Molecular characterization of the spike protein of an avian infectious bronchitis virus (IBV) with cell culture tropism and identification of the underlying determinants

Avian infectious bronchitis virus (IBV) is the causative agent of the disease infectious bronchitis in chickens which occurs globally and causes significant economic damage. The control of IBV remains a challenge and vaccination against IBV is performed extensively. Because of the restricted species tropism of IBV, vaccine strains have to be cultivated in embryonated chicken eggs (ECEs), which represent a laborious and expensive propagation method. The possibility of in vitro propagation of IBV vaccines in cell culture is highly desired. Usually, IBV strains cannot be propagated in cell lines apart from one exception: The Beaudette strain allows in vitro propagation in cell lines and thus has already been extensively characterized. Certain amino acids in the Beaudette spike protein were identified to be responsible for the ability of Beaudette to infect cell lines. However, so far cell culture-adapted IBV strains based on the Beaudette strain have failed to confer sufficient protection against challenge because of the apathogenic nature of the Beaudette strain. IB66HP, an IBV strain that was adapted to EB66® cells, represents a novel IBV strain with cell culture tropism. The objective of this thesis was to determine the molecular determinants responsible for the cell culture tropism of IB66HP in comparison to the determinants described for IBV Beaudette. An IBV reverse genetic system was used to generate several recombinant IBVs (rIBVs) for in depths analysis of the influence of the IB66HP spike protein sequence in the context of the egg-restricted IBV H52.