Production of Mosaic Turnip Crinkle Virus-Like Particles Derived by Coinfiltration of Wild-Type and Modified Forms of Virus Coat Protein in Plants

Production of Plant Virus-Derived Hybrid Nanoparticles Decorated with Different Nanobodies

Viral nanoparticles (VNPs) are self-assembled nanometric complexes whose size and shape are similar to those of the virus from which they are derived. VNPs are arousing great attention due to potential biotechnological applications in fields like nanomedicine and nanotechnology because they allow the presentation of polypeptides of choice linked to the virus structural proteins. Starting from tobacco etch virus (TEV), a plant plus-strand RNA virus that belongs to the genus Potyvirus (family Potyviridae), here we describe the development of recombinant hybrid VNPs in Nicotiana benthamiana plants able of exposing simultaneously different proteins on their surface. This system is based on the synergic coinfection of TEV and potato virus X (PVX; Potexvirus), in which PVX provides a second TEV CP in trans allowing a mixed assembly. We first generated genetically modified hybrid VNPs simultaneously displaying green and red fluorescent proteins on their surface. A population of decorated and nondecorated CPs resulting from the insertion of the picornavirus F2A ribosomal escape peptide was required for viral particle assembly. Correct assembly of the recombinant mosaic VNPs presenting the exogenous peptides was successfully observed by immunoelectron microscopy. We next achieved the production of hybrid VNPs expressing a nanobody against SARS-CoV-2 and a fluorescent reporter protein, whose functionality was demonstrated by ELISA and dot-blot assay. Finally, we engineered the production of hybrid multivalent VNPs carrying two different nanobodies against distinct epitopes of the same SARS-CoV-2 antigenic protein, emulating a nanobody cocktail. These plant-produced recombinant mosaic VNPs, which are filamentous and flexuous in shape, presenting two different fused proteins on the surface, represent a molecular tool with several potential applications in biotechnology.

Plant virus-derived nanoparticles decorated with genetically encoded SARS-CoV-2 nanobodies display enhanced neutralizing activity

Viral nanoparticles (VNPs) are a new class of virus-based formulations that can be used as building blocks to implement a variety of functions of potential interest in biotechnology and nanomedicine. Viral coat proteins (CP) that exhibit self-assembly properties are particularly appropriate for displaying antigens and antibodies, by generating multivalent VNPs with therapeutic and diagnostic potential. Here, we developed genetically encoded multivalent VNPs derived from two filamentous plant viruses, potato virus X (PVX) and tobacco etch virus (TEV), which were efficiently and inexpensively produced in the biofactory Nicotiana benthamiana plant. PVX and TEV-derived VNPs were decorated with two different nanobodies recognizing two different regions of the receptor-binding domain (RBD) of the SARS-CoV-2 Spike protein. The addition of different picornavirus 2A ribosomal skipping peptides between the nanobody and the CP allowed for modulating the degree of VNP decoration. Nanobody-decorated VNPs purified from N. benthamiana tissues successfully recognized the RBD antigen in enzyme-linked immunosorbent assays and showed efficient neutralization activity against pseudoviruses carrying the Spike protein. Interestingly, multivalent PVX and TEV-derived VNPs exhibited a neutralizing activity approximately one order of magnitude higher than the corresponding nanobody in a dimeric format. These properties, combined with the ability to produce VNP cocktails in the same N. benthamiana plant based on synergistic infection of the parent PVX and TEV, make these green nanomaterials an attractive alternative to standard antibodies for multiple applications in diagnosis and therapeutics.

Citrus sudden death-associated virus as a new expression vector for rapid in planta production of heterologous proteins, chimeric virions, and virus-like particles

The more we understand the strategies used by viruses for protein expression, the more possibilities we have to exploit viruses as expression vectors for heterologous protein production. Advances in the development of virus-based expression systems have been possible due to generation of many virus infectious clones, especially those derived from plant viruses, which have the capability for rapid and high-level transient expression of proteins in plant cells, a robust and low-cost bioreactor. In this work, we generated new replicative virus expression vectors based on a previously constructed citrus sudden death-associated virus (CSDaV) infectious cDNA clone. These vectors were generated to express the reporter green fluorescent protein (GFP) in Nicotiana benthamiana leaves by taking advantage of the expression strategies used by CSDaV to produce its structural proteins. We show that higher amounts of GFP can be produced from a coat protein (CP)-independent CSDaV-based vector, compared to levels of GFP expressed from a widely used non-replicative vector (pEAQ series); or GFP can be produced in fusion with the major CSDaV CP (CPp21) to be incorporated into chimeric virions. However, GFP-recombinant CSDaV virions do not appear uniformly assembled, but more likely as mosaic particles. Cryo-electron microscopy analysis from this work revealed the structures of the wild-type and the GFP-recombinant CSDaV virions, but it was not able to reveal where exactly the GFP is displayed in the chimeric virions. We show though that the incorporation of GFP-CPp21 fusion protein into virions occurs solely due to its interaction with free/non-fused CPp21, independent of other viral proteins. Therefore, individual co-expression of GFP-CPp21 and CPp21 in the same plant cells leads to the production of chimeric virus-like particles (VLPs), while GFP-CPp21 fusion protein itself is not able to self-assemble into VLPs. The new CSDaV-based expression vectors may provide an alternative platform for use in molecular farming, either for production of heterologous proteins or as scaffold for heterologous protein display.

Production of Potyvirus-Derived Nanoparticles Decorated with a Nanobody in Biofactory Plants

Viral nanoparticles (VNPs) have recently attracted attention for their use as building blocks for novel materials to support a range of functions of potential interest in nanotechnology and medicine. Viral capsids are ideal for presenting small epitopes by inserting them at an appropriate site on the selected coat protein (CP). VNPs presenting antibodies on their surfaces are considered highly promising tools for therapeutic and diagnostic purposes. Due to their size, nanobodies are an interesting alternative to classic antibodies for surface presentation. Nanobodies are the variable domains of heavy-chain (VHH) antibodies from animals belonging to the family Camelidae, which have several properties that make them attractive therapeutic molecules, such as their small size, simple structure, and high affinity and specificity. In this work, we have produced genetically encoded VNPs derived from two different potyviruses—the largest group of RNA viruses that infect plants—decorated with nanobodies. We have created a VNP derived from zucchini yellow mosaic virus (ZYMV) decorated with a nanobody against the green fluorescent protein (GFP) in zucchini (Cucurbita pepo) plants. As reported for other viruses, the expression of ZYMV-derived VNPs decorated with this nanobody was only made possible by including a picornavirus 2A splicing peptide between the fused proteins, which resulted in a mixed population of unmodified and decorated CPs. We have also produced tobacco etch virus (TEV)-derived VNPs in Nicotiana benthamiana plants decorated with the same nanobody against GFP. Strikingly, in this case, VNPs could be assembled by direct fusion of the nanobody to the viral CP with no 2A splicing involved, likely resulting in fully decorated VNPs. For both expression systems, correct assembly and purification of the recombinant VNPs was confirmed by transmission electron microscope; the functionality of the CP-fused nanobody was assessed by western blot and binding assays. In sum, here we report the production of genetically encoded plant-derived VNPs decorated with a nanobody. This system may be an attractive alternative for the sustainable production in plants of nanobody-containing nanomaterials for diagnostic and therapeutic purposes.

Plant-expressed virus-like particles reveal the intricate maturation process of a eukaryotic virus

Many virus capsids undergo exquisitely choreographed maturation processes in their host cells to produce infectious virions, and these remain poorly understood. As a tool for studying virus maturation, we transiently expressed the capsid protein of the insect virus Nudaurelia capensis omega virus (NωV) in Nicotiana benthamiana and were able to purify both immature procapsids and mature capsids from infiltrated leaves by varying the expression time. Cryo-EM analysis of the plant-produced procapsids and mature capsids to 6.6 Å and 2.7 Å resolution, respectively, reveals that in addition to large scale rigid body motions, internal regions of the subunits are extensively remodelled during maturation, creating the active site required for autocatalytic cleavage and infectivity. The mature particles are biologically active in terms of their ability to lyse membranes and have a structure that is essentially identical to authentic virus. The ability to faithfully recapitulate and visualize a complex maturation process in plants, including the autocatalytic cleavage of the capsid protein, has revealed a ~30 Å translation-rotation of the subunits during maturation as well as conformational rearrangements in the N and C-terminal helical regions of each subunit.