The Vero cell-adapted infectious bronchitis virus strain rHV80 exhibits high immunogenicity and broad tissue tropism

Infectious bronchitis virus (IBV) is a coronavirus that naturally infects chickens and causes damage to multiple tissues and organs, including the respiratory tract. Vaccination remains the primary method for preventing and controlling infectious bronchitis (IB). Progress toward cell-based, attenuated live vaccines has been hindered by IBV's poor replication in vitro. In previous work, we utilized the rH120 strain as the parental strain and domesticated it to derive the IBV strain HV80, which replicates efficiently in Vero cells. Using reverse genetics, we then constructed its infectious clone, rHV80. To assess its potential as a cell-adapted attenuated live vaccine, we evaluated the immunogenicity of the rHV80 strain in 1-day-old SPF chicks. We found that the strain elicits a robust immune response after immunization or infection. However, serial passage broadened the strain's tissue tropism. Compared with the parental strain, the rHV80 strain infects additional tissues, including the pectoralis, heart, and testis. It produces brain lesions that lead to neurological signs and late-stage mortality in chicks, compromising its safety as a live attenuated vaccine. This study shows that using in vitro cell-adapted strains of IBV as seed material for live attenuated vaccines can pose safety risks. Meanwhile, the results offer a theoretical foundation for the rational development of live attenuated IBV vaccines.