Plant Viruses as Adjuvants for Next-Generation Vaccines and Immunotherapy

Vaccines are the cornerstone of infectious disease control and prevention. The outbreak of SARS-CoV-2 has confirmed the urgent need for a new approach to the design of novel vaccines. Plant viruses and their derivatives are being used increasingly for the development of new medical and biotechnological applications, and this is reflected in a number of preclinical and clinical studies. Plant viruses have a unique combination of features (biosafety, low reactogenicity, inexpensiveness and ease of production, etc.), which determine their potential. This review presents the latest data on the use of plant viruses with different types of symmetry as vaccine components and adjuvants in cancer immunotherapy. The discussion concludes that the most promising approaches might be those that use structurally modified plant viruses (spherical particles) obtained from the Tobacco mosaic virus. These particles combine high adsorption properties (as a carrier) with strong immunogenicity, as has been confirmed using various antigens in animal models. According to current research, it is evident that plant viruses have great potential for application in the development of vaccines and in cancer immunotherapy.

Current Advances of Plant-Based Vaccines for Neurodegenerative Diseases

Neurodegenerative diseases (NDDs) are characterized by the progressive degeneration and/or loss of neurons belonging to the central nervous system, and represent one of the major global health issues. Therefore, a number of immunotherapeutic approaches targeting the non-functional or toxic proteins that induce neurodegeneration in NDDs have been designed in the last decades. In this context, due to unprecedented advances in genetic engineering techniques and molecular farming technology, pioneering plant-based immunogenic antigen expression systems have been developed aiming to offer reliable alternatives to deal with important NDDs, including Alzheimer's disease, Parkinson's disease, and multiple sclerosis. Diverse reports have evidenced that plant-made vaccines trigger significant immune responses in model animals, supported by the production of antibodies against the aberrant proteins expressed in the aforementioned NDDs. Moreover, these immunogenic tools have various advantages that make them a viable alternative for preventing and treating NDDs, such as high scalability, no risk of contamination with human pathogens, cold chain free production, and lower production costs. Hence, this article presents an overview of the current progress on plant-manufactured vaccines for NDDs and discusses its future prospects.

Complexes Formed via Bioconjugation of Genetically Modified TMV Particles with Conserved Influenza Antigen: Synthesis and Characterization

Recently we obtained complexes between genetically modified Tobacco Mosaic Virus (TMV) particles and proteins carrying conserved influenza antigen such as M2e epitope. Viral vector TMV-N-lys based on TMV-U1 genome was constructed by insertion of chemically active lysine into the exposed N-terminal part of the coat protein. Nicotiana benthamiana plants were agroinjected and TMV-N-lys virions were purified from non-inoculated leaves. Preparation was analyzed by SDS-PAGE/Coomassie staining; main protein with electrophoretic mobility of 21 kDa was detected. Electron microscopy confirmed the stability of modified particles. Chemical conjugation of TMV-N-lys virions and target influenza antigen M2e expressed in E. coli was performed using 5 mM 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide and 1 mM N-hydroxysuccinimide. The efficiency of chemical conjugation was confirmed by Western blotting. For additional characterization we used conventional electron microscopy. The diameter of the complexes did not differ significantly from the initial TMV-N-lys virions, but complexes formed highly organized and extensive network with dense "grains" on the surface. Dynamic light scattering demonstrated that the single peaks, reflecting the complexes TMV-N-lys/DHFR-M2e were significantly shifted relative to the control TMV-N-lys virions. The indirect enzyme-linked immunosorbent assay with TMV- and DHFR-M2e-specific antibodies showed that the complexes retain stability during overnight adsorption. Thus, the results allow using these complexes for immunization of animals with the subsequent preparation of a candidate universal vaccine against the influenza virus.

Optimization of a Plasmodium falciparum circumsporozoite protein repeat vaccine using the tobacco mosaic virus platform

RTS,S/AS01 is a circumsporozoite protein (CSP)-based malaria vaccine that confers partial protection against malaria in endemic areas. Recent reports have elucidated structures of monoclonal antibodies that bind to the central (NPNA) repeat region of CSP and that inhibit parasite invasion. Antigen configuration and copy number of CSP repeats displayed on a tobacco mosaic virus (TMV) particle platform were studied. A TMV vaccine containing CSP repeats displayed as a loop induced 10× better antibody titer than a nearly full-length CSP in mice. In rhesus model, this translated to a 5× improvement in titer. Rhesus antibodies potently inhibited parasite invasion up to 11 mo after vaccination. An optimized epitope-focused, repeat-only CSP vaccine may be sufficient or better than the existing CSP vaccines.

Plasmodium falciparum vaccine RTS,S/AS01 is based on the major NPNA repeat and the C-terminal region of the circumsporozoite protein (CSP). RTS,S-induced NPNA-specific antibody titer and avidity have been associated with high-level protection in naïve subjects, but efficacy and longevity in target populations is relatively low. In an effort to improve upon RTS,S, a minimal repeat-only, epitope-focused, protective, malaria vaccine was designed. Repeat antigen copy number and flexibility was optimized using the tobacco mosaic virus (TMV) display platform. Comparing antigenicity of TMV displaying 3 to 20 copies of NPNA revealed that low copy number can reduce the abundance of low-affinity monoclonal antibody (mAb) epitopes while retaining high-affinity mAb epitopes. TMV presentation improved titer and avidity of repeat-specific Abs compared to a nearly full-length protein vaccine (FL-CSP). NPNAx5 antigen displayed as a loop on the TMV particle was found to be most optimal and its efficacy could be further augmented by combination with a human-use adjuvant ALFQ that contains immune-stimulators. These data were confirmed in rhesus macaques where a low dose of TMV-NPNAx5 elicited Abs that persisted at functional levels for up to 11 mo. We show here a complex association between NPNA copy number, flexibility, antigenicity, immunogenicity, and efficacy of CSP-based vaccines. We hypothesize that designing minimal epitope CSP vaccines could confer better and more durable protection against malaria. Preclinical data presented here supports the evaluation of TMV-NPNAx5/ALFQ in human trials.

Enhanced Stability and Controlled Delivery of MOF-Encapsulated Vaccines and Their Immunogenic Response In Vivo

Vaccines have an innate tendency to lose their structural conformation upon environmental and chemical stressors. A loss in conformation reduces the therapeutic ability to prevent the spread of a pathogen. Herein, we report an in-depth study of zeolitic imidazolate framework-8 and its ability to provide protection for a model viral vector against denaturing conditions. The immunoassay and spectroscopy analysis together demonstrate enhanced thermal and chemical stability to the conformational structure of the encapsulated viral nanoparticle. The long-term biological activity of this virus-ZIF composite was investigated in animal models to further elucidate the integrity of the encapsulated virus, the biosafety, and immunogenicity of the overall composite. Additionally, histological analysis found no observable tissue damage in the skin or vital organs in mice, following multiple subcutaneous administrations. This study shows that ZIF-based protein composites are strong candidates for improved preservation of proteinaceous drugs, are biocompatible, and are capable of controlling the release and adsorption of drugs in vivo.