Effects of immune suppression in murine models of disseminated Candida glabrata and Candida tropicalis infection and utility of a synthetic peptide vaccine

Liposomal Fba and Met6 peptide vaccination protects mice from disseminated candidiasis

Epitopes from the Candida cell surface proteins Fba and Met6 are putative vaccine targets for invasive candidiasis. Here, we describe a Candida vaccine approach in which short peptides derived from Fba and Met6 are used in spontaneous nanoliposome antigen particle (SNAP) format. SNAP was enabled by the interaction of cobalt porphyrin phospholipid in liposomes with three histidine residues on the N-terminus of synthetic short peptide immunogens from Fba (F-SNAP), Met6 (M-SNAP), or bivalent Fba and Met6 (FM-SNAP). Liposomes were adjuvanted with synthetic monophosphoryl lipid and QS-21. In mice, immunization with F-SNAP, M-SNAP, or FM-SNAP induced antigen-specific IgG responses and mixed Th1/Th2 immunity. The duplex FM-SNAP vaccine elicited stronger antibody responses against each peptide, even at order-of-magnitude lower peptide dosing than a comparable adjuvanted, conjugate vaccine. Enzyme-linked immunosorbent spot analysis revealed the induction of antigen-specific, cytokine-producing T cells. Compared to F-SNAP or M-SNAP, higher production of TNFα, IL-2, and IFNγ was observed with re-stimulation of splenocytes from bivalent FM-SNAP-immunized mice. When vaccinated BALB/c mice were challenged with Candida auris, analysis of the fungal burden in the kidneys showed that SNAP vaccination protected from disseminated candidiasis. In a lethal fungal exposure model in A/J mice, F-SNAP, M-SNAP, and FM-SNAP vaccination protected mice from candidiasis challenge. Together, these results show that further investigation into the SNAP adjuvant platform is warranted using Fba and Met6 epitopes for a pan-Candida peptide vaccine that provides multifaceted protective immune responses.

IMPORTANCE

This study introduces a promising vaccine strategy against invasive candidiasis, a severe fungal infection, by targeting specific peptides on the surface of Candida. Using a novel approach called spontaneous nanoliposome antigen particle (SNAP), we combined peptides from two key Candida proteins, Fba and Met6, into a vaccine. This vaccine induced robust immune responses in mice, including the production of protective antibodies and the activation of immune cells. Importantly, mice vaccinated with SNAP were shielded from disseminated candidiasis in experiments. These findings highlight a potential avenue for developing a broad-spectrum vaccine against Candida infections, which could significantly improve outcomes for patients at risk of these often deadly fungal diseases.

Anti-Candida Antibodies of Patients with Invasive Candidiasis Inhibit Growth, Alter Cell Wall Structure, and Kill Candida albicans In Vitro

Invasive candidiasis (IC), caused by Candida yeasts, particularly Candida albicans, poses a significant threat with high mortality rates. Diagnosis is challenging due to Candida's common presence in human microbiota. To address this, our research group developed an immunofluorescence assay detecting Candida albicans Germ Tube Antibodies (CAGTA) in IC patients. CAGTA, indicative of invasive processes, is associated with a lower mortality rate in ICU patients. Based on this premise, this study aims to provide results regarding the lack of knowledge about the potential activity of CAGTA against invasive infections in humans caused by the fungus Candida albicans. Therefore, in order to characterize the activity of CAGTA produced by patients with IC, we used sera from 29 patients with IC caused by either C. albicans or non-albicans Candida species. Whole serum IgG antibodies were fractionated into anti-blastospores, CAGTA-enriched, and purified CAGTA and the assessments included XTT colorimetric assays for metabolic activity, CFU counts for viability, and microscopy for growth, viability, and morphological analysis. The CAGTA-enriched IgG fraction significantly reduced the metabolic activity and viability of C. albicans compared to anti-blastospores. Purified CAGTA altered germ tube cell wall surfaces, as revealed by electron microscopy, and exhibited fungicidal properties by DiBAC fluorescent staining. In conclusion, antibodies in response to invasive candidiasis have antifungal activity against Candida albicans, influencing metabolic activity, viability, and cell wall structure, leading to cell death. These findings suggest the potential utility of CAGTA as diagnostic markers and support the possibility of developing immunization protocols against Candida infections.

Novel Intravenous Immunoglobulin Therapy for the Prevention and Treatment of Candida auris and Candida albicans Disseminated Candidiasis

Disseminated candidiasis is a life-threatening disease and remains the most common bloodstream infection in hospitalized patients in the United States. Despite the availability of modern antifungal therapy, the crude mortality rate in the last decade has remained unacceptably high. Novel approaches are urgently needed to supplement or replace current antifungal therapies. In our study, we show that human intravenous immunoglobulin (IVIG) can provide protection against Candida auris and Candida albicans disseminated infections in A/J and C57BL/6 mouse models. The protective efficacy of IVIG is evidenced by the prolonged survival of mice with invasive candidiasis that were treated with human IVIG alone or in combination with amphotericin B. Our previous studies have led to the identification of a panel of Candida cell surface peptide and glycan epitopes, which are targeted by protective mouse monoclonal antibodies (mAbs) against invasive candidiasis. Of interest, the peptide- and glycan-specific IgGs could be detected in all 18 human IVIG samples. In particular, the specific IVIG lots with the highest protective peptide- and glycan-related IgGs provided the best protection. The combination of IVIG and amphotericin B had enhanced efficacy in protection compared to monotherapy against both multidrug-resistant (MDR) C. auris and C. albicans, with evidence of significantly prolonged survival and lower fungal burdens in targeted organs. This study provides evidence that the protective effects of IVIG were associated with the protective antibodies found in normal human donor sera against pathogenic Candida, and IVIG can be a novel therapy or adjunctive therapy with modern antifungal drugs against disseminated candidiasis. IMPORTANCE Since current antifungal treatments are ineffective in the immunocompromised population and no vaccine is available for humans, hope remains that antibody preparations selected for specific fungal antigens may make it possible to reduce the incidence and mortality of invasive candidiasis. Intravenous immunoglobulin (IVIG) has long been approved as a standard treatment for patients with immunodeficiency disorders who are also susceptible to fungal infection. IVIG has been widely used as prophylaxis or supplemental treatment for sepsis and septic shock; however, this form of adjunctive therapy lacks convincing data to establish its efficacy. In this study, 18 samples from commercial IVIG preparations were screened and evaluated by enzyme-linked immunosorbent assays (ELISAs); Candida peptide- and glycan-specific IgGs were detected with various titers among all IVIG lots. Importantly, significantly reduced organ fungal burdens and mortality were demonstrated in IVIG-treated mouse models of invasive candidiasis. IVIG lots with higher titers of Candida-specific IgGs provided better protection. These findings are important in (i) selecting Candida-specific IVIG therapy that may overcome several shortcomings of conventional IVIG therapy by targeting specific antigens responsible for disease pathogenesis, (ii) enhancing protective efficacy, and (iii) validating data from our previous studies and those of others showing that antibodies combined with conventional antifungal drugs provided enhanced resistance to disease. To our knowledge, this study is the first to demonstrate that human IVIG samples contain protective IgGs targeting Candida cell surface antigens and can be a novel therapy or adjunctive therapy with modern antifungal drugs against disseminated candidiasis.

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.

Synthetic conjugate peptide Fba-Met6 (MP12) induces complement-mediated resistance against disseminated Candida albicans

The fungal genus Candida includes common commensals of the human mucosal membranes, and the most prevalently isolated species, C. albicans, poses a threat of candidemia and disseminated infection associated with an unacceptably high mortality rate and an immense $4 billion burden (US) yearly. Nevertheless, the demand for a vaccine remains wholly unfulfilled and increasingly pressing. We developed a double-peptide construct that is feasible for use in humans with the intention of preventing morbid infection by targeting epitopes derived from fructose bisphosphate aldolase (Fba) and methionine synthase (Met6) which are expressed on the C. albicans cell surface. To test the applicability of the design, we vaccinated mice via the intramuscular (IM) route with the conjugate denoted Fba-Met6 MP12 and showed that the vaccine enhanced survival against a lethal challenge. Because overall endpoint IgG1 and IgG2a antibody titers were robust and these mouse subclasses are associated with protective functionality, we investigated the potential of Fba and Met6 specific antibodies to facilitate the well-defined anti-Candida response by complement, which opsonizes fungi for degradation by primary effectors. Notably, reductions in the fungal burdens and enhanced survival were both abrogated in MP12-vaccinated mice that were pre-challenge dosed with cobra venom factor (CVF), a complement depleting factor. Altogether, we demonstrated that complement is relevant to MP12-based protection against disseminated C. albicans, delineating that a novel, multivalent targeted vaccine against proteins on the surface of C. albicans can enhance the natural response to infection.

Lanosterol 14α-demethylase (CYP51)/histone deacetylase (HDAC) dual inhibitors for treatment of Candida tropicalis and Cryptococcus neoformans infections

Invasive fungal infections remain a challenge due to lack of effective antifungal agents and serious drug resistance. Discovery of antifungal agents with novel antifungal mechanism is important and urgent. Previously, we designed the first CYP51/HDAC dual inhibitors with potent activity against resistant Candida albicans infections. To better understand the antifungal spectrum and synergistic mechanism, herein new CYP51/HDAC dual inhibitors were designed which showed potent in vitro and in vivo antifungal activity against C. neoformans and C. tropicalis infections. Antifungal mechanism studies revealed that the CYP51/HDAC dual inhibitors acted by inhibiting various virulence factors of C. tropicalis and C. neoformans and down-regulating resistance-associated genes. This study highlights the potential of CYP51/HDAC dual inhibitors as a promising strategy for the discovery of novel broad-spectrum antifungal agents.

Epitope-Based Peptide Vaccine Design against Fructose Bisphosphate Aldolase of Candida glabrata: An Immunoinformatics Approach

Background

Candida glabrata is a human opportunistic pathogen that can cause life-threatening systemic infections. Although there are multiple effective vaccines against fungal infections and some of these vaccines are engaged in different stages of clinical trials, none of them have yet been approved by the FDA.

Aim

Using immunoinformatics approach to predict the most conserved and immunogenic B- and T-cell epitopes from the fructose bisphosphate aldolase (Fba1) protein of C. glabrata. Material and Method. 13 C. glabrata fructose bisphosphate aldolase protein sequences (361 amino acids) were retrieved from NCBI and presented in several tools on the IEDB server for prediction of the most promising epitopes. Homology modeling and molecular docking were performed.

Result

The promising B-cell epitopes were AYFKEH, VDKESLYTK, and HVDKESLYTK, while the promising peptides which have high affinity to MHC I binding were AVHEALAPI, KYFKRMAAM, QTSNGGAAY, RMAAMNQWL, and YFKEHGEPL. Two peptides, LFSSHMLDL and YIRSIAPAY, were noted to have the highest affinity to MHC class II that interact with 9 alleles. The molecular docking revealed that the epitopes QTSNGGAAY and LFSSHMLDL have the lowest binding energy to MHC molecules.

Conclusion

The epitope-based vaccines predicted by using immunoinformatics tools have remarkable advantages over the conventional vaccines in that they are more specific, less time consuming, safe, less allergic, and more antigenic. Further in vivo and in vitro experiments are needed to prove the effectiveness of the best candidate's epitopes (QTSNGGAAY and LFSSHMLDL). To the best of our knowledge, this is the first study that has predicted B- and T-cell epitopes from the Fba1 protein by using in silico tools in order to design an effective epitope-based vaccine against C. glabrata.

A/J mice are more susceptible than C57BL/6 to acetaminophen-induced hepatotoxicity

Acetaminophen (APAP) is commonly used to treat fever and pain. However, when in overdose is the predominant cause of hepatotoxicity. Despite advances in understanding the mechanisms of APAP-induced hepatotoxicity, the management of acute liver failure remains a challenge. Thus, more relevant experimental models are crucial to provide a better understanding of this condition. The aim of this study is to evaluate the effect of APAP-induced hepatotoxicity on A/J mice using C57BL/6 as reference experimental model. Eight- to ten-week-old male A/J and C57BL/6 mice were treated with APAP (300 or 500 mg/kg) by intraperitoneal injection. After 24 h total blood leukocyte counting, plasma levels of alanine amino transferase (ALT) and aspartate amino transferase (AST), histopathological analysis of liver, lung and kidney were evaluated. A/J mice presented reduction in circulating leukocytes concomitant with the increase in plasma levels of ALT and AST, and liver necrosis when treated with 300 and 500 mg/kg of APAP. C57BL/6 mice presented similar results only with 500 mg/kg of APAP. Our results show that A/J mice have a marked susceptibility to the effects of APAP and could be considered as an experimental model to study APAP-induced toxicity.

Chitosan hydrogel loaded with recombinant protein containing epitope C from HSP90 of Candida albicans induces protective immune responses against systemic candidiasis

We reported previously a recombinant protein (rP-HSP90C) containing epitope C from heat shock protein 90 of Candida albicans mediates protective immune responses against systemic candidiasis. However, it exhibits weak immunogenicity. Therefore, we evaluated the potential and mechanisms of thermosensitive chitosan hydrogel (CH-HG) as an adjuvant in rP-HSP90C vaccine. CH-HG synthesized by ionic cross-linking showed buffering capacity and control-released rP-HSP90C in vitro. In comparison to naked rP-HSP90C, CH-HG-loaded rP-HSP90C (CH-HG/rP-HSP90C) not only evoked a long-lasting rP-HSP90C-specific IgG, but also enhanced Th1, Th2, Th17 responses and the ratio of Th1/Th2 in vivo; Meanwhile, CH-HG/rP-HSP90C provoked a stronger CTL response than rP-HSP90C. Notably, CH-HG increased the protective immune responses against systemic candidiasis in rP-HSP90C-immunized mice since CH-HG/rP-HSP90C enhanced the survival rate of infected mice, and diminished the CFUs in kidneys compared to rP-HSP90C, which were similar to that of QuilA. Further in vitro investigation displayed CH-HG upgraded the expressions of costimulators, MHCs and cytokines in BMDCs compared to rP-HSP90C；CH-HG also promoted cellular uptake, endosomal escape and "cross-presentation" of rP-HSP90C. In addition, it recruited immune cells at the injection site. Our study demonstrated that CH-HG can be an efficient adjuvant in fungal vaccines.

Host-pathogen interaction in Candida glabrata infection: current knowledge and implications for antifungal therapy

INTRODUCTION

The opportunistic fungal pathogen Candida glabrata poses a clinical challenge in the successful treatment of invasive Candida infections, owing to its low inherent susceptibility toward azole antifungals and the recent acquisition of coresistance toward azole and echinocandin drugs. Compared to other prevalent Candida bloodstream pathogens, C. glabrata neither exhibits secreted proteolytic activity nor invokes a strong immune response in a variety of host cells and is less virulent. It also does not form true hyphae, and the success of C. glabrata, therefore, as a prevalent human fungal pathogen, appears to be built upon a distinct set of virulence attributes.

AREAS COVERED

The focus of this review is to outline, in brief, the interaction of C. glabrata with the host, deduced from the knowledge gained from different in vitro, ex vivo, and in vivo model systems. In addition, we briefly discuss the current antifungals, antifungal resistance mechanisms, and the development of new antifungal therapies, along with the available information on the host response.

EXPERT OPINION

A detailed understanding of stresses, selection pressures and differential immune responses in the presence and absence of antifungals that C. glabrata encounters in varied niches of the host, is required to design effective antifungal therapy.

Moonlighting Proteins at the Candidal Cell Surface

The cell wall in Candida albicans is not only a tight protective envelope but also a point of contact with the human host that provides a dynamic response to the constantly changing environment in infection niches. Particularly important roles are attributed to proteins exposed at the fungal cell surface. These include proteins that are stably and covalently bound to the cell wall or cell membrane and those that are more loosely attached. Interestingly in this regard, numerous loosely attached proteins belong to the class of “moonlighting proteins” that are originally intracellular and that perform essentially different functions in addition to their primary housekeeping roles. These proteins also demonstrate unpredicted interactions with non-canonical partners at an a priori unexpected extracellular location, achieved via non-classical secretion routes. Acting both individually and collectively, the moonlighting proteins contribute to candidal virulence and pathogenicity through their involvement in mechanisms critical for successful host colonization and infection, such as the adhesion to host cells, interactions with plasma homeostatic proteolytic cascades, responses to stress conditions and molecular mimicry. The documented knowledge of the roles of these proteins in C. albicans pathogenicity has utility for assisting the design of new therapeutic, diagnostic and preventive strategies against candidiasis.

Experimental Mouse Models of Disseminated Candida auris Infection

Disseminated candidiasis is a life-threatening disease and remains the most common bloodstream infection in hospitalized patients in the United States. Despite the availability of modern antifungal therapy, crude mortality in the last decade has remained unacceptably high. In particular, Candida auris is a multidrug-resistant, health care-associated fungal pathogen and has recently emerged as the first fungal pathogen to cause a global public health threat. A reliable animal model for disseminated C. auris candidiasis is therefore needed to study the unique aspects of this little-known host-pathogen interaction. In this study, we established an inbred A/J intravenous model as an appropriate model for human disseminated C. auris infection. We found that C5 deficiency in A/J mice results in a complex phenotype characterized by rapid fungal proliferation in target organs and the development of a unique and rapidly fatal response. In contrast, C57BL/6J mice and mice deficient in neutrophil elastase (NE^-/-) survived high-dose C. auris intravenous challenge, even with cyclophosphamide (CY)-induced immunosuppression. Our study is the first to provide insight into the role of C5 in the host responses to C. auris invasive infection and establishes an inbred A/J mouse model of systemic C. auris infection without CY-induced immunosuppression.IMPORTANCE In the last decade, Candida auris has emerged globally as a multidrug-resistant fungal pathogen. Although C. auris was initially isolated from the external ear canal, it can cause outbreaks of invasive infections with very high mortality and comorbidities. Recent reports highlight the ongoing challenges due to organism misidentification, high rates of multifungal drug resistance, and unacceptably high patient mortality. The assessment of C. auris virulence in a specific genetic deficiency mouse model of invasive C. auris infection in this study contributes to the little knowledge of host defense to C. auris infection, which holds promise as a model for investigating the pathogenesis of C. auris invasive infection, exploring the immune responses elicited by the fungus, evaluating the possible induction of immunity to the infection, and targeting candidates for an antifungal vaccine.

Design of a mimotope-peptide based double epitope vaccine against disseminated candidiasis

Hematogenously disseminated candidiasis in humans is the third leading cause of nosocomial bloodstream infections in the US. There is no FDA approved antifungal vaccine or prophylactic/therapeutic antibody for use in humans. We first reported novel synthetic peptide and glycopeptide vaccines against Candida albicans cell surface epitopes that protect mice against disseminated candidiasis. We showed that antibodies specific for the peptide Fba (derived from C. albicans cell surface protein fructose bisphosphate aldolase) or for C. albicans cell surface glycan epitope β-1, 2-mannotriose [β-(Man)₃]) are both protective. This is an important step forward in vaccine design against disseminated candidiasis in humans. However, given the complexity of oligosaccharide synthesis, in this study we performed a new strategy for use of peptide mimotopes that structurally mimic the protective glycan epitope β-(Man)₃ as surrogate immunogens that substitute for the glycan part of glycopeptide [β-(Man)₃-Fba] vaccine. All five selected mimotopes are immunogenic in mice and three mimotopes were able to induce protection in mice against disseminated candidiasis. Furthermore, immunization with three mimotope-peptide conjugate vaccines was also able to induce specific antibody responses, and importantly, protection against disseminated candidiasis in mice. Therefore, our new design of a mimotope-peptide based double epitope vaccine against candidiasis is a potential vaccine candidate that is economical to produce, highly efficacious and safe for use in humans.