Spike protein derived from an apathogenic IBV strain confers attenuated phenotype to a nephropathogenic IBV strain

Infectious bronchitis virus (IBV), a Gammacoronavirus, causes the economically damaging disease of poultry, infectious bronchitis. Development of IBV live attenuated vaccines (LAVs) is dependent on the supply of embryonated hens' eggs and attenuation through serial passaging, upwards of 80 times. Vaccine development needs to keep pace with IBV evolution; an issue compounded as cross protection between individual IBV serotypes is unpredictable and often limited. Rational attenuation alongside the ability to propagate LAVs in cell culture offers significant advantages in terms of adaptability, cost, and speed. Most field IBV strains exhibit restricted in vitro tropism, being unable to replicate in primary or continuous cells without serial passage and adaptation. Beaudette, an attenuated IBV strain, is capable of replication in Vero cells, an ability conferred by the spike (S) glycoprotein. In this study, utilizing a recombinant IBV based on the nephropathogenic IBV strain D388, in which the S sequence is replaced with the equivalent sequence derived from Beaudette, we determine that the Beaudette spike can confer the ability to replicate in Vero cells and Beaudette's associated attenuated in vivo phenotype to a distantly related IBV strain. Assessment of infectious progeny and viral RNA in vivo suggests that attenuation was conferred by a tissue tropism-independent molecular mechanism. Analysis of innate immune responses revealed a lower intensity and less inflammatory profile. Our findings support the use of the Beaudette S protein as an avenue for the development of rationally attenuated, cell culture-propagated rIBV vaccines and identify the S protein as a contributor to nephropathogenicity.IMPORTANCEInfectious bronchitis virus (IBV), a Gammacoronavirus of chickens, causes significant economic losses worldwide. Current live attenuated vaccines (LAVs) are generated through extensive serial passage of virulent IBV strains in specific-pathogen-free (SPF) embryonated hen's eggs, upwards of 80 passages. This process is slow, dependent on the availability of SPF embryonated eggs, and the molecular basis of attenuation remains poorly understood. Development of rationally attenuated, cell-culture-based IBV vaccines would enable more rapid responses to emerging viral variants/strains. Using a recombinant IBV, we demonstrated the spike glycoprotein from the attenuated IBV strain Beaudette can confer attenuation and the ability to replicate in Vero cells to a distantly related virulent IBV strain. Attenuation was not linked to changes in tissue tropism but was associated with altered host responses. Our research provides new insights into IBV pathogenesis and presents an avenue for rational attenuation that can be exploited for future development of cell-based IBV vaccines.