Construction and Immunological Characterization of Recombinant Marek's Disease Virus Expressing IBDV VP2 Fusion Protein

A transfer plasmid vector, pUC18-US10-VP2, was constructed by inserting the gene of enhancer green fluorescent protein (eGFP) fused to VP2 gene of very virulent infectious bursal disease virus (IBDV) JS strain into the US10 fragment of the Marek's disease virus (MDV) CVI988/Rispens. The recombinant virus, designated as rMDV, was developed by co-transfection of CEF with the transfer plasmid vector and simultaneous infection with the CVI988/Rispens virus. The PCR and IFA results indicated that rMDV was stable after 31 passages. Chickens vaccinated with rMDV were protected from challenge with 100 LD50 of IBDV JS. The protection ratios of the chickens vaccinated with the 1 000, 2 000, and 5 000 PFU of the rMDV were 50%, 60%, and 80%, respectively. It was interesting that the average histopathology BF lesion score of the chicken group immunized with 5 000 PFU of rMDV by one-time vaccination was almost the same as that of the chicken group vaccinated with IBDV live vaccine NF8 strain twice (2.0/1.5). There was no difference in protection between the two groups (P> 0.05), but there was significant difference between the groups immunized with 5 000 PFU of rMDV and normal MDV vaccine. This result demonstrated that the rMDV-expressing VP2 fusion protein was one of the most effective vaccine candidates against IBDV in SPF chickens.