Particulate inorganic adjuvants: recent developments and future outlook

Adjuvant physiochemistry and advanced nanotechnology for vaccine development

Vaccines comprising innovative adjuvants are rapidly reaching advanced translational stages, such as the authorized nanotechnology adjuvants in mRNA vaccines against COVID-19 worldwide, offering new strategies to effectively combat diseases threatening human health. Adjuvants are vital ingredients in vaccines, which can augment the degree, extensiveness, and longevity of antigen specific immune response. The advances in the modulation of physicochemical properties of nanoplatforms elevate the capability of adjuvants in initiating the innate immune system and adaptive immunity, offering immense potential for developing vaccines against hard-to-target infectious diseases and cancer. In this review, we provide an essential introduction of the basic principles of prophylactic and therapeutic vaccination, key roles of adjuvants in augmenting and shaping immunity to achieve desired outcomes and effectiveness, and the physiochemical properties and action mechanisms of clinically approved adjuvants for humans. We particularly focus on the preclinical and clinical progress of highly immunogenic emerging nanotechnology adjuvants formulated in vaccines for cancer treatment or infectious disease prevention. We deliberate on how the immune system can sense and respond to the physicochemical cues (e.g., chirality, deformability, solubility, topology, and chemical structures) of nanotechnology adjuvants incorporated in the vaccines. Finally, we propose possible strategies to accelerate the clinical implementation of nanotechnology adjuvanted vaccines, such as in-depth elucidation of nano-immuno interactions, antigen identification and optimization by the deployment of high-dimensional multiomics analysis approaches, encouraging close collaborations among scientists from different scientific disciplines and aggressive exploration of novel nanotechnologies.

PLGA Nanoparticle-Based Dissolving Microneedle Vaccine of Clostridium perfringens ε Toxin

Epsilon toxin (ETX) is an exotoxin produced by type B and D Clostridium perfringens that causes enterotoxemia or necrotic enteritis in animals such as goats, sheep, and cattle. Vaccination is a key method in preventing such diseases. In this study, we developed a new type of dissolving microneedle patch (dMN) with a nanoparticle adjuvant for enhanced immune response to deliver the rETXY196E-C protein vaccine. We chose FDA-approved poly(lactic-co-glycolic acid) (PLGA) to prepare nanospheres as the vaccine adjuvant and introduced dimethyldioctadecylammonium bromide (DDAB) to make the surface of PLGA nanoparticles (PLGA NPs) positively charged for antigen adsorption. PLGA NPs with a diameter of 100~200 nm, a surface ZETA potential of approximately +40 mV, and good safety were successfully prepared and could effectively adsorb rETXY196E-C protein. Using non-toxic and antibacterial fish gelatin as the microneedle (MN) matrix, we prepared a PLGA-DDAB dMN vaccine with good mechanical properties that successfully penetrated the skin. After immunization of subcutaneous (SC) and dMN, antibody titers of the PLGA and Al adjuvant groups were similar in both two immune ways. However, in vivo neutralization experiments showed that the dMN vaccines had a better protective effect. When challenged with 100 × LD50 GST-ETX, the survival rate of the MN group was 100%, while that of the SC Al group was 80%. However, a 100% protective effect was achieved in both immunization methods using PLGA NPs. In vitro neutralization experiments showed that the serum antibodies from the dMN and SC PLGA NPs groups both protect naive mice from 10 × LD50 GST-ETX attack after being diluted 20 times and could also protect MDCK cells from 20 × CT50 GST-ETX attack. In conclusion, the PLGA-DDAB dMN vaccine we prepared has good mechanical properties, immunogenicity, and protection, and can effectively prevent ETX poisoning. This provides a better way of delivering protein vaccines.

Immunogenicity of Different Types of Adjuvants and Nano-Adjuvants in Veterinary Vaccines: A Comprehensive Review

Vaccination is the best way to prevent and reduce the damage caused by infectious diseases in animals and humans. So, several vaccines are used for prophylactic purposes before the pathogen infects, while therapeutic vaccines strengthen the immune system after infection with the pathogen. Adjuvants are molecules, compounds, or macromolecules that enhance non-specific immunity and, in collaboration with antigen(s), can improve the body's immune responses and change the type of immune response. The potential and toxicity of adjuvants must be balanced to provide the safest stimulation with the fewest side effects. In order to overcome the limitations of adjuvants and the effective and controlled delivery of antigens, attention has been drawn to nano-carriers that can be a promising platform for better presenting and stimulating the immune system. Some studies show that nanoparticles have a more remarkable ability to act as adjuvants than microparticles. Because nano-adjuvants inactively target antigen-presenting cells (APCs) and change their chemical surface, nanoparticles also perform better in targeted antigen delivery because they cross biological barriers more easily. We collected and reviewed various types of nano-adjuvants with their specific roles in immunogenicity as a prominent strategy used in veterinary vaccines in this paper.

Protective and vaccine dose-sparing efficacy of Poly I:C-functionalized calcium phosphate nanoparticle adjuvants in inactivated influenza vaccination

Adjuvants are required to increase the immunogenicity and efficacy of vaccination and enable vaccine dose sparing. Polyinosinic-polycytidylic acid (Poly I:C), a toll-like receptor 3 agonist, is a promising adjuvant candidate that can induce cell-mediated immune responses; however, it remains unlicensed owing to its low stability and toxicity. Calcium phosphate (CaP), a biocompatible and biodegradable nanoparticle, is widely used in biomedicine for stable and targeted drug delivery. In this study, we developed Poly I:C-functionalized CaP (Poly-CaP) and evaluated its vaccine adjuvant efficacy in vitro and in vivo. A half dose of Poly-CaP nanoparticles showed similar efficacy to a full dose of soluble Poly I:C in stimulating bone marrow-derived dendritic cells and macrophages to secrete proinflammatory cytokines and express their activation markers. Immunization with a half dose of inactivated influenza vaccine in the presence of Poly I:C or Poly-CaP adjuvants induced sufficient antigen-specific humoral responses after boost immunization. Immunization with Poly I:C, CaP, or Poly-CaP-adjuvanted with a half dose of influenza vaccine showed comparable protective efficacy against lethal virus infection, with lower weight loss and virus titer than a full dose of influenza vaccine. The Poly-CaP adjuvant was effective in stimulating antigen-specific CD4+ T cell proliferation in the lungs. Collectively, our results showed that the Poly-CaP adjuvant enhanced antigen-specific cell-mediated immunity and humoral immune responses with vaccine dose-sparing effects, suggesting its potential as a novel vaccine adjuvant candidate.

Preparation and preliminary evaluation study of [131I]iodocolchicine-gallic-AuNPs: a potential scintigraphic agent for inflammation detection

BACKGROUND

Nanomedicine offers great potential for scintigraphic diagnostic imaging with lower risk and higher quality compared to other traditional techniques.

OBJECTIVES

This work aimed to develop and evaluate gold nanoparticles combined with gallic acid (gallic-AuNPs) and [¹³¹I]iodocolchicine as a scintigraphic probe for inflammation.

METHODS

[¹³¹I]iodocolchicine-gallic-AuNPs were synthesized via chemical reduction method where gallic acid was used as reducing agent and [¹³¹I]iodocolchicine was used as stabilizing agent. Then a characteristic profile for the synthesized nano-platform was performed including size analysis, zeta potential, radiochemical yield and in-vivo biodistribution in inflammation bearing mice.

RESULTS AND CONCLUSION

This platform was successfully synthesized with good stability, appropriate particle size (10 nm diameter for AuNPs), and high radiochemical purity for [¹³¹I]iodocolchicine (96.79%). The in-vivo study indicated that [¹³¹I]iodocolchicine-gallic-AuNPs accumulated with a high target to non-target ratio in intravenous injection and high retention value in intra-inflammation injection in inflammation model. The obtained data supported the usefulness of the new platform ([¹³¹I]iodocolchicine-gallic-AuNPs) as a tracer for the detection and localization of inflammation.

VaxArray immunoassay for the multiplexed quantification of poliovirus D-antigen

Next generation poliovirus vaccines are critical to reaching global poliovirus eradication goals. Recent efforts have focused on creating inactivated vaccines using attenuated Sabin strains that maintain patient safety benefits and immunogenicity of conventional inactivated vaccines while increasing manufacturing safety and lowering production costs, and on developing novel oral vaccines using modified Sabin strains that provide critical mucosal immunity but are further attenuated to minimize risk of reversion to neurovirulence. In addition, there is a push to improve the analytical tools for poliovirus vaccine characterization. Conventional and Sabin inactivated poliovirus vaccines typically rely on standard plate-based ELISA as in vitro D-antigen potency assays in combination with WHO international standards as calibrants. While widely utilized, the current D-antigen ELISA assays have a long time to result (up to 72 h), can suffer from lab-to-lab inconsistency due to non-standardized protocols and reagents, and are inherently singleplex. For D-antigen quantitation, we have developed the VaxArray Polio Assay Kit, a multiplexed, microarray-based immunoassay that uses poliovirus-specific human monoclonal antibodies currently under consideration as standardized reagents for characterizing inactivated Sabin and Salk vaccines. The VaxArray assay can simultaneously quantify all 3 poliovirus serotypes with a time to result of less than 3 h. Here we demonstrate that the assay has limits of quantification suitable for both bioprocess samples and final vaccines, excellent reproducibility and precision, and improved accuracy over an analogous plate-based ELISA. The assay is suitable for adjuvanted combination vaccines, as common vaccine additives and crude matrices do not interfere with quantification, and is intended as a high throughput, standardized quantitation tool to aid inactivated poliovirus vaccine manufacturers in streamlining vaccine development and manufacturing, aiding the global polio eradication effort.

•

Multiplexed D-antigen immunoassay for all 3 poliovirus serotypes

•

Has <3 h time to result and compares well to 3-day plate-based ELISA

•

Assay shows high specificity and is reactive to sIPV, cIPV, and OPV

•

Applicable to in-process samples, final IPV and combination vaccine formulations

•

High accuracy and precision for both sIPV and cIPV over multiple users and days

Tannic Acid Exhibits Adjuvant Activity by Enhancing Humoral and Cell-Mediated Immunity Against BSA as a Protein Antigen

BACKGROUND

Immunization or vaccination is the process of inducing artificial immunity against an antigen taking advantage of the mechanisms of immunological memory. Current vaccines include substances known as adjuvants, which tend to improve the immunogenicity of the antigen, reduce the antigen quantity employed, and boost the immune response in weak responders. Unfortunately, only a few vaccine adjuvants are approved for human use.

OBJECTIVE

Thus, the objective of this study was to investigate the effect of Tannic acid on humoral and cell-mediated immunity against bovine serum albumin (BSA) as a protein antigen in Wistar rats.

METHOD

In order to establish the Tannic acid concentration to test it as an adjuvant, the lethal dose 50 and maximum non-toxic dose were calculated through cytotoxicity and hemolytic assays with J774 A.1 cell line and rat erythrocytes by resazurin reduction method and UV/vis spectrophotometry. Thirty Wistar rats were divided into 5 groups that included two controls without antigen and three treatment groups of adjuvants plus BSA as a protein antigen. The rats were immunized in a 30-day scheme. Blood samples were collected for humoral immunity analysis by means of immunoglobulin quantification, isotyping and antigen-antibody precipitation inhibition analysis. Rat peritoneal macrophages and splenocytes were isolated for cell-mediated immunity analysis by means of nitric oxide quantification from adjuvant stimulated peritoneal macrophages and lymphocytes proliferation assay.

RESULTS

Tannic acid was capable of increasing the immunogenicity of the antigen; besides, it was able to stimulate cell-mediated immunity by means of increased lymphocyte proliferation. Moreover, Tannic acid improved the humoral response by means of increased specific antibodies titers. These activities may be attributed to pattern recognition receptors stimulation.

CONCLUSION

Tannic acid was considered biocompatible when tested in vivo because the concentration tested did not show cytotoxicity or hemolytic effect, and there was no detrimental effect observed on the animals' health. These results show Tannic acid as a promising candidate for vaccine adjuvant.

Entry-competent-replication-abortive African horse sickness virus strains elicit robust immunity in ponies against all serotypes

African horse sickness virus (AHSV) is an Orbivirus within the Reoviridae family, spread by Culicoides species of midges, which infects equids with high mortality, particularly in horses and has a considerable impact on the equine industry. In order to control the disease, we previously described Entry Competent Replication Abortive (ECRA) virus strains for each of the nine distinct AHSV serotypes and demonstrated their potential as vaccines, first in type I interferon receptor (IFNAR-/-) knockout mice, and then in ponies. In this report we have investigated whether or not a combination ECRA vaccine comprising nine vaccine strains as two different cocktails is as efficient in ponies and the duration of the immunity triggered by ECRA vaccines. In one study, a group of ponies were vaccinated with a cocktail of 4 vaccine strains, followed by a vaccination of the remaining 5 vaccine strains, mimicking the current live attenuated vaccine regimen. In the second study, ponies were vaccinated with a single ECRA-AHSV strain and monitored for 6 months. The first group of ponies developed neutralising antibody responses against all 9 serotypes, indicating that no cross-serotype interference occurred, while the second group developed robust neutralising antibody responses against the single serotype that were sustained at the same level throughout a 6-month study. The results support our previous data and further validate ECRA vaccines as a safe and efficacious replacement of current live vaccines.

Nanoparticle-Based Vaccines for Brucellosis: Calcium Phosphate Nanoparticles-Adsorbed Antigens Induce Cross Protective Response in Mice

Introduction

Vaccine formulation with appropriate adjuvants is an attractive approach to develop protective immunity against pathogens. Calcium phosphate nanoparticles (CaPNs) are considered as ideal adjuvants and delivery systems because of their great potential for enhancing immune responses. In the current study, we have designed nanoparticle-based vaccine candidates to induce immune responses and protection against B. melitensis and B. abortus.

Materials and Methods

For this purpose, we used three Brucella antigens (FliC, 7α-HSDH, BhuA) and two multi-epitopes (poly B and poly T) absorbed by CaPNs. The efficacy of each formulation was evaluated by measuring humoral, cellular and protective responses in immunized mice.

Results

The CaPNs showed an average size of about 90 nm with spherical shape and smooth surface. The CaPNs-adsorbed proteins displayed significant increase in cellular and humoral immune responses compared to the control groups. In addition, our results showed increased ratio of specific IgG2a (associated with Th1) to specific IgG1 (associated with Th2). Also, immunized mice with different vaccine candidate formulations were protected against B. melitensis 16M and B. abortus 544, and showed same levels of protection as commercial vaccines (B. melitensis Rev.1 and B. abortus RB51) except for BhuA-CaPNs.

Discussion

Our data support the hypothesis that these antigens absorbed with CaPNs could be effective vaccine candidates against B. melitensis and B. abortus.

Gold nanoparticles for preparation of antibodies and vaccines against infectious diseases

INTRODUCTION

Vaccination remains very effective in stimulating protective immune responses against infections. An important task in antibody and vaccine preparation is to choose an optimal carrier that will ensure a high immune response. Particularly promising in this regard are nanoscale particle carriers. An antigen that is adsorbed or encapsulated by nanoparticles can be used as an adjuvant to optimize the immune response during vaccination. a very popular antigen carrier used for immunization and vaccination is gold nanoparticles, with are being used to make new vaccines against viral, bacterial, and parasitic infections.

AREAS COVERED

This review summarizes what is currently known about the use of gold nanoparticles as an antigen carrier and adjuvant to prepare antibodies in vivo and design vaccines against viral, bacterial, and parasitic infections. The basic principles, recent advances, and current problems in the use of gold nanoparticles are discussed.

EXPERT OPINION

Gold nanoparticles can be used as adjuvants to increase the effectiveness of vaccines by stimulating antigen-presenting cells and ensuring controlled antigen release. Studying the characteristics of the immune response obtained from the use of gold nanoparticles as a carrier and an adjuvant will permit the particles' potential for vaccine design to be increased.

Gold nanoparticles: A plausible tool to combat neurological bacterial infections in humans

Management of bacterial infections of central nervous system is a major challenge for the scientists all over the world. Despite the development of various potential drugs, the issue of central nervous system infections persists in the society. The main constraint is the delivery of drugs across the blood brain barrier and only a few drugs after meeting the stringent criteria could cross the blood brain barrier. On the other hand, certain bacterial pathogens could easily enter the brain by using several factors and mechanisms by crossing the blood brain barriers. Interestingly, in the recent past, gold nanoparticles have shown immense potential to overcome the issues associated with the treatment of central nervous system infections, especially due to their inherent ability to cross the blood brain barrier. Initially, the present review summarized the recent updates on the pathogenesis and factors involved in neurological bacterial infections, including the mechanism used by bacterial pathogens to cross the blood brain barriers. Thereafter, the emphasis of the review was on providing current information on gold nanoparticles pertinent to their applicability for the treatment of neurological infections. After discussing the background of neurological bacterial infections, the characteristic features, antibacterial properties, mechanisms of antibacterial action and ability to cross the blood brain barrier of gold nanoparticles have been summarized. Some of the features of gold nanoparticles that make them an ideal candidate for brain delivery are biocompatibity, stability, ability to get synthesized in different sizes with facile methods, surface affinity towards various functional groups, spontaneous crossing of blood brain barrier without applying any external field and most importantly, easy non-invasive tracing by CT imaging. The current updates on the development of gold nanoparticles based therapeutic strategies for the prevention and treatment of central nervous system infections have been discussed in the present study. However, further investigation would be required to translate these preclinical outcomes into clinical applications. Nevertheless, we could safely state that the information gathered and discussed in the present review would benefit the scientists working in the field of neuro-nanotechnology.

Aluminum (Oxy)Hydroxide Nanosticks Synthesized in Bicontinuous Reverse Microemulsion Have Potent Vaccine Adjuvant Activity

Insoluble aluminum salts such as aluminum (oxy)hydroxide are commonly used as vaccine adjuvants. Recently, there is evidence suggesting that the adjuvant activity of aluminum salt-based materials is tightly related to their physicochemical properties, including nanometer-scale size, shape with long aspect ratio, and low degree of crystallinity. Herein, for the first time, the bicontinuous reverse microemulsion (RM) technique was utilized to synthesize stick-like monodisperse aluminum (oxy)hydroxide nanoparticles with a long aspect ratio of ∼10, length of ∼80 nm, and low degree of crystallinity (denoted as Al-nanosticks). Moreover, the relationship between the physicochemical properties of Al-nanosticks and the bicontinuous RM was discussed. Compared to the commercial Alhydrogel, which contains micrometer-scale aluminum oxyhydroxide particular aggregates with moderate degree of crystallinity, the Al-nanosticks are more effective in adsorbing and delivering antigens (e.g., ovalbumin, OVA) into antigen-presenting cells, activating inflammasomes, and potentiating OVA-specific antibody responses in a mouse model. It is concluded that the aluminum (oxy)hydroxide nanosticks synthesized in the bicontinuous RM are promising new aluminum salt-based vaccine adjuvants.

Applications of Gold Nanoparticles in Nanomedicine: Recent Advances in Vaccines

Nowadays, gold is used in (nano-)medicine, usually in the form of nanoparticles, due to the solid proofs given of its therapeutic effects on several diseases. Gold also plays an important role in the vaccine field as an adjuvant and a carrier, reducing toxicity, enhancing immunogenic activity, and providing stability in storage. An even brighter golden future is expected for gold applications in this area.

Novel chimeric virus-like particles vaccine displaying MERS-CoV receptor-binding domain induce specific humoral and cellular immune response in mice

Middle East respiratory syndrome coronavirus (MERS-CoV) has continued spreading since its emergence in 2012 with a mortality rate of 35.6%, and is a potential pandemic threat. Prophylactics and therapies are urgently needed to address this public health problem. We report here the efficacy of a vaccine consisting of chimeric virus-like particles (VLP) expressing the receptor binding domain (RBD) of MERS-CoV. In this study, a fusion of the canine parvovirus (CPV) VP2 structural protein gene with the RBD of MERS-CoV can self-assemble into chimeric, spherical VLP (sVLP). sVLP retained certain parvovirus characteristics, such as the ability to agglutinate pig erythrocytes, and structural morphology similar to CPV virions. Immunization with sVLP induced RBD-specific humoral and cellular immune responses in mice. sVLP-specific antisera from these animals were able to prevent pseudotyped MERS-CoV entry into susceptible cells, with neutralizing antibody titers reaching 1: 320. IFN-γ, IL-4 and IL-2 secreting cells induced by the RBD were detected in the splenocytes of vaccinated mice by ELISpot. Furthermore, mice inoculated with sVLP or an adjuvanted sVLP vaccine elicited T-helper 1 (Th1) and T-helper 2 (Th2) cell-mediated immunity. Our study demonstrates that sVLP displaying the RBD of MERS-CoV are promising prophylactic candidates against MERS-CoV in a potential outbreak situation.

•

We constructed a chimeric parvovirus virus-like particles displaying the MERS-CoV receptor-binding domain on the surface.

•

We found that the chimeric virus-like particles induced RBD-specific, neutralizing antibody responses in mice.

•

Splenocytes from immunized mice had considerably higher secretion of both Th1- and Th2-type cytokines.

•

Our results reveal that chimeric virus-like particles induce both specific humoral and cell-mediated immunity.

Nanomaterial-Based Vaccine Adjuvants

Vaccination is a biological process that administrates antigenic materials to stimulate an individual's immune system to develop immunity to a specific pathogen. It is the most effective tool to prevent illness and death from infectious diseases or diseases leading to cancers. Because many recombinant and synthetic antigens are poorly immunogenic, adjuvant is essentially added to vaccine formula that can potentiate the immune responses, offer better protection against pathogens and reduce the amount of antigens needed for protective immunity. To date, there are nearly 100 different types of adjuvants associated with about 400 vaccines that are either commercially available or under development. Among these adjuvants, many of them are particulates and nano-scale in nature. Nanoparticles represent a wide range of materials with novel physicochemical properties that exhibit immunostimulatory effects. However, the mechanistic understandings on how their physicochemical properties affect immunopotentiation remain elusive. In this article, we aim to review current development status of nanomaterial-based vaccine adjuvants, and further discuss their acting mechanisms, understanding of which will benefit the rational design of effective vaccine adjuvants with improved immunogenicity for prevention of infectious disease as well as therapeutic cancer treatment.

Biomineralized vaccine nanohybrid for needle-free intranasal immunization

Frequent outbreaks and the rapid global spread of infectious diseases have increased the urgent need for massive vaccination especially in countries with limited resources. Intranasal vaccination facilitates the mass vaccination via needle-free delivery of vaccine through nasal mucosal surfaces. Inspired by the strong capability of calcium phosphate (CaP) materials to adhere to cells and tissues, we propose to improve nasal vaccination by using a biomineralization-based strategy. The vaccine nanohybrid was obtained by covering the viral surface with CaP nanoshell, which changed the physiochemical properties of original vaccine, resulting in the increase of mucosal adhesion to the nasal tissues. The core-shell structure was beneficial for the receptor-independent uptake and the induction of elevated local IgA response within the nasal cavity. Moreover, the vaccine complex elicited enhanced systemic antibody response that neutralized wild type of dengue virus and promoted the systemic cellular immune responses. This achievement presents the potential of CaP based vaccine biomineralization for the fabrication of needle-free vaccine formulation.