Interferon-gamma influences immunity elicited by vaccines against very virulent Marek's disease virus

Vaccination of chickens with herpesvirus of turkey (HVT) confers only partial protection against challenge with a very virulent Marek's disease virus (MDV). Here, we evaluated the ability of recombinant chicken interferon-gamma (rChIEN-gamma) to enhance protective efficacy of HVT against the very virulent MDV strain, RB1B. The bioactivity of IFN-gamma expressed by a plasmid expression vector was confirmed by its ability to stimulate a chicken macrophage cell line (HD11) to produce nitric oxide (NO) in vitro. The administration of HVT with 5 mu g of pcDNA:chIFN-gamma plasmid reduced the incidence of tumor development significantly when compared to vaccinated birds (77.7% in the HVT + empty vector group and 80% in HVT group versus 33.3% in the HVT + chIFN-gamma group) and significantly increased IFN-gamma expression in the splenocytes of the protected group, suggesting that rChIFN-gamma increases the potency of HVT against MDV. Further analysis demonstrated that the protected birds that received HVT vaccine and/or plasmid had lower MDV genome load and lower amounts of transcripts for meq and vIL-8 than in the birds without lesions. Similarly, lower expression of IL-10, IL-18 and IL-6 was observed in the chickens without lesions compared to the chickens that had lesions, suggesting an inverse association between up-regulation of these cytokines and vaccine-induced immunity. In conclusion, IFN-gamma can positively influence immunity conferred by HVT vaccination against challenge with a very virulent Marek's disease virus (vvMDV) in chickens.