend on the identification of fungal surface proteins which are highly immunogenic and have no or least side effects in human subjects. The present review discusses why it is essential to look for anti-fungal vaccines and how vaccines, in general, and mRNA-based vaccines, in particular, can be the right choice in tackling the problem of rising anti-fungal resistance.

Fungal Vaccine Development: State of the Art and Perspectives Using Immunoinformatics

Fungal infections represent a serious global health problem, causing damage to health and the economy on the scale of millions. Although vaccines are the most effective therapeutic approach used to combat infectious agents, at the moment, no fungal vaccine has been approved for use in humans. However, the scientific community has been working hard to overcome this challenge. In this sense, we aim to describe here an update on the development of fungal vaccines and the progress of methodological and experimental immunotherapies against fungal infections. In addition, advances in immunoinformatic tools are described as an important aid by which to overcome the difficulty of achieving success in fungal vaccine development. In silico approaches are great options for the most important and difficult questions regarding the attainment of an efficient fungal vaccine. Here, we suggest how bioinformatic tools could contribute, considering the main challenges, to an effective fungal vaccine.

Standardization and Key Aspects of the Development of Whole Yeast Cell Vaccines

In the context of vaccine development, improving antigenic presentation is critical for the activation of specific immune responses and the success of immunization, in addition to selecting an appropriate target. In this sense, different strategies have been developed and improved. Among them is the use of yeast cells as vehicles for the delivery of recombinant antigens. These vaccines, named whole yeast vaccines (WYVs), can induce humoral and cellular immune responses, with the additional advantage of dispensing with the use of adjuvants due to the immunostimulatory properties of their cell wall components. However, there are some gaps in the methodologies for obtaining and validating recombinant strains and vaccine formulations. The standardization of these parameters is an important factor for WYVs approval by regulatory agencies and, consequently, their licensing. This review aimed to provide an overview of the main parameters to consider when developing a yeast-based vaccine, addressing some available tools, and highlighting the main variables that can influence the vaccine production process.

Vaccines against candidiasis: Status, challenges and emerging opportunity

Candidiasis is a mycosis caused by opportunistic Candida species. The occurrence of fungal infections has considerably increased in the last few years primarily due to an increase in the number of immune-suppressed individuals. Alarming bloodstream infections due to Candida sp. are associated with a higher rate of morbidity and mortality, and are emerged as major healthcare concerns worldwide. Currently, chemotherapy is the sole available option for combating fungal diseases. Moreover, the emergence of resistance to these limited available anti-fungal drugs has further accentuated the concern and highlighted the need for early detection of fungal infections, identification of novel antifungal drug targets, and development of effective therapeutics and prophylactics. Thus, there is an increasing interest in developing safe and potent immune-based therapeutics to tackle fungal diseases. In this context, vaccine design and its development have a priority. Nonetheless, despite significant advances in immune and vaccine biology over time, a viable commercialized vaccine remains awaited against fungal infections. In this minireview, we enumerate various concerted efforts made till date towards the development of anti-Candida vaccines, an option with pan-fugal vaccine, vaccines in the clinical trial, challenges, and future opportunities.

Genetic Vaccination as a Flexible Tool to Overcome the Immunological Complexity of Invasive Fungal Infections

The COVID-19 pandemic has highlighted genetic vaccination as a powerful and cost-effective tool to counteract infectious diseases. Invasive fungal infections (IFI) remain a major challenge among immune compromised patients, particularly those undergoing allogeneic hematopoietic bone marrow transplantation (HSCT) or solid organ transplant (SOT) both presenting high morbidity and mortality rates. Candidiasis and Aspergillosis are the major fungal infections among these patients and the failure of current antifungal therapies call for new therapeutic aids. Vaccination represents a valid alternative, and proof of concept of the efficacy of this approach has been provided at clinical level. This review will analyze current understanding of antifungal immunology, with a particular focus on genetic vaccination as a suitable strategy to counteract these diseases.

Apoptotic Effect of Saccharomyces cerevisiae on Human Colon Cancer SW480 Cells by Regulation of Akt/NF-ĸB Signaling Pathway

Drug resistance is one of the major problems, which causes recurrence of cancers. Therefore, complementary treatments are needed to improve the impacts of chemotherapy agents. The effect of probiotics as cancer-preventing agents through involvement in the activation of apoptotic pathways has been established. The present study sought to investigate how the heat-killed form of Saccharomyces cerevisiae (as a probiotic) could affect the Akt/NF-kB-induced apoptosis in colon cancer cells, the SW480 cell line. The cytotoxic effects of heat-killed yeast (HKY) and 5-fluorouracil (5-FU, as a positive control drug) were assayed using the MTT method. Morphological changes followed by apoptosis were examined using DAPI staining. The transcription and translation level of apoptosis genes were explored with qRT-PCR and western blotting. The data were analyzed using GraphPad Prism V6.0 Software. The results showed that HKY could induce apoptosis in colon cancer cell line through downregulation of p-Akt1, Rel A, Bcl-XL, pro-caspase 3, and pro-caspase 9 expressions, and upregulation of BAX, cleaved caspase-3, and cleaved caspase-9. Besides, Akt protein expression was not affected. It is noticeable that HKY had a better modulating effect on BAX expression compared with 5-FU. It was able to modulate Akt/NF-kB signaling pathway followed by the apoptotic cascade.

Yeast-based vaccines: New perspective in vaccine development and application

In presently licensed vaccines, killed or attenuated organisms act as a source of immunogens except for peptide-based vaccines. These conventional vaccines required a mass culture of associated or related organisms and long incubation periods. Special requirements during storage and transportation further adds to the cost of vaccine preparations. Availability of complete genome sequence, well-established genetic, inherent natural adjuvant and non-pathogenic nature of yeast species viz. Saccharomyces cerevisiae, Pichia pastoris makes them an ideal model system for the development of vaccines both for public health and for on-farm consumption. In this review, we compile the work in this emerging field during last two decades with major emphases on S. cerevisiae and P. pastoris which are routinely used worldwide for expression of heterologous proteins with therapeutic value against infectious diseases along with possible use in cancer therapy. We also pointed towards the developments in use of whole recombinant yeast, yeast surface display and virus-like particles as a novel strategy in the fight against infectious diseases and cancer along with other aspects including suitability of yeast in vaccines preparations, yeast cell wall component as an immune stimulator or modulator and present status of yeast-based vaccines in clinical trials.

Coccidioidomycosis: Epidemiology, Fungal Pathogenesis, and Therapeutic Development

Purpose of Review

Coccidioidomycosis can result from the inhalation of infectious spores of Coccidioides species (spp.) immitis or posadasii. Clinical manifestations range from mild flu-like disease to severe disseminated infection that can require life-long therapy. Burden of this mycosis is high in the southwest region of the USA where it is well characterized, and in many areas of Mexico and Latin America where it is inadequately characterized. Here, we provide historical data and current knowledge on Coccidioides spp. pathogenesis as well as recent progress in therapeutic and vaccine development against coccidioidomycosis.

Recent Findings

The virulence mechanisms of Coccidioides spp. are largely unknown; however, production and regulation of a spherule glycoprotein, ammonium production, and melanization have all been proposed as integral factors in Coccidioides spp.' pathogenesis. Therapeutic options are limited and not 100% effective, but individualized treatment with triazoles or amphotericin B over the course of pulmonary or disseminated infection can be effective in resolution of coccidioidomycosis. Human immunization has not been achieved but efforts are ongoing.

Summary

Advances in therapeutic and vaccine development are imperative for the prevention and treatment of coccidioidomycosis, especially for those individuals at risk either living or traveling to or from endemic areas.

Fungal vaccines, mechanism of actions and immunology: A comprehensive review

Fungal infections include a wide range of opportunistic and invasive diseases. Two of four major fatal diseases in patients with human immunodeficiency virus (HIV) infection are related to the fungal infections, cryptococcosis, and pneumocystosis. Disseminated candidiasis and different clinical forms of aspergillosis annually impose expensive medical costs to governments and hospitalized patients and ultimately lead to high mortality rates. Therefore, urgent implementations are necessary to prevent the expansion of these diseases. Designing an effective vaccine is one of the most important approaches in this field. So far, numerous efforts have been carried out in developing an effective vaccine against fungal infections. Some of these challenges engaged in different stages of clinical trials but none of them could be approved by the United States Food and Drug Administration (FDA). Here, in addition to have a comprehensive overview on the data from studied vaccine programs, we will discuss the immunology response against fungal infections. Moreover, it will be attempted to clarify the underlying immune mechanisms of vaccines targeting different fungal infections that are crucial for designing an effective vaccination strategy.

The Elusive Anti-Candida Vaccine: Lessons From the Past and Opportunities for the Future

Candidemia is a bloodstream fungal infection caused by Candida species and is most commonly observed in hospitalized patients. Even with proper antifungal drug treatment, mortality rates remain high at 40–50%. Therefore, prophylactic or preemptive antifungal medications are currently recommended in order to prevent infections in high-risk patients. Moreover, the majority of women experience at least one episode of vulvovaginal candidiasis (VVC) throughout their lifetime and many of them suffer from recurrent VVC (RVVC) with frequent relapses for the rest of their lives. While there currently exists no definitive cure, the only available treatment for RVVC is again represented by antifungal drug therapy. However, due to the limited number of existing antifungal drugs, their associated side effects and the increasing occurrence of drug resistance, other approaches are greatly needed. An obvious prevention measure for candidemia or RVVC relapse would be to immunize at-risk patients with a vaccine effective against Candida infections. In spite of the advanced and proven techniques successfully applied to the development of antibacterial or antiviral vaccines, however, no antifungal vaccine is still available on the market. In this review, we first summarize various efforts to date in the development of anti-Candida vaccines, highlighting advantages and disadvantages of each strategy. We next unfold and discuss general hurdles encountered along these efforts, such as the existence of large genomic variation and phenotypic plasticity across Candida strains and species, and the difficulty in mounting protective immune responses in immunocompromised or immunosuppressed patients. Lastly, we review the concept of “trained immunity” and discuss how induction of this rapid and nonspecific immune response may potentially open new and alternative preventive strategies against opportunistic infections by Candida species and potentially other pathogens.

Targeting Glycans on Human Pathogens for Vaccine Design

Glycosylation is an important post-translational modification that is required for structural and stability purposes and functional roles such as signalling, attachment and shielding. Many human pathogens such as bacteria display an array of carbohydrates on their surface that are non-self to the host; others such as viruses highjack the host-cell machinery and present self-carbohydrates sometimes arranged in a non-self more immunogenic manner. In combination with carrier proteins, these glycan structures can be highly immunogenic. During natural infection, glycan-binding antibodies are often elicited that correlate with long-lasting protection. A great amount of research has been invested in carbohydrate vaccine design to elicit such an immune response, which has led to the development of vaccines against the bacterial pathogens Haemophilus influenzae type b, Streptococcus pneumonia and Neisseria meningitidis. Other vaccines, e.g. against HIV-1, are still in development, but promising progress has been made with the isolation of broadly neutralizing glycan-binding antibodies and the engineering of stable trimeric envelope glycoproteins. Carbohydrate vaccines against other pathogens such as viruses (Dengue, Hepatitis C), parasites (Plasmodium) and fungi (Candida) are at different stages of development. This chapter will discuss the challenges in inducing cross-reactive carbohydrate-targeting antibodies and progress towards carbohydrate vaccines.

Yeast Expressing Gp43 Protein as a Vaccine Against Paracoccidioides brasiliensis Infection

Paracoccidioidomycosis (PCM) represents the most frequent systemic mycosis in Latin American. The disease is caused by the pathogenic thermally dimorphic fungus Paracoccidioides brasiliensis, and is initially characterized by pulmonary lesions, which can subsequently disseminate to other organs, resulting in secondary injuries. Although its high incidence, there is no commercially available vaccine against fungal diseases. A novel strategy, using Saccharomyces cerevisiae yeast as a vehicle for immunization against PCM, was recently successfully described. Herein, we describe strategies for the construction of the suitable S. cerevisiae vaccine, and protocols of administration and evaluation of the efficacy of the vaccine against experimental PCM.

Heat-Killed Yeast as a Pan-Fungal Vaccine

Fungal infections continue to rise worldwide. Antifungal therapy has long been a mainstay for the treatment of these infections, but often can fail for a number of reasons. These include acquired or innate drug resistance of the causative agent, poor drug penetration into the affected tissues, lack of cidal activity of the drug and drug toxicities that limit therapy. In some instances, such as coccidioidal meningitis, therapy is life-long. In addition, few new antifungal drugs are under development. In light of this information a preventative vaccine is highly desirable. Although numerous investigators have worked toward the development of fungal vaccines, none have become commercially available for use in humans. In the course of our studies, we have discovered that heat-killed yeast (HKY) of Saccharomyces cerevisiae can be used as a vaccine and have shown that it has efficacy in the prevention and reduction of five different fungal infections when used experimentally in mice, which raises the possibility of a pan-fungal vaccine preparation. In our studies we grow S. cerevisiae in broth and heat-kill the organism at 70 ° C for 3 h. The number of dead yeast cells is adjusted and mice are vaccinated subcutaneously beginning 3-7 weeks prior to infection. After infection, efficacy is assessed on the basis of survival and residual burden of the fungus in the target organs. Alternatively, efficacy can be assessed solely on fungal burden at a predetermined time postinfection. Although itself it is unlikely to be moved toward commercialization, HKY can be used a positive control vaccine for studies on specific molecular entities as vaccines, and as a guidepost for the key elements of potential, more purified, pan-fungal vaccine preparations.

Nanoyeast and Other Cell Envelope Compositions for Protein Studies and Biosensor Applications

Rapid progress in disease biomarker discovery has increased the need for robust detection technologies. In the past several years, the designs of many immunoaffinity reagents have focused on lowering costs and improving specificity while also promoting stability. Antibody fragments (scFvs) have long been displayed on the surface of yeast and phage libraries for selection; however, the stable production of such fragments presents challenges that hamper their widespread use in diagnostics. Membrane and cell wall proteins similarly suffer from stability problems when solubilized from their native environment. Recently, cell envelope compositions that maintain membrane proteins in native or native-like lipid environment to improve their stability have been developed. This cell envelope composition approach has now been adapted toward stabilizing antibody fragments by retaining their native cell wall environment. A new class of immunoaffinity reagents has been developed that maintains antibody fragment attachment to yeast cell wall. Herein, we review recent strategies that incorporate cell wall fragments with functional scFvs, which are designed for easy production while maintaining specificity and stability when in use with simple detection platforms. These cell wall based antibody fragments are globular in structure, and heterogeneous in size, with fragments ranging from tens to hundreds of nanometers in size. These fragments appear to retain activity once immobilized onto biosensor surfaces for the specific and sensitive detection of pathogen antigens. They can be quickly and economically generated from a yeast display library and stored lyophilized, at room temperature, for up to a year with little effect on stability. This new format of scFvs provides stability, in a simple and low-cost manner toward the use of scFvs in biosensor applications. The production and "panning" of such antibody cell wall composites are also extremely facile, enabling the rapid adoption of stable and inexpensive affinity reagents for emerging infectious threats.

Immunotherapy of Fungal Infections

Fungal organisms are ubiquitous in the environment. Pathogenic fungi, although relatively few in the whole gamut of microbial pathogens, are able to cause disease with varying degrees of severity in individuals with normal or impaired immunity. The disease state is an outcome of the fungal pathogen's interactions with the host immunity, and therefore, it stands to reason that deep/invasive fungal diseases be amenable to immunotherapy. Therefore, antifungal immunotherapy continues to be attractive as an adjunct to the currently available antifungal chemotherapy options for a number of reasons, including the fact that existing antifungal drugs, albeit largely effective, are not without limitations, and that morbidity and mortality associated with invasive mycoses are still unacceptably high. For several decades, intense basic research efforts have been directed at development of fungal immunotherapies. Nevertheless, this approach suffers from a severe bench-bedside disconnect owing to several reasons: the chemical and biological peculiarities of the fungal antigens, the complexities of host-pathogen interactions, an under-appreciation of the fungal disease landscape, the requirement of considerable financial investment to bring these therapies to clinical use, as well as practical problems associated with immunizations. In this general, non-exhaustive review, we summarize the features of ongoing research efforts directed towards devising safe and effective immunotherapeutic options for mycotic diseases, encompassing work on antifungal vaccines, adoptive cell transfers, cytokines, antimicrobial peptides (AMPs), monoclonal antibodies (mAbs), and other agents.

Proteomic Analysis of Pathogenic Fungi Reveals Highly Expressed Conserved Cell Wall Proteins

We are presenting a quantitative proteomics tally of the most commonly expressed conserved fungal proteins of the cytosol, the cell wall, and the secretome. It was our goal to identify fungi-typical proteins that do not share significant homology with human proteins. Such fungal proteins are of interest to the development of vaccines or drug targets. Protein samples were derived from 13 fungal species, cultured in rich or in minimal media; these included clinical isolates of Aspergillus, Candida, Mucor, Cryptococcus, and Coccidioides species. Proteomes were analyzed by quantitative MS^E (Mass Spectrometry-Elevated Collision Energy). Several thousand proteins were identified and quantified in total across all fractions and culture conditions. The 42 most abundant proteins identified in fungal cell walls or supernatants shared no to very little homology with human proteins. In contrast, all but five of the 50 most abundant cytosolic proteins had human homologs with sequence identity averaging 59%. Proteomic comparisons of the secreted or surface localized fungal proteins highlighted conserved homologs of the Aspergillus fumigatus proteins 1,3-β-glucanosyltransferases (Bgt1, Gel1-4), Crf1, Ecm33, EglC, and others. The fact that Crf1 and Gel1 were previously shown to be promising vaccine candidates, underlines the value of the proteomics data presented here.

Whole recombinant Pichia pastoris expressing HPV16 L1 antigen is superior in inducing protection against tumor growth as compared to killed transgenic Leishmania

The development of an efficient vaccine against high-risk HPV types can reduce the incidence rates of cervical cancer by generating anti-tumor protective responses. Traditionally, the majority of prophylactic viral vaccines are composed of live, attenuated or inactivated viruses. Among them, the design of an effective and low-cost vaccine is critical. Inactivated vaccines especially heat-killed yeast cells have emerged as a promising approach for generating antigen-specific immunotherapy. Recent studies have indicated that yeast cell wall components possess adjuvant activities. Moreover, a non-pathogenic protozoan, Leishmania tarentolae (L.tar) has attracted a great attention as a live candidate vaccine. In current study, immunological and protective efficacy of whole recombinant killed Pichia pastoris and Leishmania tarentolae expressing HPV16 L1 capsid protein was evaluated in tumor mice model. We found that Pichia-L1, L.tar-L1 and Gardasil groups increase the IgG2a/IgG1 ratio, indicating a relative preference for the induction of Th1 immune responses. Furthermore, subcutaneous injection of killed Pichia-L1 generated the significant L1-specific IFN-γ immune response as well as the best protective effects in vaccinated mice as compared to killed L.tar-L1, killed Pichia pastoris, killed L.tar and PBS groups. Indeed, whole recombinant Leishmania tarentolae could not protect mice against C3 tumor mice model. These data suggest that Pichia-L1 may be a candidate for the control of HPV infections.

Evaluating Common Humoral Responses against Fungal Infections with Yeast Protein Microarrays

We profiled the global immunoglobulin response against fungal infection by using yeast protein microarrays. Groups of CD-1 mice were infected systemically with human fungal pathogens (Coccidioides posadasii, Candida albicans, or Paracoccidioides brasiliensis) or inoculated with PBS as a control. Another group was inoculated with heat-killed yeast (HKY) of Saccharomyces cerevisiae. After 30 days, serum from mice in the groups were collected and used to probe S. cerevisiae protein microarrays containing 4800 full-length glutathione S-transferase (GST)-fusion proteins. Antimouse IgG conjugated with Alexafluor 555 and anti-GST antibody conjugated with Alexafluor 647 were used to detect antibody-antigen interactions and the presence of GST-fusion proteins, respectively. Serum after infection with C. albicans reacted with 121 proteins: C. posadasii, 81; P. brasiliensis, 67; and after HKY, 63 proteins on the yeast protein microarray, respectively. We identified a set of 16 antigenic proteins that were shared across the three fungal pathogens. These include retrotransposon capsid proteins, heat shock proteins, and mitochondrial proteins. Five of these proteins were identified in our previous study of fungal cell wall by mass spectrometry (Ann. N. Y. Acad. Sci. 2012, 1273, 44-51). The results obtained give a comprehensive view of the immunological responses to fungal infections at the proteomic level. They also offer insight into immunoreactive protein commonality among several fungal pathogens and provide a basis for a panfungal vaccine.

Whole glucan particles as a vaccine against systemic coccidioidomycosis

We reported previously that yeast-derived whole glucan particles (WGPs), with or without conjugation to BSA, used as a vaccine protected against systemic aspergillosis in mice. Here, we examined their utility as a potential vaccine against coccidioidomycosis. WGPs were prepared from Saccharomyces cerevisiae; conjugation with BSA (WGP-BSA) was done using 1-cyano-4-dimethylaminopyridinium tetrafluoroborate-mediated conjugation. Heat-killed S. cerevisiae (HKY) was used as a positive-control vaccine. CD-1 mice were vaccinated with WGPs or WGP-BSA, HKY or PBS once weekly, beginning 21 days prior to infection. Mice were infected intravenously with arthroconidia of Coccidioides posadasii. In the low-mortality study, 50 % of PBS-treated controls died. Only WGP-BSA at 0.6 mg per dose induced significant protection compared with PBS treatment. All surviving mice were infected in all three organs examined. Those given WGP-BSA at 0.6 mg per dose had fewer c.f.u. in liver and lungs (P = 0.04), and those given WGPs at 6 mg per dose had fewer in lungs (P < 0.02), compared with PBS. In the high-mortality study, 90 % of PBS mice died. Vaccination with HKY, and WGPs or WGP-BSA at 6 or 12 mg per dose significantly prolonged survival (P ≤ 0.05). No surviving mice were free of infection. HKY and WGP-BSA at 12 mg per dose reduced c.f.u. in the liver and lungs (P < 0.05) and WGP-BSA at 6 mg per dose reduced c.f.u. in the lungs (P < 0.05); unconjugated WGPs did not reduce infection. WGPs or WGP-BSA acted as a vaccine that protected against mortality caused by coccidioidomycosis. Thus, WGP protection against coccidioidomycosis and aspergillosis provides the basis for development of a pan-fungal vaccine.

Saccharomyces cerevisiae expressing Gp43 protects mice against Paracoccidioides brasiliensis infection

The dimorphic fungus Paracoccidioides brasiliensis is the etiological agent of paracoccidioidomycosis (PCM). It is believed that approximately 10 million people are infected with the fungus and approximately 2% will eventually develop the disease. Unlike viral and bacterial diseases, fungal diseases are the ones against which there is no commercially available vaccine. Saccharomyces cerevisiae may be a suitable vehicle for immunization against fungal infections, as they require the stimulation of different arms of the immune response. Here we evaluated the efficacy of immunizing mice against PCM by using S. cerevisiae yeast expressing gp43. When challenged by inoculation of P. brasiliensis yeasts, immunized animals showed a protective profile in three different assays. Their lung parenchyma was significantly preserved, exhibiting fewer granulomas with fewer fungal cells than found in non-immunized mice. Fungal burden was reduced in the lung and spleen of immunized mice, and both organs contained higher levels of IL-12 and IFN-γ compared to those of non-vaccinated mice, a finding that suggests the occurrence of Th1 immunity. Taken together, our results indicate that the recombinant yeast vaccine represents a new strategy to confer protection against PCM.

Fungal vaccines and immunotherapeutics

Concomitant with the increased prevalence of immunocompromised persons, invasive fungal infections have become considerably more frequent in the last 50 years. High mortality rates caused by invasive mycoses and high morbidity because of intractable mucosal infections have created an unmet need for innovative prophylactic and therapeutic strategies against fungal pathogens. Several immunotherapeutics and vaccines are in development to address this need, although one has yet to reach the clinic. This review focuses on past and current immunotherapeutic and vaccine strategies being tested to either prevent or treat fungal infections, as well as the challenges associated with their development.

Whole glucan particles as a vaccine against murine aspergillosis

Vaccination with heat-killed Saccharomyces cerevisiae (HKY) protects against experimental infection by pathogenic fungi of five genera. Here we tested whether purified Saccharomyces cell wall β-glucan could induce protection against systemic aspergillosis. CD-1 mice were given three weekly vaccine doses subcutaneously prior to intravenous infection with Aspergillus fumigatus. Mice received PBS, 2.5 mg HKY, whole glucan particles (WGP), WGP conjugated to BSA (0.06 to 12 mg per dose), a soluble medium molecular mass (MMW) β-glucan alone or MMW-BSA (≤24 mg per dose). Survival and c.f.u. were determined, and cytokine induction and anti-β-glucan antibodies were assessed in vaccinated mice. Neither soluble MMW glucan, nor MMW-BSA was effective. HKY protected in two studies (survival and c.f.u. were reduced in brain and kidney organs, P<0.004). Six or 12 mg WGP or WGP-BSA prolonged survival (P≤0.004) and reduced c.f.u. in each organ (P≤0.015) in both experiments; 0.6 mg WGP or WGP-BSA prolonged survival (P≤0.015) and reduced c.f.u. (P≤0.015) in one experiment. Cytokine profiles in serum and bronchoalveolar lavage from uninfected vaccinated mice showed an innate and adaptive immune profile (i.e. upregulation of colony stimulating factors, interferons, TNF-α, chemokines such as MCP-1, MIP-1α, RANTES and KC, and Th17-activating cytokines such as IL-6, IL-1β, IL-17). No anti-β-glucan antibodies were in the sera, suggesting an adaptive T cell-mediated, not a B cell-mediated, protective response. Vaccination with WGP or WGP-BSA proved protective against systemic aspergillosis, equivalent to that of HKY, supporting the potential of particulate β-glucans, alone or conjugated, as vaccines against aspergillosis.

Killed Saccharomyces cerevisiae protects against lethal challenge of Cryptococcus grubii

Heat-killed Saccharomyces cerevisiae (HKY) vaccination protects mice against aspergillosis, coccidioidomycosis, mucormycosis, or candidiasis. We studied HKY protection against murine cryptococcosis. Once weekly subcutaneous HKY doses (S, 6 × 10(7); 2S, 1.2 × 10(8); 3S, 2.4 × 10(8)) began 28 (×3), 35 (×4), or 42 (×6) days prior to intravenous Cryptococcus grubii infection. Survival through 28 days, and CFU in the organs of survivors, were compared to saline-vaccinated controls. In the initial experiment, S, S×4, or 2S reduced brain CFU; liver or spleen CFU was reduced by S×4 or 2S. In a more lethal second experiment, 2S×6, 2S, or 3S×4 improved survival, and HKY regimens reduced CFU in the brain, liver, or spleen, with 2S×6, 2S, or 3S×4 most efficacious. Dose size appears more important than the number of doses: Regimens >S were superior, and 2S and 2S×6 were equivalent. 2S and 3S were equivalent, suggesting doses >2S do not provide additional protection. HKY protects against Cryptococcus, supporting components of HKY as a basis for the development of a panfungal vaccine.

Heat-killed yeast protects diabetic ketoacidotic-steroid treated mice from pulmonary mucormycosis

Previous studies have shown that vaccination with heat-killed yeast, Saccharomyces cerevisiae (HKY), protects mice against systemic candidiasis, aspergillosis, cryptococcosis or coccidioidomycosis. Here we sought to define the potential use of HKY as a vaccine to protect mice from mucormycosis. Mice were vaccinated with different regimens of HKY prior to induction of diabetes. Diabetic ketoacidotic (DKA) mice were then treated with steroids prior to intratracheal challenge with Rhizopus oryzae. All regimens of HKY vaccine improved survival of DKA mice and reduced fungal burden in the primary target organ, lungs, as determined by qPCR. Furthermore, compared to mice vaccinated with diluent, vaccination with HKY substantially increased the mouse immune response as determined by detection of increased anti-Rhizopus antibody titers. Our results show that HKY protects steroid-treated DKA mice from pulmonary R. oryzae infection. Considering its demonstrated efficacy against other fungal infections, HKY is a promising candidate for development as a panfungal vaccine.

Recent advances in our understanding of the environmental, epidemiological, immunological, and clinical dimensions of coccidioidomycosis

Coccidioidomycosis is the endemic mycosis caused by the fungal pathogens Coccidioides immitis and C. posadasii. This review is a summary of the recent advances that have been made in the understanding of this pathogen, including its mycology, genetics, and niche in the environment. Updates on the epidemiology of the organism emphasize that it is a continuing, significant problem in areas of endemicity. For a variety of reasons, the number of reported coccidioidal infections has increased dramatically over the past decade. While continual improvements in the fields of organ transplantation and management of autoimmune disorders and patients with HIV have led to dilemmas with concurrent infection with coccidioidomycosis, they have also led to advances in the understanding of the human immune response to infection. There have been some advances in therapeutics with the increased use of newer azoles. Lastly, there is an overview of the ongoing search for a preventative vaccine.

Coccidioidomycosis in armadillo hunters from the state of Ceará, Brazil

Coccidioidomycosis is a systemic mycosis with a variable clinical presentation. Misdiagnosis of coccidioidomycosis as bacterial pneumopathy leads to inappropriate prescription of antibiotics and delayed diagnosis. This report describes an outbreak among armadillo hunters in northeastern Brazil in which an initial diagnosis of bacterial pneumonia was later confirmed as coccidioidomycosis caused by Coccidioides posadasii. Thus, this mycosis should be considered as an alternative diagnosis in patients reporting symptoms of pneumonia, even if these symptoms are only presented for a short period, who are from areas considered endemic for this disease.

Protein targets for broad-spectrum mycosis vaccines: quantitative proteomic analysis of Aspergillus and Coccidioides and comparisons with other fungal pathogens

Aspergillus species are responsible for most cases of fatal mold infections in immunocompromised patients, particularly in those receiving hematopoietic stem cell transplants. Experimental vaccines in mouse models have demonstrated a promising avenue of approach for the prevention of aspergillosis, as well as infections caused by other fungal pathogens, such as Coccidioides, the etiological agent of valley fever (coccidioidomycosis). Here, we investigated the hyphal proteomes of Aspergillus fumigatus and Coccidioides posadasii via quantitative MS(E) mass spectrometry with the objective of developing a vaccine that cross-protects against these and other species of fungi. Several homologous proteins with highly conserved sequences were identified and quantified in A. fumigatus and C. posadasii. Many abundant proteins from the cell wall of A. fumigatus present themselves as possible cross-protective vaccine candidates, due to the high degree of sequence homology to other medically relevant fungal proteins and low homologies to human or murine proteins.

Expression and purification of an immunogenic dengue virus epitope using a synthetic consensus sequence of envelope domain III and Saccharomyces cerevisiae

A synthetic consensus gene was designed based on residues of the amino acid sequences of dengue envelope domain III (scEDIII) from all four serotypes, and codon optimization for expression was conducted using baker's yeast, Saccharomyces cerevisiae. The synthetic gene was cloned into a yeast episomal expression vector, pYEGPD-TER, which was designed to direct cloned gene expression using the glyceraldehyde-3-phosphate dehydrogenase (GPD) promoter, a functional signal peptide of the amylase 1A protein from rice, and the GAL7 terminator. PCR and back-transformation into Escherichia coli confirmed the presence of the scEDIII gene-containing plasmid in the transformants. Northern blot analysis showed the presence of the scEDIII-specific transcript. Western blot analysis indicated that expressed scEDIII, with mobility similar to purified EDIII from E. coli, was successfully secreted into the culture media. Quantitative ELISA revealed that the recombinant scEDIII comprised approximately 0.1-0.6% of cell-free extract. In addition, 0.1-0.6 mg of scEDIII protein per liter of culture filtrate was detected on day 1 and peaked on day 3 after cultivation. The secreted scEDIII protein can be purified to ≥90% purity with 85% recovery using a simple ion-exchange FPLC followed by molecular weight cut-off. Upon administration of the purified protein to mice, mouse sera contained antibodies that were specific to all four serotypes of dengue virus. Moreover, a balanced immune response against all four serotypes was observed, suggesting that it may be possible to develop an effective tetravalent dengue vaccine using S. cerevisiae.

Progress Toward a Human Vaccine Against Coccidioidomycosis

Coccidioidomycosis (San Joaquin Valley fever) is a human respiratory disease caused by a soil-borne mold, and is recognized as an intransigent microbial infection by physicians who treat patients with the potentially life-threatening, disseminated form of this mycosis. Epidemiological studies based on surveys of skin-test reactivity of people who reside in the endemic regions of the Southwestern US have shown that at least 150,000 new infections occur annually. The clinical spectrum of coccidioidomycosis ranges from an asymptomatic insult to a severe pulmonary disease in which the pathogen may spread from the lungs to the skin, bones, brain and other body organs. Escalation of symptomatic infections and increased cost of long-term antifungal treatment warrant a concerted effort to develop a vaccine against coccidioidomycosis. This review examines recently reported strategies used to generate such a vaccine and summarizes current understanding of the nature of protective immunity to this formidable disease.

Universal fungal vaccines: could there be light at the end of the tunnel?

The complex nature of fungal pathogens, the intricate host-pathogen relationship and the health status of subjects in need of antifungal vaccination continue to hamper efforts to develop fungal vaccines for clinical use. That said, the rise of the universal vaccine concept is hoped to revive fungal vaccine research by expanding the pool of vaccine candidates worthy of clinical evaluation. It can do so through antigenic commonality-based screening for vaccine candidates from a wide range of pathogens and by reassessing the sizable collection of already available experimental and approved vaccines. Development of experimental vaccines protective against multiple fungal pathogens is evidence of the utility of this concept in fungal vaccine research. However, universal fungal vaccines are not without difficulties; for instance, development of vaccines with differential effectiveness is an issue that should be addressed. Additionally, rationalizing the development of universal fungal vaccines on health or economic basis could be contentious. Herein, universal fungal vaccines are discussed in terms of their potential usefulness and possible drawbacks.

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.

Vaccination with mannan protects mice against systemic aspergillosis

Invasive aspergillosis is a major cause of mortality in immunocompromised patients and therapeutic options are often limited, thus a vaccine would be desirable. We presently studied acid-stable cell-wall mannan (α-1, 6-linked backbone highly branched with α-1, 2; α-1, 3; and β-1, 2-linked manno-oligomers) derived from C. albicans, with or without conjugation to bovine serum albumin (BSA), as a vaccine against systemic aspergillosis. Mice were vaccinated subcutaneously with mannan or mannan-BSA conjugate weekly 3 times, ending 2 weeks prior to infection with A. fumigatus conidia. Results showed that the protection induced by mannan is dose-dependent; 12 mg unconjugated mannan alone or > 0.3 mg mannan-BSA consistently enhanced survival (P < 0.05). Fungal burdens in brains and kidneys were reduced after > 0.3 mg of mannan-BSA (all P < 0.05). Mannan-induced protection was improved about 40-fold by conjugation of BSA to mannan. Mannan-BSA (500 kDa) was more protective than 40 kDa mannan-BSA. Mannan is a candidate for a cross-protective conjugate fungal vaccine.

Fungal cell wall vaccines: an update

This discussion is intended to be an overview of current advances in the development of fungal cell wall vaccines with an emphasis on Candida; it is not a comprehensive historical review of all fungal cell wall vaccines. Selected, more recent, innovative strategies for developing fungal vaccines will be highlighted. Both scientific and logistical obstacles related to the development of, and clinical use of, fungal vaccines will be discussed.

Cross-protective TH1 immunity against Aspergillus fumigatus and Candida albicans

T cell–mediated heterologous immunity to different pathogens is promising for the development of immunotherapeutic strategies. Aspergillus fumigatus and Candida albicans, the 2 most common fungal pathogens causing severe infections in immunocompromised patients, are controlled by CD4+ type 1 helper T (TH1) cells in humans and mice, making induction of fungus-specific CD4+ TH1 immunity an appealing strategy for antifungal therapy. We identified an immunogenic epitope of the A fumigatus cell wall glucanase Crf1 that can be presented by 3 common major histocompatibility complex class II alleles and that induces memory CD4+ TH1 cells with a diverse T-cell receptor repertoire that is cross-reactive to C albicans. In BALB/c mice, the Crf1 protein also elicits cross-protection against lethal infection with C albicans that is mediated by the same epitope as in humans. These data illustrate the existence of T cell–based cross-protection for the 2 distantly related clinically relevant fungal pathogens that may foster the development of immunotherapeutic strategies.

Immune responses induced by heat killed Saccharomyces cerevisiae: a vaccine against fungal infection

Heat-killed Saccharomyces cerevisiae (HKY) used as a vaccine protects mice against systemic aspergillosis and coccidioidomycosis. Little is known about the immune response induced by HKY vaccination, consequently our goal was to do an analysis of HKY-induced immune responses involved in protection. BALB/c mice were vaccinated subcutaneously 3 times with HKY, a protective reagent, and bronchoalveolar lavage fluid, spleen, lymph nodes, and serum collected 2-5 weeks later. Cultured spleen or lymph node cells were stimulated with HKY. Proliferation of HKY-stimulated spleen or lymph node cells was tested by Alamar Blue reduction and flow cytometry. Cytokines from lymphocyte supernatants and antibody to glycans in serum collected from HKY-vaccinated mice were measured by ELISA. The results show that HKY promoted spleen cell and lymph node cell proliferation from HKY-vaccinated mice but not from PBS-vaccinated control mice (all P<0.05). Cytokine measurement showed HKY significantly promoted IFNγ, IL-6 and IL-17A production by spleen cells and lymph node cells (all P<0.05 and P<0.01, respectively). Cytokine production by HKY-stimulated cells from PBS-vaccinated mice was lower than those from HKY-vaccinated (P<0.05). Cytokines in BAL from HKY-vaccinated were higher, 1.7-fold for IFNγ and 2.1-fold for TNFα, than in BAL from PBS-vaccinated. Flow cytometry of lymphocytes from HKY-vaccinated showed 52% of CD3(+) or 56% of CD8(+) cells exhibited cell division after stimulation with HKY, compared to non-stimulated controls (26 or 23%, respectively) or HKY-stimulated cells from PBS-vaccinated (31 or 34%). HKY also induced antibody against Saccharomyces glucan and mannan with titers 4- or 2-fold, respectively, above that in unvaccinated. Taken together, the results suggested that HKY vaccination induces significant and specific Th1 type cellular immune responses and antibodies to glucan and mannan.

Universal vaccines: shifting to one for many

Human vaccines, with their exquisite antigenic specificity, have greatly helped to eliminate or dramatically abate the incidence of a number of historical and current plagues, from smallpox to bacterial meningitis. Nonetheless, as new infectious agents emerge and the number of vaccine-preventable diseases increases, the practice and benefits of single-pathogen- or disease-targeted vaccination may be put at risk by constraints of timely production, formulation complexity, and regulatory hurdles. During the last influenza pandemic, extraordinary efforts by vaccine producers and health authorities have had little or no influence on disease prevention or mitigation. Recent research demonstrating the possibility of protecting against all influenza A virus types or even phylogenetically distant pathogens with vaccines based on highly conserved peptide or saccharide sequences is changing our paradigm. “Universal vaccine” strategies could be particularly advantageous to address protection from antibiotic-resistant bacteria and fungi for which no vaccine is currently available.