Vaccine progress

Pre-Exposure With Extracellular Vesicles From Aspergillus fumigatus Attenuates Inflammatory Response and Enhances Fungal Clearance in a Murine Model Pulmonary Aspergillosis

Aspergillus fumigatus is a ubiquitous and saprophytic filamentous fungus and the main etiologic agent of aspergillosis. Infections caused by A. fumigatus culminate in a strong inflammatory response that can evolve into respiratory failure and may be lethal in immunocompromised individuals. In the last decades, it has been demonstrated that extracellular vesicles (EVs) elicit a notable biological response in immune cells. EVs carry a variety of biomolecules, therefore are considered potential antigen delivery vehicles. The role of EVs as a strategy for modulating an effective response against infections caused by A. fumigatus remains unexplored. Here we investigate the use of EVs derived from A. fumigatus as an immunization tool to induce a more robust immune response to A. fumigatus pulmonary infection. In order to investigate that, male C57BL/6 mice were immunized with two doses of EVs and infected with A. fumigatus. Pre-exposure of mice to EVs was able to induce the production of specific IgG serum for fungal antigens. Besides that, the immunization with EVs reduced the neutrophilic infiltrate into the alveoli, as well as the extravasation of total proteins and the production of proinflammatory mediators IL-1β, IL-6, and CXCL-1. In addition, immunization prevented extensive lung tissue damage and also improved phagocytosis and fungus clearance. Noteworthy, immunization with EVs, associated with subclinical doses of Amphotericin B (AmB) treatment, rescued 50% of mice infected with A. fumigatus from lethal fungal pneumonia. Therefore, the present study shows a new role for A. fumigatus EVs as host inflammatory response modulators, suggesting their use as immunizing agents.

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.

Fungal vaccines

Invasive fungal disease continues to be a cause of significant life-threatening morbidity and mortality in humans, particularly in those with a diminished immune system, such as with haematological malignancies. The mainstay of treating such life-threatening fungal infection has been antifungal drugs, including azoles, echinocandins and macrocyclic polyenes. However, like antibiotic resistance, antifungal resistance is beginning to emerge, potentially jeopardizing the effectiveness of these molecules in the treatment of fungal disease. One strategy to avoid this is the development of fungal vaccines. However, the inability to provoke a sufficient immune response in the most vulnerable immunocompromised groups has hindered translation from bench to bedside. This review will assess the latest available data and will investigate potential Aspergillus antigens and feasible vaccine techniques, particularly for vaccination of high-risk groups, including immunocompromised and immunosuppressed populations.

Comparative systems analysis of the secretome of the opportunistic pathogen Aspergillus fumigatus and other Aspergillus species

Aspergillus fumigatus and multiple other Aspergillus species cause a wide range of lung infections, collectively termed aspergillosis. Aspergilli are ubiquitous in environment with healthy immune systems routinely eliminating inhaled conidia, however, Aspergilli can become an opportunistic pathogen in immune-compromised patients. The aspergillosis mortality rate and emergence of drug-resistance reveals an urgent need to identify novel targets. Secreted and cell membrane proteins play a critical role in fungal-host interactions and pathogenesis. Using a computational pipeline integrating data from high-throughput experiments and bioinformatic predictions, we have identified secreted and cell membrane proteins in ten Aspergillus species known to cause aspergillosis. Small secreted and effector-like proteins similar to agents of fungal-plant pathogenesis were also identified within each secretome. A comparison with humans revealed that at least 70% of Aspergillus secretomes have no sequence similarity with the human proteome. An analysis of antigenic qualities of Aspergillus proteins revealed that the secretome is significantly more antigenic than cell membrane proteins or the complete proteome. Finally, overlaying an expression dataset, four A. fumigatus proteins upregulated during infection and with available structures, were found to be structurally similar to known drug target proteins in other organisms, and were able to dock in silico with the respective drug.

Linear Epitopes of Paracoccidioides brasiliensis and Other Fungal Agents of Human Systemic Mycoses As Vaccine Candidates

Dimorphic fungi are agents of systemic mycoses associated with significant morbidity and frequent lethality in the Americas. Among the pathogenic species are Paracoccidioides brasiliensis and Paracoccidioides lutzii, which predominate in South America; Histoplasma capsulatum, Coccidioides posadasii, and Coccidioides immitis, and the Sporothrix spp. complex are other important pathogens. Associated with dimorphic fungi other important infections are caused by yeast such as Candida spp. and Cryptococcus spp. or mold such as Aspergillus spp., which are also fungal agents of deadly infections. Nowadays, the actual tendency of therapy is the development of a pan-fungal vaccine. This is, however, not easy because of the complexity of eukaryotic cells and the particularities of different species and isolates. Albeit there are several experimental vaccines being studied, we will focus mainly on peptide vaccines or epitopes of T-cell receptors inducing protective fungal responses. These peptides can be carried by antibody inducing β-(1,3)-glucan oligo or polysaccharides, or be mixed with them for administration. The present review discusses the efficacy of linear peptide epitopes in the context of antifungal immunization and vaccine proposition.

Challenges and opportunities of using liquid chromatography and mass spectrometry methods to develop complex vaccine antigens as pharmaceutical dosage forms

Liquid chromatographic methods, combined with mass spectrometry, offer exciting and important opportunities to better characterize complex vaccine antigens including recombinant proteins, virus-like particles, inactivated viruses, polysaccharides, and protein-polysaccharide conjugates. The current abilities and limitations of these physicochemical methods to complement traditional in vitro and in vivo vaccine potency assays are explored in this review through the use of illustrative case studies. Various applications of these state-of-the art techniques are illustrated that include the analysis of influenza vaccines (inactivated whole virus and recombinant hemagglutinin), virus-like particle vaccines (human papillomavirus and hepatitis B), and polysaccharide linked to protein carrier vaccines (pneumococcal). Examples of utilizing these analytical methods to characterize vaccine antigens in the presence of adjuvants, which are often included to boost immune responses as part of the final vaccine dosage form, are also presented. Some of the challenges of using chromatographic and LC-MS as physicochemical assays to routinely test complex vaccine antigens are also discussed.

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.

Protective Effector Cells of the Recombinant Asp f3 Anti-Aspergillosis Vaccine

An Aspergillus fumigatus vaccine based on recombinant Asp f3-protein has the potential to prevent aspergillosis in humans, a devastating fungal disease that is the prime obstacle to the success of hematopoietic cell transplantation. This vaccine protects cortisone acetate (CA)-immunosuppressed mice from invasive pulmonary aspergillosis via CD4(+) T cell mediators. Aside from these mediators, the nature of downstream fungicidal effectors is not well understood. Neutrophils and macrophages protect immunocompetent individuals from invasive fungal infections, and selective neutrophil depletion rendered mice susceptible to aspergillosis whereas macrophage depletion failed to increase fungal susceptibility. We investigated the effect of neutrophil depletion on rAsp f3-vaccine protection, and explored differences in pathophysiology and susceptibility between CA-immunosuppression and neutrophil depletion. In addition to being protective under CA-immunosuppression, the vaccine also had a protective effect in neutrophil-depleted mice. However, in non-immunized mice, a 10-fold higher conidial dose was required to induce similar susceptibility to infection with neutrophil depletion than with CA-immunosuppression. The lungs of non-immunized neutrophil-depleted mice became invaded by a patchy dense mycelium with highly branched hyphae, and the peribronchial inflammatory infiltrate consisted mainly of CD3(+) T cells and largely lacked macrophages. In contrast, lungs of non-immunized CA-immunosuppressed mice were more evenly scattered with short hyphal elements. With rAsp f3-vaccination, the lungs were largely clear of fungal burden under either immunosuppressive condition. We conclude that neutrophils, although important for innate antifungal protection of immunocompetent hosts, are not the relevant effectors for rAsp f3-vaccine derived protection of immunosuppressed hosts. It is therefore more likely that macrophages represent the crucial effectors of the rAsp f3-based vaccine.

Immunotherapy against invasive mold infections

Invasive infections due to filamentous fungi, such as Aspergillus spp., Zygomycetes, Scedosporium and Fusarium spp., cause significant morbidity and mortality in immunocompromised patients with hematological malignancies, recipients of hematopoietic stem cell transplants and those with chronic granulomatous disease. Despite antifungal therapy, the outcome is often unfavorable in these patients; immune restoration is considered as the cornerstone of successful treatment. Important aspects of human immune response against fungi include effective innate immune response expressed as effective phagocytic functions and a balance between proinflammatory and regulatory adaptive immune responses. A number of immunomodulatory approaches, including the administration of enhancing cytokines, adoptive transfer of pathogen-specific T lymphocytes and granulocyte transfusions have been investigated as adjunctive treatments against serious mold infections. Despite encouraging in vitro and in vivo data, current clinical evidence is not sufficient to allow firm recommendations on the use of these immunomodulatory modalities in serious mold infections.

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.

CD4+ T cells mediate the protective effect of the recombinant Asp f3-based anti-aspergillosis vaccine

The mortality and morbidity caused by invasive aspergillosis present a major obstacle to the successful treatment of blood cancers with hematopoietic cell transplants. Patients who receive hematopoietic cell transplants are usually immunosuppressed for extended periods, and infection with the ubiquitous mold Aspergillus fumigatus is responsible for most cases of aspergillosis. Previously, we demonstrated that vaccination with recombinant forms of the A. fumigatus protein Asp f3 protected cortisone acetate-immunosuppressed mice from experimentally induced pulmonary aspergillosis. Here, we investigated the vaccine's protective mechanism and evaluated in particular the roles of antibodies and T cells. After vaccination, Asp f3-specific preinfection IgG titers did not significantly differ between surviving and nonsurviving mice, and passive transfer of anti-Asp f3 antibodies did not protect immunosuppressed recipients from aspergillosis. We experimentally confirmed Asp f3's predicted peroxisomal localization in A. fumigatus hyphae. We found that fungal Asp f3 is inaccessible to antibodies, unless both cell walls and membranes have been permeabilized. Antibody-induced depletion of CD4+ T cells reduced the survival of recombinant Asp f3 (rAsp f3)-vaccinated mice to nonimmune levels, and transplantation of purified CD4+ T cells from rAsp f3-vaccinated mice into nonimmunized recipients transferred antifungal protection. In addition, residues 60 to 79 and 75 to 94 of Asp f3 contain epitopes that induce proliferation of T cells from vaccinated survivors. Vaccine-primed CD4+ T cells are not expected to clear the fungal pathogen directly; however, they may locally activate immunosuppressed phagocytes that elicit the antifungal effect.