Glycoprotein-C-gene-deleted recombinant infectious laryngotracheitis virus expressing a genotype VII Newcastle disease virus fusion protein protects against virulent infectious laryngotracheitis virus and Newcastle disease virus

To develop an alternative vectored vaccine against both Newcastle disease virus (NDV) and infectious laryngotracheitis virus (ILTV), the glycoprotein C (gC) gene was first deleted from an avirulent ILTV. Based on this gC-deleted ILTV mutant, a recombinant ILTV expressing the fusion protein (F) of a genotype VII NDV (designated ILTV-Delta gC-F) was then constructed. Expression of the NDV F protein in ILTV-Delta gC-F-infected LMH cells was examined with an immunofluorescence assay and western blotting. The F gene was stably maintained in the genome of ILTV-Delta gC-F and the F protein was stably expressed. Compared with the parental virus, ILTV-Delta gC-F demonstrated an increased penetration capacity in vitro, and an increased replication rate in vitro and in vivo. Both the parental virus and ILTV-Delta gC-F were avirulent in chickens. Vaccination of specific-pathogen-free chickens with ILTV-Delta gC-F induced ILTV-specific antibodies, detected with an enzyme-linked immunosorbent assay (ELISA), and provided complete clinical protection against virulent ILTV, although viral shedding and replication were detected in the respiratory tract in the early stage of infection in a very small number of birds. Vaccination with ILTV-Delta gC-F also provided significant protection against challenge with a virulent genotype VII NDV, although the level of NDV-specific antibodies detected with an ELISA was low. Notably, the numbers of birds that were positive for the virulent genotype VII NDV and the replication of the challenge virus NDV in selected target tissues were significantly lower in the ILTV-Delta gC-F-vaccinated chickens than in the control birds. Our results indicate that ILTV-Delta gC-F has potential utility as a bivalent candidate vaccine against both infectious laryngotracheitis and Newcastle disease.