Natural and Synthetic Saponins as Vaccine Adjuvants

Saponin TQL1055 adjuvant-containing vaccine confers protection upon Mycobacterium tuberculosis challenge in mice

Tuberculosis (TB), caused by the intracellular pathogen Mycobacterium tuberculosis (Mtb), affects the lungs of infected individuals (pulmonary TB) but can also affect other sites (extrapulmonary TB). The only licensed vaccine Mycobacterium bovis bacillus Calmette-Guerin (BCG) protects infants and young children but exhibits variable efficacy in protecting against adult pulmonary TB. Poor compliance and prolonged treatment regimens associated with the use of chemotherapy has contributed to the development of multidrug-resistant (MDR) and extensively drug-resistant (XDR) Mtb. Thus, there is an urgent need for the design of more effective vaccines against TB. The development of safe and novel adjuvants for human use is critical. In this study, we demonstrate that saponin-based TQL1055 adjuvant when formulated with a TLR4 agonist (PHAD) and Mtb specific immunodominant antigens (ESAT-6 and Ag85B) and delivered intramuscularly in mice, the SA-TB vaccine induced potent lung immune responses. Additionally, the SA-TB vaccine conferred significant protection against Mtb infection, comparable with levels induced by BCG. These findings support the development of a SA-TB vaccine comprising TQL1055, as a novel, safe and effective TB vaccine for potential use in humans.

Comparison of the Immunogenicity of the LZ901 Vaccine and HZ/su Vaccine in a Mouse Model

Herpes zoster (HZ) is an infectious disease caused by the reactivation of varicella zoster virus (VZV), with 68% of cases occurring in adults over 50 years of age. HZ/su (Shingrix®) was approved by the Food and Drug Administration in 2017 for the prevention of HZ in individuals ≥ 50 years of age and showed very good protection from HZ. However, due to the use of the adjuvant AS01B, adverse reactions caused by Shingrix are a concern. Aluminum hydroxide is the most commonly used adjuvant and is widely used in a variety of vaccines. We developed a recombinant zoster vaccine (code: LZ901) consisting of a tetramer of VZV glycoprotein E (gE) and a human Fc fusion protein expressed in CHO cells, an immune complex-like molecule that can be adsorbed with an aluminum hydroxide adjuvant. We compared the immunogenicity of LZ901 with that of HZ/su in BALB/c mice. The results showed that LZ901 induced levels of gE-specific IgG antibodies comparable to those induced by HZ/su, and the results of FAMA titers further demonstrated their similar neutralizing antibody abilities. Most importantly, LZ901 induced higher levels of cell-mediated immunity (CMI) (which plays a decisive role in the efficacy of zoster vaccines) than HZ/su in BALB/c mice. The numbers of cytokine-producing T cells in LZ901-vaccinated mice were significantly greater than those in v-vaccinated mice, and the proportions of CD4+ and CD8+ T cells producing at least two types of cytokines in LZ901-vaccinated mice were significantly greater than those in HZ/su-vaccinated mice.

Application of plant-derived products as adjuvants for immune activation and vaccine development

Vaccines are one of the most important means to prevent and control the epidemic of infectious diseases. Commercial vaccines not only include corresponding antigens, but also need vaccine adjuvants. Immune adjuvants play an increasingly important role in the research, development and manufacture of vaccines. Adjuvants combined with antigens can improve the stability, safety and immune efficiency of vaccines. Some substances that can enhance the immune response have been found in nature(mainly plants) and used as adjuvants in vaccines to improve the immune effect of vaccines. These plant-derived immune adjuvants often have the advantages of low toxicity, high stability, low price, etc., providing more possibilities for vaccine development. We summarized and analyzed the advantages, application research, particulate delivery systems, existing problems and future research focus of botanical adjuvant. It is hoped to provide new ideas for the research and development of immune adjuvants in the future.

Current Progress in the Science of Novel Adjuvant Nano-Vaccine-Induced Protective Immune Responses

Vaccinations are vital as they protect us from various illness-causing agents. Despite all the advancements in vaccine-related research, developing improved and safer vaccines against devastating infectious diseases including Ebola, tuberculosis and acquired immune deficiency syndrome (AIDS) remains a significant challenge. In addition, some of the current human vaccines can cause adverse reactions in some individuals, which limits their use for massive vaccination program. Therefore, it is necessary to design optimal vaccine candidates that can elicit appropriate immune responses but do not induce side effects. Subunit vaccines are relatively safe for the vaccination of humans, but they are unable to trigger an optimal protective immune response without an adjuvant. Although different types of adjuvants have been used for the formulation of vaccines to fight pathogens that have high antigenic diversity, due to the toxicity and safety issues associated with human-specific adjuvants, there are only a few adjuvants that have been approved for the formulation of human vaccines. Recently, nanoparticles (NPs) have gain specific attention and are commonly used as adjuvants for vaccine development as well as for drug delivery due to their excellent immune modulation properties. This review will focus on the current state of adjuvants in vaccine development, the mechanisms of human-compatible adjuvants and future research directions. We hope this review will provide valuable information to discovery novel adjuvants and drug delivery systems for developing novel vaccines and treatments.

The predominant Quillaja Saponaria fraction, QS-18, is safe and effective when formulated in a liposomal murine cancer peptide vaccine

Extracts of the Chilean soapbark tree, Quillaja Saponaria (QS) are the source of potent immune-stimulatory saponin compounds. This study compared the adjuvanticity and toxicity of QS-18 and QS-21, assessing the potential to substitute QS-18 in place of QS-21 for vaccine development. QS-18, the most abundant QS saponin fraction, has been largely overlooked due to safety concerns. We found that QS-18 spontaneously inserted into liposomes, thereby neutralizing hemolytic activity, and following administration did not induce local reactogenicity in a footpad swelling test in mice. With high-dose intramuscular administration, transient weight loss was minor, and QS-18 did not induce significantly more weight loss compared to a liposome vaccine adjuvant system lacking it. Two days after administration, no elevation of inflammatory cytokines was detected in murine serum. In a formulation including cobalt-porphyrin-phospholipid (CoPoP) for short peptide sequestration, QS-18 did not impact the formation of peptide nanoparticles. With immunization, QS-18 peptide particles induced higher levels of cancer neoepitope-specific and tumor-associated antigen-specific CD8+ T cells compared to QS-21 particles, without indication of greater toxicity based on mouse body weight. T cell receptor sequencing of antigen-specific CD8+ T cells showed that QS-18 induced significantly more T cell transcripts. In two murine cancer models, vaccination with QS-18 peptide particles induced a similar therapeutic effect as QS-21 particles, without indication of increased toxicity. Antigen-specific CD8+ T cells in the tumor microenvironment were found to express the exhaustion marker PD-1, pointing to the rationale for exploring combination therapy. Taken together, these data demonstrate that QS-18, when formulated in liposomes, can be a safe and effective adjuvant to induce tumor-inhibiting cellular responses in murine models with potential to facilitate or diminish costs of production for vaccine adjuvant systems. Further studies are warranted to assess liposomal QS-18 immunogic, reactogenic and toxicological profiles in mice and other animal species.

Vaccine adjuvants: current status, research and development, licensing, and future opportunities

Vaccines represent one of the most significant inventions in human history and have revolutionized global health. Generally, a vaccine functions by triggering the innate immune response and stimulating antigen-presenting cells, leading to a defensive adaptive immune response against a specific pathogen's antigen. As a key element, adjuvants are chemical materials often employed as additives to increase a vaccine's efficacy and immunogenicity. For over 90 years, adjuvants have been essential components in many human vaccines, improving their efficacy by enhancing, modulating, and prolonging the immune response. Here, we provide a timely and comprehensive review of the historical development and the current status of adjuvants, covering their classification, mechanisms of action, and roles in different vaccines. Additionally, we perform systematic analysis of the current licensing processes and highlights notable examples from clinical trials involving vaccine adjuvants. Looking ahead, we anticipate future trends in the field, including the development of new adjuvant formulations, the creation of innovative adjuvants, and their integration into the broader scope of systems vaccinology and vaccine delivery. The article posits that a deeper understanding of biochemistry, materials science, and vaccine immunology is crucial for advancing vaccine technology. Such advancements are expected to lead to the future development of more effective vaccines, capable of combating emerging infectious diseases and enhancing public health.

Chemical and biological characterization of vaccine adjuvant QS-21 produced via plant cell culture

Many vaccines, including those using recombinant antigen subunits, rely on adjuvant(s) to enhance the efficacy of the host immune responses. Among the few adjuvants clinically approved, QS-21, a saponin-based immunomodulatory molecule isolated from the tree bark of Quillaja saponaria (QS) is used in complex formulations in approved effective vaccines. High demand of the QS raw material as well as manufacturing scalability limitation has been barriers here. We report for the first-time successful plant cell culture production of QS-21 having structural, chemical, and biologic, properties similar to the bark extracted product. These data ensure QS-21 and related saponins are broadly available and accessible to drug developers.

BODIPY-Based Analogue of the TREM2-Binding Molecular Adjuvant Sulfavant A, a Chemical Tool for Imaging and Tracking Biological Systems

Recently, we described synthetic sulfolipids named Sulfavants as a novel class of molecular adjuvants based on the sulfoquinovosyl-diacylglycerol skeleton. The members of this family, Sulfavant A (1), Sulfavant R (2), and Sulfavant S (3), showed important effects on triggering receptor expressed on myeloid cells 2 (TREM2)-induced differentiation and maturation of human dendritic cells (hDC), through a novel cell mechanism underlying the regulation of the immune response. As these molecules are involved in biological TREM2-mediated processes crucial for cell survival, here, we report the synthesis and application of a fluorescent analogue of Sulfavant A bearing the 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene moiety (Me4-BODIPY). The fluorescent derivative, named PB-SULF A (4), preserving the biological activity of Sulfavants, opens the way to chemical biology and cell biology experiments to better understand the interactions with cellular and in vivo organ targets and to improve our comprehension of complex molecular mechanisms underlying the not fully understood ligand-induced TREM2 activity.

Vaccine Strategies to Elicit Mucosal Immunity

Vaccines are essential tools to prevent infection and control transmission of infectious diseases that threaten public health. Most infectious agents enter their hosts across mucosal surfaces, which make up key first lines of host defense against pathogens. Mucosal immune responses play critical roles in host immune defense to provide durable and better recall responses. Substantial attention has been focused on developing effective mucosal vaccines to elicit robust localized and systemic immune responses by administration via mucosal routes. Mucosal vaccines that elicit effective immune responses yield protection superior to parenterally delivered vaccines. Beyond their valuable immunogenicity, mucosal vaccines can be less expensive and easier to administer without a need for injection materials and more highly trained personnel. However, developing effective mucosal vaccines faces many challenges, and much effort has been directed at their development. In this article, we review the history of mucosal vaccine development and present an overview of mucosal compartment biology and the roles that mucosal immunity plays in defending against infection, knowledge that has helped inform mucosal vaccine development. We explore new progress in mucosal vaccine design and optimization and novel approaches created to improve the efficacy and safety of mucosal vaccines.

Enhancing breadth and durability of humoral immune responses in non-human primates with an adjuvanted group 1 influenza hemagglutinin stem antigen

Seasonal influenza vaccines must be updated annually and suboptimally protect against strains mismatched to the selected vaccine strains. We previously developed a subunit vaccine antigen consisting of a stabilized trimeric influenza A group 1 hemagglutinin (H1) stem protein that elicits broadly neutralizing antibodies. Here, we further optimized the stability and manufacturability of the H1 stem antigen (H1 stem v2, also known as INFLUENZA G1 mHA) and characterized its formulation and potency with different adjuvants in vitro and in animal models. The recombinant H1 stem antigen (50 µg) was administered to influenza-naïve non-human primates either with aluminum hydroxide [Al(OH)3] + NaCl, AS01B, or SLA-LSQ formulations at week 0, 8 and 34. These SLA-LSQ formulations comprised of varying ratios of the synthetic TLR4 agonist 'second generation synthetic lipid adjuvant' (SLA) with liposomal QS-21 (LSQ). A vaccine formulation with aluminum hydroxide or SLA-LSQ (starting at a 10:25 µg ratio) induced HA-specific antibodies and breadth of neutralization against a panel of influenza A group 1 pseudoviruses, comparable with vaccine formulated with AS01B, four weeks after the second immunization. A formulation with SLA-LSQ in a 5:2 μg ratio contained larger fused or aggregated liposomes and induced significantly lower humoral responses. Broadly HA stem-binding antibodies were detectable for the entire period after the second vaccine dose up to week 34, after which they were boosted by a third vaccine dose. These findings inform about potential adjuvant formulations in clinical trials with an H1 stem-based vaccine candidate.

Bestatin, A Pluripotent Immunomodulatory Small Molecule, Drives Robust and Long-Lasting Immune Responses as an Adjuvant in Viral Vaccines

An inactivated whole-virus vaccine is currently used to prevent foot-and-mouth disease (FMD). Although this vaccine is effective, it offers short-term immunity that requires regular booster immunizations and has several side effects, including local reactions at the vaccination site. To address these limitations, herein, we evaluated the efficacy of bestatin as a novel small molecule adjuvant for inactivated FMD vaccines. Our findings showed that the FMD vaccine formulated with bestatin enhanced early, intermediate-, and particularly long-term immunity in experimental animals (mice) and target animals (pigs). Furthermore, cytokines (interferon (IFN)α, IFNβ, IFNγ, and interleukin (IL)-29), retinoic acid-inducible gene (RIG)-I, and T-cell and B-cell core receptors (cluster of differentiation (CD)28, CD19, CD21, and CD81) markedly increased in the group that received the FMD vaccine adjuvanted with bestatin in pigs compared with the control. These results indicate the significant potential of bestatin to improve the efficacy of inactivated FMD vaccines in terms of immunomodulatory function for the simultaneous induction of potent cellular and humoral immune response and a long-lasting memory response.

Deciphering triterpenoid saponin biosynthesis by leveraging transcriptome response to methyl jasmonate elicitation in Saponaria vaccaria

Methyl jasmonate (MeJA) is a known elicitor of plant specialized metabolism, including triterpenoid saponins. Saponaria vaccaria is an annual herb used in traditional Chinese medicine, containing large quantities of oleanane-type triterpenoid saponins with anticancer properties and structural similarities to the vaccine adjuvant QS-21. Leveraging the MeJA-elicited saponin biosynthesis, we identify multiple enzymes catalyzing the oxidation and glycosylation of triterpenoids in S. vaccaria. This exploration is aided by Pacbio full-length transcriptome sequencing and gene expression analysis. A cellulose synthase-like enzyme can not only glucuronidate triterpenoid aglycones but also alter the product profile of a cytochrome P450 monooxygenase via preference for the aldehyde intermediate. Furthermore, the discovery of a UDP-glucose 4,6-dehydratase and a UDP-4-keto-6-deoxy-glucose reductase reveals the biosynthetic pathway for the rare nucleotide sugar UDP-D-fucose, a likely sugar donor for fucosylation of plant natural products. Our work enables the production and optimization of high-value saponins in microorganisms and plants through synthetic biology approaches.

A Brighton Collaboration standardized template with key considerations for a benefit/risk assessment for the Novavax COVID-19 Vaccine (NVX-CoV2373), a recombinant spike protein vaccine with Matrix-M adjuvant to prevent disease caused by SARS-CoV-2 viruses

Novavax, a global vaccine company, began evaluating NVX-CoV2373 in human studies in May 2020 and the pivotal placebo-controlled phase 3 studies started in November 2020; five clinical studies provided adult and adolescent clinical data for over 31,000 participants who were administered NVX-CoV2373. This extensive data has demonstrated a well-tolerated response to NVX-CoV2373 and high vaccine efficacy against mild, moderate, or severe COVID-19 using a two-dose series (Dunkle et al., 2022) [1], (Heath et al., 2021) [2], (Keech et al., 2020) [3], (Mallory et al., 2022) [4]. The most common adverse events seen after administration with NVX-CoV2373 were injection site tenderness, injection site pain, fatigue, myalgia, headache, malaise, arthralgia, nausea, or vomiting. In addition, immunogenicity against variants of interest (VOI) and variants of concern (VOC) was established with high titers of ACE2 receptor-inhibiting and neutralizing antibodies in these studies (EMA, 2022) [5], (FDA, 2023) [6]. Further studies on correlates of protection determined that titers of anti-Spike IgG and neutralizing antibodies correlated with efficacy against symptomatic COVID-19 established in clinical trials (p < 0.001 for recombinant protein vaccine and p = 0.005 for mRNA vaccines for IgG levels) (Fong et al., 2022) [7]. Administration of a booster dose of the recombinant protein vaccine approximately 6 months following the primary two-dose series resulted in substantial increases in humoral antibodies against both the prototype strain and all evaluated variants, similar to or higher than the antibody levels observed in phase 3 studies that were associated with high vaccine efficacy (Dunkle et al., 2022) [1], (Mallory et al., 2022) [4]. These findings, together with the well tolerated safety profile, support use of the recombinant protein vaccine as primary series and booster regimens.

Mechanism of Innate Immune Response Induced by Albizia julibrissin Saponin Active Fraction Using C2C12 Myoblasts

Albizia julibrissin saponin active fraction (AJSAF), is a prospective adjuvant with dual Th1/Th2 and Tc1/Tc2 potentiating activity. Its adjuvant activity has previously been proven to be strictly dependent on its spatial co-localization with antigens, highlighting the role of local innate immunity in its mechanisms. However, its potential targets and pathways remain unclear. Here, its intracellular molecular mechanisms of innate immune response were explored using mouse C2C12 myoblast by integrative analysis of the in vivo and in vitro transcriptome in combination with experimental validations. AJSAF elicited a temporary cytotoxicity and inflammation towards C2C12 cells. Gene set enrichment analysis demonstrated that AJSAF regulated similar cell death- and inflammatory response-related genes in vitro and in vivo through activating second messenger-MAPK-CREB pathways. AJSAF markedly enhanced the Ca2+, cAMP, and reactive oxygen species levels and accelerated MAPK and CREB phosphorylation in C2C12 cells. Furthermore, Ca2+ chelator, CREB inhibitor, and MAPK inhibitors dramatically blocked the up-regulation of IL-6, CXCL1, and COX2 in AJSAF-treated C2C12 cells. Collectively, these results demonstrated that AJSAF induced innate immunity via Ca2+-MAPK-CREB pathways. This study is beneficial for insights into the molecular mechanisms of saponin adjuvants.

Four new triterpene saponins from Cephalaria speciosa and their potent cytotoxic and immunomodulatory activities

Four new triterpene saponins, namely speciosides A-D (1-4) along with six known saponins were isolated from the n-butanol extract of Cephalaria speciosa. In addition to these, three new prosapogenins (2a-4a) were obtained after alkaline hydrolysis. Elucidation of the structures of the isolated compounds was carried out by 1D, 2D NMR, HR-ESI/MS and GC-MS analyses. Cytotoxic activity was investigated on A549, CCD34-Lu, MDA-MB-231, PC-3, U-87MG, HeLa, HepG-2 cells by MTT method. Additionally, the immunomodulatory effect of compounds was evaluated for macrophage polarization with/without inactivated IBV D274 antigen treatment on THP-1 cells originated macrophage cells in terms of M1 or M2. According to the cytotoxicity results, compound 1 and prosapogenin 2a exhibit significant cytotoxicity than doxorubicin by comparison. The results demonstrated that saponin molecules treated THP-1 originated macrophages were induced M1 and/or M2 polarization. Additionally, macrophage cells treated with/without IBV D274 antigen contained saponin compounds were triggered significantly M2 polarization relative to M1. Notably, monodesmosidic saponins (1 and 2a-4a) in comparison with bisdesmosidic ones (2-4) demonstrated the most effect on M2 polarization. In conclusion, the results showed that all the isolated new saponins and their prosapogenins have immunomodulatory potential on macrophage cells increasing immune response without significant cytotoxic effect on THP-1 originated macrophages.

Subcutaneous, Oral, and Intranasal Immunization of BALB/c Mice with Leishmania infantum K39 Antigen Induces Non-Protective Humoral Immune Response

Visceral leishmaniasis is a high-burden disease caused by parasites of the Leishmania genus. The K39 kinesin is a highly antigenic protein of Leishmania infantum, but little is known about the immune response elicited by this antigen. We evaluated the humoral immune response of female BALB/c mice (n = 6) immunized with the rK39-HFBI construct, formed by the fusion of the K39 antigen to a hydrophobin partner. The rK39-HFBI construct was administered through subcutaneous, oral, and intranasal routes using saponin as an adjuvant. We analyzed the kinetics of IgG, IgG1, and IgG2a production. The groups were then challenged by an intravenous infection with L. infantum promastigote cells. The rK39-HFBI antigen-induced high levels of total IgG (p < 0.05) in all groups, but only the subcutaneous route was associated with increased production of IgG1 and IgG2a 42 days after immunization (p < 0.05), suggesting a potential secondary immune response following the booster dose. There was no reduction in the splenic parasite load; thus, the rK39-HFBI failed to protect the mice against infection under the tested conditions. The results presented here demonstrate that the high antigenicity of the K39 antigen does not contribute to a protective immune response against visceral leishmaniasis.

Toward an Improved Triterpene 3-O-Glucuronidation: The Systematic Determination of the Relative Reactivities of Glucuronyl Donors and Acceptors

3-O-β-Glucuronide triterpenes are plant-derived compounds. Some of them have been used as herbal medicine and in pharmaceuticals, such as chikusetsu saponins and Quillaja saponins. However, the demand for these materials has remained largely a challenge owing to their natural scarcity and low-yielding purification process. Therefore, a chemical triterpene 3-O-glucuronidation was conducted in this study to alleviate the surging demand on natural source. Various glucuronyl imidate donors and oleanane-type triterpene acceptors were synthesized, and the relative reactivity values (RRV) and acceptor nucleophilic constants (Aka) were systematically measured to study their influence on glucuronidation yield. As a result, applying donors in higher RRV value generally improved the production of 3-O-glucuronide triterpenes. Meanwhile, a bulky pivaloyl group was an ideal 2-O-protection to provide β-selectivity and prevented side reactions, including orthoester formation and acyl-transfer reaction. Collectively, a positive correlation was observed between reactive donors/acceptors and improved glucuronidation yields. These findings offered insights on the influence of donors' and acceptors' reactivities on 3-O-β-glucuronide triterpenes synthesis, and this knowledge would help to access saponins of interest to address future needs.

Practical Considerations for Next-Generation Adjuvant Development and Translation

Over the last several years, there has been increased interest from academia and the pharmaceutical/biotech industry in the development of vaccine adjuvants for new and emerging vaccine modalities. Despite this, vaccine adjuvant development still has some of the longest timelines in the pharmaceutical space, from discovery to clinical approval. The reasons for this are manyfold and range from complexities in translation from animal to human models, concerns about safety or reactogenicity, to challenges in sourcing the necessary raw materials at scale. In this review, we will describe the current state of the art for many adjuvant technologies and how they should be approached or applied in the development of new vaccine products. We postulate that there are many factors to be considered and tools to be applied earlier on in the vaccine development pipeline to improve the likelihood of clinical success. These recommendations may require a modified approach to some of the common practices in new product development but would result in more accessible and practical adjuvant-containing products.

Next-Generation Adjuvants: Applying Engineering Methods to Create and Evaluate Novel Immunological Responses

Adjuvants are a critical component of vaccines. Adjuvants typically target receptors that activate innate immune signaling pathways. Historically, adjuvant development has been laborious and slow, but has begun to accelerate over the past decade. Current adjuvant development consists of screening for an activating molecule, formulating lead molecules with an antigen, and testing this combination in an animal model. There are very few adjuvants approved for use in vaccines, however, as new candidates often fail due to poor clinical efficacy, intolerable side effects, or formulation limitations. Here, we consider new approaches using tools from engineering to improve next-generation adjuvant discovery and development. These approaches will create new immunological outcomes that will be evaluated with novel diagnostic tools. Potential improved immunological outcomes include reduced vaccine reactogenicity, tunable adaptive responses, and enhanced adjuvant delivery. Evaluations of these outcomes can leverage computational approaches to interpret "big data" obtained from experimentation. Applying engineering concepts and solutions will provide alternative perspectives, further accelerating the field of adjuvant discovery.

Synthesis of immunostimulatory saponins: A sweet challenge for carbohydrate chemists

Saponins are a large family of natural glycosides showing a wide range of biological activities. Current research efforts on saponins as vaccine adjuvants have been mainly focused on the development of synthetic analogs. By mimicking the immunomodulatory saponins from Quillaja saponaria (QS), less complex and readily accessible analogs have been synthesized to improve the industrial applicability and efficacy of saponins as vaccine adjuvants. Through the exploration of several structural modifications on the skeleton of QS saponins, including changes in the sugar and aglycone compositions as well as in the nature and configuration of the glycosidic bonds, structure-activity relationship (SAR) studies developed by Pr. Gin in the early 2010s were taken as a starting point for the development of a new generation of immunomodulatory candidates. In this review, the recent synthetic strategies and SAR studies of mono- and bidesmosidic QS saponins are discussed. Original concepts of vaccination including self-adjuvanticity and the development of saponin-based glycoconjugates are described. The synthesis and semi-synthesis of saponin alternatives to QS, such as Momordica saponin and onjisaponin derivatives, are also discussed in this review.

Controlled Lipid Self-Assembly for Scalable Manufacturing of Next-Generation Immune Stimulating Complexes

Immune stimulating complexes (ISCOMs) are safe and effective saponin-based adjuvants formed by the self-assembly of saponin, cholesterol, and phospholipids in water to form cage-like 30-40 nm diameter particles. Inclusion of the Toll-like receptor 4 agonist monophosphoryl lipid A (MPLA) in ISCOM particles yields a promising next-generation adjuvant termed Saponin-MPLA NanoParticles (SMNP). In this work, we detail protocols to produce ISCOMs or SMNP via a tangential flow filtration (TFF) process suitable for scalable synthesis and Good Manufacturing Practice (GMP) production of clinical-grade adjuvants. SMNP or ISCOM components were solubilized in micelles of the surfactant MEGA-10, then diluted below the critical micelle concentration (CMC) of the surfactant to drive ISCOM self-assembly. Assembly of ISCOM/SMNP particles using the purified saponin QS-21 used in clinical-grade saponin adjuvants was found to require controlled stepwise dilution of the initial micellar solution, to prevent formation of undesirable kinetically-trapped aggregate species. An optimized protocol gave yields of ~77% based on the initial feed of QS-21 and the final SMNP particle composition mirrored the feed ratios of the components. Further, samples were highly homogeneous with comparable quality to that of material prepared at lab scale by dialysis and purified via size-exclusion chromatography. This protocol may be useful for clinical preparation of ISCOM-based vaccine adjuvants and therapeutics.

Unification of medicines and excipients: The roles of natural excipients for promoting drug delivery

INTRODUCTION

Drug delivery systems (DDSs) formed by natural active compounds be instrumental in developing new green excipients and novel DDS from natural active compounds (NACs). 'Unification of medicines and excipients'(UME), the special inherent nature of the natural active compounds, provides the inspiration and conduction to achieve this goal.

AREAS COVERED

This review summarizes the typical types of NACs from herbal medicine, such as saponins, flavonoids, polysaccharides, etc. that act as excipients and their main application in DDS. The comparison of the drug delivery systems formed by NACs and common materials and the primary formation mechanisms of these NACs are also introduced to provide a deepened understanding of their performance in DDS.

EXPERT OPINION

Many natural bioactive compounds, such as saponins, polysaccharides, etc. have been used in DDS. Diversity of structure and pharmacological effects of NACs turn out the unique advantages in improving the performance of DDSs like targeting ability, adhesion, encapsulation efficiency(EE), etc. and enhancing the bioavailability of loaded drugs.

Saponin Fraction CIL1 from Lysimachia ciliata L. Enhances the Effect of a Targeted Toxin on Cancer Cells

Saponins are plant metabolites that possess multidirectional biological activities, among these is antitumor potential. The mechanisms of anticancer activity of saponins are very complex and depend on various factors, including the chemical structure of saponins and the type of cell they target. The ability of saponins to enhance the efficacy of various chemotherapeutics has opened new perspectives for using them in combined anticancer chemotherapy. Co-administration of saponins with targeted toxins makes it possible to reduce the dose of the toxin and thus limit the side effects of overall therapy by mediating endosomal escape. Our study indicates that the saponin fraction CIL1 of Lysimachia ciliata L. can improve the efficacy of the EGFR-targeted toxin dianthin (DE). We investigated the effect of cotreatment with CIL1 + DE on cell viability in a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, on proliferation in a crystal violet assay (CV) and on pro-apoptotic activity using Annexin V/7 Actinomycin D (7-AAD) staining and luminescence detection of caspase levels. Cotreatment with CIL1 + DE enhanced the target cell-specific cytotoxicity, as well as the antiproliferative and proapoptotic properties. We found a 2200-fold increase in both the cytotoxic and antiproliferative efficacy of CIL1 + DE against HER14-targeted cells, while the effect on control NIH3T3 off-target cells was less profound (6.9- or 5.4-fold, respectively). Furthermore, we demonstrated that the CIL1 saponin fraction has a satisfactory in vitro safety profile with a lack of cytotoxic and mutagenic potential.

Modular adjuvant-free pan-HLA-DR-immunotargeting subunit vaccine against SARS-CoV-2 elicits broad sarbecovirus-neutralizing antibody responses

Subunit vaccines typically require co-administration with an adjuvant to elicit protective immunity, adding development hurdles that can impede rapid pandemic responses. To circumvent the need for adjuvant in a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) subunit vaccine, we engineer a thermostable immunotargeting vaccine (ITV) that leverages the pan-HLA-DR monoclonal antibody 44H10 to deliver the viral spike protein receptor-binding domain (RBD) to antigen-presenting cells. X-ray crystallography shows that 44H10 binds to a conserved epitope on HLA-DR, providing the basis for its broad HLA-DR reactivity. Adjuvant-free ITV immunization in rabbits and ferrets induces robust anti-RBD antibody responses that neutralize SARS-CoV-2 variants of concern and protect recipients from SARS-CoV-2 challenge. We demonstrate that the modular nature of the ITV scaffold with respect to helper T cell epitopes and diverse RBD antigens facilitates broad sarbecovirus neutralization. Our findings support anti-HLA-DR immunotargeting as an effective means to induce strong antibody responses to subunit antigens without requiring an adjuvant.

Cytotoxic effects of recombinant proteins enhanced by momordin Ic are dependent on cholesterol and ganglioside GM1

Plant-derived triterpenoid saponins have been shown to play a powerful role in enhancing the cytotoxic activity of protein therapeutics. However, the mechanism of how saponins are acting is not clearly understood. In this study, momordin Ic (MIC), a triterpenoid saponin derived from Kochia scoparia (L.) Schrad., specifically enhance the antiproliferative effect of recombinant MAP30 (a type I ribosome inactivating protein, RIP) in breast cancer cells. Subsequently, the possible mechanism of how MIC enhanced the cytotoxicity of MAP30 was analyzed in detail. We observed the level of intracellular labeled MAP30 using fluorescence microscopy and flow cytometry. And a reporter protein, GAL9, was used to monitor the role of MIC in promoting endosomal escape. We found endosomal escape does not play a role for the enhancer effect of MIC while the effect of MIC on MAP30 is cholesterol dependent and that ganglioside GM1, a lipid raft marker, can competitively inhibit cytotoxicity of MAP30 enhanced by MIC. Finally, we provided some insights into the correlation between the sugar side chain of MIC and its role in enhancing of RIP cytotoxicity and altering of drug cell tropism.

Inhibiting Angiogenesis by Anti-Cancer Saponins: From Phytochemistry to Cellular Signaling Pathways

Saponins are one of the broadest classes of high-molecular-weight natural compounds, consisting mainly of a non-polar moiety with 27 to 30 carbons and a polar moiety containing sugars attached to the sapogenin structure. Saponins are found in more than 100 plant families as well as found in marine organisms. Saponins have several therapeutic effects, including their administration in the treatment of various cancers. These compounds also reveal noteworthy anti-angiogenesis effects as one of the critical strategies for inhibiting cancer growth and metastasis. In this study, a comprehensive review is performed on electronic databases, including PubMed, Scopus, ScienceDirect, and ProQuest. Accordingly, the structural characteristics of triterpenoid/steroid saponins and their anti-cancer effects were highlighted, focusing on their anti-angiogenic effects and related mechanisms. Consequently, the anti-angiogenic effects of saponins, inhibiting the expression of genes related to vascular endothelial growth factor (VEGF) and hypoxia-inducible factor 1-α (HIF-1α) are two main anti-angiogenic mechanisms of triterpenoid and steroidal saponins. The inhibition of inflammatory signaling pathways that stimulate angiogenesis, such as pro-inflammatory cytokines, mitogen-activated protein kinase (MAPKs), and phosphoinositide 3-kinases/protein kinase B (PI3K/Akt), are other anti-angiogenic mechanisms of saponins. Furthermore, the anti-angiogenic and anti-cancer activity of saponins was closely related to the binding site of the sugar moiety, the type and number of their monosaccharide units, as well as the presence of some functional groups in their aglycone structure. Therefore, saponins are suitable candidates for cancer treatment by inhibiting angiogenesis, for which extensive pre-clinical and comprehensive clinical trial studies are recommended.

Adjuvant activity of tubeimosides by mediating the local immune microenvironment

Rhizoma Bolbostemmatis, the dry tuber of Bolbostemma paniculatum, has being used for the treatment of acute mastitis and tumors in traditional Chinese medicine. In this study, tubeimoside (TBM) I, II, and III from this drug were investigated for the adjuvant activities, structure-activity relationships (SAR), and mechanisms of action. Three TBMs significantly boosted the antigen-specific humoral and cellular immune responses and elicited both Th1/Th2 and Tc1/Tc2 responses towards ovalbumin (OVA) in mice. TBM I also remarkably facilitated mRNA and protein expression of various chemokines and cytokines in the local muscle tissues. Flow cytometry revealed that TBM I promoted the recruitment and antigen uptake of immune cells in the injected muscles, and augmented the migration and antigen transport of immune cells to the draining lymph nodes. Gene expression microarray analysis manifested that TBM I modulated immune, chemotaxis, and inflammation-related genes. The integrated analysis of network pharmacology, transcriptomics, and molecular docking predicted that TBM I exerted adjuvant activity by interaction with SYK and LYN. Further investigation verified that SYK-STAT3 signaling axis was involved in the TBM I-induced inflammatory response in the C2C12 cells. Our results for the first time demonstrated that TBMs might be promising vaccine adjuvant candidates and exert the adjuvant activity through mediating the local immune microenvironment. SAR information contributes to developing the semisynthetic saponin derivatives with adjuvant activities.

Saponins: Research Progress and Their Potential Role in the Post-COVID-19 Pandemic Era

In the post-COVID-19 pandemic era, the new global situation and the limited therapeutic management of the disease make it necessary to take urgent measures in more effective therapies and drug development in order to counteract the negative global impacts caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its new infectious variants. In this context, plant-derived saponins-glycoside-type compounds constituted from a triterpene or steroidal aglycone and one or more sugar residues-may offer fewer side effects and promising beneficial pharmacological activities. This can then be used for the development of potential therapeutic agents against COVID-19, either as a therapy or as a complement to conventional pharmacological strategies for the treatment of the disease and its prevention. The main objective of this review was to examine the primary and current evidence in regard to the therapeutic potential of plant-derived saponins against the COVID-19 disease. Further, the aim was to also focus on those studies that highlight the potential use of saponins as a treatment against SARS-CoV-2. Saponins are antiviral agents that inhibit different pharmacological targets of the virus, as well as exhibit anti-inflammatory and antithrombotic activity in relieving symptoms and clinical complications related to the disease. In addition, saponins also possess immunostimulatory effects, which improve the efficacy and safety of vaccines for prolonging immunogenicity against SARS-CoV-2 and its infectious variants.

An inventory of adjuvants used for vaccination in horses: the past, the present and the future

Vaccination is one of the most widely used strategies to protect horses against pathogens. However, available equine vaccines often have limitations, as they do not always provide effective, long-term protection and booster injections are often required. In addition, research efforts are needed to develop effective vaccines against emerging equine pathogens. In this review, we provide an inventory of approved adjuvants for equine vaccines worldwide, and discuss their composition and mode of action when available. A wide range of adjuvants are used in marketed vaccines for horses, the main families being aluminium salts, emulsions, polymers, saponins and ISCOMs. We also present veterinary adjuvants that are already used for vaccination in other species and are currently evaluated in horses to improve equine vaccination and to meet the expected level of protection against pathogens in the equine industry. Finally, we discuss new adjuvants such as liposomes, polylactic acid polymers, inulin, poly-ε-caprolactone nanoparticles and co-polymers that are in development. Our objective is to help professionals in the horse industry understand the composition of marketed equine vaccines in a context of mistrust towards vaccines. Besides, this review provides researchers with a list of adjuvants, either approved or at least evaluated in horses, that could be used either alone or in combination to develop new vaccines.

Improvement influenza vaccine immune responses with traditional Chinese medicine and its active ingredients

The current influenza vaccines are unable to provide effective protection in many cases, like influenza viruses strain antigenic drift or shift, and the influenza continues to cause significant annual morbidity and mortality. Improving the immune response to influenza vaccination is an unmet need. Traditional Chinese medicine (TCM) and its active ingredients are commonly known to have immunomodulatory properties. We therefore compared influenza vaccination alone or formulated with Astragali Radix (Huangqi in Chinese), and several representative ingredients of TCM, including lentinan (polysaccharide), panax notoginseng saponins (saponin), breviscapine (flavone), andrographolide (terpenoid), and a Chinese herbal compound (kangai) for their potential to enhance immune responses to influenza vaccine in mice. We found that all these TCM-adjuvants were able to increase hemagglutination inhibition (HAI) antibody titers, splenocyte proliferation, splenic T cell differentiation, bone marrow dendritic cell maturity, and both Th1 and Th2 cytokine secretion of influenza vaccine to varying degrees, and that had the characteristics of no excessive inflammatory responses and bidirectional regulation simultaneously. Taken together, our findings show that Astragali Radix exerts a more comprehensive effect on vaccine immunity, on both innate and adaptive immunity. The effects of lentinan and andrographolide on adaptive immunity were more significant, while the effects of breviscapine on innate immunity were stronger, and the other two TCM adjuvants were weaker. As the first report of a comprehensive evaluation of TCM adjuvants in influenza vaccines, the results suggest that TCM and their active ingredients are good candidates for enhancing the immune response of influenza vaccines, and that suitable TCMs can be selected based on the adjuvant requirements of different vaccines.

Vaccine approaches for antigen capture by liposomes

INTRODUCTION

Liposomes have been used as carriers for vaccine adjuvants and antigens due to their inherent biocompatibility and versatility as delivery vehicles. Two vial admixture of protein antigens with liposome-formulated immunostimulatory adjuvants has become a broadly used clinical vaccine preparation approach. Compared to freely soluble antigens, liposome-associated forms can enhance antigen delivery to antigen-presenting cells and co-deliver antigens with adjuvants, leading to improved vaccine efficacy.

AREAS COVERED

Several antigen-capture strategies for liposomal vaccines have been developed for proteins, peptides, and nucleic acids. Specific antigen delivery methodologies are discussed, including electrostatic adsorption, encapsulation inside the liposome aqueous core, and covalent and non-covalent antigen capture.

EXPERT OPINION

Several commercial vaccines include active lipid components, highlighting an increasingly prominent role of liposomes and lipid nanoparticles in vaccine development. Utilizing liposomes to associate antigens offers potential advantages, including antigen and adjuvant dose-sparing, co-delivery of antigen and adjuvant to immune cells, and enhanced immunogenicity. Antigen capture by liposomes has demonstrated feasibility in clinical testing. New antigen-capture techniques have been developed and appear to be of interest for vaccine development.

Unglycosylated Soluble SARS-CoV-2 Receptor Binding Domain (RBD) Produced in E. coli Combined with the Army Liposomal Formulation Containing QS21 (ALFQ) Elicits Neutralizing Antibodies against Mismatched Variants

The emergence of novel potentially pandemic pathogens necessitates the rapid manufacture and deployment of effective, stable, and locally manufacturable vaccines on a global scale. In this study, the ability of the Escherichia coli expression system to produce the receptor binding domain (RBD) of the SARS-CoV-2 spike protein was evaluated. The RBD of the original Wuhan-Hu1 variant and of the Alpha and Beta variants of concern (VoC) were expressed in E. coli, and their biochemical and immunological profiles were compared to RBD produced in mammalian cells. The E. coli-produced RBD variants recapitulated the structural character of mammalian-expressed RBD and bound to human angiotensin converting enzyme (ACE2) receptor and a panel of neutralizing SARS-CoV-2 monoclonal antibodies. A pilot vaccination in mice with bacterial RBDs formulated with a novel liposomal adjuvant, Army Liposomal Formulation containing QS21 (ALFQ), induced polyclonal antibodies that inhibited RBD association to ACE2 in vitro and potently neutralized homologous and heterologous SARS-CoV-2 pseudoviruses. Although all vaccines induced neutralization of the non-vaccine Delta variant, only the Beta RBD vaccine produced in E. coli and mammalian cells effectively neutralized the Omicron BA.1 pseudovirus. These outcomes warrant further exploration of E. coli as an expression platform for non-glycosylated, soluble immunogens for future rapid response to emerging pandemic pathogens.

Current status and development prospects of aquatic vaccines

Diseases are a significant impediment to aquaculture's sustainable and healthy growth. The aquaculture industry is suffering significant financial losses as a result of the worsening water quality and increasing frequency of aquatic disease outbreaks caused by the expansion of aquaculture. Drug control, immunoprophylaxis, ecologically integrated control, etc. are the principal control strategies for fish infections. For a long time, the prevention and control of aquatic diseases have mainly relied on the use of various antibiotics and chemical drugs. However, long-term use of chemical inputs not only increases pathogenic bacteria resistance but also damages the fish and aquaculture environments, resulting in drug residues in aquatic products, severely impeding the development of the aquaculture industry. The development and use of aquatic vaccines are the safest and most effective ways to prevent aquatic animal diseases and preserve the health and sustainability of aquaculture. To give references for the development and implementation of aquatic vaccines, this study reviews the development history, types, inoculation techniques, mechanisms of action, development prospects, and challenges encountered with aquatic vaccines.

Development and Evaluation of a Novel Diammonium Glycyrrhizinate Phytosome for Nasal Vaccination

The objective of the present research was to formulate diammonium glycyrrhizinate (DG) into phytosomes (DG-P) to induce nasal immune responses and enhance absorption. Plackett- Burman design was used for process optimization, incorporating specific formulation and process variables to obtain the optimal parameters. Fourier transform infrared spectroscopy (FTIR), X-ray power diffraction (P-XRD), and transmission electron microscopy (TEM) were used for characterization. The adjuvant activity of the DG-P was evaluated by using bone marrow dendritic cells. In vitro nasal mucosal permeation and in situ nasal perfusion were also investigated to evaluate nasal absorption. The DG phytosomes were in the size range of 20~30 nm and zeta-potential range of −30~−40 mV. DG-P demonstrated 4.2-fold increased solubility in n-octanol. Coculturing bone marrow dendritic cells with DG-P led to enhanced dendritic cell maturation. Apparent permeability coefficient of the phytosomal formulation was almost four times higher than that of free DG determined by ex vivo permeation studies on excised porcine mucosa. In situ nasal perfusion studies in rats demonstrated that the nasal absorption of DG-P was significantly higher than that of free DG. Conclusively, the results confirmed that DG-P have potential for use as an adjuvant for nasal vaccine.

First-in-human assessment of safety and immunogenicity of low and high doses of Plasmodium falciparum malaria protein 013 (FMP013) administered intramuscularly with ALFQ adjuvant in healthy malaria-naïve adults

The global burden of malaria remains substantial. Circumsporozoite protein (CSP) has been demonstrated to be an effective target antigen, however, improvements that offer more efficacious and more durable protection are still needed. In support of research and development of next-generation malaria vaccines, Walter Reed Army Institute of Research (WRAIR) has developed a CSP-based antigen (FMP013) and a novel adjuvant ALFQ (Army Liposome Formulation containing QS-21). We present a single center, open-label, dose-escalation Phase 1 clinical trial to evaluate the safety and immunogenicity of the FMP013/ALFQ malaria vaccine candidate. In this first-in-human evaluation of both the antigen and adjuvant, we enrolled ten subjects; five received 20 μg FMP013 / 0.5 mL ALFQ (Low dose group), and five received 40 μg FMP013 / 1.0 mL ALFQ (High dose group) on study days 1, 29, and 57. Adverse events and immune responses were assessed during the study period. The clinical safety profile was acceptable and there were no serious adverse events. Both groups exhibited robust humoral and cellular immunological responses, and compared favorably with historical responses reported for RTS,S/AS01. Based on a lower reactogenicity profile, the 20 μg FMP013 / 0.5 mL ALFQ (Low dose) was selected for follow-on efficacy testing by controlled human malaria infection (CHMI) with a separate cohort. Trial Registration:Clinicaltrials.gov Identifier NCT04268420 (Registered February 13, 2020).

Advances in Infectious Disease Vaccine Adjuvants

Vaccines are one of the most significant medical interventions in the fight against infectious diseases. Since their discovery by Edward Jenner in 1796, vaccines have reduced the worldwide transmission to eradication levels of infectious diseases, including smallpox, diphtheria, hepatitis, malaria, and influenza. However, the complexity of developing safe and effective vaccines remains a barrier for combating many more infectious diseases. Immune stimulants (or adjuvants) are an indispensable factor in vaccine development, especially for inactivated and subunit-based vaccines due to their decreased immunogenicity compared to whole pathogen vaccines. Adjuvants are widely diverse in structure; however, their overall function in vaccine constructs is the same: to enhance and/or prolong an immunological response. The potential for adverse effects as a result of adjuvant use, though, must be acknowledged and carefully managed. Understanding the specific mechanisms of adjuvant efficacy and safety is a key prerequisite for adjuvant use in vaccination. Therefore, rigorous pre-clinical and clinical research into adjuvant development is essential. Overall, the incorporation of adjuvants allows for greater opportunities in advancing vaccine development and the importance of immune stimulants drives the emergence of novel and more effective adjuvants. This article highlights recent advances in vaccine adjuvant development and provides detailed data from pre-clinical and clinical studies specific to infectious diseases. Future perspectives into vaccine adjuvant development are also highlighted.

Saponins as Natural Emulsifiers for Nanoemulsions

The awareness of sustainability approaches has focused attention on replacing synthetic emulsifiers with natural alternatives when formulating nanoemulsions. In this context, a comprehensive review of the different types of saponins being successfully used to form and stabilize nanoemulsions is presented, highlighting the most common natural sources and biosynthetic routes. Processes for their extraction and purification are also reviewed altogether with the recent advances for their characterization. Concerning the preparation of the nanoemulsions containing saponins, the focus has been initially given to screening methods, lipid phase used, and production procedures, but their characterization and delivery systems explored are also discussed. Most experimental outcomes showed that the saponins present high performance, but the challenges associated with the saponins' broader application, mainly the standardization for industrial use, are identified. Future perspectives report, among others, the emerging biotechnological processes and the use of byproducts in a circular economy context.

LNP-CpG ODN-adjuvanted varicella-zoster virus glycoprotein E induced comparable levels of immunity with ShingrixTM in VZV-primed mice

Latent varicella-zoster virus (VZV) may be reactivated to cause herpes zoster, which affects one in three people during their lifetime. The currently available subunit vaccine Shingrix^TM is superior to the attenuated vaccine Zostavax® in terms of both safety and efficacy, but the supply of its key adjuvant component QS21 is limited. With Ionizable lipid nanoparticles (LNPs) that were recently approved by the FDA for COVID-19 mRNA vaccines as carriers, and oligodeoxynucleotides containing CpG motifs (CpG ODNs) approved by the FDA for a subunit hepatitis B vaccine as immunostimulators, we developed a LNP vaccine encapsulating VZV-glycoprotein E (gE) and CpG ODN, and compared its immunogenicity with Shingrix^TM in C57BL/6J mice. The results showed that the LNP vaccine induced comparable levels of gE-specific IgG antibodies to Shingrix^TM as determined by enzyme-linked immunosorbent assay (ELISA). Most importantly, the LNP vaccine induced comparable levels of cell-mediated immunity (CMI) that plays decisive roles in the efficacy of zoster vaccines to Shingrix^TM in a VZV-primed mouse model that was adopted for preclinical studies of Shingrix^TM. Number of IL-2 and IFN-γ secreting splenocytes and proportion of T helper 1 (Th1) cytokine-expressing CD4⁺ T cells in LNP-CpG-adjuvanted VZV-gE vaccinated mice were similar to that of Shingrix^TM boosted mice. All of the components in this LNP vaccine can be artificially and economically synthesized in large quantities, indicating the potential of LNP-CpG-adjuvanted VZV-gE as a more cost-effective zoster vaccine.

Novel Oxime-Derivatized Synthetic Triterpene Glycosides as Potent Saponin Vaccine Adjuvants

Vaccine adjuvants are key for optimal vaccine efficacy, increasing the immunogenicity of the antigen and potentiating the immune response. Saponin adjuvants such as the carbohydrate-based QS-21 natural product are among the most promising candidates in vaccine formulations, but suffer from inherent drawbacks that have hampered their use and approval as stand-alone adjuvants. Despite the recent development of synthetic derivatives with improved properties, their full potential has not yet been reached, allowing the prospect of discovering further optimized saponin variants with higher potency. Herein, we have designed, chemically synthesized, and immunologically evaluated novel oxime-derivatized saponin adjuvants with targeted structural modifications at key triterpene functionalities. The resulting analogues have revealed important findings into saponin structure-activity relationships, including adjuvant mechanistic insights, and have shown superior adjuvant activity in terms of significantly increased antibody response augmentation compared to our previous saponin leads. These newly identified saponin oximes emerge as highly promising synthetic adjuvants for further preclinical development towards potential next generation immunotherapeutics for future vaccine applications.

Ionizable Lipid Nanoparticles Enhanced the Synergistic Adjuvant Effect of CpG ODNs and QS21 in a Varicella Zoster Virus Glycoprotein E Subunit Vaccine

Varicella zoster virus (VZV) causes two diseases: varicella upon primary infection and herpes zoster when latent viruses in the sensory ganglia reactivate. While varicella vaccines depend on humoral immunity to prevent VZV infection, cell-mediated immunity (CMI), which plays a therapeutic role in the control or elimination of reactivated VZV in infected cells, is decisive for zoster vaccine efficacy. As one of the most abundant glycoproteins of VZV, conserved glycoprotein E (gE) is essential for viral replication and transmission between ganglion cells, thus making it an ideal target subunit vaccine antigen; gE has been successfully used in the herpes zoster vaccine ShingrixTM on the market. In this report, we found that ionizable lipid nanoparticles (LNPs) approved by the Food and Drug Administration (FDA) as vectors for coronavirus disease 2019 (COVID-19) mRNA vaccines could enhance the synergistic adjuvant effect of CpG oligodeoxynucleotides (CpG ODNs) and QS21 on VZV-gE, affecting both humoral immunity and CMI. Vaccines made with these LNPs showed promise as varicella vaccines without a potential risk of herpes zoster, which identifies them as a novel type of herpes zoster vaccine similar to ShingrixTM. All of the components in this LNP-CpG-QS21 adjuvant system were proven to be safe after mass vaccination, and the high proportion of cholesterol contained in the LNPs was helpful for limiting the cytotoxicity induced by QS21, which may lead to the development of a novel herpes zoster subunit vaccine for clinical application.

Th2-Oriented Immune Serum After SARS-CoV-2 Vaccination Does Not Enhance Infection In Vitro

The relatively lower protection rate of the alum-adjuvanted inactivated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines reminds us of the antibody-dependent enhancement (ADE) phenomenon observed in preclinical studies during the development of vaccines for Middle East respiratory syndrome coronavirus (MERS-CoV) and severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1). In this study, using the S1 segment of the SARS-CoV-2 spike protein or inactivated whole SARS-CoV-2 virus as an antigen and aluminum as an adjuvant, the risk of ADE of infection with T helper 2 (Th2)-oriented immune serum from mice (N=6) and humans (N=5) was examined in immune cell lines, which show different expression patterns of Fc receptors. Neither the immune serum from alum-adjuvanted S1 subunit vaccines nor inactivated SARS-CoV-2 vaccination enhanced SARS-CoV-2 S pseudotyped virus infection in any of the tested cell lines in vitro. Because both of these Th2-oriented immune sera could block SARS-CoV-2 infection without ADE of infection, we speculate that the lower protection rate of the inactivated SARS-CoV-2 vaccine may be attributed to the lower neutralizing antibody titers induced or the pulmonary eosinophilic immunopathology accompanied by eosinophilic infiltration in the lungs upon virus exposure. Adjustment of the immunization schedule to elevate the neutralizing antibody levels and skew adjuvants toward Th1-oriented responses may be considered to increase the efficacies of both inactivated and spike protein-based subunit SARS-CoV-2 vaccines.

Structure Elucidation of Triterpenoid Saponins Found in an Immunoadjuvant Preparation of Quillaja brasiliensis Using Mass Spectrometry and 1H and 13C NMR Spectroscopy

An immunoadjuvant preparation (named Fraction B) was obtained from the aqueous extract of Quillaja brasiliensis leaves, and further fractionated by consecutive separations with silica flash MPLC and reverse phase HPLC. Two compounds were isolated, and their structures elucidated using a combination of NMR spectroscopy and mass spectrometry. One of these compounds is a previously undescribed triterpene saponin (Qb1), which is an isomer of QS-21, the unique adjuvant saponin employed in human vaccines. The other compound is a triterpene saponin previously isolated from Quillaja saponaria bark, known as S13. The structure of Qb1 consists of a quillaic acid residue substituted with a β-d-Galp-(1→2)-[β-d-Xylp-(1→3)]-β-d-GlcpA trisaccharide at C3, and a β-d-Xylp-(1→4)-α-l-Rhap-(1→2)-[α-l-Arap-(1→3)]-β-d-Fucp moiety at C28. The oligosaccharide at C28 was further substituted at O4 of the fucosyl residue with an acyl group capped with a β-d-Xylp residue.

Recent Advances in the Development of Toll-like Receptor Agonist-Based Vaccine Adjuvants for Infectious Diseases

Vaccines are powerful tools for controlling microbial infections and preventing epidemic diseases. Efficient inactive, subunit, or viral-like particle vaccines usually rely on a safe and potent adjuvant to boost the immune response to the antigen. After a slow start, over the last decade there has been increased developments on adjuvants for human vaccines. The development of adjuvants has paralleled our increased understanding of the molecular mechanisms for the pattern recognition receptor (PRR)-mediated activation of immune responses. Toll-like receptors (TLRs) are a group of PRRs that recognize microbial pathogens to initiate a host’s response to infection. Activation of TLRs triggers potent and immediate innate immune responses, which leads to subsequent adaptive immune responses. Therefore, these TLRs are ideal targets for the development of effective adjuvants. To date, TLR agonists such as monophosphoryl lipid A (MPL) and CpG-1018 have been formulated in licensed vaccines for their adjuvant activity, and other TLR agonists are being developed for this purpose. The COVID-19 pandemic has also accelerated clinical research of vaccines containing TLR agonist-based adjuvants. In this paper, we reviewed the agonists for TLR activation and the molecular mechanisms associated with the adjuvants’ effects on TLR activation, emphasizing recent advances in the development of TLR agonist-based vaccine adjuvants for infectious diseases.

Caspase-1-Dependent Pyroptosis Mediates Adjuvant Activity of Platycodin D as an Adjuvant for Intramuscular Vaccines

Platycodin D (PD) is a potent adjuvant with dual Th1 and Th2 potentiating activity, but its mechanisms of action remain unclear. Here, the C2C12 myoblast cell line and mice were used as in vitro and in vivo models to identify potential signaling pathways involved in the adjuvant activity of PD. PD induced a transient cytotoxicity and inflammatory response in the C2C12 cells and in mouse quadricep muscles. A comparative analysis of microarray data revealed that PD induced similar gene expression profiles in the C2C12 cells and in the quadricep muscles, and triggered rapid regulation of death, immune, and inflammation-related genes, both in vivo and in vitro. It was further demonstrated that caspase-1-dependent pyroptosis was involved in the PD-induced cytotoxicity and inflammatory response in the C2C12 cells via the Ca²⁺–c-jun N-terminal kinase (JNK)/p38 mitogen-activated protein kinase (MAPK)–NLR family pyrin domain containing 3 (NLRP3) inflammasome signaling pathway. Consistently, the in vivo analysis revealed that a local blockage of NLRP3 and caspase-1 inhibited PD-induced cytokine production and immune cell recruitment at the injection site, and impaired the adjuvant activity of PD on antigen-specific immune responses to model antigen ovalbumin (OVA) in mice. These findings identified the caspase-1-dependent adjuvanticity of PD and expanded the current knowledge on the mechanisms of action of saponin-based adjuvants.

Immunoprophylactic properties of the Corynebacterium pseudotuberculosis-derived MBP:PLD:CP40 fusion protein

Caseous lymphadenitis (CLA) is a disease that affects small ruminants, and the best way to prevent its spread on a herd is through immunoprophylaxis. Thus, we aimed to evaluate the MBP:PLD:CP40 fusion protein as a new CLA immunogen. The fusion protein was constructed by combining Corynebacterium pseudotuberculosis PLD and CP40 proteins with maltose-binding protein (MBP) as an intrinsic adjuvant. The antigenicity, allergenic potential, prediction of B epitopes, binding to MHC receptors, and docking on the Toll-Like 2 receptor were evaluated in silico. MBP:PLD:CP40 was expressed and purified. 40 BALB/c were divided into four groups (G1 - control, G2 - Saponin, G3 - MBP:PLD:CP40, and G4 - rPLD + rCP40). Total IgG, IgG1, and IgG2a were quantified, and the expressions of cytokines after splenocyte in vitro stimulation were assessed. Mice were challenged 42 days after the first immunization. The in silico analysis showed that MBP:PLD:CP40 has immunogenic potential, does not have allergic properties, and can dock on the TRL2 receptor. MBP:PLD:CP40 stimulated the production of IgG1 antibodies in a fivefold proportion to IgG2a, and TNF and IL-17 were significantly expressed in response to the antigenic stimuli. When rPLD and rCP40 were used together for immunization, they could induce IFN-γ and IL-12, but with no detectable antibody production. The G3 and G4 groups presented a survival of 57.14% and 42.86%, respectively, while the G1 and G2 mice were all dead 15 days after the challenge. MBP:PLD:CP40 partially protected the mice against C. pseudotuberculosis infection and can be considered a potential new CLA immunogen. KEY POINTS: • The fusion protein induced more IgG1 than IgG2a antibodies; • The fusion protein also induced the expression of the TNF and IL-17 cytokines; • Mice inoculated with MBP:PLD:CP40 presented a 57.14% survival.

Bridging the Gaps between Plant and Human Health: A Systematic Review of Soyasaponins

Saponins, prominent secondary plant metabolites, are recognized for their roles in plant defense and medicinal benefits. Soyasaponins, commonly derived from legumes, are a class of triterpenoid saponins that demonstrate significant potential for plant and human health applications. Previous research and reviews largely emphasize human health effects of soyasaponins. However, the biological effects of soyasaponins and their implications for plants in the context of human health have not been well-discussed. This review provides comprehensive discussions on the biological roles of soyasaponins in plant defense and rhizosphere microbial interactions; biosynthetic regulation and compound production; immunological effects and potential for therapeutics; and soyasaponin acquisition attributed to processing effects, bioavailability, and biotransformation processes based on recent soyasaponin research. Given the multifaceted biological effects elicited by soyasaponins, further research warrants an integrated approach to understand molecular mechanisms of regulations in their production as well as their applications in plant and human health.

Effects of Varicella-Zoster Virus Glycoprotein E Carboxyl-Terminal Mutation on mRNA Vaccine Efficacy

Glycoprotein E (gE) is one of the most abundant glycoproteins in varicella-zoster virus and plays pivotal roles in virus replication and transmission between ganglia cells. Its extracellular domain has been successfully used as an antigen in subunit zoster vaccines. The intracellular C-terminal domain was reported to be decisive for gE trafficking between the endoplasmic reticulum, trans-Golgi network and endosomes and could influence virus spread and virus titers. Considering that the trafficking and distribution of mRNA vaccine-translated gE may be different from those of gE translated against the background of the viral genome (e.g., most gE in virus-infected cells exists as heterodimers with another glycoprotein, gI,), which may influence the immunogenicity of gE-based mRNA vaccines, we compared the humoral and cellular immunity induced by LNP-encapsulated mRNA sequences encoding the whole length of gE, the extracellular domain of gE and a C-terminal double mutant of gE (mutant Y569A with original motif AYRV, which targets gE to TGN, and mutants S593A, S595A, T596A and T598A with the original motif SSTT) that were reported to enhance virus spread and elevate virus titers. The results showed that while the humoral and cellular immunity induced by all of the mRNA vaccines was comparable to or better than that induced by the AS01B-adjuvanted subunit vaccines, the C-terminal double mutant of gE showed stable advantages in all of the indicators tested, including gE-specific IgG titers and T cell responses, and could be adopted as a candidate for both safer varicella vaccines and effective zoster vaccines.

Immunomodulatory Response of Toll-like Receptor Ligand-Peptide Conjugates in Food Allergy

Covalent conjugation of allergens to toll-like receptor (TLR) agonists appears to be a powerful strategy for the development of safety compounds for allergen-specific immunomodulatory response toward tolerance in allergy. In this work, we have synthesized two family of ligands, an 8-oxoadenine derivative as a ligand for TLR7 and a pyrimido[5,4-b]indole as a ligand for TLR4, both conjugated with a T-cell peptide of Pru p 3 allergen, the lipid transfer protein (LTP) responsible for LTP-dependent food allergy. These conjugates interact with dendritic cells, inducing their specific maturation, T-cell proliferation, and cytokine production in peach allergic patients. Moreover, they increased the Treg-cell frequencies in these patients and could induce the IL-10 production. These outcomes were remarkable in the case of the TLR7 ligand conjugated with Pru p 3, opening the door for the potential application of these allergen–adjuvant systems in food allergy immunotherapy.

Epigallocatechin-3-Gallate as a Novel Vaccine Adjuvant

Vaccine adjuvants from natural resources have been utilized for enhancing vaccine efficacy against infectious diseases. This study examined the potential use of catechins, polyphenolic materials derived from green tea, as adjuvants for subunit and inactivated vaccines. Previously, catechins have been documented to have irreversible virucidal function, with the possible applicability in the inactivated viral vaccine platform. In a mouse model, the coadministration of epigallocatechin-3-gallate (EGCG) with influenza hemagglutinin (HA) antigens induced high levels of neutralizing antibodies, comparable to that induced by alum, providing complete protection against the lethal challenge. Adjuvant effects were observed for all types of HA antigens, including recombinant full-length HA and HA1 globular domain, and egg-derived inactivated split influenza vaccines. The combination of alum and EGCG further increased neutralizing (NT) antibody titers with the corresponding hemagglutination inhibition (HI) titers, demonstrating a dose-sparing effect. Remarkably, EGCG induced immunoglobulin isotype switching from IgG1 to IgG2a (approximately >64-700 fold increase), exerting a more balanced TH1/TH2 response compared to alum. The upregulation of IgG2a correlated with significant enhancement of antibody-dependent cellular cytotoxicity (ADCC) function (approximately 14 fold increase), providing a potent effector-mediated protection in addition to NT and HI. As the first report on a novel class of vaccine adjuvants with built-in virucidal activities, the results of this study will help improve the efficacy and safety of vaccines for pandemic preparedness.