Virus-like Particles Produced in Plants: A Promising Platform for Recombinant Vaccine Development

The capsid proteins of many viruses are capable of spontaneous self-assembly into virus-like particles (VLPs), which do not contain the viral genome and are therefore not infectious. VLPs are structurally similar to their parent viruses and are therefore effectively recognized by the immune system and can induce strong humoral and cellular immune responses. The structural features of VLPs make them an attractive platform for the development of potential vaccines and diagnostic tools. Chimeric VLPs can be obtained by attaching foreign peptides to capsid proteins. Chimeric VLPs present multiple copies of the antigen on their surface, thereby increasing the effectiveness of the immune response. Recombinant VLPs can be produced in different expression systems. Plants are promising biofactories for the production of recombinant proteins, including VLPs. The main advantages of plant expression systems are the overall low cost and safety of plant-produced products due to the absence of pathogens common to plants and animals. This review provides an overview of the VLP platform as an approach to developing plant-produced vaccines, focusing on the use of transient expression systems.

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.

Convenient site-selective protein coupling from bacterial raw lysates to coenzyme A-modified tobacco mosaic virus (TMV) by Bacillus subtilis Sfp phosphopantetheinyl transferase

A facile enzyme-mediated strategy enables site-specific covalent one-step coupling of genetically tagged luciferase molecules to coenzyme A-modified tobacco mosaic virus (TMV-CoA) both in solution and on solid supports. Bacillus subtilis surfactin phosphopantetheinyl transferase Sfp produced in E. coli mediated the conjugation of firefly luciferase N-terminally extended by eleven amino acids forming a 'ybbR tag' as Sfp-selective substrate, which even worked in bacterial raw lysates. The enzymes displayed on the protein coat of the TMV nanocarriers exhibited high activity. As TMV has proven a beneficial high surface-area adapter template stabilizing enzymes in different biosensing layouts in recent years, the use of TMV-CoA for fishing ybbR-tagged proteins from complex mixtures might become an advantageous concept for the versatile equipment of miniaturized devices with biologically active proteins. It comes along with new opportunities for immobilizing multiple functionalities on TMV adapter coatings, as desired, e.g., in handheld systems for point-of-care detection.

Plant Viruses: New Opportunities under the Pandemic

During the pandemic, an urgent task has become to develop new vaccine platforms that will help fight the infection caused by SARS-CoV-2 and quickly respond to newly emerging pathogens. Plant viruses can make a significant contribution to the solution of this problem. Phytoviruses, having the properties of any viral particles (self-assembly, immunogenicity, nanosize), are safe for humans since plants and mammals have no common infectious agents. As a result of thermal rearrangement of the tobacco mosaic virus, spherical particles of a protein nature have been obtained, which have unique immunostimulation and adsorption properties and can play the role of a universal adjuvant platform to create vaccines. Based on these particles, a scheme for obtaining vaccine preparations is proposed. This technology resembles a toy construction set for children. The basis is spherical particles, on the surface of which there are toy blocks-antigens. The "blocks" can be removed, added, or replaced, and this does not take much time and resources. Based on spherical particles, a polyvalent vaccine candidate against COVID-19 has been created as an adjuvant platform.

Structurally Modified Plant Viruses and Bacteriophages with Helical Structure. Properties and Applications

Structurally modified virus particles can be obtained from the rod-shaped or filamentous virions of plant viruses and bacteriophages by thermal or chemical treatment. They have recently attracted attention of the researchers as promising biogenic platforms for the development of new biotechnologies. This review presents data on preparation, structure, and properties of the structurally modified virus particles. In addition, their biosafety for animals is considered, as well as the areas of application of such particles in biomedicine. A separate section is devoted to one of the most relevant and promising areas for the use of structurally modified plant viruses – design of vaccine candidates based on them.

Vaccine Candidate Against COVID-19 Based on Structurally Modified Plant Virus as an Adjuvant

A recombinant vaccine candidate has been developed based on the major coronaviruses’ antigen (S protein) fragments and a novel adjuvant—spherical particles (SPs) formed during tobacco mosaic virus thermal remodeling. The receptor-binding domain and the highly conserved antigenic fragments of the S2 protein subunit were chosen for the design of recombinant coronavirus antigens. The set of three antigens (Co1, CoF, and PE) was developed and used to create a vaccine candidate composed of antigens and SPs (SPs + 3AG). Recognition of SPs + 3AG compositions by commercially available antibodies against spike proteins of SARS-CoV and SARS-CoV-2 was confirmed. The immunogenicity testing of these compositions in a mouse model showed that SPs improved immune response to the CoF and PE antigens. Total IgG titers against both proteins were 9–16 times higher than those to SPs. Neutralizing activity against SARS-CoV-2 in serum samples collected from hamsters immunized with the SPs + 3AG was demonstrated.

Thermal remodelling of Alternanthera mosaic virus virions and virus-like particles into protein spherical particles

The present work addresses the thermal remodelling of flexible plant viruses with a helical structure and virus-like particles (VLPs). Here, for the first time, the possibility of filamentous Alternanthera mosaic virus (AltMV) virions' thermal transition into structurally modified spherical particles (SP) has been demonstrated. The work has established differences in formation conditions of SP from virions (SPV) and VLPs (SPVLP) that are in accordance with structural data (on AltMV virions and VLPs). SP originate from AltMV virions through an intermediate stage. However, the same intermediate stage was not detected during AltMV VLPs' structural remodelling. According to the biochemical analysis, AltMV SPV consist of protein and do not include RNA. The structural characterisation of AltMV SPV/VLP by circular dichroism, intrinsic fluorescence spectroscopy and thioflavin T fluorescence assay has been performed. AltMV SPV/VLP adsorption properties and the availability of chemically reactive surface amino acids have been analysed. The revealed characteristics of AltMV SPV/VLP indicate that they could be applied as protein platforms for target molecules presentation and for the design of functionally active complexes.

The pharmacology of plant virus nanoparticles

The application of nanoparticles for medical purposes has made enormous strides in providing new solutions to health problems. The observation that plant virus-based nanoparticles (VNPs) can be repurposed and engineered as smart bio-vehicles for targeted drug delivery and imaging has launched extensive research for improving the therapeutic and diagnostic management of various diseases. There is evidence that VNPs are promising high value nanocarriers with potential for translational development. This is mainly due to their unique features, encompassing structural uniformity, ease of manufacture and functionalization by means of expression, chemical biology and self-assembly. While the development pipeline is moving rapidly, with many reports focusing on engineering and manufacturing aspects to tailor the properties and efficacy of VNPs, fewer studies have focused on gaining insights into the nanotoxicity of this novel platform nanotechnology. Herein, we discuss the pharmacology of VNPs as a function of formulation and route of administration. VNPs are reviewed in the context of their application as therapeutic adjuvants or nanocarrier excipients to initiate, enhance, attenuate or impede the formulation's toxicity. The summary of the data however also underlines the need for meticulous VNP structure-nanotoxicity studies to improve our understanding of their in vivo fates and pharmacological profiles to pave the way for translation of VNP-based formulations into the clinical setting.

Various Adjuvants Effect on Immunogenicity of Puumala Virus Vaccine

Various adjuvant effects on the immunogenicity of the candidate inactivated Puumala virus vaccine were detected in BALB/c mice. Adjuvants under study were: aluminum hydroxide, spherical particles of Tobacco mosaic virus coat protein, B subunit of heat-labile enterotoxin of Escherichia coli, and low endotoxic lipopolysaccharide of Shigella sonnei. Aluminum hydroxide (1 mg/ml) did not affect neutralizing antibodies’ induction and vaccine stability during storage compared to immunization with the vaccine without adjuvant. B subunit of heat-labile enterotoxin (0.2 µg/ml), low endotoxic lipopolysaccharide (50 µg/ml), and plant virus-based spherical particles (300 µg/ml) significantly enhance the humoral immune response of vaccine (p < 0.0001). Pronounced stimulation of IL-12 and IFN-ɣ was observed when mice were immunized with vaccines both with adjuvants (except of aluminum hydroxide) and without adjuvants. It has been shown that low endotoxic lipopolysaccharide contributes not only to enhance the immune response but also to stabilize vaccine immunogenicity during at least 1 year storage.

Two approaches for the stabilization of Bacillus anthracis recombinant protective antigen

Anthrax is a zoonotic disease caused by the gram-positive spore-forming bacteria Bacillus anthracis. There is a need for safe, highly effective, long-term storage vaccine formulations for mass vaccination. However, the development of new subunit vaccines based on recombinant protective antigen (rPA) faces the problem of vaccine antigen instability. Here, the potential of simultaneous application of two different approaches to stabilize rPA was demonstrated. Firstly, we employed spherical particles (SPs) obtained from the tobacco mosaic virus (TMV). Previously, we had reported that SPs can serve as an adjuvant and platform for antigen presentation. In the current work, SPs were shown to increase the stability of the full-size rPA without loss of its antigenic properties. The second direction was site-specific mutagenesis of asparagine residues to avoid deamidation that causes partial protein degradation. The modified recombinant protein comprising the PA immunogenic domains 3 and 4 (rPA3 + 4) was stable during storage at 4 and 25°C. rPA3 + 4 interacts with antibodies to rPA83 both individually and as a part of a complex with SPs. The results obtained can underpin the development of a recombinant vaccine with a full-size modified rPA (with similar amino acid substitutions that stabilize the protein) and SPs.

Plant virus particles with various shapes as potential adjuvants

Plant viruses are biologically safe for mammals and can be successfully used as a carrier/platform to present foreign epitopes in the course of creating novel putative vaccines. However, there is mounting evidence that plant viruses, their virus-like and structurally modified particles may also have an immunopotentiating effect on antigens not bound with their surface covalently. Here, we present data on the adjuvant properties of plant viruses with various shapes (Tobacco mosaic virus, TMV; Potato virus X, PVX; Cauliflower mosaic virus, CaMV; Bean mild mosaic virus, BMMV) and structurally modified TMV spherical particles (SPs). We have analysed the effectiveness of immune response to individual model antigens (ovalbumin, OVA/hen egg lysozyme, HEL) and to OVA/HEL in compositions with plant viruses/SPs, and have shown that CaMV, TMV and SPs can effectively induce total IgG titers to model antigen. Some intriguing data were obtained when analysing the immune response to the plant viruses/SPs themselves. Strong immunity was induced to CaMV, BMMV and PVX, whereas TMV and SPs stimulated considerably lower self-IgG titers. Our results provide new insights into the immunopotentiating properties of plant viruses and can be useful in devising adjuvants based on plant viruses.

Surface characterization of the thermal remodeling helical plant virus

Previously, we have reported that spherical particles (SPs) are formed by the thermal remodeling of rigid helical virions of native tobacco mosaic virus (TMV) at 94°C. SPs have remarkable features: stability, unique adsorption properties and immunostimulation potential. Here we performed a comparative study of the amino acid composition of the SPs and virions surface to characterize their properties and take an important step to understanding the structure of SPs. The results of tritium planigraphy showed that thermal transformation of TMV leads to a significant increase in tritium label incorporation into the following sites of SPs protein: 41-71 а.a. and 93-122 a.a. At the same time, there was a decrease in tritium label incorporation into the N- and C- terminal region (1-15 a.a., 142-158 a.a). The use of complementary physico-chemical methods allowed us to carry out a detailed structural analysis of the surface and to determine the most likely surface areas of SPs. The obtained data make it possible to consider viral protein thermal rearrangements, and to open new opportunities for biologically active complex design using information about SPs surface amino acid composition and methods of non-specific adsorption and bioconjugation.

TMV Particles: The Journey From Fundamental Studies to Bionanotechnology Applications

Ever since its initial characterization in the 19th century, tobacco mosaic virus (TMV) has played a prominent role in the development of modern virology and molecular biology. In particular, research on the three-dimensional structure of the virus particles and the mechanism by which these assemble from their constituent protein and RNA components has made TMV a paradigm for our current view of the morphogenesis of self-assembling structures, including viral particles. More recently, this knowledge has been applied to the development of novel reagents and structures for applications in biomedicine and bionanotechnology. In this article, we review how fundamental science has led to TMV being at the vanguard of these new technologies.

Assessment of structurally modified plant virus as a novel adjuvant in toxicity studies

Spherical particles (SPs) generated by thermally denatured tobacco mosaic virus (TMV) coat protein can act as an adjuvant, as they are able to enhance the magnitude and longevity of immune responses to different antigens. Here, the toxicity of TMV SPs was assessed prior to it being offered as a universal safe adjuvant for the development of vaccine candidates. The evaluation included nonclinical studies of a local tolerance following the single administration of TMV SPs, and of the local and systemic effects following repeated administrations of TMV SPs. These were conducted in mice, rats and rabbits. General health status, haematology and blood chemistry parameters were monitored on a regular basis. Also, reproductive and development toxicity were studied. No significant signs of toxicity were detected following single or repeated administrations of the adjuvant (TMV SPs). The absence of toxicological effects following the injection of TMV SPs is promising for the further development of recombinant vaccine candidates with TMV SPs as an adjuvant.

Study of rubella candidate vaccine based on a structurally modified plant virus

A novel rubella candidate vaccine based on a structurally modified plant virus - spherical particles (SPs) - was developed. SPs generated by the thermal remodelling of the tobacco mosaic virus are promising platforms for the development of vaccines. SPs combine unique properties: biosafety, stability, high immunogenicity and the effective adsorption of antigens. We assembled in vitro and characterised complexes (candidate vaccine) based on SPs and the rubella virus recombinant antigen. The candidate vaccine induced a strong humoral immune response against rubella. The IgG isotypes ratio indicated the predominance of IgG1 which plays a key role in immunity to natural rubella infection. The immune response was generally directed against the rubella antigen within the complexes. We suggest that SPs can act as a platform (depot) for the rubella antigen, enhancing specific immune response. Our results demonstrate that SPs-antigen complexes can be an effective and safe candidate vaccine against rubella.

Thermal conversion of filamentous potato virus X into spherical particles with different properties from virions

We developed a method for the fast transformation of virions of tobacco mosaic virus (TMV) in so-called spherical particles (SPs) of different sizes. These SPs turned out to be highly useful for the preparation of different kinds of important biotechnological products. In this communication, we report that a representative of the flexuous helical virus group-potato virus X (PVX), produces SPs as well, but these SPs differ from TMV SPs in several important aspects. PVX SPs may be useful biotechnological devices.

Novel roles for well-known players: from tobacco mosaic virus pests to enzymatically active assemblies

The rod-shaped nanoparticles of the widespread plant pathogen tobacco mosaic virus (TMV) have been a matter of intense debates and cutting-edge research for more than a hundred years. During the late 19th century, their behavior in filtration tests applied to the agent causing the 'plant mosaic disease' eventually led to the discrimination of viruses from bacteria. Thereafter, they promoted the development of biophysical cornerstone techniques such as electron microscopy and ultracentrifugation. Since the 1950s, the robust, helically arranged nucleoprotein complexes consisting of a single RNA and more than 2100 identical coat protein subunits have enabled molecular studies which have pioneered the understanding of viral replication and self-assembly, and elucidated major aspects of virus-host interplay, which can lead to agronomically relevant diseases. However, during the last decades, TMV has acquired a new reputation as a well-defined high-yield nanotemplate with multivalent protein surfaces, allowing for an ordered high-density presentation of multiple active molecules or synthetic compounds. Amino acid side chains exposed on the viral coat may be tailored genetically or biochemically to meet the demands for selective conjugation reactions, or to directly engineer novel functionality on TMV-derived nanosticks. The natural TMV size (length: 300 nm) in combination with functional ligands such as peptides, enzymes, dyes, drugs or inorganic materials is advantageous for applications ranging from biomedical imaging and therapy approaches over surface enlargement of battery electrodes to the immobilization of enzymes. TMV building blocks are also amenable to external control of in vitro assembly and re-organization into technically expedient new shapes or arrays, which bears a unique potential for the development of 'smart' functional 3D structures. Among those, materials designed for enzyme-based biodetection layouts, which are routinely applied, e.g., for monitoring blood sugar concentrations, might profit particularly from the presence of TMV rods: Their surfaces were recently shown to stabilize enzymatic activities upon repeated consecutive uses and over several weeks. This review gives the reader a ride through strikingly diverse achievements obtained with TMV-based particles, compares them to the progress with related viruses, and focuses on latest results revealing special advantages for enzyme-based biosensing formats, which might be of high interest for diagnostics employing 'systems-on-a-chip'.

Tobacco mosaic virus-based protein nanoparticles and nanorods for chemotherapy delivery targeting breast cancer

Drug delivery systems are required for drug targeting to avoid adverse effects associated with chemotherapy treatment regimes. Our approach is focused on the study and development of plant virus-based materials as drug delivery systems; specifically, this work focuses on the tobacco mosaic virus (TMV). Native TMV forms a hollow, high aspect-ratio nanotube measuring 300×18nm with a 4nm-wide central channel. Heat-transformation can be applied to TMV yielding spherical nanoparticles (SNPs) measuring ~50nm in size. While bioconjugate chemistries have been established to modify the TMV rod, such methods have not yet been described for the SNP platform. In this work, we probed the reactivity of SNPs toward bioconjugate reactions targeting lysine, glutamine/aspartic acid, and cysteine residues. We demonstrate functionalization of SNPs using these chemistries yielding efficient payload conjugation. In addition to covalent labeling techniques, we developed encapsulation techniques, where the cargo is loaded into the SNP during heat-transition from rod-to-sphere. Finally, we developed TMV and SNP formulations loaded with the chemotherapeutic doxorubicin, and we demonstrate the application of TMV rods and spheres for chemotherapy delivery targeting breast cancer.

The key role of rubella virus glycoproteins in the formation of immune response, and perspectives on their use in the development of new recombinant vaccines

Rubella is a highly contagious viral disease which is mostly threatens to women of reproductive age. Existent live attenuated vaccines are effective enough, but have some drawbacks and are unusable for a certain group of people, including pregnant women and people with AIDS and other immunodeficiency. Thereby the development of alternative non-replicating, recombinant vaccines undoubtedly is needed. This review discusses the protein E1 and E2 role in formation of immune response and perspectives in development of new generation recombinant vaccines using them.

Modified TMV Particles as Beneficial Scaffolds to Present Sensor Enzymes

Tobacco mosaic virus (TMV) is a robust nanotubular nucleoprotein scaffold increasingly employed for the high density presentation of functional molecules such as peptides, fluorescent dyes, and antibodies. We report on its use as advantageous carrier for sensor enzymes. A TMV mutant with a cysteine residue exposed on every coat protein (CP) subunit (TMVCys) enabled the coupling of bifunctional maleimide-polyethylene glycol (PEG)-biotin linkers (TMVCys/Bio). Its surface was equipped with two streptavidin [SA]-conjugated enzymes: glucose oxidase ([SA]-GOx) and horseradish peroxidase ([SA]-HRP). At least 50% of the CPs were decorated with a linker molecule, and all thereof with active enzymes. Upon use as adapter scaffolds in conventional "high-binding" microtiter plates, TMV sticks allowed the immobilization of up to 45-fold higher catalytic activities than control samples with the same input of enzymes. Moreover, they increased storage stability and reusability in relation to enzymes applied directly to microtiter plate wells. The functionalized TMV adsorbed to solid supports showed a homogeneous distribution of the conjugated enzymes and structural integrity of the nanorods upon transmission electron and atomic force microscopy. The high surface-increase and steric accessibility of the viral scaffolds in combination with the biochemical environment provided by the plant viral coat may explain the beneficial effects. TMV can, thus, serve as a favorable multivalent nanoscale platform for the ordered presentation of bioactive proteins.

Plant Viruses as Nanoparticle-Based Vaccines and Adjuvants

Vaccines are considered one of the greatest medical achievements in the battle against infectious diseases. However, the intractability of various diseases such as hepatitis C, HIV/AIDS, malaria, tuberculosis, and cancer poses persistent hurdles given that traditional vaccine-development methods have proven to be ineffective; as such, these challenges have driven the emergence of novel vaccine design approaches. In this regard, much effort has been put into the development of new safe adjuvants and vaccine platforms. Of particular interest, the utilization of plant virus-like nanoparticles and recombinant plant viruses has gained increasing significance as an effective tool in the development of novel vaccines against infectious diseases and cancer. The present review summarizes recent advances in the use of plant viruses as nanoparticle-based vaccines and adjuvants and their mechanism of action. Harnessing plant-virus immunogenic properties will enable the design of novel, safe, and efficacious prophylactic and therapeutic vaccines against disease.

RNA-controlled assembly of tobacco mosaic virus-derived complex structures: from nanoboomerangs to tetrapods

The in vitro assembly of artificial nanotubular nucleoprotein shapes based on tobacco mosaic virus-(TMV-)-derived building blocks yielded different spatial organizations of viral coat protein subunits on genetically engineered RNA molecules, containing two or multiple TMV origins of assembly (OAs). The growth of kinked nanoboomerangs as well as of branched multipods was determined by the encapsidated RNAs. A largely simultaneous initiation at two origins and subsequent bidirectional tube elongation could be visualized by transmission electron microscopy of intermediates and final products. Collision of the nascent tubes' ends produced angular particles with well-defined arm lengths. RNAs with three to five OAs generated branched multipods with a maximum of four arms. The potential of such an RNA-directed self-assembly of uncommon nanotubular architectures for the fabrication of complex multivalent nanotemplates used in functional hybrid materials is discussed.

Proteins immobilization on the surface of modified plant viral particles coated with hydrophobic polycations

Two hydrophobic cations based on poly-N-ethyl-vinylpyridine were used to produce biologically active complexes. The complexes obtained from tobacco mosaic virus (TMV) spherical particles (SPs), hydrophobic polycation, and a model protein were stable and did not aggregate in solution, particularly at high ionic strengths. The nucleic acid-free SPs were generated by thermal remodeling of the TMV (helical rod-shaped plant virus). The model protein preserved its antigenic activity in the ternary complex (SP-polycation-protein). Immobilization of proteins on the surface of SPs coated with hydrophobic cation is a promising approach to designing biologically active complexes used in bionanotechnologies.

Biodistribution, pharmacokinetics, and blood compatibility of native and PEGylated tobacco mosaic virus nano-rods and -spheres in mice

Understanding the pharmacokinetics, blood compatibility, biodistribution and clearance properties of nanoparticles is of great importance to their translation to clinical application. In this paper we report the biodistribution and pharmacokinetic properties of tobacco mosaic virus (TMV) in the forms of 300×18nm(2) rods and 54nm-sized spheres. The availability of rods and spheres made of the same protein provides a unique scaffold to study the effect of nanoparticle shape on in vivo fate. For enhanced biocompatibility, we also considered a PEGylated formulation. Overall, the versions of nanoparticles exhibited comparable in vivo profiles; a few differences were noted: data indicate that rods circulate longer than spheres, illustrating the effect that shape plays on circulation. Also, PEGylation increased circulation times. We found that macrophages in the liver and spleen cleared the TMV rods and spheres from circulation. In the spleen, the viral nanoparticles trafficked through the marginal zone before eventually co-localizing in B-cell follicles. TMV rods and spheres were cleared from the liver and spleen within days with no apparent changes in histology, it was noted that spheres are more rapidly cleared from tissues compared to rods. Further, blood biocompatibility was supported, as none of the formulations induced clotting or hemolysis. This work lays the foundation for further application and tailoring of TMV for biomedical applications.

Complexes assembled from TMV-derived spherical particles and entire virions of heterogeneous nature

Previously, we described some structural features of spherical particles (SPs) generated by thermal remodelling of the tobacco mosaic virus. The SPs represent a universal platform that could bind various proteins. Here, we report that entire isometric virions of heterogeneous nature bind non-specifically to the SPs. Formaldehyde (FA) was used for covalent binding of a virus to the SPs surface for stabilizing the SP-virus complexes. Transmission and high resolution scanning electron microscopy showed that the SPs surface was covered with virus particles. The architecture of SP-virion complexes was examined by immunologic methods. Mean diameters of SPs and SP-human enterovirus C and SP-cauliflower mosaic virus (CaMV) compositions were determined by nanoparticle tracking analysis (NTA) in liquid. Significantly, neither free SPs nor individual virions were detected by NTA in either FA-crosslinked or FA-untreated compositions. Entirely, all virions were bound to the SPs surface and the SP sites within the SP-CaMV complexes were inaccessible for anti-SP antibodies. Likewise, the SPs immunogenicity within the FA-treated SPs-CaMV compositions was negligible. Apparently, the SP antigenic sites were hidden and masked by virions within the compositions. Previously, we reported that the SPs exhibited adjuvant activity when foreign proteins/epitopes were mixed with or crosslinked to SPs. We found that immunogenicity of entire CaMV crosslinked to SP was rather low which could be due to the above-mentioned masking of the SPs booster. Contrastingly, immunogenicity of the FA-untreated compositions increased significantly, presumably, due to partial release of virions and unmasking of some SPs-buster sites after animals immunization.

Examination of biologically active nanocomplexes by nanoparticle tracking analysis

Nanoparticle tracking analysis (NTA) was first applied to biologically active nanocomplexes to obtain concurrent information on their size, state of aggregation, concentration, and antigenic specificity in liquid. The subject of the NTA was an immunogenic complex (a candidate nanovaccine) comprised of spherical particles (SPs) generated by thermal remodeling of the tobacco mosaic virus and Rubella virus tetraepitopes exposed on the surface of SP.

β-structure of the coat protein subunits in spherical particles generated by tobacco mosaic virus thermal denaturation

Conversion of the rod-like tobacco mosaic virus (TMV) virions into "ball-like particles" by thermal denaturation at 90-98 °C had been described by R.G. Hart in 1956. We have reported recently that spherical particles (SPs) generated by thermal denaturation of TMV at 94-98 °C were highly stable, RNA-free, and water-insoluble. The SPs were uniform in shape but varied widely in size (53-800 nm), which depended on the virus concentration. Here, we describe some structural characteristics of SPs using circular dichroism, fluorescence spectroscopy, and Raman spectroscopy. It was found that the structure of SPs protein differs strongly from that of the native TMV and is characterized by coat protein subunits transition from mainly (about 50%) α-helical structure to a structure with low content of α-helices and a significant fraction of β-sheets. The SPs demonstrate strong reaction with thioflavin T suggesting the formation of amyloid-like structures.