Peter M. Moyle

Modern subunit vaccines: development, components, and research opportunities

Traditional vaccines, based on the administration of killed or attenuated microorganisms, have proven to be among the most effective methods for disease prevention. Safety issues related to administering these complex mixtures, however, prevent their universal application. Through identification of the microbial components responsible for protective immunity, vaccine formulations can be simplified, enabling molecular‐level vaccine characterization, improved safety profiles, prospects to develop new high‐priority vaccines (e.g. for HIV, tuberculosis, and malaria), and the opportunity for extensive vaccine component optimization. This subunit approach, however, comes at the expense of decreased immunity, requiring the addition of immunostimulatory agents (adjuvants). As few adjuvants are currently used in licensed vaccines, adjuvant development represents an exciting area for medicinal chemists to play a role in the future of vaccine development. In addition, immune responses can be further customized though optimization of delivery systems, tuning the size of particulate vaccines, targeting specific cells of the immune system (e.g. dendritic cells), and adding components to aid vaccine efficacy in whole immunized populations (e.g. promiscuous T‐helper epitopes). Herein we review the current state of the art and future direction in subunit vaccine development, with a focus on the described components and their potential to steer the immune response toward a desired response.

Self-adjuvanting lipopeptide vaccines.

Despite the important role of adjuvants for vaccine development, relatively few adjuvants have been successfully incorporated into vaccines intended for human administration. This is in part due to the high toxicity associated with many experimental adjuvants. This lack of choice effectively hinders the ability to produce vaccines against many diseases, or to improve current vaccine formulations. The conjugation of immunostimulatory lipids to peptide antigens, to produce self-adjuvanting lipopeptide vaccines, has been tested in human clinical trials. These systems appear to have a number of advantages over more traditional adjuvants (e.g. alum salts) including the capacity for these vaccines to be administered via mucosal routes (e.g. orally or nasally) instead of by injection, elicitation of antigen-specific cytotoxic T-lymphocytes and mucosal immunity, as well as little-to-no observed toxicity. Several lipopeptide vaccine systems have been described in the literature, ranging from the conjugation of single fatty acid chains, to the conjugation of more complex lipids and glycolipids onto peptide antigens. The following review provides an overview of the most studied lipopeptide vaccine systems grouped into the following categories: 1) bacterial lipopeptides, including tri-palmitoyl-S-glyceryl cysteine (Pam3Cys) and di-palmitoyl-S--glyceryl cysteine (Pam2Cys); 2) the lipid-core peptide (LCP) and multiple antigen lipophilic adjuvant carrier (MALAC) systems; 3) single-chain palmitoylated peptides; and 4) glycolipids (e.g. monophosphoryl lipid A). The review also discusses the potential mechanisms of action for lipopeptide and glycolipopeptide vaccines, as well as structure activity relationships, and provides examples of studies utilising each system.

Mucosal Immunisation: Adjuvants and Delivery Systems

The mucosal administration of vaccines is an area currently receiving a high level of interest due to potential advantages offered by this technique. These advantages include the ability to administer vaccines without need for needles, thus improving patient compliance with vaccination schedules, and the capacity to induce immune responses capable of preventing infections at the site of acquisition. Despite these advantages a number of limitations exist which currently inhibit our ability to successfully develop new mucosal vaccines. As such, much research is currently focused on developing new adjuvants and delivery systems to overcome these difficulties. However, despite high levels of interest in this area, relatively few mucosal vaccine candidates have successfully progressed to human clinical trials. In the review that follows, we aim to provide the reader with an overview of the immune system with respect to induction of mucosal immune responses. Furthermore, the review provides an overview of a number of microbial (bacterial toxins, CpG DNA, cytokines/chemokines, live vectors, and virus like particles) and synthetic (microspheres, liposomes, and lipopeptides) strategies that have been investigated as adjuvants or delivery systems for mucosal vaccine development, with a focus on the delivery of vaccines via the oral route.

Modern lipid-, carbohydrate-, and peptide-based delivery systems for peptide, vaccine, and gene products.

Research related to peptide, vaccine, and gene delivery has grown exponentially over the last decade. In this review, we discuss the development of delivery systems for peptides, gene and vaccine products. Special focus is given to different lipidation and glycosylation strategies to improve the metabolic stability and membrane permeability of therapeutics, and their targeting to specific sites. The synthetic methods for preparation of the systems are also described.

Method for the synthesis of mono-ADP-ribose conjugated peptides

ADP-ribosylation is an important post-translational modification involved in processes including cellular replication, DNA repair, and cell death. Despite these roles, the functions of ADP-ribosylation, in particular mono-ADP-ribosylation, remain poorly understood. The development of a technique to generate large amounts of site-specific, ADP-ribosylated peptides would provide a useful tool for deconvoluting the biochemical roles of ADP-ribosylation. Here we demonstrate that synthetic histone H2B tail peptides, incorporating aminooxy or N-methyl aminooxy functionalized amino acids, can be site-specifically conjugated to ADP-ribose. These peptides are recognized as substrates by the ADP-ribosylation biochemical machinery (PARP1), can interact with the ADP-ribose binding proteins macroH2A1.1 and PARP9, and demonstrate superior enzymatic and chemical stability when compared to ester-linked ADP-ribose. In addition, the incorporation of benzophenone photo-cross-linkers into these peptides is demonstrated to provide a means to probe for and enrich ADP-ribose binding proteins.

Development of a liposaccharide-based delivery system and its application to the design of group A streptococcal vaccines.

Group A streptococcus (GAS) is associated with many human diseases, ranging in severity from benign to life-threatening. A promising strategy for developing vaccines against GAS involves the use of carbohydrates as carriers for peptide antigens. This study describes the optimized synthesis of d-glucose and d-galactose derived carriers, bearing an adipate linker and four tert-butoxycarbonyl protected aminopropyl groups. Prophylactic GAS vaccine candidates were synthesized by conjugating multiple copies of a single GAS M protein derived peptide antigen (either J8 or J14) onto the carbohydrate carriers. These antigens contain peptide sequences, which are highly conserved and offer the potential to prevent infections caused by up to 70% of GAS strains. Lipophilic amino acids were also conjugated to the d-glucose anomeric carbon to produce a self-adjuvanting liposaccharide vaccine. High serum IgG antibody titers against each of the incorporated peptide epitopes were detected following subcutaneous immunization of B10.BR (H-2 (k)) mice with the liposaccharide vaccine candidates.

Oral vaccine delivery - New strategies and technologies

Although most commercial vaccines are delivered by injection, there is an increasing interest in needle-free vaccine delivery for reasons including the ability to elicit immune responses at mucosal surfaces, ease of administration, and the ability to administer vaccines without the need for trained medical professionals. This review summarizes strategies and technologies that are being used to improve oral vaccine absorption. Peptides and proteins, which comprise important vaccine components, exhibit unfavorable physicochemical properties including degradation in the gastrointestinal tract, and poor transport across the intestinal wall, which hinder oral vaccine development. Approaches to overcome these obstacles aim to provide new vaccines and delivery systems that are capable of eliciting protective immune responses, and are making an impact on current vaccine development.

Group A Streptococcal vaccine candidate: contribution of epitope to size, antigen presenting cell interaction and immunogenicity

AIM Utilize lipopeptide vaccine delivery system to develop a vaccine candidate against Group A Streptococcus. MATERIALS & METHODS Lipopeptides synthesized by solid-phase peptide synthesis-bearing carboxyl (C)-terminal and amino (N)-terminal Group A Streptococcus peptide epitopes. Nanoparticles formed were evaluated in vivo. RESULTS Immune responses were induced in mice without additional adjuvant. We demonstrated for the first time that incorporation of the C-terminal epitope significantly enhanced the N-terminal epitope-specific antibody response and correlated with forming smaller nanoparticles. Antigen-presenting cells had increased uptake and maturation by smaller, more immunogenic nanoparticles. Antibodies raised by vaccination recognized isolates. CONCLUSION Demonstrated the lipopeptidic nanoparticles to induce an immune response which can be influenced by the combined effect of epitope choice and size.

Site-specific incorporation of three toll-like receptor 2 targeting adjuvants into semisynthetic, molecularly defined nanoparticles: application to group a streptococcal vaccines.

Subunit vaccines offer a means to produce safer, more defined vaccines compared to traditional whole microorganism approaches. Subunit antigens, however, exhibit weak immunity, which is normally overcome through coadministration with adjuvants. Enhanced vaccine properties (e.g., improved potency) can be obtained by linking antigen and adjuvant, as observed for synthetic peptide antigens and Toll-like receptor 2 (TLR2) ligands. As few protective peptide antigens have been reported, compared to protein antigens, we sought to extend the utility of this approach to recombinant proteins, while ensuring that conjugation reactions yielded a single, molecularly defined product. Herein we describe the development and optimization of techniques that enable the efficient, site-specific attachment of three synthetic TLR2 ligands (lipid core peptide (LCP), Pam2Cys, and Pam3Cys) onto engineered protein antigens, permitting the selection of optimal TLR2 agonists during the vaccine development process. Using this approach, broadly protective (J14) and population targeted (seven M protein N-terminal antigens) multiantigenic vaccines against group A streptococcus (GAS; Streptococcus pyogenes) were produced and observed to self-assemble in PBS to yield nanoparticules (69, 101, and 123 nm, respectively). All nanoparticle formulations exhibited self-adjuvanting properties, with rapid, persistent, antigen-specific IgG antibody responses elicited toward each antigen in subcutaneously immunized C57BL/6J mice. These antibodies were demonstrated to strongly bind to the cell surface of five GAS serotypes that are not represented by vaccine M protein N-terminal antigens, are among the top 20 circulating strains in developed countries, and are associated with clinical disease, suggesting that these vaccines may elicit broadly protective immune responses.

Endosome Escape Strategies for Improving the Efficacy of Oligonucleotide Delivery Systems

Gene therapy requires safe and effective vectors to deliver genes to their target site of action. Non-viral gene delivery systems have attracted growing attention due to their low toxicity, low immunogenicity and ease of production compared to viral vectors. Most non-viral gene delivery systems enter cells via endocytic pathways, and their escape from endosomes is therefore crucial for successful transfection. Several reagents have been developed to promote endosomal escape, including peptides, polymers and lipids. Among these, endosome-disrupting peptides have been used in many studies, and have proven to be one of the most promising approaches to overcome endosomal entrapment and lysosomal degradation. This review provides an up-to-date summary of strategies for enhancing endosomal escape, with a focus on the modification of endosome-disrupting peptides to further increase the efficient delivery of oligonucleotides.