A methodology to universally attenuate avian coronavirus strains

Avian coronavirus infectious bronchitis virus (IBV) is the etiological agent of infectious bronchitis, an acute highly contagious economically relevant respiratory disease of poultry. IBV is prevalent in all types of poultry flocks worldwide and is responsible for economic losses, welfare problems in chickens and a potential risk to food security. IBV predominantly causes respiratory disease but can also infect and damage other organs such as the kidneys or the reproductive tract (resulting in loss of egg production and/or egg quality). Infection with IBV increases susceptibility to secondary bacterial infections.

Although live attenuated vaccines (LAV) and inactivated vaccines (only used as a boost after LAV) are universally used in the control of IBV, the protection gained by use of vaccination can be lost either due to vaccine breakdown or the emergence of a new IBV strain that is not related to the vaccine used. Existing vaccines alone do not offer cross-protection between the many different circulating serotypes of IBV. Current IBV LAV are produced by multiple passages of virulent field viruses in embryonated chicken eggs, a costly and time-consuming process. Spontaneous mutations arise throughout the IBV genome, some of which lead to attenuation of the virus. However, once the virus is used to inoculate chickens the mutations within the LAV may back-mutate resulting in virulent virus. Our research has highlighted that very few consensus level changes are acquired during the passaging process thereby presenting a short route back to virulence. Additionally, this method does not allow for rapid vaccine development/production in response to emerging strains.

Our previous research resulted in the generation of a recombinant IBV (rIBV) known as M41-R, based on a pathogenic strain M41-CK. M41-R is attenuated in vivo and in ovo by two amino acid changes in residues of replicase non-structural proteins (nsps) 10 and 14, which were found to be conserved among not only IBV strains but members of the wider coronavirus family. Vaccination of specific-pathogen-free (SPF) chickens with M41-R induced 100% protection following homologous challenge with virulent M41-CK, meeting the industry standards set by the European Pharmacopoeia. No clinical disease was observed in vaccinated birds post challenge and additionally no challenge virus replication was detected.

This project will assess the potential of these replicase mutations as a universal methodology to attenuate IBV for the rapid development of LAV. The project will focus on two IBV strains of economic importance, M41 and QX(D388), which are different serotype, genotype and also protectotype. Our previous research into the replicase mutations has been primarily academic and has utilised SPF Rhode Island Red (RIR) chickens, which are not used by the poultry industry as layers, breeders or broilers. However, these chickens offer an excellent tool for research purposes as they are a closed population and therefore more homogeneous than outbred birds used in a commercial setting. The aim of this project is to start bridging the gap between academia and industry by aiming to answer questions that are required for vaccine licensing including the effect of vaccine dose on safety, whether the vaccines elicit protection against homologous challenge in commercial birds and determine the risk of reversion.