Polymer-peptide hybrids as a highly immunogenic single-dose nanovaccine

Evaluation of novel single-dose vaccine candidates against gonadotropin-releasing hormone (GnRH) in mice

Immunocontraceptive vaccines targeting gonadotropin-releasing hormone (GnRH) are in high demand for controlling population growth and managing the temperament of both wild and domesticated animals. Achieving adequate efficacy, especially with a single-dose immunization, requires potent vaccines and adjuvants. However, commercial vaccines face challenges: some, like Gonacon®, have regulatory issues with veterinary authorities, while others, like Bopriva®, lack potency and require multiple booster doses. Thus, there is a critical need for highly effective vaccines with robust adjuvants suitable for easy, less invasive single-dose administration. Recent studies have shown that peptide vaccines adjuvanted with 15-mer polyleucine (L15) or poly(methyl acrylate) (PMA) demonstrate potent immunogenicity, comparable to complete Freund's adjuvant (CFA), after a single dose against Streptococcus pyogenes. Our research aims to evaluate the performance of these promising vaccine adjuvant as single-dose contraceptive vaccines in mice, compared to well-established commercial and experimental adjuvants, such as AddaVax®, Incomplete Freunds Adjuvant (IFA), and CFA. To develop a vaccine with sufficient HLA coverage for cattle immunocontraception, we evaluated a peptide vaccine incorporating three cattle-compatible helper T cell epitopes. We evaluated the immunogenicity of constructs in mice to that of a construct with the universal mouse-compatible PADRE (P) helper T cell epitope. Various vaccines were prepared to investigate: (A) the impact of incorporating cattle-compatible helper T cells on immunogenicity, and (B) the effectiveness of different adjuvant systems compared to CFA. The vaccines were administered subcutaneously to C57BL/6 mice, and serological assays revealed that the L15/Quil A-based vaccine system was highly immunogenic, with performance comparable to CFA without the need for reactogenic mycobacterial components. Our vaccines significantly reduced serum progesterone levels in mice, making the L15/Quil A system a strong candidate for single-dose anti-fertility application, followed by PMA, and AddaVax® adjuvanted GnRH.

Multiepitope Subunit Peptide-Based Nanovaccine against Porcine Circovirus Type 2 (PCV2) Elicited High Antibody Titers in Vaccinated Mice

Porcine circovirus 2 (PCV2) infection is one of the most serious threats to the swine industry. While the disease can be prevented, to some extent, by commercial PCV2a vaccines, the evolving nature of PCV2 necessitates the development of a novel vaccine that can compete with the mutations of the virus. Thus, we have developed novel multiepitope vaccines based on the PCV2b variant. Three PCV2b capsid protein epitopes, together with a universal T helper epitope, were synthesized and formulated with five delivery systems/adjuvants: complete Freund's adjuvant, poly(methyl acrylate) (PMA), poly(hydrophobic amino acid), liposomes and rod-shaped polymeric nanoparticles built from polystyrene-poly(N-isopropylacrylamide)-poly(N-dimethylacrylamide). Mice were subcutaneously immunized with the vaccine candidates three times at three-week intervals. All vaccinated mice produced high antibody titters after three immunizations as analyzed by the enzyme-linked immunosorbent assay (ELISA), while mice vaccinated with PMA-adjuvanted vaccine elicited high antibody titers even after a single immunization. Thus, the multiepitope PCV2 vaccine candidates designed and examined here show strong potential for further development.

Peptide-Based Vaccine against SARS-CoV-2: Peptide Antigen Discovery and Screening of Adjuvant Systems

The SARS-CoV-2 virus has caused a global crisis, resulting in 0.5 billion infections and over 6 million deaths as of March 2022. Fortunately, infection and hospitalization rates were curbed due to the rollout of DNA and mRNA vaccines. However, the efficacy of these vaccines significantly drops a few months post immunization, from 88% down to 47% in the case of the Pfizer BNT162 vaccine. The emergence of variant strains, especially delta and omicron, have also significantly reduced vaccine efficacy. We propose peptide vaccines as a potential solution to address the inadequacies of the current vaccines. Peptide vaccines can be easily modified to target emerging strains, have greater stability, and do not require cold-chain storage. We screened five peptide fragments (B1-B5) derived from the SARS-CoV-2 spike protein to identify neutralizing B-cell peptide antigens. We then investigated adjuvant systems for efficient stimulation of immune responses against the most promising peptide antigens, including liposomal formulations of polyleucine (L10) and polymethylacrylate (PMA), as well as classical adjuvants (CFA and MF59). Immune efficacy of formulations was evaluated using competitive ELISA, pseudovirion neutralization, and live virus neutralization assays. Unfortunately, peptide conjugation to L10 and PMA dramatically altered the secondary structure, resulting in low antibody neutralization efficacy. Of the peptides tested, only B3 administered with CFA or MF59 was highly immunogenic. Thus, a peptide vaccine relying on B3 may provide an attractive alternative to currently marketed vaccines.

Current Prospects in Peptide-Based Subunit Nanovaccines

Vaccination renders protection against pathogens via stimulation of the body's natural immune responses. Classical vaccines that utilize whole organisms or proteins have several disadvantages, such as induction of undesired immune responses, poor stability, and manufacturing difficulties. The use of minimal immunogenic pathogen components as vaccine antigens, i.e., peptides, can greatly reduce these shortcomings. However, subunit antigens require a specific delivery system and immune adjuvant to increase their efficacy. Recently, nanotechnology has been extensively utilized to address this issue. Nanotechnology-based formulation of peptide vaccines can boost immunogenicity and efficiently induce cellular and humoral immune responses. This chapter outlines the recent developments and advances of nano-sized delivery platforms for peptide antigens, including nanoparticles composed of polymers, peptides, lipids, and inorganic materials.

Polyacrylate-GnRH Peptide Conjugate as an Oral Contraceptive Vaccine Candidate

Contraceptive vaccines are designed to elicit immune responses against major components of animal reproductive systems. These vaccines, which are most commonly administered via injection, typically target gonadotropin-releasing hormone (GnRH). However, the need to restrain animals for treatment limits the field applications of injectable vaccines. Oral administration would broaden vaccine applicability. We explored contraceptive vaccine candidates composed of GnRH peptide hormone, universal T helper PADRE (P), and a poly(methylacrylate) (PMA)-based delivery system. When self-assembled into nanoparticles, PMA-P-GnRH induced the production of high IgG titers after subcutaneous and oral administration in mice. PADRE was then replaced with pig T helper derived from the swine flu virus, and the vaccine was tested in pigs. High levels of systemic antibodies were produced in pigs after both injection and oral administration of the vaccine. In conclusion, we developed a simple peptide–polymer conjugate that shows promise as an effective, adjuvant-free, oral GnRH-based contraceptive vaccine.

Poly(hydrophobic amino acid)-Based Self-Adjuvanting Nanoparticles for Group A Streptococcus Vaccine Delivery

Peptide antigens have been widely used in the development of vaccines, especially for those against autoimmunity-inducing pathogens and cancers. However, peptide-based vaccines require adjuvant and/or a delivery system to stimulate desired immune responses. Here, we explored the potential of self-adjuvanting poly(hydrophobic amino acids) (pHAAs) to deliver peptide-based vaccine against Group A Streptococcus (GAS). We designed and synthesized self-assembled nanoparticles with a variety of conjugates bearing a peptide antigen (J8-PADRE) and polymerized hydrophobic amino acids to evaluate the effects of structural arrangement and pHAAs properties on a system's ability to induce humoral immune responses. Immunogenicity of the developed conjugates was also compared to commercially available human adjuvants. We found that a linear conjugate bearing J8-PADRE and 15 copies of leucine induced equally effective, or greater, immune responses than commercial adjuvants. Our fully defined, adjuvant-free, single molecule-based vaccine induced the production of antibodies capable of killing GAS bacteria.

Immunogenicity Assessment of Cell Wall Carbohydrates of Group A Streptococcus via Self-Adjuvanted Glyco-lipopeptides

Identifying the immunogenic moieties and their precise structure of carbohydrates plays an important role for developing effective carbohydrate-based subunit vaccines. This study assessed the structure-immunogenicity relationship of carbohydrate moieties of a single repeating unit of group A carbohydrate (GAC) present on the cell wall of group A Streptococcus (GAS) using a rationally designed self-adjuvanted lipid-core peptide, instead of a carrier protein. Immunological evaluation of fully synthetic glyco-lipopeptides (particle size: 300-500 nm) revealed that construct consisting of higher rhamnose moieties (trirhamnosyl-lipopeptide) was able to induce enhanced immunogenic activity in mice, and GlcNAc moiety was not found to be an essential component of immunogenic GAC mimicked epitope. Trirhamnosyl-lipopeptide also showed 75-97% opsonic activity against four different clinical isolates of GAS and was comparable to a subunit peptide vaccine (J8-lipopeptide) which illustrated 65-96% opsonic activity.

Peptide-Polymer Conjugation Via Copper-Catalyzed Alkyne-Azide 1,3-Dipolar Cycloaddition

Dendrimers are structurally well-defined, artificial polymers with physicochemical characteristics that often imitate biomacromolecules. Consequently, they are encouraging candidates for the delivery of peptide-based vaccines. We developed a synthetic protocol for conjugating a peptide antigen derived from human papillomavirus (HPV) E7 protein to a poly(t-butyl acrylate) dendrimer to construct a vaccine candidate. The synthetic pathway utilized copper-catalyzed alkyne-azide 1,3-dipolar cycloaddition (CuAAC) click reaction, and resulted in a 76% substitution ratio of the 8-arm dendrimer. The obtained peptide-polymer construct was self-assembled, dialyzed, and characterized by microanalysis and dynamic light scattering.

Protein and Peptide Biomaterials for Engineered Subunit Vaccines and Immunotherapeutic Applications

Although vaccines have been the primary defense against widespread infectious disease for decades, there is a critical need for improvement to combat complex and variable diseases. More control and specificity over the immune response can be achieved by using only subunit components in vaccines. However, these often lack sufficient immunogenicity to fully protect, and conjugation or carrier materials are required. A variety of protein and peptide biomaterials have improved effectiveness and delivery of subunit vaccines for infectious, cancer, and autoimmune diseases. They are biodegradable and have control over both material structure and immune function. Many of these materials are built from naturally occurring self-assembling proteins, which have been engineered for incorporation of vaccine components. In contrast, others are de novo designs of structures with immune function. In this review, protein biomaterial design, engineering, and immune functionality as vaccines or immunotherapies are discussed.

Polyacrylate-Peptide Antigen Conjugate as a Single-Dose Oral Vaccine against Group A Streptococcus

Group A Streptococcus (GAS)-associated rheumatic heart disease is a leading cause of death caused by GAS infection. While antibiotics can treat the infection in most cases, growing antibiotic resistance, late medical intervention, and recurrent infection are major obstacles to the effective treatment of GAS-associated diseases. As GAS infection typically originates from the bacterial colonization of mucosal tissue in the throat, an oral vaccine that can generate both systemic and mucosal immune responses would solve problems associated with traditional medical interventions. Moreover, orally delivered vaccines are more easily administered and less expensive for mass immunization. In this study, the B-cell epitope J8, derived from GAS M protein, and universal T-helper Pan HLA-DR-binding epitope peptide (PADRE), were conjugated to poly (methyl acrylate) (PMA) to form a self-assembled nanoparticle vaccine candidate (PMA-P-J8). Strong systemic and mucosal immune responses were induced upon single oral immunization of mice with the conjugate. The antibodies generated were opsonic against GAS clinical isolates as measured after boost immunization. Thus, we developed a simple conjugate as an effective, adjuvant-free oral peptide-based vaccine.

Recent Advances in the Development of Peptide Vaccines and Their Delivery Systems Against Group A Streptococcus

Group A Streptococcus (GAS) infection can cause a variety of diseases in humans, ranging from common sore throats and skin infections, to more invasive diseases and life-threatening post-infectious diseases, such as rheumatic fever and rheumatic heart disease. Although research has been ongoing since 1923, vaccines against GAS are still not available to the public. Traditional approaches taken to develop vaccines for GAS failed due to poor efficacy and safety. Fortunately, headway has been made and modern subunit vaccines that administer minimal bacterial components provide an opportunity to finally overcome previous hurdles in GAS vaccine development. This review details the major antigens and strategies used for GAS vaccine development. The combination of antigen selection, peptide epitope modification and delivery systems have resulted in the discovery of promising peptide vaccines against GAS; these are currently in preclinical and clinical studies.

Structure-activity relationship of group A streptococcus lipopeptide vaccine candidates in trimethyl chitosan-based self-adjuvanting delivery system

Synthetic peptide vaccines based on epitopes derived from the conserved region of M-protein are proving to be a realistic option for protection against group A streptococcus (GAS). However, peptide epitopes alone are poorly immunogenic due to lack of pathogen-associated structural patterns. Therefore, we developed a GAS peptide vaccine based on combined lipidic TLR 2 agonist and self-adjuvanting polymers. We synthesized three α-poly-l-glutamic acid (PGA) conjugated lipopeptides composed of 2-amino-d,l-hexadecanoic acid, GAS B-cell peptide epitope J8 (QAEDKVKQSREAKKQVEKALKQLEDKVQ) and universal T-helper epitope PADRE (AKFVAAWTLKAAA) in different spatial arrangements. The anionic lipopeptide conjugates formed nanoparticles via ionic-complexation with a cationic polymer, trimethyl chitosan (TMC). We demonstrated that the spatial arrangement of vaccine components has a significant influence on peptide conformation and particle formation and, as such, contributes to the differential efficacy and opsonin-mediated killing potential of nanovaccines. Nanoparticles carrying branched helical lipopeptide with T-helper epitope on free N-termini (NP3) stimulated the most potent humoral immune responses. Lipopeptides without TMC (LP1-LP3) and TMC nanoparticles of peptide alone (without lipid) NP (P1) were poor inducers of antibody production, indicating that both TMC and lipid are required to induce a strong opsonic immune response.

Liposomal formulation of polyacrylate-peptide conjugate as a new vaccine candidate against cervical cancer

Peptide-based vaccines have been proposed as a therapeutic strategy for many infectious diseases, including human papilloma virus (HPV)-related cervical cancer. Peptide-based vaccines are a better treatment option than traditional chemotherapeutic agents and surgery, as they rely on the use of the body’s immune system to fight cancer cells, resulting in minimal risk of side effects. However, to increase the efficacy of peptide-based vaccines, the application of potent adjuvant and a suitable delivery system is essential. In this study, we developed a self-adjuvanting delivery system based on a combination of polymer and liposomes, for a therapeutic vaccine against cervical cancer. Peptide epitope (8Qm) derived from HPV-16 E7 protein was conjugated to dendritic poly(tert-butyl acrylate) as a primary delivery system and incorporated into cationic liposomes, which served as a secondary delivery system. Our vaccine candidate was able to kill established HPV-16 E7-positive tumor (TC-1) cells in mice following a single immunization. The immunized mice had 80% survival rate after two months. In contrast, both polymer-8Qm conjugate and liposomes bearing 8Qm failed to eradicate TC-1 tumors. The survival rate of mice was only 20% when immunized with 8Qm formulated with standard incomplete Freund’s adjuvant.

Self-assembling anticaries mucosal vaccine containing ferritin cage nanostructure and glucan-binding region of S. mutans glucosyltransferase effectively prevents caries formation in rodents

Anticaries protein vaccines that induce a mucosal immune response are not effective. Therefore, development of effective and convenient anticaries vaccines is a priority of dental research. Here we generated self-assembling nanoparticles by linking the glucan-binding region of Streptococcus mutans glucosyltransferase (GLU) to the N-terminal domain of ferritin to determine whether these novel nanoparticles enhanced the immunogenicity of an anticaries protein vaccine against GLU in rodents. We constructed the expression plasmid pET28a-GLU-FTH and purified the proteins from bacteria using size-exclusion chromatography. BALB/c mice were used to evaluate the ability of GLU-ferritin (GLU-FTH) nanoparticles to induce GLU-specific mucosal and systemic responses. The protective efficiency of GLU-FTH nanoparticles was compared with that of GLU alone or a mixture of GLU and poly(I:C) after administering an intranasal infusion to Wistar rats. The phagocytosis and maturation of dendritic cells (DCs) exposed in vitro to the nanoparticles were assessed using flow cytometry. The GLU-FTH nanoparticle vaccine elicited significantly higher levels of GLU-specific antibodies compared with GLU or a mixture of GLU and poly(I:C). Immunization with GLU-FTH achieved lower caries scores compared with those of the other vaccines. Administration of GLU-FTH nanoparticles enhanced phagocytosis by DCs and their maturation. Thus, self-assembling GLU-FTH is a highly effective anticaries mucosal vaccine that enhanced antibody production and inhibited S. mutans infection in rodents.

Linear and branched polyacrylates as a delivery platform for peptide-based vaccines

AIM

Peptide-based vaccines are designed to carry the minimum required antigen to trigger the desired immune responses; however, they are usually poorly immunogenic and require appropriate delivery system.

RESULTS

Peptides, B-cell epitope (J14) derived from group A streptococcus M-protein and universal T-helper (PADRE) epitope, were conjugated to a variety of linear and branched polyacrylates. All produced conjugates formed submicron-sized particles and induced a high level of IgG titres in mice after subcutaneous immunization. These polymer-peptide conjugates demonstrated high opsonization capacity against group A streptococcus clinical isolates.

CONCLUSION

We have successfully demonstrated that submicron-sized polymer-peptide conjugates were capable of inducing strong humoral immune responses after single immunization.

Induction of high titred, non-neutralising antibodies by self-adjuvanting peptide epitopes derived from the respiratory syncytial virus fusion protein

Respiratory syncytial virus (RSV) causes severe lower respiratory tract illness in infants and young children. The significant morbidity and mortality rates associated with RSV infection make an effective RSV vaccine development a priority. Two neutralising antibody binding sites, Ø and II, located on the pre-fusion RSV F glycoprotein are prime candidates for epitope-focused vaccine design. We report on a vaccine strategy that utilises a lipid core peptide (LCP) delivery system with self-adjuvanting properties in conjunction with either the antigenic site Ø or II (B cell epitopes) along with PADRE as a T helper cell epitope. These LCP constructs adopted the desired helical conformation in solution and were recognised by their cognate antibodies D25 and Motavizumab, specific for site Ø and II on RSV F protein, respectively. The LCP constructs were capable of eliciting higher levels of antigen specific antibodies than those induced by antigens administered with complete Freund's adjuvant, demonstrating the potent adjuvanting properties of LCP delivery. However, the antibodies induced failed to recognise native F protein or neutralise virus infectivity. These results provide a note of caution in assuming that peptide vaccines, successfully designed to structurally mimic minimal linear B cell epitopes, will necessarily elicit the desired immune response.

Chitosan nanoparticles based nanovaccines for cancer immunotherapy

Cancer immunotherapy based on tumor vaccine is very promising and intriguing for carcinoma treatment. Herein, antitumor nanovaccines consisting of self-assembled chitosan (CS) nanoparticles and two-component mucin1 (MUC1) glycopeptide antigens were reported. Two different kinds of polyanionic electrolyte [sodium tripolyphosphate (TPP) and γ-poly-L-glutamic acid (γ-PGA)] were combined with chitosan polymers to fabricate the diameter of nearly 400–500 nm CS nanoparticles by electrostatic interactions. The nanovaccines were constructed by physically mixing MUC1 glycopeptide antigens with CS nanoparticles, which reduced vaccine constructing complexity compared with traditional chemical total synthetic vaccines. Immunological studies revealed that the CS/γ-PGA nanoparticle could dramatically enhance the immunogenicity of peptide epitope and produce significantly high titers of IgG antibody which was even better than Freund’s adjuvant-containing vaccines.

Nanopharmaceuticals as a solution to neglected diseases: Is it possible?

The study of neglected diseases has not received much attention, especially from public and private institutions over the last years, in terms of strong support for developing treatment for these diseases. Support in the form of substantial amounts of private and public investment is greatly needed in this area. Due to the lack of novel drugs for these diseases, nanobiotechnology has appeared as an important new breakthrough for the treatment of neglected diseases. Recently, very few reviews focusing on filiarasis, leishmaniasis, leprosy, malaria, onchocerciasis, schistosomiasis, trypanosomiasis, and tuberculosis, and dengue virus have been published. New developments in nanocarriers have made promising advances in the treatment of several kinds of diseases with less toxicity, high efficacy and improved bioavailability of drugs with extended release and fewer applications. This review deals with the current status of nanobiotechnology in the treatment of neglected diseases and highlights how it provides key tools for exploring new perspectives in the treatment of a wide range of diseases.

Polymer-Protein Core-Shell Nanoparticles for Enhanced Antigen Immunogenicity

Nanoengineered vaccine platforms can be modeled after viruses and other pathogens with highly organized and repetitive structures that trigger the host immune system. Here we demonstrated a pyridine-grafted poly(ε-caprolactone)-based polymer-protein core-shell nanoparticles (PPCS-NPs) platform can effectively trigger the host immune system and lead to significantly higher antibody titers.

The application of self-assembled nanostructures in peptide-based subunit vaccine development

•

Smaller polymer-peptide conjugates-based nanoparticles are often more immunogenic.

•

Lipid-antigen conjugates-based nanoparticles can interact with immune receptors.

•

Peptides with β-sheet conformation usually form nanofibers.

•

α-Helical and random coil peptides tend to self-assemble into nanoparticles.

•

Peptide-based nanostructures are usually poorer inducers of immune responses.

Peptide based-vaccines are becoming one of the most widely investigated prophylactic and therapeutic health care interventions against a variety of diseases, including cancer. However, the lack of a safe and highly efficient adjuvant (immune stimulant) is regarded as the biggest obstacle to vaccine development. The incorporation of a peptide antigen in a nanostructure-based delivery system was recently shown to overcome this obstacle. Nanostructures are often formed from antigens conjugated to molecules such as polymers, lipids, and peptide, with the help of self-assembly phenomenon. This review describes the application of self-assembly process for the production of peptide-based vaccine candidates and the ability of these nanostructures to stimulate humoral and cellular immune responses.

Comparison of Fluorinated and Nonfluorinated Lipids in Self-Adjuvanting Delivery Systems for Peptide-Based Vaccines

Safe immunostimulants (adjuvants) are essential for the development of highly potent peptide-based vaccines. This study examined for the first time whether fluorinated lipids could stimulate humoral immunity in vivo when conjugated to peptide antigen. The impact of fluorination on humoral immunity was tested using a library of peptide-based vaccine candidates against the group A streptococcus (GAS). The fluorinated constructs stimulated similar mouse IgG titers to those elicited by complete Freund's adjuvant (CFA) and were higher than those produced in mice that received the nonfluorinated constructs.

Peptide-based synthetic vaccines

Classically all vaccines were produced using live or attenuated microorganisms or parts of them. However, the use of whole organisms, their components or the biological process for vaccine production has several weaknesses. The presence of immunologically redundant biological components or biological impurities in such vaccines might cause major problems. All the disadvantageous of traditional vaccines might be overcome via the development of fully synthetic peptide-based vaccines. However, once minimal antigenic epitopes only are applied for immunisation, the immune responses are poor. The use of an adjuvant can overcome this obstacle; however, it may raise new glitches. Here we briefly summarise the current stand on peptide-based vaccines, discuss epitope and adjuvant design, and multi-epitope and nanoparticle-based vaccine approaches. This mini review discusses also the disadvantages and benefits associated with peptide-based vaccines. It proposes possible methods to overcome the weaknesses of the synthetic vaccine strategy and suggests future directions for its development.

Polyacrylate-based delivery system for self-adjuvanting anticancer peptide vaccine

Vaccination can provide a safe alternative to chemotherapy by using the body's natural defense mechanisms to create a potent immune response against tumor cells. Peptide-based therapeutic vaccines against human papillomavirus (HPV)-related cancers are usually designed to elicit cytotoxic T cell responses by targeting the HPV-16 E7 oncoprotein. However, peptides alone lack immunogenicity, and an additional adjuvant or external delivery system is required. In this study, we developed new polymer-peptide conjugates to create an efficient self-adjuvanting system for peptide-based therapeutic vaccines. These conjugates reduced tumor growth and eradicated E7-positive TC-1 tumors in mice after a "single shot" immunization, without the help from an external adjuvant. The new conjugates had a significantly higher anticancer efficacy than the antigen formulated with a commercial adjuvant. Furthermore, the polymer-peptide conjugates were promptly taken up by antigen presenting cells, including dendritic cells and macrophages, and efficiently activated CD4(+) T-helper cells and CD8(+) cytotoxic T lymphocyte cells.

Recent advances in peptide-based subunit nanovaccines

Vaccination is the most efficient way to protect humans against pathogens. Peptide-based vaccines offer several advantages over classical vaccines, which utilized whole organisms or proteins. However, peptides alone are not immunogenic and need a delivery system that can boost their recognition by the immune system. In recent years, nanotechnology-based approaches have become one of the most promising strategies in peptide vaccine delivery. This review summarizes knowledge on peptide vaccines and nanotechnology-based approaches for their delivery. The recently reported nano-sized delivery platforms for peptide antigens are reviewed, including nanoparticles composed of polymers, peptides, lipids, inorganic materials and nanotubes. The future prospects for peptide-based nanovaccines are discussed.

Self-adjuvanting vaccine against group A streptococcus: application of fibrillized peptide and immunostimulatory lipid as adjuvant

Peptides are of great interest to be used as vaccine antigens due to their safety, ease of manufacturing and specificity in generating immune response. There have been massive discoveries of peptide antigens over the past decade. However, peptides alone are poorly immunogenic, which demand co-administration with strong adjuvant to enhance their immunogenicity. Recently, fibril-forming peptides such as Q11 and lipoamino acid-based carrier have been identified to induce substantial immune responses when covalently linked to peptide epitope. In this study, we have incorporated either Q11 or lipoamino acids to a peptide epitope (J14) derived from M protein of group A streptococcus to develop self-adjuvanting vaccines. J14, Q11 and lipoamino acids were also conjugated together in a single vaccine construct in an attempt to evaluate the synergy effect of combining multiple adjuvants. Physicochemical characterization demonstrated that the vaccine constructs folded differently and self-assembled into nanoparticles. Significantly, only vaccine constructs containing double copies of lipoamino acids (regardless in conjugation with Q11 or not) were capable to induce significant dendritic cells uptake and subsequent J14-specific antibody responses in non-sizes dependent manners. Q11 had minimal impact in enhancing the immunogenicity of J14 even when it was used in combination with lipoamino acids. These findings highlight the impact of lipoamino acids moiety as a promising immunostimulant carrier and its number of attachment to peptide epitope was found to have a profound effect on the vaccine immunogenicity.

Oral delivery of nanoparticle-based vaccines

Most infectious diseases are caused by pathogenic infiltrations from the mucosal tract. Therefore, vaccines delivered to the mucosal tissues can mimic natural infections and provide protection at the first site of infection. Thus, mucosal, especially, oral delivery is becoming the most preferred mode of vaccination. However, oral vaccines have to overcome several barriers such as the extremely low pH of the stomach, the presence of proteolytic enzymes and bile salts as well as low permeability in the intestine. Several formulations based on nanoparticle strategies are currently being explored to prepare stable oral vaccine formulations. This review briefly discusses several molecular mechanisms involved in intestinal immune cell activation and various aspects of oral nanoparticle-based vaccine design that should be considered for improved mucosal and systemic immune responses.

Recent progress in adjuvant discovery for peptide-based subunit vaccines

Peptide-based subunit vaccines are of great interest in modern immunotherapy as they are safe, easy to produce and well defined. However, peptide antigens produce a relatively weak immune response, and thus require the use of immunostimulants (adjuvants) for optimal efficacy. Developing a safe and effective adjuvant remains a challenge for peptide-based vaccine design. Recent advances in immunology have allowed researchers to have a better understanding of the immunological implication of related diseases, which facilitates more rational design of adjuvant systems. Understanding the molecular structure of the adjuvants allows the establishment of their structure-activity relationships which is useful for the development of next-generation adjuvants. This review summarizes the current state of adjuvants development in the field of synthetic peptide-based vaccines. The structural, chemical and biological properties of adjuvants associated with their immunomodulatory effects are discussed.

Peptide conjugation via CuAAC 'click' chemistry

The copper (I)-catalyzed alkyne azide 1,3-dipolar cycloaddition (CuAAC) or ‘click’ reaction, is a highly versatile reaction that can be performed under a variety of reaction conditions including various solvents, a wide pH and temperature range, and using different copper sources, with or without additional ligands or reducing agents. This reaction is highly selective and can be performed in the presence of other functional moieties. The flexibility and selectivity has resulted in growing interest in the application of CuAAC in various fields. In this review, we briefly describe the importance of the structural folding of peptides and proteins and how the 1,4-disubstituted triazole product of the CuAAC reaction is a suitable isoster for an amide bond. However the major focus of the review is the application of this reaction to produce peptide conjugates for tagging and targeting purpose, linkers for multifunctional biomacromolecules, and reporter ions for peptide and protein analysis.