Production of plant-expressed virus-like particle vaccines against infectious bronchitis coronavirus and vaccine efficacy in chickens

Abstract

Infectious Bronchitis Virus (or IBV) is the root cause of extremely infectious respiratory disease that occurs in chickens. It has a high mutation rate that causes the emergence of multiple variants that are difficult to control, leading to major worldwide economic losses to the poultry industry. In this study, virus-like particles (VLPs) were produced using Agrobacteriummediated transient protein expression in Nicotiana benthamiana plants as potential vaccines against IBV. Modifying the full-length IBV Spike (S) protein resulted in high levels of VLPs resembling native IBV particles visualised under transmission electron microscopy. The highest VLP expression levels were obtained when the native IBV S protein transmembrane (TM) domain and cytosolic tail (CT) sequences were substituted with the equivalent sequences of the Fusion (F) protein of Newcastle Disease Virus (NDV), and co-infiltrated with the NDV matrix protein. The second highest VLP expression levels were obtained when the TM and CT were substituted with the equivalent sequences of the Influenza A Virus (IAV) Haemagglutinin (HA) protein, and co-infiltrated with the IAV M2 protein. The lowest VLP expression levels were obtained when the chimeric modified S protein with its native TM and CT was coinfiltrated with the IBV Membrane, Envelope, and Nucleocapsid proteins. There was seroconversion in SPF chickens that received a single intramuscular immunisation of 5 μg or 20 μg (S protein content) adjuvanted IBV VLPs, with geometric mean S protein specific haemagglutination inhibition (HI) titres of 9.1 log2 and 10.5 log2 respectively after two weeks. In a challenge study with the live virus, the VLP vaccinated chickens elicited S protein specific antibodies at a level comparable to those elicited by those vaccinated with a live-attenuated vaccine mix (IBV Ma-5 and 4-91 vaccines) with geometric mean HI titres of 6.8 and 7.2 log2 respectively. The VLP vaccine was able to reduce both oropharyngeal and cloacal viral shedding between 3 and 7 days post challenge, which was similar to the reduction seen in the birds vaccinated with the live-attenuated vaccine mix. The birds vaccinated with the plantproduced VLP vaccine showed higher tracheal ciliary motility than those vaccinated with the live vaccines, with no adverse vaccine effects observed. These plant-produced IB VLPs have great potential for the global poultry industry as they are highly immunogenic, safe, and they can easily be updated to antigenically match any circulating IBV variant both speedily and cost-effectively.