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.
