Immunogenic profile of a plant-produced nonavalent African horse sickness viral protein 2 (VP2) vaccine in IFNAR-/- mice

A safe, highly immunogenic multivalent vaccine to protect against all nine serotypes of African horse sickness virus (AHSV), will revolutionise the AHS vaccine industry in endemic countries and beyond. Plant-produced AHS virus-like particles (VLPs) and soluble viral protein 2 (VP2) vaccine candidates were developed that have the potential to protect against all nine serotypes but can equally well be formulated as mono- and bi-valent formulations for localised outbreaks of specific serotypes. In the first interferon α/β receptor knock-out (IFNAR-/-) mice trial conducted, a nine-serotype (nonavalent) vaccine administered as two pentavalent (5 μg per serotype) vaccines (VLP/VP2 combination or exclusively VP2), were directly compared to the commercially available AHS live attenuated vaccine. In a follow up trial, mice were vaccinated with an adjuvanted nine-serotype multivalent VP2 vaccine in a prime boost strategy and resulted in the desired neutralising antibody titres of 1:320, previously demonstrated to confer protective immunity in IFNAR-/- mice. In addition, the plant-produced VP2 vaccine performed favourably when compared to the commercial vaccine. Here we provide compelling data for a nonavalent VP2-based vaccine candidate, with the VP2 from each serotype being antigenically distinguishable based on LC-MS/MS and ELISA data. This is the first preclinical trial demonstrating the ability of an adjuvanted nonavalent cocktail of soluble, plant-expressed AHS VP2 proteins administered in a prime-boost strategy eliciting high antibody titres against all 9 AHSV serotypes. Furthermore, elevated T helper cells 2 (Th2) and Th1, indicative of humoral and cell-mediated memory T cell immune responses, respectively, were detected in mouse serum collected 14 days after the multivalent prime-boost vaccination. Both Th2 and Th1 may play a role to confer protective immunity. These preclinical immunogenicity studies paved the way to test the safety and protective efficacy of the plant-produced nonavalent VP2 vaccine candidate in the target animals, horses.

Protective immunity of plant-produced African horse sickness virus serotype 5 chimaeric virus-like particles (VLPs) and viral protein 2 (VP2) vaccines in IFNAR-/- mice

Next generation vaccines have the capability to contribute to and revolutionise the veterinary vaccine industry. African horse sickness (AHS) is caused by an arbovirus infection and is characterised by respiratory distress and/or cardiovascular failure and is lethal to horses. Mandatory annual vaccination in endemic areas curtails disease occurrence and severity. However, development of a next generation AHSV vaccine, which is both safe and efficacious, has been an objective globally for years. In this study, both AHSV serotype 5 chimaeric virus-like particles (VLPs) and soluble viral protein 2 (VP2) were successfully produced in Nicotiana benthamiana ΔXT/FT plants, partially purified and validated by gel electrophoresis, transmission electron microscopy and liquid chromatography-mass spectrometry (LC-MS/MS) based peptide sequencing before vaccine formulation. IFNAR^-/- mice vaccinated with the adjuvanted VLPs or VP2 antigens in a 10 µg prime-boost regime resulted in high titres of antibodies confirmed by both serum neutralising tests (SNTs) and enzyme-linked immunosorbent assays (ELISA). Although previous studies reported high titres of antibodies in horses when vaccinated with plant-produced AHS homogenous VLPs, this is the first study demonstrating the protective efficacy of both AHSV serotype 5 chimaeric VLPs and soluble AHSV-5 VP2 as vaccine candidates. Complementary to this, coating ELISA plates with the soluble VP2 has the potential to underpin serotype-specific serological assays.

Plant-produced chimeric virus-like particles - a new generation vaccine against African horse sickness

BACKGROUND

African horse sickness (AHS) is a severe arthropod-borne viral disease of equids, with a mortality rate of up to 95% in susceptible naïve horses. Due to safety concerns with the current live, attenuated AHS vaccine, alternate safe and effective vaccination strategies such as virus-like particles (VLPs) are being investigated. Transient plant-based expression systems are a rapid and highly scalable means of producing such African horse sickness virus (AHSV) VLPs for vaccine purposes.

RESULTS

In this study, we demonstrated that transient co-expression of the four AHSV capsid proteins in agroinfiltrated Nicotiana benthamiana dXT/FT plants not only allowed for the assembly of homogenous AHSV-1 VLPs but also single, double and triple chimeric VLPs, where one capsid protein originated from one AHS serotype and at least one other capsid protein originated from another AHS serotype. Following optimisation of a large scale VLP purification procedure, the safety and immunogenicity of the plant-produced, triple chimeric AHSV-6 VLPs was confirmed in horses, the target species.

CONCLUSIONS

We have successfully shown assembly of single and double chimeric AHSV-7 VLPs, as well as triple chimeric AHSV-6 VLPs, in Nicotiana benthamiana dXT/FT plants. Plant produced chimeric AHSV-6 VLPs were found to be safe for administration into 6 month old foals as well as capable of eliciting a weak neutralizing humoral immune response in these target animals against homologous AHSV virus.

Plant-produced Bluetongue chimaeric VLP vaccine candidates elicit serotype-specific immunity in sheep

Bluetongue (BT) is a hemorrhagic non-contagious, biting midge-transmitted disease of wild and domestic ruminants that is caused by bluetongue virus (BTV). Annual vaccination plays a pivotal role in BT disease control in endemic regions. Due to safety concerns of the current BTV multivalent live attenuated vaccine (LAV), a safe efficacious new generation subunit vaccine such as a plant-produced BT virus-like particle (VLP) vaccine is imperative. Previously, homogenous BTV serotype 8 (BTV-8) VLPs were successfully produced in Nicotiana benthamiana plants and provided protective immunity in sheep. In this study, combinations of BTV capsid proteins from more than one serotype were expressed and assembled to form chimaeric BTV-3 and BTV-4 VLPs in N. benthamiana plants. The assembled homogenous BTV-8, as well as chimaeric BTV-3 and chimaeric BTV-4 VLP serotypes, were confirmed by SDS-PAGE, Transmission Electron microscopy (TEM) and protein confirmation using liquid chromatography-mass spectrometry (LC-MS/MS) based peptide sequencing. As VP2 is the major determinant eliciting protective immunity, the percentage coverage and number of unique VP2 peptides detected in assembled chimaeric BT VLPs were used as a guide to assemble the most appropriate chimaeric combinations. Both plant-produced chimaeric BTV-3 and BTV-4 VLPs were able to induce long-lasting serotype-specific neutralizing antibodies equivalent to the monovalent LAV controls. Antibody levels remained high to the end of the trial. Combinations of homogenous and chimaeric BT VLPs have great potential as a safe, effective multivalent vaccine with the ability to distinguish between vaccinated and infected individuals (DIVA) due to the absence of non-structural proteins.

A Dual Laser Scanning Confocal and Transmission Electron Microscopy Analysis of the Intracellular Localization, Aggregation and Particle Formation of African Horse Sickness Virus Major Core Protein VP7

The bulk of the major core protein VP7 in African horse sickness virus (AHSV) self-assembles into flat, hexagonal crystalline particles in a process appearing unrelated to viral replication. Why this unique characteristic of AHSV VP7 is genetically conserved, and whether VP7 aggregation and particle formation have an effect on cellular biology or the viral life cycle, is unknown. Here we investigated how different small peptide and enhanced green fluorescent protein (eGFP) insertions into the VP7 top domain affected VP7 localization, aggregation, and particle formation. This was done using a dual laser scanning confocal and transmission electron microscopy approach in conjunction with analyses of the solubility, aggregation, and fluorescence profiles of the proteins. VP7 top domain modifications did not prevent trimerization, or intracellular trafficking, to one or two discrete sites in the cell. However, modifications that resulted in a misfolded and insoluble VP7-eGFP component blocked trafficking, and precluded protein accumulation at a single cellular site, perhaps by interfering with normal trimer-trimer interactions. Furthermore, the modifications disrupted the stable layering of the trimers into characteristic AHSV VP7 crystalline particles. It was concluded that VP7 trafficking is driven by a balance between VP7 solubility, trimer forming ability, and trimer-trimer interactions.