Towards producing infectious bronchitis virus-like particles in plants as potential vaccine candidates

Abstract

Infectious Bronchitis Virus (IBV) causes a highly contagious respiratory disease that affects chickens and other poultry. This OIE-listed disease has global economic implications with losses per flock estimated at 10% to 20% of the market value. The South African poultry industry makes up the largest subsector within the country’s agricultural sector, producing an annual turnover of over R40 billion. Therefore, it would be beneficial to the poultry industry to prevent economic losses due to diseases such as IBV. Current autogenous IBV vaccines are effective but require the passage of live viruses in embryonated chicken eggs. Therefore, safe, efficacious, new-generation vaccines are imperative. Biopharming is the production of recombinant pharmaceutical proteins by making use of plants as bioreactors. Plants are capable of producing large quantities of target proteins cost-effectively, offering attractive alternatives for the production of vaccines, antigens for diagnostics, and other pharmaceuticals. The aim of this project was to produce infectious bronchitis virus-like particles (VLPs) displaying the major surface antigen, the Spike protein, in Nicotiana benthamiana plants by transiently co-expressing and assembling the structural proteins of IBV. VLPs are multiprotein structures that mimic the authentic virus, while lacking the genome, resulting in safe, efficacious, DIVA (distinguish between infected and vaccinated animals) compliant vaccine candidates that induce both cellular and humoral immune responses. In addition, the speed of gene synthesis and production of VLPs in plants will facilitate a rapid update of the IBV spike protein that is tailored and antigenically matched, in vaccine formulations. The native as well as recombinant versions of the spike glycoprotein of QX-like IBV were individually cloned into a plant expression vector, as were the membrane, envelope and nucleocapsid proteins. Various Agrobacterium strains were tested to identify the most appropriate strain to mediate the production of IB VLPs in plants. IBV structural proteins were successfully expressed in N. benthamiana ΔXT/FT plant leaf material, as confirmed by immunological detection, but low yields were obtained. No VLPs were detected using transmission electron microscopy, indicating that VLPs may not have assembled. In future, alternative designs of the spike protein and the addition of molecular chaperone genes will potentially elevate the numbers of VLPs produced, resulting in a commercially viable vaccine product.