Tolerogenic nanovaccines for the treatment of type I allergic diseases

The high prevalence of type I allergic diseases such as allergic rhinitis, allergic asthma, food allergies, allergic conjunctivitis, and atopic dermatitis has emerged as a significant public health concern globally. Failure of immune tolerance to ordinarily harmless substances or stimulation, and subsequent induction of T helper 2 cells by antigen-presenting cells evokes the allergic immune response, which results in persistent inflammation, tissue damage, and organ function impairment. Current therapeutic approaches for allergic diseases include avoiding allergen exposure, corticosteroids, biologics, etc. However, these strategies only relieve allergic symptoms but hardly prevent the deteriorative progression and may have adverse effects on patients. With the rapid development of nanotechnology and immunology, emerging tolerogenic nanovaccines represent novel approaches with the potential to cure type I allergic diseases rather than merely alleviate symptoms. In this review, we expound the burgeoning field of tolerogenic nanovaccines against type I allergic diseases, highlight various types of antigens employed in constructing allergen extracts, protein/peptide and nucleic acid-based tolerogenic nanovaccines, and discuss their application in allergic rhinitis, allergic asthma, food allergies, allergic conjunctivitis, and atopic dermatitis.

Establishment of an RNA-based transient expression system in the green alga Chlamydomonas reinhardtii

Chlamydomonas reinhardtii, a unicellular green alga, is a prominent model for green biotechnology and for studying organelles' function and biogenesis, such as chloroplasts and cilia. However, the stable expression of foreign genes from the nuclear genome in C. reinhardtii faces several limitations, including low expression levels and significant differences between clones due to genome position effects, epigenetic silencing, and time-consuming procedures. We developed a robust transient expression system in C. reinhardtii to overcome these limitations. We demonstrated efficient entry of in vitro-transcribed mRNA into wall-less cells and enzymatically dewalled wild-type cells via electroporation. The endogenous or exogenous elements can facilitate efficient transient expression of mRNA in C. reinhardtii, including the 5' UTR of PsaD and the well-characterized Kozak sequence derived from the Chromochloris zofingiensis. In the optimized system, mRNA expression was detectable in 120 h with a peak around 4 h after transformation. Fluorescently tagged proteins were successfully transiently expressed, enabling organelle labeling and real-time determination of protein sub-cellular localization. Remarkably, transiently expressed IFT46 compensated for the ift46-1 mutant phenotype, indicating the correct protein folding and function of IFT46 within the cells. Additionally, we demonstrated the feasibility of our system for studying protein-protein interactions in living cells using bimolecular fluorescence complementation. In summary, the established transient expression system provides a powerful tool for investigating protein localization, function, and interactions in C. reinhardtii within a relatively short timeframe, which will significantly facilitate the study of gene function, genome structure, and green biomanufacturing in C. reinhardtii and potentially in other algae.

Future Directions of Allergen Immunotherapy for Allergic Rhinitis: Experts' Perspective

Allergen immunotherapy (AIT) is broadly used all over the world as the only available disease-modifying treatment option. The aim of this experts' perspective is to address 7 important unmet needs for the further direction of AIT and to provide the readership with the authors' positions on these topics. An international group of experts in the field of AIT have formulated 7 important aspects for the future position of AIT, performed a current literature review, and proposed a consented position on these topics. The aspects discussed and consented by the authors include: (1) alternative routes of allergen application in AIT, (2) potential of recombinant vaccines, (3) the role of allergy diagnosis based on component-resolved diagnosis for AIT composition, (4) the impact of COVID-19 vaccination for further innovations in AIT, (5) potential of combining biologics to AIT, (6) future innovations in high-risk children/adolescents, and (7) the future regulatory position on AIT. Important unmet needs and topics for AIT have been addressed in this expert review. The authors' views and personal position on these 7 aspects have also been elaborated.

The global mRNA vaccine patent landscape

In light of their quick development and low risk, mRNA vaccines are gradually replacing traditional vaccines. In order to characterize the patent landscape of mRNA vaccines, this study collated mRNA vaccine-related applications that have been registered since 1962. Accordingly, the 1852 patent families were discussed in relation to their temporal distribution, geographic scope, organizational assignees, and co-patenting activities. mRNA vaccines were shown to demonstrate promise in infectious disease, cancer immunotherapy, and allergic disease, with a focus on lipid nanoparticles. Notably, these vaccines are being developed against a backdrop of fierce industrial competition and intensive collaboration with a rise in applications. The findings of this study highlighted cutting-edge inventions, key players, and collaboration dynamics among institutions. By understanding the landscape of mRNA vaccine patents, researchers and those in industry may better comprehend the latest trends in this area, which may also assist relevant decision-making by academics, government officials, and industrial leaders.

mRNA vaccines: the most recent clinical applications of synthetic mRNA

Synthetic mRNA has been considered as an emerging biotherapeutic agent for the past decades. Recently, the SARS-CoV-2 pandemic has led to the first clinical use of synthetic mRNA. mRNA vaccines showed far surpassing influences on the public as compared to other vaccine platforms such as viral vector vaccines and recombinant protein vaccines. It allowed rapid development and production of vaccines that have never been achieved in history. Synthetic mRNA, called in vitro transcribed (IVT) mRNA, is the key component of mRNA vaccines. It has several advantages over conventional gene-expressing systems such as plasmid DNA and viral vectors. It can translate proteins in the cytoplasm by structurally resembling natural mRNA and exhibit various protein expression patterns depending on how it is engineered. Another advantage is that synthetic mRNA enables fast, scalable, and cost-effective production. Therefore, starting with the mRNA vaccine, synthetic mRNA is now in the spotlight as a promising new drug development agent. In this review, we will summarize the latest IVT mRNA technology such as new mRNA structures or large-scale production. In addition, the nature of the innate immunogenicity of IVT mRNA will be discussed along with its roles in the development of vaccines. Finally, the principles of the mRNA vaccine and the future direction of synthetic mRNA will be provided.

Immunological Mechanisms of Vaccine-Induced Protection against SARS-CoV-2 in Humans

The SARS-CoV-2 infection spread rapidly throughout the world and appears to involve in both humoral and cell-mediated immunity. SARS-CoV-2 is attached to host cells via binding to the viral spike (S) proteins and its cellular receptors angiotensin-converting enzyme 2 (ACE2). Consequently, the S protein is primed with serine proteases TMPRSS2 and TMPRSS4, which facilitate the fusion of viral and cellular membranes result in the entry of viral RNA into the host cell. Vaccines are urgently required to combat the coronavirus disease 2019 (COVID-19) outbreak and aid in the recovery to pre-pandemic levels of normality. The long-term protective immunity is provided by the vaccine antigen (or pathogen)-specific immune effectors and the activation of immune memory cells that can be efficiently and rapidly reactivated upon pathogen exposure. Research efforts aimed towards the design and development of vaccines for SARS-CoV-2 are increasing. Numerous coronavirus disease 2019 (COVID-19) vaccines have passed late-stage clinical investigations with promising outcomes. This review focuses on the present state and future prospects of COVID-19 vaccines research and development, with a particular emphasis on immunological mechanisms of various COVID-19vaccines such as adenoviral vector-based vaccines, mRNA vaccines, and DNA vaccines that elicits immunological responses against SARS-CoV-2 infections in humans.

RNA Nanotechnology-Mediated Cancer Immunotherapy

RNA molecules (e.g., siRNA, microRNA, and mRNA) have shown tremendous potential for immunomodulation and cancer immunotherapy. They can activate both innate and adaptive immune system responses by silencing or upregulating immune-relevant genes. In addition, mRNA-based vaccines have recently been actively pursued and tested in cancer patients, as a form of treatment. Meanwhile, various nanomaterials have been developed to enhance RNA delivery to the tumor and immune cells. In this review article, we summarize recent advances in the development of RNA-based therapeutics and their applications in cancer immunotherapy. We also highlight the variety of nanoparticle platforms that have been used for RNA delivery to elicit anti-tumor immune responses. Finally, we provide our perspectives of potential challenges and opportunities of RNA-based nanotherapeutics in clinical translation towards cancer immunotherapy.

Gene Therapy

Gene therapy medicinal products (GTMPs) are one of the most promising biopharmaceuticals, which are beginning to show encouraging results. The broad clinical research activity has been addressed mainly to cancer, primarily to those cancers that do not respond well to conventional treatment. GTMPs to treat rare disorders caused by single-gene mutations have also made important advancements toward market availability, with eye and hematopoietic system diseases as the main applications.Nucleic acid-marketed products are based on both in vivo and ex vivo strategies. Apart from DNA-based therapies, antisense oligonucleotides, small interfering RNA, and, recently, T-cell-based therapies have been also marketed. Moreover, the gene-editing tool CRISPR is boosting the development of new gene therapy-based medicines, and it is expected to have a substantial impact on the gene therapy biopharmaceutical market in the near future.However, despite the important advancements of gene therapy, many challenges have still to be overcome, which are discussed in this book chapter. Issues such as efficacy and safety of the gene delivery systems and manufacturing capacity of biotechnological companies to produce viral vectors are usually considered, but problems related to cost and patient affordability must be also faced to ensure the success of this emerging therapy. Graphical Abstract.

DNA and mRNA vaccination against allergies

Allergen-specific immunotherapy, which is performed by subcutaneous injection or sublingual application of allergen extracts, represents an effective treatment against type I allergic diseases. However, due to the long duration and adverse reactions, only a minority of patients decides to undergo this treatment. Alternatively, early prophylactic intervention in young children has been proposed to stop the increase in patient numbers. Plasmid DNA and mRNA vaccines encoding allergens have been shown to induce T helper 1 as well as T regulatory responses, which modulate or counteract allergic T helper 2-biased reactions. With regard to prophylactic immunization, additional safety measurements are required. In contrast to crude extracts, genetic vaccines provide the allergen at high purity. Moreover, by targeting the encoded allergen to subcellular compartments for degradation, release of native allergen can be avoided. Due to inherent safety features, mRNA vaccines could be the candidates of choice for preventive allergy immunizations. The subtle priming of T helper 1 immunity induced by this vaccine type closely resembles responses of non-allergic individuals and-by boosting via natural allergen exposure-could suffice for long-term protection from type I allergy.

Poly(β-amino ester)-co-poly(caprolactone) Terpolymers as Nonviral Vectors for mRNA Delivery In Vitro and In Vivo

The production of new proteins with messenger RNA (mRNA) has gained a broad interest due to its potential for addressing a wide range of diseases. Here, the design and characterization of novel ionizable poly(β-amino ester)-co-poly(caprolactone) terpolymers, synthesized via the combination of the ring opening polymerization and the Michael step-growth polymerization, are reported. The versatility of this method is demonstrated by varying the number of caprolactone units attached to each poly(β-amino ester) (PBAE) terpolymer. The ability of the novel poly-caprolactone (PCL)-based PBAE materials to deliver mRNA is shown to depend on the physiochemical characteristics of the material, such as lipophilicity, as well as the formulation method used to complex the polymer with the oligonucleotide. This latter variable represents a previously unstudied aspect of PBAE library screens that can play an important role in identifying true top candidates for nucleic acid delivery. The most stable terpolymer is injected intravenously (IV) in mice and shows a transfection efficacy several times higher than the polyethylenimine (PEI) which is focused in the spleen, opening the possibility to use these biodegradable carriers in the intravenous delivery of antigen-encoding mRNA for cancer immunotherapy and vaccination.

Comparison of the Expression Kinetics and Immunostimulatory Activity of Replicating mRNA, Nonreplicating mRNA, and pDNA after Intradermal Electroporation in Pigs

Synthetic mRNA is becoming increasingly popular as an alternative to pDNA-based gene therapy. Currently, multiple synthetic mRNA platforms have been developed. In this study we investigated the expression kinetics and the changes in mRNA encoding cytokine and chemokine levels following intradermal electroporation in pigs of pDNA, self-replicating mRNA, and modified and unmodified mRNA. The self-replicating mRNA tended to induce the highest protein expression, followed by pDNA, modified mRNA, and unmodified mRNA. Interestingly, the self-replicating mRNA was able to maintain its high expression levels during at least 12 days. In contrast, the expression of pDNA and the nonreplicating mRNAs dropped after respectively one and two days. Six days after intradermal electroporation a dose-dependent expression was observed for all vectors. Again, also at lower doses, the self-replicating mRNA tended to show the highest expression. All the mRNA vectors, including the modified mRNA, induced elevated levels of mRNA encoding cytokines and chemokines in the porcine skin after intradermal electroporation, while no such response was noticed after intradermal electroporation of the pDNA vector.

Induction of Protective Immunity against Toxoplasma gondii in Mice by Nucleoside Triphosphate Hydrolase-II (NTPase-II) Self-amplifying RNA Vaccine Encapsulated in Lipid Nanoparticle (LNP)

RNA-based vaccine represents an irresistible and safe immunization strategy with decreasing theoretical risks of genomic integration and malignant cell transformation. To our knowledge, however, there is no report about development of RNA vaccine against Toxoplasma gondii infection. We have previously demonstrated that the recombinant T. gondii nucleoside triphosphate hydrolase-II (NTPase-II) protein is able to provide protective Th1 cell-mediated immunity against T. gondii. Herein, we evaluated the immunogenic potential of a self-amplifying RNA vaccine-encoding T. gondii NTPase-II gene, RREP-NTPase-II, delivered by a synthetic lipid nanoparticle (LNP). Immunization of mice with naked RREP-NTPase-II induced a strong cellular and humoral immune response with high-IgG antibody titers and IFN-γ production. The immunized mice displayed significantly prolonged survival time and reduction in brain parasite load (46.4%) compared with control group. Furthermore, mice vaccinated with RREP-NTPase-II-encapsulated LNP displayed significantly enhanced protection against acute infection as well as chronic infection with PRU cyst, which shows 62.1% reduction in brain cyst burden in comparison to control group. These results suggest that the combination of self-amplifying RNA and LNP ion would be beneficial to the development of a safe and long-acting vaccine against toxoplasmosis.

mRNA-Based Therapeutics - Advances and Perspectives

In this review we discuss features of mRNA synthesis and modifications used to minimize immune response and prolong efficiency of the translation process in vivo. Considerable attention is given to the use of liposomes and nanoparticles containing lipids and polymers for the mRNA delivery. Finally we briefly discuss mRNAs which are currently in the clinical trials for cancer immunotherapy, vaccination against infectious diseases, and replacement therapy.

Generation and Evaluation of Prophylactic mRNA Vaccines Against Allergy

Due to the worldwide increase in allergies and a limited efficacy of therapeutic interventions, the need for prophylactic vaccination against allergies has been recognized. mRNA and DNA vaccines have demonstrated their high potential for preventing allergic sensitization by inducing an immunological bias that prevents TH2 sensitization. However, only mRNA vaccines fulfill the stringent safety requirements for vaccination of healthy children. In this chapter, we describe the generation of conventional as well as self-replicating mRNA vaccines and methods to test their prophylactic efficacy in animal models.

Potential of nanoparticles for allergen-specific immunotherapy - use of silica nanoparticles as vaccination platform

INTRODUCTION

Allergen-specific immunotherapy is the only curative approach for the treatment of allergies. There is an urgent need for improved therapies, which increase both, efficacy and patient compliance. Novel routes of immunization and the use of more advanced vaccine platforms have gained heightened interest in this field. Areas covered: The current status of allergen-specific immunotherapy is summarized and novel routes of immunization and their challenges in the clinics are critically discussed. The use of nanoparticles as novel delivery system for allergy vaccines is comprehensively reviewed. Specifically, the advantages of silica nanoparticles as vaccine carriers and adjuvants are summarized. Expert opinion: Future allergen-specific immunotherapy will combine engineered hypoallergenic vaccines with novel routes of administration, such as the skin. Due to their biodegradability, and the easiness to introduce surface modifications, silica nanoparticles are promising candidates for tailor-made vaccines. By covalently linking allergens and polysaccharides to silica nanoparticles, a versatile vaccination platform can be designed to specifically target antigen-presenting cells, render the formulation hypoallergenic, and introduce immunomodulatory functions. Combining potent skin vaccination methods, such as fractional laser ablation, with nanoparticle-based vaccines addresses all the requirements for safe and efficient therapy of allergic diseases.

The messenger's great message for vaccination

Poly ribonucleic acid (RNA) is the only polymer capable to recapitulate all processes of life: containment of genetic information, enzymatic activities and capacity to create defined 3D structures. Since it has a remarkable chemical stability (at neutral or acidic pH) and can be modified to enhance/reduce particular features (e.g., stability in biological RNase containing milieus or recognition by immune sensors), it is a particularly versatile and ideal active pharmaceutical ingredient. However, the utilization of RNA as a gene vehicle (messenger RNA, mRNA) for therapy has only recently been exploited. Within this scope, mRNA-based vaccines designed to trigger anti-cancer, anti-virus or anti-allergy immune responses have been developed. Modifications of mRNA vectors and implementation of adequate formulations have allowed to turn this natural superlative biological molecule into a safe active pharmaceutical ingredient that can virtually address any medical need including vaccination or immunotherapy. This is the newest great message delivered by this messenger.

Materials for non-viral intracellular delivery of messenger RNA therapeutics

Though therapeutics based on messenger RNA (mRNA) have broad potential in applications such as protein replacement therapy, cancer immunotherapy, and genomic engineering, their effective intracellular delivery remains a challenge. A chemically diverse suite of delivery materials with origins as materials for cellular transfection of DNA and small interfering RNAs (siRNAs) has recently been reported to have promise as non-viral delivery agents for mRNA. These materials include covalent conjugates, protamine complexes, nanoparticles based on lipids or polymers, and hybrid formulations. This review will highlight the use of delivery materials for mRNA, with a specific focus on their mechanisms of action, routes of administration, and dosages. Additionally, strategies in which these materials can be adapted and optimized to address challenges specific to mRNA delivery are also discussed. The technologies included have shown varying promise for therapeutic use, specifically having been used to deliver mRNA in vivo or exhibiting characteristics that could make in vivo use a possibility. In so doing, it is the intention of this review to provide a comprehensive look at the progress and possibilities in applying nucleic acid delivery technology specifically toward the emerging area of mRNA therapeutics.

Prophylactic mRNA Vaccination against Allergy Confers Long-Term Memory Responses and Persistent Protection in Mice

Recently, mRNA vaccines have been introduced as a safety-optimized alternative to plasmid DNA-based vaccines for protection against allergy. However, it remained unclear whether the short persistence of this vaccine type would limit memory responses and whether the protective immune response type would be maintained during recurrent exposure to allergen. We tested the duration of protective memory responses in mice vaccinated with mRNA encoding the grass pollen allergen Phl p 5 by challenging them with recombinant allergen, 3.5, 6, and 9 months after vaccination. In a second experiment, vaccinated mice were repeatedly challenged monthly with aerosolized allergen over a period of 7 months. Antibody and cytokine responses as well as lung inflammation and airway hyperresponsiveness were assessed. mRNA vaccination induced robust TH1 memory responses for at least 9 months. Vaccination efficiently suppressed TH2 cytokines, IgE responses, and lung eosinophilia. Protection was maintained after repeated exposure to aerosolized allergen and no TH1 associated pathology was observed. Lung function remained improved compared to nonvaccinated controls. Our data clearly indicate that mRNA vaccination against Phl p 5 induces robust, long-lived memory responses, which can be recalled by allergen exposure without side effects. mRNA vaccines fulfill the requirements for safe prophylactic vaccination without the need for booster immunizations.

mRNA transcript therapy

mRNA is the central molecule of all forms of life. It is generally accepted that current life on Earth descended from an RNA world. mRNA, after its first therapeutic description in 1992, has recently come into increased focus as a method to deliver genetic information. The recent solution to the two main difficulties in using mRNA as a therapeutic, immune stimulation and potency, has provided the basis for a wide range of applications. While mRNA-based cancer immunotherapies have been in clinical trials for a few years, novel approaches; including, in vivo delivery of mRNA to replace or supplement proteins, mRNA-based generation of pluripotent stem cells, or genome engineering using mRNA-encoded meganucleases are beginning to be realized. This review presents the current state of mRNA drug technologies and potential applications, as well as discussing the challenges and prospects in mRNA development and drug discovery.

Self-replicating alphavirus RNA vaccines

Recombinant nucleic acids are considered as promising next-generation vaccines. These vaccines express the native antigen upon delivery into tissue, thus mimicking live attenuated vaccines without having the risk of reversion to pathogenicity. They also stimulate the innate immune system, thus potentiating responses. Nucleic acid vaccines are easy to produce at reasonable cost and are stable. During the past years, focus has been on the use of plasmid DNA for vaccination. Now mRNA and replicon vaccines have come into focus as promising technology platforms for vaccine development. This review discusses self-replicating RNA vaccines developed from alphavirus expression vectors. These replicon vaccines can be delivered as RNA, DNA or as recombinant virus particles. All three platforms have been pre-clinically evaluated as vaccines against a number of infectious diseases and cancer. Results have been very encouraging and propelled the first human clinical trials, the results of which have been promising.

Potent immune responses in rhesus macaques induced by nonviral delivery of a self-amplifying RNA vaccine expressing HIV type 1 envelope with a cationic nanoemulsion

Self-amplifying messenger RNA (mRNA) of positive-strand RNA viruses are effective vectors for in situ expression of vaccine antigens and have potential as a new vaccine technology platform well suited for global health applications. The SAM vaccine platform is based on a synthetic, self-amplifying mRNA delivered by a nonviral delivery system. The safety and immunogenicity of an HIV SAM vaccine encoding a clade C envelope glycoprotein formulated with a cationic nanoemulsion (CNE) delivery system was evaluated in rhesus macaques. The HIV SAM vaccine induced potent cellular immune responses that were greater in magnitude than those induced by self-amplifying mRNA packaged in a viral replicon particle (VRP) or by a recombinant HIV envelope protein formulated with MF59 adjuvant, anti-envelope binding (including anti-V1V2), and neutralizing antibody responses that exceeded those induced by the VRP vaccine. These studies provide the first evidence in nonhuman primates that HIV vaccination with a relatively low dose (50 µg) of formulated self-amplifying mRNA is safe and immunogenic.

A development that may evolve into a revolution in medicine: mRNA as the basis for novel, nucleotide-based vaccines and drugs

Recent advances strongly suggest that mRNA rather than DNA will be the nucleotide basis for a new class of vaccines and drugs. Therapeutic cancer vaccines against a variety of targets have been developed on this basis and initial clinical experience suggests that preclinical activity can be successfully translated to human application. Likewise, prophylactic vaccines against viral pathogens and allergens have demonstrated their activity in animal models. These successes could be extended preclinically to mRNA protein and gene replacement therapy as well as the induction of pluripotent stem cells by mRNA encoded transcription factors. The production of mRNA-based vaccines and drugs is highly flexible, scalable and cost competitive, and eliminates the requirement of a cold chain. mRNA-based drugs and vaccines offer all the advantages of a nucleotide-based approach at reduced costs and represent a truly disruptive technology that may start a revolution in medicine.

Synthetic mRNAs for manipulating cellular phenotypes: an overview

Availability of high quality synthetic mRNAs (syn-mRNAs) has enabled progress in their applications. Important structural features and quality requirements are discussed. Developments in the application of mRNA-mediated manipulation of cells are presented (i) mRNA-directed expression of antigens in dendritic cells for vaccination projects in oncogenesis, infectious disease and allergy prevention; (ii) reprogramming of human fibroblasts to induced pluripotent stem cells with their subsequent differentiation to the desired cell type; (iii) applications in gene therapy.

Development of nucleic acid vaccines: use of self-amplifying RNA in lipid nanoparticles

Self-amplifying RNA or RNA replicon is a form of nucleic acid-based vaccine derived from either positive-strand or negative-strand RNA viruses. The gene sequences encoding structural proteins in these RNA viruses are replaced by mRNA encoding antigens of interest as well as by RNA polymerase for replication and transcription. This kind of vaccine has been successfully assayed with many different antigens as vaccines candidates, and has been shown to be potent in several animal species, including mice, nonhuman primates, and humans. A key challenge to realizing the broad potential of self-amplifying vaccines is the need for safe and effective delivery methods. Ideally, an RNA nanocarrier should provide protection from blood nucleases and extended blood circulation, which ultimately would increase the possibility of reaching the target tissue. The delivery system must then be internalized by the target cell and, upon receptor-mediated endocytosis, must be able to escape from the endosomal compartment into the cell cytoplasm, where the RNA machinery is located, while avoiding degradation by lysosomal enzymes. Further, delivery systems for systemic administration ought to be well tolerated upon administration. They should be safe, enabling the multiadministration treatment modalities required for improved clinical outcomes and, from a developmental point of view, production of large batches with reproducible specifications is also desirable. In this review, the concept of self-amplifying RNA vaccines and the most promising lipid-based delivery systems are discussed.

Nucleic acid vaccines: prospects for non-viral delivery of mRNA vaccines

INTRODUCTION

Nucleic acid-based vaccines are being developed as a means to combine the positive attributes of both live-attenuated and subunit vaccines. Viral vectors and plasmid DNA vaccines have been extensively evaluated in human clinical trials and have been shown to be safe and immunogenic, although none have been licensed for human use. More recently, mRNA-based vaccine alternatives have emerged and might offer certain advantages over their DNA-based counterparts.

AREAS COVERED

This review describes the two main categories of mRNA vaccines: conventional non-amplifying and self-amplifying mRNA. It summarizes the initial clinical proof-of-concept studies and outlines the preclinical testing of the next wave of innovations for the technology. Finally, this review highlights the versatile functionality of the mRNA molecule and introduces opportunities for future improvements in vaccine design.

EXPERT OPINION

The prospects for mRNA vaccines are very promising. Like other types of nucleic acid vaccines, mRNA vaccines have the potential to combine the positive attributes of live attenuated vaccines while obviating many potential safety limitations. Although data from initial clinical trials appear encouraging, mRNA vaccines are far from a commercial product. These initial approaches have spurred innovations in vector design, non-viral delivery, large-scale production and purification of mRNA to quickly move the technology forward. Some improvements have already been tested in preclinical models for both prophylactic and therapeutic vaccine targets and have demonstrated their ability to elicit potent and broad immune responses, including functional antibodies, type 1 T helper cells-type T cell responses and cytotoxic T cells. Though the initial barriers for this nucleic acid vaccine approach seem to be overcome, in our opinion, the future and continued success of this approach lies in a more extensive evaluation of the many non-viral delivery systems described in the literature and gaining a better understanding of the mechanism of action to allow rational design of next generation technologies.

Allergens are not pathogens: why immunization against allergy differs from vaccination against infectious diseases

Vaccination against infectious diseases has been one of the major breakthroughs in human medical history, saving the lives of millions of people each year. More recently, prophylactic vaccination against non-infectious diseases such as cancer, Alzheimer’s disease, diabetes, and type I allergy is being investigated. Particularly in case of IgE-driven allergic disorders, which afflict almost a quarter of the population in highly developed countries, preventative measures would represent a major improvement for patients’ health as well as an economic relief for public health services. As an alternative to allergen-specific immunotherapy, prophylactic vaccination against type I allergic diseases could slow down or even stop the progress of the allergy pandemic. Allergen-encoding gene-based vaccines, i.e., plasmid DNA and mRNA vaccines, provide the advantage of purity over crude allergen extracts, which involve the risk of de novo sensitizations. Furthermore, these formulations have been demonstrated to induce T helper 1 as well as T regulatory immune responses—a pre-requisite for prophylactic intervention against allergies. However, prophylactic vaccines against environmental allergens strikingly differ from conventional vaccines against infectious diseases or therapeutic approaches concerning the underlying immunological mechanisms.

Messenger RNA-based vaccines: progress, challenges, applications

Twenty years after the demonstration that messenger RNA (mRNA) was expressed and immunogenic upon direct injection in mice, the first successful proof-of-concept of specific protection against viral infection in small and large animals was reported. These data indicate wider applicability to infectious disease and should encourage continued translation of mRNA-based prophylactic vaccines into human clinical trials. At the conceptual level, mRNA-based vaccines-more than other genetic vectors-combine the simplicity, safety, and focused immunogenicity of subunit vaccines with favorable immunological properties of live viral vaccines: (1) mRNA vaccines are molecularly defined and carry no excess information. In the environment and upon physical contact, RNA is rapidly degraded by ubiquitous RNases and cannot persist. These characteristics also guarantee tight control over their immunogenic profile (including avoidance of vector-specific immune responses that could interfere with repeated administration), pharmacokinetics, and dosing. (2) mRNA vaccines are synthetically produced by an enzymatic process, just requiring information about the nucleic acid sequence of the desired antigen. This greatly reduces general complications associated with biological vaccine production, such as handling of infectious agents, genetic variability, environmental risks, or restrictions to vaccine distribution. (3) RNA can be tailored to provide potent adjuvant stimuli to the innate immune system by direct activation of RNA-specific receptors; this may reduce the need for additional adjuvants. The formation of native antigen in situ affords great versatility, including intracellular localization, membrane association, posttranslational modification, supra-molecular assembly, or targeted structural optimization of delivered antigen. Messenger RNA vaccines induce balanced immune responses including B cells, helper T cells, and cytotoxic T lymphocytes, rendering them an extremely adaptable platform. This article surveys the design, mode of action, and capabilities of state-of-the-art mRNA vaccines, focusing on the paradigm of influenza prophylaxis.

Nonviral delivery of self-amplifying RNA vaccines

Despite more than two decades of research and development on nucleic acid vaccines, there is still no commercial product for human use. Taking advantage of the recent innovations in systemic delivery of short interfering RNA (siRNA) using lipid nanoparticles (LNPs), we developed a self-amplifying RNA vaccine. Here we show that nonviral delivery of a 9-kb self-amplifying RNA encapsulated within an LNP substantially increased immunogenicity compared with delivery of unformulated RNA. This unique vaccine technology was found to elicit broad, potent, and protective immune responses, that were comparable to a viral delivery technology, but without the inherent limitations of viral vectors. Given the many positive attributes of nucleic acid vaccines, our results suggest that a comprehensive evaluation of nonviral technologies to deliver self-amplifying RNA vaccines is warranted.