Pathogenicity and vaccine efficacy of a thymidine kinase gene deleted infectious laryngotracheitis virus expressing the green fluorescent protein gene.

The deletion of the thymidine kinase (TK) gene of herpesviruses causes a reduction in their virulence. However, the effects of the TK gene in infectious laryngotracheitis virus (ILTV) have not been clearly elucidated. In the present study, we constructed a TK gene-deleted recombinant ILTV expressing the green fluorescent protein (GFP) gene as a marker. The GFP gene was inserted in place of the TK gene in both virulent and low virulence strains of ILTV. The GFP gene in the recombinants was expressed in chicken kidney cells, LMH cells and in the chorioallantoic membrane of embryonated chicken eggs. The recombinants produced cytopathic effects in chicken kidney cells and LMH cells and formed pocks in the chorioallantoic membrane of embryonated chicken eggs. The growth rate of the recombinant in chicken kidney cells was similar to that of wild type viruses. The recombinants showed a reduction of virulence compared to that of parental viruses and induced protection against virulent ILTV in specific pathogen free chickens. The recombinant expressing GFP gene may be a candidate for a genetically engineered vaccine and provide a means to study growth kinetics and mechanism of latent infection and reactivation of ILTV. In this study, we confirmed that the TK gene is directly related to virulence of ILTV. This is the first report to show the evidence that the TK gene is a major gene related to virulence of ILTV.