Modification of the ADP-ribose-1"-monophosphatase domain in recombinant infectious bronchitis virus affects viral replication in vitro and attenuates the virus in vivo

The gammacoronavirus infectious bronchitis virus (IBV) is responsible for an acute respiratory disease in domestic fowl, which has high economic impact and welfare implications in the poultry industry. The IBV non-structural protein, nsp3, is a multifunctional protein containing several putative domains, including an ADP-ribose-1’-monophosphatase (ADRP) domain conserved among coronaviruses. Inactivation of the ADRP domain in alpha- and betacoronaviruses is associated with reduced pathogenicity in vivo and altered interferon response and cytokine profiles in the host, without affecting viral replication in vitro. Therefore, recombinant viruses lacking ADRP functions have been proposed as ideal candidates for live attenuated vaccines. A recombinant IBV (rIBV) was generated in the backbone of the pathogenic M41-K strain containing a mutation in the ADRP domain catalytic core, known to abolish ADRP function in other coronaviruses. The ADRP-defective rIBV was characterised in vitro and in vivo; conversely to previously described ADRP-defective coronaviruses, in vitro analysis showed a reduction of viral replication, and the rIBV displayed a distinctive plaque phenotype. No reversion of the mutation occurred after serial passages of the virus in primary avian cell culture, nor in ex vivotracheal organ cultures which wereutilisedas a surrogate for in vivo stability testing. Pathogenicity experiments conducted in vivo resulted in a reduction in clinical signs in comparison to M41-K-infected birds, and tracheal ciliary activity, a marker for pathogenicity, was comparable to mock infected birds. These data support the role of ADRP as a pathogenic determinant and demonstrate the potential of ADRP-defective rIBV as a promising candidate vaccine.