Protective Efficacy of a Recombinant Newcastle Disease Virus against Infectious Bronchitis

Abstract

A previous study has reported that Newcastle disease virus (NDV) recombinant LaSota strain (rLS) expressing infectious bronchitis virus (IBV) Arkansas-type (Ark) trimeric spike ectodomain (Se) (rLS/ArkSe) provides suboptimal protection against IBV challenge. This study was aimed at developing rLS expressing chicken granulocyte-macrophage colony-stimulating factor (GMCSF) and IBV Ark Se in an attempt to enhance vaccine effectiveness. We first compared protection conferred by vaccination with rLS/ArkSe and rLS/ArkSe.GMCSF. Vaccinated chickens were challenged with virulent Ark-type IBV, and protection was assessed by clinical signs, viral load, and tracheal histomorphometry. Results showed that challenged chickens immunized with rLS co-expressing GMCSF and the Se had significantly reduced tracheal viral load and tracheal lesions compared to chickens vaccinated with rLS/ArkSe. In a second experiment, we evaluated enhancement of cross-protection by a Massachusetts (Mass) serotype attenuated vaccine after rLS/ArkSe.GMCSF priming or boosting. Vaccinated chickens were challenged with Ark-type IBV, and protection was evaluated. Results showed that priming or boosting with the recombinant virus significantly increased cross-protection conferred by Mass vaccine against Ark virulent challenge. Greater reductions of viral loads in both trachea and lachrymal fluids were observed in chickens primed with rLS/ArkSe.GMCSF and boosted with Mass. Consistently, Ark Se antibody levels measured with recombinant Ark Se-protein-coated ELISA plates 14 days after boost were significantly higher in these chickens. Unexpectedly, the inverse vaccination scheme, i.e., priming with Mass and boosting with the recombinant vaccine, proved somewhat less effective. We concluded that a prime and boost strategy using rLS/ArkSe.GMCSF and the Mass attenuated vaccine, ubiquitously used world-wide, provides enhanced cross-protection. Thus, rLS/GMCSF co-expressing the Se of regionally relevant IBV variants could be used in combination with live Mass vaccines to protect against these regionally circulating variant strains.