Considerable differences in vaccine immunogenicities and efficacies related to the diluent used for aluminum hydroxide adjuvant

The Road Ahead: Advancing Antifungal Vaccines and Addressing Fungal Infections in the Post-COVID World

In impoverished nations, the COVID-19 pandemic has led to a widespread occurrence of deadly fungal diseases like mucormycosis. The limited availability of effective antifungal treatments and the emergence of drug-resistant fungal strains further exacerbate the situation. Factors such as systemic steroid use, intravenous drug misuse, and overutilization of broad-spectrum antimicrobials contribute to the prevalence of hospital-acquired infections caused by drug-resistant fungi. Fungal infections exploit compromised immune status and employ intricate mechanisms to evade immune surveillance. The immune response involves the innate and adaptive immune systems, leading to phagocytic and complement-mediated elimination of fungi. However, resistance to antifungals poses a challenge, highlighting the importance of antifungal prophylaxis and therapeutic vaccination. Understanding the host-fungal immunological interactions and developing vaccines are vital in combating fungal infections. Further research is needed to address the high mortality and morbidity associated with multidrug-resistant fungal pathogens and to develop innovative treatment drugs and vaccines. This review focuses on the global epidemiological burden of fungal infections, host-fungal immunological interactions, recent advancements in vaccine development and the road ahead.

Promising immunotherapeutic targets for treating candidiasis

In the last twenty years, there has been a significant increase in invasive fungal infections, which has corresponded with the expanding population of individuals with compromised immune systems. As a result, the mortality rate linked to these infections remains unacceptably high. The currently available antifungal drugs, such as azoles, polyenes, and echinocandins, face limitations in terms of their diversity, the escalating resistance of fungi and the occurrence of significant adverse effects. Consequently, there is an urgent need to develop new antifungal medications. Vaccines and antibodies present a promising avenue for addressing fungal infections due to their targeted antifungal properties and ability to modulate the immune response. This review investigates the structure and function of cell wall proteins, secreted proteins, and functional proteins within C. albicans. Furthermore, it seeks to analyze the current advancements and challenges in macromolecular drugs to identify new targets for the effective management of candidiasis.

Nanoalum adjuvanted vaccines: small details make a big difference

Purified vaccine antigens offer important safety and reactogenicity advantages compared with live attenuated or whole killed virus and bacterial vaccines. However, they require the addition of adjuvants to induce the magnitude, duration and quality of immune response required to achieve protective immunity. Aluminium salts have been used as adjuvants in vaccines for almost a century. In the literature, they are often referred to as aluminium-based adjuvants (ABAs), or aluminium salt-containing adjuvants or more simply "alum". All these terms are used to group aluminium suspensions that are very different in terms of atomic composition, size, and shape. They differ also in stability, antigen-adsorption, and antigen-release kinetics. Critically, these parameters also have a profound effect on the character and magnitude of the immune response elicited. Recent findings suggest that, by reducing the size of aluminium from micro to nanometers, a more effective adjuvant is obtained, together with the ability to sterile filter the vaccine product. However, the behaviour of aluminium nanoparticles in vaccine formulations is different from microparticles, requiring specific formulation strategies, as well as a more detailed understanding of how formulation influences the immune response generated. Here we review the current state of art of aluminium nanoparticles as adjuvants, with a focus on their immunobiology, preparation methods, formulation optimisation and stabilisation.

The Role of B-Cells and Antibodies against Candida Vaccine Antigens in Invasive Candidiasis

Systemic candidiasis is an invasive fungal infection caused by members of the genus Candida. The recent emergence of antifungal drug resistance and increased incidences of infections caused by non-albicans Candida species merit the need for developing immune therapies against Candida infections. Although the role of cellular immune responses in anti-Candida immunity is well established, less is known about the role of humoral immunity against systemic candidiasis. This review summarizes currently available information on humoral immune responses induced by several promising Candida vaccine candidates, which have been identified in the past few decades. The protective antibody and B-cell responses generated by polysaccharide antigens such as mannan, β-glucan, and laminarin, as well as protein antigens like agglutinin-like sequence gene (Als3), secreted aspartyl proteinase (Sap2), heat shock protein (Hsp90), hyphally-regulated protein (Hyr1), hyphal wall protein (Hwp1), enolase (Eno), phospholipase (PLB), pyruvate kinase (Pk), fructose bisphosphate aldolase (Fba1), superoxide dismutase gene (Sod5) and malate dehydrogenase (Mdh1), are outlined. As per studies reviewed, antibodies induced in response to leading Candida vaccine candidates contribute to protection against systemic candidiasis by utilizing a variety of mechanisms such as opsonization, complement fixation, neutralization, biofilm inhibition, direct candidacidal activity, etc. The contributions of B-cells in controlling fungal infections are also discussed. Promising results using anti-Candida monoclonal antibodies for passive antibody therapy reinforces the need for developing antibody-based therapeutics including anti-idiotypic antibodies, single-chain variable fragments, peptide mimotopes, and antibody-derived peptides. Future research involving combinatorial immunotherapies using humanized monoclonal antibodies along with antifungal drugs/cytokines may prove beneficial for treating invasive fungal infections.

Advances in Fungal Peptide Vaccines

Vaccination is one of the greatest public health achievements in the past century, protecting and improving the quality of life of the population worldwide. However, a safe and effective vaccine for therapeutic or prophylactic treatment of fungal infections is not yet available. The lack of a vaccine for fungi is a problem of increasing importance as the incidence of diverse species, including Paracoccidioides, Aspergillus, Candida, Sporothrix, and Coccidioides, has increased in recent decades and new drug-resistant pathogenic fungi are emerging. In fact, our antifungal armamentarium too frequently fails to effectively control or cure mycoses, leading to high rates of mortality and morbidity. With this in mind, many groups are working towards identifying effective and safe vaccines for fungal pathogens, with a particular focus of generating vaccines that will work in individuals with compromised immunity who bear the major burden of infections from these microbes. In this review, we detail advances in the development of vaccines for pathogenic fungi, and highlight new methodologies using immunoproteomic techniques and bioinformatic tools that have led to new vaccine formulations, like peptide-based vaccines.

NDV-3A vaccination prevents C. albicans colonization of jugular vein catheters in mice

NDV-3A, a novel fungal vaccine undergoing clinical trials, contains a recombinant version of the Candida albicans rAls3 N-terminus protein (rAls3p-N) in aluminum hydroxide. In a Phase 1b/2a clinical trial, NDV-3A protected women from recurrent vulvovaginal candidiasis. Here, we reveal that active immunization in mice with NDV-3A induces high titers of anti-rAls3p-N antibodies that interfere with C. albicans ability to adhere to and invade endothelial cells, and form biofilm in vitro. Anti-rAls3p-N antibodies also significantly inhibit yeast dispersal from the hyphal layers of biofilms. Compared to placebo, NDV-3A vaccination inhibited C. albicans dissemination to kidneys and prevented colonization of central venous catheters in mice. Overall, these preclinical studies suggest that NDV-3A may serve as an immunotherapeutic strategy for prevention of infections on indwelling medical devices.

Unraveling the enigma: elucidating the relationship between the physicochemical properties of aluminium-based adjuvants and their immunological mechanisms of action

Aluminium salts are by far the most commonly used adjuvants in vaccines. There are only two aluminium salts which are used in clinically-approved vaccines, Alhydrogel® and AdjuPhos®, while the novel aluminium adjuvant used in Gardasil® is a sulphated version of the latter. We have investigated the physicochemical properties of these two aluminium adjuvants and specifically in milieus approximating to both vaccine vehicles and the composition of injection sites. Additionally we have used a monocytic cell line to establish the relationship between their physicochemical properties and their internalisation and cytotoxicity. We emphasise that aluminium adjuvants used in clinically approved vaccines are chemically and biologically dissimilar with concomitantly potentially distinct roles in vaccine-related adverse events.

Human Anti-Als3p Antibodies Are Surrogate Markers of NDV-3A Vaccine Efficacy Against Recurrent Vulvovaginal Candidiasis

A Phase 1b/2a clinical trial of NDV-3A vaccine containing a Candida albicans recombinant Als3 protein formulated with alum protected women <40 years old from recurrent vulvovaginal candidiasis (RVVC). We investigated the potential use of anti-Als3p sera as surrogate marker of NDV-3A efficacy. Pre- and post-vaccination sera from subjects who experienced recurrence of vulvovaginal candidiasis (R) vs. those who were recurrence-free [non-recurrent (NR)] were evaluated. Anti-Als3p antisera obtained were evaluated for (1) titer and subclass profile and (2) their ability to influence C. albicans virulence traits including hyphal elongation, adherence to plastic, invasion of vaginal epithelial cells, biofilm formation on plastic and catheter material, and susceptibility to neutrophil killing in vitro. Serum IgG titers in NR patients were consistently higher than in R patients, particularly for anti-Als3 subclass IgG2. Sera from vaccinated NR patients reduced hyphal elongation, adhesion to plastic, invasion of vaginal epithelial cells, and biofilm formation significantly more than pre-immune sera, or sera from R- or placebo-group subjects. Pre-adsorption of sera with C. albicans germ tubes eliminated these effects, while heat inactivation did not. Finally, sera from NR subjects enhanced neutrophil-mediated killing of C. albicans relative to pre-immune sera or sera from R patients. Our results suggest that higher Als3p antibody titers are associated with protection from RVVC, attenuate C. albicans virulence, and augment immune clearance of the fungus in vitro. Thus, Als3p serum IgG antibodies are likely useful markers of efficacy in RVVC patients vaccinated with NDV-3A.

Molecular attributes of conjugate antigen influence function of antibodies induced by anti-nicotine vaccine in mice and non-human primates

Anti-nicotine vaccines aim to prevent nicotine entering the brain, and thus reduce or eliminate the reward that drives nicotine addiction. Those tested in humans to date have failed to improve quit rates over placebo, possibly because antibody (Ab) responses were insufficient to sequester enough nicotine in the blood in the majority of subjects. We have previously shown in mice that the carrier, hapten and linker used in the nicotine conjugate antigen each influence the function (nicotine-binding capacity) of the Ab induced. Herein we have evaluated immunogenicity in mice of 27 lots of NIC7-CRM, a conjugate of 5-aminoethoxy-nicotine (Hapten 7) and a mutant nontoxic form of diphtheria toxin (CRM197), that differed in three antigen attributes, namely hapten load (number of haptens conjugated to each molecule of CRM197), degree of conjugate aggregation and presence of adducts (small molecules attached to CRM197 via a covalent bond during the conjugation process). A range of functional responses (reduced nicotine in the brain of immunized animals relative to non-immunized controls) were obtained with the different conjugates, which were adjuvanted with aluminum hydroxide and CpG TLR9 agonist. Trends for better functional responses in mice were obtained with conjugates having a hapten load of 11 to 18, a low level of high molecular mass species (HMMS) (i.e., not aggregated) and a low level of adducts and a more limited testing in cynomolgus monkeys confirmed these results. Thus hapten load, conjugate aggregation and presence of adducts are key antigen attributes that can influence Ab function induced by NIC7-CRM.

Increasing the potency of an alhydrogel-formulated anthrax vaccine by minimizing antigen-adjuvant interactions

Aluminum salts are the most widely used vaccine adjuvants, and phosphate is known to modulate antigen-adjuvant interactions. Here we report an unexpected role for phosphate buffer in an anthrax vaccine (SparVax) containing recombinant protective antigen (rPA) and aluminum oxyhydroxide (AlOH) adjuvant (Alhydrogel). Phosphate ions bind to AlOH to produce an aluminum phosphate surface with a reduced rPA adsorption coefficient and binding capacity. However, these effects continued to increase as the free phosphate concentration increased, and the binding of rPA changed from endothermic to exothermic. Crucially, phosphate restored the thermostability of bound rPA so that it resembled the soluble form, even though it remained tightly bound to the surface. Batches of vaccine with either 0.25 mM (subsaturated) or 4 mM (saturated) phosphate were tested in a disease model at batch release, which showed that the latter was significantly more potent. Both formulations retained their potency for 3 years. The strongest aluminum adjuvant effects are thus likely to be via weakly attached or easily released native-state antigen proteins.

Vaccines for invasive fungal infections

Morbidity and mortality from invasive fungal infections remain unacceptably high despite availability of new antifungal agents, underscoring the need for more effective preventative strategies. Due to our enhanced understanding of the host defense and pathogenetic mechanisms that lead to invasive fungal infections, it should be feasible to develop vaccines targeting these infections. A common immunological theme across many vaccine candidates for invasive fungal infections has been the need to activate a cell-based, pro-inflammatory, Th1 or Th17 immune response to improve phagocytic killing of the fungi. Since neutralization of virulence factor functions has not been required for many active vaccines to function, the antigenic repertoire available for testing should not be limited to virulence factors. With expansion of our fundamental understanding of the immunology of fungal infections, the biggest barrier to development of fungal vaccines is the lack of available capital to translate discoveries made at the bench into biological agents used at the bedside. Continued education on the importance and feasibility of vaccination for such infections, combined with continued development of vaccine antigens and adjuvants, is necessary.

Vaccines with aluminum-containing adjuvants: optimizing vaccine efficacy and thermal stability

Aluminum-containing adjuvants have been used to enhance the immune response against killed, inactivated, and subunit antigens for more than seven decades. Nevertheless, we are only beginning to gain important insight as to what may be some very fundamental parameters for optimizing their use. For example, there is evidence that the conventional approach of maximizing antigen binding (amount and/or strength) may not result in an optimal immune response. Adsorption of antigen onto the adjuvant has recently been suggested to decrease the thermal stability of some antigens; however, whether adsorption-induced alterations to the structure and/or stability of the antigen have consequences for the elicited immune response is unclear. Finally, the thermal stability of vaccines with aluminum-containing adjuvants is not robust. Optimizing the stability of these vaccines requires an understanding of the freeze sensitivity of the adjuvant, freeze and heat sensitivity of the antigen in the presence of the adjuvant, and perhaps most important, how (or whether) various approaches to formulation can be used to address these instabilities. This review attempts to summarize recent findings regarding issues that may dictate the success of vaccines with aluminum-containing adjuvants.

Immunological reactivity of blood from healthy humans to the rAls3p-N vaccine protein

We determined reactivity of human blood to a vaccine based on the recombinant N-terminus of candidal Als3p (rAls3p-N) in preparation for future clinical trials. Healthy donor plasma had high immunoglobulin G titers (median, 1:51,200) and lower immunoglobulin A (median, 1:3,200) and immunoglobulin E (median, 1:128) titers to rAls3p-N by enzyme-linked immunosorbent assay. rAls3p-N stimulated interferon gamma (IFN-gamma) and interleukin (IL)-17, but not IL-4, from donor lymphocytes by enzyme-linked immunosorbent spot assay and IL-12 p70, IFN-gamma, IL-17, and IL-10 by cytometric bead array. Donors reacted to diverse immunodominant epitopes. Thus, facile humoral and cellular assays can monitor immune responses to the rAls3p-N vaccine in planned clinical trials.

Th1-Th17 cells mediate protective adaptive immunity against Staphylococcus aureus and Candida albicans infection in mice

We sought to define protective mechanisms of immunity to Staphylococcus aureus and Candida albicans bloodstream infections in mice immunized with the recombinant N-terminus of Als3p (rAls3p-N) vaccine plus aluminum hydroxide (Al(OH₃) adjuvant, or adjuvant controls. Deficiency of IFN-γ but not IL-17A enhanced susceptibility of control mice to both infections. However, vaccine-induced protective immunity against both infections required CD4+ T-cell-derived IFN-γ and IL-17A, and functional phagocytic effectors. Vaccination primed Th1, Th17, and Th1/17 lymphocytes, which produced pro-inflammatory cytokines that enhanced phagocytic killing of both organisms. Vaccinated, infected mice had increased IFN-γ, IL-17, and KC, increased neutrophil influx, and decreased organism burden in tissues. In summary, rAls3p-N vaccination induced a Th1/Th17 response, resulting in recruitment and activation of phagocytes at sites of infection, and more effective clearance of S. aureus and C. albicans from tissues. Thus, vaccine-mediated adaptive immunity can protect against both infections by targeting microbes for destruction by innate effectors.

The bacterium Staphylococcus aureus and the fungus Candida are the second and third leading cause of bloodstream infections in hospitalized patients. A vaccine to prevent such infections would be of enormous public health benefit. The leading hypothesis to explain why vaccines have not been successfully developed against these infections is that the microbes causing the infections are highly complex, and use multiple weapons (so-called “virulence factors”) to cause disease in humans. Therefore, a vaccine targeting either infection would have to neutralize many of these virulence factors at the same time. We have been developing a vaccine that simultaneously targets both types of infections. Our vaccine is based on a single virulence factor used by Candida, which has a similar shape to virulence factors used by S. aureus. In the current study, we report that our vaccine induces specialized cells in the immune system to more effectively call in reinforcements to kill the organisms. These data demonstrate that vaccines against both organisms can be developed even if they do not work by neutralizing multiple virulence factors, and therefore open the door to a far wider array of vaccine types against both infections.