Comparison of two different PEGylation strategies for the liposomal adjuvant CAF09: Towards induction of CTL responses upon subcutaneous vaccine administration

Liposomes, transfersomes and niosomes: production methods and their applications in the vaccinal field

One of the most effective strategies to fight viruses and handle health diseases is vaccination. Recent studies and current applications are moving on antigen, DNA and RNA-based vaccines to overcome the limitations related to the conventional vaccination strategies, such as low safety, necessity of multiple injection, and side effects. However, due to the instability of pristine antigen, RNA and DNA molecules, the use of nanocarriers is required. Among the different nanocarriers proposed for vaccinal applications, three types of nanovesicles were selected and analysed in this review: liposomes, transfersomes and niosomes. PubMed, Scopus and Google Scholar databases were used for searching recent papers on the most frequently used conventional and innovative methods of production of these nanovesicles. Weaknesses and limitations of conventional methods (i.e., multiple post-processing, solvent residue, batch-mode processes) can be overcome using innovative methods, in particular, the ones assisted by supercritical carbon dioxide. SuperSomes process emerged as a promising production technique of solvent-free nanovesicles, since it can be easily scaled-up, works in continuous-mode, and does not require further post-processing steps to obtain the desired products. As a result of the literature analysis, supercritical carbon dioxide assisted methods attracted a lot of interest for nanovesicles production in the vaccinal field. However, despite their numerous advantages, supercritical processes require further studies for the production of liposomes, transfersomes and niosomes with the aim of reaching well-defined technologies suitable for industrial applications and mass production of vaccines.

Method of manufacturing CAF®09 liposomes affects immune responses induced by adjuvanted subunit proteins

The ability to induce antigen-specific CD4+ and CD8+T-cell responses is one of the fundamental requirements when developing new efficacious vaccines against challenging infectious diseases and cancer. However, no adjuvants are currently approved for human subunit vaccines that induce T-cell immunity. Here, we incorporated a Toll-like receptor 4 agonist, i.e., the ionizable lipidoid L5N12, in the liposomal cationic adjuvant formulation 09 (CAF®09), and found that modified CAF®09 liposomes possess preserved adjuvant function as compared to unmodified CAF®09. CAF®09 consists of the cationic lipid dimethyldioctadecylammonium (DDA), monomycoloyl glycerol analogue 1 (MMG-1), and polyinosinic:polycytidylic acid [poly(I:C)]. By using the microfluidic mixing technology for liposome preparation, we gradually replaced DDA with L5N12, while keeping the molar ratios of MMG-1 and poly(I:C) constant. We found that this type of modification resulted in colloidally stable liposomes, which were significantly smaller and displayed reduced surface charge as compared to unmodified CAF®09, prepared by using the conventional thin film method. We showed that incorporation of L5N12 decreases the membrane rigidity of CAF®09 liposomes. Furthermore, vaccination with antigen adjuvanted with L5N12-modified CAF®09 or antigen adjuvanted with unmodified CAF®09, respectively, induced comparable antigen-specific serum antibody titers. We found that antigen adjuvanted with L5N12-modified CAF®09 induced antigen-specific effector and memory CD4+ and CD8+T-cell responses in the spleen comparable to those induced when unmodified CAF®09 was used as adjuvant. However, incorporating L5N12 did not have a synergistic immunopotentiating effect on the antibody and T-cell responses induced by CAF®09. Moreover, vaccination with antigen adjuvanted with unmodified CAF®09, which was manufactured by using microfluidic mixing, induced significantly lower antigen-specific CD4+ and CD8+T-cell responses than vaccination with antigen adjuvanted with unmodified CAF®09, which was prepared by using the thin film method. These results show that the method of manufacturing affects CAF®09 liposome adjuvanted antigen-specific immune responses, which should be taken into consideration when evaluating immunogenicity of subunit protein vaccines.

The Role of Mucoadhesion and Mucopenetration in the Immune Response Induced by Polymer-Based Mucosal Adjuvants

Mucus is a viscoelastic gel that acts as a protective barrier for epithelial surfaces. The mucosal vehicles and adjuvants need to pass through the mucus layer to make drugs and vaccine delivery by mucosal routes possible. The mucoadhesion of polymer particle adjuvants significantly increases the contact time between vaccine formulations and the mucosa; then, the particles can penetrate the mucus layer and epithelium to reach mucosa-associated lymphoid tissues. This review presents the key findings that have aided in understanding mucoadhesion and mucopenetration while exploring the influence of physicochemical characteristics on mucus-polymer interactions. We describe polymer-based particles designed with mucoadhesive or mucopenetrating properties and discuss the impact of mucoadhesive polymers on local and systemic immune responses after mucosal immunization. In future research, more attention paid to the design and development of mucosal adjuvants could lead to more effective vaccines.

Enhanced Influenza Immunity by Nasal Mucosal Administration of the TPGS-Modified Liposomal Vaccine

Influenza infection is difficult to prevent, control, and treat because of rapid viral mutation, fast disease progression, and high mortality. Vaccination is the main means by which to prevent and control influenza, but effectiveness is limited in that poor cellular uptake and weak immunogenicity of vaccines provides less than optimal host protection. Liposomal influenza vaccines are a promising strategy to overcome these limitations and the use of liposomal immune modulators and intranasal administration of liposomal influenza vaccines may be a means by which to improve influenza protection. The cationic lipids, i.e., dimethyldioctadecylammonium (DDA), 1,2-dioctadecanoyl-sn-glycero-3-phosphocholine (DSPC), and D-α-tocopherol polyethylene glycol 1000 (TPGS) can form blank liposomes, which can incorporate influenza antigens to produce an influenza vaccine (DDA-DSPC-TPGS). Herein, this vaccine was shown to induce dendritic cell maturation, increase host cellular uptake of the vaccine, and enhance immune responses both in vitro and in vivo. The addition of TPGS, as an amphiphilic immune adjuvant, significantly reduced the toxicity of the DDA liposomal influenza vaccine. Further, the polyethylene glycol component and tocopherol structure of TPGS enhanced the cellular uptake of the vaccine by means of stealth properties and the capacity to inhibit cellular efflux. After nasal mucosal immunization, enhanced cellular uptake rates and abundant immune cells in the nasopharyngeal-associated lymphoid tissue promoted the production of immunoglobulin A, immunoglobulin G1, and interferon-γ, which in turn mediated a more robust immune response against influenza virus. In summary, the DDA-DSPC-TPGS influenza vaccine is a safe and effective means by which to activate the immune system. The results herein provide an effective strategy by which to overcome current difficulties associated with the prevention and treatment of influenza.

Bifunctional lipids in tumor vaccines: An outstanding delivery carrier and promising immune stimulator

Cancer is still a major threat for human life, and the cancer immunotherapy can be more optimized to prolong life. However, the effect of immunotherapy is not encouraging. In order to achieve outstanding immune effect, it is necessary to strengthen antigens uptake of antigen presenting cells. Adjuvants were added to vaccines to achieve this purpose, which could be divided into two types: as an immunostimulatory molecule, the innate immunities of the body were triggered; or as a delivery carrier, and antigens were cross-delivery through the "cytoplasmic pathway" and released at a specific location. This paper reviewed the relevant research status of tumor vaccine immune adjuvants in recent years. Among the review, the function, combination strategies and derivatives of lipid A were discussed in detail. In addition, some suggestions on the existing problems and research direction of lipids as tumor vaccine adjuvants were put forward.

Lipid-Based Nanoparticles for Delivery of Vaccine Adjuvants and Antigens: Toward Multicomponent Vaccines

Despite the many advances that have occurred in the field of vaccine adjuvants, there are still unmet needs that may enable the development of vaccines suitable for more challenging pathogens (e.g., HIV and tuberculosis) and for cancer vaccines. Liposomes have already been shown to be highly effective as adjuvant/delivery systems due to their versatility and likely will find further uses in this space. The broad potential of lipid-based delivery systems is highlighted by the recent approval of COVID-19 vaccines comprising lipid nanoparticles with encapsulated mRNA. This review provides an overview of the different approaches that can be evaluated for the design of lipid-based vaccine adjuvant/delivery systems for protein, carbohydrate, and nucleic acid-based antigens and how these strategies might be combined to develop multicomponent vaccines.

Mimicking Pathogens to Augment the Potency of Liposomal Cancer Vaccines

Liposomes have emerged as interesting vehicles in cancer vaccination strategies as their composition enables the inclusion of both hydrophilic and hydrophobic antigens and adjuvants. In addition, liposomes can be decorated with targeting moieties to further resemble pathogenic particles that allow for better engagement with the immune system. However, so far liposomal cancer vaccines have not yet reached their full potential in the clinic. In this review, we summarize recent preclinical studies on liposomal cancer vaccines. We describe the basic ingredients for liposomal cancer vaccines, tumor antigens, and adjuvants, and how their combined inclusion together with targeting moieties potentially derived from pathogens can enhance vaccine immunogenicity. We discuss newly identified antigen-presenting cells in humans and mice that pose as promising targets for cancer vaccines. The lessons learned from these preclinical studies can be applied to enhance the efficacy of liposomal cancer vaccination in the clinic.

Cationic Nanoparticle-Based Cancer Vaccines

Cationic nanoparticles have been shown to be surprisingly effective as cancer vaccine vehicles in preclinical and clinical studies. Cationic nanoparticles deliver tumor-associated antigens to dendritic cells and induce immune activation, resulting in strong antigen-specific cellular immune responses, as shown for a wide variety of vaccine candidates. In this review, we discuss the relation between the cationic nature of nanoparticles and the efficacy of cancer immunotherapy. Multiple types of lipid- and polymer-based cationic nanoparticulate cancer vaccines with various antigen types (e.g., mRNA, DNA, peptides and proteins) and adjuvants are described. Furthermore, we focus on the types of cationic nanoparticles used for T-cell induction, especially in the context of therapeutic cancer vaccination. We discuss different cationic nanoparticulate vaccines, molecular mechanisms of adjuvanticity and biodistribution profiles upon administration via different routes. Finally, we discuss the perspectives of cationic nanoparticulate vaccines for improving immunotherapy of cancer.

The sixth revolution in pediatric vaccinology: immunoengineering and delivery systems

Infection is the predominant cause of mortality in early life, and immunization is the most promising biomedical intervention to reduce this burden. However, very young infants fail to respond optimally to most vaccines currently in use, especially neonates. In 2005, Stanley Plotkin proposed that new delivery systems would spur a new revolution in pediatric vaccinology, just as attenuation, inactivation, cell culture of viruses, genetic engineering, and adjuvantation had done in preceding decades. Recent advances in the field of immunoengineering, which is evolving alongside vaccinology, have begun to increasingly influence vaccine formulation design. Historically, the particulate nature of materials used in many vaccine formulations was empiric, often because of the need to stabilize antigens or reduce endotoxin levels. However, present vaccine delivery systems are rationally engineered to mimic the size, shape, and surface chemistry of pathogens, and are therefore often referred to as "pathogen-like particles". More than a decade from his original assessment, we re-assess Plotkin's prediction. In addition, we highlight how immunoengineering and advanced delivery systems may be uniquely capable of enhancing vaccine responses in vulnerable populations, such as infants. IMPACT: Immunoengineering and advanced delivery systems are leading to new developments in pediatric vaccinology. Summarizes delivery systems currently in use and development, and prospects for the future. Broad overview of immunoengineering's impact on vaccinology, catering to Pediatric Clinicians and Immunologists.

Liposome Formulations as Adjuvants for Vaccines

Development of liposome-based formulations as vaccine adjuvants has been intimately associated with, and dependent on, and informed by, a fundamental understanding of biochemical and biophysical properties of liposomes themselves. The Walter Reed Army Institute of Research (WRAIR) has a fifty-year history of experience of basic research on liposomes; and development of liposomes as drug carriers; and development of liposomes as adjuvant formulations for vaccines. Uptake of liposomes by phagocytic cells in vitro has served as an excellent model for studying the intracellular trafficking patterns of liposomal antigen. Differential fluorescent labeling of proteins and liposomal lipids, together with the use of inhibitors, has enabled the visualization of physical locations of antigens, peptides, and lipids to elucidate mechanisms underlying the MHC class I and class II pathways in phagocytic APCs. Army Liposome Formulation (ALF) family of vaccine adjuvants, which have been developed and improved since 1986, and which range from nanosize to microsize, are currently being employed in phase 1 studies with different types of candidate vaccines.

Applying Microfluidics for the Production of the Cationic Liposome-Based Vaccine Adjuvant CAF09b

Subunit vaccines require particulate adjuvants to induce the desired immune responses. Pre-clinical manufacturing methods of adjuvants are often batch dependent, which complicates scale-up for large-scale good manufacturing practice (GMP) production. The cationic liposomal adjuvant CAF09b, composed of dioctadecyldimethylammonium bromide (DDA), monomycoloyl glycerol analogue 1 (MMG) and polyinosinic:polycytidylic acid [poly(I:C)], is currently being clinically evaluated in therapeutic cancer vaccines. Microfluidics is a promising new method for large-scale manufacturing of particle-based medicals, which is scalable from laboratory to GMP production, and a protocol for production of CAF09b by this method was therefore validated. The influence of the manufacture parameters [Ethanol] (20–40% v/v), [Lipid] (DDA and MMG, 6–12 mg/mL) and dimethyl sulfoxide [DMSO] (0–10% v/v) on the resulting particle size, colloidal stability and adsorption of poly(I:C) was evaluated in a design-of-experiments study. [Ethanol] and [DMSO] affected the resulting particle sizes, while [Lipid] and [DMSO] affected the colloidal stability. In all samples, poly(I:C) was encapsulated within the liposomes. At [Ethanol] 30% v/v, most formulations were stable at 21 days of manufacture with particle sizes <100 nm. An in vivo comparison in mice of the immunogenicity to the cervical cancer peptide antigen HPV-16 E7 adjuvanted with CAF09b prepared by lipid film rehydration or microfluidics showed no difference between the formulations, indicating adjuvant activity is intact. Thus, it is possible to prepare suitable formulations of CAF09b by microfluidics.

Rational design of adjuvants for subunit vaccines: The format of cationic adjuvants affects the induction of antigen-specific antibody responses

A range of cationic delivery systems have been investigated as vaccine adjuvants, though few direct comparisons exist. To investigate the impact of the delivery platform, we prepared four cationic systems (emulsions, liposomes, polymeric nanoparticles and solid lipid nanoparticles) all containing equal concentrations of the cationic lipid dimethyldioctadecylammonium bromide in combination with the Neisseria adhesin A variant 3 subunit antigen. The formulations were physicochemically characterized and their ability to associate with cells and promote antigen processing (based on degradation of DQ-OVA, a substrate for proteases which upon hydrolysis is fluorescent) was compared in vitro and their vaccine efficacy (antigen-specific antibody responses and IFN-γ production) and biodistribution (antigen and adjuvant) were evaluated in vivo. Due to their cationic nature, all delivery systems gave high antigen loading (> 85%) with liposomes, lipid nanoparticles and emulsions being <200 nm, whilst polymeric nanoparticles were larger (~350 nm). In vitro, the particulate systems tended to promote cell uptake and antigen processing, whilst emulsions were less effective. Similarly, whilst the particulate delivery systems induced a depot (of both delivery system and antigen) at the injection site, the cationic emulsions did not. However, out of the systems tested the cationic emulsions induced the highest antibody responses. These results demonstrate that while cationic lipids can have strong adjuvant activity, their formulation platform influences their immunogenicity.

Medicinal Chemistry and Methodological Advances in the Development of Peptide-Based Vaccines

The evolution of rapidly proliferating infectious and tumorigenic diseases has resulted in an urgent need to develop new and improved intervention strategies. Among the many therapeutic strategies at our disposal, our immune system remains the gold-standard in disease prevention, diagnosis, and treatment. Vaccines have played an important role in eradicating or mitigating the spread of infectious diseases by bolstering our immunity. Despite their utility, the design and development of new, more effective vaccines remains a public health necessity. Peptide-based vaccines have been developed for a wide range of established and emerging infectious and tumorigenic diseases. New innovations in epitope design and selection, synthesis, and formulation as well as screening techniques against immunological targets have led to more effective peptide vaccines. Current and future work is geared toward the translation of peptide vaccines from preclinical to clinical utility.

Liposomal delivery of CRISPR/Cas9

Liposomes are one of the most widely investigated carriers for CRISPR/Cas9 delivery. The surface properties of liposomal carriers, including the surface charge, PEGylation, and ligand modification can significantly affect the gene silencing efficiency. Three barriers of systemic CRISPR/Cas9 delivery (long blood circulation, efficient tumor penetration, and efficient cellular uptake/endosomal escape) are analyzed on liposomal carriers with different surface charges, PEGylations, and ligand modifications. Cationic formulations dominate CRISPR/Cas9 delivery and neutral formulations also have good performance while anionic formulations are generally not proper for CRISPR/Cas9 delivery. The PEG dilemma (prolonged blood circulation vs. reduced cellular uptake/endosomal escape) and the side effect of repeated PEGylated formulation (accelerated blood clearance) were discussed. Effects of ligand modification on cationic and neutral formulations were analyzed. Finally, we summarized the achievements in liposomal CRISPR/Cas9 delivery, outlined existing problems, and provided some future perspectives. Liposomes are one of the most widely investigated carriers for CRISPR/Cas9 delivery. The surface properties of liposomal carriers, including the surface charge, PEGylation, and ligand modification can significantly affect the gene silencing efficiency. Three barriers of systemic siRNA delivery (long blood circulation, efficient tumor penetration, and efficient cellular uptake/endosomal escape) are analyzed on liposomal carriers with different surface charges, PEGylations, and ligand modifications. Cationic formulations dominate CRISPR/Cas9 delivery and neutral formulations also have good performance while anionic formulations are generally not proper for CRISPR/Cas9 delivery. The PEG dilemma (prolonged blood circulation vs. reduced cellular uptake/endosomal escape) and the side effect of repeated PEGylated formulation (accelerated blood clearance) were discussed. Effects of ligand modification on cationic and neutral formulations were analyzed. Finally, we summarized the achievements in liposomal CRISPR/Cas9 delivery, outlined existing problems, and provided some future perspectives.