Examining the potential of rationally designed recombinant Infectious Bronchitis Virus as a vaccine against Infectious Bronchitis

Abstract

Infectious Bronchitis Virus (IBV) is a gammacoronavirus that is prevalent in poultry flocks worldwide. Infection results in Infectious Bronchitis (IB), an economically important disease characterised by snicking, rales, lethargy, loss of tracheal ciliary activity, reduced weight gain and reduced egg production. Vaccination is commonly practiced using both live attenuated and inactivated vaccines. Live attenuated vaccines are generated by serial passage of a pathogenic field strain in embryonated chicken eggs; the molecular mechanism is unknown. A fine balance needs to be achieved between the loss of pathogenic in vivo phenotype and the retention of immunogenicity. Due to the unpredictability of the process and the time required there is a drive to rationally design and rationally attenuate recombinant IBV (rIBV) that can be subsequently utilised as live attenuated vaccines. Vaccine induced protection is predominantly associated with the spike (S) glycoprotein, which consists of two subunits, the immunodominant S1 and the highly conserved S2. Whilst the latter is considered to be less immunogenic, it has been reported to play a role in protective immunity. In this thesis rIBVs, based on the apathogenic rIBV, Beau-R, that express heterologous S1 or S2 subunits BeauR-QX(S1), BeauR-M41(S1), BeauR-M41(S2), or S glycoproteins, BeauR-M41(S) and BeauR-4/19(S) are investigated as vaccines against either homologous or heterologous challenge. The rIBV expressing a heterologous S glycoprotein, BeauR-M41(S) offered increased protection against homologous challenge as defined by the retention of ciliary activity in comparison to those expressing heterologous S1 or S2 subunits, BeauR-M41(S1) and BeauR-M41(S2) respectively. The protection induced however fell short of the standards set by the European Pharomopiea and further investigation demonstrated that rIBV expressing heterologous S glycoproteins, BeauRM41( S) and BeauR-4/91(S) could not protect against a heterologous QX challenge. One possible explanation is the limited replication of the vaccine viruses in vivo. Investigation of the Beau-R vaccine backbone determined that replication is temperature sensitive and highly restricted at 41°C, the core body temperature of a chicken. As a consequence a rIBV based on the laboratory strain M41-CK was subsequently investigated for its potential to act as a vaccine backbone.